Lithium difluoro(oxalato)borate (LiDFOB) was investigated as an electrolyte additive for high-voltage lithium-ion batteries in order to decrease the decomposition of the electrolyte. As a typical high-voltage cathode material, LiCoPO4 was tested in the LiDFOB-containing electrolyte, exhibiting higher reversible charge/discharge capacity and better cyclic stability. The effect of LiDFOB on the formation of a stable interphase film was investigated through cyclic voltammetry and X-ray photoelectron spectroscopy. LiDFOB was helpful to form a stable interphase film and passivate the cathode surface; therefore, the decomposition of the electrolyte was inhibited accordingly.