Enhanced ion-transport characteristics of pyrrolidinium-based electrolytes with Mg(FSA)₂

Organic ionic plastic crystals (OIPCs) are gaining attention as next-generation solid electrolytes owing to their excellent thermal and electrochemical stability and high ionic conductivity. However, reports on the application of OIPCs to Mg electrolytes are limited. In this study, Mg-ion conductors based on the bis(fluorosulfonyl)amide (FSA) anion were prepared by adding Mg(FSA)₂ to the pyrrolidinium-based OIPC N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C₂epyr][FSA]). We then evaluated the Mg transference number (t_Mg²⁺), ionic conductivity, phase transition behaviour, and Mg-ion solvation number (with FSA anions) of the resultant materials to investigate how the Mg-salt concentration influences their ionic conduction characteristics and thermal stability. The [C₂epyr][FSA]/Mg(FSA)₂ composites were solid at room temperature when the Mg-salt concentration was below 10 mol% but became liquid at concentrations above 10 mol%. Among the composites investigated in this study, [C₂epyr][FSA]/Mg(FSA)₂ (10 mol%) showed the highest ionic conductivity, which reached 1.78 × 10^-3 S cm^-1 at 25 °C. When the Mg-ion concentration was above 10 mol%, the t_Mg²⁺ of [C₂epyr][FSA]/Mg(FSA)₂ was below 0.16. By contrast, when the Mg-salt concentration was below 10 mol%, the t_Mg²⁺ of [C₂epyr][FSA]/Mg(FSA)₂ was 0.26 or higher, surpassing those of the liquid-state composites. Cyclic voltammetry measurements revealed that [C₂epyr][FSA]/Mg(FSA)₂ (5 mol%) exhibited the redox behaviour of Mg at 60 °C. These results indicate that FSA-anion-based composites function more effectively as Mg-ion conductors in the solid state than in the liquid state.