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 Cu1.8S/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 Cu1.8S/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 Na3V2(PO4)(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.

Automated summary

Rechargeable sodium-ion batteries using Cu1.8S/C composite material as the negative electrode active material in combination with a concentrated electrolyte have shown stable cycling performance with high capacity. The formation of a stable NaF-rich solid electrolyte interphase (SEI) on the anode surface contributed by NaFSI and FEC additives plays a crucial role in achieving this. Overall, these findings open new opportunities for the development of safer rechargeable sodium-ion batteries.