A new ionic liquid-based electrolyte with both high conductivity and electrochemical stability

Ionic liquids with non-flammability and wide electrochemical window are the promising candidates for the next-generation high-energy lithium metal batteries (LMBs). However, their high viscosity and low ionic conductivity lead to a limited cycling performance, hindering their commercial application. Herein, a new ionic liquid-based electrolyte (ILE) composed of lithium bis(fluorosulfonyl)imide (LiFSI), N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₃FSI), ethyl methyl carbonate (EMC) and fluorinated ethylene carbonate (FEC) is designed to address this barrier. The new ILE exhibits high conductivity, which is twice that of pure ILE. In addition, FEC that is weakly coordinated to Li⁺ increases the combination between Li⁺ and FSI⁻ indirectly, inducing a deeper co-decomposition of FSI⁻ and FEC, thereby leading to a construction of LiF-rich solid electrolyte interphase (SEI). Consequently, Li/Cu battery maintains a higher lithium stripping/plating Coulombic efficiency (CE) of 96.3 % at 250 cycles and Li/LiFePO₄ battery reaches a 93.2 % capacity retention at 150 cycles with an average CE of 99.8 %. The designed ILE with both high conductivity and electrochemical stability opens up a new insight for the rational design of electrolytes applied in next-generation LMBs.