With the aim of attaining a better understanding of the thermal stabilities of solid electrolyte interphase (SEI) layers with lithium salts, a series of lithium alkyl carbonates, including lithium methyl carbonate (LMC) and lithium ethyl carbonate (LEC), were synthesized as model reference SEI components. The effects of the lithium salts, lithium hexafluorophosphate (LiPF6) and lithium bis(oxalate) borate (LiBOB), on the thermal stabilities of the lithium alkyl carbonates were then investigated via the differential scanning calorimetry (DSC) method. It was demonstrated that small and broad exothermic peaks near 100 °C for pure lithium alkyl carbonates were drastically increased, likely due to vigorous synergic exothermal reactions between the lithium alkyl carbonates and LiPF6. In contrast, LiBOB did not show increased peaks.