Probing Interfacial Dynamics in Solid-State Lithium Metal Batteries
Author(s)
Lewis, John A.
Advisor(s)
McDowell, Matthew
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Abstract
Solid-state batteries (SSBs) are a promising technology to surpass the energy density and safety of conventional lithium-ion batteries. These devices replace the flammable liquid electrolyte with a more stable solid-state electrolyte (SSE) that can conduct lithium ions. The rigid mechanical properties of SSEs are also promising for enabling the energy dense lithium metal anode, which is plagued by dendrite formation and dead lithium in liquid electrolytes. Despite advances towards SSEs with high ionic conductivity, the understanding and control over solid electrode/SSE interfaces have emerged as major challenges in the development of SSBs. Chemo-mechanical degradation is expected to be more severe in SSBs compared to conventional liquid-electrolyte-batteries because the SSE cannot reconfigure like liquids. Understanding chemical transformations at interfaces, mechanical damage, and lithium filament growth is therefore critical for engineering SSBs.
This dissertation investigated the underlying mechanisms of these interfacial phenomenon to better inform the design of SSBs. First, severe SSE decomposition caused by electrochemical side reactions with lithium metal was found when the reacted species exhibited mixed ionic-electronic conduction. Continuous decomposition ultimately resulted in fracture due to the build-up of internal stress, and this process was accelerated when operating at higher rates. Second, the dynamic evolution of Li/SSE interfaces was probed using operando X-ray tomography. 3D images of SSBs were obtained during operation, which were then processed using segmentation to quantify how phases change and link their behavior directly to the measured electrochemistry. Analysis revealed that the significant loss of interfacial contact was responsible for cell failure. Third, the relationships between unstable Li metal deposition and electrochemical parameters, such as current density and areal capacity, were investigated. A new metric called the threshold capacity was introduced and used to evaluate lithium deposition behavior in SSBs. Cycling of cells with areal capacity controlled to be well below the threshold capacity greatly improved cell lifetime, while approaching the threshold capacity resulted in rapid short circuiting. Fourth, the mechanisms of anode-free SSBs were investigated, in which lithium metal was deposited onto a bare copper current collector during the first charge. Stripping lithium from the copper current collector was found to cause significant degradation that severely limited cycling lifetime. Lastly, the energy densities of various battery chemistries were calculated at the cell-level. Alloy anodes in SSBs were shown to have competitive energy densities, and their mechanistic advantages over alloy anodes with liquid electrolytes and lithium metal SSBs are discussed.
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Date
2022-05-03
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Text
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Dissertation