Application of boron-salt-dominated dual-salt synergistic ether-based electrolyte in high voltage sodium ion batteries

Sodium-ion batteries (SIBs) are regarded as a significant development direction in the field of large-scale energy storage in the future due to their low cost, high safety, and excellent rate performance. Nevertheless, the current low energy density of SIBs has consistently restricted their commercialization process. Ether-based solvents possess high reduction stability and excellent compatibility with the negative electrode. However, their oxidation resistance is relatively poor. Consequently, it is difficult for SIBs based on ether-based solvents to enhance the energy density by raising the cut-off voltage. In this study, a strategy of jointly constructing a high-voltage-resistant interface layer with sodium difluoro(oxalato)borate (NaDFOB) and sodium bis(fluorosulfonyl)imide (NaFSI) is proposed to prepare a borate salt-dominated high-voltage-resistant ether-based electrolyte. This electrolyte effectively ameliorates the incompatibility issue between the ether-based solvent and the positive electrode under high voltage conditions. During the battery cycling process, DFOB⁻ preferentially decomposes on the surface of the positive electrode and collaborates with the decomposition products of FSI⁻ to construct a fluorine-rich cathode-electrolyte interphase (CEI) mainly composed of boron sulfides, which can effectively inhibit the side reactions between the electrolyte and the positive electrode particles, enabling stable cycling of SIBs with ether-based electrolytes at high cut-off voltages. The assembled NaNi_1/3Fe_1/3Mn_1/3O₂//hard carbon (HC) full cell based on this ether-based electrolyte exhibits a capacity retention rate of 84% after 800 cycles at a cut-off voltage of 4.0 V, 80% after 370 cycles at a cut-off voltage of 4.2 V, and even retains a capacity retention rate of 80% after 180 cycles at an ultra-high cut-off voltage of 4.5 V.