A Complex-Anion-Doped Ionic-Liquid Electrolyte for High-Voltage Cathodes in Lithium-Ion Batteries

High-voltage cathodes can increase the energy density of lithium-ion batteries, but they impose stringent requirements on electrolyte oxidation resistance and safety. Traditional carbonate-based electrolytes can undergo oxidative decomposition at high potentials, which limits their application to high-voltage cathodes. Ionic liquids (ILs) are considered to be promising candidates for the construction of high-safety and high-voltage electrolytes due to their extremely low vapor pressure, low flammability and wide electrochemical window. In this paper, a multi-anion modification strategy was introduced into the ionic liquid electrolyte to create a complex-anion-doped ionic liquid electrolyte (CAD-ILE), which modifies the local ionic environment while retaining the main spectral features of the ionic liquid. On a formulation-component basis, CAD-ILE contains 60 mol % EMIMTFSI, 37 mol % LiTFSI, and 3 mol % auxiliary salts (LiFSI, LiPF6, LiBOB, LiBF4, LiDFP, LiOTF, LiDFOB, and LiNO3; 0.375 mol % each). This formulation is associated with a thin, LiF-containing CEI on the cathode surface. The capacity retention of the CAD-ILE electrolyte is 71% after 200 cycles at 4.6 V in a LCO||Li half cell (only 32% in commercial electrolyte), and 87% after 120 cycles at 4.55 V in a LCO||graphite full cell. This work supports a simple and effective high-voltage electrolyte design strategy.