Polymer-assisted deeply supercooled alkali-metal salts: Cation effects on ion transport and sodium electrolyte applications

Solvent-free molten salt electrolytes are attractive for advanced rechargeable batteries because they combine liquid-state processability, low flammability, and selective cation transport; however, alkali-metal salts crystallize near room temperature. Here, we report polymer-assisted deeply supercooled salts (DSSs) prepared by adding poly(methyl methacrylate) to alkali-metal bis(fluorosulfonyl)amide salts, M[FSA] (M = Li, Na, and K), and clarify how cation species governs supercooled-state stability and ionic conductivity. At an 80/20 mol% salt/polymer composition, all systems formed supercooled liquids, but supercooled-state lifetimes decreased with increasing cation size. Spectroscopy and calculations revealed stronger cation-polymer interactions for smaller cations. In contrast, ionic conductivity increased with cation size, from 5.7 × 10 −5 S cm −1 for Li-DSS to 8.8 × 10 −4 S cm −1 for K-DSS at 60°C. Fragility analysis showed that m -fragility increased from Li-DSS (60) to Na-DSS (107) and K-DSS (138), indicating increasingly fragile dynamics with increasing cation size. The Na-based DSS showed an apparent oxidative limit of 5.3 V vs. Na/Na + under a current-density criterion of 10 µA cm −2 , and a sodium transference number of 0.87 at 80°C, and it sustained sodium plating/stripping for 400 h at 70°C. These results demonstrate cation-dependent coordination and fragility as design parameters for high-transference-number DSS electrolytes.