The cycleability and rate performance of Li-ion batteries are mainly influenced by the characteristics of the solid electrolyte interphase (SEI) film. The use of diphenyloctyl phosphate (DPOF) was investigated as an SEI-forming additive in electrolyte. Cyclic voltammetry (CV), three-electrode electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used for the analyzes. The cell with DPOF had better cell performance than that without DPOF in cycle life tests and rate performance tests. In the three-electrode EIS tests, the impedance of the graphite anode electrode of the cell with DPOF was lower than that of the base electrolyte. In the XPS analysis, Li3PO4, which is known as inorganic substance that improves lithium ion conductivity, was formed on the SEI film by reduction of DPOF. From these results, the rate performance of the cell with DPOF was improved by the SEI film composed of Li3PO4.