Sodium-Difluoro(oxalato)Borate-Based Electrolytes for Long-Term Cycle Life and Enhanced Low-Temperature Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIBs) are emerging as a promising alternative for next-generation energy storage solutions, driven by the economic and environmental benefits of abundant sodium resources. A pivotal aspect of SIB advancement is the development of advanced electrolytes, which remains a formidable challenge. Herein, a facile and scalable synthesis method for low-cost sodium-difluoro(oxalato)borate (NaDFOB) is reported and explored its application as a standalone electrolyte salt for SIBs. The NaDFOB-based ether electrolyte demonstrates exceptional electrochemical stability, solvent compatibility, and the unique capacity to form a dense, robust solid-electrolyte interphase layer on electrode surfaces. As a result, the Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode with NaDFOB-based electrolyte exhibits ultrahigh cycling stability with a remarkable capacity retention of 98.7% after 1000 cycles. Furthermore, an Ah-level hard carbon (HC)//NFPP pouch cell using NaDFOB-based ether electrolyte shows an impressive cycle life of 500 cycles, coupled with an average Coulombic efficiency of 99.9%. The pouch cells also maintain superior electrochemical performance across a broad temperature range from −40 to 60 °C, showcasing the electrolyte's versatility. This work contributes significant insights into the strategic design and application of innovative salts, paving the way for longer-lasting SIBs with enhanced electrochemical performance.