Propensity of 21700 cylindrical cells to thermal runaway under slight overcharging cycling – A numerical study

Abstract

Slight overcharge of lithium-ion batteries (LIBs) could occur due to inadequate design of battery management system or unexpected malfunction of charger. In comparison with other abuse conditions, the evolution of thermal runaway (TR) under slight overcharging often requires a lengthy incubation period, necessitating relatively long periods for experiments. Numerical simulations with validated models offer a viable alternative to evaluate such risks in modules/packs and associated mitigation measures. Since a current-cutting device is usually installed inside the LIB module/packs, the current will be cut off once the overcharge reaches a certain level, therefore, slight overcharging is of more potential relevance. However, previous simulations mainly focused on TR process induced by continuous overcharging, the conditions associated with slight overcharging have been overlooked. As slight overcharging is difficult to detect, the generated heat could easily accumulate as the cycle process prevails, result in unwanted cell temperature increases with increasing propensity for TR. A three-dimensional (3-D) predictive tool for the transition of cylindrical 21700 cell from slight overcharging cycle to TR has been developed by implementing the published models for heat generation from various thermal decomposition reactions and ISC into in-house version of the opensource computational fluid dynamics (CFD) code OpenFOAM. The code is validated with newly conducted experiments involving both normal and slight overcharging cycles. The validated model has then been used to conduct further parametric studies to investigate the effects of the overcharge voltage, current-rates and convective cooling on the propensity and run up time to TR as well as the maximum cell temperature during TR for cells with different cathode materials under different ambient temperatures. This work forms a sound basis for evaluating TR propagation in modules/packs under slight overcharging/discharging cycles to aid the development of mitigative measures to improve LIB process safety in practical applications.