Research on performance constraints and electrolyte optimization strategies for lithium-ion batteries at low temperatures

Abstract

Lithium-ion batteries (LIBs) are extensively utilized in electronic devices, electric vehicles, and energy storage systems to meet the growing energy demand, due to their high energy density, extended lifespan, and absence of the memory effect. However, their high performance is significantly diminished at low temperatures. Recent research indicates that the low-temperature performance of LIBs is constrained by the sluggish diffusion of Li⁺ in the electrolyte, across the interfaces, and within the electrodes. At lower temperatures, the rise in electrolyte viscosity results in a slower ion transport rate, which is a key factor affecting battery performance. The electrolyte primarily consists of lithium salts, solvents, and additives, and improvements in these three aspects are crucial for the creation of electrolytes with excellent low-temperature performance. This review systematically introduces the factors responsible for the decline in LIBs performance at low temperatures, including reduced ionic conductivity in the electrolyte, increased Li⁺ desolvation energy in the electrolyte, slow transfer kinetics at the interface, on the anode significant lithium plating and dendrite formation, and slow Li⁺ diffusion within the electrode material. Advancements in research on lithium salts, solvents, additives, and novel electrolytes are methodically presented, comprising localized high-concentration electrolytes, weakly solvating electrolytes, liquefied gas electrolytes, and polymer electrolytes. Finally, the challenges that must be addressed in current low-temperature LIBs are identified, and potential future developments in this field are anticipated.

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1. Introduction

As global energy demand continues to rise, the excessive use of fossil fuels poses a significant threat to environmental sustainability. Environmental issues such as global warming and the depletion of natural resources, which are linked to fossil fuels, have attracted widespread attention. In response to these challenges, the scientific community is actively researching efficient energy systems to replace traditional internal combustion engines. Lithium-ion batteries (LIBs) have garnered considerable attention in recent years as a potential alternative energy source. LIBs have successfully become the leading power source for portable electronics over the past few decades and are now widely used in electric vehicles, with the potential to become a beneficial substitute in reducing fossil fuel dependency and promoting sustainable energy development in the future.^1–4 LIBs possess advantages such as high energy density, extended cycle life, and no memory effect.^5,6 However, they exhibit significant sensitivity to temperature fluctuations. Their operating temperature range is relatively narrow, with discharge temperatures limited to −20 °C to 50 °C and charging temperatures limited to 0 °C to 45 °C.⁷ For example, in regions with high altitudes, high elevations, or extreme latitudes, the performance of LIBs is particularly affected by low temperatures. Another field where low-temperature environments pose challenges for LIB applications is aerospace, where temperatures on Mars can drop to −120 °C.⁸ Especially during charging under low-temperature conditions, lithium dendrites may form on the anode, potentially piercing the separator and causing internal short circuits within the battery. This phenomenon can lead to violent reactions in the electrolyte, generating excessive heat and gas, increasing internal pressure, and even causing severe safety issues such as fires or explosions.^9,10 Therefore, in addressing the practical requirements of LIBs, it is essential not only to pursue higher energy density but also to consider a broader operating temperature range and reliable safety measures.

Temperature is a vital aspect of determining the stability and safety of LIBs. Currently, commercial LIBs utilizing ethylene carbonate (EC) and graphite anodes perform poorly at low temperatures (Fig. 1),¹¹ primarily due to three factors: (1) in low-temperature environments, the ionic conductivity of liquid electrolytes decreases significantly, and viscosity increases;^12,13 (2) electrochemical impedance increases, particularly charge transfer impedance (R_ct) and solid electrolyte interface (SEI) impedance (R_SEI). At low temperatures, R_ct and R_SEI account for the majority of the total impedance of LIBs, significantly reducing the desolvation rate of Li⁺ in the electrolyte and the diffusion speed at the electrolyte–electrode interface;^14–17 (3) the migration and diffusion rates of Li⁺ in electrode materials slow down considerably at low temperatures.¹⁸ The combined effect of these three factors leads to a marked decline in the performance of LIBs under low-temperature conditions. Moreover, lithium metal deposition on the anode is more likely to occur during low-temperature charging, especially at high rates. This exacerbates the irreversible reactions between lithium metal and the electrolyte, leading to electrolyte disintegration and the formation of additional SEI layers. Subsequently, the SEI on the electrode surface thickens, resulting in higher impedance and greater polarization of the LIBs. These side reactions accelerate the growth of lithium dendrites, which can lead to both decreased LIBs performance and potentially increased safety risks.^19–22 Li et al.²³ were the first to introduce low-temperature electron microscopy for observing and non-destructively analyzing the architecture, composition, and morphology of the SEI and lithium dendrite on the anode. This innovative method provides critical support for accurately assessing the repercussions of electrolytes on SEI formation. Given that the Li⁺ desolvation process in the electrolyte cannot be directly characterized by instruments, molecular dynamics simulations offer an effective means to deeply analyze the desolvation process along with the solvation structure in the electrolyte.^24,25 To ensure excellent performance of LIBs under low-temperature conditions, developing new electrolytes characterized by low viscosity, rapid Li⁺ desolvation rates, high ionic conductivity, and stable SEI formation is imperative.

Fig. 1 Limitations encountered by lithium-ion batteries in low-temperature environments.

In this review, we investigate the primary factors responsible for the performance decline of LIBs under low-temperature environments. Our focus is on the analysis of Li⁺ transport processes within the electrolyte, at the electrolyte–electrode interface, and throughout the electrode materials, to clarify the factors affecting LIBs' low-temperature performance. Subsequently, it explores methods to boost the performance of LIBs in low-temperature conditions by optimizing the electrolyte system, with a focus on lithium salts, solvents, and additives. Additionally, the review introduces some novel electrolyte systems, encompassing localized high-concentration electrolytes, weakly solvating electrolytes, liquefied gas electrolytes, and solid polymer electrolytes. Finally, it summarizes the challenges of preparing high-performance LIBs electrolytes under low-temperature conditions and discusses the future development directions for LIBs electrolytes.

2. Factors affecting the low-temperature performance of LIBs

As the ambient temperature decreases, there is a sharp drop in ionic conductivity in the electrolyte, resulting in slow Li⁺ transfer; increased impedance at the electrolyte–electrode interface and limited desolvation kinetics; and decreased diffusion capacity of Li⁺ in the electrode material. These are the main factors for the increased polarization and capacity decay of LIBs in low-temperature environments. In addition, uneven lithium metal deposition occurs on the electrode interface, which triggers excessive decomposition of the local electrolyte, resulting in a thick, organic-rich SEI in LIBs throughout charge and discharge processes, ultimately harming the electrode and exhausting the electrolyte.^8,26–28

2.1. Li⁺ transport in electrolyte

In low-temperature environments, the ionic conductivity of LIBs electrolyte is essential since it determines the Li⁺ transport speed within the electrolyte, further affecting the performance of LIBs in low-temperature environments.²⁹ To probe the factors influencing ionic conductivity under low-temperature conditions, researchers have used molecular dynamics simulations to identify viscosity as the primary factor affecting ion migration within the electrolyte.³⁰ The relationship between viscosity and ionic conductivity can be determined through a defined equation. Ionic conductivity, which represents the ability of ions to conduct in the electrolyte, is defined by the following equation:³¹

σ = Σ_in_iμ_iZ_ie

where σ represents ionic conductivity, n_i is the number of free ions, μ_i is the ion mobility, Z_i is the charge number of the ion, and e is the elementary charge of an electron. The ionic conductivity of the electrolyte, as derived from the equation, is mainly influenced by ion mobility (μ_i), the number of free ions (n_i), and the charge state (Z_i). For the composition of the electrolyte, the number of free ions (n_i) and the charge state (Z_i) remain constant, while ion mobility (μ_i) can be represented as:³¹

μ_i = 1/6πηr_i

where η is the electrolyte viscosity and r_i is the solvated ion radius. From the above formula, it can be seen that the ion mobility has an inverse relationship with the electrolyte viscosity, that is, the ion conductivity is also inversely related to the electrolyte viscosity.

However, the decrease in temperature causes an increase in electrolyte viscosity, which consequently leads to a decline in ionic conductivity (Fig. 2a).³² The solvation structure within the electrolyte is also subject to temperature variations. As the temperature decreases, the molecular dynamics slow down, making molecular interactions increasingly difficult to disrupt. This phenomenon is reflected in a macroscopic reduction in the volume of the solvated structure. When this volume undergoes significant compression, further reductions become progressively more difficult. The coefficient of thermal expansion (TEC) quantifies the change in volume associated with each 1 °C variation at a specific temperature and serves as a measure of looseness among the various components within the electrolyte. A higher TEC suggests that the electrolyte can be compressed more easily, implying longer interaction distances between its constituents. Conversely, a lower TEC indicates a denser state characterized by relatively tighter interactions.³⁵ As temperature decreases, the TEC diminishes due to reduced distances between components, resulting from the strong intermolecular attractions present in commercial electrolytes (Fig. 2b).³³ This reduction not only impacts ion mobility but also increases the energy barriers during desolvation processes, thereby affecting the performance of LIBs under low-temperatures conditions. In addition, the dielectric constant is an important factor influencing electrolyte performance. It indicates the ability of solvent molecules to polarize under an external electric field, reflecting the extent to which a substance responds to such a field. The dielectric constant has a significant influence on the interactions within the solution, which is closely linked to the electrolyte's physicochemical properties.³⁶ In most cases, electrolytes with higher dielectric constants tend to exhibit higher ionic conductivity. However, the strong intermolecular forces inherent to high-dielectric solvents often lead to increased viscosity. Therefore, achieving optimal ionic conductivity must strike a balance between electrolyte viscosity and dielectric constant (Fig. 2c).^32,37

Fig. 2 (a) Curves of electrolyte viscosity and conductivity with temperature. Based on ref. 32. (b) TEC at different temperatures of the electrolyte. Based on ref. 33. (c) The relationship between solvation structure and physical properties of low temperature electrolyte. Based on ref. 32. (d) Optical image of the electrolyte at low temperature. Based on ref. 34. (e) Tendency of solvent in EC-based electrolyte to enter the Li⁺ solvated sheath at low temperature. Based on ref. 33.

Besides ionic conductivity, the freezing point of the electrolyte is another significant factor. Under low-temperature conditions, the freezing point of the electrolyte can influence the performance and reliability of the battery. Most commercial carbonate-based electrolytes possess a high freezing point, which means they solidify at low temperatures, rendering them unsuitable for use in cold environments (Fig. 2d).³⁴ Therefore, developing electrolytes with low viscosity and low freezing points is particularly crucial.³⁸ The performance of LIBs is impacted by the desolvation of solvated Li⁺ in the electrolyte, especially at extreme temperatures, where the energy barrier for Li⁺ desolvation predominates. Desolvation involves the removal of solvent molecules from the solvation shell of Li⁺, enabling its insertion into the electrode. This process is highly temperature-dependent. At lower temperatures, the reduced molecular mobility slows down the desolvation kinetics, and the desolvation process of Li⁺ becomes kinetically slower. Thus, optimizing the solvation structure of Li⁺ by adjusting the coordination strength of Li⁺ with solvents or anions is vital.³⁹ The interactions between electrolyte components, specifically ion–ion and ion–solvent interactions, are classified as electrostatic interactions. According to classical physics principles, these two types of electrostatic forces can be described by the following equations:^33,40,41

U_ion–ion = −1/4πε × Z₁Z₂e²/r

U_ion–dipole = −1/4πε × Zeμ cos θ/r²

where U is the electrostatic potential energy, ε is the dielectric constant, r is the distance, μ is the molecule dipole moment of the solvent, and θ is the angle between the ion and the dipole center. In the electrolyte, the solvation structure of Li⁺ is primarily governed by the interactions between the anion and the solvent, with the observed equilibrium resulting from these interactions at room temperature. Although a reduction in temperature does not directly affect these interactions, it decreases the overall volume of the electrolyte, thereby shortening the distance r between its components and enhancing the interactions. According to the aforementioned formulas, the ion–ion electrostatic potential energy varies with r⁻¹, while the ion–dipole interaction varies with r⁻², indicating that as the temperature decreases, the increase in the latter interaction outpaces that of the former. Consequently, the solvent is likely to dominate in its competition with anions, gaining access to the inner solvation shell of Li⁺. Additionally, the dipole moment of the solvent constitutes a vital element in determining the strength of the dipole interaction. EC possessing a higher dipole moment, enhances ion–dipole interactions, further promoting the tendency for solvents to enter the Li⁺ solvation sheath at lower temperatures (Fig. 2e). Conversely, when employing solvents with lower dipole moments, anions are more capable of entering the inner solvation layer at low temperature.³³

Moreover, in lithium-ion battery electrolytes, solvent–solvent interactions have a profound impact on the overall performance of the electrolyte.⁴² These interactions not only regulate the arrangement and distribution of solvent molecules but also significantly influence the coordination environment of Li⁺ within the solvation structure. Specifically, the dipole–dipole interactions between solvent molecules can modulate the thermodynamic (redox stability) and kinetic (Li⁺ desolvation kinetics) properties of the electrolyte by altering the compactness of the solvation shell.^43,44 When strong interactions exist between solvent molecules, they tend to attract each other and form an orderly arrangement, thereby weakening the binding strength between individual solvent molecules and Li⁺. This occurs because the solvent molecules are more inclined to interact with each other rather than directly coordinate with Li⁺, simplifying the process by which Li⁺ detaches from the solvation shell. This effect helps reduce the desolvation energy barrier of Li⁺, making it easier for Li⁺ to migrate to the electrode surface for electrochemical reactions. By tuning solvent–solvent interactions, the electrolyte's solvation structure can be optimized to facilitate efficient Li⁺ desolvation at low temperatures, enhancing battery performance while also preventing Li⁺ and solvent co-intercalation into the graphite anode, thereby protecting electrode integrity and improving overall battery stability.⁴⁵ Xie et al.⁴⁶ designed a PC/CPME (PC: propylene carbonate; CPME: cyclopentylmethyl ether) electrolyte and demonstrated that solvent–solvent interactions play a crucial role in regulating the thermodynamic and kinetic properties of the electrolyte. The presence of interactions between PC and CPME was confirmed through two-dimensional nuclear magnetic resonance (NMR), which helps in modulating the Li⁺-solvent/anion interactions and the Li⁺ solvation structure. This modulation significantly reduces the desolvation energy barrier of Li⁺(PC)_x(CPME)_y clusters and increases the energy gap between the orbitals of Li⁺(PC)_x(CPME)_y clusters. Consequently, it achieves rapid Li⁺ desolvation kinetics and excellent thermodynamic reduction stability. Additionally, the electrode interfacial behavior is significantly improved, with the graphite‖LiFePO₄ full cell retaining a high specific capacity of 112 mA h g⁻¹ even at 5C.

Building upon these insights, solvent–solvent interactions can be further optimized to enhance the compatibility of electrolytes with alloying anodes such as antimony (Sb), which typically suffer from issues like large volume expansion during cycling.^47,48 While these interactions do not directly reduce the volume expansion of alloying anodes, they help mitigate the associated issues by improving the overall stability of the electrolyte and reducing side reactions at the interface between the electrolyte and the anode material. For example, the improved solvation structure allows for better Li⁺ desolvation and more efficient charge transfer, even at low temperatures, which is particularly crucial for high-energy-density anode materials like Sb.⁴⁹ Moreover, these interactions help preserve the integrity of the SEI layer, enabling more efficient migration of Li⁺, thus enhancing the cycling stability and rate performance of the battery. By maintaining a stable SEI layer despite the volume changes in alloy-based anodes, this approach contributes to preventing the degradation of the anode material and improving the overall battery life.^50,51

In conclusion, optimizing solvent–solvent interactions within the electrolyte is a promising strategy to enhance the electrochemical performance of LIBs, especially for anode materials such as antimony. This optimization not only improves the battery's performance at low temperatures but also enhances the long-term stability of the electrode–electrolyte interface, thereby improving cycling performance and efficiency, ensuring a more stable and efficient battery system.

2.2. Li⁺ transport at the electrolyte–electrode interface

Lithium dendrites are a significant factor in the degradation of lithium metal and graphite anodes, potentially causing internal short circuits and subsequent thermal runaway within batteries, thus significantly impacting their performance.^20,52 The formation of lithium dendrites is notably exacerbated in low-temperature environments. This phenomenon can be ascribed to the reduced migration rate of Li⁺ ions as the temperature decreases, resulting in a relatively weaker electric field formed on the electrode surface.⁵³ An increase in overpotential is necessary to maintain sufficient electric field strength for facilitating Li⁺ migration. However, an increase in overpotential brings about the formation of smaller and more numerous crystal nuclei on the electrode surface. This is because overpotential is inversely proportional to the size of the nuclei, and directly proportional to their number and density. Consequently, at low temperatures and low ion migration rates, numerous small-sized nuclei readily form and spatially cluster to create dendrites.⁵³ In low-temperature environments, the formation mechanism of the SEI is significantly affected. The decomposition rate of solvents in the electrolyte decreases, but due to the low potential of the anode, some incomplete reduction reactions still occur. These reactions generate byproducts, such as inorganic compounds (Li₂CO₃ and LiF) and organic compounds (ROCO₂Li), which gradually deposit on the electrode surface, forming a non-uniform and porous SEI layer.^54,55 At low temperatures, the decreased mobility of solvent molecules leads to slower SEI formation and results in a looser, more porous structure, increasing its non-uniformity. Since the ionic conductivity of the SEI is relatively low, an excessively thick SEI layer can significantly raise the impedance of the battery, thereby reducing its overall performance. Additionally, the slower desolvation process of Li⁺ at low temperatures can cause more solvent molecules to be embedded in the SEI layer, leading to swelling and reduced stability. This instability further weakens the protective function of the SEI.⁵⁶ As lithium dendrites grow, the SEI layer on their surface regenerates and thickens continuously, which increases internal resistance and lowers coulombic efficiency. The combined effects of these phenomena further degrade the performance of lithium-ion batteries and elevate safety risks. The lithium dendrites at the electrode interface can be observed via low-temperature electron microscopy. The growth of these dendrites at the interface can also be assessed using low-temperature electron energy loss spectroscopy (Fig. 3a–c).^23,57 Maraschky et al.⁶¹ propose that a reduction in the coefficient of Li⁺ diffusion and a decrease in the maximal Li⁺ migration concentration within the SEI both promote the development of lithium dendrites. Particularly under cold conditions, the continuous emergence of lithium dendrites at the electrode surface not only has the likelihood of penetrating the separator, giving rise to a short circuit in the LIBs, but can also persistently prompt SEI formation, which covers the dendrites. This results in a continuous increase in the thickness of the SEI, leading to a higher R_SEI and a lower coulombic efficiency.

Fig. 3 (a and b) Low temperature transmission electron microscope (TEM) images of different systems of artificial graphite surface at low magnification. (c) Low temperature electron energy loss spectrum of Li 1s. Based on ref. 57. (d) Curve of R_ct and R_SEI of different electrolytes with temperature. Based on ref. 58. (e) Dendrite growth process in EC-based electrolyte. Based on ref. 59. (f) Impedance of EC-based electrolyte at different temperatures. (g and h) The activation energies of R_ct and R_SEI. Based on ref. 60.

Furthermore, desolvated Li⁺ must diffuse through the SEI/cathode-electrolyte interface (CEI) film to interact with the electrode. Compared to Li⁺ transport processes within the electrolyte, ion diffusion within the solid-state SEI/CEI film is significantly hindered at low temperatures. Therefore, enhancing the diffusion rate of ions within the SEI/CEI film is vital for improving the performance of LIBs in low-temperature environments. While research on the SEI film is well-established, studies on the CEI film are relatively scarce. The process of Li⁺ intercalation into the anode at the electrolyte–electrode interface can be subdivided into two steps: (1) Li⁺ transport through the SEI, and (2) charge transfer. At room temperature, the transit time of Li⁺ through the SEI is generally considered to be between 10⁻⁵ and 10⁻³ s, while the charge transfer time ranges from 2 to 100 s.⁵⁸ At low temperatures, both R_ct and R_SEI increase as the temperature decreases (Fig. 3d). The SEI thickens at low temperatures, further reducing its ionic conductivity. This is because SEI formed from common organic electrolytes is non-uniform and highly porous, primarily composed of ROCO₂Li and Li₂CO₃. These characteristics increase the impedance to interfacial Li⁺ migration, leading to spatially uneven Li⁺ flux, which further induces dendrite growth (Fig. 3e).⁵⁹ In carbonate-based commercial electrolytes below 0 °C, R_SEI significantly becomes the limiting step in transport rates.⁵⁸ Zhang et al.¹⁴ have shown that the temperature dependence of resistance is almost unaffected by the battery voltage, with R_ct increasing significantly as the temperature decreases. Below −20 °C, R_ct constitutes the majority of the total resistance in the battery, and the cyclic performance of LIBs is primarily limited by higher R_ct, mainly due to the slow kinetics of the battery reactions. The temperature dependence of R_ct is governed by the activation energy, following the thermally activated process described by the Arrhenius equation:⁶²

1/R_ct = A₀ exp(−E_a/RT)

where A₀ is a constant, E_a represents the activation energy, T is the temperature, and R is the gas constant. From the equation, it becomes apparent that the R_ct is determined by the temperature of the charge transfer process and its activation energy. A reduction in temperature combined with a rise in activation energy both lead to a higher R_ct. In particular, the impedance of EC-based commercial electrolytes increases significantly as the temperature decreases. Fitting the data to the Arrhenius equation clearly indicates that ion transport requires a high activation energy in both the charge transfer and SEI regions, which limits performance (Fig. 3f–h).⁶⁰ Thus, lowing the activation energy involved in the charge transfer process at low temperatures is the key to minimizing charge impedance. Nan et al.⁶³ developed an electrolyte based on LiFSI/EMC/TTE (molar ratio 2 : 3.3 : 3.3; LiFSI: lithium bis(fluorosulfonyl)imide; TTE: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether; EMC: ethyl methyl carbonate; denoted as DE), and the activation energies for the charge transfer and Li⁺ transport processes through SEI were obtained by fitting. These were notably lower compared to those of the commercial electrolyte composed of LiPF₆/EC/DMC (LiPF₆: lithium hexafluorophosphate; DMC: dimethyl carbonate), indicating enhanced kinetics of the charge transfer and Li⁺ transport process through SEI. This also shows that the impedance of Li⁺ transport within SEI is reduced with the lowering of its activation energy.

In summary, slow ion transport kinetics is the primary factor limiting the performance of LIBs in low-temperature environments. The fundamental reason is the enhanced interactions between ions and solvents in the electrolyte under cold conditions, particularly the significant increase in solvent–solvent interactions. Given this situation, adjusting the electrolyte system to ameliorate this condition is entirely feasible. This includes tweaking the electrolyte components to achieve a more suitable balance between ion transport rates and lithium salt solubility at low temperatures. Other strategies such as optimizing ion transport rates or controlling the fluidity of the electrolyte could also be explored to enhance low-temperature performance further.

2.3. Transport of Li⁺ within electrode materials

Ion transport within electrode materials involves the intercalation and deintercalation of Li⁺.⁶⁴ In the cathode, Li⁺ intercalates from the electrolyte into the cathode material and deintercalates back into the electrolyte during the charging process. Conversely, in the anode, Li⁺ intercalates from the electrolyte into the anode material and deintercalates back into the electrolyte during discharge. However, reductions in temperature significantly impair the diffusion capabilities of ions within the electrode materials.^65,66 Slow ion diffusion dynamics under low temperatures conditions can hinder the ions from reaching the active sites in the electrode materials, leading to a reduction in the capacity of LIBs.⁶⁷ Therefore, improving the transport kinetics of Li⁺ within electrode materials is a critical issue that needs addressing, involving aspects at both the cathode and anode materials.

In terms of anode materials, graphite is one of the most commonly employed anode materials in LIBs under low-temperature conditions. At low temperatures, the transport speed of Li⁺ within graphite layers is slow, leading to poor performance. Senyshyn et al.¹⁸ found that changes in the thermodynamic stability of graphite anodes at low temperatures can lead to increased energy barriers, potentially obstructing Li⁺ transport pathways and further limiting the performance of LIBs. Researchers have proposed various strategies to optimize the performance of graphite anodes. One common method involves regulating the graphite structure, such as by increasing interlayer distances to enhance Li⁺ transport kinetics. Li et al.⁶⁸ proposed a slightly expanded spherical graphite as the host material, and achieved a good balance between high capacity and long-term cycle stability in lithium ion/metal storage by precisely regulating the degree of graphite sheet expansion. This adjustment was achieved through a chemical lithiation-mild expansion process (Fig. 4a). After ethanol treatment, cracks appeared on the particle surface, and under H₂ gas pressure, the cracks in the layered structure further expanded. The degree of expansion was influenced by the gas flow rate and volume, with different lithium levels leading to varying degrees of expansion (Fig. 4b). The expanded graphite anode exhibited a specific capacity 1.5 times that of conventional graphite, with significantly increased capacity (96 mA h g⁻¹), even when cycled at 0.2C and −20 °C, and a capacity degradation of only 0.05% per cycle. Zhao et al.⁷¹ used an oxidation modification method to increase the interlayer spacing of mesocarbon microbeads (MCMB), facilitating Li⁺ transport kinetics. The modified MCMB electrode material still attained a specific capacity of 100 mA h g⁻¹ at −40 °C, effectively improving its electrochemical performance under low temperatures. Dendritic tubular nitrogen-doped graphite anodes were fabricated using chemical vapor deposition (Fig. 4c). By introducing C–N functional groups with predominantly pyridine and pyrrole defects, the material formed curved knots and expanded plane branches. By introducing nitrogen atoms, the conductivity of the material was improved, along with an increase in the interlayer distances. This design significantly promoted Li⁺ intercalation, deintercalation, and diffusion kinetics at low temperatures, thereby enhancing cyclic performance (Fig. 4d).⁶⁹ Moreover, the embedding of nanoscale tin can increase the distance between graphite layers, thereby improving the rate of Li⁺ transport and enhancing the performance of the battery under low-temperature conditions.⁷² Another method to overcome the drawbacks of graphite anodes is using alternative anode materials, for instance Li₄Ti₅O₁₂.⁷³ Compared to graphite, it has good reversibility of Li⁺ intercalation/deintercalation, while accompanied by significant volume changes. Although the volume change might increase mechanical stress between the electrode material particles, negatively affecting the cyclic performance of LIBs, the structural stability and high reversibility of Li₄Ti₅O₁₂ help mitigate this impact. The high working voltage of 1.55 V means that during charging, the voltage of LIBs is unlikely to drop below the lithium plating potential, which reduces the likelihood of lithium dendrite formation and enhancing safety. However, Li₄Ti₅O₁₂ electrode materials have poor electronic conductivity and small Li⁺ diffusion coefficients, which severely limit their electrochemical kinetics, especially at low temperatures.⁷⁴ Ho et al.⁷⁰ embedded Li₄Ti₅O₁₂ nanoparticles into a hierarchical macropore–mesoporous shell carbon network (HCMS carbon), producing Li₄Ti₅O₁₂/HCMS carbon nanocomposite materials (Fig. 4e). HCMS carbon is an ideal carrier for Li₄Ti₅O₁₂, which provides short and interconnected Li⁺ diffusion paths between Li₄Ti₅O₁₂ particles and the Li⁺ storage layer, as well as sufficient channels for continuous, multidirectional electron flow into Li₄Ti₅O₁₂ particles, achieving excellent low-temperature performance at high rates. At −20 °C, the composite material still exhibits a specific capacity of 42 mA h g⁻¹ at 10C, which is 6.4 times that of commercial Li₄Ti₅O₁₂ (Fig. 4f). In summary, by regulating the graphite structure, embedding nanoparticles, and using alternative anode materials, the performance and stability of LIBs in low-temperature environments can be effectively improved. These strategies provide effective means to overcome the limitations of graphite anodes under low temperatures and offer significant insights into enhancing the performance and safety of LIBs in low-temperature conditions.

Fig. 4 (a) Diagram depicting the synthesis process of slightly expanded spherical graphite. (b) Scanning Electron Microscope (SEM) images of different degree of lithium. Based on ref. 68. (c) Branched nitrogen-doped graphite tubular foam anode and proposed lithium storage channel. (d) Performance during cycling of BNG tubular foam at −10 °C at 0.1C current density. Based on ref. 69. (e) Cross-section diagram of Li₄Ti₅O₁₂/HCMS carbon nanocomposite. (f) Multiplication performance of Li₄Ti₅O₁₂/HCMS carbon nanocomposites and commercial Li₄Ti₅O₁₂ at −20 °C. Based on ref. 70.

In low-temperature environments, cathode materials of LIBs also face a series of challenges, affecting the performance and stability of LIBs. To optimize electrochemical performance in cold temperatures, particularly improving the ionic/electronic conductivity and structural stability of cathode materials, various modifications are made to LIBs cathode materials. Surface coatings represent a viable approach to enhance the performance of the cathode material. Research shows that the introduction of small amounts of surface coatings can significantly enhance the conductivity of LIBs, reduce internal impedance, and mitigate side reactions at the electrode–electrolyte interface, thus efficiently improving the charge/discharge capacity and boosting the energy density of LIBs in low-temperature environments.^75,76 Chen et al.⁷⁷ used Ketjen Black (KB, EC-600JD) carbon as a carbon source to coat Li₃V₂(PO₄)₃ for the synthesis of a nanocomposite material. The KB carbon coating optimized the surface structure of the electrode and increased the contact area of the electrolyte with electrode, thereby promoting Li⁺ transport rate. It also protected the material structure, achieving a discharge capacity of 92 mA h g⁻¹ at a 4C rate at −30 °C. Additional innovative coating methods have been presented to further leverage the benefits of the carbon protective layer, such as using carbon nanotube-based coatings and forming bridging networks of graphene nanofibers.^78,79 Oxide coatings also help enhance the electrochemical performance of cathode materials under low-temperature conditions. For example, using Li₂O–B₂O₃ coatings on LiNi_1/3Co_1/3Mn_1/3O₂ significantly reduced R_ct and increased the discharge capacity from 37.2 to 101.9 mA h g⁻¹ at −20 °C, with a capacity retention rate of 93.4% after 50 cycles.⁸⁰ Besides carbon and oxides, additional materials have been researched as coating options, such as conductive polymers and external element doping.^62,81 In summary, improvements to cathode materials and the application of coating technologies can effectively enhance the performance of LIBs at low temperatures.

3. Improving low-temperature performance of LIBs by adjusting electrolyte composition

The electrolyte, as a critical component of LIBs, plays an essential role in ionic conductivity, solvation structure, and the formation capabilities of the SEI/CEI, significantly impacting the transport characteristics of Li⁺ between the anode and cathode. The limitations on Li⁺ transport in the electrolyte at low temperatures are considerably greater than those within the electrode materials, indicating that modifying the electrolyte is crucial for improving the performance of LIBs in cold environments. The low-temperature performance of the electrolyte is largely influenced by the viscosity and freezing point of the solvent, the extent of dissociation of the lithium salts, and the film-forming capability of additives. This section will discuss lithium salts, solvents, additives, and novel electrolyte systems from four perspectives.

3.1. Lithium salts

3.1.1. Optimizing single lithium salt. Lithium salts influence the performance of electrolytes under cold conditions by modifying the dissociation of anions and the formation capabilities of the SEI. Currently, LiPF₆ is extensively utilized as a commercial electrolyte lithium salt due to its ability to create a stable SEI on graphite anodes and its high ionic conductivity at room temperature. Nevertheless, when LiPF₆ is exposed to moisture, the resultant hydrofluoric acid corrodes the cathode active material, leading to capacity decay in LIBs.⁸² Therefore, finding alternative lithium salts becomes a pathway to improving low-temperature performance.

Research indicates that some lithium salts, such as lithium hexafluoroarsenate (LiAsF₆), lithium tetrafluoroborate (LiBF₄), lithium bis(oxalate)borate (LiBOB), and lithium difluoro(oxalate)borate (LiDFOB), exhibit good ionic conductivity and film-forming capabilities at low temperatures and have the capability to replace LiPF₆. Shiao et al.⁸³ found that a 1 mol per L LiAsF₆-EC/EMC/MA/TOL (MA: methyl acetate; TOL: toluene) electrolyte had an ionic conductivity of 1.830 mS cm⁻¹ at −40 °C and 1.100 mS cm⁻¹ at −50 °C. Although LiBF₄ has a lower ionic conductivity compared to LiPF₆, it can provide a smaller R_ct, making LiBF₄ a high-quality lithium salt for low-temperature LIBs electrolytes.⁸⁴ However, the hydrolysis sensitivity, relatively low ionic conductivity, and suboptimal film-forming capability of LiBF₄ hinder its widespread application.⁸⁵ Similarly, despite LiBOB has good film-forming ability, its high viscosity and poor solubility in carbonate solvents limit its application in low-temperature LIBs.⁸⁶ In contrast, LiDFOB brings together the benefits of LiBF₄ and LiBOB. Its chemical structure consists of half-molecules of both salts, enabling the formation of highly stable SEI films on the anode and better solubility in linear carbonate solvents, resulting in lower solution viscosity and thereby optimizing LIBs for better performance in cold environments.⁸⁷ Yang et al.⁸⁸ dissolved 1 mol per L LiDFOB in PC/EC/EMC, and at −30 °C batteries using LiDFOB-based electrolyte retained 86% of their capacity at 20 °C with lower polarization. However, LiDFOB continuously forms SEI at low temperatures, resulting in higher interface impedance. Hence, the performance of pure LiDFOB-based electrolytes at low temperatures remains limited. LiFSI and lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) have garnered considerable focus owing to their ideal thermal stability and lower R_ct.^26,88 Fan et al.⁸⁹ developed an ultra-low-temperature electrolyte comprised of 1.28 mol per L LiFSI-FEC/FEMC-D2 (FEC: fluoroethylene carbonate; FEMC: methyl (2,2,2-tri-fluoroethyl) carbonate; D2: tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane), which provided a capacity of 96 mA h g⁻¹ (56% of the capacity under room temperature conditions) at −85 °C in NCA‖Li batteries. Additionally, Xu et al.²⁶ developed an electrolyte comprised of 0.75 mol per L LiTFSI-DIOX (DIOX: 1,3-dioxane) with a freezing point below −100 °C, maintaining 60% of room temperature capacity at a 0.1C rate at −80 °C. However, it is important to note that FSI⁻ and TFSI⁻ anions can corrode the aluminum current collector of the cathode, worsening the performance of LIBs at both room temperature as well as in cold conditions.⁸² In summary, optimizing the selection of lithium salts in the electrolyte can significantly improve the performance of LIBs in cold conditions, thereby promoting their broader application in cold climate conditions.

3.1.2. Mixed lithium salt strategy. Although various lithium salts are being studied as alternatives to LiPF₆, they each have their own shortcomings and have not displaced LiPF₆ in commercial applications. Thus, blending multiple lithium salts is viewed as a potential strategy to enhance the performance of LIBs at low temperatures (Fig. 5a).^90,96 To address the inadequate film-forming capability of LiBF₄, Lv et al.⁹¹ dissolved a mixture of 0.3 mol per L LiBF₄ and 0.7 mol per L LiPF₆ in DMC/EMC/BA/EC (BA: butyl acrylate), which formed a porous and thin CEI on the cathode surface (Fig. 5b and c), facilitating Li⁺ transport and thereby augmenting the performance of LIBs in low-temperature conditions. At −40 °C, this electrolyte still achieved a discharge capacity of 85 mA h g⁻¹ during cycling. The LiBF₄–LiBOB (molar ratio 9 : 1) salt mixture developed by Zhang et al.⁹⁷ demonstrated superior performance in a PC/EC/EMC ternary solvent, enabling Li‖LiFePO₄ batteries to achieve a discharge capacity of up to 100 mA h g⁻¹ at 1C and −10 °C. Zhao et al.⁹⁸ explored the electrochemical behavior of different molar ratios of LiDFOB and LiBF₄ mixed salts. Results showed that the LiDFOB/LiBF₄ (molar ratio 1 : 1)-EC/DEC/DMS (DEC: diethyl carbonate; DMS: dimethyl sulfite) electrolyte system performed best at −40 °C, reaching a discharge capacity of 82.5 mA h g⁻¹, and maintaining 55.7% of the capacity recorded at room temperature and showing nearly 100% capacity retention after 50 cycles.

Fig. 5 (a) Advantages of mixed lithium salts in electrolytes. Based on ref. 90. (b and c) TEM analysis of various electrolytes following 50 cycles at −40 °C. Based on ref. 91. (d) Schematic diagram of forming passivation film on aluminum electrode using electrolytes of different lithium salts. Based on ref. 92. (e) Comparison of electrolyte conductivity at various temperatures. (f) Electrochemical performance of Li‖NCM523 battery at −40 °C when charged to 4.6 V with current density of 1C. Based on ref. 93. (g) Optical and SEM images at the graphite anode at −15 °C. Based on ref. 58. (h) −40 °C, discharge capacity of batteries with different carbonate based electrolyte. Based on ref. 94. (i) Schematic diagram of the change of commercial binary and designed decimal high entropy electrolyte with temperature decreasing. Based on ref. 95.

Concerning the issue of LiTFSI and LiFSI causing corrosion to the aluminum current collector of the cathode, Park et al.⁹² proposed a solution using an electrolyte comprised of 0.8 mol per L LiFSI and 0.2 mol per L LiDFOB-EC/DEC, which effectively suppressed the corrosion of the aluminum current collector. The excellent inhibitory capability of LiDFOB in the LiFSI-based electrolyte against aluminum current collector corrosion is caused by the formation of a passivation layer composed of Al–F, Al₂O₃, and B–O (Fig. 5d). Additionally, the dual-salt electrolyte composed of LiTFSI and LiDFOB offered favorable ionic conductivity and outstanding electrochemical performance at low temperatures.⁹⁹ Lin et al.⁹³ developed a 2 mol per L LiTFSI/LiDFOB (molar ratio 4.3 : 1)-TMS/EA-10% FEC (TMS: sulfolane; EA: ethyl acetate) electrolyte, which provided good ionic conductivity at low temperatures (Fig. 5e) and maintained excellent electrochemical performance under conditions of charging to 4.6 V at a 1C current density at −40 °C (Fig. 5f). Yang et al.¹⁰⁰ prepared the electrolyte by dissolving 1.5 mol per L lithium triflate (LiOTF) and 0.2 mol per L LiPF₆ in a blend of diethylene glycol dimethyl ether (DEGDME) and 1,3-dioxolane (DOL) (volume ratio 1 : 1). This electrolyte exhibited low interface impedance and small charge transfer activation energy, allowing NCM‖graphite full cells to maintain 58.3% compared to room temperature at −60 °C. Beyond binary lithium salts, Zhang et al.¹⁰¹ developed a ternary lithium salt system of LiFSI/LiBOB/LiPF₆, which enhanced the rate performance and cycle stability of LiFePO₄ cathodes and graphite anodes. In summary, the mixed lithium salt strategy provides a novel solution to overcome the issues of performance degradation in traditional electrolytes at low temperatures and offers a new perspective for designing high-performance LIBs electrolytes.

3.2. Solvents

Carbonate solvents, such as EC and DMC, are commonly utilized in commercial electrolytes because of their high dielectric constants and good chemical stability. However, their relatively high melting points lead to significant increases in the viscosity of the electrolyte and rapid declines in conductivity under low-temperature conditions, which severely limits the low-temperature performance of LIBs.¹⁰² Thus, using low-melting-point solvents in electrolytes is considered to be a strategy to enhance the low-temperature performance of LIBs. Additionally, adding suitable co-solvents to carbonate-based electrolytes to reduce the melting point and viscosity can also enhance the low-temperature performance of LIBs.

Carboxylate solvents exhibit lower viscosities and melting points in contrast to carbonate solvents, making them important alternative solvents to carbonates.¹⁰³ Due to its advantageous dielectric constant and low melting point of −87.5 °C, methyl propionate (MP) is a commonly used carboxylate. Holoubek et al.¹⁰⁴ selected MP as the primary solvent in the electrolyte and added FEC as an additive, enabling the battery to sustain optimal performance at temperatures as low as −65 °C and at a current density of 10C. Similarly, Cho et al.¹⁰⁵ proposed a 1 mol per L LiPF₆-MP/FEC (volume ratio 9 : 1) carboxylate-based electrolyte, which demonstrated significantly improved electrochemical cycling performance at −20 °C, greatly surpassing that of commercial standard carbonate-based electrolytes. Additionally, Yao et al.⁵⁸ studied an EC- and additive-free electrolyte composed of LiFSI/MA/FE/D3 (FE: fluorinated ether; D3: 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether), which by anion decomposition provided a SEI as stable as those formed in EC-based systems, and the anion-derived SEI had better Li⁺ transport properties at low temperatures. Due to improved interfacial transport performance, this electrolyte effectively suppressed lithium dendrite formation during low-temperature charging (Fig. 5g). Fluorinated carboxylate solvents are also considered good electrolyte solvents because fluorinated organic solvents have an extensive electrochemical stability window, non-flammability, and low melting points. Yang et al.¹⁰⁶ developed a novel fluorinated electrolyte composed of LiTFSI and ethyl trifluoroacetate (ETFA). Li⁺–ETFA has very low binding energy and freezing point. The weaker binding energy between ETFA solvent and Li⁺ promotes the desolvation process at low temperatures and maintains a liquid state at extremely low temperatures, providing better electrochemical performance in LIBs than carbonate-based electrolytes at low temperatures.

Besides carboxylate solvents, early studies focused on adding co-solvents to EC-based electrolytes to reduce the melting point and viscosity, thereby enhancing LIBs' performance under low-temperature conditions. Smart et al.¹⁰⁷ reported a ternary mixed electrolyte containing 1 mol per L LiPF₆ in EC/DEC/DMC, which had an ionic conductivity of 1.0 mS cm⁻¹ at −40 °C and produced a stable SEI. Then, adding varying quantities of EMC to EC/DEC/DMC further developed a quaternary all-carbonate high ionic conductivity electrolyte. Among these, the most promising electrolytes were 1.0 mol per L LiPF₆-EC/DEC/DMC/EMC (volume ratio 1 : 1 : 1 : 2) and 1.0 mol per L LiPF₆-EC/DEC/DMC/EMC (volume ratio 1 : 1 : 1 : 3).⁹⁴ At −40 °C, the electrolyte with a volume ratio of 1 : 1 : 1 : 3 exhibited superior performance in cold conditions (Fig. 5h), allowing the MCMB‖LiNiCoO₂ battery to achieve 65% of room temperature capacity. The benefits of this electrolyte were additionally confirmed in different battery systems. Liao et al.⁶⁵ studied the performance of the LiFePO₄/C cathode in 1.0 mol per L LiPF₆-EC/DEC/DMC/EMC (volume ratio 1 : 1 : 1 : 3) electrolyte, showing that Li‖LiFePO₄/C batteries maintained 84.8%, 66.9%, and 51.3% of room temperature capacity at 0 °C, −20 °C, and −40 °C, respectively.

Carboxylate solvents can also be added as co-solvents to carbonate-based electrolytes. Zhang et al.¹⁰⁸ incorporated methyl butyrate (MB) to the electrolyte, designing an electrolyte composed of 1 mol per L LiTFSI-DEC/EC/MB (volume ratio 1 : 1 : 1), resulting in a significant reduction in electrolyte and an increase in the ionic conductivity from 1.38 to 3.44 mS cm⁻¹ at −20 °C. This electrolyte system formed a uniform SEI film on the anode, effectively suppressing the development of lithium dendrites. Similarly, the addition of ethyl acetate (EA) diminished the charge transfer activation energy in carbonate-based electrolyte (EC/DMC/EMC) from 48.36 to 33.01 kJ mol⁻¹ at −20 °C, accelerating the charge transfer process and reducing the interfacial impedance at low temperatures.¹⁰⁹ Zhang et al.⁹⁵ blended various carbonate and carboxylate solvents, designing a novel high-entropy electrolyte 1 mol per L LiPF₆-EC/DEC/PC/EMC/EP/EA/MB/BA/MP/PB (EP: ethyl propionate; PB: propyl butyrate), which exhibited excellent ionic conductivity and capacity retention under ultra-low temperatures conditions down to −130 °C (Fig. 5i). This research provides new insights for designing LIBs electrolytes suitable for ultra-low temperature environments.

In addition to common carbonate and carboxylate solvents, new solvent components are being developed. Isoxazole (IZ) is a five-membered aromatic heterocyclic compound with a high boiling point, low melting point, and two electronegative heteroatoms (nitrogen and oxygen).¹¹⁰ IZ, with its low viscosity and high dipole moment facilitate efficient Li⁺ transport in IZ-based electrolytes, even at low temperatures. Tan et al.¹³ observed that the ionic conductivity of IZ-based electrolytes was almost three times that of commercial electrolytes. However, IZ alone cannot form a stable SEI on the anode side to prevent co-intercalation of the solution. By adding LiDFOB and FEC additives, the IZ-based electrolyte achieved excellent long cycling performance, reaching a capacity of 187.5 mA h g⁻¹ at −20 °C.

The types of common solvents in LIBs and their improvement strategies have been summarized, including a discussion on the performance of carboxylate solvents under low-temperature conditions, and ways to enhance the performance of electrolytes through the addition of different co-solvents. Additionally, the potential applications of novel solvents such as IZ and the research results on high-entropy electrolytes are introduced. These studies provide new ideas and directions for enhancing the performance of LIBs in cold conditions.

3.3. Additives

Due to the advantages of minimal dosage, rapid effectiveness, and low cost, the application of additives presents an effective and economical approach to enhance the low-temperature performance of LIBs and aid in the formation of a stable SEI (Fig. 6a).^111,115 Common additives include carbonates, sulfites, and lithium salts.

Fig. 6 (a) Illustration of the effect of additives on the graphite anode. Based on ref. 111. (b) PMBS bond length and potential mechanisms for formation on the electrode surface. Based on ref. 112. (c) The structure formula of EBC and the HOMO and LUMO levels of EC, EBC and VC. Based on ref. 57. (d) Cyclic Voltammetry curves of three electrolytes. (e) The rate performance of three electrolytes. Based on ref. 113. (f) Schematic illustration of SEIs formed on graphite in different electrolyte. (g) C 1s, O 1s, and Li 1s spectra of graphite electrodes after 3 cycles in different electrolyte. Based on ref. 114.

Among carbonate additives, vinylene carbonate (VC) stands out as one of the earliest to be commercially applied. During the discharge process, it is reduced to a radical anion, which facilitates the formation of a polymer that is beneficial for SEI formation. This further inhibits the solvent decomposition on the anode surface and enhances interfacial stability. Therefore, adding 1% VC to the electrolyte can significantly improve the first-cycle coulombic efficiency and cycle life of the battery.¹¹⁶ FEC is also widely used commercially. Research has indicated that the addition of 2% FEC by volume can suppress the decomposition of part of the electrolyte solvent and form an excellent SEI layer on the electrode surface, reducing battery impedance and significantly enhancing the specific capacity and cycle stability of the battery.^115,117 Besides VC and FEC, Zhu et al.¹¹⁸ introduced a mixed additive composed of 16% BA and 10% EC by volume, which facilitated the establishment of a steady SEI on the electrode surface. This also resulted in enhanced low-temperature ionic conductivity and discharge capacity, achieving a discharge capacity of 108.94 mA h g⁻¹ at −40 °C. DMS, aside from being used as a solvent, can also serve as an additive in electrolytes. The reduction products of DMS have weak binding ability with Li⁺, which facilitates the formation of a stable and uniform SEI with high ionic conductivity on the anode surface, thereby providing excellent low-temperature performance.¹¹⁹ Additionally, propyl 4-methylbenzenesulfonate (PMBS), a new type of additive, has been developed (Fig. 6b). Adding PMBS to the electrolyte aids in forming a stable and low-impedance SEI and CEI, due to PMBS's propensity to decompose preferentially relative to solvent molecules. The LIBs containing 2% PMBS exhibited superior performance, with the capacity retention rate increasing from 55.37% to 98.02% after 100 cycles at −10 °C.¹¹² Qian et al.⁵⁷ designed and synthesized a novel carbonate molecule called erythritol bis(carbonate) (EBC), which merges two EC-like structures into a single molecule and exhibits a lower LUMO energy level (Fig. 6c). A lower LUMO energy level for EBC compared to EC, indicating that it decomposes before EC is reduced. However, the weaker solvation of Li⁺ by EBC does not involve a substantial chemical reaction with Li⁺, which limits its reduction and thus forms a low-impedance and stable SEI.

Moreover, a small amount of lithium salt can also be used as an additive in electrolytes. Li et al.¹²⁰ proposed that adding lithium difluorophosphate (LiPO₂F₂) at a mass fraction of 2% to a 1 mol per L LiPF₆-DMC/EMC/PC/FEC electrolyte could enhance the electrochemical performance of LIBs under low-temperature conditions. LiPO₂F₂ addition encourages the development of a uniform, LiF-rich SEI layer on the electrode surface. This LiF-rich SEI possesses excellent ionic transport properties, facilitating the transport of Li⁺ at low temperatures. Yang et al.¹¹³ added lithium nitrate (LiNO₃) and FEC to an ether-based electrolyte. The addition of LiNO₃ further enhanced the reduction peak of FEC near 1.5 V (Fig. 6d), indicating the co-decomposition of FEC and LiNO₃. Moreover, the addition of FEC effectively addressed the issue of solvent co-intercalation in ether-based electrolytes. The combined addition of FEC and LiNO₃ not only preserved the preferential decomposition characteristic of FEC, but also introduced LiN_xO_y with high ionic conductivity into the SEI, aiding the rapid transport kinetics of Li⁺, and enhancing the electrochemical performance of LIBs (Fig. 6e). Additionally, LiBF₄ can serve not only as a substitute salt for LiPF₆, but also as a supplement salt. Qin et al.¹¹⁴ studied the addition of 0 M, 0.05 M and 0.1 M LiBF₄ into a 1 M LiPF₆-EC/EMC/MB (2 : 2 : 6 by volume) electrolyte, labeled as E0, E1 and E2, respectively. The results showed that the addition of trace amounts of LiBF₄ enhances the coordination between PF₆⁻ and Li⁺, weakens the interaction between EC and Li⁺, and forms a dense, LiF-rich SEI. The electrolyte containing LiBF₄ exhibited a higher capacity, indicating that more anions were reduced and a large amount of SEI was accumulated. In the E0 electrolyte, the decomposition of PF₆⁻ was limited, and the graphite edge mainly grew in amorphous regions scattered with a few crystalline particles. In contrast, in the E1 electrolyte, a continuous film formed on the graphite surface, primarily composed of LiF and LiBO₂, with LiF being the dominant component. However, in the E2 electrolyte, which had a higher concentration of LiBF₄, the inorganic components in the SEI were mainly LiBO₂, which could not form an effective protective film, leading to poor passivation performance (Fig. 6f). The SEI formed in the E1 electrolyte, as identified through XPS analysis, contained the highest proportion of inorganic substances, including higher contents of Li₂CO₃ and LiF, further verifying its high interfacial stability and dynamic advantages (Fig. 6g).

This suggests that by regulating the solvation structure of LiBF₄, an appropriate amount of LiBF₄ plays a role in forming a highly protective, LiF-rich SEI, thereby enhancing LIB performance. Similarly, LiDFOB can also be used as a supplement salt for LiPF₆. As an additive, LiDFOB preferentially decomposes on the surface of the lithium anode and high nickel cathode. DFOB⁻ preferentially reduces and oxidizes, forming a strong, boron-rich interfacial film with high electronic insulation, which effectively inhibits the consumption of active lithium and the decomposition of the electrolyte. On the cathode surface, key CEI components such as BF₃, BF₂OH and BF₂OBF₂ form strong coordination bonds with lattice oxygen, inhibiting the loss of lattice oxygen and slowing down the irreversible structural degradation process. On the lithium anode, the SEI, rich in BF₂CH₂CH₂COOLi and BF₂OCH₂CH₂CH₂CH₂OBF₂, generated by LiDFOB decomposition, has low adhesion, reduces the energy barrier for lithium deposition/stripping, promotes uniform lithium deposition, and inhibits the formation of lithium dendrites.¹²¹ Through continuous research and innovation, electrolyte additives are poised to provide crucial support and assurance for the performance improvement and application expansion of future LIBs.

3.4. Constructing a novel electrolyte system

3.4.1. Local high concentration electrolyte. High concentration electrolytes (HCE) and localized high concentration electrolyte (LHCE) systems have garnered considerable research interest in recent years. HCEs are highly appreciated for their oxidative stability, thermal stability, ability to form quality SEI, and unique solvation structures. In the solvation structure of HCE, lithium salts exist in various forms within the lithium salt-solvent complexes, such as solvent-separated ion pairs (SSIP), contact ion pairs (CIPs), and aggregates (AGGs).¹²² However, HCEs encounter obstacles at low temperatures, including high viscosity, reduced wettability, and poor ionic transport, restricting the application of HCE-based electrolyte systems in LIBs at low temperatures. Researchers have proposed a new approach involving the addition of inert solvents to HCEs, forming LHCEs (Fig. 7a).^123,130 The inert diluent, while not dissolving the lithium salt, is miscible with the solvent, thus reducing the concentration of lithium salts without affecting the original Li⁺ solvation structure in HCEs.

Fig. 7 (a) Solvation structure of electrolytes with low concentration, high concentration and local high concentration. Based on ref. 123. (b) Distribution analysis of various solvation structures and representative MD snapshots. (c) Design concept of solvation at room temperature and low temperature. Based on ref. 124. (d) Low temperature performance at 0.2C discharge rate. Based on ref. 125. (e) TEM images of graphite negative electrodes. Based on ref. 126. (f) Formation of special solvated structures in weakly solvated electrolytes. Based on ref. 127. (g) Mechanism of solvation structure of polyoxymethylene ether electrolyte. Based on ref. 128. (h) Diagram of Li⁺ binding energy of solvation of DME and DMM as a function of coordination number. (i) Desolubilization and activation energy of two electrolytes. Based on ref. 129. (j and k) NMR spectra of different electrolytes. Based on ref. 2.

Holoubek et al.¹²⁴ proposed an LHCE based on LiFSI/dimethoxyethane (DME) with bis(2,2,2-trifluoroethyl) ether (BTFE) as the diluent. By modifying the molar ratio between lithium salt and solvent, an ionic pairing transition occurs when the local concentration of lithium salts exceeds 4 mol L⁻¹. This transition alters the balance between the inner and outer layers of the solvation structure, resulting in the formation of an HCE rich in AGGs (Fig. 7b). This altered structure facilitates the formation of a fluoride-rich SEI, thus promoting uniform Li deposition in Li/LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) batteries at low temperatures (Fig. 7c). Regulating the solvation structures of ionic pairs positively impacts the reversibility of Li metal under low-temperature conditions. Zhang et al.¹²⁵ proposed three types of LHCEs: 1.4 mol per L LiFSI-DMC/TTE (molar ratio 2.2 : 3), 1.4 mol per L LiFSI-DMC/VC/TTE (molar ratio 2 : 0.2 : 3), and 1.4 mol per L LiFSI-DMC/EC/TTE (molar ratio 2 : 0.2 : 3), designated as AE001, AE002, and AE003, respectively. Compared to conventional carbonate-based electrolytes, these three LHCEs significantly enhance battery performance over an extensive temperature range. The LHCE containing a small amount of EC showed superior low-temperature performance, achieving a discharge capacity of 160.7 mA h g⁻¹ at −30 °C and a 0.2C rate (Fig. 7d), possibly due to the ability of EC to establish a thinner and more robust SEI. Additionally, a new short-chain fluorinated diluent known as 1,1,2,2-tetrafluoroethyl methyl ether (TFME) has been developed. An LHCE was prepared by adding TFME to a MA-based electrolyte, enabling LIBs to maintain high performance in low-temperature environments, delivering 80.85% of their room temperature discharge capacity at 0.2C and −50 °C.¹³¹ Jiang et al.¹²⁶ used BTFE as the diluent in a 1.5 mol per L LiFSI and DME-based electrolyte for the production of an LHCE. This LHCE was able to form a predominantly inorganic, uniform, and robust SEI (Fig. 7e), effectively inhibiting the co-intercalation of ether solvents into the graphite anode. The graphite‖Li cells exhibited superior electrochemical performance, delivering a discharge capacity of 90 mA h g⁻¹ at 0.1C at −20 °C. Under room temperature conditions, the cells achieved 220 mA h g⁻¹ at 4C, demonstrating high-rate capabilities. The LHCE demonstrated excellent cycling stability, maintaining approximately 85.5% of its initial capacity after 200 cycles at 4C. 1,2-bis(1,1,2, 2-tetrafluoroethoxy)ethane (BTFEE), as a diluent with weak solvation ability, has been used in the study of electrolyte systems.¹³² Due to its weak solvation ability with salts, BTFEE is primarily used as a diluent, though it can also partially participate in the dissolution process, thereby influencing the makeup of the electrode–electrolyte interfaces, particularly the formation of inorganic compounds rich in Li₂O and LiF. Unlike traditional non-coordination diluents, BTFEE can form weak coordination with Li⁺ in LHCE and participate in the first solvation sheath structure. By optimizing the ratio of DME, which has strong solvation ability, to BTFEE, which has weak solvation ability, an inorganic-rich SEI and CEI can be effectively customized, significantly improving the cycle stability of the battery. In terms of electrochemical performance, lithium metal batteries with a NCM811 cathode, with a loading of 4 mA h cm⁻², retained 80% of their initial capacity after 470 cycles when cycled in the voltage range of 2.8–4.4 V.¹³² Lai et al.¹³³ designed a locally high-concentration electrolyte based on tetrahydrofuran (THF), selecting THF as the alkaline solvent due to its low viscosity (0.53 cP), low melting point (−108 °C), and high solubility for LiTFSI. This electrolyte exhibits lower viscosity and higher conductivity at low temperatures compared to traditional high-concentration electrolytes, which helps improve the Li⁺ diffusion at low temperatures. In addition to high oxidation stability, the higher anion ratio in the solvated structure further facilitates the charge transfer process.

To further enhance the performance of the battery at low temperatures, researchers introduced 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoro-propylether (HFE) as a diluent to create LHCE from HCE. The addition of HFE not only diminishes the viscosity of HCE by destroying the 3D network structure, but also preserves the unique solvation structure of HCE due to the HFE's weak coordination ability with Li⁺. This LHCE system effectively combines the high ion mobility characteristic of low-concentration electrolytes with the unique solvation properties of high-concentration electrolytes. Furthermore, this electrolyte demonstrates superior low-temperature performance, delivering over 70% capacity at −70 °C and maintaining 72.5 mA h g⁻¹ (≈77.1%) capacity for 200 cycles at a 1C rate at −40 °C. Even when the rate increases to 5C, the battery still operates well at −40 °C. This work shows that solvation regulation significantly influences the kinetics of cells at low temperatures and offers a design method for future electrolyte development. In summary, the formation of LHCEs by adding inert solvents is a successful strategy to optimize the performance of HCEs in cold conditions. These studies provide valuable insights for extending LIBs' applications to extreme environments.

3.4.2. Weak solvent electrolyte. In traditional electrolytes, the strong binding capacity of Li⁺ with solvents prevents anions from coordinating more extensively with Li⁺, hindering the formation of an anion-derived SEI. At low temperatures, the fluidity of the electrolyte decreases, leading to a decline in ionic conductivity and a slowdown in chemical reaction rates on the electrode surfaces. This further exacerbates the degradation of the SEI and contributes to the decline in LIBs performance. Using weakly solvating electrolytes composed of solvents with weak solvation binding energy is an effective approach. Due to their special solvation structures, these electrolytes commonly feature CIPs and AGGs, which promote the formation of an inorganically-rich SEI at the electrode surface (Fig. 7f).¹²⁷ This SEI is more stable and conductive, especially at low temperatures. Cao et al.¹³⁴ designed a weakly solvating electrolyte in which the fluorinated solvent could form weaker interactions, such as hydrogen bonds and van der Waals forces, and also interacted less intensively with Li⁺, thereby reducing the solvation capacity of Li⁺ to address the desolvation challenges. This electrolyte, composed of 1 mol per L LiFSI, FEC, bis(2,2,2-trifluoroethyl) carbonate (BTFC), and weakly solvating ETFA with a fluorinated structure, further weakens the solvation capacity of the electrolyte while effectively lessening its viscosity and melting point. Full cells using this electrolyte attained a high specific capacity of 103.5 mA h g⁻¹ at −20 °C and showed stable performance over 100 cycles. By changing the ion–dipole interaction in the Li⁺ solvent sheath, FEC, BTFC solvent molecules and FSI⁻ anions preferentially decompose to form an SEI rich in LiF and other inorganics, which effectively prevents the continuous reduction of organic solvents and the structural damage of electrodes. Liu et al.¹²⁸ systematically designed a class of weakly solvated polyoxymethylene ether electrolyte for LIBs by controlling the length of the –O–CH₂–O– fragment. The –O–CH₂–O– fragment in the solvent is capable of forming a weak four-membered ring Li⁺ coordination structure, and an increased number of fragments can anchor the solvent to Li⁺ without significantly sacrificing ionic dissociation ability (Fig. 7g).

Cyclic voltammetry confirms that the designed molecules can achieve reversible solvent-free co-intercalation of Li⁺ into a graphite cathode at conventional concentrations. Due to the enhanced “anchoring effect” and the increased chain length, the oxidation stability of the electrolyte is improved. Among them, dipolyformaldehyde dimethyl ether (DDE) showed both high oxidation stability and excellent electrode reaction kinetics. Additionally, the weak solvation ability of DDE enables anions to participate in the solvation structure, facilitating the formation of an inorganic-rich CEI/SEI interface on the electrode surface. In terms of electrochemical performance, Gr‖NCM811 Ah-class pouch cells with the DDE-based electrolyte retained 70.85% capacity after 1000 cycles at room temperature and 75.86% capacity after 400 cycles at −20 °C, demonstrating excellent cycle stability and low-temperature performance. Yang et al.¹³⁵ designed a 1 mol per L LiTFSI-ETFA/FEC (volume ratio 7 : 3) weakly solvating electrolyte, which demonstrated excellent low-temperature performance by maintained 78% of room temperature capacity at −30 °C in a graphite‖LiFePO₄ battery. Ma et al.¹²⁹ compared two ether solvents, dimethoxymethane (DMM) and DME. As shown in Fig. 7h, regardless of coordination number, the interaction of DMM with Li⁺ is weaker than that of DME. Compared to DME-based electrolytes, the solvation sheath of the electrolyte with DMM containes fewer and weaker coordinating solvents, thus DMM-based electrolytes exhibit faster desolvation kinetics (Fig. 7i). Rapid desolvation in weakly solvating electrolytes achieves smoother lithium deposition and higher coulombic efficiency, significantly enhancing the safety of battery operation under low-temperature conditions. Moreover, Li‖SPAN full cell shows a high initial discharge capacity at −40 °C (422.3 mA h g⁻¹) and excellent capacity retention (63.8% after 120 cycles), indicating that the weakly solvating electrolyte effectively supports stable and efficient performance even under extreme conditions. Liu et al.¹³⁶ developed a weakly solvated electrolyte system with IZ as the main solvent, consisting of 1.0 M LiFSI dissolved in IZ, fluorobenzene (FB) as a non-coordinating solvent, and FEC as a film-forming additive. Through the dipole–dipole interaction between FB and IZ, this system effectively reduces the desolvation energy of Li⁺. Its weak solvation ability allows more anions to enter the solvation sheath, promoting the formation of CIPs and AGGs, thereby enhancing the Li⁺ transport rate and maintaining high ionic conductivity across a wide temperature range. At −20 °C, this electrolyte system achieves a reversible capacity of 200.9 mA h g⁻¹ in a graphite half-cell, nearly three times that of traditional EC-based electrolytes. Chen et al.² designed a novel weakly solvating electrolyte with a “drag effect” by introducing bis(2,2,2-trifluoroethyl) ethyl phosphonate (TFEP) and EMC into a LiPF₆–PC-based electrolyte. TFEP, with a higher degree of fluorination, has weaker Li⁺ coordinating capabilities, which allows PF₆⁻ to co-squeeze into the first solvation sheath layer of Li⁺, forming an anion-induced ion–solvent coordination structure (AI-ISC). Alternatively, TFEP acts as a flame-retardant solvent, significantly reducing the electrolyte's flammability. However, the interaction of Li⁺ with TFEP is too weak to break the coordination bonds of Li⁺–PC and Li⁺–PF₆⁻, making it challenging for Li⁺ to break free from the solvation complex. Thus, a third solvent, EMC, which exhibits a stronger affinity for Li⁺ and a comparable coordination capability to TFEP, is incorporated into the aforementioned PC–TEFP. This addition effectively facilitates Li⁺ transport without disrupting the AI-SIC structure, thereby reducing the strength of interactions between Li⁺–PC and Li⁺–PF₆⁻. The “drag effect” markedly lowers the binding energy between Li⁺–PC and Li⁺–PF₆⁻, as demonstrated by nuclear magnetic resonance characterization (Fig. 7j and k). Due to its low Li⁺ desolvation energy, elevated Li⁺ transference number, and strong electrochemical compatibility of the AI-ISC structure, the weakly solvating electrolyte endows the graphite anode with outstanding cycle stability (100% capacity retention after 400 cycles), superb low-temperature performance (approximately 79% and 62.5% capacity retention at −40 °C and −50 °C, respectively), and safety features (verified through overcharge, squeeze, thermal shock, and nail penetration tests).²

Overall, employing weakly solvating electrolytes is an effective strategy to overcome the low-temperature performance limitations of traditional electrolytes in LIBs. This research direction will continue to advance battery technology, providing more stable and reliable energy solutions for next-generation electric vehicles and portable electronic devices.

3.4.3. Liquefied gas electrolyte. In addition to the aforementioned electrolyte systems, recent research on liquefied gas electrolytes with low melting points and enhanced safety has garnered significant attention for their potential to improve the performance of LIBs. Among these, electrolytes based on fluoromethane (FM) are particularly noted for their low melting points and low viscosity. However, the low lithium salt solubility and high polarization of FM limit its practical application. Researchers have introduced THF as a co-solvent to address this issue, successfully enhancing the solubility of lithium salts and forming a uniform SEI rich in LiF (Fig. 8a), thereby achieving excellent low-temperature performance.¹³⁷ Nevertheless, the operational temperature range and oxidative stability of this electrolyte still require further improvement. Yang et al.¹³⁸ discovered that using acetonitrile (AN) as a co-solvent with FM in the preparation of liquefied gas electrolytes resulted in higher lithium salt solubility (1.2 mol per L LiTFSI) and a noticeably wider temperature range (−60 °C ∼55 °C). In these liquefied gas electrolytes, a uniformly thick and more extensively covering CEI was formed on the surface of NMC cathode materials (Fig. 8b and c). The XPS analysis revealed that the CEI formed in carbonate-based electrolytes exhibits organic compound characteristics, consisting of decomposition products from the carbonate-based electrolytes (Fig. 8d). On the contrary, the CEI formed in liquefied gas electrolytes includes LiF, S–O, and N–O species derived from the decomposition of LiTFSI and FM, facilitating the formation of a protective interfacial phase, which further enhances battery performance. Under the conditions of 3 mA cm⁻² and 3 mA h cm⁻², the average coulombic efficiency for lithium metal cycling over 200 cycles reached 99.4%.¹³⁸ Researchers have tested the safety of batteries using liquefied gas electrolytes through nail penetration tests. Traditional LIBs undergo internal short circuits at the moment of nail penetration, leading to a rapid local temperature increase and thus thermal runaway. Batteries using liquefied gas electrolytes rapidly vaporize the internal electrolyte upon nail penetration, causing a swift drop in battery temperature, followed by a gradual return to room temperature. The rapid vaporization of the electrolyte obstructs the internal ionic pathways, thereby preventing internal short circuits caused by nail penetration.¹⁴¹ Subsequently, liquefied gas electrolytes based on difluoromethane (DFM) were also proposed.¹⁴¹ Compared to FM-based electrolytes, DFM-based liquefied gas electrolytes exhibit lower pressure, lower flammability, and a wider operational temperature range, opening new directions and possibilities for research on liquefied gas electrolytes. Yin et al.¹⁴² developed a new liquefied gas electrolyte based on dimethyl ether (Me₂O) and PC, aimed at improving the electrochemical performance of batteries under extreme temperature conditions. The electrolyte maintains high ionic conductivity (>3.5 mS cm⁻¹) over a wide temperature range from −70 °C to 60 °C. Me₂O, with its low desolvation energy, enhances the performance of the electrolyte at both high rates and low temperatures. Additionally, because the C–O–C bond in Me₂O has higher Lewis basicity than the C–F bond, its ability to dissolve salts is superior to other gaseous solvents, such as FM and DFM, while its excellent compatibility with lithium metal makes it suitable for applications across a wide temperature range. To further optimize the performance of the electrolyte, the researchers replaced DME in the traditional LiBF₄–DME–PC formulation with Me₂O. By optimizing the ratio of Me₂O to PC, the electrolyte's transmission and discharge performance was significantly improved. Due to the weak affinity between Me₂O and Li⁺, anions take a more dominant role in the solvation structure, enhancing the electrolyte's desolvation process and improving its rate performance and low-temperature operability. Thanks to its rapid desolvation and transport dynamics, the optimized electrolyte achieves high utilization of CF_x, exhibits exceptional rate performance at room temperature and −60 °C, and demonstrates exceptional energy output within the extended temperature window of −70 °C to 55 °C. In Li/CF_x cells using a 4.3 mg cm⁻² loading CF_x cathode, the cell delivered 780 mA h g⁻¹ (91% room-temperature capacity retention) under a 10 mA g⁻¹ discharge current at −60 °C. Furthermore, when 50 mg cm⁻² CF_x was utilized, the cell still displayed 706 mA h g⁻¹ (84% room-temperature capacity retention) at −60 °C, and the average discharge voltage remained above 2.1 V.¹⁴²

Fig. 8 (a) Solubility test and solvation mechanism diagram of liquefied gas electrolyte. Based on ref. 137. (b and c) Cyclic performance of two electrolytes at low temperature at 0.1C. (d) XPS map of two electrolytes interfacial membranes. Based on ref. 138. (e) Schematic diagram of Li⁺ transport mode for PEO based electrolytes. Based on ref. 139. (f) Stress–strain curves of F-PCEE at 25 °C, −10 °C, −20 °C and −30 °C. (g) Conductivity of two electrolytes at different temperatures. (h) Cycle performance of the battery at −10 °C and 0.1C. Based on ref. 140.

In summary, as a novel type of electrolyte system, liquefied gas electrolytes offer good low-temperature performance and enhanced safety, providing new insights and solutions for the development of lithium-ion batteries. However, further research and improvements are still needed to meet the demands of practical applications.

3.4.4. Polymer electrolyte. Compared to traditional liquid electrolytes, polymer electrolytes exhibit superior interfacial stability and enhanced safety, making them excellent candidate electrolytes for LIBs. Xiang et al.¹⁴³ presented a novel polymerized 1,3-dioxolane (PDE) electrolyte, formulated with a multifunctional tris(pentafluorophenyl)borane (TB) additive. The addition of TB led to the formation of a PDE that exhibited excellent flame resistance, substantially extended operational temperature range, and boosted oxidative stability. This polymer electrolyte also possesses the capability to form a highly stable and LiF-rich SEI film, high ionic conductivity, and reduced interfacial impedance. It demonstrated exceptional performance in low-temperature conditions in Li–S batteries, maintaining a capacity of about 700 mA h g⁻¹ at −20 °C, with the capacity recoverable after 24 hours of rest at low temperatures.¹⁴³ Succinonitrile (SN) was introduced into a poly(ethylene oxide) (PEO)-based electrolyte, forming a uniform solid-state polymer electrolyte (Homo-SPE).¹³⁹ By varying the molar ratio of SN and PEO to 1 : 4, rapid Li⁺ transport channels were formed within the Homo-SPE (Fig. 8e), resulting in a hundredfold increase in ionic conductivity, rendering it applicable in mid to low-temperature environments. Compared to the original PEO-based electrolyte, the NMR ⁷Li spectrum of Homo-SPE shifted forward by 0.50 ppm, with the peak moving to a higher field indicating weakened interactions of PEO–Li⁺. Therefore, the addition of SN could mitigate the affinity between PEO and Li⁺, further explaining the mechanism of rapid Li⁺ transport in Homo-SPE. This electrolyte exhibited good cyclic performance and high capacity retention (97.3%).¹³⁹ Park et al.¹⁴⁰ developed a fluorine-containing plastic-crystal embedded elastic electrolyte (F-PCEE) for the stable operation of solid lithium metal batteries at −10 °C. F-PCEE has a bicontinuous structure, consisting of an SN-based plastic crystal phase embedded in a 2,2,3,4,4,4-hexafluorobutyl acrylate (HFBA) based elastomer matrix. The electrolyte exhibits excellent performance at −10 °C, with Li⁺ conductivity reaching up to 0.23 mS cm⁻¹, and enhanced elastic properties with a fracture strain of about 300% (Fig. 8f and g), effectively overcoming the tradeoff between ionic conductivity and mechanical properties. Additionally, F-PCEE facilitates the formation of a LiF-rich SEI on lithium metal surfaces, enabling full cells with lithium metal anodes and NCM811 cathodes to achieve excellent cycling performance at −10 °C. After 150 cycles at 0.1C, the battery's capacity retention rate reaches 85.3%, with a cutoff voltage of up to 4.5 V (Fig. 8h), representing one of the best cycling performances among solid polymer electrolyte-based lithium metal batteries at −10 °C, particularly for nickel-rich cathode systems. In contrast, poly(butyl acrylate)-based PCEE (BA-PCEE) exhibits lower ion conductivity and mechanical properties at −10 °C, leading to rapid capacity decay in full cells.¹⁴⁰ Motivated by the high-efficiency water purification and soil stabilization properties of aquatic plants, Liu et al.¹⁴⁴ prepared poly(1,3-dioxolane) (PDOL) through in situ ring-opening polymerization of DOL. This process created a 3D desolvation area, which improved Li⁺ desolvation at the interface. This resulted in the production of an amorphous gel polymer electrolyte with high Li⁺ ionic conductivity (5.73 mS cm⁻¹). A greater number of anions contribute to the solvation structure, culminating in the formation of a stable SEI derived from anions, which enhances Li⁺ transport through the SEI.¹⁴⁴ Despite the many advantages of polymer electrolytes, such as lightweight, ultra-thin, and flexibility, they still face some challenges at low temperatures. Compared to liquid electrolytes, polymer electrolytes generally have poorer ionic conductivity and performance at low temperatures. Therefore, although polymer electrolytes hold potential, further research and improvements are needed to enable their widespread application in cold environments (Table 1).

Table 1 New electrolyte system and electrochemical performance

Electrolyte system	Electrolyte	Cell system	Capacity (LT)	Capacity (25 °C)	Ref.
High-concentration	3 mol per L LiPF6/EA/FEC (9 : 1 by vol.)	Graphite‖NCM811	0.85 A h (−40 °C, 0.1C)	0.9 A h (300 cycles, 2C)	21
	5 mol per L LiTFSI/THF	Li‖LMO	50.1 mA h g^−1 (−40 °C, 0.1C)	300 cycles, 84.1% (1C)	133
	LiFSI/DME (1 : 1.4 by mol)	Li‖NCM333	—	500 cycles, 92% (1C)	145
Local high-concentration	3.3 mol per L LiFSI/AN/TTE (1 : 1 by vol.)	Graphite‖NCM111	110 mA h g^−1 (−30 °C, 0.1C)	250 cycles, 94% (0.5C)	146
	1.4 mol per L LiFSI/DMC/TTE (2.2 : 3 by mol)	Graphite‖NCM811	106.5 mA h g^−1 (−30 °C, 0.2C)	400 cycles, 100% (1/3C)	125
	1.4 mol per L LiFSI/DMC/VC/TTE (2 : 0.2 : 3 by mol)	Graphite‖NCM811	154.9 mA h g^−1 (−30 °C, 0.2C)	400 cycles, 98.6% (1/3C)	125
	1.4 mol per L LiFSI/DMC/EC/TTE (2 : 0.2 : 3 by mol)	Graphite‖NCM811	160.7 mA h g^−1 (−30 °C, 0.2C)	600 cycles, 94.2% (1/3C)	125
	1.5 mol per L LiFSI/DME/BTFE (1 : 2 by vol.)	Graphite‖Li	90 mA h g^−1 (−20 °C, 0.1C)	200 cycles, 85.5% (4C)	126
	2 mol per L LiPF6 + 0.04 mol per L LiDFOB/DMC/HFE (1 : 1 by vol.)	Graphite‖Li	240 mA h g^−1 (−20 °C, 0.1C)	210 mA h g^−1 (2C)	147
Weak solvation	1 mol per L LiTFSI/ETFA/FEC (7 : 3 by vol.)	Graphite‖Li	183 mA h g^−1 (−30 °C, 0.05C)	200 cycles, 84.5% (5C)	135
	1 mol per L LiFSI/DMM	Li‖LTO	100 cycles, 75.9% (−20 °C, 0.2C)	200 cycles, 77.3% (0.5C)	129
	1 mol per L LiPF6/VC ethyl fluoroacetate (3 : 97 by vol.)	Graphite‖LiCoO2	2.87 A h (−40 °C, 0.1C)	400 cycles, 87.7% (1C)	148
Liquefied gas	1.2 mol per L LiTFSI + 1 mol per L AN in FM	Li‖NCM622	89 mA h g^−1 (−60 °C, 1/15C)	500 cycles, 96.5% (1/3C)	138
	0.3 mol per L LiFSI + 0.35 mol per L DME in DFM	Li‖NCM622	108 mA h g^−1 (−20 °C, 0.05C)	151 mA h g^−1 (0.05C)	141
Polymer	2 mol per L LiTFSI/DOL + 3 wt% TB	Li–S	700 mA h g^−1 (−20 °C, 0.5C)	550 cycles, 62.3% (0.2C)	143
	1 g PEO + 0.203 g LiTFSI + 0.45 g SN	Li‖LiFPO4	141.5 mA h g^−1 (0 °C, 0.03C)	750 cycles, 93.3% (0.5C)	139

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4. Conclusions

Over the past few decades, the performance decline of LIBs in low-temperature environments has been a focal point of concern. To address this challenge, scientists have invested considerable efforts in the development of electrolytes suitable for low temperatures, dedicating substantial resources to this pursuit. This paper provides a comprehensive review of the causes for the performance decline of LIBs in low-temperature environments and highlights the latest research progress in aspects ranging from lithium salts, solvents, additives to novel electrolyte systems.

Firstly, by improving the formula of the electrolyte, reducing its melting point and viscosity, effectively preventing the solidification of the electrolyte at low temperature, and accelerating the transmission rate of Li⁺. Secondly, the formation of an inorganically-rich SEI at the electrolyte–electrode interface effectively reduces interfacial impedance, thereby facilitating Li⁺ transport at the interface and enhancing the low-temperature performance of the battery. Moreover, innovative designs in electrode material structures also contribute to enhancing the performance of LIBs in cold conditions.

Despite significant progress in accelerating Li⁺ transport and constructing stable electrolyte–electrode interfaces, several challenges remain under low-temperature conditions. Firstly, the desolvation of Li⁺ at low temperatures is a critical performance limitation, necessitating further research into Li⁺ solvation structures with low desolvation energy. Secondly, while an SEI rich in LiF and with low impedance is crucial for low-temperature LIBs, its formation mechanism and the Li⁺ migration mechanism still require in-depth investigation. Thirdly, the role of electrode materials in low-temperature LIBs is also critical. It is necessary to ensure rapid Li⁺ transport within the electrode materials, considering the compatibility between the electrolyte and electrode materials.

In future research, characterization techniques and theoretical studies will play an increasingly important role. Novel in situ characterization techniques, such as in situ Raman spectroscopy, electrochemical SEM and TEM, can monitor reactions and structural changes inside the battery in real time, providing more data to support an in-depth understanding of the behavior mechanism of the electrolyte. At the same time, computational methods such as molecular dynamics simulations and density functional theory will play a key role in the prediction and design of new low-temperature electrolytes. By combining these experimental and theoretical approaches, scientists are expected to develop more systematic and effective design strategies, laying the foundation for the future development of more efficient, stable and economical low-temperature electrolytes. Overall, the development of low-temperature electrolytes is not only crucial for improving the application of LIBs in extreme environments, but it will also have a broad impact on the fields of electric vehicles, aerospace, and energy storage. Through continuous technological innovation and in-depth theoretical exploration, future lithium-ion batteries will be able to maintain excellent performance across a wider temperature range, further promoting the commercialization of low-temperature electrolytes.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Author contributions

Changlin Liu: conceptualization, writing – original draft. Lizhi Sheng: writing – review & editing, funding acquisition. Lili Jiang: project administration, review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the Department of Science and Technology of Jilin Province (20240304104SF, 20240304103SF), Research and Innovation Fund of Beihua University for the Graduate Students (Major Project 2023012). Lizhi Sheng is grateful to the China Scholarship Council for the financial support (No. 202108220125).

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