Hybrid Solvating Electrolytes for Practical Metal Batteries

Sodium-metal batteries have gained widespread attention due to higher abundance and lower cost of sodium. Their applications depend on good stability of electrolytes against sodium-metal anode and cathode simultaneously. Here, we propose hybrid solvating electrolytes (HSEs), composed of both strongly and weakly solvating solvents of sodium salts, to tune the solubility, solvation structure, and electrochemical decomposition properties. Fifty HSEs are prepared by mixing various pre-labeled solvent molecules, and changing the molar ratio among solvents, validating the mixture selection requirements and correlations between salt/solvent types and their mixture-dependent performance, including oxidative stability, Coulombic efficiency, and cycling overpotential. A model hybrid solvent formed by mixing weakly solvating N,N-dimethyltrifluoromethane sulfonamide (DMTMSA) with strongly solvating tetrahydrofuran (THF) demonstrates strong beyond-rule-of-mixture effects, showing extraordinarily stable cycling performance against Na3V2(PO4)3, Na0.44MnO2 and NaNi1/3Fe1/3Mn1/3O2 cathodes, and Na-metal anode. Spectroscopic analysis and molecular dynamics simulations reflect the corresponding change in ion-dipole interaction and solvation structures, leading to distinct electrochemical cycling behavior for sodium metal. The strong-weak hybrid solvating principle can also be extended to prepare HSEs for lithium-metal batteries. Our work presents a guiding principle for electrolyte design to enable practical alkali-metal batteries.