Recycling of Li-ion batteries (LIBs) is becoming an urgent issue. However, the chemical composition and the hazard of off-gas produced during the recycling process still remain unclarified due to the complicated reactions during thermal runaway (TR). In order to meet the legislative requirements to carry out an environmentally friendly recycling, this manuscript aims to undertake quantitative analysis and toxicity evaluation of the off-gas produced in mechanical treatment and thermal treatment of LIBs. The measurements were carried out by online Fourier transform infrared spectroscopy (FTIR) and ion chromatograph (IC). The volume of total off-gas was calculated and its toxicity was evaluated by USA’s Protective Action Criteria. The influences of treatment method, state-of-charge (SOC), atmosphere, and type of cathode were investigated.