Decoupling Ion Transport and Desolvation via Spatially Heterogeneous Solvation Structure for Wide-Temperature Sodium-Ion Batteries

Despite competitive room-temperature performance, sodium-ion batteries suffer from sluggish kinetics and unstable interphases at ultralow temperatures. Herein, a single-ether (diethylene glycol dibutyl ether, DGDE)-based electrolyte featuring a spatially heterogeneous solvation structure across both the bulk and interfacial regions is successfully constructed by introducing a strongly polar sulfonate ester additive, 2,2,2‑trifluoroethyl trifluoromethanesulfonate (TTMS). In the bulk, DGDE chelates Na⁺ via its multiple coordination sites to form a solvent‑separated ion pair dominated solvation structure, thereby enhancing ion dissociation and ionic conductivity. At the electrode–electrolyte interface, TTMS preferentially adsorbs onto the cathode surface, reconstructing the electric double layer into a compact, anion-rich configuration dominated by contact ion pairs and aggregates. Meanwhile, TTMS in the inner Helmholtz plane provides desolvation-active sites, lowering the charge-transfer barrier and enabling the formation of a robust, inorganic-rich interphase. This spatially heterogeneous solvation structure enables the decoupling of fast bulk ion transport and rapid interface desolvation. Consequently, at −40°C, the Na||NaNi_1/3Fe_1/3Mn_1/3O₂ cell with the optimized electrolyte delivers an initial specific capacity of 109.9 mAh g⁻¹ and sustains reversible cycling for 140 cycles with a capacity retention of 87.3%. Moreover, the cell demonstrates reliable electrochemical operation over a wide-temperature range from −60°C to 55°C.