Comparative study on lithium borates as corrosion inhibitors of aluminum current collector in lithium bis(fluorosulfonyl)imide electrolytes

Abstract Lithium bis(fluorosulfonyl)imide (LiFSI) is a promising salt that can possibly overcome the limitations of lithium hexafluorophosphate (LiPF₆) in current Li-ion batteries (LIBs). Aluminum (Al) corrosion issue, however, is a major bottleneck for the wide use of LiFSI. This study investigates lithium borate salts as Al corrosion inhibitors in LiFSI electrolytes. Through a systematic comparison among lithium tetrafluoroborate (LiBF₄), lithium bis(oxalato)borate (LiBOB), and lithium difluoro(oxalato)borate (LiDFOB), and LiPF₆, the inhibition ability of the additives is revealed to be in the following order: LiDFOB > LiBF₄ ≈ LiPF₆ > LiBOB. In particular, the inhibition effect of LiDFOB is outstanding; the anodic behavior of Al in 0.8 M LiFSI + 0.2 M LiDFOB ethylene carbonate (EC)-based electrolyte is comparable to that of corrosion-free 1 M LiPF₆ solution. The superior inhibition ability of LiDFOB is attributed to the formation of a passive layer composed of Al-F, Al₂O₃, and B-O species, as evidenced by X-ray photoelectron spectroscopy (XPS) measurements. A LiCoO₂/graphite cell with 0.8 M LiFSI + 0.2 M LiDFOB electrolyte exhibits a rate capability comparable to a cell with 1 M LiPF₆ solution, whereas a cell with 0.8 M LiFSI solution without LiDFOB suffers from poor power performance resulting from severe Al corrosion.