Poly(ionic liquid) electrolytes at an extreme salt concentration for solid-state batteries

Abstract

Polymer-in-salt electrolytes offer a promising solution to the critical challenge of low Li-ion conductivity in solvent-free solid polymer electrolytes. One crucial aspect of their development is maintaining good stability and high conductivity of molten salts within a polymer system. Remarkably, cationic poly(ionic liquids) (polyIL) have emerged as a promising option. The high salt concentration in polyIL not only helps enhance ionic conductivity but also pushes the charge carrier ion transference number beyond 0.5. Nevertheless, stabilizing molten salt remains a challenging hurdle. Here, we report a novel poly(ionic liquid)s-in-salt system with an exceptionally high Li-salt content of up to 90 mol% by integrating a crystallization-resistive Li salt through an asymmetric anion. The resulting electrolyte maintains a stable amorphous phase and achieves considerable conductivity of 9.0×10^− 5 S cm^− 1 and an impressive Li transference number of 0.81 at 80°C. This leads to substantial improvements in electrolyte performance in prototype Li cells, including reduced interfacial resistance, lowered polarization, and a stable Li deposition/dissolution profile up to 0.5 mA cm^− 2. This work provides a valuable opportunity to revisit polymer-in-salt electrolytes at an extremely high salt concentration, contributing new insights into the relationships between high salt concentrations, coordination structures, glass transitions, conductivity, and the decoupling/coupling of ion transport from structural dynamics. It also emphasises the unique role of cationic polymers and opens new prospects for the future design of polymer-in-salt electrolytes.