Reversible Copper Sulfide Conversion in Nonflammable Trimethyl Phosphate Electrolytes for Safe Sodium‐Ion Batteries

Rechargeable sodium‐ion batteries are considered promising candidates for low‐cost and large‐scale energy storage systems. However, the limited energy density, cyclability, and safety issues remain challenges for practical applications. Herein, investigation of the Cu 1.8 S/C composite material as the negative electrode active (conversion) material in combination with a concentrated electrolyte composed of a 3.3 m solution of sodium bis(fluorosulfonyl)imide (NaFSI) in trymethyl phosphate and fluoroethylene carbonate (FEC) as the additive is reported on. Such a combination enables the stable cycling of the conversion‐type Cu 1.8 S/C electrode material for hundreds of cycles with high capacity (380 mAh g −1 ). Both the salt (NaFSI) and the additive (FEC) contribute to the formation of a stable NaF‐rich solid electrolyte interphase (SEI) on the anode surface. A full cell using the Na 3 V 2 (PO 4 ) 3 /C cathode also demonstrates stable cycling performance for 200 cycles with a promising Coulombic efficiency (CE) (99.3%). These findings open new opportunities for the development of safer rechargeable sodium‐ion batteries.