In this paper, overcharge behaviors of a large format lithium-ion battery with Li(Ni0.6Co0.2Mn0.2)O-2 cathode for electric vehicles are investigated, and the overcharge-induced degradation and failure mechanisms of the whole overcharge process are studied using both in-situ and ex-situ techniques. The whole overcharge-to-thermal runaway process can be characterized as four stages. In stage I, no obvious capacity fade is observed due to the excessive capacity of both cathode and anode. In stage II, once the state of charge (SoC) exceeds 130%, the decomposition of active materials at elevated potential dominates the capacity decay. Meanwhile, the loss of lithium inventory occurs resulting from lithium plating and other lithium involved side reactions concomitantly with the oxidation of electrolyte. In stage III, when the cell is overcharged up to 145% SoC, the structures of both the cathode and anode collapse and the component of cathode changes. As a result, cell temperature begins to rise more dramatically, which adversely leads to more aggressive side reactions, causing cell failure. In stage IV, the cell ruptures and thermal runaway happens, as a result of internal short circuit. This work provides deeper insights into the degradation and failure mechanisms of lithium-ion battery during the whole overcharge process. (C) 2018 The Electrochemical Society.