Electronic Structure Elucidation of Two-Step Electrolyte Decomposition Mechanism on Li(x)Mn(1.5)Ni(0.5)O(4) (001) Surfaces

Abstract

High voltage cathode oxide mateirals like the Li(x)Mn(1.5)Ni(0.5)O(4) spinel can potentially enable high energy density lithium batteries for transportation storage applications. However, they rapidly degrade the organic liquid electrolyte. Customized solvent molecules and passivation coatings/strategies have been devised to alleviate this problem. Drawing from previous theoretical work and new hybrid functional Density Functional Theory (DFT) predictions, we argue in this presentation that the degradation mechanism has more than one step on the spinel (001) surface. First, standard battery liquid solvent molecules already decompose at low potentials, and even on the surfaces of spinel oxides without nickel doping, which operate at modest voltages. The organic fragments cover up catalytically active transition metal sites, yielding a functional form of cathode electrolyte interphase (CEI). Later, at high voltages, the decomposed solvent fragments further break down to release carbon dioxide and other species, leaving the catalytic sites uncovered and leading to continuous reactions with the liquid electrolyte. Mechanistic comparison with organic reactions on platinum surfaces will be made. However, the precise voltage range of breakdown offset cannot yet be predicted. These steps underscore the importance of understanding CEI degradation reaction, not just electrolyte oxidation, and will help inform new passivation strategies to enable high voltage cathode materials. This work was supported by Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.