Thermal Stability and Electrochemical Properties of Fluorine Compounds as Nonflammable Solvents for Lithium-Ion Batteries

Differential scanning calorimetry study demonstrated that mixing of fluoro-ethers and fluoro-carbonates improved the thermal stability of 0.67 mol/L –ethylene carbonate (EC)/diethyl carbonate (DEC)/propylene carbonate (PC) (1:1:1 by volume). The oxidation currents were smaller in the fluorine compound-mixed electrolyte solutions than in 0.67 mol/L –EC/DEC/PC, which also shows a high stability of the fluorine compound-mixed electrolyte solutions against electrochemical oxidation. Electrochemical reduction of fluorine compounds took place at the higher potentials than EC, DEC, and PC, as suggested by the highest occupied molecular orbital and lowest unoccupied p-molecular orbital energies of the fluorine compounds. However, charge/discharge experiments using natural graphite (NG) electrodes showed that the fluorine compounds increased first coulombic efficiencies due to the quick formation of the solid electrolyte interphase on NG in PC-containing solvents.