In this investigation, it was shown that a probability of thermal runaway in commercial lithium-ion cells of the type 18650 grows with number increase of charge/discharge cycles and increase of cells state of charge (SOC). Notably, experiments in an accelerating rate calorimeter (ARC) showed that with the number growth of cells charge/discharge cycles, it is observed a considerable decline of an initiation temperature of exothermic reactions of thermal runaway and increase of released energy. Additional ARC-experiments with the following analysis of the gas released showed that in a course of cells cycling in anode graphite, hydrogen is accumulated. It was proven in experiments that a recombination of released-from-graphite-anode atomic hydrogen is exactly that powerful exothermic reaction, which increases the released energy in the beginning of the thermal runaway and decreases the temperature of its initiation. Thus, the reason for the initiation of thermal runaway in lithium-ion cells is a powerful exothermic reaction of recombination of atomic hydrogen accumulated in anode graphite in a during of cells cycling. The possible mechanism of initiation thermal runaway has been proposed corresponding to all the experimental results obtained. (C) The Author(s) 2018. Published by ECS.