Electrolytes with Tailored Solvent–Solvent Interactions for Flame‐Retardant Stable Sodium‐Metal Batteries.

Abstract Sodium metal batteries (SMBs), which possess abundant sodium resources and high energy density, have attracted widespread attention. However, the continuous reaction between the electrolyte and the sodium metal anode, along with the formation of an unstable solid electrolyte interphase (SEI), leads to rapid capacity decay and the safety hazard of potential ignition. In this work, designing a low‐cost and flame‐retardant electrolyte with solvent–solvent interactions is achieved by introducing sodium‐difluoro(oxalato)borate (NaDFOB) as a single salt into the ester‐based electrolyte on the basis of trimethyl phosphate. Theoretical research combined with experimental study disclose through the solvent–solvent interactions, an ion‐aggregate‐rich solvation structure is formed at low concentrations, leading to the formation of a gradient SEI enriched with inorganic compounds such as B and F on the anode. This effectively suppresses interfacial reactions and sodium dendrite growth, significantly improving the cycling stability along with the optimizing the safety of SMBs. The Na||Na 3 V 2 (PO 4 ) 3 battery using this electrolyte maintains a high‐capacity retention of 93% after 5000 cycles (320 days) at 1C. This approach provides a reliable solution for the application of flame‐retardant electrolytes in SMBs, which also sheds light on the designing principle of advanced battery systems.