Anion-Enriched Solvation Engineering of Ester-Based Electrolytes for Sodium Metal Batteries at -40 °C

Sodium metal batteries (SMBs) are recognized for their high theoretical capacity, abundant availability, and low electrochemical potential. However, the slow Na⁺ de-solvation kinetics and the poor solid electrolyte interface (SEI) of commercial electrolytes hinder their application at low temperatures. Herein, we propose a novel ultralow-temperature, non-flammable ester-based electrolyte system incorporating trimethyl phosphate (TEP), which exhibits a low melting point, low viscosity, and low polarity. The results of theoretical calculations indicate that the addition of TEP significantly alters the proportions of aggregates (AGGs) solvated structures and reduces the de-solvation energy barrier for sodium ions. Additionally, research on interface structure reveals that the increase in AGGs configurations promotes the formation of an inorganic-rich SEI layer. Consequently, the TEP-based electrolyte enables stable cycling for over 1500 h in symmetric cells at -40 °C, while achieving a capacity retention rate of 98.4% after 500 cycles at 0.5 C in Na||Na₃V₂(PO₄)₃ full cells.