Effect of Side‐Chain Modifications on Salt‐Concentrated Poly(Ionic Liquid) Electrolytes

Poly(ionic liquid)‐in‐salt (PolyIL‐in‐salt) systems offer a promising pathway towards next‐generation, safe, and high‐performance all‐solid‐state alkali metal batteries. This study investigates the effect of side‐chain modifications on ion transport in both neat and salt‐containing polymer systems based on polydiallyldimethylammonium bistriflimide. Three types of side chains, i.e., methoxyethane, butyl, and isobutyl chains, were introduced to replace one methyl group to evaluate the influences of functional group, side‐chain length, and branching. All modified polymers exhibit decreased glass transition temperature and increased anion diffusion in the neat state, which can be associated with weakened polycation‐anion interaction. In salt‐concentrated systems, ion diffusion varied between lithium and sodium systems, with a possible correlation to the change in free volume. Notably, the methoxyethane‐substituted system shows the highest ionic diffusivity for both salts. This work provides valuable insights for the rational design of PolyIL‐in‐salt electrolytes with enhanced ionic conductivity.