The evolution of gas in lithium-ion batteries (LIBs) at a charged state is one of the main problems in the industry because it causes significant distortion or swelling of the batteries. The mechanism of the gas-generating reaction related to the cathode at a charged state of LIBs was investigated. A side reaction between the electrolyte solution and free lithium compounds, such as Li2CO3 or LiOH in the cathode, is considered as the main cause of gas evolution at early stages of the storage test. Both Li2CO3 and LiOH generated CO2 mainly by a HF-mediated reaction, but the evolution of CO2 could be triggered by addition of H2O without a fluorine source for LiOH. Ni-based cathode materials generated more gas than conventional LiCoO2 at the initial stage because they contain more free lithium compounds, but the rate of gas evolution slowed down with time, suggesting that Ni-based active materials might be more appropriate for reducing long-term gas evolution in LIBs if the free lithium compounds could be removed effectively from the surface.