Enhancing the Interfacial Stability of High‐Energy Si/Graphite || LiNi$_{0.88}$Co$_{0.09}$Mn$_{0.03}$O$_2$ Batteries Employing a Dual‐Anion Ionic Liquid‐based Electrolyte

The poorly flammable room-temperature ionic liquid-based electrolyte composed of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr$_{14}$FSI) with fluoroethylene carbonate (FEC) as an additive is investigated towards its compatibility with the LiNi$_{0.88}$Co$_{0.09}$Mn$_{0.03}$O$_{2}$ (NCM88) cathode and a high-capacity Si/graphite (SiG) anode, revealing a remarkably stable performance in lithium-ion cells. Interestingly, this dual-anion electrolyte with FEC additive forms a stable electrode-electrolyte interphase on both sides, which suppresses the morphological degradation of the electrode materials and continuous electrolyte decomposition. Consequently, lithium-ion cells using such dual-anion ionic liquid-based electrolyte display significantly improved cycling stability compared to conventional carbonate ester-based electrolyte, achieving a high specific energy of 385 Wh kg$^{-1}$ (based on both cathode and anode active materials weight) with a capacity retention of 74% after 200 cycles at 0.2 C, demonstrating the possibility to realize safe and high energy density LIBs.