| Author: | Liu, Yanhui |
| Title: | Fundamental study of battery fire safety under the low ambient pressure |
| Advisors: | Huang, Xinyan (BEEE) Fu, Xiao (BEEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Lithium ion batteries Lithium ion batteries -- Safety measures Fire prevention Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xiii, 112 pages : color illustrations |
| Language: | English |
| Abstract: | The lithium-ion battery (LIB) plays a pivotal role in the global transition towards a clean and sustainable energy landscape. It improves the utilization and storage of renewable energy sources and drives transportation electrification. As the LIB application continuously expands, ensuring their fire safety has become increasingly significant, especially in special conditions like aerospace and high-altitude environments. While extensive research efforts have been devoted to studying LIB fires under standard atmospheric pressure, the fire characteristics of LIBs under low ambient pressure remain inadequately addressed. Therefore, this work attempts to improve the understanding of LIB fire safety in low-pressure conditions and provide new insights into effective countermeasures. This thesis follows a manuscript-style format, starting with an introductory chapter that outlines the research background and objectives. The following chapters consist of independent papers that have either been published or are intended for submission to peer-reviewed academic journals. The concluding chapter summarises the overall findings and suggests possible directions for future research. Chapter 1 introduces the research background and motivations of this thesis. While LIBs provide convenience and sustainability to modern society, their safety concerns are becoming more pronounced with the growing market demand. Unlike conventional fires, LIB fires exhibit greater complexity in the ignition routes and spread rates, rendering them challenging to control. Although the application of LIBs is increasing in low-pressure environments such as aerospace and high-altitude regions, our understanding of LIB fires in such conditions is still limited. Consequently, a fundamental study is imperative to systematically investigate the LIB fire characteristics and explore the associated hazards under low ambient pressure. Chapter 2 investigates the fire initiation of LIB stacks under reduced atmospheric pressure. A novel low-pressure chamber is devised to create a controlled setting (20 to 100 kPa), where the classical hot-plate test method is employed to provoke the thermal runaway of 18650-type LIB stacks (up to 9 cells). Each cell is adjusted to a state of charge (SOC) of 30%, the maximum SOC value for LIBs during low-pressure air transport. The influences of environmental pressure and the number of cells on the initiation of thermal runaway are comprehensively examined. The entire process of LIB combustion includes three stages: heating, venting, and thermal runaway. Decreasing ambient pressure can enhance safety venting and reduce the intensity of the exothermic reactions within cells, which is confirmed by the surface morphology of cell electrodes. The overall risk of fire increases with higher ambient pressure and larger LIB stack size, as indicated by the lower minimum temperature threshold for thermal runaway (255 °C to 385 °C). Furthermore, a simplified heat transfer model is established to explain the trend of thermal runaway criteria and the impact of the low-pressure environment. This chapter provides novel insights into the effects of pressure and stack size on LIB fire ignition, which can aid in improving fire safety measures for storing and transporting large-scale LIB stacks under varying pressure conditions. Chapter 3 evaluates the fire propagation within open-circuit LIB stacks. The fire experiment takes place in an advanced low-pressure chamber, where ambient pressure can be decreased to 0.1 kPa. The one-dimensional layer-to-layer propagation rate of thermal runaway is used to quantify the development of fire risks in LIBs. The fire test is conducted in the further developed low-pressure chamber, where the ambient pressure can be reduced to 0.1 kPa. The one-dimensional layer-to-layer thermal runaway propagation rate is defined to quantify the development of LIB fire hazards. Results indicate that the thermal runaway propagation rate decreases with decreasing SOC level and ambient pressure. As the SOC decreases from 100% to 30%, the thermal runaway propagation rate decreases from 1.73 [layer/min] to 0.30 [layer/min] at 100 kPa. For 30% SOC cells, the thermal runaway propagation rate decreases by about 23% as the environmental pressure decreases by about 80% (20 kPa), eventually dropping to zero at 0.1 kPa. The X-ray computed tomography imaging helps to prove that low ambient pressure can weaken both external flaming combustion and internal thermal runaway reactions during the venting stage. When we reduce the environmental pressure, such synergistic effect increases the thermal runaway onset temperature from 200 °C to 310 °C, reduces the maximum surface temperature from 800 °C to 400 °C, and lowers the burning mass loss fraction from 32% to 10%. The findings verify the mitigating role of ambient pressure on LIB thermal runaway and can help to assess the fire safety of open-circuit LIB piles in storage and transport. Chapter 4 explores the coupling influences of ambient pressure, ambient temperature, and electrical connection mode on fire propagation over a linear LIB module. Similarly, the propensity of thermal runaway propagation for the open-circuit LIB array is much lower and only occurs at high ambient temperature and ambient pressure. For parallel-connected LIB modules, the thermal conductivity through the tab is significant, leading to a complicated thermal runaway propagation. As ambient pressure decreases, the rate of thermal runaway propagation first increases due to the reduced environmental cooling (i.e., thermal controlled). It then falls due to lower remaining electrolytes after venting (i.e., venting controlled). For the investigated LIB module, the corresponding pressure of maximum thermal runaway propagation speed is 60 kPa. The maximum time for the thermal runaway propagating from one cell to the next cell is about 7 min. Finally, a heat transfer analysis is proposed based on the experimental data to reveal such dual effects of pressure on thermal runaway behaviour and explain the trend of thermal runaway propagation. This chapter further highlights the effect of ambient pressure on thermal runaway intensity, which can deepen the understanding of LIB fire safety under low ambient pressure and inspire the thermal safety design of the LIB modules with electrical connections. Chapter 5 develops a numerical model to predict LIB thermal runaway across different sub atmospheric pressures. The model integrates heat release from chemical reactions, gas generation, and venting. The effects of ambient pressure on mass loss and energy dissipation during venting are also considered. Coil heating tests validate the simulation results and help determine critical parameters for quantifying energy loss. Based on the validated model, the impacts of ambient pressure, cell heating rate, and safety valve threshold on LIB thermal failure are examined. Before safety venting, the internal cell pressure is raised initially by electrolyte vaporisation and then by gases produced from chemical reactions. As the safety valve threshold rises from 1.2 MPa to 2.2 MPa, the gas from SEI decomposition increases from 73.5% to 82.3% at the moment of safety venting. When we reduce the environmental pressure, the incubation period between venting and thermal runaway increases. In other words, lowering the ambient pressure allows more emergency response time before thermal runaway. The developed model approach and simulations improve our understanding of thermal runaway under low ambient pressures and provide novel insights for ensuring battery safety in storage and transportation. Chapter 6 provides a comprehensive overview of the outcomes and critical contributions while offering perspectives on potential areas for future research. |
| Rights: | All rights reserved |
| Access: | open access |
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