Construction of high-throughput interface phase using boron containing anions to regulate solvation structure and achieve high-performance sodium metal batteries

The commercialization of sodium metal batteries (SMBs) remains challenging due to the instability of the solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI), resulting in rapid capacity degradation under harsh cycling conditions. In this paper, a novel electrolyte additive, sodium difluoro-oxalate borate (NaDFOB), was developed to address these challenges. NaDFOB effectively improved the stability of the SEI/CEI by forming a solid electrolyte interphase enriched in NaF and B₂O₃. This modification was shown to significantly inhibit cathodic transition metal leaching and anodic sodium dendrite growth. The capacity retention of Hard Carbon (HC) || NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM) cylindrical battery containing BE-NaDFOB electrolyte was 94.1% after 1000 cycles of charging and discharging at 0.5 C. The electrolyte with BE-NaDFOB electrolyte is capable of stable operation at –40 ^oC~60 ^oC in Na || NFM batteries. This work explained the effect of the introduction of the additive NaDFOB on the solvation structure of the bulk electrolyte, the decomposition mechanism of NaDFOB that formed a uniform solid electrolyte interface film (CEI/SEI) on the surface of the anode and cathode, and the rapid desolvation of sodium ions. The rapid desolvation of sodium ions contributed to the formation of a high-flux SEI (Refers to the movement and distribution of sodium ions at the solid-state incremental electrolyte (SEI) during deposition or stripping).