Graphite anode could initialize the thermal runaway of lithium-ion cells due to its spontaneous exothermic reactions above the critical temperature. In order to explore the mechanisms of these reactions, a series of experiments were carried out with on-line coupled thermogravimetric analyser–Fourier transform infrared spectrometer (TGA–FTIR). The reactions under study essentially involved metallic lithium reacting with electrolyte and the thermal decomposition of natural graphite GDR at various states of charge (SOC). It is found that metallic lithium reacts with ethylene carbonate (EC) violently at around 180°C producing CO2 and dilithio butylene dialkoxide which could further react with LiPF6 and/or PF5 to produce POF3. At the relatively low temperature (about 60°C), CO2 produced from both 0% SOC and 100% SOC GDR samples is attributed to the decomposition of organic compounds in solid electrolyte interface (SEI) film. In addition, the amount of POF3 produced from 0% SOC GDR sample is more than that from 100% SOC GDR sample. Further, fully intercalated GDR sample shows a structural collapse, a solid-state exothermic reaction at ∼260°C. Based on the TGA–FTIR results obtained from these exothermic reactions, detailed thermal decomposition paths are proposed in this paper.