Exothermic reactions at elevated temperatures (T) between Li-intercalated C6Li x negative electrodes and nonaqueous electrolytes play a crucial role in the thermal runaway of lithium-ion batteries. However, despite intensive studies so far, the origin of the reactions has not been fully understood, particularly from the viewpoint of a material balance. In this paper, we performed differential scanning calorimetry (DSC) analyses up to 450 °C for samples with x = 0.22, 0.45, 0.67, and 0.89, which were prepared from a graphited mesophase-pitch-based carbon fiber. The DSC profiles for C6Li x with 1 M LiPF6 dissolved in ethylene carbonate (EC)/diethylene carbonate (DEC) solution (EC/DEC = 3/7 by volume) were found to be divided into four different T regions regardless of x. That is, Region (I) below 150 °C, Region (II) for 150 °C < T ≤ 240 °C, Region (III) for 240 °C < T ≤ 270 °C, and Region (IV) above 270 °C. By combining with results for X-ray diffraction measurements and scanning electron microscopic analyses, we have rationalized the change in enthalpy (ΔH) of each Region taking into account the given material balance. Strategies for inhibiting the thermal runaway of LIBs are also discussed.