Understanding of working mechanism of lithium difluoro(oxalato) borate in Li||NCM85 battery with enhanced cyclic stability

Despite the significant advances achieved in recent years, the development of efficient electrolyte additives to mitigate the performance degradation during long-term cycling of high-energy density lithium||nickel-rich (Li||Ni-rich) batteries remains a significant challenge. To achieve a rational design of electrolytes and avoid unnecessary waste of resources due to trial and error, it is crucial to have a comprehensive understanding of the underlying mechanism of key electrolyte components, including salts, solvents, and additives. Herein, we present the utilization of lithium difluoro(oxalate) borate (B) (LiDFOB), a B-containing lithium salt, as a functional additive for Li||LiNi0.85Co0.1Mn0.05O2 (NCM85) batteries, and comprehensively investigate its mechanism of action towards enhancing the stability of both anode and cathode interfaces. The preferential reduction and oxidation decomposition of DFOB- leads to the formation of a robust and highly electronically insulating boron-rich interfacial film on the surface of both the Li anode and NCM85 cathode. This film effectively suppresses the consumption of active lithium and the severe decomposition of the electrolyte. Furthermore, the presence of B elements in the cathode-electrolyte interfacial film, such as BF3, BF2OH, and BF2OBF2 compounds, can coordinate with the lattice oxygen of the cathode, forming strong coordination bonds. This can significantly alleviate lattice oxygen loss and mitigate detrimental structural degradation of the Ni-rich cathode. Consequently, the Li||NCM85 battery cycled in LiDFOB-containing electrolyte displays superior capacity retention of 74% after 300 cycles, even at a high charge cut-off voltage of 4.6 V. The comprehensive analysis of the working mechanisms of LiDFOB offers valuable insights for the rational design of electrolytes featuring multifunctional lithium salts or additives for high energy density lithium metal batteries.