An additive in the trimethyl phosphate (TMP)-based non-flammable electrolyte was investigated in order to improve the poor reductive stability for the graphite anode. We found that the charge–discharge characteristic of the TMP-based non-flammable electrolyte was improved by adding ethylene ethyl phosphate (EEP) which had function of non-flammability. The solid electrolyte interface (SEI) layers formed on the graphite anode were analyzed with X-ray photoelectron spectroscopy (XPS) and Fourier transformation type infrared spectroscopy (FT-IR). Moreover, evolved gases in the battery were analyzed with gas chromatography/mass spectrometry (GC/MS) to understand the electrochemical reaction in SEI formation. We concluded that the SEI layers including phosphorus created by the EEP additive suppressed the reductive decomposition of TMP.