Breaking the solubility limit of sodium difluorophosphate with anion receptor: Enabling high-voltage, zero-gas-emission sodium-ion pouch cell

In contrast to well-known methods that use high-polarity solvents to dissolve poorly soluble sodium salts, an “anion-anion receptor synergy” strategy is developed to overcome the solubility limit of sodium difluorophosphate (NaDFP) and generate functional electrolytes by introducing the anion receptor reagent tris(pentafluorophenyl)borane (TFBB). The resulting electrolyte can effectively enhance the high-voltage and safety performance of pouch NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM)/hard carbon (HC) sodium-ion batteries (SIBs); a capacity retention of 90 % after 550 cycles under a charge cutoff voltage of 4.0 V is successfully achieved, and no visible gas production is observed. The performance enhancement is ascribed to avoiding excessive dissolution of the anode and cathode interfacial films caused by traditional highly polar electrolytes and optimizing the two electrode‒electrolyte interfaces and solvation structure benefited from the as-designed electrolyte composition. This work provides a functional electrolyte with promising application prospects as well as new insights regarding the use of poorly soluble additives to design functional multicomponent electrolytes for SIBs.