Differential Scanning Calorimetry (DSC) and Thermal Gravimetry (TGA) combined with gas analysis are used to identify the main decomposition processes and to develop reaction kinetic models for thermal runaway modelling of Graphite – Lithium Nickel–Manganese–Cobalt-Oxide (NMC 111) cells. Heating rates of 5, 10 and 15 °C min−1 with multiple replications are performed to determine the frequency factor, activation energy and the heat of reaction of the different sub-processes. It is found that both the anode and cathode decompose in multiple parallel and consecutive reactions between 5 °C and 600 °C. A double breakdown mechanism of the protecting Solid Electrolyte Interface (SEI) is suggested to describe the anode decomposition reactions. For the cathode, decomposition and evaporation of ethylene carbonate (EC), decomposition of NMC with the liberation of oxygen, combustion of EC with the liberated oxygen, decomposition of binder, decomposition of EC and combustion of carbon additive reactions are identified and modelled. The proposed model can be used to simulate thermal runaway initiation methods in a realistic way.