Cathode Electrolyte Interphase-Forming Additive for Improving Cycling Performance and Thermal Stability of Ni-Rich LiNi_<i>x</i>Co_<i>y</i>Mn_{1–<i>x</i>–<i>y</i>}O₂ Cathode Materials

High-capacity Ni-rich LiNixCoyMn1–x–yO2 (NCM) has been investigated as a promising cathode active material for improving the energy density of lithium-ion batteries (LIBs); however, its practical application is limited by its structural instability and low thermal stability. In this study, we synthesized tetrakis(methacryloyloxyethyl)pyrophosphate (TMAEPPi) as a cathode electrolyte interphase (CEI) additive to enhance the cycling characteristics and thermal stability of the LiNi0.8Co0.1Mn0.1O2 (NCM811) material. TMAEPPi was oxidized to form a uniform Li+-ion-conductive CEI on the cathode surface during initial cycles. A lithium-ion cell (graphite/NCM811) employing a liquid electrolyte containing 0.5 wt % TMAEPPi exhibited superior capacity retention (82.2% after 300 cycles at a 1.0 C rate) and enhanced high-rate performance compared with the cell using a baseline liquid electrolyte. The TMAEPPi-derived CEI layer on NCM811 suppressed electrolyte decomposition and reduced the microcracking of the NCM811 particles. Our results reveal that TMAEPPi is a promising additive for forming stable CEIs and thereby improving the cycling performance and thermal stability of LIBs employing high-capacity NCM cathode materials