Mechanical stresses and strains developing locally within the microstructure of active ion-battery-electrode materials during charge-discharge cycles can compromise their long-term stability. In this context, crystalline compounds exhibiting low volume changes are of particular interest. Atomistic simulations can be employed to quantify the volume change of the crystal structure upon intercalation and deintercalation of ions and to elucidate the local mechanisms underlying the global structural response. While density functional theory (DFT) offers a robust and accurate framework for such calculations, its computational cost limits its applicability for large-scale screening of diverse intercalation structures and sites. In this work, we present a workflow designed to prioritize candidate materials for subsequent detailed characterization. The workflow calculates the volume change upon intercalation using atomic-level features and a machine-learning model for bond-length prediction. The bond-length predictions are based on the assumption that bonds between the same ionic species in similar local coordination environments exhibit comparable lengths across different crystallographic structures. The model was trained on a DFT-generated dataset, which inherently defines the chemical space in which reliable predictions can be expected. We demonstrate the workflow's utility by screening approximately 1,175,000 transition-metal oxides and fluorides, followed by DFT validation of the most promising candidates. The proposed workflow enables filtering of large candidate sets and accelerates the potential discovery of low volume change intercalation materials for batteries.

The intriguing physics and rich application potential of strongly correlated first-row transition metal oxide compounds result from the complex interplay of several factors that influence the electronic structure. To shed light on the effect of composition, structure, and correlation strength, we apply a well-established charge self-consistent combination of density functional theory and dynamical mean field theory, which has proven to give electron binding energies in good agreement to experimentally derived excitation spectra. For paramagnetic NiO and CoO, we analyze the effect of rock-salt and zincblende structures and their different ligand fields on the spectral functions. By varying the value of the interaction parameter U, different correlation strengths among the transition-metal 3d electrons are considered, as well as the effect of additionally accounting for correlations in the oxygen 2p orbitals by a self-interaction-correction pseudopotential scheme.

Li containing transition metal oxides are known as good ionic conductors. Performing classical molecular dynamics simulations, the diffusion behavior of Li ions is investigated in crystalline and amorphous phases with the stoichiometries Li$_2$ZrO$_3$ and LiNbO$_3$. We first demonstrate the stability of the crystal structures for the used interatomic potential model and then analyze the amorphous phases, which result from melt-and-quench simulations, in terms of radial distribution functions. Diffusivities of Li ions in those systems are obtained from a statistical Arrhenius analysis of mean square displacement curves at different temperatures. The crystalline phase of Li$_2$ZrO$_3$ exhibits two well-defined migration mechanisms: vacancy-mediated migration is dominant below and a site exchange of Li ions above a crossover region between about 1700 and 1800 K. The latter mechanism also prevails in the amorphous phases of Li$_2$ZrO$_3$ with a strongly reduced activation energy, which is due to a smaller equilibrium separation of Li ions as in the crystal structure. This migration mechanism is found in amorphous LiNbO$_3$, too.

The stability of the cathode is of crucial importance for the service life of lithium-ion batteries (LIB). In the cathode, the lattice parameter usually changes during (de-)insertion of lithium, which causes various signs of aging that have a negative effect on the capacity of the cell. This problem can be tackled using a specific type of materials called zero-strain (ZS) materials. These materials do not show any (de-)intercalation-induced structural changes and stresses. Such a zero-strain effect was already demonstrated with fluorine-containing materials [1] and even for the similar material K0.6FeF3 [2] for sodium-ion batteries (SIB). In this case, potassium ions are removed in the first cycle and the vacant lattice sites can be used for the incorporation of sodium ions, while keeping the structural integrity of the host material. Complementary to experiments, theoretical atomistic simulations are powerful to develop design criteria for further ZS-materials. Combining experiments and simulations in this work, the structural behavior of the material K0.5FeF3 was investigated when used as a cathode material in a LIB. The synthesis of K0.5FeF3 was achieved by a Rapid Microwave-Enhanced Solvothermal Process, which allowed to obtain the material in a TTB-type (Tetragonal Tungsten Bronze) phase with a high degree of purity. The material was then processed into electrodes, which were galvanostatically cycled against lithium metal. It was shown that the material is suitable for LIB as a cathode material. X-ray diffraction experiments showed that the lattice parameter changed by less than 1% after the incorporation of Li-Ions, in good agreement with the theoretical calculations conducted in parallel. Lattice Parameter Charged Discharged a[Å] 12.74±0.024 12.73±0.006 c[Å] 3.99±0.008 4.00±0.002 Volume [Å3] 647.57±2.67 647.75±0.76 This work was supported by the German Research Foundation (DFG) BI 1636/7-1 and EL 155/29-1. References [1] De Biasi, L., et al. Journal of Power Sources, 2017, 362. Jg., S. 192-201. [2] Han, Y., et al. Journal of Materials Chemistry A, 2016, 4. Jg., Nr. 19, S. 7382-7389. Figure 1: Diffractograms of an electrode with K0.5FeF3, which was measured before use and an electrode after the first successful discharge, down to a voltage of 1.8 V. Figure 1

Oxygen vacancy formation energies play a major role in the electric field‐assisted abnormal grain growth of technologically relevant polycrystalline perovskite phases. The underlying effect on the atomic scale is assumed to be a redistribution of cationic and anionic point defects between grain boundaries (GBs) and the bulk interior regions of the grains due to different defect formation energies in the structurally different regions, accompanied by the formation of space charge zones. Using atomistic calculations based on classical interatomic potentials, optimized structures of the symmetric tilt GBs Σ5(210)[001] and Σ5(310)[001], and of the asymmetric tilt GB (430)[001]||(100)[001] in the electroceramic perovskite materials SrTiO3, BaTiO3, and BaZrO3, are derived and discussed. Profiles of oxygen vacancy formation energies across those GBs are presented and their dependence on composition and GB type is discussed.

battery; cell manufacturing; cell ageing; workflow; multiscale models; multiphysics models

Artificial electrostatic potentials can be present in supercells constructed for atomistic simulations of surfaces and interfaces in ionic crystals. Treating the ions as point charges, we systematically derive an electrostatic formalism for model systems of increasing complexity, both neutral and charged, and with either open or periodic boundary conditions. This allows to correctly interpret results of classical atomistic simulations which are directly affected by the appearance of these potentials. We demonstrate our approach at the example of a strontium titanite (SrTiO$_3$) supercell containing an asymmetric tilt grain boundary. The formation energies of charged oxygen vacancies and the relaxed interface structure are calculated based on an interatomic rigid-ion potential, and the results are analyzed in consideration of the electrostatic effects.

Solid oxide fuel cells (SOFC) and solid oxide electrolyzer cells (SOEC), which transform chemical into electrical energy and vice versa, have the potential to make a significant contribution to the efforts of overcoming present problems of the energy economy in the near future. An optimal functionality of these devices requires a high catalytic activity at the electrodes, which strongly depends on point defect concentrations and on the capability of the material to allow for fast charge transfer reactions. Promising anode materials regarding these requirements are perovskite compounds (ABO\(_3\)), where the transition-metal ion on the B site can adopt different oxidation states by accepting and releasing electrons during the oxygen reactions at the SOEC/SOFC surfaces. For LaFeO\(_3\), a typical representative of this material class, we present results regarding the phase stability and point defect formation energies derived by density functional theory GGA+U calculations. The influence of point defects on the electronic charge-carrier concentrations as a function of the oxygen partial pressure is studied and compared for the perovskite materials LaFeO\(_3\), LaMnO\(_3\) and CaMnO\(_3\). In addition to the scientific results, the performance of the DFT calculations applied for these studies on the ForHLR I computer cluster is reported.

Oxygen vacancies have been identified to play an important role in accelerating grain growth in polycrystalline perovskite-oxide ceramics. To advance the fundamental understanding of growth mechanisms at the atomic scale, classical atomistic simulations were carried out to investigate the atomistic structures and oxygen vacancy formation energies at grain boundaries in the prototypical perovskite-oxide material SrTiO3. In this paper, we focus on two symmetric tilt grain boundaries, namely, Sigma 5 (310)[001] and Sigma 5 (210)[001]. A one-dimensional continuum model is adapted to determine the electrostatic potential induced by charged lattice planes in atomistic structure models containing grain boundaries and point defects. By means of this model, electrostatic artifacts, which are inherent to supercell models with periodic or open boundary conditions, can be considered and corrected properly. We report calculated formation energies of oxygen vacancies on all the oxygen sites across boundaries between twomisoriented grains, and we analyze and discuss the formation-energy values with respect to local charge densities at the vacant sites.

We examine the ionic migration of Li in LATP [Li1+xAlxTi2−x(PO4)3] solid electrolytes from an atomistic viewpoint by means of density functional theory calculations. We vary the Al content and investigate its effects on the crystal structure of LATP and on the migration energy landscape of interstitial Li ions. The energy profiles governing the Li diffusion are found to be systematically influenced by the position of Al ions in direct vicinity of the migration path, and we derive a simplified classification scheme of three universal energy profile shapes. The overall influence of the Al/Ti-ratio on the Li migration is analyzed by a separation into chemical and geometrical aspects. This work provides a solid basis for a resource-efficient computational examination of the ionic conductivity of Li in LATP with varying Al/Ti concentrations.

The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition metal ions. Such relations are of high interest as they offer the possibility to influence the catalytic activity of perovskites for the application in solid-oxide fueland electrolyser cells. Combining theoretical and experimental approaches, we explore the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing non-stoichiometric LaFeO3 (LFO). In the theoretical part, formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the DFT+U results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe2O3 and La2O3) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases in detail by X-ray diffraction and dedicated microscopy methods, namely scanning electron microscopy (SEM) and (scanning) transmission electron Microscopy ((S)TEM) in combination with energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectrometry (EELS). Instead of a variation of the La/Fe ratio, a mixture of two phases, Fe2O3/LaFeO3, was observed resulting in an invariant charge state of Fe, which is in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previously published studies on this material system.

Knowledge about the formation energies of compounds is essential to derive phase diagrams of multicomponent phases with respect to elemental reservoirs. The determination of formation energies using common (semi-)local exchange-correlation approximations of the density functional theory (DFT) exhibits well-known systematic errors if applied to oxide compounds containing transition metal elements. In this work, we generalize, reevaluate, and discuss a set of approaches proposed and widely applied in the literature to correct for errors arising from the over-binding of the O2 molecule and from correlation effects of electrons in localized transition-metal orbitals. The DFT+U method is exemplarily applied to iron oxide compounds, and a procedure is presented to obtain the U values, which lead to formation energies and electronic band gaps comparable to the experimental values. Using such corrected formation energies, we derive the phase diagrams for LaFeO3, Li5FeO4, and NaFeO2, which are promising materials for energy conversion and storage devices. A scheme is presented to transform the variables of the phase diagrams from the chemical potentials of elemental phases to those of precursor compounds of a solid-state reaction, which represents the experimental synthesis process more appropriately. The discussed workflow of the methods can directly be applied to other transition metal oxides.

We examine the ionic migration of Li in LATP [Li_1+xAl_x Ti_2-x(PO_4)_3] solid electrolytes from an atomistic viewpoint by means of density functional theory calculations. We vary the Al content and investigate its effects on the crystal structure of LATP and on the migration energy landscape of interstitial Li ions. The energy profiles governing the Li diffusion are found to be systematically influenced by the position of Al ions in direct vicinity of the migration path, and we derive a simplified classification scheme of three universal energy profile shapes. The overall influence of the Al/Ti-ratio on the Li migration is analyzed by a separation into chemical and geometrical aspects. This work provides a solid basis for a resource-efficient computational examination of the ionic conductivity of Li in LATP with varying Al/Ti concentrations.

Compounds crystallizing in the structure of NaZr2(PO4)3 (NZP) are considered as promising materials for solid state electrolytes in Li-ion batteries. Using density functional theory (DFT), a systematic computational screening of 18 NZP compounds, namely, LiX2(LO4)3 with X=Ti, V, Fe, Zr, Nb, Ru, Hf, Ta, Os, and L=P, Mn, is performed with respect to their activation energies for vacancy-mediated Li migration. It is shown how the different ionic radii of the cationic substitutions influence structural characteristics such as the octahedron volumes around Li ions on the initial-state and transition-state sites, which affect the activation energies (“composition–structure–property” relationships). The prevalent assumption that structural bottlenecks formed by triangularly arranged oxygen atoms at a certain location along the migration path determine the energy barriers for Li migration is not supported by the DFT results. Instead, the ionic neighborhood of the migrating ion in the initial and in the transition state needs to be taken into account to relate the structure to the activation energies. This conclusion applies to Na-containing NZP compounds as well.

We extend a canonical-thermodynamic method for computing intrinsic point defect concentrations as a function of chemical stoichiometry and temperature to include extrinsic defect contributions, while applying this method to study defects in CuInSe 2 . This method relies on a large set of defect formation energies calculated from first principles, which require corrections for known errors arising from spurious interactions. Guided by recent experimental work exhibiting the complex interplay between Na, K, and Cd incorporation, we examine the behavior of dominant defects as the material composition varies under experimentally relevant conditions. In addition to identifying the regions of composition-parameter space relevant to the incorporation of impurities Na, K, and Cd, and stable against the formation of secondary compounds, we also study defect kinetics susceptible to the presence of these impurities. From this analysis, we propose a simple model for the enhanced diffusion of Cu-vacancies mediated by K, which could lead to the enhanced incorporation of Cd observed experimentally.

Cu $_2$ ZnSnS $_4$ (CZTS) is a highly promising absorber material for efficient and low-cost solar cells. In a cost-effective method of production of high-quality CZTS, nanocrystals of precursor compounds, solved in an amine solution, are deposited on a substrate and sulfurized. This can generally lead to considerable residual carbon in the final samples, in the form of separate amorphous layers as observed experimentally, but also incorporated in the CZTS matrix. In this study, preferred configurations, bonding characteristics, formation energies, and ionization levels of carbon-related defects in CZTS are calculated, including interstitials, single- and double-carbon antisites, and complexes of those with intrinsic defects. For different growth conditions of carbon, the set of dominant defects is determined, and the resulting total carbon concentrations are discussed with respect to experimental values.

Solid state electrolytes (SSEs) can become a key component for the development of novel reliable, safe, and highly efficient Li-ion batteries. This work focuses on the vacancy-mediated diffusion of Li ions through solid compounds with NZP crystal structures [e.g. LiTi2(PO4)3 (LTP); NZP stands for NaZr2(PO4)3], which is a promising class of materials for the application as SSEs. Since this crystal structure is known to be stable for many combinations of elements on the cation positions, the activation energies for vacancy jumps were calculated in this work for a variety of NZP-type compounds with different compositions. First-principles calculations based on density functional theory were performed to determine the migration barrier heights, and to correlate their values to structural characteristics. In addition, the bond valence method was applied to the NZP-type compounds, which not only helps to identify diffusion networks and transition points, but which can also be valuable for predicting qualitative trends by systematic compositional screening.

Recent experimental work has revealed the distinct and beneficial role of K incorporation on the fabrication of increasingly efficient thin-film photovoltaic devices with Cu(In,Ga)Se2 (CIGS) absorber layers. This has been attributed, in part, to improved CdS/CIGS heterojunction quality due to the enhanced diffusion of Cd into the near-interface region of CIGS. In this work, we try to distinguish the role of K compared to Na in enhancing Cd incorporation in CuInSe2 (CIS) based on first-principles calculations. Using a canonical method for calculating defect concentrations as a function of temperature and material stoichiometry, we identify experimentally relevant conditions under which a simple model for Na and K kinetics can lead to such an effect. We argue that a sufficiently low migration barrier for K diffusion mediated by Cu vacancies can lead to Cu-depletion near the CdS interface, allowing Cd to occupy greater numbers of vacant Cu sites.

Energetically favorable configurations of defect structures in CIGS are studied with a Metropolis-Hastings lattice Monte Carlo (MC) method. Empirical models or the binding energy of arbitrary CIGS configurations are tested and fit to ab-initio data. The MC simulations yield energetically favorable crystal structures for the optimized binding energy model. These structures reveal temperature-dependent phases of indium-gallium segregation and copper-vacancy segregation. These results are then used in kinetic lattice Monte Carlo (KLMC) simulations to study cation diffusion.