Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Chemical and Materials Engineering
Dr. Yang-Tse Cheng
Lithium-ion batteries (LIBs) are a staple in today’s society. From our cellphones, laptops, power tools, the ever-growing electric vehicle, and many more application, LIBs are more important to us than most realize. They provide the best combination of both high-energy and high-power density compared to other battery types such as Ni-Cd, Ni-MH, or the lead acid batteries used in our cars. Plus, LIBs are much safer. However, as new technologies grow and are developed, the demand for higher energy and power density, better safety, lower costs, and longer life increases. One way to achieve the ever-increasing demands is to replace the traditional graphite electrode with a pure lithium metal anode. However, lithium metal batteries have their challenges. Another way to meet the demands especially in terms of safety is to switch from liquid electrolytes to solid electrolytes. But these too have their challenges. This dissertation will thus focus on understanding fundamentals and testing simple solutions, such as external pressure, in order to overcome some of the challenges with these two next generation energy storage materials.
First, the effect of stack pressure was investigated on lithium metal cells using liquid electrolyte. We show that stack pressure is an important environmental factor that can help improve the cycle life of lithium metal batteries. However, we also cycled at an extremely high pressure and although we were able to minimize the cell expansion during cycling, the high pressure can be more detrimental than helpful. Next, we investigated the effect of external pressure on lithium metal batteries using solid electrolyte, Li6.4La3Zr1.4Ta0.6O12 (LLZTO). Prior to electrochemical cycling, we show that when pressure in applied then removed, the interfacial impedance between LLZTO and lithium metal decreases with time. The irreversible decrease of interfacial resistance can be understood by a gradual reduction of the total energy of the system, including strain energy and interfacial energy. Also, under external pressure exceeding ~25 MPa, lithium can be squeezed into LLZTO, fracturing the ceramic solid electrolyte. Lastly, utilizing nanoindentation, we investigated the mechanical property changes in LLZTO due to electrochemical cycling. We report that the elastic modulus, hardness, and fracture toughness of LLZTO do not change due to electrochemical cycling. However, lithium metal is observed to plate or penetrate the solid electrolyte along a certain plane and forms a “honeycomb’ like microstructure upon fracture. An attempt to use nanoindentation across the lithium-rich cross-section proved inconclusive, and thus other techniques in the future should be used to study this lithium-rich cross-section.
The electrochemical-mechanical studies presented on these two next generation materials may benefit the community by: (1) Suggesting that to achieve the desired thickness change, lithium microstructure, and cycle life, extreme pressures can be more detrimental and instead pressure needs to be designed in combination with other methods such as electrolyte and its additives. (2) Unveiling the fundamental interactions at the interface between garnet-type solid electrolytes and lithium metal. (3) Understanding the mechanical changes to LLZTO can help in engineering and designing methods to minimize the penetration of lithium dendrites and improve the overall performance of this solid electrolyte.
Digital Object Identifier (DOI)
I would like to acknowledge that this work was supported by the Vehicle Technologies Office of the U.S. Department of Energy Battery Materials Research (BMR) Program under Contract Number DE-EE0007787 from 2017-2019 and Contract Number DD-EE0008863 from 2020-2021.
I would also like to acknowledge support from the UK Energy Research Priority Area program in 2021.
Meyer, Andrew, "An Electrochemical-Mechanical Investigation of Next Generation Lithium Metal Electrode and Solid Electrolyte" (2022). Theses and Dissertations--Chemical and Materials Engineering. 137.