The thermal stability and chemical structure of solid electrolyte interphase (SEI) formed on a natural-graphite negative-electrode in ethylene carbonate (EC) and dimethyl carbonate (DMC)-based electrolyte was investigated by thermogravimetry-differential thermal analysis combined with mass spectrometry (TG-DTA/MS) and X-ray photoemission spectroscopy (XPS). Due to the decomposition of SEI, two CO2 evolution peaks at around 330 and 430 °C were detected in TG-MS studies with continuous CO2 background. The continuous CO2 background was attributed to the gradual decomposition of oxygen-containing polymeric species of SEI. Another two dominant components of SEI, lithium alkyl carbonate and lithium oxalate, were found to contribute to the CO2 peaks at 330 and 430 °C separately. The effects of charging-depth, current density and cycle number on the CO2 distribution and XPS spectra were studied. It was found that lithium oxalate was reduction product of lithium alkyl carbonate during the intercalation of lithium ions. The reduction reaction could be accelerated by elevated temperature. The transformation of SEI chemical structure showed direct effect on the thermal stability of SEI. At the same time, lithium carbonate was also found in SEI on the graphite electrode after long cycles, while it was negligible in the electrode subjected to short cycles.
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