Suppression of Interphase Dissolution Via Solvent Molecule Tuning for Sodium Metal Batteries

Solvent molecule tuning is used to alter the redox potentials of solvents or ion-solvent binding energy for high-voltage or low-temperature electrolytes. Herein, an electrolyte design strategy that effectively suppresses solid electrolyte interphase (SEI) dissolution and passivates highly-reactive metallic Na anode via solvent molecule tuning is proposed. With rationally lengthened phosphate backbones with ─CH₂ ─ units, the low-solvation tris(2-ethylhexyl) phosphate (TOP) molecule effectively weakens the solvation ability of carbonate-based electrolytes, reduces the free solvent ratio, and enables an anion-enriched primary Na⁺ ion solvation sheath. The decreased free solvent and compact lower-solubility interphase established in this electrolyte prevent electrodes from continuous SEI dissolution and parasitic reactions at both room temperature (RT) and high temperature (HT). As a result, the Na/Na₃ V₂ (PO₄ )₃ cell with the new electrolyte achieves impressive cycling stability of 95.7% capacity retention after 1800 cycles at 25 °C and 62.1% capacity retention after 700 cycles at 60 °C. Moreover, the TOP molecule not only maintains the nonflammable feature of phosphate but also attains higher thermal stability, which endows the electrolyte with high safety and thermal stability. This design concept for electrolytes offers a promising path to long-cycling and high-safety sodium metal batteries.