Enhancing the Interfacial Stability of High‐Energy Si/Graphite||LiNi_0.88Co_0.09Mn_0.03O₂ Batteries Employing a Dual‐Anion Ionic Liquid‐based Electrolyte

Abstract The poorly flammable room‐temperature ionic liquid‐based electrolyte composed of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and N‐butyl‐N‐methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₄FSI) with fluoroethylene carbonate (FEC) as an additive is investigated towards its compatibility with the LiNi_0.88Co_0.09Mn_0.03O₂ (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.