Date of Award

9-5-2024

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Anodes;Batteries;Diffusion-induced-stresses;Graphite;Lithium-ions;Silicon

Supervisor

Reza Riahi

Supervisor

Giuseppe, Claudio Elia, Gerbaldi

Abstract

Lithium-ion batteries, due to the growing shift towards renewable energy sources, are becoming particularly promising, offering one of the highest energy densities among their competitors. Anyway, one of the primary issues related to them lies in the damage incurred through electric cycling. The damage, caused by the repeated insertion and extraction of lithium ions within the active materials induces mechanical stresses, called diffusion-induced stresses (DIS), leading to crack propagation, material degradation and subsequent decline in performance. The objective of this thesis is to develop a detailed electrochemical-mechanical model for battery cell using COMSOL Multiphysics. The model aims to accurately estimate the stresses within the battery anode during the lithiation process. Through this model, different anode materials such as graphite and silicon will be separately investigated to understand their influence on stress development. Furthermore, the study will delve into key parameters such as active anode particle size, anode thickness and charging rate to comprehend their effects on stress development within the anode. The results show that, the developed electrochemical-mechanical battery model is able to accurately reproduce the experimental cell behavior. Specifically, the voltage vs. time curve is reproduced within an error margin of 3% for the graphite anode cell and 2% for the silicon one. Regarding the mechanical model, it precisely replicates the experimental results, staying within an error margin below 2% for the graphite anode and 5% for the silicon. The maximum stress for graphite is 53.69 MPa, while for silicon it is 1.5 GPa. While, at the end of the cycle, the residual stress is 8.33 MPa for graphite and 0.19 GPa for silicon. The stress increases over time during charging, but both materials exhibit a decrease in slope in the later phases, while during discharging a decrease is experienced. Experimental results show that these stresses did not cause fragmentation of the electrode but caused permanent deformation as some lithium ions remained trapped inside the anode. Considering the entire electrode thickness, a trend can be recognized, where during the charging phase the maximum stress is always at the electrolyte interface. However, during the last phase of discharging, it is inverted with the maximum stress at the current collector interface. The results of the parametric studies show that, for graphite, the most detrimental factor for increasing stress is a high charging rate, leading to a 69.7% increase, while the most beneficial is having a lower particle radius, inducing a 32.24% decrease. For silicon, the highest charging rate is the most dangerous, generating a 65.3% increase, while the lowest rate is the most beneficial, provoking a negative variation of 33.3%.

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