Fluorination and alkyl chain length effects on the oxidative stability of carbonate-based electrolytes

Fluorinated carbonate electrolytes are promising candidates for high-voltage lithium-ion batteries because of their enhanced oxidative stability. However, the effects of fluorination, alkyl chain length, and Li⁺ coordination on their oxidative stability remain unclear. This study investigates the oxidation potentials and electronic properties of a series of fluorinated carbonate electrolytes and Li⁺ complexes. Among the isolated electrolytes, di-2,2,2-trifluoroethyl carbonate had the highest predicted oxidation potential, whereas bis(4,4,4-trifluorobutyl) carbonate had the lowest. Increased fluorination enhances oxidation resistance, whereas longer alkyl chains reduce oxidative stability. Frontier molecular orbital analysis supports these trends. Topological analysis reveals non-covalent electrostatic interactions between the electrolytes and Li⁺, and Li⁺ coordination further improves oxidative stability. The oxidation potentials of both isolated electrolytes and electrolyte/Li⁺ complexes lie beyond the operating voltage range of conventional lithium-ion batteries, indicating excellent resistance to oxidative decomposition. Spin density analysis identifies the preferred oxidation sites in the oxidized species. The calculated oxidation potential of ethyl-2,2,2-trifluoroethyl carbonate agreed with the available experimental value, particularly at the BMK and G4MP2 levels. These findings provide molecular-level insights into the design of fluorinated carbonate electrolytes for next-generation high-voltage lithium-ion batteries.

Geometries and oxidation potentials were calculated using B3PW91 and BMK with the aug-cc-pVTZ basis set; G4MP2 calculations were used to assess the oxidation-potential predictions. Solvent effects were treated with the SMD continuum model using a dielectric constant representative of an ethylene carbonate/ethyl methyl carbonate mixture. Optimized structures were verified by vibrational-frequency calculations. HOMO energies, natural bond orbital charges, and Hirshfeld spin densities were analyzed. Quantum chemical calculations were performed with Gaussian 16, and electron-density topology was analyzed with Multiwfn.