Lithium-ion batteries pose high risks of failure when subjected to fast charging due to accumulated degradation from side reactions. Venting is a common failure behaviour that results in the release of gases and depressurization. However, the connection between fast-charging degradation and venting characteristics remains unclear. In this study, fast-charging experiments are conducted on LiNi0.6Mn0.2Co0.2O2 (NMC622)/graphite prismatic cells (51 Ah) with controlled cooling, followed by adiabatic thermal runaway tests. The degraded cell, after 4C charging, shows venting at a temperature 34.4 °C lower than a fresh cell. Gas analysis of the degraded anode-electrolyte partially reactive system in hot-box tests reveals that the reaction between plated lithium and electrolyte was a primary contributor to the additional gases. The fast-charging protocols are conducted under natural convection with less heat dissipation, causing battery swelling and venting without thermal runaway when subjected to excessive cycling at off-limit charge rates (5C and 5.6C). The temperature decreasing rates during rest after each charging process slow down or stabilize for a certain period, highlighting the role of exothermic side reactions. This experimentally demonstrates that venting failures can be directly triggered by the exothermic, gas generating reaction during fast charging. These findings inform future designs for safe and rapid battery charging.