Investigation of thermal propagation in electric vehicle high voltage batteries

Abstract

Understanding the Li-ion battery, which is essentially driving the industry, is important given the expansion of the vehicle electrification market, rising environmental and governmental laws, urbanization trends, rising fuel prices, and a developing client market. The increasing demand for electric vehicles (EVs) has released the need for efficient and reliable energy storage solutions. Lithium-ion batteries have emerged as a leading technology due to their high energy density, long cycle life, and lightweight design. The effort to save modern society from energy crises and environmental pollution relies on clean energy. In this study, a brief explanation of lithium-ion materials was given including cathode materials, anode materials, electrolytes, and separators. The cathode materials, such as lithium cobalt oxide (LCO), lithium nickel manganese cobalt oxide (NMC), and lithium iron phosphate (LFP), significantly impact battery performance by influencing energy density, power capability, and safety considerations. On the other hand, anode materials such as graphite and silicon play a crucial role in capacity and rate capabilities, and the thesis gives the characteristics, challenges, and opportunities associated with different anode materials. Furthermore, the study delves into the underlying mechanisms behind ageing phenomena. Factors such as side reactions, cycling, storage conditions and contamination contribute to battery ageing. As a result of ageing, capacity fades and impedance increases. Moreover, this thesis also addresses the thermal runaway (TR) mechanism which is a key safety concern in lithium-ion batteries. It explores the conditions that can lead to thermal runaway, such as overcharging, high temperature, and mechanical abuse. Various strategies to mitigate thermal runaway, including advanced cell designs, thermal management systems, protection systems and packaging are evaluated for their effectiveness in enhancing battery safety. Foams are a key component in providing thermal management when building a high voltage (HV) battery pack module. Correct foam selection provides a better thermal management system, safer system, longer life cycle and easier module production. In the study, thermal propagation (TP) behavior in HV batteries was investigated and the effects of parameters such as thermal conductivity and compression force-displacement of three different foam materials on thermal dissipation were evaluated. After the determination of optimum foam by experiments, pack-level thermal propagation was tested and investigated. To optimize the TP duration, experiments were carried out and the effects of short circuits and pressure were examined through the tests. As a result, the precautions to be taken were determined and the importance of investigating thermal propagation in electric vehicle HV batteries was emphasized.