Electro-Chemo-Mechanical Phenomena in Solid-State Battery Electrodes and Interfaces

Abstract

Expanding our capacity for energy storage is vital to the worldwide push to address climate change by transitioning to renewable energy. Improving battery technologies by increasing energy density, cycle life, charge rates, and safety will accelerate this change. Solid-state batteries (SSBs) are a promising technology for step improvements in each of these areas. Despite marked progress over the past decade, significant hurdles still need to be overcome to enable the widespread commercialization of SSBs.

The primary goal of this thesis is to gain a more fundamental understanding of the electro-chemo-mechanical phenomena that govern SSB performance. By improving our understanding of these underlying mechanisms, we can more efficiently design battery systems that address each of these challenges. In this thesis, operando techniques are used to examine model systems to elucidate the mechanisms that both limit (1) cycle life and (2) charge rates.

Cycle life in Li batteries is largely limited by undesirable side reactions and/or structural changes that both consume the Li reservoir and degrade the performance of the electrodes and electrolytes. This is especially detrimental in Li metal batteries, where the Li anode is highly reactive. The first section of this thesis aims to understand undesirable reactions that occur at the Li metal/solid electrolyte (SE) interface in SSBs. Specifically, the role of interfacial chemistry is investigated by both changing the SE used and adding interlayers between the Li and SE. It is observed that adjusting the interfacial chemistry can limit the impact of these side reactions. Operando video microscopy and operando x-ray photoelectron spectroscopy are used to investigate the time-dependent interplay between interfacial chemistry and morphology. Distinct differences in the chemical evolution are observed between SEs that form stable interlayers after reacting with Li metal and those that continue to react, leading to battery failure. Additionally, design rules are established for artificial interlayers used to stabilize the Li/SE interface.

Increasing charge rates in SSBs requires improvement in both the Li metal anode, where high current densities can lead to short-circuiting, and in composite electrodes, where slow Li transport into the depth of the electrode hinders fast charging. The second aim of this thesis probes both the mechanical properties that lead to Li penetration of the SE (short-circuiting) and the electrode properties that slow down Li transport. By synchronizing operando video microscopy with cycling data of both Li electrodes and composite electrodes, the microscale impact of fast charging is observed and correlated to signatures in the voltage traces. Molten Li electrodes are used as a model system to show that the mechanical properties of Li play a crucial role in cell shorting. Graphite composite electrodes are used to observe heterogeneous lithiation of the electrode caused by Li transport limitations in both the SE and active material phases.

In summary, this thesis improves our understanding of the mechanisms that limit both cycle life and charge rates in SSBs. Design rules based on these insights are given that can aid in the development of SSBs that last longer and can charge faster.