CF₃-Substituted Ethylene Carbonates for High-Voltage/High-Energy Rechargeable Lithium Metal-LiNi_0.8Co_0.1Mn_0.1O₂ Batteries

The development of advanced liquid electrolytes for high-voltage/high-energy rechargeable Li metal batteries is an important strategy to attain an effective protective surface film on both the Li metal anode and the high-voltage composite cathode. Herein, we report a study of two CF₃-substituted ethylene carbonates as components of the electrolyte solutions for Li metal|NCM811 cells. We evaluated trifluoromethyl ethylene carbonate (CF₃-EC) and trans-ditrifluoromethylethylene carbonate Di-(CF₃)-EC as cosolvents and additives to the electrolyte solutions. Using CF₃-substituted ethylene carbonates as additives to a fluoroethylene carbonate (FEC)-based electrolyte solution enables improved capacity retention of high-power Li metal|NCM811 cells. The composition of the products from the transformations of CF₃-EC and Di-(CF₃)-EC in Li|NCM811 cells was studied by FTIR, XPS, and ¹⁹F NMR spectroscopy. We concluded that fluorinated Li alkyl carbonates are the main reaction products formed from these cyclic carbonates during the cycling of Li|NCM 811 cells, and fragmentation of the ring with the formation of CO₂, CO, or olefins is not characteristic of CF₃-substituted ethylene carbonates. The NCM 811 cathodes and Li metal anodes were characterized by X-ray diffraction, SEM, XPS, and FTIR spectroscopy. The role of CF₃-substituted ethylene carbonate additives in stabilizing high energy density secondary batteries based on Li metal anodes was discussed. A bright horizon for developing sustainable rechargeable batteries with the highest possible energy density is demonstrated.