Investigations on the essential causes of the degrading properties of ternary lithium-ion batteries with different nickel content during thermal runaway stage

Ternary lithium-ion batteries (LIBs) with higher energy density are more vulnerable to thermal runaway (TR) owing to the interior material structure, particularly under abusive conditions. Among various abusive conditions, overheating abuse is widely utilized to trigger TR. In this paper, the thermal-electrochemical characteristics correlations of ternary lithium power cells (Li(Ni0.5Co0.2Mn0.3)O2/C, NCM523; Li(Ni0.6Co0.2Mn0.2)O2/C, NCM622; and Li(Ni0.8Co0.1Mn0.1)O2/C, NCM811) under adiabatic and TR conditions were systematically studied. Primarily, in order to clarify the impact of nickel content and discharge rate on temperature, the heat generation behaviors of the aforementioned three cells were explored under adiabatic conditions at ambient/higher temperature experimental conditions. Secondly, the thermal-electrochemical properties of three ternary cells during the TR process triggered by heating abuse were assessed. Moreover, the variations trends of voltage, current, and characteristic temperature with time were carefully examined, and the mechanism of TR occurrence and chain reactions are explained. Eventually, the microscopic irreversible damage to the active materials caused by the increased heat production and electrochemical performance deterioration were analyzed. The experimental results indicated that the maximum temperature and temperature rising rate of NCM811 under adiabatic conditions came to the highest value. The autogenous heat onset temperature T1 of NCM811 reached 78.4 oC, which was 2.9 oC and 2.4 oC lower than that of NCM523 and NCM622, respectively. The peak temperature T3 of NCM811 during the TR process achieved 674.44 oC, which was 99.5 oC and 3.81 oC higher than that of NCM523 and NCM622, respectively. Hence，the thermal safety of NCM811 was the worst. Notably, the corresponding micro results showed NCM811 suffered the most severe irreversible damage as a result of TR. This damage was mostly manifested in the collector and negative electrode materials being severely peeled off, as well as the obvious aggregation of negative electrode active materials. The relevant research will provide systematic theoretical guidance and experimental data support for the improvement of thermal safety performance of ternary lithium batteries and modules with high energy density.