A C80 calorimeter was used to study the thermal behaviors of LixC6 and LixC6 in 1.0 M LiPF6/EC + DEC electrolyte. C80 results show that LixC6 alone shows one exothermic peak, which is attributed to the solid electrolyte interphase (SEI) decomposition. Four exothermic peaks were detected in LixC6-1.0 M LiPF6/EC + DEC electrolyte samples. These four peaks are attributed to SEI decomposition, Li-electrolyte reaction as well as new SEI film formation, new SEI film decomposition, and Li with PVDF/other products reactions. The apparent activation energy of LixC6 and LixC6-electrolyte at different states of charge was calculated, and it was found that with intercalated lithium increasing, the activation energy shows a decreasing trend. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2139955] All rights reserved.