Date: Monday, Feb 16th
Presenter: Dr. Kai He, Postdoctoral Research Associate, Center for Functional Nanomaterials, Brookhaven National Laboratory
Understanding the physical and chemical processes occurring at nanoscale environments is essential to a variety of scientific disciplines. Advanced transmission electron microscopy (TEM) combined with in situ experimental approach represents a powerful characterization method to provide atomic-scale spatial resolution, quantitative chemical information, real-time visualization, and quantum-mechanics phase change detectability. Here, I will present our recent studies towards understanding the nanoscale dynamic phenomena in electrochemical systems and device functionalities for energy storage using complementary in situ electron microscopy, spectroscopy, holography, and tomography at all relevant length scales and dimensions. I will start from the nickel oxide electrode material for lithium-ion batteries, in which the kinetic pathways of electrochemical reactions were tracked in real time and three dimensions to reveal the reaction modality. We identified the transition from near-surface to interior redox and unlocked the intrinsic limiting mechanism for fast-charging electrode materials. Furthermore, the reaction kinetics were compared between lithium and sodium chemistries in both oxides and fluorides materials, where novel phenomena occurred due to different surficial and interfacial properties. Combined with synchrotron x-ray and DFT modeling, we were able to get a complete portrait of the entire reaction, linking electrochemistry, nanostructures and overall performance all together. Our findings herein uncover new nanoscale clues to optimizing battery performance, while the methodology can be equally valid for diverse scientific areas. An outlook for future electron microscopy development and its implementation for solving grand challenges in multi-scale science will also be discussed.
Dr. Kai He is a postdoctoral research associate at the Center for Functional Nanomaterials in Brookhaven National Laboratory. He received his PhD from Arizona State University in 2010, and then worked as the Ellen Williams Fellow at University of Maryland. His research expertise includes atomic-resolution transmission electron microscopy, spectroscopy, holography, tomography, and their in-situ implementations in the nanostructured magnetic, electronic, and energy materials. He has published over 30 papers in leading journals and received multiple academic awards from the Microscopy Society of America, IEEE Magnetics Society, and Chinese government.