Hybrid Solvating Electrolytes for Practical Sodium Metal Batteries

Sodium-metal batteries could be competitive against Li-metal batteries ["Earth-Abundant Na-Mg-Fe-Mn-O Cathode with Reversible Hybrid Anionic and Cationic Redox," 10.1002/aenm.202300746; 10.1039/D4EE01867B], but 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 with sodium-cation salts, to tune the solvation ability and modify the electrochemical properties. The composition-dependent performance, including coulombic efficiency and cycling overpotential, follows the volcano plot paradigm, revealing the “Sabatier optimum” for rational design of HSEs. Spectroscopic analysis reflects the corresponding change of ion-dipole interaction and solvation structures. A model hybrid solvent 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 with Na3V2(PO4)3, and Na0.44MnO2 cathodes and Na-metal anode. In addition, seventeen HSEs, including up to five hybrid solvents, are prepared and studied, presenting a guiding principle about the electrolyte design for practical sodium-metal batteries.