Progress and obstacles in electrode materials for lithium-ion batteries: a journey towards enhanced energy storage efficiency

Abstract

This review critically examines various electrode materials employed in lithium-ion batteries (LIBs) and their impact on battery performance. It highlights the transition from traditional lead-acid and nickel–cadmium batteries to modern LIBs, emphasizing their energy density, efficiency, and longevity. It primarily focuses on cathode materials, including LiMn₂O₄, LiCoO₂, and LiFePO₄, while also exploring emerging materials such as organosulfides, nanomaterials, and transition metal oxides & sulfides. It also presents an overview of the anode materials based on their mechanism, e.g., intercalation–deintercalation, alloying, and conversion-type anode materials. The strengths, limitations, and synthesis techniques associated with each material are discussed. This review also delves into cathode materials, such as soft and hard carbon and high-nickel systems, assessing their influence on storage performance. Additionally, the article addresses safety concerns, recycling strategies, environmental impact evaluations, and disposal practices. It highlights emerging trends in the development of electrode materials, focusing on potential solutions and innovations. This comprehensive review provides an overview of current lithium-ion battery technology, identifying technical challenges and opportunities for advancement to promote efficient, sustainable, and environmentally responsible energy storage solutions. This review also examines the issues confronting lithium-ion batteries, including high production costs, scarcity of materials, and safety risks, with suggestions to address them through doping, coatings, and incorporation of nanomaterials.

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1. Introduction

Energy serves as a fundamental necessity in today's world, and the United Nations has identified the provision of affordable and clean energy as the seventh Sustainable Development Goal (SDG). Since the beginning of civilization, humanity has been dependent on energy sources. Initially, the focus was primarily on non-renewable resources. However, the continuous growth in global energy demand and the awareness about the depletion of fossil fuel reserves highlighted the critical necessity for renewable energy alternatives. Additionally, the growing environmental concerns associated with carbon dioxide emissions and rising fuel prices have intensified the drive for the advancement of hybrid and electric vehicles.¹ In this regard, guaranteeing a secure and sustainable energy supply is arguably our most urgent societal challenge. Therefore, collaborative initiatives are being pursued to find solutions that mitigate air pollution from internal combustion engines, while simultaneously advancing effective technologies for the generation and storage of electrical energy derived from solar and wind sources. Rechargeable batteries are viewed as the most promising alternatives in this scenario.² They store chemical energy, which can be converted into electrical energy on demand.^3,4

The first rechargeable battery was developed in the mid-19th century, with Gaston Plante's lead-acid batteries debuting in 1859.⁵ The quest for higher energy density, enhanced efficiency, and more environmentally friendly options spurred the evolution of rechargeable batteries, transitioning from lead-acid to nickel–cadmium (NiCd) and ultimately to lithium-ion technology. While lead-acid batteries remain in use, their limited lifespan, low energy density, and considerable weight have prompted the search for superior alternatives. Nickel–cadmium batteries offered improved energy density, durability, and performance across various temperatures, yet their environmental impact and health risks due to cadmium toxicity posed significant challenges. A pivotal breakthrough occurred in the 1990s with the advent of lithium-ion batteries, which have since become the predominant battery technology, largely due to their reduced environmental footprint.⁵

The shift from lead-acid batteries to nickel–cadmium and subsequently to lithium-ion batteries was driven by pressing practical requirements rather than solely by technological progress. As society increasingly prioritized mobility, electrification, and sustainability, there arose a demand for energy storage solutions that were lighter, more efficient, and environmentally friendly. Innovations in materials science, particularly in the development of electrodes and electrolytes, largely addressed the limitations of earlier battery technologies. Additionally, the worldwide pursuit of cleaner technologies and the expansion of electric and portable electronics significantly accelerated the adoption of lithium-ion systems.⁶

LIB is rechargeable utilizing the reversible intercalation of Li⁺ into electrically conducting electrodes to store energy. Typically, one electrode functions as the cathode, often composed of lithium cobalt oxide, while the anode is usually made of graphite. These materials are capable of absorbing and releasing lithium ions. The electrolyte, a specialized liquid that enables the transfer of Li⁺ ions between the electrodes, commonly consists of a mixture of dimethyl carbonate and ethylene carbonate. A separator is also present to allow the flow of Li⁺ ions while preventing direct contact between the electrodes. During charging, lithium is oxidized, prompting the power supply to drive Li⁺ ions from the cathode to the anode through the electrolyte and separator. To maintain charge balance, electrons travel from the cathode to the anode via an external circuit instead of through the electrolyte. This entire process is reversed during discharging, where Li⁺ ions migrate from the anode to the cathode through the electrolyte, and electrons follow the same path through the external circuit, generating power from the battery.⁷

LIBs have been a game changer in the battery market because of their distinct advantages over other commercially available rechargeable batteries in the context of their high specific energy, energy density and efficiency, longer cyclability, and extended shelf life.⁸ LIBs are particularly advantageous for electric vehicles primarily because of their elevated energy density, allowing for better energy storage in a lighter and more compact form. They also deliver superior power output relative to their mass compared to traditional rechargeable batteries.⁹ Presently, the majority of all hybrid vehicles in the market utilize Ni-MH batteries. However, research shows that this trend may change within a decade, with projections indicating that 70% of hybrids, 100% of plug-in hybrids, and almost all electric vehicles (EVs) will depend on LIBs.¹⁰ A comparison of the energy density of LIBs with other materials and batteries can be seen in Fig. 1.

Fig. 1 Comparison of the energy density of different materials.

The idea behind the first LIB was based on the reversible transmission of Li⁺ ions between anode and cathode materials of varying potentials. This mechanism has led to the common designation of “rocking chair battery”, reflecting the oscillatory movement of lithium ions between the two electrodes.¹¹ The ability of the series of lithium incorporated transition metal oxides (TMOs) with the LiFeO₂ structure to intercalate and de-intercalate Li-ions reversibly at comparatively high potentials marked a ground-breaking discovery. Nickel and cobalt oxides and their mixtures with Al, Mn, and Fe were also found to demonstrate this capability. Among these materials, LiCoO₂ emerged as the active material for LIBs. Later on, a new form of MnO₂ was prepared from LiMn₂O₄ (LMO) that exhibited reversible oxidation and reduction reactions in a nonaqueous electrolyte at elevated potentials, making it an effective option for various commercial battery applications.^12,13 The recognition of materials suited for use as anode materials proved to be more intricate; initially, graphite and carbon-based materials were utilized, but they faced challenges due to the co-intercalation of solvent molecules, leading to solvent degradation and damage to the carbon structure. Eventually, petroleum coke (PC) was recognized superior to graphite due to its resistance to reduction and solvent co-intercalation, and the addition of ethylene carbonate further enhanced the stability of both petroleum coke and graphite.^14,15

Currently, a variety of cathode materials are employed in batteries alongside the traditional options LCO and LMO. A newer alternative, lithium nickel manganese cobalt oxide (NMCO), is available in several formulations based on the ratios of nickel, manganese, and cobalt, including 111, 532, and 811, with the 111 and 532 variants being the most commonly utilized. Recently, lithium iron phosphate (LFP), characterized by its tunnel-like structure, has garnered significant interest from researchers. Each of these materials presents distinct advantages and disadvantages, influencing their specific applications.¹⁶

Despite significant advancements of LIBs over recent decades, they continue to exhibit certain shortcomings. The overall production costs of these batteries have fallen to approximately $190 per kW h, yet this figure remains considerably above the target of $125. The increasing demand for LIBs has heightened the strain on the availability of critical materials, including lithium carbonate. As the use of these batteries broadens, maintaining an adequate supply of necessary materials will become a considerable challenge. Furthermore, safety concerns related to lithium-ion batteries continue to exist, particularly regarding effective heat dissipation management, which poses challenges in high-temperature environments or within extensive battery setups.¹⁷ An increase in battery temperature can trigger undesirable side reactions that may result in thermal runaway, characterized by the generation of excessive heat that is difficult to control. This situation poses risks of mechanical failure, electrical malfunctions, thermal damage, and even potential explosions. Consequently, it is crucial to comprehend the behaviour of LIBs under these hazardous conditions to enhance their safety.^18,19 This review article seeks to address the aforementioned safety issues by exploring various materials for electrodes and electrolytes and assessing their performance. Additionally, it offers a comprehensive overview of the impact of various electrode materials on the overall battery efficiency of LIBs.

2. Cathode materials used in LIBs

2.1 TMOs as cathode materials

Transition metal oxides are being extensively studied as promising materials for cathodes in LIBs due to their ability to provide high energy densities, a wide range of electrochemical characteristics, and a lower cost in comparison to other options. These materials typically comprise transition metals such as Co, Ni, Mn, or Fe bonded with oxygen.²⁰ The key advantages of using TMOs stem from their ability to undergo reversible lithium insertion and extraction during charge/discharge cycles, which makes them suitable for energy storage applications. The fundamental working principle of TMOs-based cathodes in LIBs is based on the processes of intercalation and de-intercalation of Li⁺ ions within the layers of oxide structures. The TMOs' oxidation states, such as Ni²⁺/Ni³⁺ in nickel-based oxides, Mn³⁺/Mn⁴⁺ in manganese oxides, and Co³⁺/Co⁴⁺ in cobalt oxides, play a critical role in facilitating these reversible processes.²¹ This enables the cathode materials to store energy during charging and release it during discharge, thus powering devices such as EVs and portable electronics. These cathodes also exhibit relatively high voltage outputs, typically between 3.5 V and 4.5 V, which is favourable for achieving high energy densities. Several TMO materials have been investigated for LIBs as listed in Table 1.

Table 1 TMOs as cathode materials in LIBs

Cathode materials	Method of preparation	Structure	Reversible capacity (mA h g^−1)	Cycle number	Coulombic efficiency (%)	Ref.
α-LiFeO2	Solid state method	Cubic with space group Fm3̄m	500	30	>95	22
NCM@LiFeO2 and Fe2O3	Sol–gel method followed by calcination	Layered structure	180	600	≈99	23
α-LiFeO2/graphene nanocomposites	Hierarchical assembly	Cubic structure	238.9	50	>95	24
0.7Li2MnO3.0.3LiFeO2	Dry synthesis	Layered structure	160	50	>95	25
α-LiFeO2 nanorods	Solid state method	Nanorods	139.8	50	>85	26
LiNi0.6Mn0.37Fe0.03O2	Coprecipitation method	Layered R3m	185.3	500	>90	27
α-LiFeO2/C composites	Solvothermal method	Cubic with Fm3̄m	135.5	100	>90	28
Nanosized lithium cobalt oxide N-LCO	High-temperature solid-state reaction	Nanosized porous structure	168.7	300	>90	29
LiCoO2 sintered disk cathode	High-temperature sintering method	Sintered thick and dense structure	135.8	>50	Good coulombic efficiency	30
LiCoO2 nano polyhedron on Au-coated Cu foam	Microwave-assisted route followed by annealing	Hexagonal layered structure	136.4	600	>95	31
Li1.2Mn0.54Co0.13Ni0.13O2 microspheres	Co-precipitation method	Layered structure with super-lattice reflections	202	100	>95	32
LiNiO2 doped with 0.5% Mg and 0.3% Cu	Co-precipitation method	Rhombohedral	213	200	>80	33
LiNiO2	By varying the partial pressure of oxygen during the calcination process	A structure having t2g/eg symmetry	204	100	92.5	34
LiNiO2	Precipitation followed by calcination	The rhombohedral unit cell (R3m space group)	231.7	400	91.3	35
Mg/Mn–LiNiO2	The co-precipitation method followed by high-temperature calcination	Layered with rhombohedral unit cell	216	350	86	36
LiNiO2 with LiYO2–YO3 coating	The co-precipitation method followed by calcination	Layered with rhombohedral unit cell	225	100	93.4	37
LiNi0.95Mn0.05O2	The co-precipitation method followed by calcination	Layered structure	189.6	100	≈99	38
Niobium treated LiNiO2	Facile incorporation of Nb into LiNiO2	Layered structure	160	500	≈99	39
LiNi0.95Al0.04Mg0.01O2	Solid state interface elemental inter-diffusion strategy	Layered structure	200.8	200	79.46	40
Selenium coated LiNiO2	Solid-phase mixing combined with low-temperature sintering	Layered structure	200.1	300	>95	41
Pre-oxidized LiNiO2	Precursor peroxidation method	Layered structure	190.9	100	>90	42
LiNiO2 with Al2O3 artificial coating layer	Atomic layer deposition	Layered hexagonal structure	170	50	>95	43
Al^3+ and Ti^4+ doped LiNiO2	Sol–gel method	Layered structure with R3̄m space group	223	300	81.59	44
LiMnO2 and rGO nanocomposite cathode	One pot hydrothermal route	Layered structure	185.6	100	>95	45
LiMnO2–LiMn2O3 composites	Hydrothermal method	Orthorhombic structure	265	80	>95	46
LiMnO3 nanowires	Low-temperature reduction process	Nanowires	120.1	100	>90	47
Li0.98MnO1.992	Tuneable electrochemical conversion using protonated Mn3O4	Spinel layered heterostructures	250.5	1000	98.8	48
Li2Se–LiTiO2	Two-step solid-phase synthesis method	Cubic LiTiO2 in which Li2Se are uniformly distributed	398	100	>95	49
LiNi0.6Co0.2Mn0.2O2	Ball-milling and tempering method	Rock salt type	≈180	200	80.01	50
LiCu2O2	Solid phase sintering technique	Crystalline structure	423	>50	Good coulombic efficiency	51

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Layered and spinel oxides have received special attention due to their high efficiency as cathode materials used in commercial LIBs. These oxides exhibit high capacitance and decent rate capability. Some of the layered and spinel oxides are discussed in the subsequent sections. Fig. 2 shows the schematic illustration of different cathode materials used for LIBs.

Fig. 2 Cathode materials for LIBs.

2.1.1 LiCoO₂ (LCO) as cathode material. LCO was initially proposed as a cathode material by Goodenough and Mizushima in 1980.⁵² It continued to be a prevalent material in commercial Li-ion batteries because of its attributes such as higher discharge potential, energy density, cycling stability, and facile bulk preparation. One of the key advantages of LCO is its ideal layered structure when it is in a fully charged state. The difference in ionic radii between Li⁺ and Co³⁺ facilitates this layered structure that allows for rapid 2D diffusion of lithium ions.⁵³ The layered architecture of LiCoO₂ contributes to its structural integrity during the processes of lithium ion intercalation and deintercalation, remaining stable even with the cycling of up to 0.5 Li⁺ ions per formula unit. This compound boasts an impressive energy density of up to 840 W h kg⁻¹ at 4.55 V, making it particularly well-suited for compact electronic devices that demand significant energy storage. Its ability to sustain consistent performance over numerous charge–discharge cycles ensures a prolonged battery life. Furthermore, LiCoO₂ maintains a stable and reliable voltage profile throughout the discharge cycle, which is crucial for sensitive electronic devices requiring a steady voltage supply.

Theoretically, LCO can achieve a capacitance of approx. 274 mA h g⁻¹ when all the Li⁺ ions are de-intercalated from its structure. However, in practice, it achieves only around 140 mA h g⁻¹ within the potential range of 3.0–4.2 V.⁵⁴ LiCoO₂ exhibits greater volumetric density of 4.2 g cm⁻³ compared to numerous other cathode materials. This characteristic allows for greater energy storage within a compact volume, rendering it ideal for portable and miniaturized devices. Its widespread application in 3C devices—namely computers, communication tools, and consumer electronics—highlights its significance. Furthermore, the high energy density of LiCoO₂ suggests promising possibilities for use in electric vehicle applications.⁵⁵

LCO can be prepared using various methods such as sol–gel method, solid-state reaction, co-precipitation, combustion synthesis, freeze-drying, mechano-chemical processes, hydrothermal methods, and microwave synthesis.^53,56 Among these methods, the solid-state reaction methodology is most commonly used. It involves grinding lithium and cobalt salts and heating the mixture at high temperatures. The synthesis method significantly influences the structure of the material and, consequently, affects the electrochemical performance. For instance, low-temperature synthesis often results in a high initial capacity, followed by rapid capacity degradation because of disordered Co³⁺ and Li⁺ ions placement, resulting in a spinel-like LCO phase.⁵⁷

Along with its merits LCO also faces some challenges as a cathode material. Its main issue is the phase transition during the deintercalation of Li⁺, causing lattice distortions. In particular, a reduction in the lattice dimensions at around 50% Li⁺ extraction (beyond 4.2 V) results in capacity fading due to non-uniform strain, leading to particle fractures. Cobalt dissolution at high voltages (above 4.2 V) is another issue, leading to further capacity loss. This is partly triggered by oxygen loss from the lattice during deep Li⁺ extraction.^58–60 The instability of LCO at high voltages and the loss of active elements such as Li⁺ and Co also cause compatibility problems with the electrolyte, binders, and conductive additives, affecting the whole battery system. These issues lead to reduced electrochemical performance and can even pose safety concerns. The interface between LCO and other components, like the electrolyte, is also a significant challenge. Moreover, disruption of the solid electrolyte interface due to oxidation of the electrolyte leads to interface instability and capacity fading. The commonly used electrolyte salt, LiPF₆, has low thermal stability and is sensitive to hydrolysis. At high voltages, it decomposes and triggers side reactions with the cathode, which worsens battery performance and raises safety risks, including thermal runaway.^13,61 These high-voltage challenges can be addressed by doping and coating. Doping can improve the structure of LCO, enhancing its stability. In contrast, coating materials like Al₂O₃, CePO₄, FePO₄, Li₄Ti₅O₁₂, SnO₂, TiO₂, ZnO, and ZrO₂ can improve conductivity, stabilize the material, and choke unwanted reactions between the electrolyte and electrode and accounts for improved C_s and longer cycle life.⁶² Another practical approach to enhance LCO's cycle life and discharge capacity is metal doping, using elements such as Al, Cr, Ni, Ti, or Mg.⁶³

Future advancements in LiCoO₂ cathode materials will leverage innovative engineering strategies to enhance performance, stability, and safety. One promising approach is advanced defect engineering, which employs techniques such as high-temperature shock to deliberately introduce controlled imperfections in the material. These defects can potentially boost electrochemical performance and enhance lithium-ion mobility. Additionally, morphology engineering plays a vital role by designing specific particle shapes and structures, such as core–shell configurations, which can significantly improve structural stability and prolong cycle life. Furthermore, there is a growing demand for precise control over doping and surface coating processes to ensure uniform distribution and optimal amounts of modifying agents. This precision will further enhance the electrochemical properties and long-term durability of LiCoO₂, thereby increasing its efficiency and reliability for next-generation lithium-ion batteries.⁶⁴

2.1.2 LiMn₂O₄ (LMO) as cathode material. Spinel LiMn₂O₄ is an appealing choice due to its cost-effectiveness, low toxicity, and admirable voltage profile. The abundance and cost-effectiveness of manganese render LiMn₂O₄ a more economical and accessible alternative to cobalt-based cathode materials. Additionally, manganese's environmental advantages and lower toxicity align with the increasing emphasis on sustainable energy solutions. The spinel structure of LiMn₂O₄ promotes rapid lithium ion diffusion, enhancing both charging and discharging rates. These attributes position LiMn₂O₄ as a promising candidate for widespread use in electric vehicle batteries. However, they are not recommended for use under high temperature because their efficiency drops at around 60 °C, as their manganese dissolves into the acidic liquid electrolyte solution. This efficiency loss needs to be addressed for large scale utilization of LMOs in high-temperature applications.^65,66

Traditionally, LMOs are synthesized through solid-state reactions involving oxides or carbonates having lithium and manganese ions. These reactions are followed by calcination at high temperatures. Though this process is widely used, it suffers from several drawbacks, including inhomogeneity, irregular particle morphology, large particle sizes, broad size distribution, long heating times, and the need for multiple grinding and annealing steps.^67,68 To address these limitations several alternate methods such as Pechini process, sol–gel, emulsion, and citric acid methods, have been developed by producing spinel materials with smaller, more uniform particle sizes. Preparing LMO by spray drying technique offers several benefits including simplicity, low cost, and scalability for large production quantities. Spray drying allows for the fabrication of microscale particles and enables the synthesis of high-purity materials at lower temperatures. It also allows the formation of homogeneous multi-component systems by blending precursor solutions.^69–71

Despite these advances in synthesis, the issue of efficiency loss at elevated temperatures remains a challenge. Researchers have attempted to address this challenge by coating the surface of LMO with inorganic materials such as Al₂O₃, AlPO₄, AlF₃, ZrO₂, and SiO₂ to shield the LMO from direct exposure to the electrolyte, thereby enhancing its stability. Overly thick coatings affect the material's electrochemical performance.^72,73 Therefore, to avoid resistance researchers use electrically active materials as coatings. For instance, electrically conductive carbon coatings enhance LMO's electrochemical performance and room temperature stability. Nevertheless, when LMO coated with carbon is tested at 60 °C, its performance rapidly deteriorates under the same experimental conditions.⁷⁴ Surface coatings with electrochemically active materials like LiCoO₂, LiNi_0.5Mn_1.5O₄, LiNi_0.05Mn_1.95O₄ and Li₄Ti₅O₁₂, have been found to demonstrate some improvements in room-temperature performance. But still these approaches are less successful at higher temperatures. For instance, LiMn₂O₄ coated with Li₄Ti₅O₁₂ show a discharge capacity of approximately 90 mA h g⁻¹, showing less than 80 percent of its original capacity after 40 cycles at 55 °C.⁷⁵

2.1.3 LiFePO₄ (LEF) as cathode material. LFP is both economically viable and environmentally sustainable, boasting a theoretical capacity of 170 mA h g⁻¹ and a stable discharge voltage of 3.4 V in relation to the Li/Li⁺ system. It also demonstrates commendable cycling performance and thermal stability.^76–78 Nevertheless, LFP encounters certain obstacles, such as slow electronic conduction and a reduced diffusion rate of Li⁺ ions, which hinder its overall performance. To address these issues, researchers have explored various modifications. A common approach to improve its conductivity involves coating LFP particles with highly conducting materials such as carbon, metals, or metal oxides. This coating helps enhance electron flow, allowing for better battery performance.^79,80 Reducing the particle size of LiFePO₄ has also been explored to increase surface area, thereby improving lithium-ion diffusion.⁸¹ In one particular research, LiFePO₄/C cathode was developed with minute particle size by mixing lithium acetate, iron oxalate, ammonium phosphate, and glucose followed by thorough grinding with water and a PVA binder for 8 hours. The resulting mixture was dried using a spray drier with hot air and then heated in two steps: 300 °C for 5 h, then 600 °C for 10 h in an inert N₂ atmosphere to form the final product. After cooling, the material was ground again for 8 hours, producing LiFePO₄/C powder. Electrodes were fabricated by blending LiFePO₄ with carbon and a binder, applied onto aluminium, and tested in a battery utilizing the counter electrode of Li metal and adding an electrolyte solution. The battery's electrochemical properties were evaluated through charging and discharging between 2.0 and 4.2 volts, along with electrochemical impedance spectroscopy. The battery's performance was tested at various charge/discharge rates (C-rates), with promising results. At C/5, the battery delivered 134.6 mA h g⁻¹; at 1C, it provided 129.5 mA h g⁻¹; and at 5C, it yielded 120.1 mA h g⁻¹. Further, at 5C, it maintained 95.6% of its original capacity even after 50 cycles, demonstrating a minimum of 0.08% capacity loss per cycle and indicating strong cycling stability.^82–84

A composite cathode of LiFePO₄ and polypyrrole (PPy–LiFePO₄) was explored for examining the influence of conductive coatings on the battery performance of LiFePO₄. Three variations of the PPy–LiFePO₄ composites were created, incorporating 5%, 10%, and 20% PPy content. The pure LiFePO₄ exhibited crystal sizes ranging from 2 to 5 μm, whereas the 10% PPy–LiFePO₄ sample featured LiFePO₄ crystals enveloped in a cotton-like PPy layer, which effectively reduced resistance and enhanced conductivity. In terms of discharge capacity, pure LiFePO₄ demonstrated a limited capacity of 112 mA h g⁻¹ due to its inadequate electrical conductivity. In contrast, while PPy alone showed an even lower capacity of 69 mA h g⁻¹, the 10% PPy–LiFePO₄ sample outperformed both, as the PPy coating significantly improved conductivity, facilitating a more efficient utilization of LiFePO₄. Regarding cycling performance, the 10% PPy–LiFePO₄ electrode outperformed the others after multiple charge and discharge cycles. Pure LiFePO₄ lost capacity quickly due to its poor conductivity, while the PPy coating improved lithium-ion transfer, enhancing both capacity and cycling stability.⁸⁴ The comparison of the LiCoO₂, LiMn₂O₄, and LiFePO₄ is given in Table 2.

Table 2 Comparison of LiCoO₂, LiMn₂O₄ and LiFePO₄ as cathodes

Property	LiCoO2	LiMn2O4	LiFePO4
Theoretical capacity	∼140 mA h g^−1	∼120 mA h g^−1	∼170 mA h g^−1
Energy density	High	Moderate	Moderate
Cycle life	Moderate (∼500–1000 cycles)	Moderate (∼500–1000 cycles)	Long (>2000 cycles with good retention)
Thermal stability	Low – prone to thermal runaway	Better than LiCoO2, but still moderate	Excellent – stable at high temperatures
Environmental impact	High (toxic cobalt, difficult recycling)	Lower than LiCoO2, but Mn dissolution can be an issue	Low (non-toxic, environmentally benign)
Safety	Moderate to low – can overheat	Better safety profile than LiCoO2	Very safe – minimal risk of fire or explosion
Cost	High (due to cobalt content)	Lower cost than LiCoO2	Low (iron and phosphate are abundant)
Electronic conductivity	High	Moderate	Low (requires carbon coating or doping)
Rate capability	Moderate	Good	Moderate (can be improved with nanostructuring)
Low-temperature performance	Poor to moderate	Poor to moderate	Poor
Structural stability	Moderate – suffers from capacity fading	Good – spinel structure provides better stability	Excellent – stable olivine structure
Applications	Mobile phones, laptops, consumer electronics	Power tools, electric vehicles (low-cost variants)	EVs, solar storage, grid storage

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2.2 Nanomaterials as cathodes

Commercially available lithium-ion batteries utilize transition metal oxides as cathode materials, facilitating the intercalation of lithium ions. Significant research efforts have focused on enhancing these lithium-ion cells' electrochemical performance through various cathode materials. In this context, nanomaterials have surfaced as a promising alternative.⁸⁵ Recent progress in nanoscience and nanotechnology has led many researchers to concentrate on nano-architectonic cathode materials, aiming to create lithium-ion cells that demonstrate superior charge and discharge capacity, better rate performance, lower costs, and longevity. Research indicates that nanostructured materials can enhance the thermodynamics and kinetics of electrochemical reactions to a certain degree. Nanomaterials reduce the diffusion distance for electrons or ions and provide an increased surface area for electrode reactions.^86,87 To overcome the deficiencies of conventional cathode materials, nano-sized cathode materials with distinct morphologies have been prepared. They range from 0D nanoparticles to 1D structures including nanorods, nanowires, nanotubes, 2D constituting nano-sheets or nano-plates, and 3D hierarchical nanostructures. These have been prepared using grinding, solution synthesis, and sol–gel methods. The materials demonstrate distinctive lithium-ion storage capacity and superior electrochemical performance; characterized by enhanced rate capability, charge/discharge capacity, cyclic stability, and safety features. Nevertheless, there are also disadvantages associated with them. The oxidation of nanoparticles is enhanced, resulting in greater electrolyte decomposition and the formation of a thicker solid electrolyte interphase (SEI) layer on the nanoparticle surface.^88,89 Nanostructured cathode materials enhance the kinetic properties of lithium-ion cells by facilitating shorter pathways for Li⁺ ion diffusion. Nevertheless, this characteristic is not the sole determinant of the kinetic efficiency of these cells. Consequently, investigating the impact of nanoscale structures on the electrochemical performance of Li⁺ ion cells represents a complex and intriguing field of research.^90–93

Nanosheets are a game-changer in energy storage due to their distinctive characteristics. Olivine-type transition metal phosphates, specifically LiMPO₄ where M represents metals like Fe, Co, Mn, or Ni, have demonstrated considerable potential as effective cathode materials for LIBs. Their advantages include impressive thermal stability, elevated energy density, eco-friendliness, and low raw material costs. However, these materials are not without their drawbacks, particularly in terms of low electronic conductivity and slow kinetics for lithium ion diffusion. Despite these challenges, their overall promise in energy storage applications remains noteworthy. To test the impact of nanosize on these properties, LiMPO₄ nanosheets with thicknesses 2.7–4.6 nm were synthesized using a liquid phase exfoliation approach. Exfoliation was succeeded by solvothermal lithiation conducted in supercritical fluids under conditions of high pressure and elevated temperature. It produced ultrathin monodispersed nanosheets, and their electrochemical performance was tested by measuring the time required for Li⁺ ions diffusion and rate capabilities. The diffusion time of Li⁺ ion over a length of <5 nm was found to be 2.5, 25, and 250 microseconds for LiFePO₄, LiMnPO₄, and LiCoPO₄ nanosheets, respectively, and this was 5 times higher in magnitude than that of their corresponding bulk materials, showcasing the potential of nanotechnology in energy storage. The energy density and rate capabilities for LiFePO₄ nano sheets were 18 kW kg⁻¹ and 90 W h kg⁻¹, which were also higher than the corresponding bulk materials, further highlighting the impressive performance of nanosheets.⁹⁴

The 3D crystal structures of spinel LiMn₂O₄ are promising contenders as cathode materials for LIBs as they are economical, environmentally friendly, and exhibit excellent voltage profiles.^95,96 At the same time, they are hindered by a lower power density, which arises from significant polarization effects at elevated charge–discharge rates.^97,98 To address this issue, researchers are developing lithium intercalated compounds in nanoscale dimensions and various morphologies, such as nanoparticles,⁹⁹ nanowires,¹⁰⁰ nanotubes,¹⁰¹ hollow spheres,¹⁰² mesoporous materials, and so on.¹⁰³ In one such attempt, highly crystalline spinel LiMn₂O₄ with representative morphologies of 1D nanorods, 3D nano thorn microspheres, and 3D hollow nanospheres were prepared using topochemical methods, and their electrochemical properties were investigated and compared. It was found that LiMn₂O₄ nanorods and nano-thorns showed two distinct discharge plateaus (4.1 V and 3.95 V), while hollow nano spheres showed less distinct plateaus and lower capacity. The diffusion rate of Li⁺ in hollow nanospheres was higher than in nanorods and nano-thorn microspheres because of the high surface area. Their cycling performance was also better than that of other LiMn₂O₄ structures. Hollow nanospheres retained 99% of their capacity after 100 cycles, while nanorods and nano-thorn microspheres retained 90% and 98%. Regarding rate performance, hollow nanospheres retained 89% of their capacity at a higher C-rate (5C), while it was 85% for nanorods and 83% for nano-thorn microspheres. This better electrochemical performance of these nanostructures compared to other structures of LiMn₂O₄ was perhaps due to high crystallinity and larger surface area. It enhanced Li⁺ ion diffusion because of the smaller particle size.¹⁰⁴

2.3 Organosulfides as cathode materials

The functional groups of organic compounds can be modified to adjust their properties and electrochemical behaviour, making them highly customizable. Among organic compounds, organosulfides have emerged as promising alternatives to traditional cathode materials. Organodisulfides contain a single sulfur–sulfur (S–S) bond, which can break and reform as the battery charges and discharges, making them rechargeable. However, compared to traditional TMO cathode materials, organodisulfides have struggled to compete due to their limited capacities and energy densities. Despite this slow progress, recent advancements have revealed the potential of organosulfides as cathode materials. In particular, organosulfides with longer sulfur chains are more appealing owing to their high capacity and the beneficial characteristics of organic electrode materials. These contain both polymers (–S–R–S_n–, where n ≥ 1) and linear molecules (R–S_n–R, where n ≥ 3). These materials help improve lithium–sulfur battery performance by reducing electrolytes, enhancing discharge voltage, and improving discharge product adsorption through functional groups. These features make them promising candidates for addressing some of the inherent problems associated with sulfur cathodes in Li–S batteries.^105,106

Numerous organodisulfides were developed for use in batteries with TETD initially proposed by Visco and colleagues for applications in rechargeable batteries.¹⁰⁷ However, TETD exhibited low electrochemical efficiency and slow reaction kinetics, resulting in significant polarization during the charging and discharging processes. Consequently, the achieved potential was only 36% of its theoretical potential. Since then, researchers have studied other organodisulfides due to their promising theoretical capacities. For example, 2,5-dimercapto-1,3,4-thiadiazole (DMcT) is a well-known organosulfide with a theoretical capacity of 362 mA h g⁻¹. However, DMcT exhibits subpar performance at ambient temperature owing to its lower charge conductivity, which restricts the effective use of the active material in the battery.¹⁰⁸ In another study, researchers prepared a new organodisulfide called TPQD (2,3,4,6,8,9,10,12-octathia biscyclopenta[b,c]-5,11-anthraquinone-1,7-dithione) to utilize as a high-performing cathode material for LIBs. Due to its strong backbone structure, TPQD did not dissolve in ether-based electrolytes like DME/DOL and stayed stable throughout the battery's charge and discharge cycles. The compound had a theoretical capacity of about 324 mA h g⁻¹, based on the expectation of a six-electron transfer process from the disulfide and quinone (BQ) groups. The inclusion of BQ group into TPQD resulted in enhancing energy density and accelerating the reaction kinetics; attributed to its π-conjugation structure.

When TPQD was evaluated as a cathode in lithium-ion batteries, it underwent charging and discharging within a voltage range of 1.4 to 3.5 V. In the initial cycle, the discharge capacity achieved 251.7 mA h g⁻¹, whereas the charge capacity was recorded at 227.6 mA h g⁻¹, resulting in an efficiency of 91.7%. The researchers ascribed the initial loss in capacity to the formation of a SEI or the depletion of Li products. Over the first ten cycles, the capacity slightly increased as the material activated. After 50 cycles, the TPQD electrode stabilized at around 200 mA h g⁻¹. The TPQD electrode demonstrated strong cycle stability and retained 130 mA h g⁻¹ following 200 cycles at a rate of 5C with a minimal capacity loss per cycle. The resistance to charge transfer also decreased after the first few cycles, improving the performance. Cyclic voltammetry tests showed that the carbonyl bonds performed well at higher current rates, while the disulfide bonds reacted more slowly, limiting the overall capacity at those rates. Overall, the TPQD-Li system demonstrated improved rate capability, good specific capacity, steady cyclability, and high coulombic efficiency compared to previously known disulfide-based electrodes. The results offered a novel strategy for enhancing the slow kinetics of organodisulfide materials and guided future designs for development of high-performance organic electrodes in LIBs.¹⁰⁹

2.4 TMSs as cathodes for LIBs

The 2e⁻ electrochemical reaction (CuS + 2Li⁺ + 2e⁻ → Li₂S + Cu) allow CuS to attain a theoretical capacity of 560 mA h g⁻¹. Two key strategies, namely complexing and nano-structuring, are commonly utilized to improve electrochemical performance while tackling issues such as volume expansion and the electronic insulating characteristics of the discharge product (Li₂S). To combat challenges such as expansion of volume, synthesis of nanostructures has frequently been employed to facilitate the conversion of reaction-based materials. Cheng et al., employed a simple in situ melt diffusion method to synthesize a mesoporous network of CuS sheets and particles on top of 3D monolithic Cu foam without using a binder for LIB. For the first and 100th cycles at 0.2C (1C = 560 mA h g⁻¹), the synthesized cathode has an exceptional capacity of 185.1 and 468.3 mA h g⁻¹. This CuS cathode outperformed many others already available in the market thanks to its outstanding rate capability, which allowed the capacity recovery to reach 83.9% after cycling at various current rates.¹¹⁰ Another possibility is 1D nano-structures, which can provide the desired axial electronic conduction and a shorter ion diffusion channel. Furthermore, it facilitates improved accommodation of volumetric changes during reversible charging and discharging. A solvothermal approach that does not require a template was used by Feng et al. to synthesize CuS nanowire bundles.¹¹¹ At room temperature and a rate of 0.2C, the initial releasing capacity was 831 mA h g⁻¹, and the corresponding coulombic efficiency was 62.3%. At 0.2C and 0.4C, the reversible capacities following 100 cycles were approximately 570 and 200 mA h g⁻¹.

Wang and co-workers synthesized nanosized cobalt sulfide powders at ambient temperature. The exploration of cobalt sulfide powder as a prospective cathode material for LIBs was conducted via electrochemical testing. The findings from CV measurements indicated that the Li/Co₉S₈ cell undergoes a discharge process characterized by two distinct steps.¹¹² Hollow spheres of mesocrystal Co₉S₈ exhibited a capacity of 254.9 mA h g⁻¹ at a rate of 100 mA g⁻¹ following 100 cycles and were prepared by Jin et al. via solvothermal method.¹¹³ NiS was subjected to charging/discharging at 80 °C, utilizing a PEO solid polymer electrolyte. The ex situ XRD measurements indicated that NiS transformed various phases during the discharging process, including the formation of nickel, Ni₃S₂, and sulphur. The start discharge capacity was reported as 580 mA h g⁻¹ for the NiS cathode material at a 1.5 V voltage of vs. Li/Li⁺. The cycling performance was impressive, maintaining 93% of starting discharge capacity following 200 cycles.¹¹⁴

Porous FeS₂ nanoparticles wrapped by rGO were synthesized using a solvothermal method and employed as a cathode for LIBs. It demonstrated good cyclic stability and rate capability. Furthermore, FeS₂ composite was found to maintain a discharge capacity of more than 170 mA h g⁻¹ under high current density conditions, following 2000 cycles.¹¹⁵ Zhang and co-workers synthesized a 1D porous FeS₂@C nanowires which proved to be a highly effective cathode material. These nanowires not only promoted efficient electron transport but also improved diffusion kinetics and acted as a reliable buffer against volume expansion. The unique combination of porous nanostructures and the surrounding amorphous carbon resulted in FeS₂@C nanowires exhibiting remarkable lithium storage capabilities, achieving a capacitance of 889 mA h g⁻¹ at a current rate of 0.1 A g⁻¹ and 521 mA h g⁻¹ at 10 A g⁻¹. The discharge energy density is 1225 W h kg⁻¹ at 2 A g⁻¹ and remains at 637 W h kg⁻¹ even after 1000 cycles, surpassing the performance LCO cathode.¹¹⁶ Pan et al. presented a hydrolysis-sulfurization approach for fabricating FeS₂/C nanotube arrays utilizing glucose carbonization and a nanowire template based on sacrificial Co₂(OH)₂CO₃. Self-supported FeS₂/C nanotubes exhibited improved cycling performance and significant high-rate capability, delivering capacities of 735 mA h g⁻¹ at 0.25C and 482 mA h g⁻¹ at 1.5C shown in Fig. 3, surpassing those of pristine FeS₂ reported in previous studies. The architecture of the composite nanotube arrays contributed positively to electrochemical enhancement by leveraging the benefits of a sizeable electrode–electrolyte surface area, improved electrical conductivity, adequate strain accommodation, and increased structural stability.¹¹⁷

Fig. 3 Electrochemical comparison of FeS₂@C and FeS₂ cells: cyclic voltammogram of the (a) FeS₂@C and (b) FeS₂ cells at a scan rate of 0.1 mV s⁻¹ within a voltage range of 1.0 and 3.0 V vs. Li/Li⁺; (c) cycling stability and the respective coulombic efficiencies of FeS₂@C and FeS₂ cells at a current density of 0.1 A g⁻¹ overs 100 cycles; (d) rate capabilities of FeS₂@C and FeS₂ cells at varying current densities from 0.2 A g⁻¹ to 10 A g⁻¹; (e) discharge energy density of the FeS₂@C cell at a current density of 2 A g⁻¹ vs. cycle number shown alongside the theoretical discharge energy density of LiCoO₂ cathode (550 W h kg⁻¹ based on the mass of LCO). This figure has been adapted from ref. 116 with permission from Elsevier, copyright 2016.

Nanostructured composite cathode materials that facilitate solid-state lithium-ion batteries were found to have sustained high energy and high-rate performance throughout cycling. The particle size of titanium sulfide (TiS₂) was minimized through planetary ball-milling in order to improve the efficacy of nanostructured composite cathodes. The enhanced use of active materials and rapid kinetics achieved through the reduction of the size of TiS₂ leading to a high reversible battery capacity at both conditions i.e., room and raised temperatures. Using nano-sized TiS₂ in high-power solid-state batteries resulted in a power density above 1000 W kg⁻¹ for more than 50 cycles, reaching a peak power density surpassing 1400 W kg⁻¹.¹¹⁸ Table 3 enlists some transition metal sulphides used as cathode materials in LIBs.

Table 3 Transition metal sulfides as cathode materials for LIBs

Cathode material	Method of preparation	Structure	Reversible capacity (mA h g^−1)	Cycle number	Coulombic efficiency (%)	Ref.
Defect-rich FeS2 nanoflowers	Self-assembly method	Nanoflowers	841	3000	>95	119
FeS/N–C	Simple pyrolysis method	Nanoflowers	729	500	>95	120
S/FeS2–C cathode	Template synthesis followed by carbon coating, calcination and sulfidation	Yolk–shell morphology	761.2	350	>90	121
Lamellar iron sulfide with interlayer solvated cations	Solvothermal decomposition	Lamellar structure	450	50	>90	122
Hierarchical FeS2/C nanospheres	Controllable solvothermal method	Nanospheres	676	100	>85	123
LiNi0.8Co0.1Mn0.1O2 layered oxide cathode with LiS and NaS	Wet chemical treatment	Layered structure having surface passivation	181	100	>70	124
NiS2/Ketjen black@S composites	Solvothermal method followed by wet chemical precipitation	Cubic pyrite NiS2 nanospheres uniformly dispersed in composite matrix	461	500	99.1	125
Li2S	Electrochemical activation process	Stable crystal lattice structure	1052	400	50–90	126
S/sulfide composite cathode	Slurry coating process	Solid film	1169	30	Good coulombic efficiency	127
Fe0.34Co0.33Ni0.33S2	Hydrothermal method	Hollow nanotubes assembled by nanoparticles	1350	300	>95	128
Li2S	Coating lithium sulfide with polymerizable electrolyte additive	Particles coated with layer of electrolyte additive	1052	100	99.5	129
FeS2	Use of chelating type binders in a carbonate-based electrolyte	Particles stabilized by chelating type binders	527.3	300	>95	130
Lithium rich layered titanium sulfides	Chemical lithiation of TiS2	Layered structure	265	60	Good coulombic efficiency	131
Vacancy rich titanium sulfide trapped in a hollow carbon nano cage	Template-free arc-discharge method	Nanoparticles with abundant sulfur vacancies	1118.8	200	96.71	132
Lithium rich Li2TiS3	High energy ball-milling method	Nanoscale cubic rock-salt	423.2	100	112.2 during first cycle	133
Titanium disulfide with sulphur cluster vacancies			650	300	Good coulombic efficiency	134
VS2	Hydrothermal synthesis	Hierarchical spheres composed of nanosheets with porous structure	215	100	>98	135
VS2–VO2@S	Hydrothermal and high temperature calcination method	Heterostructures	588.9	500	∼60	136
Amorphous titanium polysulfide composites	High energy ball milling followed by liquid phase synthesis	Amorphous structure	507.4	100	40–50	137
Li2S@graphene	Burning Li foils in CS2 vapours	Nanoparticles encapsulated by a few layers of graphene	1160	100	Good coulombic efficiency	138

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2.5 Nickel-rich cathode materials

The performance of lithium-ion batteries is heavily influenced by the choice of cathode materials, which significantly impact energy density, cycle life, and overall cost. Among various cathode chemistries, high nickel-rich layered oxides, such as LiNi_1−xM_xO₂ (where M represents Co, Mn, and Al), have garnered considerable attention due to their impressive specific capacity exceeding 200 mA h g⁻¹, lower cost compared to cobalt-rich alternatives, and their ability to fulfil the energy requirements of grid storage and electric vehicles (EVs). These high nickel-rich cathodes typically derive from layered transition metal oxides based on the α-NaFeO₂ structure, which crystallizes in the R3̄m space group. The redox processes involving Ni²⁺/Ni³⁺/Ni⁴⁺ are crucial for capacity generation, although cation mixing often occurs due to the similar ionic radii of Li⁺ and Ni²⁺, which can obstruct lithium ion diffusion and reduce capacity rates. In this context, manganese and aluminium serve as stabilizers—Mn⁴⁺ enhances structural integrity while Al³⁺ contributes to thermal stability—whereas cobalt plays a vital role in improving conductivity and maintaining structural robustness.^139,140

The synthesis methods for nickel-rich cathodes significantly influence their properties. Various techniques have been developed to control particle morphology, size distribution, and chemical uniformity. Among these, the solid-state method stands out as a traditional approach where metal oxide or carbonate precursors are mechanically blended and subsequently calcined at elevated temperatures. While this method is cost-effective and scalable, it often leads to inferior chemical homogeneity.¹⁴¹ Co-precipitation, commonly used in industrial applications, involves the lithiation and calcination of transition metal hydroxides or carbonates precipitated under controlled conditions, ensuring high purity and uniform particle size.¹⁴² The sol–gel technique combines metal salts with organic chelators, such as citric acid, to form a gel that is later sintered, resulting in small, uniform particles and precise stoichiometric control.¹⁴³ Additionally, hydrothermal and single-crystal synthesis methods utilize high-pressure crystallization to yield single-crystal particles with minimal grain boundaries.¹⁴⁴

Recent studies have focused on two important members of this family, LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) and LiNi_0.85Co_0.1Al_0.05O₂ (NCA8515), to overcome issues with thermal instability, transition metal dissolution, and structural degradation. A thorough investigation was conducted to shed light on the structural and electrochemical distinctions between NCM811 and NCA8515. NCA8515 demonstrated superior cycling stability over NCM811 despite having a more significant nickel concentration. NCM only kept 44% of its initial capacity after 500 cycles at 55 °C, whereas NCA kept 57%. NCA demonstrated better structural retention with reduced voltage decay (0.2427 V) than NCM (0.2794 V). According to first-principles calculations, the Al–O bonds in NCA8515 (2.8354 eV) are stronger than the Mn–O bonds in NCM811 (2.7976 eV). Weaker bonds in NCM encourage the movement of transition metals, the creation of oxygen vacancies, and structural collapse. These results underscore the importance of bonding stability and the selection of dopants in determining the longevity and safety of cathodes.¹⁴⁵

High nickel-rich cathodes show potential; but their commercial viability is limited by a number of significant issues. Anisotropic volume variations during cycling, particularly in the H2 → H3 phase transition, create mechanical stress that leads to particle cracking. Electrolyte infiltration through these cracks speed up adverse side reactions and degradation. To lessen this problem, single-crystal particles or coatings like LiO₃ have been suggested.¹⁴⁶ At elevated voltages exceeding 4.3 V, particularly under high-temperature conditions, nickel and manganese are likely to leach into the electrolyte. This accelerates capacity fading by contaminating the solid electrolyte interphase (SEI) and depleting active material. Cation mixing is another issue that reduces ionic conductivity and overall battery performance by blocking lithium diffusion channels caused by Ni²⁺ migration into Li²⁺ sites. It has been demonstrated that Rb⁺ or Na⁺ doping increases the Li-layer gap, resolving this problem.¹⁴⁷ Exothermic reactions brought on by oxygen leakage at high temperatures (>200 °C) as a result of broken metal–oxygen bonds can result in thermal runaway. The thermal decomposition threshold is raised when magnesium and aluminium are co-doped.

Researchers have employed various material design strategies to overcome the aforementioned issues. One such approach is ion-doping, specifically cationic doping with Al³⁺, which enhances structural stability and strengthens oxygen bonds. Another promising dopant, V⁵⁺, contributes to the stabilization of the layered structure. Additionally, anion doping with F⁻ reinforces metal–oxygen interactions, substitutes for oxygen, and consequently mitigates thermal instability and oxygen loss.¹⁴⁸ Coating the surface with inorganic materials or polymers can serve as an effective strategy. Substances such as Li₂ZrO₃ and La₄NiLiO₈ create protective coatings that inhibit nickel dissolution and lower interfacial resistance.^149,150 Polyacrylic acid (PAA) and its associated polymers serve as effective barriers against moisture and carbon dioxide.^151,152 Excellent performance has been demonstrated by core–shell designs with a high-Ni core that provides substantial capacity, and a manganese-rich shell, which enhances stability. A full-gradient NCM has been shown to retain 90% capacity after 1000 charge–discharge cycles.

Future efforts should focus on the development of advanced multi-element coatings, in situ characterization techniques, and the integration of solid-state batteries to bridge the divide between laboratory achievements and commercial implementation. The use of multiple dopants, such as Al–Mg–F, may yield synergistic advantages in both performance and stability.¹⁴⁸ Advanced materials that offer both conductivity and protective features, including PEDOT and self-healing polymers, can deliver significant benefits. Real-time monitoring techniques can uncover dynamic structural changes during operation. Additionally, by pairing solid electrolytes like sulfides or oxides with high-nickel cathodes, it is possible to mitigate the degradation associated with liquid electrolytes, thereby enabling safer high-voltage operations.^153–156

3. Commonly used anode materials in LIBs

A dense surface film forms on the electrode when Li comes into contact with polar aprotic solvents commonly found in electrolytes. This film impedes proper passivation, crucial for protecting the electrode's surface.^157–159 Lithium dendrites begin forming as the battery undergoes multiple charge and discharge cycles, posing a significant issue. These dendrites can penetrate the separator between the anode and cathode, creating severe safety risks and causing a substantial loss of lithium over time. Graphite remains the most commonly used anode material in LIBs. However, its relatively low capacitance restricts its energy storage potential, which lags meeting the growing demand for batteries with high performance. As a result, researchers are exploring alternative anode materials to improve battery performance.

For a lithium battery to maintain thermodynamic stability, the energy difference between the LUMO and the HOMO of the electrolyte must be greater than the voltage difference between the anode and cathode. Specifically, the potential energy of the anode must be lower than the LUMO of the electrolyte, and the cathode's potential energy must be higher than that of the HOMO. This ensures the electrolyte's stability during operation, helping to prolong the battery's lifespan and reduce degradation over time.¹⁶⁰ In simpler terms, selecting suitable electrode materials is essential to ensure they do not destabilize the electrolyte, which would otherwise reduce the battery's performance and lifespan.

3.1 Intercalation-type anode materials

Oxides of TMs and other materials with 2D or 3D layered structures have been studied for their capability to intercalate and deintercalate Li⁺ ions into their crystal structure, just like graphite as shown in Fig. 4. These materials are interesting because they can absorb and release lithium ions without damaging or destroying their crystal structure. This property is crucial because it makes them suitable for applications like LIBs, where maintaining the material's integrity throughout the charging and discharging cycles is essential.

Fig. 4 Intercalation type anode materials with the schematic illustration of the intercalation mechanism for LIBs.

3.1.1 Graphite intercalation compounds (GICs). The formation of GICs entails the introduction of atomic or molecular layers of different chemical species between the graphite's carbon layers. The covalent bonds between the carbon atoms within the layers, along with van der Waals forces maintain the structural integrity of the layers.¹⁶¹ Numerous GICs have been investigated since their initial synthesis in 1841, with a diverse array of intercalants, including acidic oxides, metal chlorides, fluorides, alkali metals, bromides, metal oxides, oxyhalides, and strong acids, among many others. These compounds have been extensively explored for their unique structural, electronic, and optical properties, with applications in conductivity, superconductivity, catalysis, hydrogen storage, displays, and polarizers.¹⁶²

GICs with lithium-ion storage intercalants, such as oxides of metals and chlorides of metal, show potential as leading anode materials for LIBs, offering significantly better cycling stability than other alternatives. FeCl₃–GIC was prepared using the melt-salt method with flake natural graphite (FNG) as the host material, and it was chosen as a model.¹⁴¹ FeCl₃ molecules were intercalated uniformly into the graphite layers to form FeCl₃–GIC, which was tested in a half-cell comprising the Li/FeCl₃–GIC. In the first cycle, the material delivered 665 mA h g⁻¹ discharge and 506 mA h g⁻¹ charge, reaching 76% coulombic efficiency. A reversible capacity amounting to 506 mA h g⁻¹ was consistently maintained in the following cycles, far surpassing the graphite's theoretical capacity. The exceptional efficiency of FeCl₃–GIC arises due to its unique structure. The intercalated FeCl₃ sandwiched between graphene layers offered a barrier for volume changes during the lithiation process. Additionally, the graphene layers boost electrical conductivity, while the expanded interlayer spacing promotes rapid lithium-ion diffusion. The redox reaction of FeCl₃ occurred in a favourable potential range (0.8–1.2 V vs. Li/Li⁺), offering stability during high-rate discharge processes. Furthermore, FeCl₃–GIC demonstrated robust cycling performance, sustaining a reversible 480 mA h g⁻¹ capacity with complete retention over 400 cycles at a current density of 100 mA g⁻¹.¹⁶³

3.2 TMOs as anode materials

TMOs are promising anode materials for LIBs. Compared to commercial graphite with a limited theoretical capacity, TMOs offer comparatively higher capacities because of the ability of TM ions to undergo multiple valence state changes.^164,165 Besides their high capacity, TMOs present advantages such as cost-effectiveness, safe operating potentials, and the ease of synthesizing nanosized materials with varied structures. These benefits have spurred substantial research into developing advanced materials used as anodes for LIBs.^166,167 However, a significant challenge with TMOs is their poor cycle stability. During the lithiation (insertion of Li ions) and delithiation (removal of Li ions) processes, the material experiences repeated and considerable volume changes that disrupt the contact between the counter electrode and active material. This decreases battery performance and efficiency over time.¹⁶⁸ Table 4 represents several TMOs utilized as anode materials for LIBs.

Table 4 TMOs as anode materials for LIBs

Anode material	Method of preparation	Structure	Reversible capacity (mA h g^−1)	Cycle number	Coulombic efficiency (%)	Ref.
Co3O4 nanoparticles	Thermal decomposition of MOF precursor	Cubic spinel structure	913	60	68	169
Janus-structured nanocomposites with Fe3O4 nanoparticles and polydopamine on each side of graphene oxide sheets	Langmuir Schaefer technique	Super lattice like out of plane section of the multi-layered nanocomposites	639	1800	>95	170
(CoNiZnFeMnLi)3O4	Solid phase method	Spinel structure	605	100	65.22	171
Fe3O4@C/rGO composites	In situ plantation	Lamellar structure	849	120	79.19	172
Phosphorous doped graphene framework with porous Fe2O3 nano framework	Thermal treatment	Hierarchical structure	1051	100	74.6	173
MnO2@HCN (hollow carbon nanospheres)	Chemical reduction method	Core–shell porous structure	604	200	Good coulombic efficiency	174
MnO@N-doped hollow carbon nanotubes	Self-template method	Pea-pod like configuration	966.7	300	62.7	175
Fe3O4 with partially reduced graphene oxide	Facile redox deposition method	Hybrid structure	2136	100	Good coulombic efficiency	176
Mesoporous α-Fe2O3 hierarchical tubes	Salt impregnation followed by air calcination	Mesoporous structure	880.7	800	67.9	177
(CrMnFeNiCu)3O4	Hydrothermal method	Spinel structure	913	250	99	178
SnFe2O4/rGO composites	One pot solvothermal method	Inverse spinel structure with face-centred cubic lattice	1010	300	80	179
SnO2/Co3O4/rGO composites	Chemical precipitation followed by calcination and hydrothermal treatment	Nanocubes anchored onto rGO sheets	727	100	63.4	180
SnO2/CSC/rGO composites	Co-precipitation method	Nanoparticles anchored on graphene sheets	410.5	100	70.87	181
SnOx/TiO2@N-doped carbon	Mild reduction reaction	Hierarchical microspheres	826.3	300	72.3	182
SnO2/TiO2 composites	Pulsed-laser deposition	Films with nanostructures	809.3	100	Good coulombic efficiency	183
SnO2/ZnO@PPy hollow nanotubes	Electrospinning method	Hollow nanotubes	626.1	100	61.12	184
SnO2 nanoparticles decorated three-dimensional carbon network	Internally NaCl template directing and freeze drying followed by pyrolysis	3D hierarchical honeycomb like carbon network	798	1000	62.5	185
SnO2 quantum dots in a C and GO network	Solid-state ball-milling followed by water-phase self-assembly	Hierarchical network of C and GO having QD in it	1156	350	58.8	186
SnO2 embedded rGO composites	CTAB assisted hydrothermal method followed by reduction	Particles embedded in sheet configuration	598	200	64.8	187
Sn/SnO2/N-carbon doped superstructures	Facile electrospray carbonization strategy	Nanoparticles embedded in a matrix configuration	747.9	200	73.6	188
Carbon coated TiNb2O7	Soft-template synthesis followed by hydrothermal carbon coating process	Monoclinic crystallographic structure	2136	100	98	189
Co3O4@nitrogen doped graphitic carbon cabalistic oxide/MXene	Reduction under inert conditions followed by subsequent oxidation	2D nanosheets	830	500	67.6	190
(Fe2.5Ti0.5)1.04O4/C/MXene	Solvothermal self-assembly followed by ultrasound hybridizing followed by high-temperature annealing	2D layered architecture	757.2	800	76.45	191
Mentha aquatic-derived graphitic C TiO2	Biogenic single precursor approach	Hybrid structure	597	1000	>99	192
Mg^2+ and Bi^3+ doped lithium titanium oxide	Ball milling assisted solid-state method	Spinel cubic structure	200	50	67.9	193
Ni–NiO–MoO2/rGO composites	In situ construction of nanostructures	Heterostructures having nanosheets with nano holes	910	220	98	194
Ni and NiO nanocomposites	Direct pyrolysis of a Ni oxalate precursor	Porous prismatic morphology	633.7	100	78.5	195
NiO–Ni foam	Freeze casting followed by thermal oxidation	Porous Ni foam with uniform NiO layer on the surface	550	50	70.9	196
PEDOT/MnO2 coated on porous carbon nanofibers	Electrospinning followed by carbonization and electrodeposition	Electrode having crosslinked and rough surface	1191	20	>90	197
(FeNiCrMnZn)3O4	Ball milling followed by calcination	Hexagonal space group R3̄m	386.7	185	67.9	198

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Layered vanadium oxides, including LiV₃O₈, V₂O₅, VO₂, and V₆O₁₃, are highly valued as intercalation-type electrode materials because they can undergo multiple valence changes from V⁵⁺ to V²⁺. These materials feature unique layered or tunnel-like structures that accommodate numerous Li⁺ ions with minimal lattice strain, making them particularly attractive for battery applications.¹⁹⁹ Despite their advantages, vanadium oxides have high redox potentials, exceeding 2.5 V, making them unsuitable anodes. Recently, co-precipitation was used to synthesise 40–50 nm Cu₃V₂O₈ NPs. Despite of having discharge capacity of 462 mA h g⁻¹ for the initial 10 cycles, these NPs reached 773 mA h g⁻¹ after 50 cycles. The reversible production and breakdown of SEI film may improve Li storage at the contact. With 218 mA h g⁻¹ discharge capacity at 1000 mA g⁻¹ current density, the Cu₃V₂O₈ NPs anode performed well. Li storage was examined using ex situ XRD, which demonstrated that Cu₃V₂O₈ disintegrates into Cu metal and Li₃VO₄ at 0.01 V. Li₃VO₄ hosts lithium ions in following cycles via intercalation. This revolutionary “in situ” electrochemical compositing provides crucial insights for creating LIB anode materials.²⁰⁰

3.3 Lepidocrocite-type layered titanate structures

Layered titanates have attracted considerable attention in technology due to their advantageous physical characteristics, ability for ion exchange, and potential for electrochemical intercalation. Recent studies indicate that various layered titanates can facilitate reversible reduction intercalation with lithium and sodium ions. Some of these materials, with theoretical capacities exceeding 200 mA h g⁻¹, are capable of inserting alkali metal cations at shallow potentials, in some cases below 0.5 V. A high-capacity, reversible titanate anode could advance sodium-ion battery (SIB) technology, as graphite anodes that are commonly used in LIBs are not well-suited for intercalation of sodium due to larger size than Li ions. Although hard carbon materials have demonstrated high reversible sodium capacities, legitimate safety concerns are associated with their use. In contrast, titanates offer additional advantages, including higher densities (∼3.5 g cm⁻³) than graphite (2.25 g cm⁻³) and other carbon-based electrode materials. This enhances energy density in both lithium-ion and sodium-ion systems, providing a safer alternative. Moreover, titanium's affordability, non-toxicity, and wide availability make these materials especially appealing for industrial uses, especially in industrial scale batteries for EVs and grid applications. The crystal structures of trititanate and lepidocrocite titanate are shown in Fig. 5.

Fig. 5 Structure models of (A) trititanate and (B) lepidocrocite titanate. The unit cells are indicated in lines. This figure has been reproduced from ref. 201.

The investigation of the electrochemical properties of lepidocrocite-type titanates, specifically those derived from K_0.8Ti_1.73Li_0.27O₄ demonstrates improved performance upon substituting potassium ions with sodium. This ion exchange results in a higher capacitance in both sodium and lithium half-cells, while preserving the overall structure of the titanate. However, it is noteworthy that the symmetry of the lattice is modified, as sodium occupies different positions within the crystal framework. Sodium's smaller ionic size allows more alkali metal cations to be inserted during discharge, enhancing reversible capacity in both types of cells. Lithium insertion occurs in lithium cells at 0.8 V versus. Li⁺/Li, while Na intercalation occurs at 0.5 V versus. Na⁺/Na in sodium cells. The voltage profiles for both cells are sloping, indicating single-phase processes. Synchrotron XRD confirms the formation of a lithiated lepidocrocite structure during lithium discharge, with minimal volume change. The study demonstrates that lepidocrocite titanates can be tailored for application as anode materials in both LIBs and SIBs through compositional adjustments and ion exchange processes.²⁰²

3.4 Alloy-type anode materials

Certain elements such as Si and Sb, Sn, Zn, In, Bi, and Cd,^203,204 are known to form Li alloys. These alloys are considered promising anode materials as shown in Fig. 6.

Fig. 6 Pictorial illustration of metals that can form an alloy with lithium.

Lithium alloying and dealloying reactions typically occur at lower potentials (≤1.0 V vs. Li), which ensures reversible capacity during lithium cycling. Silver–tin composite alloys are emerging anode materials. With the appropriate composition, the Ag–Sn composite system showed a high capacity and long cycle life, even though the system did not form the typical active/inactive composite structure since all components were active towards lithium. Efforts were made to elucidate the reaction mechanism of the composite alloys containing Li and to investigate the improved cycling performance by analysing the structural alterations during Li insertion and desertion. With optimized compositions and morphological structure, the Ag₅₂Sn₄₈ and Ag₄₆Sn₅₄ composite electrodes delivered approximately 800 mA h g⁻¹, maintaining a reversible over 350 mA h g⁻¹ capacity for over 50 cycles. The Ag₅₂Sn₄₈ electrode preserved a reversible capacity of around 200 mA h g⁻¹ even after 300 cycles. Structural changes in the Ag₅₂Sn₄₈ electrode during Li charge/discharge were analyzed using XRD. The results revealed that the composite alloy undergoes changes to form a ternary lithium inserted phase upon lithium insertion and restores to a structure containing β-Sn, Ag₃Sn, and residual Ag₂LiSn phases after Li extraction. The improved cycle life is attributed to its composite structure and the formation of ternary lithium inserted phase formed during the initial cycle. However, the residual Ag₂LiSn has been reported to likely contribute towards increasing the irreversible capacity loss.²⁰⁵

Silicon is also used in LIBs because of its higher lithium storage capacity of 4200 mA h g⁻¹ and lower potential for lithium insertion.²⁰⁶ The challenge faced with Si-based anodes is the substantial volume expansion—up to 400%—during lithium-ion insertion. This expansion can lead to cracking and fragmentation of the Si structure, resulting in capacity loss. To seek the solution to this issue, researchers have turned to various nano-shaped silicon materials like Si nanowires, nanoparticles, and nanotubes, which can better handle the volume fluctuations during charging and discharging cycles.²⁰⁷ Among these, nanowire core–shell structures have gained particular interest. They can be engineered to ensure an efficient electrical pathway. At the same time, the outer shell, typically a thin Si layer, provides a large surface area to facilitate faster lithium-ion insertion. Moreover, using carbon nanofibers (CNF) as the core in a heterogeneous core–shell structure instead of C–Si further improves the battery's performance and stability.^208,209

A recent development involves an alloy-forming method, where researchers transformed the alloy formed from hollow Si–Cu nanotubes to amorphous core–shell nanowires formed from functionalized Si core with CuO shell. Applying a simple hydrogen annealing process reduced the CuO cores and diffused into the a-Si shell, forming the alloy nanotubes. These hollow nanotubes offer high capacity, self-conductive properties, and excellent mechanical support for the anode. This innovative structure achieved a high capacitance after 1000 cycles at 3.4 A g⁻¹ (or 20 A g⁻¹), retaining a capacity of approx. 84% (or 88%) without requiring a conductive agent. Notably, the structure endured high-speed charging rates of 70 A g⁻¹ for 35 cycles (equivalent to a full charge in 30 seconds) while retaining 88% of its capacity. This Si–Cu alloy nanotube design represents a breakthrough in unlocking the full potential of silicon-based LIBs.²¹⁰

Since the initial examination in the 1970s and 1980s, Li storage metals went on to see a significant surge in interest, especially from 1995 onwards. This is primarily due to the discovery that the cycling stability of these metals significantly improves when they are used in alloys or nano-dispersed composites. Most research in this area has focused on metals like tin, aluminium, and silicon, as well as their various compounds. Some other metals such as antimony and bismuth have also been promising, as shown by early studies in 1975, followed by thermodynamic and kinetic research and more recent cycling studies.^211,212 Pure antimony and bismuth tend to have poor stability, often showing capacity losses after just a few cycles. This has been demonstrated in several studies. One notable improvement is using skinny Bi films, which offer better stability but are only a few micrometres thick. However, these films' practical use in whole cells is limited because their thinness restricts the total capacity that can be achieved.²¹³ The performance of these metals can be significantly enhanced by forming alloys. Among antimony-containing alloys, the one that has shown the highest stability and cycling capacity so far is tin–antimony (SnSb). This alloy has also been studied in various forms, such as a multi-phase alloy (Sn/SnSb), as a single-phase intermetallic compound, and in more recent times, as a composite combined with carbon nanotubes or mesocarbon microbeads. Other notable tin alloys include indium–antimony, cobalt–antimony, and CoFe₃Sb₁₂, though these represent only a small selection of the materials being explored for better cycling performance and stability.²¹⁴

Sn/SnSb, Sn/SnSb/Bi and Sn/Bi multi-phase materials were prepared using NaBH₄ or Zn employed as a reducing agent and tested as potential anode materials for lithium-ion batteries through galvanostatic cycling. NaBH₄ produced more stable and finer materials, while Zn resulted in purer materials that exhibited lower irreversible capacity. The reversible capacities observed were approximately 600 mA h g⁻¹, 500 mA h g⁻¹ and 350–400 mA h g⁻¹ for Sn/SnSb, Sn/SnSb/Bi and Sn/Bi respectively. This difference in stability is partly due to the influence of intermetallic phases as the material undergoes the lithiation process.²¹⁵

3.5 Conversion type anode materials

Some materials function as anodes through a redox or “conversion” reaction with lithium. This conversion reaction is not only limited to oxides but applies to other materials like fluorides, sulfides, and nitrides. Usually, Li₂O is electrochemically inactive and does not decompose into metal and oxygen. Lithium can cycle within the material at suitable voltages for the metal, enabling large, stable reversible capacities over multiple charge–discharge cycles. Nanotechnology has helped to develop materials that have been evaluated for lithium cycling via different pathways.²¹⁶ In this regard some key challenges must be overcome for conversion materials to be considered for potential commercialization, including voltage hysteresis, extended sloping regions in the discharge profile ascribed to redox reactions, poor cycling stability, rate capability, and notable loss of capacity within the first cycle.²¹⁷

Voltage hysteresis, which refers to the variation in voltage during charge and discharge cycles, can reduce the overall efficiency of the battery. The sloping regions, as observed in the discharge profile, indicate that the voltage gradually decreases over time, making it harder to maintain a stable and predictable energy output. Inconsistent cycling stability means the material's performance degrades over repeated charging and discharging cycles, while rate instability indicates the material's inability to handle high charging or discharging speeds. Lastly, the capacity fading during the first cycle can significantly reduce the overall capacity of the battery, making it less efficient in the long term. Addressing these issues is critical for successfully commercializing conversion materials in LIBs.²¹⁸ Fig. 7 represents the comparative analysis of theoretical capacities of different metals when used as anode. Extensive research into alternative anode materials for LIBs has resulted in an improved understanding of materials chemistry and electrochemistry and, hence, has provided new insights over the past decade.²¹⁶

Fig. 7 Comparative analysis of theoretical capacities of different metals when used as anode.

3.6 Transition metal sulfides as anodes

3.6.1 Cu-based sulfides. Cu₂S and CuS nanorods have been found to demonstrate stable reversibility when Li is inserted and extracted during the cycles and show excellent rate capability. These electrodes retain reasonable capacitance as well at outstanding current densities (3200 mA g⁻¹), regardless of the rate. While, at high current rates, the CuS electrode retained over 370 mA h g⁻¹, and the Cu₂S one retained its discharge capacity at 260 mA h g⁻¹. After 100 cycles, the CuS and Cu₂S electrodes achieved 472 and 313 mA h g⁻¹ discharge capacities, preserving 92% and 96% of the starting capacity at 100 mA g⁻¹. The simplicity of production and their improved electrochemical characteristics make them suitable negative electrodes for future LIBs.²¹⁹ Zhou et al. developed CuS nanorods measuring lesser than 10 nm employing a straightforward sol–gel method, eliminating the need for post-heating treatment. These as-synthesized nanorods showed a hexagonal covellite CuS structure, which is responsible for demonstrating a significant reversible capacity, cyclic stability, and high coulombic efficiency.²²⁰

Carbon materials like CNTs, graphene, and rGO demonstrate various chemical and physical benefits, such as exceptional electronic conductivity, outstanding mechanical strength, and superior structural flexibility. As a result of this, incorporating carbon materials enhances electrochemical performance to cope the challenges of volume change and inadequate electronic conduction in metal sulfides. By using a one-pot hydrothermal approach, Tao and colleagues synthesized a CuS/graphene composite that demonstrated enhanced cyclability and reversible capacity as compared to the pristine CuS. The composite maintained approximately 296 mA h g⁻¹ after 25 cycles at 50 mA g⁻¹.²²¹ Likewise, Yuan et al. prepared CuS/rGO composite as an anode for LIBs. The composite exhibited outstanding electrochemical lithium storage capacities when ether- and carbonate-based electrolytes were employed. At current densities of 100 mA g⁻¹, the two electrolytes demonstrated a reversible capacity of 422 and 390 mA h g⁻¹ following 70 cycles. Following 200 charge/discharge cycles at 500 mA g⁻¹, the composite demonstrated a discharge capacity of 390 mA h g⁻¹ in the ether-based electrolyte, achieving a capacity retention of 96.7%, as illustrated in Fig. 8.

Fig. 8 Charge/discharge profiles of (a) copper sulfide and (b) CuS/rGO at a current density of 100 mA g⁻¹. Cyclic voltammetric curves of (c) CuS and (d) CuS/rGO between a V range of 1.0–3.0 V. This figure has been adapted from ref. 222 with permission from Elsevier, copyright 2016.

The impressive electrochemical characteristics of the electrode was due to the synergistic interaction between the rGO and nanoflowers CuS.²²² CuS/graphene was reported to be synthesized as an anode via a microwave irradiation method that used a one-pot strategy carried out at room temperature conditions. As compared to the pure CuS, the CuS/graphene provided a notably improved reversible capacity and enhanced stability during Li ion charge/discharge cycles.²²³ In the same vein, CNTs are known for their effective conductive network that boosts the electrochemical characteristics. Wang et al.²²³ employed a straightforward microwave-assisted pathway to produce CuS nanospheres with CNTs interconnected to it. This composite material exhibited strong electrochemical performance. Following 450 cycles, the different percentage compositions of CuS/CNT, i.e., CuS/0.1CNT and CuS/0.5CNT, exhibited discharge capacities of over 437 and 569 mA h g⁻¹ at a rate of 400 mA g⁻¹, surpassing performance of controlled samples. Moreover, it was observed that the CuS/_0.5CNT retained a capacity of approximately 400 mA h g⁻¹ at 6400 mA g⁻¹. Inner electron transport, as well as reaction kinetics, are enhanced by the conducting network of CNTs, leading to higher reversible capacities and rate capabilities. Thus, by utilizing the favourable characteristics of rGO, such as its superior conductivity and mechanical strength, issues related to inadequate electronic conductivity and expanded lattice structure of Li₂S have been effectively addressed. A considerable amount of effort has been dedicated, and numerous strategies have been suggested to develop these materials.

Zhao and co-workers introduced a unique self-templating thermolysis approach, to synthesize Cu₂S hollow spheres with various shell coatings, utilizing the core's thermal decomposition characteristics and the shell's protective attributes. The method is different from conventional wet processing techniques. This synthetic approach has been applied to three distinct coatings, which include carbon, TiO₂, and MoS₂, resulting in the preparation of their respective hollow spheres, which were subsequently employed as efficient anodes for LIBs, demonstrating outstanding cycling stability. The electrochemical performance of Cu_2−xS@C was meticulously examined.²²⁴ The Cu_2−xS@C electrode demonstrated boosted electrocatalytic performance compared to pure Cu_2−xS and delivered a discharge capacity of 309 and 277 mA h g⁻¹ following 300 cycles.²²⁵

3.6.2 Co-based sulfides. Various cobalt sulfides, including Co₉S₈, CoS, Co₃S₄, and CoS₂, have been tested as anodes. Yan et al. reported CoS₂ to achieve a 1st discharge capacity of 1280 mA h g⁻¹ at 50 mA g⁻¹.²²⁶ Jiao with his co-workers synthesized CoS₂ hollow spheres through a solvothermal method and found these to exhibit a discharge capacity of 1210 mA h g⁻¹ with good cycling stability, suggesting its potential for long-term applications as an anode material for LIBs.²²⁷ Likewise, the electrocatalytic properties of Co₉S₈ nanotubes indicated a reasonable discharge capacity, suggesting its potential use as an electrode material for LIBs.²²⁸ Numerous investigations into the application of Co₉S₈ in LIBs have been published. Numerous strategies have been implemented to increase conductivity, manage mechanical stress, and optimize the electrochemical activity of Co₉S₈. A simple NaCl method was employed to create 3D Co₉S₈@CNNs anchored in carbon nanosheet networks to enhance conductivity and electrochemical activity. By integrating the benefits of porous and nanoscale structures, 3D Co₉S₈@CNNs inhibited the clustering of nanostructures, offered a wealth of electrochemically active sites. Consequently, the synthesized 3D networks demonstrated exceptional electrochemical performance when employed as anodes.²²⁹

Hollow cobalt sulfide carbon nanocages showed excellent mechanical flexibility and significant structural stability, resulting in outstanding Li-ion storage capabilities. Operating between a V range of 1.0–3.0 V, it showed an energy density of 707 W h kg⁻¹, reasonable rate capabilities, and reversible capacities. Furthermore, it demonstrated stable cycling performance at 1C, showing approximately 26% capacity loss after an extensive 150 cycles, maintaining capacity retention of 365 mA h g⁻¹.²⁰⁴ Qu et al., presented a synthetic pathway for CoS NPs incorporated into Co₉S₈@C through carbonization followed by a sulfidation process. The material demonstrated improved rate stability and high capacity with a C_s of 1565 mA h g⁻¹ at 0.1C (Fig. 9). Moreover, the battery demonstrated a consistent reversible capacity of 606 mA h g⁻¹ above 300 cycles at 1C.²³⁰

Fig. 9 (a) Cyclic voltammetric curves of Co₉S₈@C nanocomposite obtained at 0.1 mV s⁻¹ within V of 0.01 and 3 V. At 0.1C (b) charge/discharge curves for Co₉S₈@C nanocomposite at different cycles and (c) cycling performance along with the coulombic efficiency of Co₉S₈@C in comparison to pristine Co₉S₈. (d) Comparison of Co₉S₈@C nanocomposite electrode rate performance at different current densities with reference to pure Co₉S₈. (e) The measured cycling performance and associated coulombic efficiency of the Co₉S₈@C nanocomposite. This figure has been adapted from ref. 230 with permission from Elsevier, copyright 2016.

A 1D MWCNT@a-C@Co₉S₈ nanocomposite with amorphous layer acting as a linker between the MWCNTs and Co₉S₈ was assessed as an anode for LIBs. It was found to exhibit notable advantages in higher capacity and longevity, outperforming MWCNTs@Co₉S₈ composite and pristine Co₉S₈. The reversible capacity remained at 662 mA h g⁻¹ after 120 cycles conducted at 1 A g⁻¹ current density. This nanocomposite's effective synthesis and outstanding performance provided many opportunities for exploring other sulfides as innovative anodes for LIBs.²³¹ Liu et al., synthesized Co₉S₈/GNS and evaluated its performance as an anode for LIBs. In comparison to bulk material, the as-prepared nanocomposites showed better electrocatalytic performance, cyclic durability with a reversible capacity of 1480 mA h g⁻¹. The meticulously designed structure of Co₉S₈/GNS nanocomposite enhanced its electrochemical characteristics.²³² The incorporation of C materials facilitated the reduction of Co₉S₈ nanoparticle clustering by providing abundant active sites and efficiently handling mechanical stress during the cycling of Li-ion batteries. To enhance the electrochemical characteristics of carbon materials, heteroatoms were introduced into their structures. Fang et al. synthesized Co₉S₈ nanoparticles incorporated within nitrogen-rich hollow carbon shells and demonstrated outstanding lithium storage capabilities at elevated charge/discharge rates. The outstanding battery performance resulted from the hollow structures combined with the nitrogen-rich carbon encapsulation.²³³

A notable capacity recovery occurred during this cycling process, linked to increased capacitance achieved through successive electro-chemical milling. The formation of a carbon shell on the hollow nanospheres improved the reversible capacities at higher rates, achieving approximately 896 mA h g⁻¹ up to 800 cycles at 2 A g⁻¹. The outstanding Li-ion storage capabilities highlighted the significant potential of sulfides as anodes with high capacity, rapid charge rates, and extended lifespan.²³⁴ The CNTs incorporated TMSs show potential for use as anode materials for LIBs. However, their mechanisms and electrochemical performance are not yet fully understood. Developing high-performance materials from these composites requires an in-depth knowledge of their mechanism at the nanoscale. Wang et al. synthesized CoS/CNTs nanocomposites using a solvothermal method and probed their potential as an anode material for LIBs. The findings indicated that the material demonstrated robust cyclic stability and maintained a discharge rate of 780 mA h g⁻¹ following 50 cycles at 100 mA g⁻¹. The outstanding electrochemical performance was attributed to the effective integration of the hybrid structure and enhanced electron transport facilitated by CNTs. The CoS/CNTs nanocomposites exhibited remarkable rate capabilities and superior performance, positioning them as effective anode materials for LIBs.²³⁵ Tan et al. developed CoS NFs–rGO electrodes, which demonstrated a discharge capacity of 939 mA h g⁻¹ after the 100th cycle at a current rate of 100 mA g⁻¹, while sustaining a coulombic efficiency greater than 98%.²³⁶

The self-assembled interweaving CoS₂/CNTs/graphene (CCG) was synthesized through the hydrothermal method. The architecture of CCG employed 1D CNTs and 2D graphene nanosheets to establish a conductive and porous network that mitigated the CoS₂ aggregation. CCG functioned as the anode material for lithium-ion batteries, exhibiting superior performance relative to uncoated CoS₂. It attained an initial discharge capacity of 993 mA h g⁻¹ at a current rate of 100 mA g⁻¹, along with a rated capacity of 212 mA h g⁻¹ at 200 mA g⁻¹.²³⁷ A novel nanocomposite with durable hollow frameworks made of N-doped CNTs and highly active CoS₂ nanoparticles enhanced Li storage capacity as a LIB anode. The material had a capacitance of 937 mA h g⁻¹ at 1 A g⁻¹ and a cycle life of over 160 cycles.²³⁸ CoS₂ nanoparticles, self-assembled and wrapped by CoS₂ QDs anchored onto graphene nanosheets, were synthesized through hydrothermal reaction. The composite demonstrated exceptional electrochemical performance. It has 1048 mA h g⁻¹ reversible capacitance and 831 mA h g⁻¹ capacity after 300 cycles at 1 A g⁻¹ current density. The rated capacity reaches 411 mA h g⁻¹ at a current density of 10 A g⁻¹, marking it as the highest among CoS₂-based materials under these elevated current density conditions. The exceptional performance was attributed to the distinctive structure featuring CoS₂ nanoparticles uniformly enveloped by graphene nanosheets rich in CoS₂ quantum dots.²³⁹ A hierarchical structure of CoS₂/G was reported as an outstanding material for Li-ion batteries. The CoS₂/G electrode material showed a remarkable rate capability of around 398 mA h g⁻¹ at a current density of 500 mA g⁻¹. The initial characterization of the material was achieved through X-ray absorption spectroscopy, revealing that CoS₂/G was transformed into metallic Co and LiS₂ upon discharge to 0.01 V.²⁴⁰

Guo et al., employing the solvothermal technique, graphene nanosheets (GNs) were systematically arranged in layers around monodisperse CoS₂ nanocages. The adaptable GNs within the multilayer composite incorporate monodisperse CoS₂ nanocages, facilitated buffer volume changes, and prevented agglomeration throughout electrochemical cycling. Monodisperse nanocages exhibited resilience to volume fluctuations, facilitated a reduction in lithium diffusion, and maintained a non-agglomerated state. The sample exhibited an impressive capacity of 800 mA h g⁻¹ after 150 cycles at a rate of 100 mA g⁻¹, and a capacity of 697 mA h g⁻¹ following 300 cycles at 500 mA g⁻¹, alongside commendable capacity retention and outstanding rate capability, making it a promising candidate for use as an anode in Li-ion batteries.²⁴¹ Similarly, the CoS₂/rGO nanocomposite also functions as a highly efficient anode material for LIBs. It demonstrates high efficiency with a reversible capacity retention of 640 mA h g⁻¹ after 50 cycles at 100 mA g⁻¹.²⁴² A successful alcoholysis approach was used to synthesize hollow CoS₂@C nanosheet aggregates for LIB anodes. The sample had a good rate capacity and consistent capacity of 720 mA h g⁻¹ following 200 cycles. The aggregates' nano-scaled CoS₂ nanosheets ensured the electrode's high capacity and quick Li-ion diffusion.²⁴³ Wang and co-workers synthesized ultra-small CoS₂ NPs in N-rich carbon, demonstrating encouraging Li-ion storage capabilities with minimal capacity loss at elevated charge/discharge rates. When the current density increased to 2500 mA g⁻¹, a reversible capacity of 410 mA h g⁻¹ was attained. The remarkable and consistent performance was credited to the synergistic interaction between the NPs and N-rich carbon shells.²⁴⁴ Similar hollow spheres with a different synthesis method were reported by Zhang et al.²³⁸

3.6.3 Mo-based sulfides. Molybdenum disulfide (MoS₂) possesses a layered architecture, with a plane of molybdenum atoms interposed between layers of sulfide ions. The bulk MoS₂ comprises stacked monolayers interconnected by weak van der Waals forces. Due to the facile intercalation and extraction of Li⁺ ions from these layered structures, it has been widely investigated as an electrode material for LIBs.²⁴⁵ Lou and his co-workers synthesized Mo-glycerate solid spheres and transformed them into hierarchical MoS₂ nanospheres via sulfidation. Due to their hollow interior and distinctive ultrathin subunits, the synthesized MoS₂ hollow nanospheres demonstrated remarkable efficiency as an anode material for LIBs. MoS₂ exhibited a noteworthy capacity of around 1100 mA h g⁻¹ at a current density of 0.5 A g⁻¹, demonstrating excellent rate retention and extended cycle life.²⁴⁶

MoS₂ has drawn the attention of researchers in the domain of electrochemical energy storage. Yu et al. synthesized N-doped carbon shells, incorporated them in MoS₂ nanosheets, and referred to them as C@MoS₂ nanoboxes. Adding N-doped carbon shells increased the hybrid structure's conductivity and prevented MoS₂ nanosheet aggregation. Ultrathin MoS₂ nanosheets increase electrochemical reaction active sites. As an anode material for LIBs, these C@MoS₂ nanoboxes showed a capacitance of 1000 mA h g⁻¹, excellent cyclic stability for up to 200 cycles, and good rate performance.²⁴⁷ Recent studies indicated that, alongside the atomic composition and arrangement, the dimensions significantly influence the properties and performances of specific materials. Recently, a significant focus has been on the extensive study of 2D graphene. The graphene displays distinctive characteristics not found in bulk graphite, including mechanical strength and optical properties. The 2D TM dichalcogenides, characterized by their layered structure akin to graphene, have garnered considerable interest. Research on 2D monolayer or few-layer MoS₂ has been undertaken for energy-related applications due to its unique properties and layered architecture, which include batteries, fuel cells, and photoelectrochemical systems.²⁴⁸

3.6.4 Fe-based sulphides as anodes in LIBs. Iron sulfides have attracted considerable attention for use in LIBs. Pyrite possesses several advantages, including a high theoretical energy density of approximately 1313 W h kg⁻¹, commendable thermal stability, affordability, natural abundance, and non-toxic properties. In contrast to other metal sulfides, pyrite has been utilized effectively in Li–FeS₂ batteries for an extended period. Furthermore, the potential for developing commercial FeS₂-based LIBs appears more promising than alternative metal sulfides.^249,250 Marcasite-FeS₂ exhibited stronger bond Fe–S bonds and reduced S–S interactions; it is likely to be a more effective electrode material than pyrite-FeS₂. Fan et al. developed a hierarchical composite of m-FeS₂ and carbon nanofibers (m-FeS₂/CNFs) featuring a grape-cluster architecture through hydrothermal synthesis, showcasing exceptional performance as an electrode material for lithium-ion batteries. It achieved a reversible capacity of 1399.5 mA h g⁻¹ following 100 cycles at 100 mA g⁻¹ at 10 A g (Fig. 10). The hierarchical m-FeS₂ microspheres, constructed from small FeS₂ nanoparticles within the m-FeS₂/CNF composite, transformed into a mimosa with open leaves during Li⁺ insertion and reverted upon removal.²⁵¹

Fig. 10 (a) CV curves of m-FeS₂/CNFs at 0.1 mV s⁻¹. (b) The discharge/charge curves for m-FeS₂/CNFs at 0.1 A g⁻¹. (c) Comparison of the cycling performance of pure m-FeS₂, pure CNFs, and m-FeS₂/CNFs composite at 0.1 A g⁻¹. (d and e) Rate capabilities of m-FeS₂/CNFs composite anode material at different current densities within a voltage range of 0.01–3 V. (f) The specific capacities comparison of the m-FeS₂/CNFs composite electrode with FeS electrodes reported in the literature for lithium-ion batteries. This figure has been adapted from ref. 251 with permission from the American Chemical Society, copyright 2017.

Lee and co-workers synthesized FeS₂/rGO using a solvothermal process demonstrating remarkable performance, particularly under high current densities. Impedance measurements indicated that functionalizing FeS₂ with rGO markedly reduces the charge transfer resistance and enhances its structural stability. The as-synthesized material demonstrated a capacity of 970 mA h g⁻¹ at a current rate of 890 mA g⁻¹ following 300 cycles. LIB showed a capacity of 380 mA h g⁻¹ at exceptionally high current density of 8900 mA g⁻¹ after undergoing 2000 cycles.²⁵² FeS₂ nanocrystals were synthesized within carbon capsules by Xu et al. within pre-pared hierarchical porous carbon. The high-performance composite, featuring micro-porous shells and interconnected macro-porous carbon capsules, demonstrated exceptional ionic and electrical conductivities and a practical buffering effect on the volume changes of FeS₂. FeS₂@HPC demonstrated a notable C_s of 907 mA h g⁻¹ during the initial cycle thereby maintaining a capacity of 720 mA h g⁻¹ following 100 cycles at a rate of 1C.²⁵³

3.6.5 Other transition metal-based sulphides in LIBs. 1D nanostructures exhibit distinct properties that render them advantageous for utilization as active materials for LIBs. Initially, the 1D structure facilitates adapts to volume changes associated with lithium charge/discharge cycles in a specific direction. Additionally, the gaps between the adjacent structures can serve as channels for electrolytes, facilitating their diffusion into the electrode, thus extending the lifespan of batteries. Furthermore, the 1D nanostructure arrays offer large surface-to-volume ratio when compared with bulk materials and enhance the availability of active sites and facilitate fast diffusion of Li⁺ ions.²⁵⁴ In addition to previously discussed materials, other TM sulphides, particularly 2D materials like TiS₂, WS₂, and VS_2, are being considered as favourable anodes for LIBs. TiS₂ is recognized for its exceptional electronic and ionic conductivity, that makes it ideal for energy applications. Leveraging the rapid Li⁺ diffusion rates, TiS₂ is primarily investigated for its applications in high-power batteries.²⁵⁵ For the successful implementation of solid-state LIBs in future EVs, it is crucial to fabricate electrodes that are stable and deliver high energy density and power outputs. To tackle these problems, Sun et al. reported that TiS₂ anode in the “Water in salt” electrolyte has high electrochemical reversibility. The electrochemical reversibility of TiS₂ was much improved, and the hydrolysis side reaction was effectively controlled in the aqueous electrolyte by using the “Water in salt” electrolyte. Combined with a LiMn₂O₄ cathode, the LiMn₂O₄/TiS₂ fuel cell exhibited a notable energy density of 78 W h kg⁻¹ at 1.7 V and commendable performance.²⁵⁶ WS₂ is a layered TM dichalcogenide stacked in 2D. The material exhibited an identical crystal structure to MoS₂, characterized by S–W–S layers held together by van der Waals interactions, and is highly favoured for Li⁺ ion intercalation, signifying its potential use in energy storage devices. The semiconducting characteristics of WS₂, combined with the insulating properties of discharge products Li₂S, result in sluggish kinetics. Consequently, numerous initiatives have been undertaken to enhance electric conductivity.²⁵⁷

Liu et al. reported WS₂ nanoflowers@rGO in which rGO served as a cross-link, thereby enabling a 3D conductive network while also safeguarding from the effects of volume changes in WS₂ that occur during charge discharges cycles. The distinctive configuration of the nano-composite facilitated improved ion and electron diffusion while simultaneously bolstering electrode stability, resulting in a high-capacity anode for LIBs. The WS₂–NF@rGO achieved a C_s of 730 mA h g⁻¹ following 150 cycles at 100 mA g⁻¹ current density while sustaining a capacity exceeding 260 mA h g⁻¹ at 2 A g⁻¹.²⁵⁸ A WS₂ nanosheet@ carbon fibre cloth (CFC) composite have shown promise for developing advanced flexible anodes in LIBs. The presence of a metallic phase in WS₂ allowed the 1T@2H WS₂@CFC electrode to achieve a specific capacity of 1130 mA h g⁻¹ after 200 cycles at a current density of 0.1 A g⁻¹, along with an impressive rate capability of 510 mA h g⁻¹ after 800 cycles at 2 A g⁻¹, while experiencing a minimal initial discharge capacity loss of 12.64%. Furthermore, the integrated capacitive charge storage within the 1T@2H WS₂@CFC electrode was examined, revealing its advantages for rapid charge storage and sustained cyclability over extended periods.²⁵⁹

The multi-layered nanostructured WS₂, combined with enhanced electronic conductivity and volume accommodating effect of CMK-3, resulted in a material that exhibited optimal discharge capacity, enhanced rate capability, and improved cyclability when compared with pristine WS₂. The material exhibited a capacitance of 720 mA h g⁻¹ over 100 cycles when tested for LIBs at a current density of 100 mA g⁻¹. This remarkable performance highlighted the immense potential of the WS₂@CMK-3 nanocomposite.²⁶⁰ The successful integration of graphene sheets with CNF remains an elusive goal. To address this, Fan and co-workers presented a novel and straightforward method for preparing flexible graphene CNF (GCNF) membranes, wrapping the CNF with conductive graphene sheets, significantly enhancing electrical conductivity. Carbonization of the resulting hybrid membranes made of GO and electro-spun polyacrylonitrile (oPAN) NFs was the only demanding step in that approach. Hierarchical WS₂/GCNF hybrid membranes were fabricated using a highly conductive GCNF membrane as a template, featuring multi-layered WS₂ nanosheets grown uniformly onto a NFs, serving as efficient anode for LIBs. The WS₂/GCNF nanocomposite membrane exhibited a large specific surface area, extensive porous structure, notable electrical conductivity, and robust material strength. These characteristics facilitated rapid Li⁺ diffusion, efficient electron transfer, and enhanced stability. The optimized WS₂/GNF composite membrane demonstrated an initial charge capacity of 1128.2 mA h g⁻¹ at a current rate of 0.1 A g⁻¹, along with notable cyclic stability, and marinating 95% of its capacity after 100 charge/discharge cycles.²⁶¹

A simple solvothermal approach, followed by freeze-drying and then post-annealing, was used to synthesize a hybrid nanoarchitecture aerogel with a 3D scaffold structure made out of organized microchannels and CNTs–reduced graphene oxide (rGO). The 3D-ordered microchannel structures facilitated effective electronic transport and offered superior ionic conductive pathways, improving the performance of the material. The WS₂/CNT–rGO aerogel nanostructure achieved a C_s of 749 mA h g⁻¹ at a current density of 100 mA g⁻¹, and a coulombic efficiency of 53.4% in the first cycle.²⁶² An intercalation-transformation approach was used to successfully synthesize a multilayer-cake WS₂/C nanocomposite. This process involved sandwiching a few layers of WS₂ and carbon in an alternating sequence, which demonstrated promising performance, due to enhanced chare transport properties of the interlayer carbon and its distinctive “regular” and “alternate” architecture. The nanocomposite exhibited an impressive capacity of 829.4 mA h g⁻¹ at a current rate of 0.3 A g⁻¹ following 140 cycles, maintaining a stable capacity of approximately 326.8 mA h g⁻¹ even at a higher current density of 8.0 A g⁻¹.²⁶³

Fang et al. presented a straightforward one-pot method for synthesizing hierarchical VS₂/graphene nanosheets (VS₂/GNS) composites. The composite's structure featured 50 nm thick VS₂ sheets uniformly distributed onto graphene whereby the composite demonstrated optimal rate capability and cyclability, maintaining 181.1 mA h g⁻¹ even after 200 cycles at 0.2C. Comprehensive investigations indicated that VS₂/GNS nanocomposite demonstrated commendable reversible performance, delivering a C_s of 528 mA h g⁻¹ following 100 cycles at 200 mA g⁻¹. The electrochemical performance of the composite serving as an anode is ascribed to the synergistic effect between VS₂ and GNS, which facilitates rapid electron transport between the two, as well as supports facile diffusion of Li-ions within the electrode structure. Furthermore, GNS offered a structural framework that supported the growth of 2D VS₂ nanosheets, that results in alleviating the effects of volume changes, thereby enhancing cyclic stability. These composites showed potential as electrode materials for the upcoming generations of rechargeable batteries.²⁶⁴

VS₄ showed promise as an anode material due to its distinct chain-like structure, elevated levels of sulfur, and unique electrochemical behaviour. Here, amorphous carbon coated with MWCNTs served as the basis for the VS₄ nanoparticles generated via a solvothermal process. The unique structure allows modified CNTs to improve charge transportation within the composite and externally, a feat that was challenging to accomplish with alternative structures. The composite outperformed the numerous TM sulfides in terms of cyclic stability and rate capability owing to these enhancements in charge transfer. The reversible capacity achieved was 922 mA h g⁻¹ following 100 cycles at a rate of 0.5 A g⁻¹. A 1000 cycle at 2 or 5 A g⁻¹ didn't affect the 576 or 401 mA h g⁻¹ capacity (Fig. 11).²⁶⁵

Fig. 11 (a) Cyclic voltammograms (b) discharge/charge profiles of MWCNTs@a-C/VS₄ (c) cycling stability at a current density of 0.5 A g⁻¹ (d) rate capability comparison among MWCNTs@a-C/VS₄, MWCNTs + VS₄, and MWCNTs/VS₄ (e) schematic diagram depicting the electron and charge transfer mechanisms for the three composite types (MWCNTs@a-C/VS₄, MWCNTs + VS₄ and MWCNTs/VS₄). This figure has been adapted from ref. 265 with permission from Elsevier, copyright 2017.

Jang et al. introduced a colloidal method for synthesizing ultrathin ZrS₂ nanodiscs, which were approximately 1.6 nm thick and comprised of two S–Zr–S unit cells. The S–Zr–S layers within individual discs were connected through weak van der Waal forces, allowing each UT-ZrS₂ disc to create spaces serving as host sites for Li-ion intercalation. The UT-ZrS₂, due to its nanosized particles, resulted in a 230% increase in discharge capacity and significant enhancement in stability compared to its bulk counterpart. The impact of nanosized UT-ZrS₂ was particularly substantial during charge/discharge cycles, with an observed average recovery of 88% in reversible capacity for UT-ZrS₂ discs with a width of 20 nm.²⁶⁶

Osaka and co-workers synthesized SnS₂ nanoparticles, which achieved a 620 mA h g⁻¹ of discharge capacity and reduced degradation of the electrode. The findings suggested that increasing the surface area enhanced Li-ion intercalation–deintercalation within the active material while at the same time reducing the stress during the cycles that improved the electrochemical pathways and, thus, electrode kinetics.²⁶⁷ Liu et al. proposed a flowerlike SnS₂ structure synthesized via a solvothermal process. The investigation focused on the impact of synthetic parameters, such as the choice of electrolyte and thiourea concentration, on the morphology of the resulting products. The material demonstrated superior electrochemical properties. The flower-like SnS₂ structures achieved a higher reversible capacity of approximately 502 mA h g⁻¹ and exhibited more stable cyclic retention by the 50th cycle compared to SnS₂ nanoplates.²⁶⁸ 3D SnS₂ microspheres were produced using the facile hydrothermal method and used as an anode material. These SnS₂ microspheres, formed through the self-assembly of layered nanosheets, demonstrated boosted Li⁺ storage capacity with a stable cycle life and retained an optimal discharge capacity following 100 charge/discharge cycles. Their exceptional performance was attributed to the distinctive 3D hierarchical architectures of these SnS₂ microspheres.²⁶⁹ Hollow microspheres of SnS₂ and 3D nanoflakes-based SnS₂ hollow microspheres were hydrothermally synthesized and exhibited improved performance as an anode material.²⁷⁰ A gram-scale and template-free solvothermal route was used to synthesize ultra-long SnS₂ nanobelts with a production yield of up to approximately 98%. The SnS₂ nanobelts thus produced outperformed the graphite anode when its C_s (640 mA h g⁻¹) and cyclic stability were compared, which was 560 mA h g⁻¹ after 50 cycles.²⁷¹

Crystalline SnS₂ nanosheets were made via a one-step solvothermal method and integrated into negative electrodes for LIBs. Compounds of SnS₂ synthesized in ethylene glycol displayed a nanosheet morphology with an approximate thickness of 2 nm and demonstrated remarkable capacity retention of around 95% following 50 cycles at 0.5C.²⁷² Zhai et al. synthesized ultra-thin hexagonal SnS₂ nanosheets through a straightforward hydrothermal reaction, demonstrating a reasonably enhanced reversible capacity with an optimal cyclic stability, achieving an impressive capacity retention of 96% following 50 cycles.²⁷³ Liu et al. used a straightforward chemical bath deposition method to fabricate SnS₂ nanowall (NW) arrays onto the copper foils. The NW arrays demonstrated enhanced Li ion storage capacities when employed as an anode material. At 0.3C, the NW arrays delivered a C_s of 700 mA h g⁻¹ following 40 cycles and maintained a steady 400 mA h g⁻¹ capacity as it is subjected to a high rate of 1.2C.²⁷⁴ Similarly, Zhong et al. was successful in synthesizing layered SnS₂ nanosheets directly onto the Sn foil with improved electrochemical performance for LIBs.²⁷⁵ Other than nanosheets, NPs of tin sulfides (SnS and SnS₂) were also synthesized hydrothermally and resulted in superior electrochemical performance and long cyclic life. However, their discharge capacities still need to be improved.²⁷⁶ Du et al. proposed commercial scale synthesis of 2D SnS₂ nanoplates, which resulted in notable electrochemical behaviour and capacity retention following 30 cycles.²⁷⁷

MWCNTs and carbon in metallic nanocomposites can significantly enhance the electrochemical properties. Carbon-coated SnS₂ NPs were synthesized by Kim et al.²⁷⁸ via a solvothermal process, resulting in a significant electrochemical response. Huang and co-workers successfully synthesized Porous 3D SnS₂ nanoflowers incorporating acetylene black by using a facile solvothermal route aided by PEG. The material displayed a boosted cyclic stability and rate capability, achieving an average reversible capacity of up to 525 mA h g⁻¹ at a high current rate over 70 cycles. The electrochemical performance results dues to the synergistic effect of acetylene black and the distinctive structure of SnS₂. Acetylene black enhanced the electron transfer and stabilized the electrode structure by mitigating the impact of volume changes. In addition to that, the porous architecture of SnS₂ contributed to stabilizing the electrode structure and enhanced the shuttling of Li⁺ ions.²⁷⁹ Kang et al. focused on 3D MWCNTs/SnS₂ heterostructured electrodes, which exhibited remarkable rate capabilities at high rates. The significance of MWCNTs was extensively examined to trace the roots of the improved rate capabilities.²⁸⁰ Graphene nanocomposites loaded with SnS₂ NPs were synthesized hydrothermally. The electrochemical characterizations indicated a significant reversible capacity exceeding 800 mA h g⁻¹ at 0.1C and maintained 200 mA h g⁻¹ at a 5C rate.²⁸¹ Ji et al. concluded that graphene–SnS₂ nanocomposites demonstrated superior performance in LIBs as anode materials. Some of the recent transition metal sulfide-based anode materials for LIBs are listed in Table 5.²⁸¹

Table 5 Transition metal sulfides-based anode materials for LIBs

Anode material	Method of preparation	Structure	Reversible capacity (mA h g^−1)	Cycle number	Coulombic efficiency (%)	Ref.
rGO@NiS composites	Solvothermal synthesis	Particles uniformly distributed in 3D network	1328.7	120	69.1	282
NiTi2S4 ternary sulfide	High energy mechanical milling	Nanoparticles with monoclinic crystal structure	635	50	89	283
α/γ-MnS embedded in N-doped layered carbon	One-step annealing process	Nanosheets	1042	1000	67.6	284
Nickel cobalt manganese sulfide/C composite	Gas sulfidation process	Metal sulfides embedded in C matrix	900.4	200	70.1	285
Ni3S2/Ni@VCMN	Plasma-assisted chemical vapour deposition	Polycrystalline Ni3S2 nanostructures capping Ni nanoparticles forming a “broccoli like” morphology	1113	100	92.8	286
Ni3S2/Ti3C2Tx/C	One-step solvothermal reaction	Nanoparticles supported on sheets	1150.8	400	58.9	287
Ni3S2@PPy	One-step reflux method	Nanoparticles grown on foam	396.5	100	84.6	288
Nanostructured polycrystalline Ni3S2	Powder vapour transport technique	Flower shaped nanostructures	931	30	98–99	289
Ni-doped Ni3S2 nanoflakes intertexture grown on GO	Two step solvothermal reaction	Nanoflakes	741.8	120	94.77	290
Ni3S2/NiSe2 hollow spheres	In situ gas phase selenization method	Hollow spheres	709	200	Good coulombic efficiency	291
Ni3S2@Ni foam	Facile hydrothermal method	3-D porous structure		400	98.3	292
Petal-like Ni3S2 nanosheets on 3D carbon nanotube foams	One-pot solvothermal method	Petal-like nanosheets on 3D nanotube foams	968.2	900	40.3	293
Ni3S2 functionalized V2O3 nanospheres	One-step solvothermal method	Nanocrystals with nanospheres	550	100	Good coulombic efficiency	294
Ni9S8/Ni3S2/NiS1.03 particles with homogenous bonds to 3D rGO sheets	Facile hydrothermal method followed by freeze drying and calcination	Particles supported on sheets	826.3	1000	100	295
VS2/rGO composites	Solvothermal method	Honey comb like structure	737	150	80.57	296
VS2 nanosheets/graphene composites	Solvothermal method	Monodispersed porous bowl shaped nanosheets	889	500	≈60	297
3D VS2/rGO heterostructure	In situ assembly of nanosheets	Heterostructure	644.02	140	Good coulombic efficiency	298
Hierarchical composite of VS2/CNTs/C	Solvothermal method	3D hierarchical structure	1075	900	∼40	299
VS2@C/TiO2	Electrospinning carbonization followed by hydrothermal method	Branch leaf structure	324.2	200	∼100	300
Vertically aligned VS2 on graphene	Scalable solvothermal and post annealing process	3D heterostructure	989	1000	64	301
Vanadium sulfide decorated at C network	Slurry-coating method	3D porous conductive network	188	2500	∼40	302
CoS2–MnS@C	Thermal decomposition or pyrolysis	Nanoparticles on C matrix	814	100	55	303
CoS2–MnS@rGO	Thermal decomposition or pyrolysis	Nanoparticles on rGO matrix	1324	100	58	303
CoS2–MnS@CNTs	Thermal decomposition or pyrolysis	Nanoparticles on CNTs matrix	1017	100	55	303
α-MnS nanoparticles anchored on holey graphene	Ex situ strategy	Nanoparticles anchored on holey graphene sheets	870.5	200	66	304
MnS–MoS2 heterostructure	One-step solvothermal method	Particles embedded on matrix	1246.2	500	40–50	305
α-MnS@axial multichannel carbon nanofibers	Electro bowling spinning technique	Nanoparticles embedded in axial multichannel nanofibers	772	200	72.9	306
Mn2SnS4@ZnS@Nitrogen doped carbon	Dual modification method	Nanocubes coated with ZnS and encapsulated in NC shell	1508.6	500	77.8	307
Mn–Sn bimetallic sulfides doped C nanorods	Simple solvothermal method	Heterostructures	719.6	1000	70.9	308
Hierarchical MoS2@αMnS nanocomposites	One-pot hydrothermal method	Nanosheets wrapped by nanoparticles forming a rose like morphology	1498	120	78.1	309

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3.7 Soft and hard-carbon anodes

Hard carbon (HC) is attracting interest as a potential anode material for next-generation LIBs because of its distinctive microstructure and improved electrochemical properties. In contrast to graphite, hard carbon exhibits a non-graphitizable nature characterized by a disordered structure. This structure encompasses defects, micropores, and expanded interlayer spacing, all of which enhance lithium-ion storage capacity and facilitate quicker ion diffusion. Nonetheless, obstacles such as low initial coulombic efficiency (ICE), significant irreversible capacity, and voltage hysteresis hinder its potential for commercial application.³¹⁰ Curved, randomly orientated graphitic sheets with short-range order but no long-range order make up hard carbon. Li⁺ intercalation is facilitated by the increased interlayer gap (0.37–0.42 nm), and more Li⁺ storage sites are provided by the defects and functional groups of the structure. Hard carbon can outperform graphite in terms of low-temperature performance (down to −50 °C) and larger capacity (up to 1000 mA h g⁻¹), all while minimizing electrode expansion during cycling, which enhances safety. However, irreversible Li⁺ consumption during SEI generation and pore trapping is the reason for its low ICE (≤80%). Remaining hydrogen-terminated aromatic fragments that bind Li⁺ ions cause voltage hysteresis.³¹¹

Hard carbons utilize three primary mechanisms for Li⁺ storage: the filling of nanopores, adsorption at defect sites and functional groups, and intercalation within graphitic nanodomains. These mechanisms are supported by Raman, XRD, and SAXS, providing evidence for Li⁺ absorption in nanopores and the reversible structural changes that occur during this process. Hard carbons are classified according to their predecessors, which include compounds generated from biomass, pitch, and resin. High purity and adjustable porosity are features of resin-based carbons, which have capacities of up to 1294 mA h g⁻¹. Pitch-based carbons, which are derived from petroleum pitch or pre-oxidized coal tar, normally have capacities of about 829 mA h g⁻¹. With their high carbon yield and heteroatom doping, biomass-derived carbons—such as those found in cellulose, lignin, and starch—offer sustainable alternatives.^312,313 Heteroatom doping (such as nitrogen, phosphorus, and sulphur) is one optimization technique for hard carbons that enhances capacity and interlayer spacing. Graphitization is avoided by pre-oxidizing precursors, and pore engineering guarantees closed pores for Li⁺ storage that do not allow electrolyte penetration. Prelithiation makes up for the early loss of Li⁺, and controlled pyrolysis temperatures balance porosity and crystallinity. By mixing hard and soft carbon, hybrid designs seek to increase capacity and ICE.³¹⁴

Future studies should concentrate on using sophisticated characterization techniques (e.g., TEM, NMR) to understand Li⁺ storage dynamics, optimizing precursor materials for fine microstructure control, and investigating hybrid designs with metal oxides for better volumetric energy density. The best heteroatom configurations can be predicted with the use of computational modelling. Reducing manufacturing costs by using biomass and resolving challenges with consistency and purity will be crucial for industrial scaling. Hard carbons can also be used in rapid-charging applications for grid storage systems and electric vehicles by increasing low-voltage capacity and utilizing fast Li⁺ diffusion. Conclusively, hard carbon may prove to be a more effective anode material than graphite for high-energy, rapid-charging LIBs. Although there are still obstacles to overcome, improvements in pore design, doping, and precursor selection may enable it to reach its maximum potential and become an essential part of energy storage systems of the future.³¹⁵

Soft carbon, a form of carbon material with a somewhat organized structure, can change into graphite when heated to high temperatures, usually above 2500 °C. Soft carbon is better suited for various energy storage applications compared to hard carbon due to its ability to allow for varying levels of graphitization. Its disordered structure and capacity to adjust the degree of graphitization render it ideal for use in sodium-ion and potassium-ion batteries, enhancing its effectiveness in energy storage solutions.³¹⁶ A key feature of soft carbon is its controllable graphitization, which enhances electronic conductivity while potentially reducing interlayer spacing and influencing ion diffusion. With a broader interlayer spacing than graphite, soft carbon allows for more rapid intercalation and deintercalation of lithium ions. Additionally, by manipulating the material's shape to create nanostructures, its stability and surface area can be improved. The incorporation of defects and heteroatom doping, such as nitrogen or sulfur, further enhances the Li⁺ storage capacity by generating more active sites.³¹⁷

Soft carbon can store lithium through two main processes: adsorption and intercalation. Li⁺ ions adsorb onto defect sites and heteroatoms, adding to the capacity and accelerating the kinetics. They also intercalate into the graphitic domains, a reversible process that could expand volume. In nanostructured or doped soft carbon, hybrid mechanisms that combine intercalation and adsorption are frequently seen. These mechanisms provide enhanced performance, including decreased diffusion barriers and metallic conductivity. For LIBs, soft carbon offers a number of benefits, such as superior rate capability because of faster Li⁺ diffusion, structural stability that can adapt to volume variations during cycling, and customization through doping and graphitization changes. There are still issues, though, like permanent capacity loss in the early cycles, which can be lessened with pre-lithiation techniques. Its low-temperature performance and the high expense of accurate synthesis are further obstacles to its widespread application.³¹⁸ Future soft carbon research should focus on advanced characterization techniques to better understand its storage mechanisms and optimize synthesis procedures. Hybrid materials that combine high-capacity and soft carbon materials with specially designed electrolytes may improve its performance. In conclusion, soft carbon is a promising anode material for LIBs because of its special qualities. Additional developments in design and manufacturing may unlock its full potential for high-performance energy storage applications.

4. Future electrode materials

The development of lithium-ion batteries depends heavily on high-energy-density cathode materials. Lithium and manganese rich (LMR) materials ((xLi₂MnO₃·(1 − x)LiMO₂), where M = Ni, Co, Mn) and Ni-rich NCM/NCA (e.g., NCM811) are among the high-voltage cathodes that are attracting interest. These materials are appealing for next-generation batteries because of their high energy density and capacity (more than 250 mA h g⁻¹). Voltage decay, structural instability, and electrolyte breakdown at high voltages are still major problems, though. Recent advancements have aimed to enhance these materials via doping and surface coatings, which aid in structural stabilization.

Sulfur cathodes offer a cost-effective solution with a remarkably high theoretical capacity of 1675 mA h g⁻¹. Despite these benefits, shuttle effect of polysulfides (Li₂S_n) results in a short cycle life. Strategies like employing interlayers, porous carbon hosts, and catalysts (such metal oxides) to speed up polysulfide conversion have been investigated in order to overcome this. Advanced electrolytes with localised high-concentration techniques are also being developed to enhance cycle stability and performance.³¹⁹ Oxygen (O₂) cathodes are lightweight and have the highest potential energy density (3500 W h kg⁻¹). Nevertheless, they encounter difficulties such electrolyte instability, adverse responses, and low reversibility. To improve the stability and performance of oxygen-based cathodes, recent studies have concentrated on protecting lithium anodes and creating stable catalysts like RuO₂.³²⁰

Silicon and lithium metal are particularly promising anode materials. Despite having a very high capacity of 3579 mA h g⁻¹ for Li₁₅Si₄, silicon confronts many difficulties because of its enormous volume expansion (more than 300%), which causes electrode pulverization. Nanostructuring (like Si nanoparticles or porous Si) and composite designs (like Si–graphite mixes) are being investigated as ways to get around this. Commercial applications of silicon already exist; for example, Tesla's “Silicon Oxide” anodes blend silicon with graphite to enhance performance.³²¹ The maximum theoretical capacity of 3860 mA h g⁻¹ and the lowest potential of −3.04 V are provided by lithium metal. However, problems including endless volume change, unstable solid electrolyte interphase (SEI), and dendritic development make it difficult to employ. The creation of 3D porous carbon hosts for regulated deposition, synthetic SEI layers with polymer coatings, and solid-state electrolytes inhibiting dendritic growth are some ways to lessen these issues. For next-generation batteries like Li–S and Li–O₂, where its large capacity can greatly improve energy storage, lithium metal is especially promising.³²²

The future of high-energy-density batteries depends on resolving a number of important issues. Understanding material behaviour and enhancing performance will require multiscale research that blends theoretical modelling (density functional theory and phase-field modelling) with experimental methods. With continuous efforts to create materials that can tolerate high current densities without degrading, fast-charging capabilities are another area of focus. Widespread adoption will also need finding economical ways to close the gap between lab-scale breakthroughs and industrial-scale production.^323,324

5. Environmental impact of LIBs production and disposal

LIBs, which have emerged as a key component of contemporary technology, power electric vehicles and portable electronics. Although these batteries are essential for allowing clean energy solutions and lowering carbon emissions while in use, there is rising concern about the environmental impact of their manufacture, disposal, and recycling. To produce LIBs, raw materials including lithium, cobalt, nickel, and graphite must be extracted, which can significantly affect the environment. Soil erosion, water pollution, and habitat destruction are all consequences of mining these resources, especially in areas with lax environmental standards. For instance, lithium extraction in Chile's Atacama Desert uses much water, endangering nearby water sources. The Democratic Republic of the Congo is the primary mining location for cobalt, which is associated with both environmental degradation and human rights abuses. The manufacture of LIBs uses a lot of energy, greatly increasing their carbon footprint. Cathode manufacturing is one of the most carbon dioxide-intensive steps in the production process, particularly when materials like lithium cobalt oxide are utilized. High emissions of greenhouse gases are the result of this. Depending on the battery type and manufacturing process, the cumulative energy demand for LIB manufacturing is estimated to be between 316 MJ kW⁻¹ h⁻¹ and 2318 MJ kW⁻¹ h⁻¹. Lithium hexafluorophosphate and ethylene carbonate are two solvents used in this production process that can emit toxic chemicals as they break down, further polluting the environment.^325,326

Though LIBs offer significant environmental benefits, particularly when employed in electric vehicles powered by renewable energy, they also present certain negative effects. The overall environmental impact largely hinges on the energy source utilized for charging; however, the integration of LIBs in electric vehicles contributes to a reduction in emissions. The environmental benefit of LIBs is lessened in areas where coal or other fossil fuels are used extensively to generate electricity because coal power is a significant source of greenhouse gas emissions. Furthermore, there are possible health dangers associated with electromagnetic radiation (EMR) from LIB-powered gadgets such as computers and cell phones. Long-term EMR exposure has been connected to a number of health problems, such as heart disease, brain tumours, and sleep disorders. The extensive usage of LIB-powered devices underscores the necessity for continued research into potential health hazards linked with continuous exposure to EMR, even though these effects are still being studied.^327,328

When LIBs approach the end of their life cycle, their impact on the environment becomes increasingly apparent. It is anticipated that the quantity of used LIBs will rise in tandem with the growing demand for portable devices and electric cars. An estimated 11.5 million tons of LIB trash will be produced worldwide by 2030. Because these batteries contain hazardous elements, including cobalt, nickel, and lithium, that can poison water supplies and seep into the soil, improper disposal poses serious environmental dangers. The hazardous elements in LIBs may be discharged into the environment if they are disposed of inappropriately, either by unlawful dumping or incineration. For instance, plants and soil creatures are poisoned by cobalt and lithium, which impacts soil health and biodiversity. Hydrofluoric acid (HF), a very hazardous chemical that can have serious adverse effects on the environment and human health, can be released when the electrolytes in LIBs break down. The hazardous chemicals released by wasted LIBs that are allowed to decompose in landfills have the potential to contaminate soil and groundwater, causing long-term environmental harm. When LIBs are improperly disposed of in open spaces, the batteries may catch fire, further releasing hazardous gases like HF and raising the possibility of air and soil contamination.³²⁹

A key strategy for mitigating the adverse environmental impacts of LIBs is their recycling. This process enables the recovery of essential metals such as cobalt, nickel, and lithium found in LIBs, thereby reducing the ecological costs associated with resource extraction. Additionally, recycling diminishes the need for energy-intensive and environmentally detrimental raw material mining. By incorporating recycled materials, the overall environmental footprint of manufacturing new batteries can be significantly reduced. Fig. 12 illustrates the environmental impacts associated with LIBs.

Fig. 12 Environmental impacts of LIBs.

5.1 Recycling of LIBs

Recycling presents a sustainable approach to reclaim valuable materials, reduce reliance on new resources, and lessen environmental effects. The recycling process involves several steps: collection, discharging, dismantling, material separation, and metal recovery. Depending on the chemistry of the batteries and the materials being targeted, three primary recycling methods are utilized: hydrometallurgy, pyrometallurgy, and the developing method of solvometallurgy. These methods are often paired with specialized techniques aimed at recovering specific components of the batteries.^330,331 Hydrometallurgy is a method that includes leaching battery materials using aqueous solutions, followed by the separation and purification of metal ions. It is the most established and commonly used technique for recycling LIBs due to its relatively low energy consumption and the capacity to produce high-purity battery-grade materials. The procedure generally utilizes leachants, including sulfuric, hydrochloric, or organic acids like citric acid, often combined with reducing agents such as hydrogen peroxide to improve the metal dissolution process. After leaching, valuable metals are extracted through methods such as precipitation, solvent extraction, or electrochemical techniques. Hydrometallurgy is particularly effective in recovering lithium, cobalt, and nickel from various cathode materials, including LiCoO₂, nickel–manganese–cobalt, and lithium manganese oxide. Recent improvements have increased leaching efficiency by optimizing temperature, solid-to-liquid ratio, and pH. The process produces minimal secondary waste and presents opportunities for solvent reuse, although chemical handling and effluent treatment are still significant concerns.^332,333

Solvometallurgy utilizes non-aqueous solvents for metal extraction and has emerged as an environmentally friendly alternative to conventional aqueous hydrometallurgy. A significant advancement in this field is the utilization of deep eutectic solvents (DES). These solvents are formed by combining a hydrogen bond donor (HBD) with a hydrogen bond acceptor (HBA), resulting in a liquid that exhibits unique solvation properties at comparatively low temperatures. Common examples of DES include combinations of choline chloride with urea or ethylene glycol. These solvents are eco-friendly, non-volatile, and relatively straightforward to produce. DES can effectively leach metals from cathodes like NMC and LCO, although their efficiency is generally lower than that of mineral acids due to their lesser acidity. Nevertheless, their effectiveness can be improved by adding more acidic HBDs, such as p-toluenesulfonic acid or citric acid. Leaching typically takes place with moderate heating, and this process can be integrated with electrochemical metal recovery.^334,335

Pyrometallurgy employs elevated temperatures to process LIBs for the recovery of metals, utilizing methods such as smelting or roasting. During this process, organic binders and electrolytes are decomposed, and metal oxides are reduced to alloyed forms that can be refined further. Also, this approach is very tolerant to battery chemistries and formats, so it works very well with mixed or unsorted waste. That enables scaling and automation potential and has led to lots of commercial operations, in particular by companies like Umicore and Glencore. Nonetheless, pyrometallurgy is energy demanding and can release dangerous gases. This often results in the loss of lithium and graphite, with additional hydrometallurgical processes needed for complete materials recovery. Newer techniques such as salt-assisted roasting and microwave-assisted reduction aim to improve lithium extraction and minimize its environmental cost.³³⁶

The diverse materials utilized in LIBs have led to the creation of specialized recycling methods aimed at specific components, enhancing both efficiency and the recovery of value. Graphite, a common component of LIB anodes, is increasingly being recovered by supercritical fluid extraction, water-based separation, or thermal treatment. Recycling graphite is essential for lowering mining pressures and promoting circular supply chains, despite its lesser economic value than cathode metals. Cathode materials are the primary focus because of their high value. Pyrometallurgical and hydrometallurgical methods are the primary approaches in this domain. However, emerging alternatives such as phytomining, which employs plants for metal extraction, and bioleaching, which utilizes microorganisms to dissolve metals, offer low-impact solutions. These innovative techniques demonstrate potential for selective and energy-efficient recovery, although further research is still required to fully understand their capabilities. Lithium salts and organic solvents, two electrolyte components that are frequently lost during high-temperature processing, are receiving more attention for recovery. Electrolyte recovery can be achieved through methods such as liquid–liquid extraction and supercritical CO₂. Additionally, the use of green solvents for the recovery of binder components, such as polyvinylidene fluoride (PVDF), enhances the sustainability of the overall process.^337,338

Despite technological progress, the recycling of LIBs continues to encounter significant challenges. The diversity in cell designs and chemical compositions complicates automated disassembly and the establishment of standardized processing methods. Additionally, fluctuating prices of metals and elevated processing costs undermine the economic viability of recycling initiatives. Stringent environmental regulations are necessary due to the potential for hazardous waste and emissions from certain procedures. In many regions, ineffective collection methods and a lack of consumer awareness further impede the supply of feedstock. Variations in global e-waste legislation also affect investment and implementation in recycling efforts. To address these challenges, collaboration among manufacturers, recyclers, and policymakers is essential. Implementing extended producer responsibility regulations and designing batteries with recycling in mind could catalyze systemic change.³³⁹ Integrating various technologies to achieve high recovery rates with minimal environmental impact is key to the future of LIB recycling. Hybrid methods that combine hydrometallurgy for purification and pyrometallurgy for bulk processing are becoming more popular. With further study, it is also anticipated that solvometallurgical processes employing DES and other environmentally friendly solvents would expand. Emerging technologies for improving recycling efficiency include blockchain for material tracking, machine learning for sorting, and automation for battery disassembly. Additionally, battery producers increasingly focus on closed-loop recycling, which involves reusing recovered materials in new batteries. The transformation of LIB recycling from a specialized sector into a widely accepted pillar of the circular economy would require investments in infrastructure, public–private collaborations, and standardized rules. A circular, zero-waste battery economy is attainable with coordinated innovation, policy, and industry cooperation activities.^340,341

5.2 Safety concerns

LIBs have received the utmost attention in meeting the energy demands of the electronic automobile industries. However, at the same time, they have safety issues because of combustible materials. Fires and explosions of lithium-ion batteries have caused global concerns because they can lead to severe harm and damage. For example, in 2020, a Tesla ignited and detonated on the highway following a collision. Such incidents can cause severe harm and damage not only to the environment but also to human life. So, safety issues can never be neglected while discussing the role of a material in battery performance. The main reasons of battery thermal runaway, which leads to fires and explosions, are mechanical damage, electrical faults, excessive heat, and internal short circuits. Mechanical abuse means when the battery is crushed or hit. It leads to direct contact between the electrodes, resulting in short circuiting and fire explosion.³⁴² Electrical abuse includes external short circuit (because of touching of wires with each other) and overcharging.³⁴³ Thermal abuse is because of exposure of the battery to high temperature for a sufficient length of time, leading to dangerous chemical reactions. Internal short circuit occurs because of malfunctioning inside the battery, causing direct contact between the opposite electrodes.³⁴⁴

The main components of the batteries that are prone to combustion are electrodes, electrolyte and separators. Substances mostly used as cathodes are transition metal oxides and transition metal sulfides³⁴⁵ while carbon, nano-carbon, alloy materials and metal oxides are used as anodes.^346,347 So, while choosing materials for cathodes and anodes, safety issues should also be considered.

Anodes, though a key component of the battery, are prone to failures because of several reasons, which can lead to thermal runaway or combustion. Due to overcharging, lithium deposition on the surface of the anode occurs, which causes dendrite formation leading to short circuit and combustion.³⁴⁸ High temperature causes decomposition of the solid electrolyte interface, which triggers unwanted chemical reactions leading to thermal runaway.³⁴⁹ Li-ion movement can cause mechanical stress on the anode material, leading to cracks in the material and deformation. Loss of anode material also occurs because of electric shock, which occurs when the anode materials lose contact with other battery components, leading to increased resistance.

Various strategies are being adopted to enhance the safety of electrode materials. The key role here is of the solid electrolyte interface present on the surface of the anode. This SEI layer directly or indirectly affects the safety of lithium-ion batteries. If this layer is unstable, it can lead to dangerous side reactions, leading to thermal runaway or failure of the battery. So, one of the possible solutions to improve safety is to create an artificial SEI layer. It can be done by coating the anode with some heat-resistant material. It can be an Al₂O₃ layer or a polymer with a high value of the thermal expansion coefficient. Another way to create this layer is by adding certain additives in the electrolyte. These additives help build the SEI layer. However, one problem with the SEI layer is that it may improve safety, but at the same time, it reduces battery capacity. So future research should aim to devise ways to develop SEI layer which not only improves the safety but also does not provide harm to the energy storage capacity.³⁵⁰

Cathode materials can also cause safety issues because of thermal instability, heat accumulation and structural collapse. At high temperatures, chemical reactions occur between the cathode and the electrolyte, which produces heat. Once heat is produced, it cannot escape easily, so accumulation of heat occurs, which causes damage to the battery. Moreover, removing lithium ions from the cathode weakens the cathode's structure, reducing the battery's capacity and its life span. Various modifications are being adopted to improve the safety issues of cathode materials. These include doping and surface/structure modification. Doping can prevent structural collapse and reduce oxygen release and thermal instability by suppressing unwanted phase transition and reducing voltage degradation. Surface modification or coating prevents side reactions which reduce oxygen release. Cathodes with core–shell morphology minimize abrupt structural changes leading to stress-induced cracks preventing side reactions and increasing thermal stability. Single-crystal cathodes are also being made which are resistant to oxygen release and enhance thermal stability. These and many other innovations can improve the safety of cathode materials by making them thermally stable and resistant to unwanted side-reactions. However, it has also been found that cathode materials that improve safety lower the energy density. For example, LFP is the safest but has the lowest energy density.³⁵¹ So, future research should aim to make cathode materials that balance safety and energy density.³⁵⁰ Fig. 13 illustrates the safety issues in Li-ion batteries.

Fig. 13 Safety issues associated with Li-ion batteries.

5.3 Future batteries

The hunt for effective, sustainable, and scalable energy storage technologies has been sparked by the exponential rise in the world's energy consumption and the pressing need to slow down climate change. LIBs, which power everything from smartphones to electric vehicles, have long dominated the technological scene. However, issues like scarce lithium supplies, exorbitant prices, safety worries, and environmental effects are driving a big change in the battery sector.³⁵² This shift directs scientific research and industry attention towards sodium-ion batteries (SIBs), solid-state batteries (SSBs), and other future battery technologies to achieve more excellent performance, sustainability, and cost-effectiveness.³⁵³ Like lithium-ion batteries, Na⁺ move between the anode and cathode during charging and discharging in sodium-ion batteries. However, regarding raw material availability and cost, using sodium—an element significantly more abundant and equally distributed than lithium—offers a decisive advantage. Because of this, SIBs are especially appealing for integrating renewable energy sources and grid-scale storage. Unlike LIBs, which require more expensive copper for the anode, SIBs are compatible with aluminium current collectors for both electrodes. This significantly lowers the battery's weight and total production cost.³⁵⁴

Despite these benefits, SIBs still have technical issues, especially with material stability and energy density. Because sodium ions are heavier and bigger than lithium ions, their energy density is lower and their ionic diffusion is slower. The longevity of batteries is further limited by the structural deterioration that cathode and anode materials frequently experience from repeated cycles of charging and discharging. The creation of suitable electrolytes that guarantee safe and effective ion transport represents another major challenge.³⁵⁵ Researchers have investigated a range of cathode and anode materials in order to address these issues. Although they have large capacities, Layered Transition Metal Oxides (LTMOs) such as Na_xTMO₂ have phase instability as a drawback for cathodes. Despite problems with crystal water and flaws, Prussian Blue Analogues (PBAs) have an open structure that facilitates Na⁺ diffusion. Despite their reputation for structural stability, polyanionic compounds like Na₃V₂(PO₄)₃ have relatively low energy densities. Hard carbon has become the most promising material on the anode side because of its consistent performance, extended cycle life, and high capacity. Other options, such as titanium-based compounds (such as Na₂Ti₃O₇), provide safer voltage profiles but necessitate additional cycling stability improvements. Despite being environmentally beneficial and sustainable, organic materials are hampered by problems with solubility in traditional electrolytes. SIBs are best suited for uses like stationary energy storage systems, renewable energy grids, and low-speed electric cars where cost, safety, and resource sustainability considerations outweigh the necessity for a very high energy density.³⁵⁶

With the promise of improved safety, energy density, and cycle life, the idea of all-solid-state batteries (ASSBs), which substitute solid-state electrolytes (SSEs) for flammable liquid electrolytes, is gaining popularity. All-Solid-State Sodium Batteries (ASSBs) combine the inherent safety and compactness of solid-state designs with sodium's cost-effectiveness and resource advantages in sodium-ion chemistry. ASSBs solve several classic battery safety issues, including thermal runaway and electrolyte leakage. They also encourage the adoption of metallic sodium anodes, which have far greater capacities than traditional anode materials in theory. This creates opportunities to increase significantly the gadget lifetime and energy density.³⁵⁷ However, ASSB development is still in its early phases and confronts significant engineering and scientific obstacles. One of the primary problems is that many solid electrolytes have limited ionic conductivity at normal temperature, which reduces the effectiveness of charge and discharge. Furthermore, poor contact and excessive interfacial resistance frequently plague the interface between solid electrodes and solid electrolytes, reducing battery performance and cycle life. Because many SSEs are brittle and prone to cracking under stress, particularly while cycling, mechanical instability is another issue.³⁵⁸

Research on ASSBs has focused on three primary types of solid-state electrolytes: oxide-based electrolytes, sulfide-based electrolytes, and polymer-based electrolytes. The production of batteries is made more difficult by oxide-based electrolytes, such as NASICON and β-alumina, which are known for their great mechanical strength and chemical stability but frequently need high-temperature sintering. Sulfide-based electrolytes, such as Na₃PS₄, have strong ionic conductivities but are also moisture-sensitive and can emit harmful gases when exposed. Polymer-based electrolytes are versatile and easy to produce but often display poorer ionic conductivities and limited heat stability. Notwithstanding these problems, ASSBs are seen to be perfect for uses requiring high performance and safety, like defence technology, aerospace systems, and high-end electric cars. To get around the drawbacks of individual components and strike the ideal balance between flexibility, conductivity, and stability, hybrid solid electrolytes—made up of a blend of polymers and ceramics—are also being created.³⁵⁹

Innovative ideas that go beyond conventional notions influence the direction of battery technology. Anode-free sodium batteries (AFSBs) are one such idea that represents a daring new design paradigm. These solutions save battery weight, volume, and production complexity by eliminating the requirement for a pre-installed anode. During charging, sodium is instead plated straight onto the current collector. To guarantee safe and effective functioning, however, problems, including uneven sodium deposition and dendritic formation, must be resolved. An additional potential approach is the use of metal–organic framework-derived anodes, which improve the electrochemical performance of SIBs by providing high porosity and conductivity. These materials can be designed for good ion transport and structural stability and show excellent tunability. Hybrid solid electrolytes are also becoming more popular to close the gap between solid-state and conventional liquid-based batteries. These systems offer a compromise that overcomes the mechanical and interfacial limitations of traditional SSEs by combining the flexibility of polymers with the high conductivity of ceramics. Furthermore, the discovery, optimization, and scaling of battery materials and systems are entirely transformed by the combination of artificial intelligence, machine learning, and autonomous research platforms like the Battery Interface Genome-Materials Acceleration Platform (BIG-MAP). Researchers can now see battery degradation, interface reactions, and ion dynamics in previously unheard-of detail thanks to methods like operando spectroscopy, real-time sensing, and digital twinning.^360,361 The shift from lithium-ion to next-generation battery technologies signifies a paradigm shift in our understanding of energy storage, striking a balance between sustainability and performance, innovation and pragmatism, and advancement and environmental responsibility. The future of batteries promises to be not only more efficient but also safer, cleaner, and more widely available, thanks to ongoing research and global collaboration.^360–364

6. Conclusions, challenges and future prospects

Lithium-ion batteries have revolutionized energy storage, particularly in the realms of electric vehicles and renewable energy, due to their superior characteristics over conventional technologies. Investigating nanomaterials, alternative organic compounds, and innovative metal alloys presents considerable potential for enhancing the performance of both cathodes and anodes. With continued innovation, the next generation of LIBs hold great promise to meet the growing global energy demands and contribute to sustainable, high-efficiency energy storage solutions. Significant research efforts are focused on improving these systems' safety, longevity, and performance. A promising development area is solid-state batteries, which address the hazards associated with liquid electrolytes that can lead to fire risks. These batteries provide higher energy densities and increased lifespans but are still under development. Recycling and second-life usage of LIBs are also of great concern to the inter-operability and compatibility of electric power trains. Indeed, effective mechanisms for recycling minimize the flow of material waste back into the natural environment and the need to mine for raw materials. In the same way, reusing batteries used in EVs for less demanding roles, such as energy storage for renewable power systems, will also help give batteries a longer life and make the whole process more sustainable. There are future possibilities for LIBs in terms of battery materials. Alternative materials for graphite are being investigated – lithium–sulfur or silicon anodes that, for instance, can have higher capacities and longer-cycle life. Emerging advances in nanotechnology and manufacturing techniques can help herald affordable ways to produce high-performance batteries. In conclusion, LIBs face tough challenges; however, the research and the resulting technological innovations point to a bright future. Safety, sustainability, and performance enhancements will be decisive factors in addressing the demand for energy storage applications, such as consumer electronics, EVs, and renewable power systems. They will remain in use in the future and can even become an essential tool for managing energy storage and usage.

The key issue of LIBs is the safety. Their high energy density leads to rapid heating. In certain circumstances, damage or improper use of these batteries may result in fires or explosions. Consequently, addressing these safety issues necessitates advanced electronic systems to monitor and control battery temperature and voltage and the design of complex and intricate battery packs. Another issue of concern is the depletion of raw materials and their environmental consequences, particularly concerning lithium, cobalt, and nickel. The extraction and processing of these resources are expected to result in environmental challenges and ethical concerns, including the employment of child labour in cobalt mining. With the increasing use of Li-ion batteries, the stress on these ingredients increases, calling for a focus on green sourcing and recycling. The constantly diminishing performance is another challenge. As mentioned, LIBs degrade with each charge cycle due to phenomena such as the formation of layers of SEI and lithium plating. This deterioration shortens battery lifespan and impacts the efficiency of electronic devices and vehicles reliant on these batteries. Addressing this issue necessitates advancements in materials and battery management strategies.

Data availability

Data are available within the manuscript in the form of figures and tables. No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

Authors declare no conflict of interest.

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