Tailored Solvation Synergy in Phosphate Ester Electrolytes Toward High-Voltage and Wide-Temperature Sodium Metal...

Phosphate esters provide a nonflammable characteristic and high oxidative stability for sodium metal battery (SMB) electrolytes. However, their widespread adoption is hindered by inherent limitations such as poor Na metal compatibility and slow kinetics, limiting their development for high-voltage, wide-temperature applications. Herein, we introduce a tailored solvation synergy strategy to overcome these drawbacks in conventional triethyl phosphate (TEP)-based electrolytes. Sodium difluoro(oxalato)borate (NaDFOB) acts as a conductive salt and a film-forming agent, promoting the formation of a uniform NaF/borate-rich interphase on both electrodes. A reduced amount of fluoroethylene carbonate (FEC) facilitates the incorporation of DFOB^- into Na⁺ solvation sheath, thereby activating their interfacial stabilization function. Furthermore, the P═O group in TEP enhances its interaction with DFOB^-, while FEC competitively coordinates with Na⁺, establishing a weak solvation environment and strong anion-participation mechanism that significantly reduces desolvation energy barriers and accelerates Na⁺ transport. As a result, Na||Na₃V₂(PO₄)₂F₃ cells demonstrate over 1600 stable cycling at 4.5 V, sustain 800 cycles at 60°C, deliver high-rate capability up to 20 C, and maintain reliable operation at -20°C. This work offers new perspectives for designing safe SMBs that operate at high voltages and across a wide temperature range through synergistic solvation regulation and interface engineering.