Low‐Cost NaNO ₃ ‐Based Phosphate Electrolytes with Exceptional Stability Toward Sustainable and High‐Safety Sodium‐Ion Batteries

Abstract

Abstract 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., NaPF 6 , NaFSI), which suffer from complex synthesis, atmospheric instability, and high production costs. Inorganic sodium salts such as NaNO 3 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 NaNO 3 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 NaNO 3 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 x O 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 Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (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 NaNO 3 in achieving sustainable energy storage solutions.