Concentration of Vacancies at Metal Oxide Surfaces: Case Study of MgO (100)

We investigate effects of doping on formation energy and concentration of oxygen vacancies at a metal oxide surface, using MgO (100) as an example. Our approach employs density-functional theory, where the performance of the exchange-correlation functional is carefully analyzed, and the functional is chosen according to a fundamental condition on DFT ionization energies. The approach is further validated by CCSD(T) calculations for embedded clusters. We demonstrate that the concentration of oxygen vacancies at a doped oxide surface is largely determined by formation of a macroscopically extended space charge region

Decoupling Substitution Effects from Point Defects in Layered Ni‐Rich Oxide Cathode Materials for Lithium‐Ion Batteries

Ni-rich LiNixCoyMnzO2 cathode materials offer high practical capacities and good rate capability, but are notorious for being unstable at high state of charge. Here, a series of such layered oxides with nickel contents ranging from 88 to 100 mol% is fabricated by sodium-to-lithium ion exchange, yielding materials devoid of ${\mathrm{Ni}}_{{\mathrm{Li}}}^ \bullet$ substitutional defects. Examining the initial charge/discharge cycle reveals effects that are specifically caused by transition-metal substitution, which would otherwise be obscured by changes in lithium-site defect concentration. Lowering the nickel content helps to stabilize the high-voltage regime, while simultaneously negatively affecting lithium diffusion. Operando X-ray diffraction indicates mitigation of volume variation during cycling and transition toward solid-solution behavior with sufficiently high cobalt and manganese contents, thus providing an explanation for the increased stability. The interplay between transition-metal substitution, kinetic hindrance, and solid-solution behavior may be a result of local inhomogeneities due to lithium-vacancy pinning, which is further elucidated through density functional theory calculations. Overall, this work sheds new light on the effects of manganese and cobalt incorporation into the transition-metal layer and their conjunction with ${\mathrm{Ni}}_{{\mathrm{Li}}}^ \bullet$ defects

From LiNiO₂ to Li₂NiO₃ : Synthesis, Structures and Electrochemical Mechanisms in Li-Rich Nickel Oxides

The Li−Ni−O phase diagram contains a variety of compounds, most of which are electrochemically active in Li-ion batteries. Other than the well-known LiNiO2, here we report a facile solid-state method to prepare Li2NiO3 and other Li-rich Ni oxides of composition Li1+xNi1−xO2 (0 ≤ x ≤ 0.33). We characterize their crystal and electronic structure, exhibiting a highly oxidized Ni state and defects of various nature (Li−Ni disorder, stacking faults, oxygen vacancies). We then investigate the use of Li2NiO3 as a cathode active material and show its remarkably high specific capacity, which however fades quickly. While we demonstrate that the initial capacity is due to irreversible O2 release, such process stops quickly in favor of more classical reversible redox mechanisms that allow cycling the material for >100 cycles. After the severe oxygen loss (∼15−20%) and prolonged cycling, the Bragg reflections of Li2NiO3 disappear. Analysis of the diffracted intensities suggests the resulting phase is a disordered rock salt-type material with high Li content, close to Li0.5Ni0.5O, never reported to date and capable of Li diffusion. Our findings demonstrate that the Li−Ni−O phase diagram has not been fully investigated yet, especially concerning the preparation of new promising materials by out-of-equilibrium methods

First-principles calculations on structure and properties of amorphous Li5P4O8N3 (LiPON)

The structural, electronic and ion transport properties of an amorphous member of the LiPON family with non-trivial composition and cross-linking are studied by means of electronic structure calculations within Density Functional Theory. By a combination of an evolutionary algorithm followed by simulated annealing and alternatively by a rapid quenching protocol, structural models of disordered Li5P4O8N3 are generated, which are characterized by a local demixing in Li-rich and Li-poor layers. These structures have a composition close to what is found experimentally in thin films and contain all the expected diversely coordinated atoms, namely triply- and doubly-coordinated nitrogens and bridging and non-bridging oxygens. The issue of ionic conductivity is addressed by calculating defect formation energies and migration barriers of neutral and charged point defects. Li+ interstitials are energetically much preferred over vacancies, both when the lithium reservoir is metallic lithium and LiCoO2. The competitive formation of neutral Li interstitials when LiPON is contacted with metallic Li results in the chemical reduction of LiPON and the disruption of the network, as recently observed in experiments

Interaction of CO with Electron-Rich Defects on MgO(100)

Electron-rich defects are believed to be important for the reactivity of simple oxides such as MgO. We use density functional theory for embedded cluster models to unravel the reactivity differences toward CO between neutral F⁰ centers with paired electrons and single excess electron sites, represented by singly charged, paramagnetic F⁺ centers and by MgO divacancies with one trapped electron. On neutral F⁰ centers, adsorption of CO into the most stable state (³A″, 1.3 eV binding for a step site) is prevented by a significant barrier (0.5 eV). This explains that the predicted strongly red-shifted IR band of 1365 cm^–1 is not observed. This shift is due to transfer of more than one electron from the defect site into the antibonding CO orbitals. In contrast, single excess electron sites readily react with a CO molecule. On a F⁺ step site, the binding energy is ∼1.2 eV and the predicted CO stretching frequency is 1734 cm^–1 within the range of observed values. Paramagnetic divacancies interact with CO in a similar fashion as F⁺ centers. The picture emerging from the calculations supports the conclusions reached from IR and EPR observations of CO in contact with electron-rich MgO surfaces

Planar gliding and vacancy condensation: the role of dislocations in the chemomechanical degradation of layered transition-metal oxides

Stacking faults driven by dislocations have been observed in layered transition-metal oxide cathodes both in cycled and uncycled materials. The reversibility of stacking-sequence changes directly impacts the material performance. Irreversible glide due to lattice invariance or local compositional changes can initiate a catastrophic sequence of degradation mechanisms. In this study we compare the chemomechanical properties of LiCoO2 and LiNiO2 by combining density functional theory and anisotropic linear elasticity theory. We calculate stacking fault energies as a function of Li content and quantify the extent to which excess Ni hinders stacking-sequence changes. We then characterize screw dislocations, which mediate stacking-sequence changes, and find a peculiarly compliant behavior of LiNiO2 due to the interaction of Jahn–Teller distortions with the dislocation strain field. Finally, we analyze the tendency of vacancies to segregate along dislocation lines. This study represents the first instance of explicit ab initio atomistic dislocation models in layered oxides and paves the way for the understanding and optimization of the chemomechanical behavior of cathode active materials during battery operation

Interfacial instability of amorphous LiPON against lithium: A combined Density Functional Theory and spectroscopic study

The chemical instability of the glassy solid electrolyte LiPON against metallic lithium and the occurrence of side reactions at their interface is investigated by combining a surface science approach and quantum-mechanical calculations. Using an evolutionary structure search followed by a melt-quenching protocol, a model for the disordered structure of LiPON is generated and put into contact with lithium. Even the static optimization of a simple model interface suggests that the diffusion of lithium into LiPON is driven by a considerable driving force that could easily take place under experimental conditions. Calculated reaction energies indicate that the reduction and decomposition of LiPON is thermodynamically favorable. By monitoring the evolution of the LiPON core levels as a function of lithium exposure, the disruption of the LiPON network alongside the occurrence of new phases is observed. The direct comparison between UV photoelectron spectroscopy measurements and calculated electronic densities of states for increasing stages of lithiation univocally identifies the new phases as Li_2O, Li_3P and Li_3N. These products are stable against Li metal and form a passivation layer which shields the electrolyte from further decomposition while allowing for the diffusion of Li ions. Interfacial instability of amorphous LiPON against lithium: A combined Density Functional Theory and spectroscopic study. Available from: https://www.researchgate.net/publication/316112203_Interfacial_instability_of_amorphous_LiPON_against_lithium_A_combined_Density_Functional_Theory_and_spectroscopic_study [accessed May 16, 2017]