Enhancing High‐Rate Capability by Introducing Phosphonate Functionalized Imidazolium Ionic Liquid into Organic Carbonate Electrolyte

A phosphonate functionalized imidazolium ionic liquid (PFIL) is synthesized and studied as an electrolyte additive for lithium ion batteries. The Li/LiFePO4 cells with addition of PFIL shows superior electrochemical performance, including an increased initial Coulombic efficiency, better capacity retention, and improved rate capability. The electrochemical mechanism of PFIL used in high‐performance batteries is investigated by using fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). Our experimental result reveals that the phosphonate groups are stronger donors than carbonate groups, thus change the solvation environment of lithium ions by competitive coordination, assisting the transferring of lithium ions. Therefore, an excellent cyclability and superior rate capability with PFIL‐based electrolyte could be explained by enhanced mobility lithium ions via coordination to phosphonate groups on imidazolium cations. This work displays that PFIL‐based electrolyte systems can be considerable potential candidates for the applications in high‐performance Li‐ion batteries. Cells of Li/LiFePO4, featuring with phosphonate‐functionalized ionic liquid as a key electrolyte additive, exhibit appreciable improvement of electrochemical properties, especially remarkable rate capability. Phosphonate groups tethered at cations of ionic liquids contribute to the improved performance by assisting transferring of lithium ions.