ChEMS Seminar: Probing Nanoscale Materials Transformation and Dynamic Phenomena by Advanced In-situ Electron Microscopy
Department of Materials Science and Engineering
Northwestern University, Evanston, IL
Abstract: Understanding the physical and chemical processes occurring at nanoscale environments is essential to a variety of scientiﬁc disciplines. Recent advancement of aberration correction has enabled routine atomic resolution imaging and spectroscopy using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) methods. Further, combined with modern in-situ approaches, they have been proven to be powerful for probing dynamic structural, chemical and functional evolutions in real time. Here, I will present our recent research effort on development of in-situ analytical S/TEM techniques and their implementations in energy storage materials, in particular, to unveil the phase transformation during electrochemical reactions upon battery charge/discharge and their correlation to realistic battery performance. We have investigated electrode materials for lithium ion batteries with different crystal structures, from simple cubic structure with no interstitial space, to spinel and layered structures with vacant openings; and we found the reaction mechanism varies from direct conversion to a combination of intercalation and conversion. More importantly, we discovered multiple lithiation modalities with distinct reaction pathways, and illustrated that the kinetic effect plays a critical role in realistic electrochemical conditions where thermodynamic nonequilibrium is generally applied. The knowledge gained from the in-situ experiments has addressed key questions to leverage the structure-pathway-property relationship and provides mechanistic insight into future battery design and manufacture.
Bio: Kai He is a research assistant professor at the Department of Materials Science and Engineering, Northwestern University. He received his doctorate from Arizona State University in 2010 and was then appointed as the Ellen Williams Fellow at the University of Maryland and research associate at the Center for Functional Nanomaterials of Brookhaven National Laboratory. His research focuses on development of advanced electron microscopy, spectroscopy and holography methodologies and uses these techniques to study atomic structures and dynamic phenomena in multifunctional nanomaterials, with particular emphasis on energy storage, nanoelectronics and magnetism applications. He has authored over 50 peer-reviewed publications, delivered over 20 invited talks, and received multiple academic awards, including the Microscopy Society of America’s Presidential Scholar Award. He also has served as symposium organizer for M&M and APS meetings.
Host: Lorenzo Valdevit