Predictive modeling of degradation mechanisms in advanced lithium batteries and barrier coatings

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Luo, Hao
Zhu, Ting
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Lithium batteries play a critical role in the emerging landscape of renewable energies. On the anode side of lithium batteries, silicon is a promising candidate to replace the currently used graphite. However, the mechanical degradation of Si anode induced by the large volume changes during charge and discharge hinders its wide applications in lithium batteries. To mitigate the degradation, Si nanowires or nanotubes with surface coatings are often used. But it remains unclear regarding the effects of coatings and structural changes on the degradation mechanisms in Si anodes. In this thesis, a predictive chemomechanical model will be developed to account for two-phase lithiation/delithiation and large volume changes during the lithiation and delithiation process. Alongside in situ transmission electron microscopy experiments, this chemomechanical model will be used to investigate the degradation mechanisms in Si nanowires, nanotubes and nanoparticles with coatings. The optimal design of coatings has been explored to maximize the benefits of Si based anodes. On the cathode side of lithium batteries, a major technical challenge is to achieve both high energy density and high power density simultaneously. To address this challenge, a mixture cathode consisting of Li1+x(NixCoyMn1-x-y)O2 (Li-excess NMC) and nano Li(NiCoMn)1/3O2 (nano NMC) has been designed by our collaborators. To evaluate this design, a continuum electrode model has been developed to characterize the thermodynamics, reaction kinetics and diffusion processes in the heterogeneous electrode structures. This model enables predictions of the electrochemical behaviors of cathodes with different particle distributions and compositions, so as to guide the optimization of cathode design. Another issue for cathode is the loss of energy density for parallel cells. The developed continuum model has then been adapted to study the origin of current distribution of two parallel cells within a battery. In addition to lithium batteries, barrier coatings are crucial for the reliable operation of flexible electronics. To characterize the strain limits of barrier coatings in flexible electronics, a singular critical onset strain value is often used. However, such metrics do not account for time-dependent or environmentally assisted cracking, which can be critical to the overall reliability of these thin-film coatings. In this thesis, the time-dependent channel crack growth behavior of silicon nitride barrier coatings on polyethylene terephthalate substrates will be investigated in dry and humid environments. To elucidate the origin of the time-dependent crack growth behavior, predictive numerical simulations will be carried out based on the continuum elastic-viscoplastic model. The integrated experiment and modeling will provide a guideline for the optimal design of reliable barrier coatings. Overall, the models and numerical producers developed in this thesis will provide a basis for further study of the reliable lithium batteries and barrier coatings.
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