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.

Mixtures of triisobutylmethylphosphonium bis(fluorosulfonyl)imide (P_1i444FSI) with different concentrations of NaFSI display composition-dependent phase behaviour with depression of the melting point upon NaFSI addition and suppression of crystallisation in the middle of the phase diagram and a higher-melting-point, NaFSI-rich mixed-phase region at compositions beyond 60 mol% NaFSI. Thermal treatment of the intermediate compositions results in complete crystallisation of the materials. All compositions showed high ionic conductivity (>10⁻⁵ S cm⁻¹) at 20 °C, and Na symmetric cells containing high concentration of NaFSI (Na|[Na_0.9(P_1i444)_0.1]FSI|Na) were cycled efficiently at 50 and 90 °C at 0.05 and 0.1 mA cm⁻², respectively. Intermediate composition (Na|[Na_0.6(P_1i444)_0.4]FSI|Na) cells were cycled at room temperature and 50 °C at 0.25 mA cm⁻² with very low and stable cell polarisation (200 mV).

The phase behaviour, ionic conductivity, electrochemical stability and diffusion coefficients of mobile components in three organic ionic plastic crystals (OIPCs): triisobutylmethylphosphonium bis(fluorosulphonyl)amide (P_1i444FSI), triisobutylmethylphosphonium bis(trifluromethanesulphonyl)amide (P_1i444NTf₂) and trimethylisobutylphosphonium bis(trifluoromethanesulphonyl)amide (P_111i4NTf₂) are compared to study the effect of the anions and cations on phase behaviour and dynamics. The FSI-based OIPC shows lower melting point and higher conductivity values most likely because of the higher degree of charge distributions and weaker ion–ion interactions compared to NTf₂ anion-based OIPCs. Cyclic voltammetry of electrolytes consisting of these OIPCs with 70 mol% sodium salt incorporated indicates stable sodium plating/stripping behaviour at 70 and 50 °C for all samples. The magnitude of the peak currents, however, are much higher for the FSI-based electrolyte.