An approach is proposed to evaluate the VC effect in a LiFePO4/graphite cell at 60 °C through liquid chromatography mass spectrometry (LC-MS) together with direct analysis in real time mass spectrometry (DART-MS). The LC-MS result shows that VC can effectively suppress the formation of phosphate esters as well as carbonate oligomers during the electrochemical cycling. It is also known from DART-MS analysis that VC assists the formation of thermally resistant oligomeric phosphate ester layer on the graphite surface, storing more solvent EC inside the layer. On the other hand, no compounds are observed on the LiFePO4 cathode surface. The formation of the denser oligomeric phosphate ester layer is found to be the reason for the improved cycle stability of the cell cycled at elevated temperature.