Committee Members

Prof. Matthew McDowell, Advisor, ME/MSE Prof. Meilin Liu, MSE Prof. Faisal Alamgir, MSE Prof. Juan-Pablo Correa-Baena, MSE Prof. Tom Fuller, ChBE

Probing Interfacial Dynamics in Solid-State Lithium Metal Batteries 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 investigates 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 is found when the reacted species exhibit mixed ionic-electronic conduction. Continuous decomposition ultimately results in fracture due to the build-up of internal stress, and this process is found to be accelerated when operating at higher current densities. Second, the dynamic evolution of Li/SSE interfaces is probed using operando X-ray tomography. 3D images of SSBs are obtained during operation, which are then processed using segmentation to quantify how phases change and link their behavior directly to the measured electrochemistry. Analysis reveals that the significant loss of interfacial contact is responsible for cell failure in this experiment. Third, the relationships between unstable filamentary lithium metal deposition and electrochemical parameters, such as current density and areal capacity, are investigated. A new metric called the threshold capacity is introduced and used to evaluate lithium deposition behavior in SSBs. Cycling of cells with areal capacity controlled to be well below the threshold capacity is found to greatly improve cell lifetime, while approaching the threshold capacity results in rapid short circuiting. Fourth, the behavior and operational mechanisms of anode-free SSBs are investigated, in which lithium metal is deposited onto a bare copper current collector during the first charge. Using sulfide SSEs, it is demonstrated that substantial amounts of lithium can be deposited onto a copper current collector at a relatively high current density of 1 mA cm-2. However, stripping lithium from the copper current collector is found to cause significant degradation that severely limits cycling lifetime. Strategies to mitigate the detrimental effects of stripping and extend cell lifetime are explored. Lastly, the energy densities of various battery chemistries are calculated at the cell-level. Alloy anodes in SSBs are shown to have competitive energy densities, and their mechanistic advantages over alloy anodes with liquid electrolytes and lithium metal SSBs are discussed.