Nonflammable Fluorinated Ester-Based Electrolytes for Safe and High-Energy Batteries with LiCoO₂

To further improve the energy density and safety of Li-ion batteries (LIBs), multifunctional electrolyte solvents are needed to replace conventional carbonate solvents. In this study, a nonflammable fluorinated ester, methyl 3,3,3-trifluoropropionate (MTFP), is evaluated as an electrolyte solvent for high-voltage Li batteries with the LiCoO₂ positive electrode. A Li/LiCoO₂ cell with an MTFP-based electrolyte exhibits superior capacity retention compared with a cell with a conventional carbonate-based electrolyte with a cutoff voltage of 4.5 V. Moreover, the LiCoO₂ composite electrode with sodium carboxymethyl cellulose and styrene-butadiene rubber as binders, instead of the commonly used poly­(vinylidene fluoride), can be cycled in the MTFP-based electrolyte without capacity loss or increase in polarization under high-voltage operation. The low-temperature performance and thermal stability of the LiCoO₂ electrode are also improved by using the MTFP-based electrolyte. The analysis by X-ray photoelectron spectroscopy of the LiCoO₂ electrode cycled in the MTFP-based electrolyte suggests that a thin and uniform passivation layer is formed on the electrode surface, resulting in excellent cyclability and thermal stability for LiCoO₂. The insights related to nonflammable electrolytes contribute to the development of high-energy LIBs without sacrificing safety.