(Henry B. Linford Award Address for Distinguished Teaching) Intricacies of High-Energy Cathodes for Lithium-Ion Batteries

Abstract

First, I take this occasion of receiving the Henry B. Linford Award for Distinguished Teaching to express my deepest gratitude to more than 250 students and postdoctoral fellows who have worked with me over the years, including the graduation of 60 Ph.D. students and 26 M.S. students. Through their hard work and dedication, my career at the University of Texas at Austin has become truly rewarding.The cathodes in lithium-ion batteries play a key role in terms of cost, energy density, cycle life, and safety. As a postdoctoral fellow working with the 2019 Chemistry Nobel Laureate John B. Goodenough in the 1980s, my career began with the identification of inductive effect in polyanion oxide cathodes LixFe2(XO)4 (X = Mo, W, and S) in drastically altering the operating voltage depending on the covalency of the X-O bond. For example, with the same structure and the same Fe2+/3+ redox couple, both LixFe2(MoO)4 and LixFe2(WO)4 operate at 3.0 V, while LixFe2(SO)4 operates at 3.6 V, compared to < 2 V for LixFe2O3. This finding served as the basis for the subsequent development of olivine LiFePO4 cathode by Goodenough’s group 10 years later in 1997. The polyanion family of materials have now become broad with a variety of compositions as electrode materials for both lithium-ion and sodium-ion batteries.The battery industry is now largely based on layered LiNi1-y-zMnyCozO2 (MNC) cathodes. My group (i) demonstrated the differences in the relative chemical instabilities of LiMO2 (M = Co, Ni, and Mn) cathodes through chemical analysis of oxygen content in chemically delithiated Li1-xMO2 in the early 2000s and (ii) correlated the results to a qualitative picture of the relative positions of the metal:3d band with respect to the top of the O2-:2p band. These results now exemplify the ability to increase the capacity by increasing the nickel content in NMC cathodes. This presentation will present the complexities and prospects of high-nickel layered oxide cathodes for next-generation lithium-ion batteries. Analogous to the layered oxide cathodes, the presentation will also focus on the intricacies involved with high-voltage spinel LiNi0.5Mn0.5O4 (LNMO) cathodes. Specifically, approaches to eliminate expensive cobalt from cathodes will be pointed out.Finally, sulfur offers a low-cost alternative to oxide cathodes with an order of magnitude higher capacity than the oxide cathodes. However, lithium-sulfur cells are hampered by poor ionic and electronic transport, polysulfide shuttling, and lithium-metal anode degradation. This presentation will focus on addressing these issues and fabricating pouch lithium-sulfur batteries with a high sulfur content and loading and low electrolyte amount, assisted by effective electrocatalysts, in order to be competitive with the current lithium-ion technology.The value of advanced analytical techniques, such as in-situ x-ray diffraction, x-ray photoelectron spectroscopy, time of flight secondary ion mass spectrometry, scanning electron microscopy, and high-resolution transmission electron microscopy in understanding the intricacies of the cathodes and mitigating the problems will be presented.