PFAS-Free Locally Concentrated Ionic Liquid Electrolytes for Lithium Metal Batteries

Locally concentrated electrolytes are promising candidates for highly reversible lithium–metal anodes (LMAs) but heavily rely on cosolvents containing −CF$_3$ and/or −CF$_2$– groups. The use of these hazardous per- and polyfluoroalkyl substances (PFAS) leads to environmental and occupational safety concerns. Herein, ionic liquids and anisole are employed as solvents and cosolvent, respectively, to construct PFAS-free locally concentrated electrolytes. Anisole not only promotes the ion transport of the electrolytes via inducing a nanophase-segregation solution structure but also modulates the solid electrolyte interphase by affecting the deposition of organic cations and anions on LMAs as well as the conversion of anions to LiF. Optimizing the anisole content enables Li plating/stripping Coulombic efficiency up to 99.71% from 99.19% achieved with the anisole-free ionic liquid electrolyte. As a result, Li/LiFePO$_4$ and Li/sulfurized-polyacrylonitrile cells employing such an electrolyte and 1.5-fold lithium metal excess achieve stable cycling for 400 and 350 cycles, respectively, with 90% capacity retention.