Fundamental study and materials design on lithium metal anodes

Commercialized nearly three decades ago, the lithium-ion (Li-ion) battery has completely transformed the portable electronics industry and enabled the modern conveniences of social connectivity and mobile services. Nonetheless, current Li-ion batteries are quickly approaching their practical energy density limits. To make a lasting impact on emerging applications such as electric vehicles and grid-scale energy storage, an intimate understanding of materials chemistry in both existing and emerging batteries ---- such as those containing metallic Li ---- needs to be developed. In particular, metallic Li offers the largest specific capacity of all Li-based anodes, however, significant safety hazards caused by its high reactivity and dendritic growth during charge and discharge are major issues that prevent its practical applications, and need to be addressed. The ultimate tool for probing the atomic structure and chemistry of battery materials is the transmission electron microscope (TEM). Unfortunately, most high-energy battery materials such as metallic Li are extremely reactive in air, and unstable upon irradiation of high-energy electrons necessary for high-resolution imaging, causing them to quickly degrade during sample preparation and observation. In this thesis, I will not only present my study on using nanoscale engineering approaches to address the problems of the lithium metal anode, but also demonstrate the successful development of new tools to discover fundamental understandings. In Chapter 1, the working mechanisms and limitations of the state-of-the-art Li-ion battery chemistries will be introduced. An overview of promising new battery chemistries based on metallic lithium anode will also be provided. In Chapter 2, I report a novel synthesis method by reacting clean molten lithium foil directly with pure nitrogen gas to generate instantaneously a pinhole free and ionically conductive alpha-Li3N film directly bonded onto Li metal foil. This Li3N coating is chemically stable, isolating the reactive metallic lithium from liquid electrolyte, prevents continuous electrolyte consumption during battery cycling and promotes dendrite-free uniform lithium plating/stripping underneath. In Chapter 3, I introduce the basic imaging and working principles of the transmission electron microscope (TEM). In Chapter 4, I discuss the nanoscale corrosion process of lithium metal inside an environmental transmission electron microscope, allowing for high resolution monitoring of gas reactions in real time with chemical and structural information. In Chapter 5, I highlight the successful application of cryogenic electron microscope (cryo-EM) techniques on battery research for the rst time, demonstrating that cryo-EM is able to preserve Li metal in its native state after battery operation and stabilize the beam-sensitive material for atomic-resolution imaging. In Chapter 6, I will present my perspectives and outlooks for the future application of cryo-EM on battery research.

Keywords: Chemical Engineering) degree committee member
Materials Science and Engineering.
Advisor: Cui Yi
Chueh William