High-Temperature Cycling Performance of Lithium-Ion Batteries with NMC Cathodes and Lithium Difluorophosphate and Vinylene Carbonate

High-temperature lithium-ion battery operation (approaching 100 °C) is challenging due to accelerated degradation of critical cell components, limiting their use in many applications. In this work, we investigate the cycling performance and thermal stability of LiNi0.33Mn0.33Co0.33O2 (NMC111) and LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes in coin cells with various additives designed for 100 °C operation. A full-factorial design of experiments varied cathode composition and electrolyte additives, including vinylene carbonate (VC) and lithium difluorophosphate (LiDFP), to statistically evaluate their effects on capacity retention, internal cell resistance, and coulombic efficiency at 100 °C. Cells with NMC111 cathodes, featuring either additive, demonstrated superior thermal stability and markedly longer cycle lives at 100 °C compared to NMC811 cells with additives. NMC811 cells exhibited a higher initial specific capacity, as expected for Ni-rich cathodes, but suffered rapid capacity fade at 100 °C. The LiDFP additive further accelerated capacity loss in NMC811 cells, whereas VC provided minimal performance improvements. Further, the addition of LiDFP generally lowered the cells’ DC resistance, while NMC111 or cells with VC exhibited higher DC resistances. Interestingly, LiDFP exhibited lower, less stable coulombic efficiencies, whereas the addition of VC led to higher, more stable coulombic efficiencies. Energy-dispersive X-ray spectroscopy showed higher fluorine content on both electrodes in cells with low impedance and low capacity retention (cells without VC or with NMC8111), suggesting that these systems promote the formation of lithium fluoride-containing compounds on the electrode interphases, which consume lithium inventory, reduce interphase impedance, and fail to passivate the electrodes. These changes were present under open-circuit conditions and, to a greater extent, during cycling at elevated temperatures, suggesting that parasitic interphase-forming reactions were both chemical and electrochemical. Overall, the results highlight that NMC111 cathodes paired with additives to improve the stability of the solid electrolyte interface and cathode electrolyte interface can enable significantly improved cycle life at 100 °C, whereas Ni-rich NMC811 cathodes are prone to faster performance degradation at elevated temperatures.