Fluorine-free electrolytes in batteries: principles, strategies, and advances

Abstract

Electrolytes play a pivotal role in battery technologies, influencing performance and safety. However, electrolytes containing fluorine present adverse environmental risks due to their high greenhouse gas emissions, which contribute to global warming. Hence, developing fluorine-free alternatives is imperative to design net-zero fluorine electrolytes. This review addresses the need for sustainable, low-toxicity electrolytes by exploring strategies for eliminating fluorine in the electrolyte system. Studies on the choice of electrolyte ingredients, such as fluorine-free salts, green solvents, safe additives, and fluorine-free binders, have demonstrated that specific electrolyte ingredients can effectively enhance battery performance and safety. Recent progress highlights significant improvements in the environmental impact and functionality of fluorine-free electrolytes (FFEs), demonstrating their potential for practical applications. Despite these advancements, challenges remain in matching the performance and stability of traditional fluorinated electrolytes. Future research encourages us to focus on developing fluorine-free materials, understanding functional degradation processes, and ensuring commercial scalability. This review provides an in-depth look at recent innovations and promotes design principles for complete fluorine elimination strategies. It guides future pathways for creating high-performance, non-flammable, low-cost, environmentally sustainable FFEs for advanced rechargeable batteries. The summary and perspectives emphasize the importance and future directions of a sustainable circular economy in advancing sustainable electrolyte engineering.

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Boligarla Vinay

Vinay Boligarla is an MS student from the Dept. of Chemical Engineering in the Nano-Electrochemistry Laboratory group at the National Taiwan University of Science and Technology, Taiwan. He earned his B.Tech. in chemical engineering from Kalasalingam Academy of Research and Education, India, in 2023. His current research focuses on developing fluorine-free electrolytes for battery applications, aiming to enhance sustainability in energy storage. He is passionate about battery technology and strives to design environmentally friendly and efficient energy systems. His dedication to innovation and practical solutions drives his pursuit of advancements in next-generation batteries, contributing to developing safer and more sustainable energy storage technologies for real-world applications.

Yosef Nikodimos

Dr Yosef Nikodimos is an Assistant Research Fellow at the National Taiwan University of Science and Technology, Taiwan. He obtained his BSc and MSc degrees in Chemistry. He earned a dual PhD in 2021, one in Chemical Engineering from the National Taiwan University of Science and Technology and another in Energy and Battery Technology from Ming Chi University of Technology. His research focuses on electrolyte design, electrode engineering, and integrating advanced materials for sustainable energy storage solutions. His work aims to address interfacial stability challenges and enhance the electrochemical performance of next-generation batteries.

Tripti Agnihotri

Dr Tripti Agnihotri is a postdoctoral fellow in the Nano-Electrochemistry Laboratory group at the National Taiwan University of Science and Technology (NTUST). She earned her PhD in Chemical Engineering from NTUST, specializing in electrolyte and separator engineering to enhance the safety and performance of anode-free lithium metal batteries (AFLMBs) with high-voltage cathodes. With a deep-rooted passion for organic chemistry and energy storage systems, her research bridges chemistry and engineering to develop next-generation battery technologies. Her academic journey reflects a steadfast dedication to sustainable electrochemical energy solutions innovation.

Shadab Ali Ahmed

Shadab Ali Ahmed is a PhD student in the Nano-Electrochemistry Laboratory group at the National Taiwan University of Science and Technology. His research focuses on cathode modification to enhance the safety and performance of lithium metal batteries. With a strong passion for organic chemistry, materials science, and energy storage systems, he integrates chemical principles with engineering advancements to develop next-generation battery technologies. His academic journey reflects a deep commitment to creating sustainable and efficient electrochemical energy solutions for the future.

Wei-Nien Su

Wei-Nien Su is a professor from the Graduate Institute of Applied Science and Technology at the National Taiwan University of Science and Technology (NTUST), Taiwan. His research interests are rooted in synthesizing and characterizing advanced materials for various electrochemical devices and energy applications. Prof. Su is currently involved in multiple international and national research programs. He received his PhD from the Wolfson School, Loughborough University (UK), and his Diplom-Ing. in Chemical Engineering from Universität Stuttgart (Germany).

Bing Joe Hwang

Bing Joe Hwang studied Chemical Engineering and received his PhD in 1987 from the National Cheng Kung University, Taiwan. Since 2006, he has been serving as a chair professor at the National Taiwan University of Science and Technology (NTUST). His research has spanned various subjects, from electrochemistry to spectroscopy, interfacial phenomena, materials science, and theoretical chemistry. He has established experimental and computational strategies for the development of new materials and for understanding interfacial phenomena. His work has led to a better understanding of electrochemical reaction mechanisms and improved ability to predict the properties of potential new materials for batteries, fuel cells, and biosensors.

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Broader context

Electrolytes are a key component of batteries, playing a crucial role in determining the performance and stability of batteries. Fluorinated electrolytes are commonly used in traditional batteries due to their high electrochemical stability and conductivity. However, these fluorinated electrolytes present significant challenges, as they are toxic and harmful to the environment. This has spurred ongoing research into finding alternatives, specifically fluorine-free electrolytes (FFEs), to develop fluorine-free batteries (FFBs) that are both environmentally friendly and cost-effective. Despite the need for FFEs, most current research still focuses on partially fluorinated components, whether in the form of salts, solvents, diluents, or additives. Only a limited number of studies have fully explored the use of FFEs. This review seeks to address this gap by highlighting the critical issues in designing FFBs and examining the latest advancements in fundamental research. It proposes several innovative design strategies that could help overcome current challenges, offering a bridge to future sustainable and efficient battery technologies. By focusing on FFEs, this review aims to contribute to developing safer batteries for both people and the planet.

1. Introduction

In recent years, the global demand for advanced energy storage technologies has experienced a remarkable surge, driven by an increasing emphasis on sustainable energy solutions and the need for efficient power systems across various sectors.¹ This growing demand is a direct consequence of the ever-expanding range of applications that rely on rechargeable batteries, which have become an integral part of modern society.² From portable electronics that power our daily lives to electric vehicles that promise a greener future and even large-scale grid storage systems that stabilize our energy infrastructure, rechargeable batteries are at the forefront of technological innovation.³ However, despite their widespread adoption, several critical challenges have impeded these technologies’ broader implementation and optimization. Among the most pressing concerns are the environmental impact and safety risks associated with the materials used in these energy storage devices.⁴

At the heart of any battery lies the electrolyte, a vital component that significantly influences the device's overall performance, safety, and longevity.⁵ The electrolyte's primary function is to facilitate the movement of ions between the battery's electrodes during charge and discharge cycles, thereby enabling the electrochemical reactions necessary for energy storage and release.⁶ Traditionally, the electrolytes used in commercial batteries have been predominantly based on fluorinated compounds.^7–9 This preference for fluorinated electrolytes stems from their exceptional chemical and thermal stability, which ensures consistent performance even under demanding conditions. Additionally, these compounds offer high ionic conductivity, which is crucial for efficient charge transfer, and the ability to form stable interphases with the battery's electrodes, thereby enhancing the device's lifespan.^10–12

Common fluorinated components found in traditional electrolytes include lithium hexafluorophosphate (LiPF₆) salts and a variety of fluorinated solvents.¹³ LiPF₆, in particular, has been a staple in the battery industry due to its favorable properties, such as good solubility in organic solvents and the formation of a protective solid electrolyte interphase (SEI) on the anode surface.^14,15 Fluorinated solvents, on the other hand, contribute to the overall stability and conductivity of the electrolyte solution.¹⁶ Despite these advantageous characteristics, the use of fluorinated electrolytes is not without significant drawbacks.^17,18 The production and disposal of fluorinated compounds present substantial environmental challenges, as they can lead to the release of toxic and persistent pollutants.¹⁹ These pollutants pose severe risks to both human health and the environment, as they can accumulate over time and cause long-term ecological damage.²⁰ Recent advances have focused on improving performance by increasing the fluorine content of electrolytes and electrode components, which has been accomplished through the use of fluorinated salts, solvents, additives, and binders.^8,21 These fluorinated chemicals, namely perfluoroalkyl and polyfluoroalkyl substances (PFAS), have played an important role in enhancing battery's thermal stability, chemical resistance, and overall performance, particularly under high-voltage conditions.⁸ However, the European Chemicals Agency (ECHA) has announced intentions to adopt a dramatic regulation change, limiting the use of almost 10 000 fluorinated compounds, including PFAS, in the battery industry.^22,23 Notably, substances like lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), poly(vinylidene fluoride) (PVDF), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), which are frequently used as salts, binders, and solvents in lithium-ion and other advanced battery systems, are especially problematic because of their toxic and bioaccumulative nature.^24,25 The internal operation of cells releases hazardous HF and POF₃ which then degrade the other battery components.²⁶ In addition, toxic HF gas emission is a concern during the recycling of traditional batteries.²⁷ These ongoing challenges influence to shut down the fluorinated electrolytes as shown in Fig. 1.^27,28 Moreover, fluorinated electrolytes are associated with safety concerns, particularly regarding their thermal stability.^29,30 In the event of thermal abuse, such as overheating or short-circuiting, fluorinated electrolytes can undergo decomposition reactions that generate hazardous gases and trigger exothermic reactions.^31–33 This can result in thermal runaway, a dangerous phenomenon where the battery's temperature rapidly escalates, potentially leading to fire or explosion.³⁴ These safety issues underscore the urgent need for safer, more environmentally friendly alternatives in battery technology.^35–37

Fig. 1 Crisis to shut down the fluorinated electrolytes: regulatory restrictions, HF shutdown, and recycling risks.

The research community has increasingly focused on developing FFEs in response to these pressing challenges.^38–40 This emerging area of study aims to address the environmental and safety issues associated with traditional fluorinated compounds while striving to maintain or even enhance the electrochemical performance of batteries.²¹ FFEs encompass various materials, including alternative salts, solvents, additives, and diluents.^41,42 Each component is carefully designed to be environmentally benign, cost-effective, and compatible with various battery chemistries. This offers a promising pathway toward more sustainable and safer energy storage solutions.⁴³

Exploring FFEs is not merely a response to current challenges but also represents a proactive approach to future-proofing battery technology.^44,45 As the world transitions toward cleaner energy sources and electric mobility, the demand for high-performance batteries will only increase.^46,47 Therefore, the development of next-generation electrolytes that do not rely on fluorine is crucial for meeting future energy needs while minimizing environmental impact and enhancing safety.^48,49 While various review studies have explored greener, safer batteries, there has been limited focus on FFEs. This comprehensive review delves into the principles underlying FFEs, the strategies employed in their design and synthesis, and the latest advances in this exciting field. By exploring the potential of fluorine-free alternatives, we aim to highlight the opportunities and challenges in creating a new paradigm for safe and sustainable energy storage.

Herein, we compare the characteristics and performance of fluorinated and FFEs, highlighting the motivations for transitioning to fluorine-free alternatives. Next, we explore various strategies for eliminating fluorine from electrolytes, including fluorine-free salts, solvents, additives, and diluents. We explore recent innovations in fluorine-free batteries (FFBs), emphasizing cutting-edge developments in material design and electrolyte formulation strategies. Furthermore, we examine the performance of FFEs in different battery chemistries, including lithium (Li), sodium (Na), magnesium (Mg), potassium (K), calcium (Ca), and zinc (Zn) batteries. The review also addresses the challenges and limitations of implementing FFEs, such as electrochemical stability and ionic conductivity. Finally, we present conclusions and outlooks on the future directions of FFE research, emphasizing the potential for these materials to enable safer, more sustainable, and high-performance battery systems. This comprehensive review aims to provide a valuable resource for researchers and industry professionals working towards developing sustainable next-generation battery technologies.

2. Fluorinated vs. fluorine-free electrolytes (FFEs)

The selection of an appropriate electrolyte is crucial for the design and performance optimization of battery systems.⁵⁰ It plays an essential role in defining the device's efficiency, safety, and longevity.^50,51 Among the various components of a battery, the electrolyte serves as the medium that facilitates the movement of ions between the anode and the cathode, thus enabling the electrochemical reactions necessary for energy storage and release.⁵² In commercial battery systems, particularly lithium-ion batteries (LIBs), fluorinated compounds have historically been the preferred electrolyte choice.⁷ These compounds are celebrated for their remarkable chemical stability, high ionic conductivity, and the formation of stable interphases with electrodes.⁵³ These properties are critical for ensuring the reliable operation of batteries across various applications, ranging from consumer electronics to electric vehicles.^54–56 However, the widespread use of fluorinated electrolytes has increasingly come under scrutiny due to their associated environmental and safety concerns. As a result, there has been a growing interest in exploring fluorine-free alternatives.^44,57,58 This section delves into the fundamental differences between fluorinated and FFEs, examining their respective properties, performance, and the challenges they present.⁵⁷

Fluorinated electrolytes typically include fluorine-rich salts, such as LiPF₆, commonly employed in LIBs. These salts are prized for their ability to efficiently facilitate ionic transport and their excellent solubility in non-aqueous solvents.^59,60 LiPF₆ is mainly known for forming a stable passivation layer, known as the SEI, on the surface of the anode.⁶¹ The SEI layer acts as a protective barrier, preventing unwanted reactions between the electrolyte and the electrode, safeguarding the electrode material and extending the battery's lifespan.^62,63 The formation of the SEI layer predominantly relies on the presence of fluorinated species derived from various sources, including the salts, solvents, and additives used in the electrolyte composition. These fluorinated components collectively contribute to the robust performance characteristics of the battery.

One of the key features of fluorinated electrolytes is their ability to enhance battery performance by providing stability and safety. For example, the incorporation of 1-fluoroethylene carbonate (FEC) as a co-solvent with dimethyl carbonate (DMC) has been demonstrated to significantly improve the stability of high-rate stripping/plating processes and prolong the operational life of batteries, especially in anode-free lithium metal battery configurations.^64–68 FEC is particularly advantageous for anodes made from silicon and lithium metal, which are susceptible to substantial volume changes and challenging chemical environments during cycling.^69,70 The SEI layer formed in the presence of FEC typically consists of compounds such as lithium fluoride (LiF), lithium oxide (Li₂O), and other fluorinated organic species.⁷¹ These components enhance the kinetics of lithiation and improve the battery's coulombic efficiency (CE), making fluorinated electrolytes a popular choice for high-performance applications.^72,73 Fluorinated electrolytes, such as those containing LiPF₆ in organic solvents, have been the standard in LIBs for several reasons.⁷⁴ Firstly, fluorinated electrolytes exhibit high ionic conductivity, facilitating efficient ion transport between the anode and the cathode. This is crucial for achieving high power density and fast charging capabilities.^44,75 Secondly, the strong C–F bond in fluorinated compounds provides excellent chemical stability.^7,76 This stability is essential for maintaining the integrity of the electrolyte under various operating conditions, including high temperatures and voltages. Thirdly, fluorinated electrolytes can form a stable SEI layer on the anode surface, which protects the electrode from further reactions with the electrolyte, thereby enhancing the battery's cycle life and safety.¹¹

Despite the numerous benefits of fluorinated electrolytes, they also present drawbacks such as generation of harmful byproducts⁶⁶ like hydrogen fluoride (HF) and phosphorus pentafluoride (PF₅).¹⁶ Over the past decade, non-aqueous electrolytes containing fluorinated ingredients have faced challenges due to their sensitivity to moisture during electrochemical reactions because of hydrolysis of fluorinated salts, solvent decomposition, or contamination. For instance, the hydrolysis of LiPF₆ with trace water exacerbates this problem by producing corrosive HF, damaging key battery components and reducing efficiency.^16,28,68,77 Additionally, releasing these gases heightens safety risks, potentially triggering thermal runaway, which can result in fire or explosion. Furthermore, the reaction generates toxic by-products, posing serious threats to both human health and the environment.^28,77,78 Although many studies emphasize the degradation mechanism of fluorinated ingredients, especially LiPF₆, the solutions to address the issue remain unclear.^79–81 For example, Di Muzio et al.'s computational studies reveal that LiBOB (lithium bis(oxalato)borate) is more resistant to hydrolysis than LiBF₄, which degrades via LiF/HF formation.⁸² The stability of the fluorine-free BOB⁻ anion enhances moisture resistance, ensuring safer, long-lasting performance.

Beyond operational hazards, the disposal and recycling of PFAS-containing battery components pose significant environmental challenges, as demonstrated in Fig. 1. The release of toxic substances such as HF and PF₅ during the disposal or recycling of spent batteries can have long-term adverse effects on the environment and public health.⁸³ The management of these batteries requires stringent safety protocols to prevent contamination and ensure safe handling. In confined spaces, such as during electric vehicle accidents, releasing toxic gases can pose immediate dangers to first responders and the public, underscoring the need for safer electrolyte alternatives.⁸⁴ Eventually, with growing concerns over the toxicity and environmental impact of PFAS, regulatory bodies like ECHA are intensifying efforts to impose stricter controls, promoting the development of fluorine-free alternatives.²⁵ Economic concerns and potential slowdowns in battery innovation further complicate regulatory efforts.

In light of these challenges, there has been a burgeoning interest in developing FFEs that can deliver comparable performance while mitigating the environmental and safety issues associated with fluorinated compounds.⁵⁴ The development of FFEs is centered around eliminating fluorine-containing compounds, which reduces the environmental footprint of batteries and enhances their safety profile.⁴⁵

FFEs are characterized by their environmentally friendly nature, producing toxic-free and persistent decomposition products as illustrated in Fig. 2. Moreover, these electrolytes are less likely to generate hazardous gases upon decomposition, thereby reducing the risk of thermal runaway and improving the overall safety of the battery system.⁸⁵ Eventually, the development of sustainable and effective energy storage systems has attracted attention, which has prompted research into cutting-edge materials that improve battery performance. For example, with recent advances in FFEs, LiBOB is a potential electrolyte salt for lithium-ion batteries due to its outstanding thermal stability and high-voltage endurance, and is a cost-effective PFAS-free alternative.²⁵ It enables a stable CEI that prevents oxidative degradation and shields cathode materials at voltages as high as 5.3 V.⁸⁶ LiBOB has been utilized as an HF scavenger, improving battery safety and cycle performance by lowering the production of dangerous free radicals.⁸⁷ Additionally, it enhances compatibility with manganese and iron cathodes, inhibits electrolyte deterioration, and promotes improved cycle stability in high-energy systems.⁸⁷ Particularly in traditional batteries, minute amounts of water in the electrolyte can lead to hydrolysis and affect the development of the solid electrolyte interface (SEI) on electrode surfaces.⁸⁰ However, the hydrolysis of LiBOB offers significant benefits.⁸⁸ LiBOB breaks down in water and forms non-toxic byproducts, creating a stable SEI that safeguards the battery's performance over time. LiBOB hydrolysis is slower and more regulated compared to other lithium salts, improving electrolyte stability.⁸⁹ This characteristic not only improves cycle stability but also reduces negative environmental consequences by limiting the production of toxic PFAS.²⁵ While LiBOB has great stability and is non-toxic, it confronts problems such as poor solubility in low-dielectric solvents and low ionic conductivity at low temperatures, resulting in increased resistance and performance decay.⁸⁸ At voltages greater than 4.7 V, the failure of the CEI causes side reactions, significantly lowering performance.⁹⁰

Fig. 2 Illustration of the fluorination and fluorine-free era for sustainable battery contribution.

Consequently, the development of effective FFEs is not without its own set of challenges. One of the primary hurdles is achieving electrochemical stability and ionic conductivity. FFEs may be more susceptible to oxidation and reduction, potentially affecting the battery's performance and cycle life.⁴⁵ Additionally, fluorine-free salts and solvents often exhibit lower ionic conductivity, which can limit the power density and efficiency of the battery.^44,45

To address these challenges, researchers have been exploring a variety of non-fluorinated salts and solvents.⁴⁴ Several anions, including those based on elements such as boron (B), carbon (C), oxygen (O), sulfur (S), chlorine (Cl), phosphorus (P), and nitrogen (N), have been investigated as potential alternatives to fluorinated compounds. While many of these anions contain fluorine, a few notable fluorine-free options have emerged, such as nitrate (NO₃), dicyanotriazolate, bis(oxalato)borate (BOB), perchlorate (ClO₄), and tris(oxalato) phosphate (TOP).⁹¹ These anions vary in their ionic conductivity and stability, with some showing promising results in laboratory settings.⁹²

A critical aspect of designing effective FFEs is selecting suitable salts and solvents to form stable interphases at the electrode–electrolyte interface.⁵⁸ The interaction between anions and solvent molecules is pivotal in determining the dissolution behavior of electrolyte salts in non-aqueous solvents.⁹³ Weakly coordinating anions with delocalized charges are particularly desirable, as they can lower the dissociation energy of salts, thereby enhancing ionic conductivity.^94,95 Furthermore, these anions can promote the formation of stable interphases at the electrode–electrolyte interface, which is crucial for maintaining the physical and electrochemical stability of the SEI layer.

Recent research has emphasized the significance of solvent structure and its influence on the reactivity of the electrolyte in lithium-metal batteries.⁹⁶ The interactions between anions and solvent molecules can significantly impact the composition and stability of the SEI layer. For instance, a shift from solvent-dominant to anion-dominant interfacial reactions can alter the composition of the SEI, transforming it from organic-rich to inorganic-rich.^54,97 This shift can enhance the physical and electrochemical stability of the SEI, thereby optimizing the performance and safety of lithium-metal battery systems.⁹⁸ Despite the progress in developing FFEs, challenges remain, such as effectively passivating aluminum cathode current collectors at high potentials. In LIBs, LiPF₆ is often favored over fluorinated salts like lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI) due to its superior ability to passivate aluminum surfaces.^75,99–107 However, recent studies have suggested that some non-fluorinated salts can effectively passivate aluminum, providing a viable alternative to fluorinated salts.^108–110

Table 1 provides a foundation for understanding the comparative landscape of fluorinated and FFEs. FFEs aim to address the limitations of fluorinated electrolytes by eliminating the use of fluorine-containing compounds.⁵⁷ The development of FFEs focuses on maintaining desirable electrochemical properties while improving environmental sustainability and safety. Key characteristics of FFEs include their environmental friendliness, as they reduce the conservation footprint of batteries by eliminating fluorinated compounds.^8,44,111 They are designed to be more benign, with less toxic and persistent decomposition products. Moreover, FFEs are less likely to produce hazardous gases upon decomposition, reducing the risk of thermal runaway and improving overall battery safety.¹¹¹ However, developing effective FFEs presents several challenges. Achieving comparable electrochemical stability without fluorine can be difficult, as FFEs may have lower resistance to oxidation and reduction, affecting the battery's performance and cycle life. Additionally, fluorine-free salts and solvents may have lower ionic conductivity than their fluorinated counterparts, potentially limiting the power density and efficiency of the battery.⁷ Therefore, future advancements in materials science, electrolyte formulations, and battery engineering will be critical to overcoming these challenges and realizing the full potential of FFEs.

Table 1 Comparison between fluorinated electrolytes and FFEs based on key characteristics and performance indicators

Fluorinated electrolytes	Characteristics	FFEs
High	Ionic conductivity	Variable (generally lower than that of fluorinated counterparts)
Excellent (due to strong C–F bonds)	Chemical stability	Variable (depends on the composition)
Decompose to harmful gases	Thermal stability	Decompose to less hazardous products
Negative (toxic and persistent pollutants)	Environmental impact	More environmentally friendly, fewer toxic by-products
Can decompose into hazardous gases (e.g., HF, PF5)	Safety concerns	Lower risk of producing harmful gases and reduced thermal runaway risk
Wide voltage window	Electrochemical stability	Variable, generally narrower than that of fluorinated counterparts
Form a stable SEI on the anode	SEI formation	SEI formation varies; alternative protective interphases may be needed
High (due to specialized manufacturing and raw materials)	Cost	Potentially lower (depending on the materials used)
Long (due to the stable SEI)	Cycle life	Variable, depending on SEI formation and electrolyte stability
Widely used in commercial LIBs	Applications	Emerging and research-focused batteries

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3. Roadmap of progress and challenges in fluorine-free electrolytes

3.1. Roadmap of progress in fluorine-free electrolytes

Several recent studies have focused on pollution-free electrolyte design as a key aspect of next-generation research in the context of climate change. FFBs are increasingly popular for their sustainable and eco-friendly properties, and they often utilize metals such as Li, Na, Mg, Al, Si, K, Ca, and Zn. These batteries aim to provide high performance, cost-efficiency, and long cycle life while eliminating harmful fluorine compounds. Innovations include lithium bis(oxalato)borate (LiBOB) for LIBs, sodium bis(oxalato)borate (NaBOB) for Na-ion batteries, and magnesium borohydride (Mg(BH₄)₂) for Mg batteries. Additionally, researchers are exploring aluminum chloride (AlCl₃) in ionic liquids, potassium bis(oxalato)borate (KBOB), calcium tetrafluoroborate (Ca (BF₄)₂), and zinc sulfate (ZnSO₄) for their respective metal batteries. Challenges such as lower ionic conductivity, thermal stability, and effective interphase formation compared to fluorinated electrolytes persist. Several studies show optimized electrolyte formulations and synergistic additives to boost stability and conductivity. Innovative strategies, including ionic liquids and customized solvent systems, are being developed to ensure that these fluorine-free alternatives meet or surpass the performance of traditional batteries while maintaining environmental safety. Earlier studies on FFBs have motivated the pursuit of high performance, and fortunately, a few studies achieved highly reliable practical prospects. Among all FFBs, FFE studies are frequently performed in lithium-based batteries. As shown in Fig. 3, the developed FFBs showed enhanced oxidative stability owing to optimized electrolyte composition, engineered robust interphases, and widened potential window, all of which help prevent breakdown under extreme conditions.

Fig. 3 Roadmap showing the progress of FFE-based batteries.^{45,58,112–127} LMBs Reproduced with permission,⁴⁵ Copyright (2024) American Chemical Society. LIBs Reproduced with permission,⁵⁸ Copyright (2020) American Chemical Society. LIBs Reproduced with permission,¹¹² Copyright (2024) Elsevier. LMBs Reproduced with permission,¹¹³ Copyright (2014) Elsevier. LIBs, Reproduced with permission,¹¹⁴ Copyright (2018) Springer Nature. Li–S battery, Reproduced with permission,¹¹⁵ Copyright (2019) Wiley-VCH. LIBs, Reproduced with permission,¹²³ Copyright (2024) Wiley-VCH. LMBs, Reproduced with permission,¹²⁶ Copyright (2025) Wiley-VCH. LIBs, Reproduced with permission,¹²² Copyright (2024) Wiley-VCH. Na battery Reproduced with permission,¹¹⁶ Copyright (2020) American Chemical Society. Na battery Reproduced with permission,¹¹⁷ Copyright (2021) American Chemical Society. Ca battery Reproduced with permission,¹¹⁸ Copyright (2021) Springer Nature. Micro-Si anode battery Reproduced with permission,¹²⁰ Copyright (2023) Wiley-VCH. KMB Reproduced with permission,¹²¹ Copyright (2023) Springer Nature. Zn battery Reproduced with permission,¹¹⁹ Copyright (2023) Wiley-VCH. Zn battery Reproduced with permission,¹²⁴ Copyright (2024) American Chemical Society. For monovalent and multivalent cations, Reproduced with permission,¹²⁵ Copyright (2025) Wiley-VCH.

To overcome PFAS toxicity, an FFE is a promising approach for future generations where all components are fluorine-free species. Recently, an FFE study was undertaken by Jiang et al. by using the borate anion in an ether solvent (1,2-dimethoxyethane (DME)), and a good cycling performance at high temperatures was demonstrated.⁴⁵ In addition, incorporating a fluorine-free binder with good self-healing capabilities at a high loading LFP cathode boosted the capacity retention (CR) of Li-LFP cells to 82%, at 60 °C, as shown in Fig. 4a.⁴⁵ The FFE study was conducted for the first time using the novel strategy as bilayer phenomena for robust SEI formation. This bilayer approach consists of organic (ROCO₂, C–C, Li₂CO₃) and inorganic components (LiB_xO_y, Li₂O, Li₃N, LiN_xO_y). While thin electrolyte–electrode interfaces (EEL) with organic components (inner layer) offer fast ion transport and stable chemical and mechanical support, thick EEL with organic components can lead to an unstable SEI and impede ion transport. The inorganic components (outer layer) function as a protective shield for the battery components; they are crucial for forming a protective SEI, which provides excellent electronic insulation, promotes ionic conductivity, and reduces electrolyte consumption. Although their FFE emphasizes a novel strategy focused on bi-layered SEI formation which offers several synergistic benefits, their sole-solvent DME has a flash point ≤ 0, meaning the resulting electrolyte is highly flammable.¹²⁸ Shuai et al. developed FFEs by using 3-methyl-2-oxazolidinone (MO) as the sole solvent, which has a high donor number (DN: 27), meaning that it has high LiNO₃ solubility to facilitate high ionic conductivity (3.2 × 10⁻³ S cm⁻¹) and promote an inorganic-rich SEI with Li₃N and LiN_xO_y species. As shown in Fig. 4b, their SEI stability enabled 91.4% capacity retention after 1000 cycles in Li‖NMC622. However, their sole solvent is limited to >1 M LiNO₃ due to increasing viscosity and deterioration in cycling performance.

Fig. 4 Electrochemical performance of fluorine-free batteries; (a) lithium metal battery Li‖LFP@1 M LiBOB + 0.5 M LiNO₃ in a DME electrolyte, Reproduced with permission,⁴⁵ Copyright (2024) American Chemical Society. (b) High-voltage lithium metal battery pouch cell Li‖NMC622@1 M LiNO₃ MO electrolyte, Reproduced with permission,¹²⁷ Copyright (2025) Elsevier. (c) High-voltage lithium metal battery Li‖NMC811@1 M LiClO₄ + 0.2 M LiNO₃ TEP electrolyte, Reproduced with permission,¹²⁶ Copyright (2025) Wiley-VCH. (d) Sodium-ion battery Prussian white|hard carbon‖Na@0.5 M NaBOB/TMP electrolyte, Reproduced with permission,¹¹⁶ Copyright (2020) American Chemical Society. (e) KMB K‖PTCDA@0.1 M KBPh₄ EC/DEC, Reproduced with permission,¹²¹ Copyright (2023) Springer Nature. (f) Calcium battery Ca‖S/C@0.5 M CMC, DME/THF (1 : 1, v/v) 0.5 to 3.2 V; dQ/dV in the inset, Reproduced with permission,¹¹⁸ Copyright (2021) Springer Nature. (g) Zinc battery Zn‖Zn@1.0 M Zn(dca)₂/DMSO electrolyte, Reproduced with permission,¹¹⁹ Copyright (2023) Wiley-VCH.

Gabert et al. studied the interfacial decomposition of the triethyl phosphate (TEP) based FFE system in LIBs, revealing that >30% of TEP caused strong Li⁺ solvation leading to graphite co-intercalation, solvent decomposition, more gas evolution and unstable SEI formation.¹²² In addition, Xu et al. demonstrated the in situ formation of heterostructure Li₃N/Li₂O interfaces by utilizing dual salts (1 M LiClO₄ + 0.2 M LiNO₃) in sole solvent TEP for high-voltage LMBs.¹²⁶ Their studies evidently showed that bilayer SEI engineering significantly contributed to several advantages; the SEI comprised of inorganic species of Li₃N/Li₂O facilitated high ionic conductivity and fast Li⁺ transport, whereas that consisting of organic species formed a protective elasticity layer to reduce the electron tunnelling. In addition, LiCl/Li₃N inorganic CEI species reinforce the interfacial layer by improving ionic conductivity and also the high-voltage Li‖NMC811 cell showed excellent electrochemical performance, achieving 82.11% capacity retention after 500 cycles at 60 °C as shown in Fig. 4c.¹²⁶ These studies all utilized TEP solvent, showing both the promise and challenges of TEP-based electrolytes, underscoring the need for strategies to mitigate decomposition while promoting non-flammable and solvation benefits.^122,126 Khan et al. have developed a biomass-based electrolyte, paving the way for the next generation to incorporate bio-based materials and achieve a sustainable scale.³⁸ Furthermore, such promising approaches need more investigations towards electrochemical stability. The development of FFE has been reported by utilizing the commonly used fluorine-free ingredients to establish a benchmark of FFEs in lithium-based battery. Although FFEs with LiB_xO_y, Li₂O, Li₃N, and LiCl species have shown promise in stabilizing lithium metal interfaces, their anionic frameworks remain constrained by poor solubility, gas evolution, and electrode corrosion, limiting their applicability to lithium-based systems. This challenge becomes more pronounced with the demand for multivalent batteries, where electrolyte compatibility plays a pivotal role. Therefore, the rational design of innovative fluorine-free anions is imperative to unlock broader energy storage systems, paving the way for next-generation multivalent battery technologies.

In this new era, sodium-based batteries are critical in propelling a cleaner, more sustainable society. The switch from lithium to sodium represents a significant technological advancement, promoting a more sustainable and resource-efficient future. Greener battery development has persisted due to using FFEs in sodium batteries. Few studies of FFEs in sodium batteries have been explored in the past years. Building on advances in lithium-based FFE systems, similar approaches have also been investigated in sodium batteries. Mogensen et al. developed a fluorine-free sustainable electrolyte based on 0.5 M NaBOB in trimethyl phosphate (TMP).¹¹⁶ The electrolyte displayed a high ionic conductivity (5 mS cm⁻¹) and superior electrochemical performance with a CE of 80% in the first cycle. It increased to 97% in the following cycles, as shown in Fig. 4b, and this combination has produced a secure and economical solution.¹¹⁶ This work was extended by incorporating N-methyl-2-pyrrolidone (NMP) as a co-solvent, which attained a high conductivity of 8.83 mS cm⁻¹.¹¹⁷ This combination boosts battery safety and efficiency, with vinylene carbonate (VC) additive further enhancing performance. Furthermore, Buckel et al. investigated the same combination (NaBOB-TMP) for long cycling by high mass loading Prussian white|hard carbon cathode.¹²⁹ Although sodium batteries have achieved total fluorine removal through sustainable strategies (non-flammable, non-toxic, eco-friendly), their FFE combinations and electrochemical performance are still constrained. The potassium metal battery (KMB) study has recently been explored in the FFE design. Previous studies of KMB development have identified challenges like dendrite formation and fast capacity fading.^121,130 Liu et al. designed an ultra-low concentrated FFE using EC and DEC carbonate solvent with 0.1 M KBPh₄. The fluorine-free anion generated a stable SEI layer and allowed reversible cycling for the PTCDA/K full cell, as shown in Fig. 4e. This promising approach could stimulate future research to deepen the understanding of anion-dominant chemistry and interfacial stability of K-batteries. Due to concerns about resource availability and safety issues with LIBs, Ca and Mg batteries are being explored as substitutes. Ca is superior to Mg in ion transport and has a higher energy density. Nevertheless, there aren’t many appropriate room temperature electrolytes for Ca. Ponrouch et al. designed an electrolyte, Ca(BF₄)₂, in EC/PC at ∼100 °C which enabled reversible plating and stripping.¹³¹ However, CaF₂ formation alongside Ca metal deposition impedes migration, restricting plating/stripping efficiency.^131,132 Conventional electrolytes create films of blockage that impede their effectiveness. Recent developments include Ca(BH₄)₂ in THF, which is fluorine-free but less stable, and Ca[B(hfip)₄]₂ (hfip = hexafluoroisopropyloxy) in DME, which has high stability but CaF₂ problems.^133–135 Creating a fluorine-free, stable, high-conductivity Ca electrolyte is essential. For example, Kisu et al. developed a Ca electrolyte devoid of fluorine that contained calcium monocarborane (CMC@ Ca[CB₁₁H₁₂]₂). It provided a broad potential range (up to 4 V vs. Ca²⁺/Ca) and high conductivity (4 mS cm⁻¹) with reversible Ca plating/stripping at room temperature. Fig. 4f shows the outstanding electrochemical performance of a Ca–S battery using a fluorine-free (0.5 M CMC, DME/THF) electrolyte system, which produced an initial discharge capacity of 805 mA h g⁻¹.

As a safer, more affordable, and plentiful substitute, zinc batteries show great promise. Though they have problems with zinc dendrites and electrolyte stability, they provide larger capacities. A novel electrolyte that exhibited superior performance, high conductivity, stability, and smooth zinc deposits is zinc dicyanamide (Zn(dca)₂) in dimethyl sulfoxide (DMSO) by Kar et al., as shown in Fig. 4g.¹¹⁹ As the concentration of Zn(dca)₂ increased, a zinc nitride (Zn₂N₃) rich SEI was formed on the anode. While sodium intercalation improved zinc's sluggish reaction kinetics in hybrid batteries with polyanion cathodes, mixed salts, such as Zn and Na, enhanced ion transport and electrochemical reversibility, and widened the potential window. They extended studies on a dual-cation electrolyte, [1.0 M Na(dca) + 1.0 M Zn(dca)₂]/DMSO (NaZn), which facilitated stable cycling over 100 cycles at a 0.1C rate in Zn|NFP (t-NaFePO₄).¹²⁴ The addition of Na(dca) improved Zn²⁺ transport, resulting in high ionic conductivity (7.9 mS cm⁻¹) and a high zinc transference number (t_(zn²⁺) = 0.83) at 50 °C, underscoring the promise of FFEs in zinc hybrid batteries. Li et al. have pioneered a novel bifunctional FFE tailored for micro-Si anodes, transforming battery efficiency by pre-lithiating the native SiO_x layer and reducing LiF-free interphase buildup.¹²⁰ Their research demonstrated remarkable outcomes, with the optimized micro-Si anode achieving over 5 mA h cm⁻², boasting a high specific capacity of 2900 mA h g⁻¹, an impressive initial coulombic efficiency of 94.7%, and a stable CR of 94.3% after 100 cycles at 0.2C. The key innovation lies in the 2 M LiBH₄/THF–MeTHF electrolyte, which chemically pre-lithiated the oxide layer, stimulating the SEI and enhancing lithium-ion transfer. This breakthrough has set the stage for developing eco-friendly, cost-effective electrolytes for next-generation batteries.¹²⁰

Remarkably, the partial fluorine electrolyte strategy has also been explored in many studies and achieved a high-performance battery.^136–139 This state-of-the-art system incorporates comparatively fewer fluorination species into electrolytes for high cycling performance, non-flammable, and high voltage applications.^{137,140–142} However, this system also shows toxicity from fluorination species, adversely affecting climate change. The existing literature studies on FFEs for corresponding alkaline battery systems strongly oppose practical outlooks of the development of novel ingredients to enhance efficiency with low cost. Therefore, future directions are highly encouraging to explore various strategies to design and develop innovative/discover studies of FFEs in all types of alkaline metal batteries.

3.2. Fluorine-free electrolytes in mono and multivalent batteries

The above roadmap discussion visibly demonstrated that FFEs transitioning from lithium-based batteries are promising for future generations. The development of FFEs in monovalent and multivalent battery systems gives prominence to exploring novel battery chemistry and, eventually, gaining insight into the pivotal challenge of pursuing sustainable energy. While lithium-based batteries have established the foundational principles for electrolyte design, extending these concepts to Na, K, and multivalent systems presents distinct difficulties due to variations in ion chemistry and solvent interactions. In monovalent systems, sodium and potassium-based electrolytes are hindered by differences in ion–solvent interactions compared to lithium. Despite similarities in the physicochemical characteristics of Li⁺, Na⁺, and K⁺, their reactivity diverges owing to differences in charge density, solvation structures, and Lewis acidity. For example, Okoshi et al. compared de-solvation energies of mono-valent Li⁺ and Na⁺ ions and di-valent Mg-ions in 26 fluorine-free solvents (carbonates, lactones, and amides), finding that Na-ions had lower energies than Li due to weaker Lewis acidity, while divalent Mg-ions had higher energies and additional interactions due to their double charge.¹⁴³ Furthermore, their extended studies on monovalent K⁺ ions revealed even lower interaction energies, reflecting K's larger ionic radius and weaker Lewis acidity, contributing to K-ion batteries’ high rate capability and low solubility compared to other monovalent (Li⁺, Na⁺) and divalent (Mg²⁺) systems.¹⁴⁴ Beyond solvent interactions, anion chemistry plays a crucial role in electrolyte performance, particularly in enabling efficient cation dissociation and transport. For instance, substituting –CF₃ in TFSI⁻ with –n-C₄F₉ in NFSI⁻ reduces cation affinity, lowering Li⁺ dissociation energy (591 → 576 kJ mol⁻¹), enhancing ion mobility and electrochemical efficiency.¹⁴⁵ FFEs eventually lead to a unique anion chemistry design by replicating existing anions for advancing monovalent cations into multivalent rechargeable batteries. The BOB⁻ anion has been extensively studied for its structural and electrochemical performance. For instance, boron-containing monovalent salts like LiBOB and NaBOB are primarily explored in FFEs,⁴⁴ while Ca(BH₄)₂ and Mg(BH₄)₂ show potential in multivalent systems, though the exact role of boron remains unclear.¹⁴⁶ Xu et al. innovatively developed the orthoboron-based anion bis(glycolato)borate (BGB), emphasizing its application for both monovalent (Li⁺, Na⁺, K⁺) and divalent (Mg²⁺, Ca²⁺) cations as shown in Fig. 5.¹²⁵ Their research highlights the superior moisture stability of LiBGB and NaBGB compared to LiBOB. BGB salts' thermal stability follows the trend Li > Na > K > Mg > Ca, with LiBGB having the highest decomposition temperature (370 °C) and CaBGB the lowest. However, >1 M LiBGB electrolytes reduce ionic conductivity due to ion-pairing and an increase in viscosity.¹²⁵ The 1 M LiBGB–TEP electrolyte shows excellent electrochemical stability. At the same time, 1 M LiBGB–TMP demonstrates the best long-term performance for Li plating-stripping. In addition to appreciation, further exploration of these anions should focus on their structural, physicochemical, and electrochemical performance, advancing electrolyte design for the transition from monovalent to multivalent rechargeable batteries.¹²⁵

Fig. 5 A novel fluorine-free bis(glycolato)borate anion-based salt for monovalent and multivalent cations, Reproduced with permission,¹²⁵ Copyright (2025) Wiley-VCH.

3.3. Challenges and limitations

Designing FFBs presents several significant challenges. Without fluorine, achieving similar stability and performance becomes difficult. FFEs often struggle with issues like lower ionic conductivity, reduced electrochemical stability, and increased susceptibility to degradation. Additionally, forming a stable SEI without fluorine is challenging, leading to problems such as lithium dendrite growth and poor cycle life. Studies are exploring various strategies, such as using alternative solvents and additives, to overcome these hurdles. However, significant advancements are still needed to match the performance of fluorinated systems.¹³⁸

3.3.1. Electrochemical stability. FFBs present significant challenges in maintaining electrochemical stability, particularly over extended cycling. The absence of fluorine limits the formation of stable inorganic compounds within the SEI, making the interphase more susceptible to breakdown under high-voltage operations. Additionally, many FFEs lack oxidative stability to prevent side reactions at elevated voltages, reducing cycle life and leading to lower efficiency. The literature studies on FFEs suggest that while some alternatives like borate-based and nitrile anion electrolytes have shown promise, they still face issues with surface reactivity and degradation distribution, particularly when interacting with high-energy electrode materials.^45,147 To safeguard the anode and extend battery life, one of the major concerns is the challenge of creating a strong SEI; in the absence of this, issues such as lithium dendrite development can compromise battery lifetime and safety.¹³⁸ Their operating range is further restricted by oxidative degradation since these electrolytes frequently lack the oxidation stability required for high-voltage applications.^45,58,147 Another issue is compatibility with different cathode materials since FFEs could perform poorly with few high-energy cathodes reducing capacity and cycle life.¹⁴⁸ A third complicating factor is temperature stability since many electrolytes without fluorine work poorly at low temperatures.⁴⁵ These factors could lead to poor electrochemical stability of the FFE system. Moreover, the design of fluorine-free additives that can replicate the protective effects of fluorinated compounds remains a key obstacle. These limitations highlight the need for continued innovation in electrolyte formulations and interphase engineering to achieve the desired stability and performance in FFB systems.

3.3.2. Ionic conductivity and efficiency. FFEs confront major hurdles in attaining high ionic conductivity owing to the lack of strongly electronegative fluorine atoms, which usually assist ion transport via strong ion–dipole interactions.⁴⁴ Without fluorine, electrolytes often have lower ion mobility and less stable SEI.¹⁴⁹ This leads to greater resistance and reduces overall battery efficiency.⁴⁵ Chemical stability in these systems can also be impaired, resulting in a shorter cycle life than fluorinated equivalents.¹¹⁷ Khan and coworkers’ studies on biomass-derived FFEs inherently suffered from lower ionic conductivity due to less efficient ion transport pathways.³⁸ Their higher viscosity and lower dielectric constants impeded lithium-ion mobility, making them less effective than traditional fluorinated electrolytes.³⁸ This limitation highlights the need for advancements to improve the performance of biomass-derived FFEs. Jiang and coworkers’ FFE study found that elevated temperatures in FFEs lead to LiNO₃ decomposition and more Li₃N formation, boosting ionic conductivity.⁴⁵ This enhances Li⁺ transport and de-solvation, enabling very fast charging due to the efficient SEI and inner Helmholtz layer.⁴⁵ Physicochemical properties of ingredients such as lower viscosity, higher dielectric constant, and donor number could facilitate the frequent ion transport to form a robust SEI to promote high efficiency.

4. Strategies for eliminating fluorine from electrolytes

Developing sustainable FFBs presents significant challenges, including the limited availability of FFE ingredients, electrochemical stability at high voltages, and inadequate interfacial engineering.¹⁵⁰ Since FFE systems are still in the initial stages, studies emphasize incorporating partially fluorinated compounds or hybrid blend approaches. In addition, studies highlight the integration of fluorine-free components such as co-salts, co-solvents, additives, or diluents within fluorinated systems to enhance performance. Examining the fluorinated electrolytes provides critical insights into existing technologies and the ongoing transition toward fluorine-free solutions. Analyzing these hybrid systems is essential for understanding their role and accelerating the systematic shift toward fully PFAS-free electrolytes, marking an essential step in fluorine elimination. This section highlights the future generation research to design and develop innovative FFE ingredients to create enormous eco-friendly batteries with efficient benefits that drain out the pollutants, making it easy to follow a sustainable circular economy.^151–153

4.1. Fluorine-free salts

The choice of lithium salt significantly impacts the stability and performance of rechargeable LIBs, notably electrolyte oxidation. Lithium salts have varied degrees of stability during oxidation processes, affecting the battery system's overall safety and efficiency. The fluorine-free lithium salt anion stabilizes and forms the SEI, influencing cycle life, voltage stability, and operational dependability.¹⁵⁴ Chemical stability varies across lithium salts, influencing their susceptibility to breakdown and side reactions, compromising electrolyte performance over time, and limiting battery life. Although the options for salts in the FFE system are limited, the most employed anions include (1) perchlorate-based salts, (2) phosphate-based salts, (3) nitrile-based salts, (4) borate-based salts, and (5) nitrate-based salts.

4.1.1. Halogen anion. Accurate anion oxidation potential predictions are essential for understanding and improving lithium battery electrolyte stability. Johansson's studies on intrinsic anion oxidation potentials used computational techniques by integrating solvation effects and anion volume correction.¹⁵⁴ The future of lithium perchlorate (LiClO₄) as an electrolyte for LIBs depends on overcoming present constraints and investigating possible benefits via more research and development.¹⁵⁵ One crucial factor is optimizing its performance through combination/developing the electrolyte design to optimize the ratio or additives to increase stability and longevity. Other factors to consider are safety implications, such as thermal behavior to prevent risks like a thermal runaway, interaction with advanced electrode materials, environmental impact, recyclability, and ensuring commercial viability through cost-effective production and scalability. Continued efforts in these areas are critical for fully realizing LiClO₄'s potential in next-generation battery technology.^156–159 Despite its favorable properties such as low cost and good solubility, LiClO₄'s safety concerns and strong oxidizing tendencies have limited its usage in commercial LIBs. However, researchers have made several trials. For example, Marom et al. found LiClO₄ to be less effective than LiPF₆, posing safety concerns with more exothermic thermal reactions.¹⁶⁰ Recently, Gallant et al. developed a quantitative titration method for designing localized high-concentration fluorine-free electrolytes and emphasized the significance of Li₂O in obtaining high CE in LIBs.¹⁶¹ The total Li₂O concentration in the cycled Li anode can now be precisely measured owing to a new titration technique that broadens the scope of chemical studies that may be used to examine the structure of the lithium SEI and anode chemistry (Na and graphite).^65,161 This technique includes a variety of phases, including B-, P-, LiF, S-, Li₂C₂, RLi, ROCO₂Li, and Li₃N-containing species, as well as inactive Li produced during cycling.¹⁶¹ Notably, Li₂O emerged as the dominating phase at high CE values over a wide range of electrolytes. The findings of the statistical studies cast doubt on the commonly accepted belief that LiF is the only crucial component of the SEI, as Li₂O concentration and CE were found to have a substantial positive connection that even outran LiF.¹⁶¹ Also, a similar high CE was obtained using an oxygenated LiClO₄ electrolyte devoid of fluorine, resulting in enhanced Li₂O generation. In addition, their study demonstrated that substituting LiTFSI with LiClO₄ enabled FFEs to achieve over 99% CE by promoting Li₂O contribution to SEI formation, resulting in higher efficiency and stability. This highlights the critical importance of Li₂O concerning LiF and emphasizes attention to a technique of SEI oxygenation that was previously neglected, offering a new design path for high-CE electrolytes that may be able to challenge conventional fluorination methods as shown in Fig. 6a, comparing studies of F-rich and O-rich salts.¹⁶¹ Similarly, Nam et al. developed a LiF-free FFE and binder by combining 1 M LiClO₄ with EC : EMC : DMC (1 : 1 : 1 wt), additives (1% VC + 1% PS + 0.1% LiBOB) and a newly synthesized aromatic polyamide (APA) binder.¹¹² Notably, the CEI layer formed in the APA–FFE system incorporated B and Cl species, which enhanced ionic conductivity and provided a stable, uniform protective layer, as shown in Fig. 6b.¹¹² Their APA–FFE system retained 75.2% capacity after 200 cycles at 1C, addressing high voltage performance concerns with high nickel NCM cathodes and underlining the requirement for fluorine-free materials to fulfill environmental laws. In the pursuit of improved performance and environmental friendliness, this strategy creates new avenues for future electrolyte designs that go beyond fluorine-based frameworks as the industry transitions to sustainable battery technologies.¹⁶¹

Fig. 6 Design of the fluorine-free halogen anion; (a) summary of CEs for LHCEs with various salts and the normalized amounts of Li₂O and LiF by capacity loss in select electrolytes, Reproduced with permission,¹⁶¹ Copyright (2024) Springer Nature. (b) Enhanced ionic conductivity in Li-ion batteries with fluorine-free ClO₄⁻ anion and aromatic binders, compared to PVDF, Reproduced with permission,¹¹² Copyright (2024) Elsevier. (c) Fluorine-free phosphate anion; (i) i-E profiles of PC solutions at 10 mV s⁻¹, (ii) discharge curves in 1 : 1 EC–THF with 0.5 mol dm⁻³ LBNB, LBBB, and LTBP for Li/V₂O₅ cells at 25 °C, 1.0 mA cm⁻², Reproduced with permission,¹⁶² Copyright (1999) The Electrochemical Society.

4.1.2. Phosphate anion. Phosphate anions and their fluorine-free alternatives have been investigated as a promising replacement for LiPF₆ in LIBs. Wietelmann et al.'s study on lithium tris(catecholato)phosphates (LiTOP) highlights several advantages over other conducting salts in lithium batteries.¹⁶³ These salts possess high thermodynamic and kinetic stability attributed to their anion symmetry, which is crucial for ensuring long-term battery performance and safety.¹⁶³ Their large anion volume facilitates effective charge distribution, thereby enhancing stability and efficiency. Compounds like Li[P(C₂O₄)₃]₂ exhibit a wide voltage window, essential for maintaining consistent performance across numerous charge–discharge cycles.^163,164 These salts demonstrate excellent conductivity and resistance to anodic decomposition, significantly contributing to overall battery efficiency and life span. With a lower molecular weight than catecholatophosphates, the TOP anion retains the beneficial chelating effect, improving kinetic performance and overall battery efficiency.¹⁶³ Their synthesis involves reacting phosphorus compounds with amine bases and dihydroxy compounds, resulting in stable hexacoordinated structures. These attributes position LiTOP as a promising candidate for enhancing secondary lithium battery performance, safety, and durability. LiTOP shows good solubility and ionic conductivity in carbonate-based solvents, with thermal stability alternating with LiPF₆ and LiBOB.¹⁶³ The TOP anion showed reversible lithium-ion intercalation with good CE and CR in graphite half-cells. Although resistant to anodic degradation, its feasibility in full-cell batteries remains unreliable due to a shortage of long-term analyses with cathode materials.¹⁶³

The chelate complex of phosphorus, particularly lithium tris[1,2-benzenediolato(2)-O,O′]phosphate (LTBP), offers unique advantages in lithium battery electrolytes, significantly enhancing battery performance.¹⁶² LTBP's chelating effect with bidentate ligands boosts thermal stability, making it more resistant to decomposition than traditional lithium salts like LiPF₆.¹⁶² This enhanced stability contributes to improved safety, ensuring more reliable and stable battery operation. Additionally, LTBP demonstrates superior electrochemical performance, exhibiting better discharge characteristics in Li/V₂O₅ cells. This leads to higher energy density and capacity, as shown in Fig. 6c-(i) and (ii).¹⁶² A comparison of the salt LTBP with its fluorinated counterpart, lithium tris[3-fluoro-1,2-benzenediolato(2-)-O,O′] phosphate (3-FLTBP), reveals that 3-FLTBP exhibits higher conductivity due to fluorine's electron-withdrawing effect. LTBP offers lower viscosity and potentially improved electrolyte stability, highlighting a balance between conductivity enhancement and practical electrolyte characteristics in lithium battery applications.¹⁶⁵

4.1.3. Nitrile anion. Fluorine-free nitrile anion salts are gaining popularity for lithium batteries due to their environmental advantages and high performance. They have better thermal and electrochemical stability, competitive ionic conductivity, and lower toxicity than fluorinated counterparts. These salts may increase SEI generation, hence enhancing cycle stability and CR. Ongoing research attempts to improve these formulations for better interaction with lithium metal anodes and cathodes, consequently promoting the development of more sustainable lithium battery systems. Heterocyclic lithium salts, such as lithium 4,5-dicyano-1,2,3-triazolate (LiDCTA), have excellent solubility and ionic conductivity in LIB solvents due to substantial charge delocalization.^165,166 Despite its reduced anodic stability in propylene carbonate (PC), LiDCTA shows potential, particularly when combined with other solvents, such as poly(ethylene glycol) dimethyl ether (PEGDME) or adiponitrile and sulfolane combinations.¹⁶⁶Ab initio calculations on lithium salts with heterocyclic anions [CF₃SON₄C_2n]⁻ (0 ≤ n ≤ 4) have shown decreased lithium ion affinity with increased CN-substitution, improved oxidative stability, and restricted bidentate coordination from C≡N groups, making them viable for high-voltage battery electrolytes.¹⁶⁷ While theoretical studies indicate low solubility and conductivity, evidence from experiments is imminent. Additional potential borate salt, lithium tetracyanoborate (LiB(CN)₄), has high oxidative stability and poor Li coordination. Experimental studies with PEGDME demonstrated outstanding stability in Li‖LiFePO₄ cells at 4 V vs. Li⁺/Li, with 99% CR after 22 cycles.¹¹³ However, its performance in glyme-based gel polymer electrolytes discloses its LiPF₆-based equivalents, with instability and low CE above 4 V vs. Li⁺/Li.¹⁶⁸ For example, Scheers et al. (2014) developed electrolytes from lithium salts with nitrile-based anions, such as LiB(CN)₄:PAN:PEGDME or LiDCTA:PAN:PEGDME, and tested them through electrospun PAN membranes, attaining up to 98% and 96% average CE after 11 to 15 cycles in Li/LiFePO₄ cells at ambient temperature.¹¹³ The addition of Al₂O₃ particles had minimal effects on the PAN membranes, suggesting potential long-term performance repercussions. The impact of solvent and Li⁺ concentration on anion stability and the challenge of substituting traditional Li-salts with fluoro-based anions without sacrificing stability are obstacles to boosting anode stability. Identifying fluorine-free salts with increased stability at the anodic interface through quantum chemical calculations, combined with experimental measurements and exploring various combinations, are all potential solutions.¹¹³ LiB(CN)₄ and LiDCTA are crucial in forming the electrolytes, contributing to ionic conductivity and stability.

4.1.4. Borate anion. LiBOB, a fluorine-free trending salt with a borate anion, has provided valuable insights into its potential applications in LIBs, and extensive studies have highlighted its chemistry, advantages, and drawbacks.^91,169–180 LiBOB's unique chemistry enables it to form stable interfaces on electrode surfaces, supporting reversible lithium-ion intercalation and enhancing the stability and performance of lithium-ion cells. Its electrochemical stability helps maintain stable capacity performance at room temperature, and it has demonstrated effectiveness in stabilizing graphene structures, which suggests benefits for enhancing electrode material stability. However, LiBOB also faces several drawbacks, one of which is its limited solubility in solvents, which means it needs both linear and cyclic carbonates for proper salt dissolution. Using only linear carbonate without cyclic carbonate may lead to a white dispersed electrolyte. At the same time, it has high solubility in ether-based solvents.^169,179 Moreover, it is worth noting that LiBOB with low dielectric constant solvent restricts its use in specific electrolyte formulations.¹⁶⁹ Furthermore, its ionic conductivity is lower than that of LiPF₆, impacting the battery's performance at low temperatures.¹⁰⁰ Additionally, impurities in LiBOB and its moisture sensitivity can lead to gas production and stability issues.^54,91 Despite these challenges, the continued research and development efforts are crucial to addressing these limitations and realizing the full potential of LiBOB in improving the reliability and performance of LIBs.¹⁸⁰ For example, Jiang et al. developed an ether-based electrolyte of 1 M LiBOB in DME/1 M LiDFOB in DME, while DOL solvents resulted in a solidified/white dispersed electrolyte, as shown in Fig. 7a.¹⁷⁹ Hernández et al. designed a fluorine-free ideal electrolyte of 0.7 M LiBOB + 2 vol% VC in a mixture solvent of EC/EMC (3 : 7) for LIBs.⁵⁸ The cycle life and discharge capacity are significantly impacted by using an electrolyte devoid of fluorine in LIBs. The study found that an FFE based on LiBOB and VC performed better than a highly fluorinated electrolyte, including LiPF₆. Specifically, the fluorine-free variant demonstrated a greater discharge capacity of 147 mA h g⁻¹ and an 84.4% CR with 99.51% CE after 200 cycles at C/10, as shown in Fig. 7b-(i) and (ii).⁵⁸ Furthermore, more oxygen-rich compounds created an SEI layer in the FFE, raising interface resistance while stabilizing the silicon-based anode for improved long-term cycling. The electrolyte without fluorine showed increased resistance and overpotential at higher currents, but overall, it improved discharge capacity and cycle life.⁵⁸

Fig. 7 Fluorine-free borate anions; (a) BOB⁻ anion solubility in ether-based solvents (DME/DOL), Reproduced with permission,¹⁷⁹ Copyright (2023) American Chemical Society. (b) FFE@NMC111/Si–graphite at C/10;(3.0–4.2) V with electrolytes LP57–1 M LiPF₆–EC/EMC(3/7) (black), LP57 + FEC + VC–1 M LiPF₆–EC/EMC(3/7) + 10 vol% FEC + 2 vol% VC (red), LiPF₆ + LiBOB + FEC + VC – 0.38 M LiPF₆–EC/EMC(3/7) + 0.5 M LiBOB + 10 vol% FEC + 2 vol% VC (yellow), and LiBOB + VC–0.7 M LiBOB–EC/EMC(3/7) + 2 vol% VC (blue); (i) CE, (ii) median discharge resistance, Reproduced with permission,⁵⁸ Copyright (2020) American Chemical Society. (c) BOB⁻ anion decomposition in a multistep breakdown on graphite anodes forms semi-carbonate-B (V) and (VI), Reproduced with permission,¹⁷³ Copyright (2003) The Electrochemical Society. (d)-(i) Lactone effect influence on 1.70 V irreversible capacity in Li-ion cells, (ii) LiBOB stability performance at 60 °C on NCA, LCO, NMC, spinel (Li_xMn₂O₄) and LiFePO₄, Reproduced with permission,¹⁸¹ Copyright (2008) The Electrochemical Society. (e) Absorbance and IR spectra of a porous GC electrode in LiClO₄/DME with LiBOB at OCP, (f) model study of LiBOB's reduction mechanism, SEI formation, and reactions with EC, Reproduced with permission,¹⁷⁸ Copyright (2024) American Chemical Society.

The SEI composition using LiBOB-based electrolytes is generally investigated by XPS and FTIR; however, sample preparation is an issue due to salt precipitation in DMC, necessitating the usage of a γ-BL/DMC combination, which might cause SEI dissolution.¹⁷³ Remarkably, the LiBOB-based SEI has an oxygen-rich layer that protects solvent intercalation and graphite degradation. This layer is ascribed to semi-carbonate-like compounds and orthoborates from anion breakdown.¹⁷³ While LiBOB forms a stable SEI on graphite that requires EC for initial layer development and BOB⁻ anion reduction, its increased cell resistance remains a constraint. However, LiBOB outperforms LiPF₆-based electrolytes at higher temperatures, extending cycle life.

Additionally, studies on the interactions of LiBOB with different cathode materials have shown promise for better thermal stability and CR.^100,173 This is particularly applicable for iron-based and silicon electrodes, which hold out the prospect of improved performance in electrolyte systems without fluorine. They have proposed a multi-step breakdown process for the BOB anion, generated by a single electron and occurring on graphite anode surfaces.¹⁷³ This technique might provide a variety of semi-carbonate-like compounds containing B in oxidation states V–VI (Fig. 7c).¹⁷³

LiBOB-based electrolytes are evaluated in Li-metal batteries with LiFePO₄ cathodes, and they outperformed fluorinated electrolytes in cycle life without dendrite development.⁵⁸ Accelerated rate calorimetry tests found higher initial temperatures of self-heating reactions for LiBOB, indicating improved thermal stability.¹⁸² Even with greater resistances and a lower rate capability, LiBOB provides greater thermal stability, safety, and sustainability. Thereafter, solvent and additive optimization is required.¹⁸² Extensive works by Kang Xu focus on modifying the electrolyte composition for LiBOB in LIBs.^169,181 Xu's studies aim to produce electrolyte formulations with enhanced solubility, ionic conductivity, and interfacial characteristics by investigating the potential of LiBOB as an alternative salt and exploring lactone-based cosolvents to dissolve it efficiently.¹⁸¹ Lactone-based co-solvents, such as γ-butyrolactone (GBL), play an important role in increasing the solubility of LiBOB in electrolytes, allowing for the development of LiBOB-rich electrolytes. However, using lactone-based cosolvents may result in an increase in irreversible capacity due to the decrease of the BOB anion on carbonaceous anode surfaces, particularly with GBL.^183,184 Furthermore, elevated temperatures may impact battery performance when utilizing electrolytes with a higher lactone concentration.¹⁸¹ This study reveals the importance of the varied properties of different lactones when designing the electrolyte for LiBOB-based systems because of the influence on the irreversible capacity and overall performance of the battery.^181,185 The voltage profiles of Li-ion cells with different lactones during the early part of the first charge cycle highlight irreversible capacities from 1.70 V reductions, as shown in Fig. 7d-(i).¹⁸¹ The cycling stability of Li-ion cells with various cathode surfaces, using 1 M LiBOB in a EC/DMC 50 : 50 electrolyte at 60 °C, was also assessed. After 40 cycles, LCO and NMC cells retained capacities better than others, indicating faster LiBOB decomposition (Fig. 7d-(ii)).¹⁸¹ This incompatibility with Co-rich cathode surfaces has been frequently reported.¹⁸⁶ A preliminary study suggests that Co may catalyze the decomposition of the BOB anion, leading to poor performance in LCO and NMC cathodes.⁹⁰ The study observed several lactones, including valerolactones, that exhibited intermediate effects, butyrolactone (BL), which significantly increased irreversible capacity, and caprolactones, which revealed irreversible decreases.¹⁸¹

Compared to LiPF₆, LiBOB has superior thermal stability up to 302 °C and produces less hazardous breakdown products.^100,181 Since high-humidity storage of LiBOB and other contaminants, such as lithium oxalate from salt manufacture, can undermine stability and increase cell impedance, it is imperative to ensure purity.¹⁸⁷ Despite these obstacles, LiBOB-based electrolytes show improved SEI development on graphite anodes, improving cell performance when compared to LiPF₆.¹⁰⁰ The electrochemical behavior shows an irreversible plateau due to oxalate ester impurities, which is modified by solvent choice, emphasizing the need for thorough salt purification and solvent selection in LiBOB electrolytes.^100,181,182 Even so, the researchers are interested in exploring the lactones in the context of ideal electrolyte design. For example, Teoh et al. recently developed a novel FFE for a lithium-ion capacitor system with a unique combination of lithium bis(oxalato)borate (LiBOB) and γ-valerolactone (GVL). The electrolyte showed a conductivity of 4.9 mS cm⁻¹ at 20 °C, indicating effective ion transport due to the high conductivity of the electrolyte.¹⁸⁸ This electrolyte exhibits a flash point of 90 °C, indicating higher thermal stability and lower flammability than the 25 °C flash point of 1 M LiPF₆ in EC/DMC, highlighting its safer, more sustainable design. A protective passivation layer was also formed on the aluminum current collector at high potentials owing to LiBOB breakdown, which was successfully prevented by the electrolyte from anodic dissolution. Further, the full cell assembly developed by activated carbon graphite achieved 25 000 cycles with 80% retention of initial capacity. In addition, LiBOB attained a high solubility of up to 2 M in GVL, which makes it easier to use as the main salt in the electrolyte.¹⁸⁸ Due to irreversible processes, a CE of 53.2% was demonstrated in the first cycle.¹⁸⁸ However, consecutive cycles consistently displayed an average CE of 96% over 5 cycles, showing the BOB anion's role in forming the SEI, as verified by XPS studies on graphite electrodes. By realizing how these lactones affect battery performance, electrolyte compositions for LiBOB-based systems may be optimized, possibly improving LIB solubility, stability, and performance.^188,189 LiBOB has shown promise for both cathodes and anodes, but its reaction mechanisms remain unclear. Melin et al. conducted an operando study using ATR-FTIR, as shown in Fig. 7e, to investigate LiBOB's reduction process, which began around 1.8 V and produced lithium oxalate and oxalatoborates, along with CO₂, that influenced the interphase on the negative electrode.¹⁷⁸ This research shed light on the reduction pathways of LiBOB and their role in interphase formation. The electrochemical reduction mechanism for LiBOB is shown in Fig. 7f, emphasizing the side reactions and SEI generation.¹⁷⁸ According to the study, there was a reduction peak at less than 1.8 V.^178,189 Advanced operando methods are used to monitor the process as LiBOB forms an SEI mostly composed of Li-oxalate and minor byproducts from reactions inside the electrolyte.¹⁷⁸ Future research directions include optimizing electrolyte composition to reduce irreversible capacity losses and improve thermal stability and investigating alternative solvent systems to scale up LiBOB-based electrolytes for commercial use.

4.1.5. Nitrate anion. Lithium nitrate (LiNO₃) is gaining attention as a key FFE for batteries due to its ability to form a stable SEI on the anode, enhancing performance and longevity by preventing lithium dendrite growth.^40,139,190 Although LiNO₃ offers excellent thermal stability and non-flammability,¹⁹¹ its low solubility in common solvents poses a challenge.^45,192 Studies are exploring various methods to improve its solubility in carbonate electrolytes, including the use of high-dielectric co-solvents, polymeric gels, solvent carriers like TEP, and solubilizers such as CuF₂ and LiBF₄.^193–196 Techniques like encapsulating nitrate nanoparticles in polymers and employing metal–organic frameworks (MOFs) are also being investigated.^197–207 Despite these advancements, achieving a uniform distribution of LiN_xO_y and Li₃N in the SEI remains difficult.¹⁹³ Innovations, including using polar solvents, adjusting solubility with electron-deficient ions, and implementing pre-reaction strategies, are designed to integrate LiNO₃ into carbonate electrolytes effectively. These approaches hold promise for developing safer and high-performance fluorine-free lithium metal batteries. However, it is crucial to address potential issues such as gas evolution to ensure these systems' overall stability and safety.

NO₃⁻, with its high donor number and dielectric constant, improves interaction with Li⁺, accelerating de-solvation and forming a Li₃N-rich SEI at the anode due to its low LUMO energy.²⁰⁸ Increasing the concentration of lithium salts in electrolytes boosts anion levels relative to solvents, promoting the creation of a strong, anion-derived SEI with better conductivity and mechanical strength, crucial for preventing dendrite formation.²⁰⁹ Two primary strategies have been developed to modulate the solvation structure and interfacial chemistry in the electrolyte: increasing the anion concentration or altering the interaction between anions and solvents without changing the overall salt concentration.^202,210 These methods facilitate forming a stable, conductive SEI rich in Li₃N and Li₂O, optimizing solvation and ensuring robust protection for the lithium metal anode.²¹¹ Incorporating LiNO₃ into carbonate electrolytes increases cycling performance by optimizing Li⁺ solvation and SEI formation.²¹² LiNO₃, along with other nitrates such as KNO₃ and NaNO₃, augments Li⁺ de-solvation and mitigates polysulfide shuttle effects, thereby improving battery efficiency.^199,213,214 The Liu research group demonstrated that an FFE integrating 3.5 M LiNO₃ into a carbonate electrolyte with DMI, without additional stirring, substantially elevates lithium-metal battery performance by generating a stable SEI enriched with lithium salts, leading to an average CE of 99.1% for 80 hours.²⁰⁹ The high donor number of DMI also aids in stabilizing the electrolyte by neutralizing PF₅ and curbing HF formation, as shown in Fig. 8a.²⁰⁹ Under severe circumstances (45 μm ultrathin Li foil and high-loading cathode of ∼1.5 mA h cm⁻²), the anode retains 96.5% of its capacity after 2200 cycles at a 5C rate and 97.9% after 240 cycles at 1C rate.²⁰⁹

Fig. 8 Fluorine-free nitrate anion; (a) schematic showing how LiNO₃/DMI inhibits PF₆ decomposition, Reproduced with permission,²⁰⁹ Copyright (2022) American Chemical Society. (b) Summary of solvent DN and CE with LiNO₃, Reproduced with permission,²¹⁵ Copyright (2023) American Chemical Society. (c) LiNO₃ solubility in carbonate solvents, Reproduced with permission,²¹⁶ Copyright (2021) Wiley-VCH. (d) Schematic of LiNO₃ solubility: (a) enhanced dissociation, (b) equilibrium state, and (c) prevention of recombination, Reproduced with permission,¹⁹² Copyright (2021) American Chemical Society. (e) Li–S@cycling performance of fluorine-free solvent in NO₃⁻ anion: (i) 1 M LiTFSI in DMI–DMI–LNO-0 M, (ii) 1 M LiTFSI + 0.2 M LiNO₃ in DMI–DMI–LNO-0.2 M, and (iii) 1 M LiTFSI+0.5 M LiNO₃ in DMI–DMI–LNO-0.5 M, Reproduced with permission,²¹⁷ Copyright (2020) Wiley-VCH. (f) Electrolyte solubility of Li chippings in 3 M LiNO₃/DMI: (a) day 0 and (b) day 20. (g) Columnar Li growth in 3 M LiNO₃/DMI: (a) initial cation/anion setup, (b) N-rich SEI on Cu, (c) small Li nuclei formation, and (d) strong SEI supporting columnar Li. (h) SEM images of Li on Cu in 3 M LiNO₃/DMI: (a), (b), (d) and (e) top view and (c) and (f) cross-sectional view at 1 and 2 mA cm⁻² for 2 h, Reproduced with permission,¹⁴⁹ Copyright (2021) American Chemical Society.

The FFE with LiNO₃ as the main salt offers the following characteristics: (i) it has a lower molar mass than LiPF₆, LiFSI, and LiTFSI, which reduces electrolyte weight at comparable concentrations.²¹⁸ (ii) NO₃⁻ exhibits high Li⁺ affinity, displacing solvent molecules and reducing their decomposition. (iii) Decreasing the nucleation overpotential and promoting granular development facilitates the homogeneous deposition of Li metal.²¹⁹ However, its use as a main salt has been restricted due to its poor solubility and dissociation in popular solvents. Zhou et al. emphasized the importance of donor number (DN) in maximizing Li plating/stripping using LiNO₃.²¹⁵ LiNO₃ is difficult for low-DN solvents to dissolve, which reduces conductivity and cycling.^215,220,221 In contrast, high-DN solvents over-coordinate with Li⁺ ions, resulting in excessive solvent breakdown. Trimethyl phosphate (TMP) (DN = 23.0 kcal mol⁻¹) and TEP (DN = 26.0 kcal mol⁻¹) were chosen due their moderate DN, as shown in Fig. 8b, demonstrating good cycling performance and offering more solvent and salt alternatives for use in FFE systems.^209,215 Interestingly, Zhang et al. developed a simple and scalable way for stabilizing water-containing FFEs by demonstrating that LiNO₃ can effectively restore the electrolyte.²²² The NO₃⁻ anion has two functions: it inhibits hexafluorophosphate anion hydrolysis and forms a SEI, and ultimately improves the electrochemical reversibility of Li plating and stripping trial.¹¹⁷ Their findings address water-related hazards in the battery industry and provide insight that excess water can be turned into functional component for sustainable, high-performance batteries, aiding the shift away from fluorinated materials.²²²

Zhao et al. demonstrated how EC molecules are liberated to dissolve LiNO₃ by strong interactions with complex anions, such as those involving the Lewis acids, as shown in Fig. 8c.²¹⁶ The substitution of ethylene glycol diacetate (EGD) with DMC in mixed solvents shows that modifying the solvation structure of Li⁺ to prevent recombination with NO₃⁻ can greatly increase solubility.^192,223 Despite its low permittivity, EGD has high LiNO₃ solubility owing to its distinctive solvation structure, which repels NO₃⁻ and inhibits recombination for dynamic equilibrium, as shown in Fig. 8d.¹⁹² Baek et al. introduced an FFE in a Li–S battery with different concentrations of LiNO₃, tested using galvanostatic methods. This study showed that greater LiNO₃ contents improved cycling performance over 50 cycles and enhanced specific capacity with better CR, as shown in Fig. 8e.²¹⁷ The constant breaking and fixing of the SEI depleted the electrolyte, causing severe lithium corrosion and reduced CE.^149,224 Unrestrained dendrite development during sustained cycling can result in short circuits and possibly detrimental battery failures. Many fluorinated electrolyte systems/LiF have been developed to combat this situation and create an artificial SEI.^225–233 However, these approaches greatly impact environmental strategies, especially for large-scale development. DMI solvent has a high DN property, which can enhance the solubility of LiNO₃. Zhou et al. developed a complete FFE system for studying dendrite growth in LMBs for the first time using a fluorine-free approach.¹⁴⁹ Owing to the strong Li⁺ and NO₃⁻ interactions that obstruct dissociation in most solvents, LiNO₃ is rarely employed as the dominant salt. DMI enables extraordinary solubility of LiNO₃, reaching up to 7 M. Compared to commercial LiPF₆-based electrolytes, FFE (DMI/LiNO₃) retained a high viscosity (51.7 cP) and moderate ionic conductivity (2.23 mS cm⁻¹) at ambient temperature at 3 M concentration.¹⁴⁹ As shown in Fig. 8f, both 3 M LiNO₃/DMI electrolyte and pure DMI solvent remained colorless even after 20 days, while submerged lithium metals maintained a shiny appearance. Even with greater current densities and areal capacities (2 mA cm⁻², 4 mA h cm⁻²), SEM analysis shows consistent and uniform lithium deposition with columnar formations perpendicular to the copper substrate surface, as shown in Fig. 8g.¹⁴⁹ Similarly, Fig. 8h demonstrates the functional degradation, suggesting strong reversibility and modest column size decrease due to faster nucleation and growth at higher current density.¹⁴⁹

Pham et al. designed an FFE with LiNO₃ as the main salt in GBL and VC pentafluoro(phenoxy)cyclotriphosphazene (FPPN) as additives for flame retardance and SEI/CEI development.²⁰² In GBL, strong Li⁺ and NO₃⁻ interactions dominate contact ion pairs and aggregates.^{202,234–237} Forming SEI/CEI layers rich in conductive inorganic species improves interfacial stability.^139,238,239 At 3.5 wt% FPPN and 5 wt% VC, the electrolyte exhibited an oxidation stability up to 5.0 V and did not catch fire. The NF-LiNO₃/GBL (0.8 M LiNO₃-GBL/FPPN (96.5 : 3.5 wt%) + 5 wt% VC) electrolyte exhibited a great CE of 98.3% in Li‖Cu configuration and an excellent CR of ∼90% after 200 cycles in Li‖NMC622, as shown in Fig. 9a. Furthermore, this study achieved partial fluorine removal, which indicates that the primary goal of the study is a non-flammable FFE design achieved by incorporating the fluorinated additive.²³⁷ In recent times, Chen et al. introduced an electron-donation modulation strategy that uses low donor-number solvents (PC) to sustain solvation and high donor-number solvents (DMSO) for LiNO₃ dissolution (Fig. 9b).¹⁴⁶ PC exhibits a minor role in Li⁺ solvation compared to NO₃⁻ and DMSO, as shown by spectroscopy and theoretical models.^237,240 The LiNO₃–DMSO@PC electrolyte showed stable cycling across 200 cycles with graphite, LiFePO₄, and LiCoO₂ electrodes.²⁴⁰ The recent studies on fluorine-free salts illuminate promising pathways for advancing novel FFE systems for practical applications. Despite the progress, there remains a notable lack of research on these fluorine-free salt strategies. This underscores the urgent need for future investigations to synthesize and develop fluorine-free salts that can effectively address the limitations of fluorinated systems and foster the evolution of sustainable battery technologies.

Fig. 9 Fluorine-free solvent-salt; (a) electrochemical performance in Li‖NMC622: discharge capacities for LiFSI/GBL, LiNO₃/GBL, and NF-LiNO₃/GBL electrolytes, Reproduced with permission,¹³⁹ Copyright (2023) Elsevier. (b) LiNO₃ dissolution in solvents: (a) DN values, (b) electron pair donation, (c) low donor-number solvents, (d) low donor-number solvents, (e) solubility picture as salt-to-solvent ratios and molarity, Reproduced with permission,²⁴⁰ Copyright (2024) Wiley-VCH.

4.2. Solvents for fluorine-free electrolytes

Solvent selectivity is crucial in electrolyte design, as 70–80% of the electrolyte's mass consists of solvents, which are vital for lithium-based battery performance and safety.^11,241 Solvents must dissolve lithium salts, have high dielectric constants, low viscosity for efficient ion transport, and ensure stability across the battery's voltage range.^242,243 Research focuses on minimizing interactions between Li⁺ ions and solvents, increasing ionic conductivity, and widening the liquid range.²⁴⁴ Using cosolvents with a low freezing point and viscosity can enhance conductivity at low temperatures.^245–249 Maintaining the SEI is essential for stability.²³⁸ Hereafter, current research focuses on solvents' physicochemical and thermomechanical properties.^243,250,251 However, given the sustainability concerns, there is a push for fluorine-free alternatives to improve safety and environmental impact, with FFEs being a promising but still developing area.^11,138 Developing innovative, high-performance FFEs is crucial for sustainable battery technologies. Fig. 10 shows possible fluorine-free diluents or solvents, emphasizing the importance of selecting optimized solvents or diluents with appropriate dielectric and viscosity properties for developing efficient FFEs.

FFEs, compared to fluorinated ones, contain a greater variety of solvents. These solvents offer superior thermal characteristics and create an optimal solvation structure.^252,253 Alkali metal batteries frequently utilize organic liquid electrolytes due to their electrochemical stability, robust ionic conductivity, prevention of irreversible capacity loss, and adaptability to many electrode materials.^254–257 The main types of solvents used in these electrolytes include (1) cyclic carbonate (CC) – linear carbonate (LC) solvents, (2) ether-based solvents (EBs), (3) ester@lactone based solvents, (4) phosphate-based solvents (PBs), (5) nitrile-based solvents (NBs), (6) sulfone-based solvents, and (7) ionic liquid-based solvents (ILBs).

Fig. 10 Comparison of dielectric constant and viscosity of different diluents or solvents for fluorine-free LHCE design.

4.2.1. Fluorine-free carbonate and ether-based solvents. The well-known cyclic carbonate solvents (CCs) include ethylene carbonate (EC) and propylene carbonate (PC). Due to its high melting point, EC exists as a solid at room temperature, limiting its application.²⁵⁸ PC, although incompatible with graphite, is stable above 4 V. A stable SEI is formed on the graphite anode by EC, which principally solvates lithium ions.^258,259 The high viscosity of Li salt solutions that solely include EC necessitates the addition of linear carbonates as co-solvents, such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), to improve the solubility and structural arrangement of the corresponding salt and functionalize to enhance the physical and electrochemical characteristics.²³⁸ However, the main drawback of LCs is their flash point (almost close to ambient temperature), which leads to ignition or causes catastrophic thermal runaway.^238,260 Therefore, non-flammable electrolytes must be used to improve battery safety without sacrificing performance.²⁶⁰ For example, to generate a good SEI on graphite, Tarascon and Guyomard introduced an electrolyte containing LiPF₆ in a mixture of EC and DMC.²⁶¹ LiPF₆ has strong ionic conductivity and antioxidant stability. However, HF's adverse effects lead to analyses for alternative salts to replace PF₆⁻ functionality. Xu's studies stated the possibility of replacing PF₆⁻ with a borate-based anion.^91,169–180 Perhaps the solubility of salts depends on solvents and the synergy of solvation structure. Jin and coworkers developed a fluorine-free solvent electrolyte by using a tri-salt combination in dual cyclic carbonate solvents such as 0.6 M LiTFSI/0.4 M LiBOB/0.05 M LiPF₆ in EC/PC, which achieved 97% of CR around 400 cycles.¹³⁶ The trisalt in the EC/PC (7 : 3, v/v) electrolyte improved the SEI by balancing Li₂CO₃ and ROCO₂Li, enhancing mechanical properties. Adding 0.05 M LiPF₆ generated sufficient LiF without disrupting this balanced ratio. However, excessive LiPF₆ (1 M) increased Li₂CO₃ and reduced ROCO₂Li, as shown in Fig. 11a. Consequently, this electrolyte produced an optimal SEI with high stability and low impedance, outperforming other combinations in cycling stability. Lithium salts are efficiently solvated by LCs, facilitating ion transport in electrolytes. DEC improves battery performance by stabilizing the SEI on lithium electrodes of moderate viscosity. In contrast, EMC provides viscosity–stability balancing across all battery chemistry. LC challenges include low oxidative stability at high voltages and breakdown, which results in gas production and reduces electrolyte longevity and safety. Eventually, optimizing the usage of LCs in lithium batteries requires addressing these problems. For example, Lee et al. introduced molecular engineering via LCs to mitigate the thermal runways of LIBs with safe electrolyte engineering.²⁶² Interestingly, this work used alkoxy substitution and alkyl-chain extension, as shown in Fig. 11b, to apply the equilibrium strategy and confirm that methoxy substituents play a vital role in Li-ion transport for developing an ideal electrolyte system for real-time perspective.²⁶² This methoxy electrolyte provided stable cycling without excess gas/heat generation upon exposure to mechanical, electrical, and thermal abuse, emphasizing its exceptional electrochemical performance. Bicarbonate solvents were identified as by-products of the EC–LC (EC/DMC, EC/EMC, EC/DEC) electrolyte system formed by transesterifying LC and EC ring-opening products.²⁶³ Its structure integrates the SEI film-forming ability of EC, the efficient de-solvation of DMC, and the high anodic stability of EMC.^264,265 Zhang et al. introduced dimethyl 2,5-dioxahexanedioate (DMDOHD) solvent as an alternative to traditional carbonate-based electrolytes.²⁶⁵ This novel electrolyte demonstrated dendrite-free lithium deposition, anodic stability up to 5.2 V, and more than 97% capacity retention after 250 cycles in Li‖LiNi_0.5Mn_1.5O₄ cells. Many studies state that carbonate-based electrolytes are promising approaches to transport lithium ions. Furthermore, these interesting work strategies and potential electrolyte optimization gives more support for developing future standards for carbonate-based solvents.

Fig. 11 Fluorine-free solvents; (a) FTIR analysis for different electrolytes – after 100 cycles of Li/NMC cells, Reproduced with permission,¹³⁶ Copyright (2019) American Chemical Society. (b) Strategy to reduce flammability and maintain ionic conductivity in LCs, Reproduced with permission,²⁶² Copyright (2023) The Royal Society of Chemistry. (c) Molecular structures, ESP maps, and properties of (a) DME, (b) DMC, (c) their blend, and (d) BMC solvent with ether and ester groups for high-voltage, safe LMBs, Reproduced with permission,²⁶⁶ Copyright (2024) Springer Nature. (d) Fluorine-free ether electrolyte solvation power of ether solvents in 1.8 M LiFSI salt, Reproduced with permission,²⁶⁷ Copyright (2023) Springer Nature. (e) Schematic of the molecular anchoring diluent electrolyte (MADE) strategy, Reproduced with permission,²⁶⁸ Copyright (2024) Springer Nature. (f) Experimental design to assess the impact of electrolyte leakage, Reproduced with permission,²⁶⁹ Copyright (2023) Wiley-VCH. (g) Methylation strategy design for fluorine-free ether electrolytes, Reproduced with permission,¹³⁸ Copyright (2024) Springer Nature.

EBs can achieve partial elimination of fluorine in electrolyte systems with outstanding performance. EBs, including DME and 1,3-dioxolane (DOL), are preferred in the development of FFEs for lithium batteries due to their low viscosity, strong ionic conductivity, and excellent solvation of lithium ions and are worthy of comparison with CC–LC based electrolytes.¹³⁷ DME is notable for its substantial ion transport and SEI production on lithium anodes. It aids lithium–sulfur and lithium–air batteries by dissolving active species. In contrast, DOL supports DME by stabilizing lithium metal, minimizing dendrite development, and improving battery safety due to its unique ring structure. EB and CC–LC mixed systems are promising approaches for ideal FFE design.^270–272 For example, in 1984 and 1987, Tobishima studied FFEs using fluorine-free salt LiClO₄ in an EB–CC mixed solvent system for a lithium battery.^270,271 These studies explored different electrolyte compositions, including 1 M LiClO₄ with different optimized ratios of EC/THF, PC/THF, EC/DME, PC/DME, EC/PC, and PC alone. Comparing the EC/ether combination to the EC/PC/ether and PC/ether systems, it was found that the conductivity and lithium cycling efficiency of the mixture rose dramatically. Because of the low viscosity of ether and the high dielectric constant of EC, the improvement in conductivity became particularly noticeable at a realistic solute concentration, such as 1 M. The adhesion of ether and EC on the deposited lithium, which has lower reactivity than PC, is responsible for increased lithium cycle efficiency for EC/ether combinations.²⁷⁰ Still, a similar approach is promising for studying different combinations of EBs and CCs incorporating electrolyte designs. Perhaps researchers grasped this in the fluorination era. However, this approach causes huge toxicity emissions.²⁷² In recent times, Chen's research group investigated the performance and safety of high-voltage lithium metal batteries (LMBs) by developing a new FFE, bis(2-methoxyethyl)carbonate (BMC).²⁶⁶ The electrostatic potential (ESP) map in Fig. 11c shows a less negative charge on ether oxygens in BMC as compared to those in DME, and less positive and negative charge was exhibited by the carbonyl carbon and carbonyl oxygen in BMC relative to DMC. This molecular hybridization improved electron distribution, resulting in increased oxidative and reductive stability, thermal stability, and flash points. BMC exhibited comparatively low solvating capacity and a limited ability to dissolve LiNO₃. In LMBs, the BMC-based electrolyte enabled stable operation at high voltages up to 4.4 V and obtained a dendrite-free battery with a high average CE of 99.4% over 10 cycles.²⁶⁶ This work stimulates future researchers to design an FFE contribution by hybridizing EB–CC–LC solvent in molecular design engineering. In contrast to ester-based substitutes, EB electrolytes have a distinct reduction stability, which makes them a good choice for organic electrolytes. Generally, EBs are classified as cyclic (DOL, THF) or linear (DME, triglyme (T3GM), and tetraglyme (T4GM)), as shown in Fig. 11d, but their strong chemical reactivity makes them prone to open-ring polymerization in the presence of Lewis acids, which has a deleterious influence on ionic conductivity.^111,269 Not many studies have been done on T3GM and T4GM, which often exhibit unsatisfactory battery performance.^273,274 Despite having robust ionic conductivity and excellent ionic solvation, DME electrolytes have poor oxidative stability and aluminum corrosion, which causes significant oxidative breakdown at 4.0 V.^45,137,138 To improve DME electrolytes’ electrochemical performance, researchers have changed their spatial arrangement. Li's research team compared non-polar and polar solvents towards a series of EBs for high-voltage practical applications and concluded a solvating power order as DPE < DEE < DME < DIG.²⁶⁷ It is noteworthy that a pre-passivated cathode (99.53%) exhibited steady cycling and greater CEs than a pristine one (98.54%) over 120 cycles when a known incompatible electrolyte (1.8 M DME) was used. The DME-based highly concentrated electrolyte (HCE) system demonstrated that cathode passivation cannot completely avoid dilute ether oxidation.²⁶⁷ Most DME molecules are prone to oxidation due to their low concentration and cation repulsion. Although the DPE-originated cathode electrolyte interphase (CEI) boosted CE by 0.99%, the solvent-rich EDL structure of DME failed to prevent ether oxidation, leading to lower CEs after exchange despite a well-formed CEI layer from the DPE electrolyte. To improve the stability of nickel-rich cathodes at 4.7 V and increase the thermal runaway temperature of LMBs, it is crucial to limit the reactivity of free solvent molecules, especially in ether solvents with low oxidation stability (<4.0 V vs. Li⁺/Li). As shown in Fig. 11e, a novel molecular anchoring method employing hydrogen-bonding interactions was proposed to suppress ether side reactions.²⁶⁸

Comprehending the influence of electrolyte constituents on solvation structure and interfacial chemistry is crucial for creating efficient electrolytes for high-energy-density LMBs. Localized high-concentration electrolytes (LHCEs) form nanometric aggregates that increase LiF content in the SEI, improving uniform Li deposition and doubling the cycle life compared to advanced ether-based LHCEs.²⁷⁵ This was achieved using a fluorine-free EB, 1,3-dimethoxypropane (DMP), which has a weaker solvating power but high Li salt solubility compared to DME.²⁷⁶ Although a fluorine-free solvent (DME) is frequently utilized in LMBs, it has drawbacks such as poor stability at high voltage and low CE. This work presents a novel design approach using 1,2-diethoxyethane (DEE) by replacing the methoxy groups in DME with further ethoxy groups, altering lithium ions' solvation structure using steric hindrance.²⁷⁷ DEE exhibited improved cycling performance and more stable interfacial characteristics due to its reduced solvation capacity. Under severe circumstances, 4 M LiFSI/DEE outperformed 4 M LiFSI/DME by achieving 182 cycles at 80% CR. At the same time, the pristine counterpart could only complete 94 cycles.²⁷⁷ For high-voltage LMBs, DEE is a viable substitute for DME, demonstrating the potential of steric effects in the electrolyte design. Jinfan et al.'s studies on a potassium ion battery introduced a fluorine-free solvent system by investigating the interaction between weak and strong ions of EBs with K⁺ intercalation at low concentrations.^278,279 This work tunes the solvent–ion interactions for a high performance electrolyte strategy. Similarly, Ma and colleagues developed green EBs with carbon chain modulation that regulates steric hindrance for an effective solvation structure.²⁶⁹ When compared to diethylene glycol dimethyl ether (DGM) and diethylene glycol diethyl ether (DGT) solvents, the electron density around the middle oxygen atom in the ESP of dipropylene glycol dimethyl ether (DMM) was lower.^111,269 This structure considerably decreased the solvent's bio-toxicity, and the tailored solvation structure produced long-lasting high-voltage electrolytes for alkali metal-ion batteries. Potassium-ion batteries benefit from a stable interphase formation on both the anode and cathode, due to preferred anion-dominated solvation structure. This determination interestingly provides molecular design ideas of a sustainable world created via novel electrolyte development, as shown in Fig. 11f.

In recent years, many research studies have focused on a series of fluorine-free ether-based electrolytes for high-voltage FFBs. Li et al. explored an FFEE system via methylation of primary ether-solvent DME in series. This eventually promoted high oxidation stability by anion reduction, as shown in Fig. 11g.¹³⁸ Methylation contributes to solving problems like cathode cracking and lithium dendrite formation in high-voltage LMBs by facilitating the development of ether-based electrolyte systems. Although fluorinated solvents extend the life of batteries by raising LiF in the SEI, there are numerous concerns about their high cost and potential adverse environmental effects. To improve oxidation stability and inhibit dendrite development, methylating DME reduces currents by increasing contact ion pair (CIP) and aggregate (AGG) formation, enhancing LiF-rich SEI, and lowering the anodic current in DEP. This methylation method strikes a compromise between antioxidation and the increase of ionic conductivity in the order of DEP < DPE < DEE < DME, making these solvents suitable alternatives to fluorinated ones.¹³⁸ In addition, molecular dynamics simulations demonstrated that Li⁺ is predominantly coordinated by ether oxygen (EO) atoms and the oxygen of the FSI⁻ anion. The coordination number for Li⁺–EO decreases monotonically, while that for Li⁺–O (FSI) increases in the solvent order of DME < DEE < DPE < DEP.¹³⁸ While the fraction of free Li⁺ cations is similar in DME and DEE, it is substantially lower in DPE and DEP. DME has the lowest fraction of Li⁺ bonded to two FSI⁻ anions, whereas DEE, DPE, and DEP have higher levels.¹³⁸ As the solvent changes, there is a visible intensification in aggregation and reduction in free FSI⁻ ions, as confirmed through Raman analysis. Methylation of DME's α-H atoms improved CE (>99.9%, 150 cycles for pouch cell). It ensured excellent cycle performance in 4.3 V Li‖NCA full cells (600 cycles, 99.8% CE for coin cell).¹³⁸ Furthermore, EBs have superior stability at Li metal anodes than CB-based electrolytes. However, their low anodic stability prevents usage in high-voltage applications.

4.2.2. Fluorine-free ester-phosphate-based solvents. Compared to conventional carbonate-based electrolytes, as shown in Fig. 12a, an ester-based solvent derived from a fluorinated/non-fluorinated carboxylate ester has been proposed for use in LIB electrolytes.^280–283 This solvent offers a wide electrochemical window, ranging from 0 to 4.73 V (vs. Li⁺/Li), low solvation energy for effective lithium-ion interaction, and liquid stability down to −120 °C. Compared to organic linear carbonates, linear esters have a carbonyl carbon and an α-carbon, the latter being the carbon adjacent to the carbonyl carbon, which makes them ideal cosolvents for LIB electrolytes. Their benefits include enhanced electrolytic conductivity, particularly at low temperatures, and the creation of a solid electrolyte interphase protective layer, which improves cell performance. Despite extensive study, there is still a need for systematic property measurements and comparisons with carbonates. Diethyl carbonate (DEC) and ethyl acetate (EA) have similar structures, and EA has qualities closer to DEC than propylene carbonate (PC).²⁸⁴ While linear esters have greater dielectric constants and lower viscosities, they are also more reactive, resulting in poorer stability against lithium metal and possible interactions with carbonates at high temperatures. Ma et al. studied fluorine-free tri-esters acting as co-solvents in MP, EA, and MB, which were tested with EC:EMC:DMC and VC in NCA/graphite–SiO cells. MP showed the best performance in cycling and high-rate charging tests, with reactions at elevated temperatures.²⁸⁰ The studies at high temperatures like 60 °C revealed that cells containing up to 20% MP had the lowest voltage loss over 500 hours, outperforming EA and MB. Compared to cells without esters, cells with esters demonstrated a comparable CR under prolonged cycling at 40 °C. Up to 40% MP inclusion was compatible with electrode materials without causing adverse reactions, as seen by the suppression of lithium plating during rapid charging by MP-enhanced Li⁺ conductivity. Yamaki et al. designed a novel electrolyte by using fluorinated ester-based solvents to enhance the thermal stability of the lithium metal anode.²⁸⁵ Smart et al. investigated the effects of several aliphatic esters, including methyl acetate (MA), EA, ethyl propionate (EP), and ethyl butyrate (BE), on the performance of lithium–graphite cells using carbonate electrolyte mixtures as a baseline.²⁴⁶ Among the electrolyte compositions studied, one was 0.75 M LiPF₆ in EC + DEC + DMC + X (1 : 1 : 1 : 1), where X stands for one of the esters. Compared to their lower molecular weight equivalents (MA and EA), which tended to form insulating films and exhibited persistent reactivity with the anode, higher molecular weight esters (EP and BE) generated more stable and less resistive SEI layers. The cells with greater molecular weight esters showed lower interfacial impedance and improved low-temperature performance. Ionic liquids containing methyl acetate groups have high conductivity and electrochemical stability.^286,287 These liquids interact well with lithium ions, which may enhance the performance of the battery electrolyte.^{286,288–290} For example, Xia et al. designed an electrolyte in partially fluorinated linear carboxylate esters (FLCEs) as co-solvents for non-flammable, high-voltage electrolytes.²⁹¹ While methyl 3,3,3-trifluoropropanoate (F1) had acceptable solubility for LiPF₆, other FLCEs could not dissolve the salt well and failed to create stable SEI films on lithium metal, necessitating their usage in combination with another solvent. High oxidation and reduction potentials shown by the FLCEs qualified them for use in high-voltage applications. In addition, F1-based electrolytes showed remarkable wettability, cyclability, and safety, demonstrating the promise of partial FLCEs as useful co-solvents to improve battery performance.

Fig. 12 (a) Summary of fluorine-free ester solvents relative to fluorinated solvents, Reproduced with permission,²⁸⁵ Copyright (2001) Elsevier. (b) TEP-modified electrolyte and Gr electrode performance, Reproduced with permission,²⁹² Copyright (2023) American Chemical Society. (c)-(i) Conventional electrolyte vs. P-MSE: stability and long-life cycling for PIBs; (ii) solvent structures with different steric hindrance, Reproduced with permission,²⁹³ Copyright (2024) Wiley-VCH. (d)-(i) Solvation structure in electrolytes; (ii) Raman spectra of P–O and S=O, Reproduced with permission,²⁹⁴ Copyright (2024) American Chemical Society. (e) Interfacial chemistry: carbonate vs. tri-anion phosphate, Reproduced with permission,²⁹⁵ Copyright (2024) Elsevier. (f) TEP effects on Li⁺ solvation, SEI, and gas generation in electrolytes, Reproduced with permission,¹²² Copyright (2024) Wiley-VCH.

Sulfone–ester (SL–EA) electrolytes possess higher anodic stability than EC-EMC electrolytes. However, their lithium cell charge–discharge performance is lower. A study focused on the sulfone–ester (SL–EA) mixed solvent with LiBF₄ solute because of LiBF₄'s greater oxidation tolerance than LiPF₆.²³⁷ Organic additives such as cycloalkanes, triethylene glycol derivatives, and VC can significantly increase cycling performances for high-voltage LiCoO₂ and LiNi_0.5Mn_1.5O₄ cells up to 4.5 V and beyond.²³⁷ To improve sulfone-ester mixed solvents for 5.6 V-class lithium batteries, further study is needed on altering solvent chemical structures or electrolyte additions. Phosphate-based solvents (PBs) have the potential for developing a non-flammable electrolyte design. In addition, the PBs such as TMP, TEP, and DMMP are widely used as flame retardants.^{279,296–298} In the study of DMMP as a flame retardant in LIBs, Feng et al. found that the graphite anode had a high CE of 84% in the 1st cycle and better cycle stability.²⁹⁹ This stability was ascribed to the electrolyte additive chloro-ethylene carbonate, which promoted the development of the SEI layer on the graphite and prevented the electrochemical reduction of DMMP.²⁹⁹ Phosphate-esters are appealing due to their high lithium salt solubility, broad voltage window, low viscosity, and nonflammability, making them prospective low-cost, high-safety solvents.³⁰⁰ However, reductive breakdown on lithium metal anodes is difficult, resulting in dendrite formation and low CE. Previous studies found that tributyl phosphate (TBP) mixed with LiFSI performed poorly because of its high viscosity and low conductivity.³⁰⁰ Incorporating DME as a cosolvent increased lithium-ion conductivity and kinetic diffusion. This novel method cleared the way for creating safer, higher-temperature electrolytes for LMBs by demonstrating exceptional electrochemical performance and safety in Li‖NMC811 cells. TEP is a non-flammable, cost-effective co-solvent and is a potential solution to the flammability of carbonate-based electrolytes in LIBs.^301–307 The study found that although TEP strongly coordinates with lithium ions, impeding graphite electrode intercalation, these problems may be minimized by enhancing TEP's competitive coordination with ethylene carbonate (EC), as shown in Fig. 12b.^308,309 After 150 cycles with a graphite anode, the resultant TEP-modified carbonate electrolyte exhibited strong ionic conductivity and nonflammability, maintaining approximately 100% of its capacity.²⁹² This formulation greatly minimized fire and explosion hazards in high-capacity pouch cells, providing a safer approach for energy storage in LIBs.³¹⁰

Phosphate-based solvents have emerged as potential energy storage options, notably in PIBs and LIBs. These electrolytes have various benefits, including nonflammability, strong ionic conductivity, and excellent electrochemical stability.^{238,279,311–313} Tris(1-propyl) phosphate (TPP) is a noteworthy phosphate solvent that balances solvation capability with salt dissociation, improving battery performance. Due to its mild solvation qualities, TPP has been chosen above alternative candidates, such as TEP and tributyl phosphate (TBP), for phosphate-based electrolytes. This feature enables better lithium or potassium ion coordination while limiting excessive solvation, which might impair conductivity. TPP's structural features also aid in creating stable electrode–electrolyte interphases, which reduce side reactions and improve battery lifetime. For example, Zhang et al. provided a new non-flammable electrolyte for PIBs that used TPP as the ideal fluorine-free solvent.²⁹³ TPP was chosen beyond TEP and TMP due to its moderate solvation capacity, as shown in Fig. 12c, which achieved a compromise between efficient ion coordination and high salt dissociation, critical for improving electrochemical performance and safety.^293,314 With a salt content of 0.6 M, the designed electrolyte had low viscosity, good ionic conductivity, and excellent oxidative stability, facilitating protective interphases that reduced side reactions. PIBs containing this electrolyte displayed extraordinary longevity by maintaining 80% capacity after 2000 cycles at 4.2 V and demonstrating TPP's promise in creating safe, high-performance energy storage technologies. Similarly, Cheng et al. developed a TMP-based electrolyte. They highlighted that the Li⁺ solvation structure analysis revealed a decrease in coordinated TMP (Li⁺-TMP) from 56% in Li⁺[TMP]_4.9[PF₆⁻] to 36% in the Li⁺[TMP]_2.5[EMC]_2.2[EC]_0.73[DTD]_0.4[PF₆⁻] electrolyte, which was due to the sequential inclusion of ethylene sulfate (DTD), EMC, and EC in the electrolyte design.²⁹⁴ Raman spectroscopy verified the attenuated Li⁺–TMP interactions with P–O vibrational frequencies, as shown in Fig. 12d.²⁹⁴ Recently, Ni et al. conducted anion comparison of carbonate and phosphate-based electrolytes by using fluorine-free single-anion, dual-anion, and tri-anion strategies, as shown in Fig. 12e, which eventually motivated the development of the multi-anion combination strategy.²⁹⁵ Moreover, optimized tri-anion by incorporating TEP achieved an excellent electrochemical performance of full cell Li|NMC811 with 98.6% CR after 300 cycles. Gebert et al. developed fluorine-free TEP-based electrolytes for interfacial investigation, and a threshold effect was observed at 30 vol% TEP when inadequate carbonate-ester solvated Li⁺ initiated poor SEI production and TEP breakdown.¹²² At increased TEP concentration (>30%), this resulted in immense ethane production and irreversible capacity loss, as shown in Fig. 12f. Perhaps additive incorporation could stabilize the TEP for safer and high-performance batteries. Therefore, fluorine-free phosphate-based solvents are a major component in developing safe flame retardant, multifunctional additives for sustainable energy systems by achieving high CR and extended cycle life.²⁷⁹

4.2.3. Fluorine-free nitriles and sulfone-based solvents. Fluorine-free formulations provide an alternative to traditional solvents, enhancing electrolyte stability and performance. Nitrile solvents, categorized into mono-nitrile and di-nitrile based on the number of cyano groups, are typically used as co-solvents or additives in electrolytes, constituting about 10% v/v or less. Acetonitrile (ACN) and butyronitrile (BN), two nitrile-based electrolyte solvents with good oxidative stability and conductivity, necessitate FEC's addition to effectively generate an SEI.^315,316 A modified electrolyte that inhibits anodic aluminum dissolution and improves stability and performance is made of aliphatic cyclic nitriles, such as cyclohexane-1-carbonitrile and cyclopentane-1-carbonitrile. With NMC622‖graphite cells, these developments enhance cycling performance and CE.

Electrolytes with great thermal stability and high flashpoints were produced by using sulfolane (SL) in adiponitrile (ADN).^317,318 Excellent oxidation stability was demonstrated by electrolytes containing LiDCTA and LiTDI up to 4.5 V.³¹⁷ Fluorine-free ASE (ADN : SL : EC at 2 : 1 : 1) electrolytes, notably LiDCTA, achieved ∼145 mA h g⁻¹ discharge capacities, stabilizing after initial losses with a CE of 97.5% (LiTDI:ASE) and 95.8% (LiDCTA:ASE) after 5–20 cycles due to side reactions in Li/LiFePO₄, as shown in Fig. 13a-(i) and (ii). The extended cut-off voltage of 4.5 V reduced reaction reversibility. However, AS (ADN : SL at 2 : 1) electrolytes had a higher average CE of 99.8% after 5–20 cycles despite slightly reduced capacities. However, adding lithium salt made EC break down into CO₂, emphasizing the need for purification.³¹⁹ Xu and Angell created various cyclic and acyclic sulfones in an effort to better understand their physical and electrochemical characteristics and evaluated them as electrolyte solvents for LIBs.³²⁰ However, their compatibility with graphitic anodes, which occasionally match the performance of carbonate-based electrolytes, is identified by their alkyl substituent structures.³²¹ Jiang et al. developed a novel aqueous deep eutectic solvent (DES) based on methylsulfonylmethane (MSM), LiClO₄, and water that exhibits increased conductivity and stability in LIBs, as shown in Fig. 13b.^322,323 The electrolyte's improved performance resulted in approximately 100% CE and 72.2% CR after 1000 cycles at 4.5C. For example, in recent times, Kreth and coworkers introduced a new FFE, 1 M LiBOB in EiPS, for use in high-temperature capacitive and faradaic energy storage devices.¹²³ The given electrolyte has superior safety characteristics (boiling point 265 °C, flash point 137 °C) and strong SEI layer formation on graphite electrodes, leading to outstanding performance at 60 °C. The capacity of the LFP half-cell in LIBs with LFP-positive electrodes was 150 mA h g⁻¹ at 0.1C and reduced to 123 mA h g⁻¹ at 5C, as shown in Fig. 13c-(i) and (ii). Less than 2% of capacity was lost across 200 cycles of long-term cycling at 1C and 60 °C, signifying the high-temperature adaptability of the electrolyte. Fig. 13c-(iii) shows the promising performance of a partially optimized LIB cell with a massive LFP cathode and graphite anode, which maintained roughly 80% of the initial capacity for the first 90 cycles.¹²³ Due to their increased viscosity compared to conventional CB systems, fluorine-free sulfone-based electrolytes are widely opted for high-voltage. Moreover, wettability issues with polypropylene or polyethylene separators are mitigated by mixing CBs or EBs or surface modification of the separator, enhancing the stability of the high voltage application. Furthermore, future directions are highly motivated to develop such solvents in bi-functional additive case studies rather than as main solvent or co-solvent to empower potential strategies.³²⁴

Fig. 13 (a)-(i) CE (2.5–4.4) V and (ii) Specific capacity at different C-rates (2.5–4.0) V for Li/LFP cells with AS, ASE, and LiDCTA ASE electrolytes, Reproduced with permission,³¹⁷ Copyright (2016) Elsevier. (b) DFT structure of MSM : LiClO₄ DES (2 : 1) and MSM : LiClO₄ : H₂O DES (2 : 1 : 1), Reproduced with permission,³²² Copyright (2019) American Chemical Society. (c) Electrochemical performance at 60 °C with 1 M LiBOB in EiPS electrolyte: (i) rate capability; (ii) 200-cycle CR at 1C in LFP; (iii) 100-cycle CR at 1C in graphite‖oversized LFP cathode (1 : 7 ratio), Reproduced with permission,¹²³ Copyright (2024) Wiley-VCH.

4.2.4. Ionic liquid-based solvents. Due to their distinct characteristics, ionic liquids (ILs) significantly eliminate fluorine from alkaline batteries. Replacing conventional carbonate-based electrolytes with ionic liquid-based electrolytes is a viable technique for enhancing battery safety.^4,325–330 ILs, which are molten salts at room temperature, have restricted flammability, good thermal stability, and a wide electrochemical stability window, which improves the safety of LIBs. Recent studies have also highlighted the stability of phosphonium cations, particularly trimethyl(isobutyl)phosphonium (P_111i4), with the FSI anion, which forms an efficient SEI with lithium metal.^331–333 ILs also have high ionic conductivity, critical for efficient ion transport. They can prevent lithium dendrite development, enhancing the reliability and durability of LMBs. For example, Karimi et al. insightfully focused on fluorine-free ILs derived from the dicyanamide (DCA) anion³³⁴ and lithium tricyanomethanide (LiTCM), which have low viscosity and significant ionic conductivity owing to their compact anion size and efficient charge delocalization. The electrolyte's ionic conductivity and viscosity were investigated. They showed that while viscosity fell from 74 mPa s to 8.9 mPa s, conductivity rose with rising temperature, from 4.1 mS cm⁻¹ at 0 °C to 27.6 mS cm⁻¹ at 60 °C, as shown in Fig. 14a-(i)–(iii).³³⁴ Durable Li–anion interactions that obstruct ion mobility are responsible for the LDPT (LiDCA–P_111i4TCM) electrolyte's lower conductivity (4.5 mS cm⁻¹) and increased viscosity (64.9 mPa s) at 20 °C with the addition of lithium salt. It is worth noticing that P_111i4TCM (PT) showed conductivity greater than that of P_111i4FSI,^330,335,336 its fluorinated equivalent, and almost on par with conventional carbonate-based electrolytes, indicating that PT is a viable option for LMBs. The LDPT electrolyte displayed a Li⁺ transference number of 0.28, which is common for liquid electrolytes. In addition, LDPT complex ions were present in the SEI on Li electrodes, which generated a polymeric outer layer and an inorganic-rich interior layer to improve Li⁺ conductivity.³³⁴ However, due to their relatively high viscosity, which can result in poor intrinsic conductivity and rate capability, difficulties persist. Furthermore, many ILs are costly and could react irreversibly with carbon anodes.³³⁷ To counter the problems, methods for reducing viscosity and improving electrochemical performance, such as using mixed (composite) solvents that blend ILs with other organic solvents, are being investigated. Furthermore, they have a lower density than bis(trifluoromethanesulfonyl)imide (TFSI)⁻-based ILs, which can reduce production costs and increase energy density.^{335,338–340} Shah et al. have explored fluorine-free IL-based electrolytes with tetrabutylammonium (N_4,4,4,4)⁺ and tetrabutylphosphonium (P_4,4,4,4)⁺ cations paired with the 2-[2-(2-methoxyethoxy)ethoxy]acetate (MEEA)⁻ anion. (P_4,4,4,4) (MEEA) showed better thermal and electrochemical stability, lower glass transition temperature, and higher ionic conductivity compared to (N_4,4,4,4)(MEEA).³⁴¹ The Li salt decreased anion mobility and conductivity, highlighting the importance of ion–ion interactions in optimizing FFEs for batteries.³⁴¹ In addition, the ionic conductivity of ILs at 20 °C with the order of conductivity being [P₄₄₄₄] [TpA] < [P₄₄₄₄] [FuA] < [P₄₄₄₄][HFuA] and the oxidation limits, as shown in Fig. 14b, which follow the trend [HFuA]⁻ < [TpA]⁻ < [FuA]⁻ were attributed to the electron delocalization in the aromatic anions [FuA]⁻ and [TpA]⁻, which improves electrochemical stability and oxidative potentials.^341,342

Fig. 14 Fluorine-free ionic liquids: (a)-(i) conductivity; (ii) viscosity vs. temperature for PT@P_111i4TCM and LiDCA–P_111i4TCM@LDPT electrolyte; (iii) linear sweep voltammograms of Li/PT/Pt and Li/LDPT/Pt cells, Reproduced with permission,³³⁴ Copyright (2022) American Chemical Society. (b) Oxidation limit of aromatic IL anions, Reproduced with permission,³⁴² Copyright (2020) American Chemical Society. (c) Schematic of the steps for production of IL from biomass, Reproduced with permission,³⁸ Copyright (2021) American Chemical Society. (d) Structure of fluorine-free ionic liquid anion, Reproduced with permission,³⁸ Copyright (2021) American Chemical Society; Reproduced with permission,³³⁴ Copyright (2022) American Chemical Society; Reproduced with permission,³⁴¹ Copyright (2020) American Chemical Society; Reproduced with permission,³⁴² Copyright (2020) American Chemical Society.

FFEs are eco-friendly alternatives to conventional toxic fluorinated electrolytes.¹⁹⁵ Perhaps the in-depth insights from inspiring works on biomass-derived IL electrolytes for future greener battery applications could pave the way for net zero emissions by mitigating hazardous substances in the battery environment. For example, Khan and colleagues explored a new method for eliminating fluorine in LIBs, called “bio-sustainability”, which entails using enormous eco-friendly, low-cost, and readily available natural materials, such as agricultural waste and biomass.³⁸ In comparison to non-aromatic anions, tetra(n-butyl)phosphonium and tetra(n-butyl)ammonium, together with aromatic furan-2-carboxylate and thiophene-2-carboxylate anions, show better thermal and electrochemical stability.^342,343 Based on the (FuA)⁻ anion, a tetra(n-butyl)phosphonium furoate [(P₄₄₄₄)(FuA)]-based fluorine-free IL was prepared and combined in different ratios with lithium 2-furoate, as shown in Fig. 14c.³⁸ Furthermore, this fluorine-free IL with specific cations–anions, listed in Fig. 14d, was thoroughly investigated for potential battery applications due to its better synergistic ion interaction, ion diffusivity, thermal stability, and electrochemical characteristics.^38,342,343 Despite the sustainability advantage of these electrolytes, their performance is limited compared to conventional electrolytes.

Liu et al. developed a PFAS-free locally concentrated ionic liquid electrolyte to study the challenges, dendrite formation, and low reversibility in both monovalent LMBs and multivalent batteries (aluminium-metal batteries (AMBs)).³⁴⁴ By incorporating fluorine-free non-solvating cosolvents such as aromatic organic cations, it is promoted to reduce viscosity, improving ion transport and anode compatibility.^345,346 Their studies enhanced the lithium stripping/plating CE to 99.7%, providing good electrochemical stability and safety compared to traditional systems.²⁵ In addition, FFE systems are insightfully demonstrated in terms of compatibility with high-energy cathodes, including Ni-rich layered oxides and sulfurized polyacrylonitrile (SPAN), enabling high-performance LMBs by improving ion transport and EEI formation in AMBs.^346–348 Their studies facilitated the transition from monovalent to multivalent batteries, significantly promoting cost-effective batteries through greener approaches. However, the interactions at the electrode interface and their impact on reversibility and performance, particularly in the aluminum-metal anode, remain unclear.³⁴⁸ Investigating ionic liquid cations, Lewis basic ligands, and non-solvating cosolvents could provide valuable insights into enhancing multivalent battery performance.³⁴⁴ Nonetheless, these studies truly herald a new era in developing IL-based formulations for greener batteries, addressing various challenges for future generations.

4.3. Additives for fluorine-free electrolytes

By improving the inherent characteristics of electrolytes, electrolyte additives, even in small amounts, can significantly increase battery performance. These additives are classified according to their specialized functions, which include producing protective coatings, stabilizing electrolytes, preventing fire and overcharging, and promoting conductivity. Fluorinated additives are highly promising in most aspects for overcoming such obstacles.¹⁵¹ However, a minimal amount of fluorine greatly impacts the environment. Under such circumstances, promoting fluorine-free additives (FFAs) in FFEs could scale-up the sustainable strategy. In addition, developing new eco-friendly functional additives influences eliminating high toxicity in the energy source field. Moreover, there are a few FFAs, such as carbonate additives (VC),⁵⁸ nitrate-based additives (LiNO₃),^139,200,349 potassium nitrate (KNO₃),²¹³ rubidium nitrate (RbNO₃),^350,351 dinitriles (adiponitrile, glutaronitrile, and succinonitrile),^352–355 phosphorus-based additives (TMP, TEP),^356–360 sulfone-based additives (SBAs),^361,362 and aromatic based additives (benzene, chlorobenzene),³⁶³ which are widely used for appropriate issues.^{58,139,142,213,364–370} These additives have received recent attention by achieving a sustainable pathway. This is essential for lithium metal anodes to enrich the SEI layer, inhibit dendrite growth, and enable smooth lithium deposition. Despite these advancements, high-performance FFAs are still limited, highlighting the need for bifunctional and multifunctional additives in future battery technologies. Bifunctional additives are essential for stabilizing the SEI layer and enhancing lithium-ion transport, addressing dendrite formation and capacity retention challenges. Multifunctional additives further optimize electrolyte properties, thereby improving battery safety, cyclability, and energy density.

For example, Zhou et al. introduced N,O-bis(trimethylsilyl)acetamide (BSA) as a bifunctional fluorine-free additive for high-voltage LMBs to overcome challenges such as dendrite growth and lithium anode expansion.¹⁴¹ Their DFT analysis revealed BSA's high reduction stability (LUMO: 0.0311 eV) and strong Li-ion affinity (−1.024 eV), promoting uniform deposition and stable SEI formation. In addition, BSA scavenged harmful HF byproducts, enhanced ionic conductivity, and reinforced the SEI with SiO_x and Li₃N components, as shown in Fig. 15a, improving mechanical strength and ion transport. Eventually, their approach proved more sustainable and cost-effective than fluorinated additives, signifying the potential of Si−O bond-based compounds.¹⁴¹ BSA enhanced SEI composition by 0.5 wt%, allowing successful cycling at a high loading (6 mg cm⁻²), 4.3 V at ∼2C, and 72.64% CR after 200 cycles.¹⁴¹ Interfacial characteristics are influenced mainly by the solvation shell of Li⁺. Hence, the cation's solvation structure must be carefully controlled to achieve reversible cycling in various batteries. This may be done by adjusting the type and concentration of solvents, additives, and anions. Moreover, adding a few additives like SBAs increases the viscosity and thermal stability issues despite improving the electrode stability and performance. Chagnes’ research group reported the synthesis of 22 bi-functional additives for LIBs.³²⁴ Their work achieved good electrode stability but limited ionic conductivity due to the high viscosity of the electrolyte. More importantly, the fast capacity fading issue associated with an anode-free battery system can eventually be solved by an FFA system.^{213,371–373} Our research group introduced a bi-functional FFA, KNO₃, in the commercial electrolyte (1 M LiPF₆ EC DEC (1 : 1 v/v)).²¹³ As shown in Fig. 15b, our study demonstrated the synergistic shielding effect of K⁺ and NO₃⁻ in countering the dendrite formation and inhibiting capacity fading, which contributed to effective cycling performance.²¹³ Similarly, Rahman et al. investigated the use of CsNO₃ as a bi-functional additive to improve the performance of LMBs by forming stable interphases.³⁷⁴ Unlike traditional fluorinated electrolytes, which form LiF-rich SEIs that hinder fast charging due to low ionic conductivity, the electrolyte with 3 wt% CsNO₃ enhances ion transport by utilizing 1,2-dimethoxyethane (DME) as the sole solvent. As shown in Fig. 15c, under the electric field, Cs⁺ and NO₃⁻ ions are transferred to the electron-rich Li metal surface and the electron-deficient NMC811 surface, facilitating protective interphase formation on both electrodes. The larger ionic radius of Cs⁺ (1.67 Å) compared to that of Li⁺ (0.59 Å) weakens the interaction with DME, forming a larger solvation shell that allows more anions to contribute to forming a robust interphase.³⁷⁴ In addition, while compared with other alkali metals (3 wt% Li, 2 wt% Na, 3 wt% K, 3 wt% Rb) nitrate additives show some improvement, CsNO₃ outperforms them, challenging the conventional role of LiF in SEI formation and demonstrating the promising potential of FFE systems for advancing sustainable batteries.³⁷⁴ This work indicates future directions to develop a fluorine-free multi-functional additive (FF-MFA) to enhance safety, eco-friendliness, and reliable battery performance. In recent years, FF-MFA has been a promising approach to enhance pristine electrolyte properties towards sustainability. Liu et al. developed a sustainable and multifunctional FF-MFA for LIBs.¹⁴² The designed solvents for FF-MFA and bis(2-methoxyethyl)methylphosphonate (BMOP) functioned as flame retardants and were compatible with graphite, while also demonstrating solvation structure dominance.¹⁴² Analysing theoretical and practical data aided in understanding how BMOP affects graphite's electrochemical performance. To investigate the decomposition reactions, the blank electrolyte (1 M LiPF₆ in DMC/EMC/EC, v : v : v = 1 : 1 : 1) and the 15% BMOP electrolyte were characterized after their Li/Cu cells were charged to 2 mA h cm⁻² at 0.5 mA cm². The blank electrolyte had produced dimethyl 2,5-dioxahexane dicarboxylate (DMDOHC), which could have contributed to oligomer formation for SEI construction. In contrast, 15% BMOP electrolyte had generated various organic by-products, including dimethyl methylphosphonate (DMMP), lithium 2-methoxyethyl methylphosphonite (LMMP), and lithium 2-methoxyethan-1-olate (LMO), as shown in Fig. 15c. These by-products and their derivatives from BMOP decomposition are expected to have improved SEI stability compared to BE. This configuration improves the graphite anode's long-term cycle stability by enhancing Li⁺ migration, lowering charge transfer impedance, and suppressing lithium dendrites and fractures.¹⁴² Kim et al. have recently introduced nano-Si₃N₄ as an innovative FF-MFA that significantly enhances the stability and performance of LMBs.³⁷⁵ Their approach precisely modulates Li⁺ solvation dynamics to promote the formation of a robust Si, N-based SEI through alloying and conversion reactions.³⁷⁵ This SEI not only improves ionic transport but also reinforces interfacial stability, both of which are critical for long-term cycling of LMBs. In addition, the nano-Si₃N₄ additive mitigates dendrite formation by facilitating uniform lithium deposition and suppresses electrolyte degradation through effective HF scavenging, thereby addressing a major safety concern associated with moisture exposure or the decomposition of fluorinated salts.³⁷⁵ Furthermore, such multi-functional approaches can significantly enhance overall performance.

Fig. 15 Fluorine-free additives: bifunctional additives’ mechanism (a) BSA in LMBs, Reproduced with permission,¹⁴¹ Copyright (2023) Wiley-VCH. (b) K⁺ prevents dendrites via electrostatic shielding, Reproduced with permission,²¹³ Copyright (2019) Elsevier. (c) CsNO₃ in LMBs, Reproduced with permission,³⁷⁴ Copyright (2023) Springer Nature. (d) Multifunctional additive BMOP degradation mechanism, Reproduced with permission,¹⁴² Copyright (2023) Elsevier.

Simultaneously, several studies have been investigating the incorporation of fluorinated or fluorine-free scavengers to neutralize species such as HF, PF₅, and H₂O in traditional batteries.³⁷⁶ Although traditional scavengers can effectively neutralize HF, they often degrade over time, limiting their long-term efficiency.³⁷⁷ Moreover, decomposed fluorinated species complicate internal operation by deteriorating the battery components, including the cathode (particularly Co-free Ni-rich), separator (high-temperature shutdown), and high-concentration, high-voltage electrolytes. These components' interaction can obscure the underlying chemical mechanisms.^28,87 Furthermore, the potential cost increase associated with adding scavenging materials, their byproducts, and interactions with other electrolyte additives in current LIB systems necessitates thorough evaluation before commercialization.³⁷⁶ Similarly, many studies have been conducted to increase the fluorine components in order to improve cycling performance. This consideration, particularly in the practical application, could intensely impair the eco-system. Here, our concern is not limited to scavengers only.

Despite the promising attributes of FFAs in terms of environmental and safety benefits, their integration into fluorinated electrolytes to meet viable electrochemical performance remains an unclear interfacial contribution.^141,142 Furthermore, incorporating FFAs in such an HF environment may not provide insight into developing an eco-friendly battery.¹⁴¹ This situation underscores the urgent need for a more sustainable approach to electrolyte design. One should focus on developing advanced FFAs that replace fluorinated species and maintain compatibility with other battery components. Such advancements are essential for realizing high-performance, cost-effective, environmentally benign batteries that meet the global demand for clean and efficient solutions.

4.4. Diluents for fluorine-free electrolytes

To overcome the high viscosity and expense of HCEs, LHCEs are incorporated into lithium batteries. LHCEs increase wettability and ionic conductivity by lowering viscosity, encouraging the battery's efficiency. They also reduce the total cost by decreasing the amount of lithium salt required. Diluent electrolytes assist in stabilizing the electrolyte, resulting in improved battery system performance and safety. Fluorinated diluents (FDs) are widely used for designing various electrolyte systems, including low-concentration electrolytes, HCEs, and LHCEs, to overcome the issue to attain high performance.^67,378–380 However, these FDs are highly toxic and adversely affect global integrity.^{111,359,381–384} Generally, the massive amount of diluent used in electrolyte design, such as FDs, presents obstacles like high toxicity, environmental pollution, and cost. Introducing non-fluorinated diluents may be a more reliable strategy. In recent years, fluorine-free diluents (FFDs) have shown promise in developing LHCEs. For example, Hai and colleagues designed an interesting LHCE using benzene as an FFD.¹⁴⁰ This system addresses the drawbacks of FDs for high-loading (9 mg cm⁻²) LMBs.

Owing to the absence of electron-withdrawing groups, benzene's (PhH) distinct conjugated structure offers exceptional oxidative (>4.7 V) and reduction stability. This diluent method eliminates fluorinated solvents' high cost, density, and environmental contamination. PhH-LHCE has strong ion conductivity, low density, cheap price, great lithium metal stability, and minimal dendrite formation compared to fluorinated electrolytes. This allows for stable cycling for 450 cycles in SC811‖Li cells, as shown in Fig. 16a.¹⁴⁰ However, benzene has a very low flash point and is highly flammable, so caution is needed.

Fig. 16 (a) Electrochemical performance of FFDs for long cycling Li-SC811 (2.8–4.3 V), Reproduced with permission,¹⁴⁰ Copyright (2024) Wiley-VCH. (b) Resonance structures: purple (with effect), orange (without effect), average CE and Kamlet–Taft (β) values, Reproduced with permission,³⁸⁵ Copyright (2022) Springer Nature. (c)-(i) Voltage profiles, (ii) cycling performance, Li/NCM811 in 0.6 M LiFSI + 0.2 M LiDFOB-1,3-DIOX electrolyte, Reproduced with permission,¹³⁷ Copyright (2024) Wiley-VCH. (d) Structural analysis of FFDs for sodium batteries, Reproduced with permission,³⁸⁶ Copyright (2024) Wiley-VCH. (e) Structural analysis of cosolvent as FFDs for sodium batteries,³⁸⁷ Reproduced with permission, Copyright (2025) Wiley-VCH.

Researchers are focusing on lithium metal electrodes for their high energy density and low electrochemical potential despite the challenge of dendritic growth. A strong SEI can prevent this issue by limiting unwanted reactions. LHCEs with non-solvating cosolvents initiate compact solvation pairs, forming a robust SEI with lower viscosity and higher lithium-ion conductivity than the HCE. Although 1,2-difluorobenzene has a higher dielectric constant and dipole moment than THF, it is less effective at solvating lithium ions.³⁸⁸ To enhance cosolvent design, new solvent descriptors that account for interactions within the electrolyte are suggested.³⁸⁹ Moon et al.'s study on FFD electrolytes motivates future researchers to explore the FFD system.³⁸⁵ This study comprised 14 diluents in a LiTFSI–EC/DEC system. It revealed that stabilization energy, reflecting Lewis basicity (β), strongly correlates with solvation structures and battery performance.³⁸⁵ Moreover, this method showed that β is a better predictor of battery performance than traditional metrics.

As shown in Fig. 16b, the Kamlet–Taft parameter is proposed for evaluating solvent miscibility in polar electrolytes, aiding in selecting optimal diluents. It was found that both 3 M LiFSI DME : Furan-(1 : 2) and 3 M LiFSI DME : PhM (anisole)-(1 : 2) performed the best, with a high CE of 99.0% for 1400 cycles and 98.5% for 500 cycles, respectively.³⁸⁵ Furan-based electrolytes showed impressive performance even at higher current densities and capacities, maintaining 99.4% efficiency for 500 cycles. Similarly, Yang and his team developed high-voltage fluorine-free ether-based electrolytes in low-concentration fluorinated salts using 1,3-dioxacyclohexane (1,3-DIOX) as a diluent. An electrolyte (0.8 M) with 0.2 M LiDFOB + 0.6 M LiFSI in 1,3-DIOX achieved a high CE of 99.47%. It supported high-loading Li‖NCM811 cells up to 4.5 V, retaining 75.2% capacity after 300 cycles, as shown in Fig. 16c-(i) and (ii). DFOB⁻ anions formed stable interphases that mitigated transition metal dissolution and gas release. This approach offered a sustainable alternative to traditional methods using hazardous fluorinated solvents. Furthermore, many studies focus on leveraging fluorination to achieve excellent performance; however, some challenges still exist. On the other hand, little investigation has been carried out into effectively removing fluorine from FFD electrolytes.

In the large-scale manufacture of alkaline metal batteries, this strategy promises to lower price and reduce environmental impact. For instance, Pai et al. studied the aromatic hydrocarbon toluene as a diluent for Na–S batteries.³⁸⁶ Their computational analysis revealed a similar solvation structure and exceptional binding energy of both FDs (TTE) and FFDs (toluene) towards the Na⁺ ion, as shown in Fig. 16d. In addition, toluene promotes an inorganic-rich SEI, which is significantly dominated by anions, further contributing to practically viable performance for Na-SPAN cells. Their fluorine-free LHCE in the sodium–sulfur battery emphasizes cost-effective and practical viable high-energy-density batteries. However, aromatic hydrocarbon-based diluents are limited in practical implementation by their low flash point and high flammability, posing significant safety concerns. Despite its promising electrochemical properties, the risks associated with benzene necessitate careful evaluation of its thermal stability and flammability before widespread adoption. Consequently, further research is needed to optimize electrolyte formulations that retain benzene's advantages while ensuring safe and scalable application. Furthermore, fluorine-free ethers often exhibit higher reductive stability and offer more possibilities for solvation structure modulation. There is interest in finding fluorine-free solvents with extremely low solvating power that exhibit remarkable compatibility with sodium metal while reasonably priced. For example, Pai et al. recently examined the LHCE by utilizing ether cosolvent DMP and cyclopentyl methyl ether (CPME)) as a diluent for sodium–sulfur (Na–SPAN) batteries.³⁸⁷ CPME has high reduction stability, weak solubility with polysulfides, and minimal solvating interactions with salt anions due to lower electron density of the five-carbon ring naphthene, which provides substantial steric hindrance and steadily contributes to anion-rich solvation shell, as shown in Fig. 16e.³⁸⁷ Furthermore, stable, realistically feasible Na–SPAN batteries have been developed, revealing a method that uses mixed fluorine-free ether electrolytes with LHCE behavior. However, this ether is highly flammable due to low flash points, rising safety issues. Although fluorine-free monovalent batteries (Li⁺, Na⁺) employing LHCEs have been extensively reported to have satisfactory performance and are cost-effective by utilizing aromatic hydrocarbons and mixed ether cosolvents, they face safety issues. These challenges signify that future advances should focus on sustainable LHCE development to promote safety and superior performance. In addition, designed LHCEs can be made viable for multivalent batteries to make cost-efficient and durable performance batteries.

Numerous studies have been conducted, focusing on fluorine-free solvents, salts, diluents, and additives across a range of formulations, as outlined in Table 2. These investigations have yielded promising results, showcasing the potential of fluorine-free alternatives in battery technology. However, despite these encouraging advancements, significant challenges remain. The path toward the commercial realization of FFBs is still long, as further research and optimization are necessary to ensure their efficiency, stability, and cost-effectiveness in large-scale applications.

Table 2 Summary of electrolytes for fluorine-free batteries

Battery	Electrolyte	Temperature	Cut-off voltage [V]	Electrochemical measurement			Ref.
				Cell configuration	Capacity [mA h g^−1]/C-rate	Avg. CE [%]/CR [%]/cycle number
Fluorine-free lithium-based battery	0.7 M LiBOB EC/EMC (1 : 1, v/v)	65	3.3–4.35	Li‖LiMn2O4	60/0.5	—	390
	1.0 M in LiBOB EC/PC (1 : 1, v/v)	60	3.5–4.2	Li‖LiMn2O4	—/0.5	—/—/100	391
	0.7 M LiBOB SL/DMS (1 : 1, v/v)	60	2.7–4.2	Li‖LiFePO4	—/0.5	—/94.0/100	392
	0.7 M LiBOB SL/DES (1 : 1, v/v)					—/66.5/100
	0.7 M LiBOB SL/DEC (1 : 1, v/v)	RT	2.7–4.2	Li‖LiFePO4	—/0.5	—/89.7/342	393
	LiB(CN)4:PAN:6% Al2O3:PEGDME	30	2.5–4.4	Li‖LiFePO4	—/10	∼97/—/15	113
	0.7 M LiBOB EC/EMC (3 : 7, v/v) + 2 vol% VC	RT	3–4.2	Silicon–graphite‖LiNi1/3Mn1/3Co1/3O2 (NMC111)	147/1	99.51/84.4/200	58
	0.305 M [Li(FuA)]0.100 mol%; [(P4444)(FuA)]0.900 mol%	20–80	—	Glassy carbon/Pt/Ag–AgCl	—	—	38
	LiClO4 : MSM : H2O (1 : 1.8 : 1 mol%)	—	2.4–3.3	Li4Ti5O12‖LiMn2O4	—/4.5	99.5/85.2/1000	322
	1 M LiBOB GVL	—	2.0–4.2	Graphite‖AC	2	—/80/25 000	188
	1 M LiBOB EiPS	60	2.0–4.2	LiFePO4|graphite|Li	—/0.1	—/80/100	123
	1 M LiBOB + 0.5 M LiNO3 DME	60	2.0–4.2	Li‖LiFePO4	—/50	—/75/500	45
	1 M LiNO3/MO	—	∼3–4.25	Li‖|NCM622	—/0.5	99/91.4/1000	127
	1 M LiClO4 + 0.2 M LiNO3 TEP	60	4.3	Li‖NCM811	—/1	99.72/82.11/500	126
		25	4.3	Li‖NCM811	—/1	99.84/80.63/800
	LiPAA 50  wt%	—	—	LiTi2(PO4)3‖LiMn2O4	—/0.5	—/—/100	114
	1 M LiClO4–2H2O–3.9PED	25	1.5–2.8	LMO‖LTO	—/0.5	93.5/92.4/300	394
	1 M LiClO4 EC : EMC : DMC (1 : 1 : 1, w/w/w) + 1% VC + 1% PS + 0.1% LiBOB	25	2.8–4.3	Gr‖NCM811	145.9/1	—/75.2/200	112
	LiTCM PEO	70	1.6–2.6	Li–S‖Li	800/0.2	100/—/1	115
	0.4 mol LiNO3/0.5 mol Li2S8 1 L DOL/DME (1 : 1, v/v)	—	1.8–2.8	CMK3|S|Li–S	800/0.1	—/—/>500 cycles	395
	3 M LiNO3 DMI	25	2.5–4.0	Li‖LiFePO4	120/—	—/—/700	149
	1 M Li2SO4 0.1 M LiOH/H2O	—	—	LiTi2(PO4)3‖LiFePO4	—/6	—/90/1000	396
	0.7 M LiBOB EC:EMC (3 : 7, v/v) + 30 vol% TEP	RT	3–4.3	NMC622‖Gr	180/0.2	—/—/100	122
	2.27 M LiClO4 0.74 Anisole (AN)/0.26DME	RT	—	Li‖Cu	—	98.9/—/—	397 and 398
	1 M LiClO4 + 3 wt% LiNO3 DOL/DME					99.1/—/—
Partial fluorination lithium-based battery	0.6 M LiFSI + 0.2 M LiDFOB 1,3-DIOX	RT	3–4.5	Li‖NCM811	—/0.4	99.47/75.2/300	137
	DME + 0.96 M LiFSI + 12 mM TFP ClO4	RT	3–4.2	Li‖NCM811	—/0.1	99.6/94/275	399
	1 M LiPF6 + 0.5 wt% BSA EC : DME : EMC (1 : 1 : 1, v/v/v)	RT	2.8–4.3	Li‖NMC622	100.99/0.2–1,2,3 cycle/2	98/72.64/200	141
	0.8 M LiNO3 + 5 wt% VC GBL/FPPN (96.5 : 3.5 wt%)	25	2.7–4.3	Li‖NMC622	143/0.619	—/90/200	139
	2.0 M LiFSI DEP	25 to 27 °C	2.8–4.3	Li‖NCA	—/0.3	99.68/80/600	138
	1 M LiPF6 + 15% BMOP DMC/EMC/EC (1 : 1 : 1, v/v/v)	—	3.0–4.3	Li‖NCM811	—/4	90.9/—/500	142
	2 M LiFSI DEE/PhH (1 : 1, v/v)	—	2.8–4.3	Li‖LiNi0.8Co0.1Mn0.1O2 (SC811)	—/0.5	99.8/87.3/450	140
	0.6 M LiTFSI/0.4 M LiBOB/0.05 M LiPF6 EC/PC (7 : 3, v/v)	RT	2.8–4.3	Li‖NMC111	140.1/1	99.7/92.8/200	136
	2 M LiFSI + 0.15 M LiDFP DX EGDBE (1 : 1, v/v)	—	2.6–4.6	Li‖NCM811	—/0.5	99.85 /80/250	400
	1 M LiFSI + 0.2 M LiNO3 MODOL	25	2.7–3.85	Li‖LFP	—/1	—/83/150	97
		−20	2.7–3.85	Li‖LFP	—/0.2	—/90/110
	LiFSI-2CPME	25	2.8–3.8	Li‖LFP	—/0.2/0.5	—/—/400	95
	1 M LiPF6 EC/DEC (1 : 1 v/v) + FEC 5 wt% + 4.0 wt% 3.5 M LiNO3/DMI	60	2.8–3.65	Li‖LiFePO4	—/5	—/95.6/2200	209
	3 Msolv LiFSI DME:AN (1 : 2, v/v)	25	2.7–4.2	Li‖LiFePO4	—/0.1	—/93.7/300	385
	2 Msolv LiFSI DME:Furan (1 : 2, v/v)					—/86.2/300
	3 Msolv LiFSI DME : ethoxybenzene(1 : 2, v/v)					—/71.4/300
Fluorine-free sodium-based battery	0.7 M NaBOB NMP/TMP (1 : 1, v/v)	55	1.3–3.6	Prussian white|hard carbon‖Na	—/0.2	—/50/100	117
	0.5 M NaBOB TMP	—	1.0–3.8	Prussian white|hard carbon|Na	—/0.2	97/—/2	401
Partial fluorine-free sodium-based battery	2 M NaFSI DMP/CPME (1 : 1, v/v)	RT	0.8–2.8	Na‖SPAN	720/0.5	—/—/300	387
	2 M NaFSI DME/toluene (5 : 3, v/v)	RT	0.8–2.8	Na‖SPAN	740/0.5	—/85/200	386
Fluorine-free potassium-based battery	0.1 M KBPh4 EC/DEC (3 : 7, v/v)	—	1.5–3.2	K‖PTCDA	—	92.6/200	121
Fluorine-free calcium-based battery	0.5 M CMC DME/THF (1 : 1, v/v)	RT	0.5–3.2	Ca‖S/C	805/0.1C per S	—	118
Fluorine-free zinc-based battery	1.0 M Zn(dca)2 DMSO	—	0.05–0.17	Zn‖Zn	—	—/—/90	119
	1.0 M Na(dca) + 1.0 M Zn(dca)2 DMSO	50	0.2–2.0	Zn‖NaFePO4	—/0.1	>99/—/100	124

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5. Paths toward greener battery materials

The transition to greener energy storage solutions is a critical step in addressing the environmental challenges posed by traditional battery technologies. A key focus in this journey is the development of fluorine-free materials and electrolytes, which offer significant advantages in terms of sustainability, safety, and environmental impact. This chapter briefly explores the path to FFB technologies, emphasizing the innovative approaches in material design and electrolyte formulation that pave the way for more eco-friendly and efficient batteries. The following sections delve into these developments, highlighting how cutting-edge research transforms the energy storage landscape by integrating sustainable practices and advanced technological solutions.

5.1. Material design

The quest for safe and ecologically friendly battery solutions has resulted in extensive research towards fluorine-free materials. Fluorine-containing compounds are commonly used in traditional batteries owing to their excellent electrochemical stability and ability to generate a stable SEI.²¹ However, the environmental effect, toxicity, and high cost of fluorine have prompted an attempt for other materials that might provide comparable performance while avoiding these downsides. Significant environmental benefits are revealed by the life cycle assessment (LCA) of batteries designed using fluorine-free components.⁴⁰² It is possible to reduce greenhouse gas emissions and harmful byproducts in the production process by removing fluorine and making it less energy-intensive.⁴⁰³ Besides reducing hazardous waste and disposal costs, the lack of fluorine makes end-of-life recycling easier. LCA demonstrates that finding and producing fluorine-free products frequently involves less ecologically harmful operations, such as rare element mining and processing, as shown in Fig. 17.^404,405 In LCA, cradle-to-gate covers extraction to production, cradle-to-grave includes the entire life cycle to disposal, and grave-to-cradle focuses on recycling for new products.⁴⁰⁵ Thus, switching to FFB technology promotes the use of safer and more abundant materials while also improving sustainability and being consistent with the ideas of the circular economy. For example, Jiang et al. pioneered a significant advancement in battery technology by developing a net-zero fluorine battery that incorporates a fluorine-free binder in its cathode material, notably enhancing cycling performance.⁴⁵ Cui et al. introduced a groundbreaking zero-strain electrode material that achieves 100% active material utilization in all-solid-state lithium batteries.⁴⁰⁶ This innovation not only boosted energy density but also extended cycle life by eliminating polymeric binders. The use of such 100% active materials is a highly promising approach for advancing FFB technology. The ongoing pursuit of FFBs necessitates a comprehensive approach that integrates cutting-edge material design, sophisticated characterization techniques, and a steadfast commitment to sustainability. Researchers are exploring alternatives such as LiF-free electrolytes and polymeric binders to develop high-performance batteries that meet rigorous environmental and safety standards. These advancements are set to transform the energy storage landscape, reducing the LCA footprint and paving the way for greener, more efficient battery technologies.

Fig. 17 Schematic illustration of LCA for fluorine-free batteries.

5.2. Electrolyte formulation

The drive towards eco-friendly battery technologies has led to exciting advancements in FFE formulations as summarized in Table 2. The FFE system has explored different strategies for various metal batteries. Xu et al. have crafted innovative FFE “solvent-in-salt” (SIS) electrolytes for sodium batteries using NaDEEP and TEOP.⁴⁰⁷ This combination enhanced ionic conductivity and thermal stability up to 270 °C, pushing the boundaries of sodium battery performance.⁴⁰⁷ In the realm of lithium batteries, Li et al. have pioneered fluorine-free solvents by methylating DME, promoting the formation of stable inorganic LiF-rich interphases.¹³⁸ This advancement addressed high-voltage stability concerns while reducing fluorine content.¹³⁸ Meanwhile, Kim et al. developed a cost-effective sodium metal anode FFE, 1 M NaBH₄ in an ether-based solvent, as shown in Fig. 18a, enabling a robust SEI layer crucial for stability and longevity.⁴⁰⁸ Moreover, Hong et al. proposed a strategy to significantly reduce the amount of fluorine used in lithium batteries by using diluted fluorinated cations (0.1 wt%) to produce robust battery interfaces with no effect on the environment, as shown in Fig. 18b.³⁹⁹

Fig. 18 FFE advancements (a) seawater battery mechanisms; M1 and M2 reduce the oxide layer to NaH and H₂, while M3 shows H₂ reacting with sodium to form NaH, Reproduced with permission,⁴⁰⁸ Copyright (2022) The Royal Society of Chemistry. (b) FFE SEI interphase with Li-fluorinated cation, Reproduced with permission,³⁹⁹ Copyright (2024) The Royal Society of Chemistry. (c) LiBOB-based electrolyte (FFE) vs. LiPF₆-based electrolyte (fluorinated electrolyte) preferential degradation on a SiGr-composite electrode, Reproduced with permission,¹⁴⁷ Copyright (2024) The Electrochemical Society.

Kisu et al. developed an FFE based on a hydrogen monocarborane cluster as a possible substitute for lithium-ion technology in calcium batteries.¹¹⁸ The FFE in alternative alkaline batteries works with outstanding performance. Liu et al. have developed an ultra-low concentration FFE electrolyte with exceptional stability and performance in a KMB.¹²¹ Furthermore, Weng et al. extended their studies to implement FFEs as safer substitutes for LiPF₆, looking at how they affected graphite and silicon electrodes.¹⁴⁷Fig. 18c shows that LiBOB-based electrolytes, while forming a thicker SEI layer, degraded more on graphite than silicon, leading to poorer performance compared to LiPF₆.^58,147 EDX mapping showed higher boron concentration on graphite, indicating preferential degradation.¹⁴⁷ This interfacial engineering study reveals the need to match electrolyte chemistry with electrode materials to improve SEI formation and battery performance.¹⁴⁷ Consequently, innovations in electrolyte formulations advance FFBs, incentivizing the circular economy, as shown in Fig. 19. Implementing a circular economy has significantly reduced waste by allowing the recovery and reuse of FFB materials throughout a product's life cycle. This strategy has been fostering resource conservation, priority, and efficiency.¹⁵²

Fig. 19 Promoting a sustainable circular economy of FFEs in FFBs.

6. Summary and future perspectives

6.1. Summary

The development of FFEs for batteries has gained significant attention in recent years due to the environmental and safety concerns associated with traditional fluorinated components. This review emphasizes the need for sustainable alternatives that maintain high electrochemical performance. The transition to greener battery materials, especially in the design and formulation of electrolytes, is crucial for reducing environmental impact while maintaining or enhancing battery performance. To this end, several strategies have been explored to eliminate fluorine, including developing fluorine-free salts based on various anions such as halogen, phosphate, nitrile, borate, and nitrate. In addition, fluorine-free solvent systems, such as carbonate, ether, ester-phosphate, nitriles, sulfones, and ionic liquids, have been investigated for their potential to replace fluorinated counterparts. Each class of salts and solvents has been evaluated for its electrochemical properties, stability, and compatibility with high-performance batteries. Moreover, fluorine-free additives and diluents have been explored as potential enhancers to improve the overall performance of FFEs further. While fluorinated electrolytes offer notable benefits in terms of stability and conductivity, they pose significant environmental risks, including greenhouse gas emissions and challenges in managing the life cycle of fluorine-containing compounds.

As concerns about climate change escalate, the development of fully “fluorine-free” electrolytes offers a compelling solution. However, “fluorine-free” is often inaccurately applied when fluorinated components remain in use, highlighting the importance of precision in terminology. This review advocates for adopting “partially fluorine-free electrolytes” where applicable, urging researchers to focus on eliminating fluorine over the long term. Achieving this goal requires optimizing ion transport, interfacial chemistry, and battery safety, aligning battery innovation with sustainability frameworks such as the 3Rs (reduce, reuse, recycle) and 3Es (energy, environment, economy). The ongoing development of FFEs has resulted in significant improvements in reducing environmental impact, aligning with the global shift towards greener technologies and cradle-to-cradle design. While there has been visible progress in developing FFEs, challenges remain, particularly in achieving adequate ionic conductivity and electrochemical stability without using fluorine-based salts and solvents. Overcoming these obstacles will require a deeper understanding of SEI formation mechanisms, degradation mechanisms, optimization of electrolyte formulations, and scaling up production processes for commercial viability. Despite these hurdles, the progress made thus far provides a solid foundation for future-generation battery technologies.

6.2. Future perspectives

As we have discussed above, FFE studies have recently been given attention, towards either partial or complete elimination of fluorine. The progress achieved on FFEs has been promising, showing significant potential for providing safer and more environmentally friendly battery systems. However, their role in enabling truly sustainable energy for the future needs attention. The future of FFEs presents a significant research opportunity as the field aims for environmentally friendly alternatives. The following are key challenges and potential research topics that should be explored in this area.

An innovation of FFBs relies on the novel anion design and selection of non-fluorinated salts, solvents, additives, diluents, and binders to optimize FFEs. Materials that withstand elevated temperatures and voltage ranges must be chosen, ensuring long-term stability. The compatibility with electrodes and separators is key to minimizing degradation and providing reliable performance throughout the battery's life cycle. In addition, limited research studies on transitioning FFEs beyond lithium-based batteries present significant challenges. However, they also offer exciting opportunities for innovation.^25,26,344 These breakthroughs would demonstrate the feasibility of creating FFBs that perform reliably under practical conditions, achieving safe, high-energy, cost-effective and environmentally friendly battery technologies.

6.2.1. Prospects and commercial viability.
6.2.1.1. Strategic innovations for fluorine-free electrolytes (FFEs). One of the main challenges in FFEs is the lack of fluorine-free ingredients, which contribute to efficient battery building with outstanding SEI/CEI construction. For instance, fluorine-free salts, including LiBOB, LiNO₃, and LiClO₄, commonly face issues with limited dissolution, gas evolution, and electrode corrosion. Eventually, a novel anion design should be developed strategically to overcome fundamental anion design challenges. Rational anion framework engineering can improve ionic conductivity and interfacial stability by incorporating electron-withdrawing groups or delocalized charge distribution. Moreover, a hybrid salt strategy that integrates multiple functional groups offers a pathway to optimize SEI formation while mitigating parasitic reactions. In addition, anion optimization should focus on enhancing solubility, oxidative stability, and electrochemical compatibility. Apart from lithium-based battery systems, the designed anion must evolve to support multivalent chemistries like the BGB anion, requiring deeper insights into solvation structures and transport properties. Beyond salt development, overcoming solvent design challenges is crucial, as non-fluorinated solvents often struggle with poor anodic stability, high viscosity, or low ionic dissociation. Solvent innovation should optimize electron-donating/withdrawing effects, steric hindrance, and solvation structure to improve ion transport and interphase formation. Addressing these challenges through molecular-level tuning and asymmetric solvent design approaches drives the next-generation batteries. Furthermore, designed novel FFEs underscore that they are free from expensive scavengers without compromising performance. Incorporating synergistic solvent–additive combinations or LHCE approaches may further mitigate stability issues, ensuring that FFEs can rival fluorinated counterparts in high-energy-density applications.
6.2.1.2. Scaling the synthesis of fluorine-free batteries (FFBs). The full potential of FFEs could be realized when integrated with advanced electrode materials, such as high-energy-density cathodes and dendrite resistant anodes. Although current fluorine-free ingredients are promising in performance, they suffer from limitations such as compatibility. Innovations in synthesis methodology necessitate developing moisture-free ingredients, which are crucial for performance and life cycle. In addition, novel material preparation methods and optimizing techniques should be needed to reduce resource use and manufacturing costs for broader adoption. For instance, fluorine-free disordered rock salt (DRX) cathodes, like Li_1.1Mn_0.8Ti_0.1O₂, demonstrate high specific capacities and strong cycling stability but require carefully optimized electrolyte composition to ensure efficient Li⁺ transport and interphase formation.⁴⁰⁹ The absence of fluorine in these systems eliminates Li–F clustering effects, thus simplifying interfacial chemistry and enhancing the long-term structural integrity of the electrode. By synergistically integrating F-free electrolyte formulations with manganese-rich DRX cathodes, the approach circumvents the limitations of conventional fluorinated systems. Furthermore, new FFB materials should be advanced in two ways: benchmarking FFEs and advanced tools, as stated below.
6.2.1.3. Comprehensive benchmarking FFEs and advanced tools for fluorine-free batteries (FFBs). Benchmarking FFBs against conventional systems is critical for validating their viability with regard to energy density, cycle life, safety, and cost-effective and scalable formulations. Advanced tools like artificial intelligence (AI) or machine learning (ML) accelerate discovery by predicting key properties such as dielectric constant, HOMO/LUMO levels, viscosity, flashpoints, solubility, and decomposition temperatures from large-scale databases. Moreover, higher energy density batteries’ operating voltages ≥4.5 V necessitate viable oxidative stability to construct a robust SEI/CEI via effective FFE design and protective cathode coatings, especially for nickel-rich cathodes prone to degradation. Simultaneously, emerging in situ and operando characterization techniques will be indispensable for elucidating real-time electrode–electrolyte interfacial evolution, thereby guiding the optimization of FFE formulations in a short-term period for sustainable technologies.

FFEs exhibit distinct LiF-free interfacial chemistry that promotes durable battery performance. Advanced techniques, such as spectroscopy, imaging, and molecular dynamics simulations, are necessary to explore the surface chemistry of these systems, revealing key insights into reaction products and interfacial phenomena. For example, Lu et al. studied the formation and evolution of the SEI by utilizing fluorine-free LiBOB-based electrolytes, with and without VC additives.⁴¹⁰ Using operando ex situ X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XRR), three stages of SEI evolution were observed: formation, densification, and thinning during delithiation, stated as “breathing” behavior.⁴¹⁰ VC was found to inhibit LiBOB decomposition and promote a smoother SEI, reduce electron tunneling, and prevent lithium trapping. Their findings suggest that LiBOB-based electrolytes form distinct SEIs compared to traditional LiPF₆ systems, with VC enhancing stability and electrochemical performance. Furthermore, this understanding emphasizes advanced tools’ utility in identifying optimal materials and configurations, allowing for the development of FFEs that enhance battery efficiency and longevity. By integrating advanced characterization tools, researchers can easily understand the complex mechanisms governing fluorine-free ingredients.

6.2.2. Enabling the transition to multivalent batteries. Lithium-based batteries are renowned for their high energy density and excellent electrochemical performance. However, challenges include high Li⁺ reactivity, low standard reduction potential, and electrolyte instability, particularly in FFE systems. Moreover, the escalating cost of lithium, driven by its limited availability and resource constraints, compounds sustainability concerns surrounding lithium-based technologies. Consequently, there is increasing attention to alternative metal batteries, which are abundant and offer improved interfacial stability and cost-effectiveness, positioning them as promising candidates for future practical applications.
6.2.2.1. Expansion of FFEs beyond lithium-based batteries. One of the most promising limits for FFEs is their application in multivalent batteries. These batteries hold significant potential due to their constituent materials' abundance and low cost, making them ideal for large-scale energy storage applications. For example, Liu et al. worked on developing fluorine-free locally concentrated ionic liquid electrolytes, which hold great promise for multivalent AMBs, including aluminum–sulfur (Al–S) batteries.^{340,345,347,348} By incorporating fluorine-free cosolvents and aromatic organic cations, these electrolytes reduce viscosity and enhance ion transport.³⁴⁰ However, further research is needed to optimize electrode/electrolyte interphase formation, molecular-level interactions, and electrolyte stability.^125,344 Improving organic cations, cosolvent design, and SEI evolution is critical to advancing the performance and stability, especially in Al–S batteries.³⁴⁴ Ongoing advancements in ionic liquids, deep eutectic solvents, and borate-based anions are key to improving ion transport and dendrite suppression. This requires the development of electrolytes specifically tailored for multivalent systems. Research must focus on understanding the de-solvation kinetics and interfacial reactions of multivalent ions, which are crucial for optimizing ion transport and ensuring stability in the electrolyte. For instance, integrating FFEs into sodium-ion and magnesium-ion batteries could provide a sustainable alternative to lithium-ion batteries. Sodium-ion batteries offer a high degree of cost-effectiveness due to the abundance of sodium. In contrast, magnesium-ion batteries have the potential for high energy density. Successful adaptations of FFEs for multivalent batteries would open new possibilities for sustainable and efficient energy storage systems that scale across various sectors.
6.2.2.2. Synergizing electrolyte and electrode innovations. The advancement of multivalent FFBs requires an integrated approach to electrolyte and electrode engineering to achieve superior electrochemical performance and stability. FFEs, utilizing alternative multivalent salts like borates (e.g., bis(glycolato)borate (BGB), bis(malonato)borate (BMB)) or sulfonimides, promote an HF-free environment while improving interfacial stability and ionic conductivity.^123,411,412 Concurrently, multivalent battery electrodes such as calcium manganese dioxide (CaMn₂O₄) for Ca-ion and SPAN for Na–S and Al–S are gaining attention for future viability. Additionally, PFAS-free binders like hydroxyl-rich siloxane nanohybrid (SNH) and recyclable alternatives such as poly(ethyl cyanoacrylate) (PECA) promote sustainable battery designs.^413,414 As these innovations continue to address existing limitations, FFBs could achieve real-world applicability, offering a viable pathway toward environmentally responsible energy storage solutions.^25,26

6.2.3. Environmental sustainability and recyclability. As the regulatory landscape surrounding PFAS, toxic and non-recyclable materials, is getting stringent, the sustainability of FFEs will be a key factor in their commercialization. Research into recycling technologies and a circular approach to battery material life cycles will drive the adoption of FFEs and support the transition to greener battery chemistry. Exploring biomass-based ingredients for electrolyte and electrode binders could offer a promising path toward large-scale application, combining performance with sustainability. Additionally, life cycle assessments (LCA) and techno-economic analyses should focus on evaluating the feasibility of large-scale implementation. Developing cost-effective production methods while maintaining high electrolyte purity and performance will be essential for commercial adoption.

Overall, the shift toward FFEs is driven by environmental concerns and the need for better battery performance. Developing these systems requires a comprehensive approach, from material selection to recycling strategies, leveraging innovative technologies to meet the demands of modern energy storage applications. Focusing on these areas will drive the next generation of sustainable and high-performing FFB technologies.

Abbreviations

1,3-DIOX	1,3-Dioxacyclohexane
3-FLTBP	Lithium tris[3-fluoro-1,2-benzenediolato(2-)-O,O′]phosphate
AC	Activated carbon
ACN	Acetonitrile
ADN	Adiponitrile
AGG	Aggregate
AMBs	Aluminium-metal batteries
BB	Butyl butyrate
BE	Ethyl butyrate
BGB	Bis(glycolato)borate
BL	Butyrolactone
BMC	Bis(2-methoxyethyl)carbonate
BMEC	Bis(2-methoxyethyl)carbonate
BMOP	Bis(2-methoxyethyl)methylphosphonate
BN	Butyronitrile
BOB	Bis(oxalato)borate
BrB	Bromobenzene
BSA	N,O-Bis(trimethylsilyl)acetamide
CCs	Cyclic carbonate solvents
CE	Coulombic efficiency
CEI	Cathode electrolyte interphase
CIB	Chlorobenzene
CIP	Contact ion pair
CMC	Calcium monocarborane
CMK3	Mesoporous carbon
CPME	Cyclopentylmethyl ether
CR	Capacity retention
CTC	Tetrachloromethane
CYH	Cyclohexane
DBB	1,3-Dibromobenzene
DBC	Dibutyl carbonate
DCA	Dicyanamide
DCB	1,3-Dichlorobenzene
DEC	Diethyl carbonate
DEE	1,2-Diethoxyethane
DEE	Ethylene glycol diethyl ether
DEP	1,2-Diethoxypropane
DES	Diethyl sulfite
DGM	Diethylene glycol dimethyl ether
DGT	Diethylene glycol diethyl ether
DMC	Dimethyl carbonate
DMDOHD	Dimethyl 2,5-dioxahexanedioate
DME	1,2-Dimethoxyethane
DMM	Dipropylene glycol dimethyl ether
DMMP	Dimethyl methylphosphonate
DMP	1,3-Dimethoxypropane
DMS	Dimethyl sulfite
DMSO	Dimethyl sulfoxide
DN	Donor number
DOL	1,3-Dioxolane
DPE	Dipropyl ether
DX	1,3-dioxane
EA	Ethyl acetate
EB	Ethylbenzene
EBs	Ether-based solvents
EC	Ethylene carbonate
ECHA	European Chemicals Agency
EEL	Electrolyte–electrode interfaces
EGD	Ethylene glycol diacetate
EGDBE	Ethylene glycol dibutyl ether
EiBS	Ethyl-iso-butyl sulfone
EiPS	Ethyl isopropyl sulfone
EMC	Ethyl methyl carbonate
EMEES	Ethyl methoxyethoxyethyl sulfone
EMES	Ethyl methoxyethyl sulfone
EsBS	Ethyl-sec-butyl sulfone
ESCP	Ethyl sulfonyl cyclopentane
ESP	Electrostatic potential
FDs	Fluorinated diluents
FEC	1-Fluoroethylene carbonate
FFA	Fluorine-free additive
FFBs	Fluorine-free batteries
FFD	Fluorine-free diluent
FFE	Fluorine-free electrolyte
FPPN	Pentafluoro(phenoxy)cyclotriphosphazene
GBL	γ-Butyrolactone
Gr	Graphite
GVL	γ-Valerolactone
HCE	Highly concentrated electrolytes
HF	Hydrogen fluoride
HPT	Heptane
IB	Iodobenzene
ILs	Ionic liquids
iPB	Cumene
KBPh4	Potassium tetraphenylborate
KMB	Potassium metal battery
LCA	Life cycle assessment
LCs	Linear carbonate solvents
LHCEs	Localized high-concentration electrolytes
Li(FuA)	Lithium furan-2-carboxylate
LIBs	Lithium-ion batteries
LiDCTA	Lithium 4,5-dicyano-1,2,3-triazolate
LiFSI	Lithium bis(fluorosulfonyl)imide
LiPAA	Lithiated substituted polyacrylic acid
LiPF6	Lithium hexafluorophosphate
LiTCM	Lithium tricyanomethanide
LiTFSI	Lithium bis(trifluoromethanesulfonyl)imide
LiTOP	Lithium tris(catecholato)phosphates
LMBs	Lithium metal batteries
LMMP	Lithium 2-methoxyethyl methylphosphonite
LMO	Lithium 2-methoxyethan-1-olate
LTBP	Lithium tris[1,2-benzenediolato(2)-O,O′]phosphate
MA	Methyl acetate
MADE	Molecular anchoring diluent electrolyte
MAN	Malononitrile
MB	Methyl butyrate
MEMS	Methoxyethyl methyl sulfone
MES	Mesitylene
MME	3-Methoxyperfluoro(2-methylpentane)
MO	3-Methyl-2-oxazolidinone
MODOL	2-Methoxy-1,3-dioxolane
MOFs	Metal–organic frameworks
MP	Methyl propionate
MSM	Methylsulfonylmethane
MTS	1-Methyltrimethylene sulfone
NaDEEP	Sodium bis(2-(2-ethoxyethoxy)ethyl)phosphate
NMP	N-Methyl-2-pyrrolidone
OOE	1,1,2,2,5,5,6,6-Octafluoro-3-oxahexane
P4444(FuA)	Tetra(n-butyl)phosphonium furan-2-carboxylate
PAN	Polyacrylonitrile
PC	Propylene carbonate
PEGDME	Poly(ethylene glycol)dimethyl ether
PEO	Polyethylene oxide
PFAS	Perfluoroalkyl and polyfluoroalkyl substances
PhE	Phenetole
PhH	Benzene
PhM	Anisole
PTCDA	Perylene-3,4,9,10-tetracarboxylic dianhydride
PVDF	Poly(vinylidene fluoride)
RT	Room temperature
S/C	Sulfur/carbon composites
SCN	Succinonitrile
SEI	Solid electrolyte interphase
SIS	Solvent-in-salt
SL	Sulfolane
SL–EA	Sulfone–ester
SPAN	Sulfurized polyacrylonitrile
T3GM	Triglyme
T4GM	Tetraglyme
TBP	Tributyl phosphate
TCM	Chloroform
TEOP	Tris(2-(2-ethoxyethoxy)ethyl)phosphate
TEP	Triethyl phosphate
TFP ClO4	N-Methyl-2,4,6-trifluoropyridinium perchlorate
THF	Tetrahydrofuran
TMP	Trimethyl phosphate
TOL	Toluene
TOP	Tris(oxalato)phosphate
TPP	Tris(1-propyl)phosphate
TriMS	Trimethylene sulfone
TTE	1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether
VC	Vinylene carbonate

Data availability

Due to the nature of a review article, the primary data supporting this article can be obtained upon contact with the corresponding authors. All the data cited from published papers can be referred to through the given copyright permission and citation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the valuable financial support from the National Taiwan University of Science and Technology, Taiwan. Financial support from the National Science and Technology Council of Taiwan (NSTC 113-2639-E-011-001-ASP, 113-2923-E-011-002, 113-2222-E-011-004; 112-2923-E-011-001, 112-2923-E-011-004-MY3; 112-2218-E-011-011), the Ministry of Education of Taiwan (the Sustainable Electrochemical Energy Development Center (SEED Center) from the Featured Areas Research Center Program), as well as the supporting facilities from National Taiwan University of Science and Technology (NTUST), National Center for High-performance Computing (NCHC), and National Synchrotron Radiation Research Centre (NSRRC), are all gratefully acknowledged. V. Boligarla thanks “The Ministry of Education (MOE) Taiwan Scholarship Program.”

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