Deep Eutectic Electrolytes Enable Anion‐Derived Interfaces for High‐Temperature Sodium‐Ion Batteries

Conventional carbonate-based electrolytes used in sodium-ion batteries (SIBs) suffer from severe high-temperature limitations, including rapid decomposition, unstable cathode-electrolyte interphase formation, and safety risks. It is therefore critical to develop thermally robust electrolytes to advance SIBs for large-scale energy storage. This study presents the design and synthesis of a deep eutectic electrolyte (denoted as NPST), comprising sodium bis(fluorosulfonyl)imide and prop-1-ene-1,3-sultone, with exceptional thermal and electrochemical stability. Benefiting from its precisely tailored solvation structure, NPST promotes the formation of an inorganic-rich anion-derived interfacial phase, thereby suppressing electrolyte decomposition, transition-metal dissolution, and free-radical-driven side reactions at high temperatures. Moreover, the anion-derived interfacial film on the negative electrode side of the NPST electrolyte suppresses the growth of sodium dendrites, facilitating the uniform deposition of sodium. Enhanced interface integrity and suppressed transition-metal dissolution are confirmed through X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry studies. Consequently, SIB full cells employing NPST retain 91.5% capacity after 3000 cycles at 60°C. This work demonstrates that engineering deep eutectic electrolytes can be a potent strategy to overcome electrolyte instability in high-temperature SIBs, advancing the next-generation interfacial design.