Fluoroethylene Carbonate: Bis(2,2,2,) Trifluoroethyl Carbonate as High Performance Electrolyte Solvent Blend for High Voltage Application in NMC811|| Silicon Oxide‐Graphite Lithium Ion Cells

LiNi_xMn_yCo_zO₂ cathode materials combined with Si‐based anode materials are current state‐of‐the‐art high energy density chemistries for lithium ion batteries. Increasing the upper cut‐off voltage is an intriguing approach to achieve even higher energy density in lithium ion batteries. However, poor oxidation stability of the state‐of‐the‐art electrolytes leads to transition metal dissolution (TMD), migration, and deposition (TMDMD) on the negative electrode, followed by sudden and rapid capacity fade. Furthermore, the chemical instability of the lithium hexafluorophosphate causes hydro‐fluoric acid to develop, which targets the native SiO_x layers on silicon anodes and breaks the chemical bond to the carboxymethylcellulose sodium salt binder. Herein, a fluorine‐rich electrolyte formulation consisting of lithium‐bis(fluorsulfonyl)imide with fluoroethylene carbonate (FEC): bis(2,2,2,) trifluoroethyl carbonate (BFEC) was applied in NMC811||10%SiO_x‐90%graphite cells to achieve high oxidation stability and prevent TMD and deposition. Up‐to‐date, this is the premier electrochemical performance reported in literature with a capacity retention of 94.5% and 92.2% with 0.5 °C and 4.5 V upper cut‐off voltage cycling at 20 and 40 °C after 100 cycles, respectively. The post mortem analysis showed that stabilization is achieved by forming inorganic‐ and salt‐rich interphases that protect the electrolyte versus decomposition at the electrode.