Phosphonate‐functionalized Ionic Liquid: A Novel Electrolyte Additive for Eenhanced Cyclic Stability and Rate Capability of LiCoO2 Cathode at High Voltage

Phosphonate‐functionalized ionic liquid (PFIL) is employed as a multifunctional additive to the base electrolyte mixture consisting of ethylene carbonate, dimethyl carbonate and LiPF6. Addition of PFIL (5 wt%) significantly improves the cyclic stability and rate capability of LiCoO2 cathode performed between 3.0 and 4.3 V (vs Li/Li+), and decreases the flammability of the base electrolyte as well. Electrochemical studies indicate that PFIL is preferentially oxidized on cathode to form robust cathode electrolyte interface (CEI), which contributes to the enhanced cyclic stability and rate capability. Morphological and surface analysis on the cycled cathodes also reveal that PFIL is a key component for the formation of thin CEI film and retention of structural integrity of cathode material. Comparison experiments verify that chemically linking between ionicity and phosphorus functionality is indispensable for the enhanced performance. Additives of individual nonfunctionalized ionic liquid, neutral phosphonate, or their mixture all fail to improve the electrochemical performances. Phosphonate‐functionalized ionic liquid (PFIL) is proposed as an effective multifunctional additive. The excellent high‐voltage stability of LiCoO2 cathode is ascribed to the stable and uniform CEI film originated from the oxidative decomposition of PFIL.