High‐Stability Ultra‐Low‐Concentration Sodium Bis(oxalate)borate‐Based Flame‐Retardant Electrolyte via Dipole‐Dipole Interaction Regulation for Low‐Cost and High‐Safety Sodium‐Ion Batteries

ABSTRACT Sodium bis(oxalato)borate (NaBOB)‐based phosphate ester electrolytes are promising, low‐cost, fluorine‐free, and flame‐retardant candidates for sodium‐ion batteries. However, their low‐concentration formulations typically suffer from poor reductive stability on carbon anodes, resulting in rapid capacity fade and low Coulombic efficiency. To address this, we investigate the role of tris(2,2,2‐trifluoroethyl) phosphate (TFEP) as a flame‐retardant diluent in regulating the solvation structure of trimethyl phosphate (TMP)‐based electrolytes. Introducing TFEP enables low‐salt‐concentration, all‐phosphate ester electrolytes based on NaBOB (NBTF). Spectroscopic and theoretical analyses reveal that the dipole–dipole interactions between TFEP and TMP enhance the reductive stability of the electrolyte. This optimized solvation structure facilitates the formation of stable electrode‐electrolyte interphases. Consequently, the Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ (NFPP) cathode achieves an initial Coulombic efficiency of 95.78% and retains 97% capacity after 400 cycles at 1 C. More importantly, the assembled Ah‐level hard carbon//NFPP pouch cell employing the NBTF electrolyte delivers long‐term stability (85.04% capacity retention after 1000 cycles with >99.9% average CE) and wide‐temperature operability (−20°C–60°C), maintaining 82.58% capacity after 200 cycles at 60°C. Through solvation structure engineering, this work achieves a synergistic balance among low salt concentration, high safety, and excellent interfacial stability, providing a new design strategy for fluorine‐free sodium salt‐based phosphate ester electrolytes.