Poly(Ionic Liquid) Electrolytes at an Extreme Salt Concentration for Solid-State Batteries

Polymer-in-salt electrolytes were introduced three decades ago as an innovative solution to the challenge of low Li-ion conductivity in solvent-free solid polymer electrolytes. Despite significant progress, the approach still faces considerable challenges, ranging from a fundamental understanding to the development of suitable polymers and salts. A critical issue is maintaining both the stability and high conductivity of molten salts within a polymer matrix, which has constrained their further exploration. This research offers a promising solution by integrating cationic poly­(ionic liquids) (polyIL) with a crystallization-resistive salt consisting of asymmetric anions. A stable polymer-in-salt electrolyte with an exceptionally high Li-salt content of up to 90 mol % was achieved, providing a valuable opportunity for the in-depth understanding of these electrolytes at an extremely high salt concentration. This work explicates how increased salt concentration affects coordination structures, glass transitions, ionic conductivity, and the decoupling and coupling of ion transport from structural dynamics in a polymer electrolyte, ultimately enhancing electrolyte performance. These findings provide significant knowledge advancement in the field, guiding the future design of polymer-in-salt electrolytes.