Balanced Cathode and Anode Interfaces Formation Performance of Sodium Difluoro(oxalato)borate Additive Enables High-Voltage Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are becoming a strong candidate for large-scale energy storage applications due to their cost-effectiveness and abundant sodium resource reserves. Ether solvents have advantages such as excellent low-temperature performance and good reduction stability. However, poor oxidation stability limits the use of ether-based electrolytes, which need to be addressed urgently. In this study, 1 M sodium tetrafluoroborate (NaBF₄) and 0.05 M sodium difluoro(oxalato)borate (NaDFOB) were added in tetraethylene glycol dimethyl ether (G4), which is named “BDG4”. BDG4 electrolyte can promote the formation of cathode electrolyte interface (CEI) layers containing NaF and B─O/B─Na inorganic components on the surface of the cathode. The dense CEI layers can prevent the solvent from undergoing oxidation reactions. Therefore, thanks to the lower highest occupied molecular orbital (HOMO) energy level of G4 and its close coordination structure with Na⁺, the electrolyte has a high-voltage stability exceeding 4.5 V versus Na⁺/Na. Therefore, BDG4 electrolyte can be stably cycled on Na₃(VO)₂(PO₄)₂F (NVOPF) and NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathode for over 250 cycles at room temperature, exhibiting a high Coulombic Efficiency (CE) exceeding 99.8%. Furthermore, BDG4 demonstrated excellent rate performance of NFM cathode, maintaining 75% capacity retention even at 4 C. Due to the stable solid electrolyte interface (SEI) layers formed by the mixture of inorganic and organic components, BDG4 electrolyte can also cycle stably on the Hard Carbon (HC) anode. Therefore, the commercial NFM||HC pouch full cell can cycle stably with 88.3% capacity retention after 80 cycles. This work reveals the role of NaDFOB and provides a feasible approach for the design of high-voltage ether electrolytes of SIBs.