Bidentate Phosphonate‐Functionalized Ionic Liquid Exhibiting Better Ability in Improving the Performance of Lithium‐Ion Battery

In this work, flame retarding phosphonate‐functionalized imidazolium ionic liquids (PFILs) were synthesized and employed as electrolyte additives for Li/LiFePO4 half‐cells. Physical and electrochemical properties, including ionic conductivity, electrochemical properties, electrochemical oxidation limit, electrochemical impedance spectroscopy (EIS) and lithium‐ion transference number (TLi+), were investigated. The results demonstrate that the addition of PFIL can improve the electrochemical performance of lithium‐ion batteries (LIBs). Hybrid electrolyte with 10 wt % bidentate phosphonate‐functionalized imidazolium ionic liquid (b‐PFIL) exhibits more excellent electrochemical oxidation stability (up to 4.77 V vs. Li/Li+), capacity retention (87.3 % after 400 cycles) and rate capability. These improvements can be ascribed to the enhanced lithium‐ion transference number (TLi+), which results from the stronger binding affinity between chelating b‐PFIL and LiPF6. The Li/LiFePO4 half‐cells with PFIL‐hybridized electrolytes exhibit excellent cycling performance stability and capacity retention. Compared to the monodentate phosphonate‐functionalized imidazolium ionic liquids (m‐PFIL), the bidentate PFIL (b‐PFIL) can more efficiently coordinate to lithium ions and decrease the electrostatic interaction between lithium ions and PF6 anions, thus makes lithium ions moving faster. However, the non‐functional imidazolium HmImPF6 does not coordinate to lithium ions but increases the viscosity of electrolyte, and thus inverse impact on the electrochemical performance is observed.