Safety aspects of lithium ion batteries

Abstract

Lithium-ion batteries (LiBs) are the highest energy density battery and are essential to the transition away from fossil fuels, and hence the decarbonisation of the planet. LiBs find major application in electric vehicles (EVs) and Battery Energy Storage Systems (BESS). LiBs in BESS can store energy from intermittent renewable generators (the sun doesn’t always shine or the wind blow) to support national grids. The high energy density of LiBs comes at a price: they contain flammable solvents, and if this chemical energy is released in an uncontrolled manner, through abuse, defects, or contamination, they generate gases which can undergo thermal runaway and ignite producing flare-like flames or explode. LiBs are comprised of cells: a cell in thermal runaway can cause other cells to enter thermal runaway (thermal propagation). Large LiBs, i.e. those in EVs (10- 100kWh), can re-ignite hours, days or weeks after apparently being extinguished and many times. This thesis is essentially in two parts, the first part, Chapters 3, 4 & 5 report on experiments carried out on single LiB modules and stacks of modules from the Nissan Leaf 2 EV. In Chapters 3 & 4 the module(s) were sent into thermal runaway by abuse to investigate the fire characteristics, gases, and provide a demonstration of LiB fires and re ignition to emergency responders. It was found that the emitted gases can easily be confused for smoke and/or steam when they actually contain toxic and flammable components. Chapter 5 introduces a novel method to estimate the Heat Release Rate (HRR) from a single module fire using optical cameras. It was found that using this method, approximately twice the heat released was accounted for. The second part, Chapter 6 follows the experiments in Chapters 3 & 4 and is a report written for the Department of Business & Industrial strategy (BEIS) concerning the safety of LiBs in domestic BESS (DLiBESS). The aim was to understand if there are additional risks associated with the use of second-hand LiBs in DLiBESS and if the current codes and standards are adequate. Part of the conclusion was that consideration should also be given to whether stricter requirements are needed for home-built (“DIY”) DLiBESS that use second-life batteries, similarly to the USA.