Liquid electrolytes play a critical role in lithium-ion batteries, serving as the medium for lithium-ion transport between the cathode and anode. The composition of these electrolytes directly influences key performance metrics such as ionic conductivity, electrochemical stability, thermal behavior, and cycle life. A well-formulated electrolyte typically consists of a lithium salt dissolved in a blend of organic solvents, each component contributing to the overall functionality of the battery system.

The most widely used solvents in lithium-ion batteries are cyclic and linear carbonates, including ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). These solvents are selected for their complementary properties. EC, a cyclic carbonate, has a high dielectric constant, which enhances salt dissociation and improves ionic conductivity. However, its high viscosity can limit ion mobility at lower temperatures. To mitigate this, linear carbonates such as DMC, DEC, or EMC are blended with EC to reduce viscosity while maintaining sufficient solvation capability. A common solvent mixture is EC:EMC or EC:DMC in varying ratios, balancing high conductivity with acceptable viscosity.

Lithium salts are another essential component, providing the source of lithium ions for charge transfer. Lithium hexafluorophosphate (LiPF6) is the dominant salt in commercial lithium-ion batteries due to its favorable combination of high ionic conductivity and moderate stability. However, LiPF6 is sensitive to moisture and elevated temperatures, leading to decomposition into hydrofluoric acid (HF), which degrades battery performance. Alternative salts such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) have been explored for their superior thermal and hydrolytic stability. LiFSI, in particular, exhibits higher conductivity and better compatibility with high-voltage cathodes, though its tendency to corrode aluminum current collectors at elevated voltages remains a challenge.

The interplay between solvent blends and lithium salts determines the electrolyte’s ionic conductivity, which is crucial for high-rate performance. Conductivity is influenced by the dissociation of lithium salts and the mobility of ions in the solution. Higher salt concentrations generally increase conductivity up to a certain point, beyond which ion pairing and increased viscosity reduce mobility. Optimal salt concentrations typically range between 0.8 M and 1.2 M, depending on the solvent system. For example, a 1 M LiPF6 solution in EC:DMC (1:1 by weight) achieves a conductivity of approximately 10–12 mS/cm at room temperature, whereas higher or lower concentrations may result in diminished performance.

Viscosity is another critical parameter, as it affects ion transport and wetting of electrode materials. Low-viscosity solvents like DMC and EMC improve electrolyte penetration into porous electrodes, enhancing interfacial contact and reducing internal resistance. However, excessive dilution of high-dielectric solvents like EC can compromise salt dissociation, leading to lower ionic conductivity. Therefore, solvent blends must be carefully optimized to balance viscosity and dielectric properties.

Thermal stability is a major concern for liquid electrolytes, particularly in high-temperature applications. Carbonate-based solvents are volatile and flammable, posing safety risks if the battery overheats. The decomposition of LiPF6 at elevated temperatures further exacerbates these risks by generating reactive species that accelerate electrolyte breakdown. To improve thermal resilience, researchers have investigated alternative solvent systems such as fluorinated carbonates or sulfones, which exhibit higher boiling points and greater resistance to oxidation. However, these alternatives often suffer from higher viscosity or poorer compatibility with electrode materials, limiting their widespread adoption.

Electrochemical stability is another key consideration, especially for high-voltage battery systems. The electrolyte must remain stable within the operating voltage window of the electrodes to prevent oxidative decomposition at the cathode or reductive degradation at the anode. Conventional carbonate-based electrolytes are stable up to approximately 4.3 V vs. Li/Li+, but higher-voltage cathodes require more robust formulations. LiFSI-based electrolytes demonstrate improved oxidative stability compared to LiPF6, enabling compatibility with nickel-rich or lithium-rich cathodes operating above 4.5 V.

A critical trade-off exists between high conductivity and electrochemical/thermal stability. While low-viscosity solvents and high salt concentrations enhance conductivity, they may compromise stability under extreme conditions. For instance, highly conductive electrolytes may exhibit faster degradation at high voltages or temperatures, reducing cycle life. Conversely, overly stable formulations may suffer from poor rate capability due to higher viscosity or lower ion mobility.

In summary, liquid electrolyte formulations for lithium-ion batteries require careful optimization of solvent blends and lithium salts to achieve the desired balance of ionic conductivity, viscosity, thermal stability, and electrochemical resilience. The choice of carbonates, their mixing ratios, and the selection of lithium salts all contribute to the overall performance and safety of the battery. While LiPF6 remains the industry standard due to its balanced properties, emerging salts like LiFSI offer promising alternatives for next-generation high-energy and high-voltage systems. Future advancements will likely focus on further improving stability without sacrificing conductivity, enabling safer and more efficient energy storage solutions.