The stability of the electrolyte formulation in low-speed power lithium batteries directly affects the battery's cycle life, safety performance, and environmental adaptability. Its optimization requires a comprehensive approach across multiple dimensions, including lithium salt selection, solvent matching, additive synergy, process control, and environmental adaptability design, to construct an electrolyte system that balances chemical, thermal, and electrochemical stability.
Lithium salts are the core component of the electrolyte, and their chemical stability is fundamental to the overall electrolyte stability. While traditional lithium hexafluorophosphate (LiPF₆) exhibits excellent conductivity, it decomposes at high temperatures, producing corrosive substances such as HF, leading to electrode material degradation. To improve stability, novel lithium salts such as lithium bis(fluorosulfonyl)imide (LiFSI) can be introduced. The anionic structure of LiFSI, through the strong electron-withdrawing effect of fluorine atoms, reduces ion association and increases lithium-ion dissociation.
Furthermore, its thermal decomposition temperature is significantly higher than that of LiPF₆, and its reduction products can form a low-impedance SEI film, effectively suppressing side reactions between the electrolyte and the electrode. Furthermore, the purity control of lithium salts is crucial. Trace impurities can trigger catalytic decomposition reactions, thus requiring high-purity raw materials and rigorous purification processes. The choice of solvent system directly affects the physicochemical stability of the electrolyte. Cyclic carbonates (such as EC) have high dielectric constants but high viscosity at low temperatures; chain carbonates (such as DMC and EMC) have low viscosity but low boiling points and are easily volatile.
By mixing different solvents, complementary performance can be achieved. For example, mixing EC and EMC in a certain proportion can ensure the ionic conductivity of the electrolyte at low temperatures while increasing the boiling point at high temperatures and reducing volatilization losses. In addition, introducing fluorinated solvents (such as FEC) can further improve stability. The fluorine atom substitution in FEC lowers the reduction potential of the solvent, allowing it to preferentially form a stable SEI film on the negative electrode surface. Simultaneously, its high flash point significantly improves the thermal safety of the electrolyte.
Additives are key auxiliary components for improving electrolyte stability. Film-forming additives (such as VC and PS) can form a dense SEI film on the electrode surface, inhibiting direct contact between the electrolyte and the electrode and reducing side reactions. For example, VC is reduced to form a polycarbonate film on the negative electrode surface, effectively preventing solvent molecule co-intercalation and extending cycle life; PS can form a protective layer on the positive electrode surface, inhibiting the dissolution of transition metal ions. Flame retardant additives (such as TMP) increase the ignition temperature of the electrolyte through gas-phase or condensed-phase flame retardant mechanisms, reducing the risk of thermal runaway. Furthermore, overcharge protection additives (such as anisole) can undergo oxidative polymerization during battery overcharging, covering the positive electrode surface, interrupting the charging process, and preventing electrolyte decomposition caused by excessive voltage.
The electrolyte preparation process also significantly affects stability. Moisture and impurities are the main causes of electrolyte decomposition; therefore, the purity of raw materials and the humidity of the production environment must be strictly controlled. For example, lithium salts must be stored under dry inert gas protection, and solvents must be dried using molecular sieves to reduce moisture content. In the mixing process, the dissolution order of solvent and lithium salt, temperature control, and stirring speed all need to be optimized to avoid decomposition reactions caused by excessively high local concentrations. In addition, strict sealing tests are required after electrolyte filling to prevent the infiltration of moisture and oxygen from the air. Environmental adaptability design is a crucial direction for improving electrolyte stability. For low-speed power lithium batteries operating under extreme temperatures, wide-temperature-range electrolytes need to be developed. In low-temperature environments, adding low-freezing-point solvents (such as vinyl nitrate) or film-forming additives can reduce electrolyte viscosity and improve ion mobility. In high-temperature environments, high-boiling-point solvents (such as DEC) and thermally stable additives (such as LiPO₂F₂) are required to inhibit electrolyte volatilization and decomposition. Furthermore, for high-humidity environments, optimizing the electrolyte formulation, reducing the use of hygroscopic components, or adding moisture-proof additives can reduce the impact of moisture on stability.
Interfacial compatibility between the electrolyte and electrode materials is a core challenge for stability. The surface characteristics of electrode materials (such as graphite and silicon-carbon anodes) directly affect the quality of SEI film formation. For example, silicon-carbon anodes are prone to SEI film rupture due to volume expansion, leading to continuous electrolyte consumption. By adding elastic film-forming additives (such as a composite system of FEC and PS), a self-healing SEI film can be formed, adapting to volume changes and maintaining interfacial stability. Furthermore, residual alkali on the surface of cathode materials (such as high-nickel ternary cathodes) can easily catalyze electrolyte decomposition, necessitating surface coating or the addition of cathode protectants (such as LiDFOB) to suppress side reactions.
Improving the stability of electrolytes in low-speed power lithium batteries is a systematic project requiring synergistic optimization across multiple aspects, including lithium salts, solvents, additives, processes, environmental adaptability, and interfacial compatibility. By selecting highly stable lithium salts, rationally combining solvent systems, precisely adding functional additives, strictly controlling the preparation process, designing wide-temperature-range formulations, and optimizing interfacial chemistry, electrolyte systems with long cycle life, high safety performance, and excellent environmental adaptability can be constructed, providing a solid guarantee for the widespread application of low-speed power lithium batteries.
