Graduation Semester and Year

2023

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Ankur Jain

Abstract

Strategies to prevent or minimize propagation of thermal runaway in a pack of Li-ion cells are critically needed to ensure safe operation, storage and transportation. While significant literature already exists on thermal runaway simulations, several key questions of practical relevance have remained unaddressed. This work presents multi-mode heat transfer simulations to predict the onset and propagation of thermal runaway in a pack of cylindrical Li-ion cells. The impact of inter-cell gap and thermal properties of the interstitial material on onset and propagation of thermal runaway is studied. It is shown that high interstitial thermal conductivity promotes thermal runaway propagation. However, too low a thermal conductivity results in heat localization in the trigger cell, also leading to thermal runaway. An optimum range of interstitial material thermal conductivity is thus identified. The impact of trigger cell position on propagation is investigated. It is shown that, depending on external conditions, either the center or the corner position may be more susceptible to propagation. Finally, it is shown that radiation and natural convection play a key role in thermal runaway propagation. It is expected that the trade-offs identified here will help minimize the onset and propagation of thermal runaway in Li-ion battery packs. The venting of hot gases due to rupture of a Li-ion cell during thermal runaway may rapidly transfer thermal energy to neighboring cells in a battery pack and cause propagation of thermal runaway. While thermal runaway has been studied extensively through both measurements and simulations, there is a lack of research on the impact of the venting process on thermal runaway propagation. This work presents a non-linear thermal-fluidic simulation of supersonic turbulent flow of hot gases ejecting from a trigger cell and spreading to neighboring cells. Assuming isentropic flow, temperature and speed of the gas flow as functions of time are estimated based on past measurements. These data are used in simulations to determine the thermal impact of the venting process on neighboring cells. The impact of various geometrical parameters of the battery pack on the spreading of venting gases is investigated. Results indicate that cell-to-cell gap, overhead gap and the location of the vent hole strongly influence the nature of propagation of thermal runaway to neighboring cells. This work develops a fundamental understanding of an important process during thermal runaway and may help in the design and optimization of safe Li-ion battery packs for energy conversion and storage. Understanding the nature of onset and propagation of thermal runaway in a Li-ion battery pack is critical for ensuring safety and reliability. This paper presents thermal runaway simulations to understand the impact of radiative heat transfer on thermal runaway onset and propagation in a pack of cylindrical Li-ion cells during transportation/storage. It is shown that radiative properties of the internal partition walls between cells commonly found in battery packs for transportation/storage play a key role in determining whether thermal runaway propagation occurs or not. Surface emissivity of the internal partitions is shown to drive a key balance between radiative heat absorbed from the trigger cell and emitted to neighboring cells. It is shown that a high thermal conductivity partition may greatly help dissipate the radiatively absorbed heat, and therefore prevent onset and propagation. Therefore, choosing an appropriate emissivity of the internal partitions may offer an effective thermal management mechanism to minimize thermal runaway. Emissivity of the cells is also shown to play a key role in radiative heat transfer within the battery pack. This work contributes towards the fundamental understanding of heat transfer during thermal runaway in a battery pack and offers practical design guidelines for improved safety and reliability. Accurate understanding of propagation of thermal runaway is of much importance for developing safe battery pack designs. Combustion of vent gases emerging from a trigger cell undergoing thermal runaway has not been studied in sufficient detail, even though the additional heat generated during combustion likely plays an important role in thermal runaway propagation. This work presents comprehensive numerical modeling and simulation of thermal runaway propagation in a pack of cylindrical cells. The model accounts for multiple coupled non-linear phenomena, including vent gas flow and combustion, radiation, and thermal runaway. Non-premixed combustion of venting gas is modeled using k-ε turbulence model and finite rate chemical kinetics. Simulation results are shown to be in good agreement with experimental data for a benchmark turbulent non-premixed jet flame. Simulations show that hot combustion products are rapidly transported in gaps between cells, leading to self-sustained thermal runaway propagation to adjacent cells. Results demonstrate the critical importance of combustion in determining the nature of propagation of thermal runaway. The vent hole location is identified as an important parameter that influences whether and the extent to which thermal runaway propagation occurs. This work contributes towards the practical understanding of thermal runaway safety of Li-ion battery packs Thermal runaway in Li-ion cells and battery packs impacts the safety and performance of electrochemical energy conversion and storage systems. Preventing the propagation of thermal runaway in large-scale battery storage and transportation is of much importance. While most of the past work addresses single cells or a small battery pack, only limited literature is available on systems of larger size. This work presents Multiphysics simulations of propagation of thermal runaway during large-scale storage and transportation of Li-ion cells. Through simulations that account for multimode heat transfer, Arrhenius heat generation, turbulent fluid flow and combustion, the propagation of thermal runaway from one pallet of cells to an adjacent pallet is studied. The model quantitatively predicts the temperature field in/around the pallets and predicts whether the adjacent pallet will also catch fire or not. Results indicate that the gap between pallets plays a key role in determining propagation. A sharp threshold value of the gap is found, beyond which, propagation does not occur. Results from this work may be helpful in ensuring thermal safety during large-scale storage and transportation of Li-ion cells, ultimately contributing towards improved electrochemical energy storage and conversion.

Keywords

Li-ion batteries, Thermal runaway, Combustion, Venting, Ejecta, Thermal conductivity

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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