SUMMARY

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 proposal, a predictive chemomechanical model will be developed to account for two-phase lithiation/delithiation and large volume changes during the operating process. Alongside in situ transmission electron microscopy experiments, this chemomechanical model will be used to investigate the degradation mechanisms in Si nanowires and nanotubes with coatings.On the cathode side of lithium batteries, a major technical challenge is to achieve both high energy density and high power density. To address this challenge, a mixture cathode consisting of Li1+x(NixCoyMn1-x-y)O2 and Li(NiCoMn)1/3O2 has been designed by our collaborators. To evaluate this design, a continuum electrode model will be developed to characterize the processes in the heterogeneous electrode structures. This model will enable predictions of the electrochemical behaviors of cathodes with different particle distributions and compositions, so as to guide the optimization of cathode design.
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 proposal, 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.