Structure plays a vital role in determining materials properties. In lithium ion cathode materials, the crystal structure defines the dimensionality and connectivity of interstitial sites, thus determining lithium ion diffusion kinetics. In most conventional cathode materials that are well-ordered, the average structure as seen in diffraction dictates the lithium ion diffusion pathways. Here, we show that this is not the case in a class of recently discovered high-capacity lithium-excess rocksalts. An average structure picture is no longer satisfactory to understand the performance of such disordered materials. Cation short-range order, hidden in diffraction, is not only ubiquitous in these long-range disordered materials, but fully controls the local and macroscopic environments for lithium ion transport. Our discovery identifies a crucial property that has previously been overlooked and provides guidelines for designing and engineering cation-disordered cathode materials.

Despite several reports on the surface phase transformations from a layered to a disordered spinel and a rock-salt structure at the surface of the Ni-rich cathodes, the precise structures and compositions of these surface phases are unknown. The phenomenon, in itself, is complex and involves the participation of several contributing factors. Of these factors, transition metal (TM) ion migration toward the interior of the particle and hence formation of TM-densified surface layers, triggered by oxygen loss, is thermodynamically probable. Here, we simulate the thermodynamic phase equilibria as a function of TM ion content in the cathode material in the context of lithium nickel oxides, using a combined approach of first-principles density functional calculations, the cluster expansion method, and grand canonical Monte Carlo simulations. We developed a unified lattice Hamiltonian that accommodates not only rock-salt like structures but also topologically different spinel-like structures. Also, our model provides a foundation to investigate metastable cation compositions and kinetics of the phase transformations. Our investigations predict the existence of several Ni-rich phases that were, to date, unknown in the scientific literature. Our simulated phase diagrams at finite temperature show a very low solubility range of the prototype spinel phase. We find a partially disordered spinel-like phase with far greater solubility that is expected to show very different Li diffusivity compared to that of the prototype spinel structure.

Understanding structural transformation at the surface of Ni-rich layered compounds is of particular importance for improving the performance of these cathode materials. In this Letter, we identify the surface phases using first-principles-based kinetic Monte Carlo simulations. We show that slow kinetics precludes the conventional Li0.5NiO2 spinel to form from its layered parent phase at room temperature. Instead, we suggest that densified phases of the types Ni0.25NiO2 and Ni0.5NiO2 can form by Ni back diffusion from the surface owing to oxygen loss at highly charged states. Our conclusion is supported by the good agreement between the simulated STEM images and diffraction patterns and previously reported experimental data. While these phases can be mistaken for spinel and rock salt structures in STEM, they are noticeably different from these common structure types. We believe that these results clarify a long-standing puzzle about the nature of surface phases on this important class of battery materials.

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The development of Mg batteries, which can potentially achieve higher energy densities than Li-ion systems, is in need of cathodes that can reversibly intercalate Mg 2+ and exhibit a higher energy density than the stateof-the-art Chevrel and thio-spinel cathodes. Recent theoretical and experimental studies indicate that the oxide spinel family presents a set of promising Mg cathodes. Specifically, in this work, we investigate Mg intercalation into the spinel-Mg x Cr 2 O 4 system. Using first-principles calculations in combination with a cluster expansion model and the nudged elastic band theory, we calculate the voltage curve for Mg insertion at room temperature and the activation barriers for Mg diffusion, respectively, at different Mg concentrations in the Cr 2 O 4 structure. Our results identify a potential limitation to Mg intercalation in the form of stable Mg-vacancy orderings in the Cr 2 O 4 lattice, which exhibit high migration barriers for Mg diffusion in addition to a steep voltage change. Additionally, we propose cation substitution as a potential mechanism that can be used to suppress the formation of the stable Mg-vacancy ordering, which can eventually enable the practical usage of Cr 2 O 4 as a Mg-cathode.

A climbing image nudged elastic band method for finding saddle points and minimum energy paths The Journal of Chemical Physics 113, 9901 (2000) In this work, we propose a new scheme to accelerate NEB calculations through an improved path initialization and associated energy estimation workflow. We demonstrate that for cation migration in an ionic framework, initializing the diffusion path as the minimum energy path through a static potential built upon the DFT charge density reproduces the true NEB path within a 0.2 Å deviation and yields up to a 25% improvement in typical NEB runtimes. Furthermore, we find that the locally relaxed energy barrier derived from this initialization yields a good approximation of the NEB barrier, with errors within 20 meV of the true NEB value, while reducing computational expense by up to a factor of 5. Finally, and of critical importance for the automation of migration path calculations in high-throughput studies, we find that the new approach significantly enhances the stability of the calculation by avoiding unphysical image initialization. Our algorithm promises to enable efficient calculations of diffusion pathways, resolving a long-standing obstacle to the computational screening of intercalation compounds for Li-ion and multivalent batteries. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

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The electronic properties of the heavy metal superconductor [Formula: see text] are reported. The estimated superconducting parameters obtained from physical properties measurements indicate that [Formula: see text] is a BCS-type superconductor. Electronic band structure calculations show that Ir d-states dominate the Fermi level. A comparison of electronic band structures of [Formula: see text] and [Formula: see text] shows that the Ir-compound has a strong spin-orbit-coupling effect, which creates a complex Fermi surface.