High-Stability Ultra-Low-Concentration Sodium Bis(oxalate)borate-Based Flame-Retardant Electrolyte via Dipole-Dipole...

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 Na₄Fe₃(PO₄)₂P₂O₇ (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.