Designing Advanced Electrolytes for High-Safety and Long-Lifetime Sodium-Ion Batteries via Anion–Cation Interaction Modulation

Safety hazards caused by flammable electrolytes have been major obstacles to the practical application of sodium-ion batteries (SIBs). The adoption of nonflammable all-phosphate electrolytes can effectively improve the safety of SIBs; however, traditional low-concentration phosphate electrolytes are not compatible with carbon-based anodes. Herein, we report an anion–cation interaction modulation strategy to design low-concentration phosphate electrolytes with superior physicochemical properties. Tris­(2,2,2-trifluoroethyl) phosphate (TFEP) is introduced as a cosolvent to regulate the ion–solvent-coordinated (ISC) structure through enhancing the anion–cation interactions, forming the stable anion-induced ISC (AI-ISC) structure, even at a low salt concentration (1.22 M). Through spectroscopy analyses and theoretical calculations, we reveal the underlying mechanism responsible for the stabilization of these electrolytes. Impressively, both the hard carbon (HC) anode and Na4Fe2.91(PO4)2(P2O7) (NFPP) cathode work well with the developed electrolytes. The designed phosphate electrolyte enables Ah-level HC//NFPP pouch cells with an average Coulombic efficiency (CE) of over 99.9% and a capacity retention of 84.5% after 2000 cycles. In addition, the pouch cells can operate in a wide temperature range (−20 to 60 °C) and successfully pass rigorous safety testing. This work provides new insight into the design of the electrochemically compatibility electrolyte for high-safety and long-lifetime SIBs.