Sodium versus Lithium: How Solvation Improves Battery Behavior

Increasing demand for lithium-ion (Li-ion) batteries necessitates diversification of battery materials to ease the strain on supply chains. Other alkali metals, such as sodium (Na), are intriguing due to their global abundance. Owing to differences in atomic size and reactivity, research on lithium is not always directly transferable to sodium batteries. Herein, we compare LiTFSI and NaTFSI electrolytes in blends of diglyme and fluoroethylene carbonate. We initially observed with optical microscopy that Na systems electrodeposit more smoothly than Li in diglyme blends, motivating our subsequent measurements that show Na⁺ interacting weakly with diglyme compared to Li⁺. This causes profound effects on solution dynamics, manifesting in faster Na⁺ transport. In support of this conclusion, we use fringe field NMR, viscosity-conductivity analysis, and molecular dynamics simulations to show that Na⁺ moves more freely than Li⁺, particularly in solvent blends dominated by diglyme. Ultrafast infrared techniques, including pump-probe anisotropy, show that Li⁺ tends to structure the solvent more rigidly than Na⁺, and this ordering effect can be sensed by distal molecules such as carbonate additives. In summary, our findings contribute to the emerging consensus that electrolytes engineered with weak solvation improve transport and charge transfer kinetics, highlighting the potential of Na-based systems as a more abundant alternate to Li-ion batteries.