The formation of a solid electrolyte interface (SEI) on the surface of graphite in a LiBC2O4F2-based electrolyte was studied by galvanostatic cycling and electrochemical impedance spectroscopy (EIS). The results show that a short irreversible plateau at 1.5-1.7 V versus Li+/Li was inevitably present in the first cycle of graphite, which is attributed to the reduction of -OCOCOO- pieces as a result of the chemical equilibrium of oxalatoborate ring-opening. This is the inherent property of LiBC2O4F2 and it is independent of the type of electrode. EIS analyses suggest that the reduced products of LiBC2O4F2 at 1.5-1.7 V participate into the formation of a preliminary SEI. Based on the distribution of the initial irreversible capacity and the correlation of the SEI resistance and graphite potential, it was concluded that the SEI formed at potentials below 0.25 V during which the lithiation takes place is most responsible for the long-term operation of the graphite electrode in Li-ion batteries. In addition, the results show that the charge-transfer resistance reflects well the kinetics of the electrode reactions, and that its value is in inverse proportion to the differential capacity of the electrode. Published by Elsevier B.V.