Effects of Li Salt and Additive Content on the Electrochemical Performance of [C4C1mim]-Based Ionic Liquid Electrolytes

Ionic liquids based on imidazolium cations have attracted attention due to their high safety and exceptional ionic conductivity. However, imidazole-based ionic liquids exhibit poor electrochemical stability due to the strong reactivity of hydrogen atoms at the C-2 position of imidazole cations. In this work, an ionic liquid 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([C4C1mim][TFSA]), characterized by a methyl-substituted C-2 position and a butyl chain, was investigated in various Li+ environments created by different lithium salt concentrations and fluoroethylene carbonate (FEC) additives. Both optimal Li+ concentrations and the addition of reasonable FEC enable the improvement of ionic conductivity to 3.32 mS cm−1 at 25 °C and a maximum electrochemical window of 5.21 V. The ionic liquid electrolyte Li[TFSA]-[C4C1mim][TFSA] at a molar ratio of 2:8 with 5 wt% FEC addition demonstrates excellent thermal stability. The corresponding Li/LiFePO4 cell exhibits a mitigated polarization growth (increasing from 0.12 V to 0.25 V over 10 cycles) with a high initial discharge capacity of 169.3 mAh g−1.