High Energy Density Metal-Oxides for Energy Storage: in silico Electrochemistry to Control Interface Chemistry

We will harness the power of modern computational chemistry to generate a first-principles understanding of the impact of the surface structures and compositions of metal-oxides (MOx) on the chemical reactions and mass transport dynamics essential to high energy density electrochemical energy storage (EES). Considerable advances in EES technologies, including fuel cells, batteries, and capacitors, are revolutionizing energy mobility and consumption, and fostering the deployment of alternative energy sources. Developed within the context of seemingly straightforward chemical phenomena that transform chemical energy into electrical energy (and vice versa), EES enables technologies that range from small, portable electronic devices through transportation to residential and large-scale electrical grid load leveling and storage. Regardless of the accolades and advances, however, EES technologies fall short of the storage efficiencies and capacities needed to transform global energy supply and consumption, and technology safety remains a paramount concern. While MOx nanocrystals are widely used in electrochemical cell electrodes, the elementary processes by which these electrodes operate remain unclear. Inhomogeneous electrode compositions and multicomponent electrolyte formulations result in operating environments that stymie efforts to resolve the intimate electrochemical reactions that occur on the MOx surface and, subsequently, at the broader electrode-electrolyte interface. Here we will exploit state-of-the-art electronic-structure methods and molecular-dynamics techniques to determine essential physicochemical processes that occur during EES with molecular-scale resolution. In particular, we will focus on the surface chemistry of layered LiNiyMnyCo1.2yO2 (NMC), which has received significant attention as a cathode MOx that can operate at over 4 V. Very few theoretical studies address the critical relationships between MOx surface composition and structure, much less the chemical reactivity and dynamics of the electrode¡Velectrolyte interface during EES. This critical knowledge gap is problematic as the rich natures of the MOx surface and the surrounding chemical environment can lead to substantial differences when compared to bulk MOx structural and electronic properties. With an overarching goal to establish first-principles mechanisms by which the MOx surface composition and structure direct interfacial chemical reactions in EES, we set the following research objectives: „X Objective 1: Determine the impact of chemical composition on the surface characteristics of NMC. We contend that MOx surface chemistry plays a critical role on electrode performance in EES applications, and that this is of particular significance for the nanocrystalline MOx used in modern battery electrodes. For NMC, we will explore the geometric, chemical, and electronic characteristics of the low-energy surfaces. Our investigations will determine how structural defects, dopants, and surface termination variations that arise due to modifications of nanocrystal NMC growth conditions and exposure to varying environments affect the surface characteristics when compared to pristine surfaces. Implicit solvent models will be used to provide a first approximation as to how the interface with high-dielectric electrolytes impacts the NMC surface properties. „X Objective 2: Reveal the impact of adsorbed electrode composites and electrolyte components on NMC surface structure and electronic states. While studies of MOx bulk and (bare) surfaces can provide important physicochemical characteristics relating to their performance in EES applications, MOx-based electrodes are often inhomogeneous composite formulations that contain carbon black and polymer binders, with the MOx interface further modified through chemical interactions with the electrolyte. We hypothesize that the chemical and physical interactions of MOx with electrode composite materials and components of the electrolyte regulate the surface structure, electronic states, and potentials in these heterogeneous environments, and that these considerations are essential to developing a full mechanistic understanding of EES. We will determine how electrode additives interact with NMC surfaces and identify surface restructuring pathways, variations in electronic states, and the impact of these surface modifications on subsequent chemical reactions. Our investigations will account for ion interactions with the modified surfaces and explore the formation of solid-electrolyte interaction (SEI) layers induced by the chemical degradation of the electrolyte solvent and salt, processes for which little understanding currently exists in the literature. Objective 3: Uncover the complex chemical dynamics of the electrode-electrolyte phase boundary. While first-principles solid–gas or implicit solvent interface studies provide important insights into the structure-property relationships of MOx surfaces and hint at the mechanisms of action during EES, they miss critical aspects of the dynamic, often kinetically controlled, nature of the in operando EES environment. We hypothesize that the nature of the MOx surface chemistry will dictate larger dynamic processes taking place at the electrode-electrolyte boundary, and that the development of such mechanistic details can be used to synthetically regulate these processes. We will develop classical molecular dynamics (MD) techniques to determine how the orientation and frequency of interactions between the electrolyte and NMC surfaces control aspects of the dynamic physicochemical processes involved in EES. These simulations will provide critical information on the mass transport dynamics of the EES environment, and serve as input for more advance ab initio MD methods that can deliver information on the dynamics of chemical reactivity.