Ion conduction and phase behaviour in dual cation polyelectrolyte blends for sodium-ion batteries†

Emerging battery technologies such as solid-state sodium batteries can benefit from new polymer electrolytes with improved sodium ion transport to optimise electrochemical performance. In this work, we propose, for the first time, the use of polyelectrolyte blends utilising a dual cation approach with a common polyanion backbone, poly(1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethanesulfonyl)imide) (polyMTFSI). Thus, three new anionic polyelectrolytes were synthesised based on polyMTFSI having three different counter cations such as sodium (Na) (polyMTFSI-Na), trimethyl(isobutyl)phosphonium (poly-MTFSIP_111i4) and diethyl (isobutyl)(methyl)phosphonium (polyMTFSI-P_122i4). The miscibility between the polyelectrolytes in blends was determined by observing a single glass transition, T_g, for different compositions. Upon the addition of bulky organic cations, an increase in the dynamics and ionic conductivity was observed. Finally, we investigated the effect of NaFSI as an additional component in a ternary electrolyte system, whereby the salt acted as a plasticizer, decreasing T_g, and further enhancing the ionic conductivity.