Abstract: Electrolyte formulations for lithium-ion batteries are disclosed that enable high-voltage operation, improved cycle life, reduced impedance, and enhanced safety. The formulations comprise an aprotic organic solvent, a lithium salt, and an epoxide functionalized organic compound, and may further include additives such as organosulfur compounds, alkyl nitriles, fluorinated aliphatic esters, cyclic carbonates, partially fluorinated phenyl phosphate esters, and fluorinated cyclic phosphazenes. These components synergistically stabilize the cathode electrolyte interface (CEI), mitigate oxidative decomposition at elevated voltages, and impart flame-retardant properties. Electrochemical energy storage devices incorporating these formulations exhibit superior performance under high-voltage and high-temperature conditions, making them suitable for applications such as electric vehicles, aerospace systems, and grid storage.