Phosphonium plastic crystal salt alloyed with a sodium salt as a solid-state electrolyte for sodium devices: phase behaviour and electrochemical performance

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).