Low-Cost NaNO₃-Based Phosphate Electrolytes with Exceptional Stability Toward Sustainable and High-Safety Sodium-Ion...

The pursuit of cost-effective and sustainable sodium-ion batteries (SIBs) necessitates innovative electrolyte designs to address the limitations of conventional sodium salts (e.g., NaPF₆, NaFSI), which suffer from complex synthesis, atmospheric instability, and high production costs. Inorganic sodium salts such as NaNO₃ offer a promising alternative due to their low cost, air stability, and thermal robustness, but their poor solubility in non-aqueous solvents and inadequate reduction stability hinder practical application. Herein, we propose a novel electrolyte system utilizing NaNO₃ as the sole sodium salt, dissolved in a dual-solvent formulation of trimethyl phosphate (TMP) and tris-(2,2,2-trifluoroethyl) phosphate (TFEP). TMP enables high NaNO₃ solubility via its high Gutmann donor number and permittivity, while TFEP modulates ion-solvent coordination to enhance reduction stability and promote the formation of a stratified inorganic NaF/NaN_xO_y-rich SEI film on hard carbon (HC) anodes. Attributed to the enhanced reduction resistance of the solvation structure and the robust SEI film, the HC electrode achieves a high initial Coulombic efficiency (78.6%) and impressive cycling stability with 99.4% capacity retention after 200 cycles, while the Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) cathode retains 87.5% capacity after 400 cycles. Additionally, HC//NFPP pouch cells confirmed the system's viability, even discharged under ignition conditions. This study provides a strategic blueprint for designing low-cost, high-safety electrolytes for advanced SIBs, highlighting the promise of NaNO₃ in achieving sustainable energy storage solutions.