Phosphonium Poly(Ionic Liquid) Electrolytes for Fast Lithium-Ion Conduction

Polymer electrolytes based on ionic liquids provide a safe solution for future solid-state high-energy-density batteries. In this work, we report a novel class of solid polymer electrolytes for fast lithium-ion conduction based on phosphonium poly(ionic liquid)s (poly(IL)s). First, a new family of poly(diallyldimethylphosphonium) poly(IL)s was synthesized by cyclopolymerization of diallyldimethylphosphonium iodide following anion exchange with different sulfonamide-based anions. The characterization of the poly(IL)s confirmed the formation of a stable 5-member ring phosphonium cationic polymer backbone. Then, solid polymer electrolytes (SPEs) were prepared by doping the phosphonium sulfonamide poly(IL)s with different contents of lithium fluorosulfonamide salt (LiFSI). The nature of the ionic interactions and lithium transport of SPEs with different anion systems was deeply investigated by FTIR, DSC, ionic conductivity, solid-state ⁷Li NMR, and molecular modeling simulations. Polymer-in-salt phosphonium SPEs showed excellent properties, with high ionic conductivities up to 1.5 × 10^-3 S cm^-1 at 80 °C, high lithium transference numbers up to 0.7, and a wide electrochemical stability window superior to state-of-the-art SPEs. Finally, we showed that phosphonium poly(IL) electrolytes allowed successful Li⁺ plating/stripping as a solid electrolyte in lithium metal symmetrical cells, showing constant overvoltage limited to 0.06 V working at 0.1 mA cm^-2 current density at 60 °C, and stable cycling without signs of short-circuiting during 200 cycles at 40 °C.