The 18,650 lithium-ion battery was charged and discharged at different rates, first in an environment with heat source, and then in one with heat-insulation, to study the effects of different thermal environments on the thermal behavior of lithium batteries. In the case of charging and discharging in adiabatic environments, the battery did not have thermal runaway, and the maximum temperature and temperature rise rate increased with the increase of the charging and discharging rates. During the 4 C discharge process, the resistance increased due to the action of the Positive Temperature Coefficient (PTC) element, and the loop current decreased, which made the highest temperature lower than that in the 3 C discharge process. In the presence of an external heat source, when charged and discharged at different rates, the battery was thermally out of control. The thermal runaway behavior of the battery during charging was a large amount of gas leakage. At the time of discharge, due to the high initial State of Charge (SOC), at the 0.5 C, 1 C, 2 C rates, there was thermal runaway following a large amount of gas leakage. The same phenomenon also occurred at the 3 C, 4 C rates, but without continuous combustion. In the environment of external heat source, results showed that the thermal runaway heat release was always greater in the charge process than in the discharge process. With the increase of the charge and discharge currents, the battery power before and after the thermal runaway remained almost the same. Therefore, the ratio of the thermal runaway heat release in the charge process to that in the discharge process kept increasing and finally approached 1. (C) 2019 Elsevier Ltd. All rights reserved.