Interfacial Reactivity Studies of Electrochemical Energy Storage Materials from First Principles

Abstract

Since their commercialization in the early 1990’s, rechargeable lithium ion batteries (LIBs) have become ever-present in consumer electronics, and the share of electric\nvehicles within the transportation sector has become much more significant. Ab initio\nmodeling techniques - namely density functional theory (DFT) - have played a signifcant role in describing the atomic scale nature of Li+ insertion and removal chemistry\nin LIB electrode materials, and have been pivotal in accelerating the design of energy\ndense battery materials based on their bulk properties. Despite these advances, there\nremains a knowledge gap with respect to understanding the many complex reactions\nthat occur at the surfaces and interfaces of rechargeable battery materials. This work\nconsiders several case studies of surface and interfacial reactions in energy storage\nmaterials, using DFT modeling techniques to develop strategies that can rationally\ncontrol the interfacial chemistry for optimal electrochemical performance.\n\n \n \n \n \n \n \n The first portion of this thesis aims to understand the role of interfacial modification strategies toward mitigating Mn dissolution from the spinel LiMn2O4 (LMO)\nsurface. First, a thermodynamic characterization of LMO surface structures is performed in order to develop models of LMO substrates for subsequent computational\nsurface science studies. A subset of these surface models are then used analyze interfacial degradation processes through delithiation-driven stress buildup and crack\nformation, as well as reaction mechanisms for ethylene carbonate and hydrofluoric\nacid to form surface Mn2+ ions that are susceptible to dissolution. Surface passivation mechanisms using protective oxide and metallic coatings are then analyzed, which elucidate an electronic structure-based descriptor for structure-sensitive atomic layer\ngrowth mechanisms and describe the changes in lithiation reactions of coated electrodes through electronic band alignment at the solid-solid interface. These studies\nof protective coatings describe previously overlooked physics at the electrode-coating\ninterface that can aid in further development of coated electrode materials. Using\nthe LMO substrate models, a thermodynamic framework for evaluating the solubility limits and surface segregation tendencies of cationic dopants is described in the\ncontext of stabilizing LMO surfaces against Mn loss. \n \n \n \n \n \n \n Next, solid-solid interfacial models are developed to evaluate the role of nanostructure in catalyzing the lithiation of NiO to form reduced Ni and Li2O as concurrent discharge products. Applying a Ni/NiO multilayer morphology, interfacial\nenergies are evaluated using DFT and implemented into a classical nucleation model\nat a heterogeneous interface. These calculations, alongside operando X-ray scattering measurements, are used to explain atomic scale mechanisms that reduce voltage\nhysteresis in metal oxide LIB conversion chemistry. \n \n \n \n \n \n \n The structure between a Li metal anode and the lithium lanthanum titanate solid\nelectrolyte are subsequently analyzed as a model system to understand potential inter-\nfacial stabilization mechanisms in solid-state batteries. This analysis combines bulk,\nsurface, and interfacial thermodynamics with ab initio molecular dynamics simulations to monitor the evolution of the interfacial structure over short time scales, which\nprovides insights into the onset of degradation mechanisms. It is shown that the reductive instability of Ti4+ is the primary driving force for interfacial decomposition\nreactions, and that a lanthanum oxide interlayer coating is expected to stabilize the\ninterface based on both thermodynamic and electronic band alignment arguments. \n \n \n \n \n \n \n In the last part of this thesis, charge transfer kinetics are studied for several\napplications using constrained DFT (cDFT) to account for electronic coupling and\nreorganization energies between donor and acceptor states. Charge hopping mechanisms to and from dichalcogenide-based electrocatalysts during O2 and CO2 reduction/evolution reactions in Li-O2 and Li-CO2 battery systems are first evaluated. Then, the role of the spatial separation Li+ vacancies and interstitials on hole and\nelectron polaron hopping in the prototypical LixCoO2 cathode is analzyed. The\nresults demonstrate that Marcus rate theories using cDFT-derived parameters can\nreproduce experimentally observed anisotropies in electronic conductivity, whereas\nconventional transition state theory analyses of polaron hopping do not. Overall, this\nproof-of-concept study provides a framework to understand how charged species are\ntransported in battery electrodes and are dependent on charge compensating defects.\n \n \n \n Finally, the key insights from these studies are discussed in the context of future\ndirections related to the understanding and design of materials for electrochemical\nenergy conversion and storage.