Committee:

1. Prof. Matthew T. McDowell - School of Materials Science and Engineering (advisor)
2. Prof. Juan Pablo Correa-Baena - School of Materials Science and Engineering
3. Prof. Claudio V. Di Leo - School of Aerospace Engineering
4. Prof. Anju Toor - School of Materials Science and Engineering
5. Prof. Gleb Yushin - School of Materials Science and Engineering

Abstract

High concentration silicon (Si) anodes (≥90%) could dramatically improve the energy density of lithium-ion batteries (LIBs) due to the large specific capacity of Si (3579 mAh g-1) compared to current graphite anodes (372 mAh g-1). However, the large number of Li atoms hosted per Si atom requires massive volumetric changes during charge and discharge of the battery, leading to pulverization of the Si and continuous harmful side reactions with the liquid electrolyte; this quickly degrades battery capacity. Solid-state batteries (SSBs) using a solid electrolyte instead of a liquid electrolyte may improve the long-term cyclic stability of Si anodes, but researchers often test Si anode SSBs under extreme applied pressures or low Si concentrations to mitigate chemo-mechanical degradation from the volumetric changes in the Si anode. These conditions are not feasible for commercially viable batteries, and so proper characterization of the chemo-mechanical degradation mechanisms of Si anode SSBs operated under application-relevant conditions is required to begin optimizing them.

Previously, I demonstrated that operando X-ray computed tomography (XCT) can be used to observe the dynamic evolution of Si anode SSBs at high Si concentrations (~99.0%) and moderate stack pressures (10 MPa). This identified local geometry, rather than bulk stress evolution, as the primary cause of fracture at the interface between Si and the solid electrolyte. Based on these findings, I propose two research objectives to improve understanding of degradation in Si anode SSBs. First, Si electrodes with optimized thickness uniformity will be fabricated and characterized at different pressures with electrochemical tests and XCT imaging. This will quantify the effect of applied pressure on the observed interfacial fracture mechanisms from the previous work and correlate this effect to electrochemical performance of the Si anodes. Finally, thickness-optimized Si electrodes will be fabricated with low concentrations (≤5%) of different binder and Li ion conducting additives and tested at low pressures through electrochemical methods and XCT imaging. This will reveal the efficacy of additives at appropriate concentrations on mitigating the previously observed chemo-mechanical degradation mechanisms. With these experiments, I will provide an educated path towards scalable, energy-dense Si anode SSBs by contributing to broader knowledge on the degradation of these anodes under appropriate operating conditions.