Electrolytes serve as the critical ionic conduction medium in lithium-ion batteries, facilitating lithium-ion transport between the cathode and anode while maintaining electronic insulation. The formulation of these electrolytes significantly impacts battery performance, safety, and longevity. Conventional lithium-ion electrolytes consist of three primary components: lithium salts, organic solvents, and functional additives. Each component is carefully selected to balance ionic conductivity, electrochemical stability, and safety.

The solvent system in liquid electrolytes typically comprises cyclic and linear carbonates, which offer a combination of high dielectric constant and low viscosity. Ethylene carbonate (EC) is a ubiquitous cyclic carbonate due to its high dielectric constant, which enhances salt dissociation, and its ability to form a stable solid-electrolyte interphase (SEI) on graphite anodes. However, EC has a high melting point, requiring blending with low-viscosity linear carbonates like dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC) to improve low-temperature performance. A common solvent blend is EC:EMC or EC:DMC in a 3:7 volume ratio, providing a balance between ion transport and SEI-forming capability. Propylene carbonate (PC) is avoided in graphite-based systems due to its tendency to co-intercalate and exfoliate graphite, though it is sometimes used in high-voltage applications with alternative anodes.

Lithium salts provide the source of lithium ions for conduction. Lithium hexafluorophosphate (LiPF6) is the most widely used salt due to its acceptable conductivity, moderate cost, and reasonable stability in carbonate solvents. However, LiPF6 is thermally unstable, decomposing into PF5 and LiF at elevated temperatures, with PF5 further reacting with solvents to form acidic byproducts that degrade battery performance. Alternative salts like lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI) offer superior thermal stability and conductivity but face challenges such as aluminum current collector corrosion at high voltages. Lithium tetrafluoroborate (LiBF4) exhibits better low-temperature performance but suffers from lower conductivity. The concentration of lithium salt typically ranges from 0.8 M to 1.2 M, as higher concentrations increase viscosity without proportional gains in conductivity.

Additives are incorporated in small quantities (typically 0.5-5% by weight) to enhance specific electrolyte properties. SEI-forming additives like vinylene carbonate (VC) and fluoroethylene carbonate (FEC) preferentially decompose before the base solvents, creating a stable passivation layer on the anode that prevents further electrolyte reduction and lithium inventory loss. VC is particularly effective in graphite systems, while FEC is favored in silicon-containing anodes due to its ability to accommodate larger volume changes. Overcharge protection additives, such as biphenyl (BP) and cyclohexylbenzene (CHB), shuttle electrons at high voltages to prevent catastrophic oxidation of the electrolyte. Flame-retardant additives like trimethyl phosphate (TMP) and phosphazenes reduce electrolyte flammability but often compromise ionic conductivity. Lithium difluorophosphate (LiPO2F2) and lithium bis(oxalato)borate (LiBOB) are dual-function additives that improve both SEI stability and cathode protection.

The SEI layer is a critical component that forms during the initial charging cycles, primarily through the reductive decomposition of electrolyte components. A well-formed SEI is ionically conductive but electronically insulating, preventing continuous electrolyte breakdown while allowing lithium-ion transport. The composition of the SEI depends on the electrolyte formulation, with EC-based electrolytes typically forming a stable layer rich in lithium ethylene dicarbonate (LEDC). Additives like VC introduce polycarbonate species that enhance SEI elasticity and adhesion. Inadequate SEI formation leads to increased impedance, capacity fade, and lithium plating risks, particularly at low temperatures or high charging rates.

Flammability remains a significant concern for conventional liquid electrolytes due to the volatile nature of organic carbonates. The flash points of common solvents like EMC and DMC are below 30°C, making them prone to ignition upon thermal runaway. Strategies to mitigate flammability include using fluorinated solvents, which exhibit higher flash points and reduced vapor pressure, or incorporating non-flammable hydrofluoroethers (HFEs) as co-solvents. However, these approaches often come with trade-offs in wettability and ion transport. Polymer electrolytes, such as those based on polyethylene oxide (PEO) complexed with lithium salts, offer improved safety by eliminating volatile solvents, but they suffer from low room-temperature conductivity (10^-6 to 10^-4 S/cm) compared to liquid electrolytes (10^-2 to 10^-3 S/cm). Gel polymer electrolytes (GPEs) strike a middle ground by immobilizing liquid electrolytes in a polymer matrix, providing enhanced mechanical stability while retaining reasonable conductivity.

Conductivity in liquid electrolytes is governed by the dissociation of lithium salts and the mobility of resulting ions. Higher dielectric constants promote salt dissociation, while lower viscosity facilitates ion mobility. EC-DMC blends achieve conductivities around 10 mS/cm at room temperature, sufficient for most applications. Temperature dependence follows an Arrhenius relationship, with conductivity dropping sharply below -20°C due to increased viscosity and reduced ion mobility. Additives like succinonitrile (SN) can improve low-temperature performance by lowering crystallization tendencies.

Degradation mechanisms in lithium-ion electrolytes include thermal decomposition, hydrolysis, and electrochemical oxidation/reduction. LiPF6 hydrolysis generates HF, which corrodes electrode materials and degrades SEI integrity. Water contamination must be kept below 20 ppm to minimize these effects. At high voltages (>4.3 V vs Li/Li+), solvents oxidize to form CO2 and other gaseous products, while repeated cycling leads to additive depletion and SEI thickening. Advanced electrolyte systems for high-voltage cathodes (>4.5 V) often incorporate sulfones or nitriles as co-solvents for their wider electrochemical windows, though these typically require compatibility-enhancing additives due to poor anode SEI formation.

Polymer electrolytes face distinct challenges in lithium-ion batteries. PEO-based systems require temperatures above 60°C to achieve practical conductivity due to crystalline domain formation. Crosslinking or copolymerization strategies can reduce crystallinity, while ceramic fillers like Al2O3 or TiO2 provide additional conduction pathways. Plasticized polymer electrolytes incorporate small amounts of liquid carbonates to boost conductivity but reintroduce some flammability risks. The interfacial stability between polymer electrolytes and electrodes is generally poorer than with liquid systems, requiring careful optimization of salt concentration and polymer chemistry.

Future developments in electrolyte formulation focus on extending electrochemical stability windows, improving safety without sacrificing performance, and enabling compatibility with next-generation electrodes. Multi-component additive packages are becoming increasingly sophisticated, targeting simultaneous improvement of SEI quality, cathode protection, and thermal stability. Computational modeling aids in screening potential new solvents and salts by predicting properties like oxidation potentials and solvation structures. While incremental improvements continue in conventional carbonate systems, fundamental shifts toward non-flammable ionic liquids or highly concentrated electrolytes may emerge as viable alternatives for specialized applications. The ideal electrolyte remains elusive, requiring case-specific optimization based on battery chemistry, operating conditions, and performance priorities.