The oxidative decomposition mechanism of lithium bis(oxalate)borate (LiBOB) as a cathode film-forming additive has been investigated using density functional theory calculations at the B3LYP/6-311++G(d) level, with the polarized continuum model. The calculated oxidation potentials of the investigated structures decreased in the following order: carbonate (including isolate EC, PC and DMC) > BOB⁻ (isolate) ≈ carbonate–BOB⁻ clusters. Charge distribution results show that the electron of the oxidized carbonate–BOB⁻ cluster was taken from BOB⁻, indicating the higher oxidation activity of BOB⁻. Decomposition mechanism analyses of the EC–BOB⁻-e cluster indicate that breakage of the BOB⁻ structure is more energetically favorable than EC. The most likely reaction path of this cluster is the ring opening reaction of BOB⁻ via two transition states, generating CO, CO₂ and radical R1 which may further terminate generating a borate-containing oligomer. This oligomer is believed to play a crucial role in suppressing further oxidative decomposition of carbonate solvents.