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

ABSTRACT 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.