Computational study of self-heating ignition of Lithium-ion batteries during storage: effects of heat transfer and multi-step kinetics

Abstract

Fire safety is a serious concern when storing a large number of Lithium-ion batteries (LIBs) stacked as an ensemble. Many such fires reported in recent years have caused severe damage to industrial facilities, public property, and even loss of life. It is crucial to understand the mechanisms and causes of these storage fires to provide insights for prevention. While previous studies mostly focused on the chemistry of LIBs and ignition while charging or discharging, this thesis explores the possibility of another fundamental cause of such fires driven by heat transfer, self-heating ignition. Three major challenges are identified for the modelling of self-heating ignition of LIB ensembles: large sizes, multi-dimensional heat transfer, and multiple chemical reactions. In this thesis, a typical LiCoO2 (LCO) battery with four-step reaction kinetics is chosen for analysis and modelling the fundamentals of self-heating ignition. Four numerical models based on COMSOL Multiphysics are developed to deal with these challenges. The numerical results show that the critical ambient temperature triggering self-heating ignition decreases significantly with the size of the battery ensemble, from 155℃ for a single cell to 45℃ for a rack of cells. The spacing and packaging materials used to separate LIBs in storage can promote self-heating ignition further decreasing the critical temperature. The increase in size and the presence of packaging materials result in slower internal heat transfer, which allows the cells to self-ignite at lower ambient temperatures. The heat from self-discharge, which is often neglected in the literature, is predicted to have minor effects on small LIB ensembles but to be dominating for a shelf of LIBs, indicating a substantial change in important chemical mechanism for different sizes. The differences resulting from different numerical models are investigated by a benchmarking analysis using two simulation tools: COMSOL and Gpyro. This thesis provides insights on the fundamental mechanism of self-heating ignition of LIBs during open-circuit storage and scientifically proves that self-heating ignition can be a cause of fires when LIBs are stacked to large sizes.