Investigating the role of mixed-cation ionic liquid electrolytes in sodium battery efficiency and stability

Abstract

This study explores the influence of mixed-cation ionic liquid (IL)-NaFSI based electrolyte systems on their physicochemical and electrochemical properties. Utilising two ionic liquids with distinct cation chemistries, (trimethyl isobutyl phosphonium) P_111i4⁺ and (N-methyl-N-propylpyrrolidinium) C₃mpyr⁺, combined in various ratios with either a constant 42 mol% NaFSI salt concentration or a saturated NaFSI concentration, we examined their thermal behaviour, ionic conductivities, and electrochemical properties over a range of temperatures. We discovered that NaFSI addition disrupts in the neat IL crystallization in both salt concentrations (42 mol% and saturated), resulting in only glass transition temperatures (T_g) that were lower for P_111i4/NaFSI than in C₃mpyrFSI/NaFSI systems indicating an influence of the cation type on the thermal properties of these IL electrolytes. In both the 42 mol% NaFSI and saturated NaFSI systems, T_g in C₃mpyrFSI/P_111i4FSI/NaFSI mixtures slightly decreased as the P_111i4FSI content increased. This was consistent with a slight increase in ionic conductivity and in cation and anion diffusion coefficients with higher P_111i4FSI content and temperature. Despite modest changes in ionic conductivity and diffusion coefficients, the electrochemical behaviour shows increased current density and earlier sodium plating initiation with increasing P_111i4FSI concentration, highlighting the potential of cation mixing to enhance electrochemical properties suggesting that cation mixing shows potential for improved electrochemical properties through optimized IL electrolyte composition. This work underscores the feasibility of optimizing IL electrolyte compositions for improved performance and stability in sodium batteries, paving the way for future research on cation chemistry effects and practical applications in high-temperature (50 °C) Na–metal-based batteries.