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Intercalation chemistry has dominated electrochemical energy storage for decades, and storage capacity worldwide has now reached the terawatt-hour level. State-of-the-art intercalation cathodes for Li-ion batteries operate within the limits of transition metal cation electrochemistry, but the discovery of anion-redox processes in recent decades suggests rich opportunities for substantially increasing stored energy densities. The diversity of compounds that exhibit anion redox in the solid state has inspired the exploration of new materials for next-generation cathodes. In this Review, we outline the mechanisms proposed to contribute to anion redox and the accompanying kinetic pathways that can occur in layered transition metal oxides. We discuss the crucial role of structural changes at both the atomic and mesoscopic scales with an emphasis on their impact on electrochemical performance. We emphasize the need for an integrated approach to studying the evolution of both the bulk structure and electrode–electrolyte interphase by combining characterization with computation. Building on the fundamental understanding of electrochemical reaction mechanisms, we discuss engineering strategies such as composition design, surface protection and structural control to achieve stable anion redox for next-generation energy storage devices.

The most promising solid electrolytes for all-solid-state Li batteries are oxide and sulfide ceramics. Current ceramic solid electrolytes are brittle and lack the toughness to withstand the mechanical stresses of repeated charge and discharge cycles. Solid electrolytes are susceptible to crack propagation due to dendrite growth from Li metal anodes and to debonding processes at the cathode/electrolyte interface due to cyclic variations in the cathode lattice parameters. In this perspective, we argue that solutions to the mechanics challenges of all-solid-state batteries can be borrowed from the aerospace industry, which successfully overcame similar hurdles in the development of thermal barrier coatings of superalloy turbine blades. Their solution was to exploit ferroelastic and transformation toughening mechanisms to develop ceramics that can withstand cyclic stresses due to large variations in temperature. This perspective describes fundamental materials design principles with which to search for solid electrolytes that are ferroelastically toughened., Comment: 8 pages, 8 figures

Author(s): Saber, Muna; Preefer, Molleigh B; Kolli, Sanjeev K; Zhang, William; Laurita, Geneva; Dunn, Bruce; Seshadri, Ram; Van der Ven, Anton

We report the successful electrochemical intercalation of F-ions into a densely packed perovskite oxide from a liquid electrolyte at room temperature. Using galvanostatic oxidation and electrochemical impedance spectroscopy coupled with operando X-ray diffraction, we show that roughly 0.5 equivalents of F-ions can be inserted onto the vacant A-site of the perovskite ReO3. Density functional theory calculations indicate that the intercalated phase is thermodynamically unfavorable compared to other less densely packed polymorphs of ReO3F. Pairing X-ray spectroscopy, neutron total scattering measurements, and magic-angle spinning 19F NMR confirms a rapid decomposition of the product on removal from the cell but nevertheless, these results clearly demonstrate that small anions like fluoride can be intercalated into solids as readily as alkali cations at room temperature, which opens new opportunities to electrochemically fluorinate many new materials.

Li-excess materials created by incorporating Li2MnO3 into traditional layered transition-metal oxides promise to substantially increase the energy density of Li-ion batteries. The precise redox mec...

We investigate phase stability in all binary alloys comprised of elements from groups 4 (Ti, Zr, Hf), 5 (V, Nb, Ta) and 6 (Cr, Mo, W) of the periodic table. First-principles calculations of the energy landscapes along crystallographic pathways that connect bcc to hcp and bcc to ω show that group 4 elements are very distinct from group 5 and 6 elements. While group 5 and 6 elements are stable in bcc, group 4 elements favor hcp and ω and are predicted to be dynamically unstable in bcc. A comprehensive first-principles investigation of the 36 refractory binary systems using statistical mechanics techniques reveals six distinct classes of alloys, each with a unique phase diagram topology. The predictions of this study are in excellent agreement with previous experimental work. One exception is a class of refractory alloys with high temperature miscibility gaps that are not predicted with the methods used in this work. Our calculations predict the stability of a low-temperature Laves phase in the Nb-V binary that has yet to be observed experimentally. The relationships between alloy chemistry and high-temperature phase stability revealed in this study provide a basis for the systematic design of multicomponent disordered refractory alloys.

The success of K-ion battery technology will rely on the development of robust cathode materials that can incorporate and shuttle high amounts of K reversibly. Recent experimental work has demonstr...

Refractory metals such as titanium can dissolve significant quantities of interstitial oxygen. The interactions between dissolved interstitial species and substitutional alloying elements remain poorly understood in refractory metals, but are increasingly recognized as being of importance, especially in high entropy refractory alloys. We performed a first-principles statistical mechanics study of the interactions between substitutionally dissolved Al and interstitial O in hcp Ti. A systematic analysis of phase stability in the Ti-Al-O composition space predicts the low temperature stability of a new ternary Ti6Al2O compound. In spite of a well-known affinity between Al and O, extensive first-principles calculations using density functional theory predict a strong nearest-neighbor repulsion between Al and O when both are dissolved in hcp Ti. The repulsion has its origin in the unique electronic structure of the Ti host. The combination of a cluster expansion Hamiltonian and Monte Carlo simulations shows that the strong short-range interaction between dissolved Al and O has important consequences for phase stability and the degree of order at elevated temperature in Ti-Al-O alloys. Monte Carlo simulations predict that an increase in the dissolved oxygen concentration leads to a reduction of the order-disorder transition temperature of α-Ti3Al. The results of this study provide guidance as to how the solubility and degree of ordering of interstitial species in refractory metals can be tailored by different alloying strategies.

PNb9O25, a Wadsley–Roth compound whose structure is obtained by appropriate crystallographic shear of the ReO3 structure, is a high-power electrode material that can reach 85% of the equilibrium ca...

Conventional Li-ion cathodes store charge by reversible intercalation of Li coupled to metal cation redox. There has been increasing interest in new materials capable of accommodating more than one Li per transition-metal center, thereby yielding higher charge storage capacities. We demonstrate here that the lithium-rich layered iron sulfide Li₂FeS₂ as well as a new structural analogue, LiNaFeS₂, reversibly store ≥1.5 electrons per formula unit and support extended cycling. Ex situ and operando structural and spectroscopic data indicate that delithiation results in reversible oxidation of Fe²⁺ concurrent with an increase in the covalency of the Fe–S interactions, followed by reversible anion redox: 2 S²⁻/(S₂)²⁻. S K-edge spectroscopy unequivocally proves the contribution of the anions to the redox processes. The structural response to the oxidation processes is found to be different in Li₂FeS₂ in contrast to that in LiNaFeS₂, which we suggest is the cause for capacity fade in the early cycles of LiNaFeS₂. The materials presented here have the added benefit of avoiding resource-sensitive transition metals such as Co and Ni. In contrast to Li-rich oxide materials that have been the subject of so much recent study and that suffer capacity fade and electrolyte degradation issues, the materials presented here operate within the stable potential window of the electrolyte, permitting a clearer understanding of the underlying processes.

The identification of materials supporting complex, tunable magnetic order at ambient temperatures is foundational to the development of new magnetic device architectures. We report the design of Mn2XY tetragonal inverse Heusler alloys that are capable of hosting magnetic antiskyrmions whose stability is sensitive to elastic strain. We first construct a universal magnetic Hamiltonian capturing the short- and long- range magnetic order which can be expected in these materials. This model reveals critical combinations of magnetic interactions that are necessary to approach a magnetic phase boundary, where the magnetic structure is highly susceptible to small perturbations such as elastic strain. We then computationally search for quaternary Mn2(X1,X2)Y alloys where these critical interactions may be realized and which are likely to be synthesizable in the inverse Heusler structure. We identify the Mn2Pt(1-z)X(z)Ga family of materials with X=Au, Ir, Ni as an ideal system for accessing all possible magnetic phases, with several critical compositions where magnetic phase transitions may be actuated mechanically.

Metallic lithium and sodium are actively investigated as anodes for all solid-state batteries. While the mechanical properties of Li and Na remain poorly understood, there is a growing consensus that they play a crucial role in determining the integrity of solid electrolytes. The mechanical properties of Li and Na are complicated by the rich variety of martensitic transformations that they undergo upon cooling below room temperature. Here, we develop an overarching crystallographic description that connects the high temperature bcc forms of Li and Na to the large number of close-packed phases that Li and Na transform to at low temperatures. First-principles calculations predict that Li and Na have unusually flat energy surfaces as a function of a minimal set of strain and shuffle order parameters that describe pathways between bcc and close-packed structures. Calculated generalized stacking fault energies of close-packed Li and Na phases are similarly very flat, indicating a negligible resistance to dislocation motion. The remarkably shallow energy surfaces of Li and Na as a function of strain and shuffle deformations suggest that these metals are unusually soft and should exhibit negligible resistance to plastic deformation.

Tin is an important component of a variety of promising anode and solid-electrolyte chemistries for Li and Na-ion batteries. Here, we report on a first-principles investigation of phase stability a...

The lithium-excess manganese oxides are a candidate cathode material for the next generation of Li-ion batteries because of their ability to reversibly intercalate more Li than traditional cathode materials. Although reversible oxidation of lattice oxygen has been proposed as the origin of this anomalous excess capacity, questions about the underlying electrochemical reaction mechanisms remain unresolved. Here, we critically analyse the O2−/O− oxygen redox hypothesis and explore alternative explanations for the origin of the anomalous capacity, including the formation of peroxide ions or trapped oxygen molecules and the oxidation of Mn. First-principles calculations motivated by the Li–Mn–O phase diagram show that the electrochemical behaviour of the Li-excess manganese oxides is thermodynamically consistent with the oxidation of Mn from the +4 oxidation state to the +7 oxidation state and the concomitant migration of Mn from octahedral sites to tetrahedral sites. It is shown that the Mn oxidation hypothesis can explain the poorly understood electrochemical behaviour of Li-excess materials, including the activation step, the voltage hysteresis and voltage fade. Reversible anion redox is widely accepted as the origin for the extra capacity of Li-excess cathode materials. Here, the authors analyse the literature and theorize that the oxidation of Mn beyond the +4 state could be responsible for the extra capacity of Li-excess Mn oxides.

Resonant inelastic X-ray scattering (RIXS) at the O K-edge is considered a prime technique to identify bulk oxidized oxygen formation, but its fundamental interpretation is not straightforward. In ...

The discovery, design, and development of new alloys have long been critical elements of advanced engineering systems. Challenged by their chemical and structural complexity, this design process is, however, often too slow. This article highlights progress in theory, computation, data, and advanced experimental techniques that are advancing our capabilities for rapid discovery and design of new multicomponent alloys. Applied across the length scales, these new capabilities support exploration across broad composition spaces; examples of new materials and associated advances in the understanding of underlying thermochemical and thermomechanical phenomena are presented. We highlight current challenges, gaps, and specific areas that, if further developed, could have future high payoff.

The recent surge of interest in K-ion batteries necessitates a fundamental understanding of phase stability and K ordering tendencies in common electrode materials. We report on a first-principles study of phase stability in layered KxCoO2 (0 ≤ x ≤ 1) in the O1/P3/O3 family of host structures and identify K ordering preferences within each host. We find that the P3 host is stable at intermediate K concentrations and exhibits a multitude of hierarchical orderings characterized by well-ordered domains separated by antiphase boundaries. We also predict the stability of a new family of layered structures at high K concentrations that are highly distorted and host both octahedrally and prismatically coordinated K within each intercalation layer.

Alloying Mg with rare-earth elements, such as Nd, Gd, or Y, enables hardening from the precipitation of metastable coherent phases during aging at low temperatures. While the aging potential of binary Mg-Nd alloys is relatively limited due to the nucleation of coarse β 1 and β precipitates at the expense of the strengthening β’’’ precipitates, binary Mg-Y alloys exhibit exceptional stability of the strengthening coherent β S ’ phase when aged at 200 o C. Through combination of high-resolution characterization, density functional theory calculations, and finite-element elasticity studies, we demonstrate that the strengthening β S ’ phase is thermodynamically stable at low temperatures (as opposed to metastable as in other Mg-RE binaries) and that misfit strains play a key role in not only controlling precipitate structure and morphology but also their unusual evolution into an interconnected network.

The high solubility of oxygen in Ti, Zr, and Hf makes it difficult to stabilize protective oxide scales on their surfaces as the subsurface regions can serve as boundless sinks that continuously dissolve oxygen. Alloying elements are crucial to reduce the oxygen solubility and diffusivity within early transition metals. Past studies have shown that most substitutional alloying additions to titanium repel interstitial oxygen. Here we use first-principles calculations to show that this repulsion is short ranged and explore several factors that are likely responsible for the repulsive interaction. Calculations of Bader charges suggest the existence of short-range Coulomb interactions due to the accumulation of charge on the substitutional solute and interstitial oxygen that is drawn from the Ti host. Misfit strains due to differences in the atomic radii of the solutes and Ti are also found to play a role. We identify a unique hybridization phenomenon between dissolved substitutional elements and interstitial oxygen within hcp Ti that leads to a repulsive interaction at short distances, similar to that between closed-shell atoms, which is especially pronounced for Al and Si solutes.

We report the feasibility of using magnetoentropic mapping for the rapid identification of magnetic cycloid and skyrmion phases in uniaxial systems, based on the GaV4S8 and GaV4Se8 model skyrmion hosts with easy-axis and easy-plane anisotropies respectively. We show that these measurements can be interpreted with the help of a simple numerical model for the spin Hamiltonian to yield unambiguous assignments for both single phase regions and phase boundaries. In the two lacunar spinel chemistries, we obtain excellent agreement between the measured magnetoentropic features and a minimal spin Hamiltonian built on Heisenberg exchange, single-ion anisotropy, and anisotropic Dzyaloshinskii-Moriya interactions. In particular, we identify characteristic high-entropy behavior in the cycloid phase that serves as a precursor to the formation of skyrmions at elevated temperatures and is a readily-measurable signature of this phase transition. Our results demonstrate that rapid magnetoentropic mapping guided by numerical modeling is an effective means of understanding the complex magnetic phase diagrams innate to skyrmion hosts. One notable exception is the observation of an anomalous, low-temperature high-entropy state in the easy-plane system GaV$_4$Se$_8$, which is not captured in the numerical model. Possible origins of this state are discussed.

Sodium-ion batteries have emerged as a promising, cost-effective energy storage solution. Critical to the success of these technologies is the efficient transport of ions within the battery electrodes. Here, the authors use first-principles techniques to examine sodium diffusion in a canonical sodium-ion battery cathode material, revealing a new, unconventional mechanism in which the mobile sodium ions are confined to boundaries between otherwise immobile, ordered regions. They provide evidence of a diffusion mechanism that occurs via the collective motion of the boundaries through the material. This behavior is dramatically different from that which is seen in analogous lithium-ion battery materials, and may extend to related candidate electrode materials for beyond-lithium-ion batteries. These mechanistic insights have important implications for the rate of sodium diffusion, which impacts battery charge/discharge speed.

Lithium-ion batteries (LIBs) featuring high energy density have received particular attention in recent years due to their promise as electrical energy storage devices for electric vehicles and portable electronic devices. However, most of the commercial LIBs use graphite anode which suffer from the formation of solid-electrolyte interface (SEI) layer. Moreover, the poor performance of graphite anodes at high C-rates limits their application for fast-charging LIBs. To address the challenges associated with graphite anodes, niobium-containing Wadsley-Roth shear phase materials have been suggested. Thanks to the redox potential of Nb4+ (above 1 V versus Li/Li+), the niobium containing Wadsley-Roth shear phase materials avoid SEI formation. Furthermore, a combination of edge-shared and corner-shared octahedra in this type of shear phase materials forms many tunnel-like regions where Li-ions can rapidly diffuse during lithiation and delithiation. In fact, niobium-containing Wadsley-Roth shear phase materials show impressive cycling performance as fast-charging battery materials and maintain more than 50 % of their theoretical capacity at C-rate of 10 C. This study aims to understand the fast-charging capability of niobium containing Wadsley-Roth shear phase materials accompanied by ion ordering in the application of LIBs. In addition, it aims to provide experimental evidence to elucidate the occurrence of Li-ion ordering in this type of shear phase materials. To do so, galvanostatic cycling and cyclic voltammetry were performed on selected niobium-containing Wadsley-Roth shear phase materials including TiNb2O7, PNb9O25, VNb9O25, and NaNb13O33. In addition, open circuit voltage and entropic potential of those materials were measured. The results show that ion ordering characterized by a tilde-shaped fluctuation in entropic potential was observed in all tested niobium containing Wadsley-Roth shear phase materials. Furthermore, the results indicate that the b-value approaching 0.5 corresponded to the first order transition accompanied by a two-phase coexistence while the b-value approaching 1 corresponded to the ordering of Li-ions. This analysis implies that the ion ordering phenomena can enhance the fast-charging capability of the material. This study provides valuable insights for fast-charging battery materials.

The Heulser intermetallic ${\mathrm{MnAu}}_{2}\mathrm{Al}$ is shown to undergo a dramatic change in net magnetization in response to plastic deformation. A mechanism is proposed involving antiferromagnetic interactions in the otherwise ferromagnetic compound (when ordered) that arise due to chemical changes at the antiphase boundaries created by the deformation. The coupling between chemical and magnetic order across antiphase boundaries is likely to occur in other ordered magnetic systems and may provide an explanation for otherwise anomalous magnetic behavior across several systems, including other Heusler intermetallics.

The anomalous capacity of Li-excess cathode materials has ignited a vigorous debate over the nature of the underlying redox mechanism, which promises to substantially increase the energy density of rechargeable batteries. Unfortunately, nearly all materials exhibiting this anomalous capacity suffer from irreversible structural changes and voltage hysteresis. Non-hysteretic excess capacity has been demonstrated in Na2Mn3O7 and Li2IrO3, making these materials key to understanding the electronic, chemical and structural properties that are necessary to achieve reversible excess capacity. Here, we use high-fidelity random-phase-approximation (RPA) electronic structure calculations and group theory to derive the first fully consistent mechanism of non-hysteretic oxidation beyond the transition metal limit, explaining the electrochemical and structural evolution of the Na2Mn3O7 and Li2IrO3 model materials. We show that the source of anomalous non-hysteretic capacity is a network of pi-bonded metal-d and O-p orbitals, whose activity is enabled by a unique resistance to transition metal migration. The pi-network forms a collective, delocalized redox center. We show that the voltage, accessible capacity, and structural evolution upon oxidation are collective properties of the pi-network rather than that of any local bonding environment. Our results establish the first rigorous framework linking anomalous capacity to transition metal chemistry and long-range structure, laying the groundwork for engineering materials that exhibit truly reversible capacity exceeding that of transition metal redox.

The generalized stacking fault energy is a key ingredient to mesoscale models of dislocations. Here we develop an approach to quantify the dependence of generalized stacking fault energies on the degree of chemical disorder in multicomponent alloys. We introduce the notion of a “configurationally-resolved planar fault” (CRPF) energy and extend the cluster expansion method from alloy theory to express the CRPF as a function of chemical occupation variables of sites surrounding the fault. We apply the approach to explore the composition and temperature dependence of the unstable stacking fault energy (USF) in binary Mo–Nb alloys. First-principles calculations are used to parameterize a formation energy and CRPF cluster expansion. Monte Carlo simulations show that the distribution of USF energies is significantly affected by chemical composition and temperature. The formalism is broadly applicable to arbitrary crystal structures and alloy chemistries and will enable the development of rigorous models for deformation mechanisms in high-entropy alloys.

The lacunar spinel family, which contains multiferroic skyrmion hosts GaV4S${}_{8}$ and GaV${}_{4}$Se${}_{8}$, is characterized by unique metal--metal bonding leading to strong coupling between crystal structure and magnetism. Synchrotron diffraction analysis of the low temperature crystal structure of GaMo${}_{4}$Se${}_{8}$, a less studied member of the family, reveals coexistence of the reported rhombohedral crystal structure and a new metastable orthorhombic structure. Computational and experimental characterization of the magnetic phase diagram of GaMo${}_{4}$Se${}_{8}$ indicates that slight crystal structure differences lead to distinct magnetic properties; the reported $R3m$ phase hosts skyrmions similar to GaV${}_{4}$Se${}_{8}$, while the novel $I\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}m2$ phase is a strongly uniaxial ferromagnet.

Oxygen participation, arising from increased transition metal–oxygen covalency during delithiation, is considered essential for the description of charge compensation in conventional layered oxides. The advent of high-resolution mapping of the O K-edge resonant inelastic X-ray scattering (RIXS) provides an opportunity to revisit the onset and extent of oxygen participation. Combining RIXS with an array of structural and electronic probes for the family of Ni-rich LiNi0.8Co0.2−yAlyO2 cathodes, we identify common charge compensation regimes that are assigned to formal transition metal redox ( 4.25 V). From O K-edge RIXS maps, we find the emergence of a sharp RIXS feature in these systems when approaching full delithiation, which has previously been associated with lattice oxidized oxygen in alkali-rich systems. The lack of transition metal redox signatures and strong covalency at these high degrees of delithiation suggest this RIXS feature is similarly attributed to lattice oxygen charge compensation as in the alkali-rich systems. The RIXS feature's evolution with state of charge in conventional layered oxides is evidence that this feature reflects the depopulation of occupied O 2p states associated with oxygen participation.

One approach to increasing the capacity of Li-ion batteries is to expand the usable voltage range over which the battery is cycled by charging the cathode to higher voltages. Layered intercalation compounds commonly used as cathodes in Li-ion batteries, however, can become susceptible to irreversible structural changes at high states of charge due to cation migration to the emptied Li layers. Here we report on the discovery of a strong correlation between the position of the Fermi level and the energy barrier for cation migration to the Li layers of fully charged layered intercalation compounds. Since the identity of the transition metal strongly influences the Fermi level, this discovery suggests that cation migration can be suppressed at high states of charge using targeted alloying strategies. First-principles calculations indicate that an increase in the concentration of Ni relative to Co or Mn should reduce, or even impede, interlayer migration in layered oxides. The insights of this study pave the way for the discovery of layered intercalation compounds that can approach their theoretical capacities in practice.

Large anharmonic vibrations often play a crucial role in dynamically stabilizing crystalline phases whose structures are unstable at low temperature. Although the average structure of such phases can be measured through diffraction experiments, their local structure remains a challenge to characterize and understand. Dynamically stabilized phases are often classified as order/disorder or displacive based on the qualitative nature of their local structure. A robust understanding of how chemistry determines this distinction in behavior, however, is lacking. This article presents a parametric study of an anharmonic vibrational model that describes the transition from a cooperative to a noncooperative Jahn-Teller distortion in layered oxides---a class of materials widely used in Li-ion and Na-ion batteries. The results illustrate how the shape of the energy landscape determines the extent to which the high-temperature phase has order/disorder vs displacive character. We find that the nature of the high-temperature phase is determined by a competition between the strength of the elastic coupling between Jahn-Teller distortions at nearby sites and the energy scale driving the Jahn-Teller distortion. A comparison of the model to energy landscapes calculated with density-functional theory suggests that the high-temperature phases of the Jahn-Teller active layered compounds $\mathrm{LiNi}{\mathrm{O}}_{2}$, $\mathrm{NaNi}{\mathrm{O}}_{2}$, $\mathrm{LiMn}{\mathrm{O}}_{2}$, and $\mathrm{NaMn}{\mathrm{O}}_{2}$ exhibit order/disorder character.

Machine learning tools such as neural networks and Gaussian process regression are increasingly being implemented in the development of atomistic potentials. Here, we develop a formalism to leverage such non-linear interpolation tools in describing properties dependent on occupation degrees of freedom in multicomponent solids. Symmetry-adapted cluster functions are used to differentiate distinct local orderings. These local features are used as input to neural networks that reproduce local properties such as the site energy. We apply the technique to reproduce a synthetic cluster expansion Hamiltonian with multi-body interactions, as well as the formation energies calculated from first-principles for the intercalation of lithium into TiS2. The formalism and results presented here show that complex multi-body interactions may be approximated by non-linear models involving smaller clusters. Robust descriptors of the degree of configurational order in a crystal can be formulated using machine learning tools. A team led by Anton Van der Ven at University of California, Santa Barbara, developed an advanced neural network implementation to build accurate lattice model Hamiltonians using a moderate number of correlation functions as descriptors. Using site-centric correlation function descriptors, the formalism can accurately model the formation energies of a synthetic multi-body binary Hamiltonian on face centered cubic crystals, as well as on Li-intercalated TiS2. As a result, complex multi-body interactions may be approximated by non-linear models involving smaller clusters. The approach can be generalized to describe any scalar property of a multi-component crystal, including its formation energy and volume, as a function of the configurational degrees of freedom.

Practical utilization of energy densities near the theoretical limit for R3m layered oxide positive electrode materials is dependent on the stability of the electrochemical performance of these materials at or near full delithiation. To develop new chemistries and novel approaches toward the improvement of the electrochemical performance of these materials at such high states of charge, a robust understanding of the failure mechanisms limiting current materials is necessary. Thorough analysis of LixCo1–yAlyO2 and LixNi1–yAlyO2 as well as LixNi0.8Co0.2O2 and LixNi0.8Co0.15Al0.05O2 (1 ≥ x ≥ 0 and 0.2 ≥ y ≥ 0) enabled the identification of key relationships between the transition metal chemistry of the electrode, its structural stability, and cycling characteristics at or near complete delithiation (4.75 V). Extensive characterization of these materials was achieved by a multitude of physical and electrochemical techniques to investigate the relative importance of surface vs bulk phenomena. The resulting in...

Navigating the high dimensional composition spaces of multi-principle element alloys, also referred to as high entropy alloys, will require new methods to construct first-principles alloy Hamiltonians. We introduce a recursive approach to parameterizing multi-component alloy Hamiltonians using interaction parameters from simpler subsystems as Bayesian informative priors. We applied this approach to perform a first-principles statistical mechanics study of the Ni-Al-Cr system. Ternary cluster expansions for the Ni-Al-Cr alloy were constructed by building on optimized Ni-Al and Ni-Cr binary cluster expansions. Monte Carlo simulations predict a sizable Cr solubility in the Ni-rich FCC based γ and γ ′ phases. The L 1 2 -ordered γ ′ phase is predicted to dissolve Cr primarily on its Al-sublattice. We also identify a family of hierarchical long-period super structure orderings as groundstates in the Ni-Cr and Al-Cr binaries. The recursive approach to parameterizing alloy Hamiltonians opens the door to rigorous first-principles treatments of the elevated temperature thermodynamics of alloys in high dimensional composition spaces.

The tremendous benefits of all-solid-state Li-ion batteries will only be reaped if the cycle-induced strain mismatch across the electrode/electrolyte interfaces can be managed at the atomic scale to ensure that structural coherency is maintained over the lifetime of the battery. We have discovered a unique strain accommodation mechanism within an epitaxially constrained high-performance bronze TiO2 (TiO2-B) electrode that relieves coherency stresses that arise upon Li insertion. In situ transmission electron microscopy observation reveals the formation of anatase shear bands within the TiO2-B crystal that play the same role that interface dislocations do to relieve growth stresses. While first-principles calculations indicate that anatase is the favored crystal structure of TiO2 in the lithiated state, its continued propagation is suppressed by the epitaxial constraints of the substrate. This discovery reveals an accordion-like mechanism relying on an otherwise undesirable structural transformation that c...

First-principles DFT methods are combined with an experimental approach to characterize the creep behavior of quinary Co-based L1 $$_2$$ -containing superalloys at elevated temperature conditions. Temperature-dependent SISF energies have been modeled, combining 0 K formation energies with vibrational free energy calculations to assess deformation mechanisms at finite temperature. Two different Co-Al-W alloys, containing the maximum possible amount of DFT-identified d-block alloying additions, were identified and cast as single crystals via the Bridgman process. Creep tests have been performed at two primary testing conditions, one at 900 $$^\circ $$ C and the other at 982 $$^\circ $$ C. Transmission scanning electron microscopy (TSEM) was performed at 30 kV in a scanning electron microscope to rapidly characterize the defect substructures. We observe a coupled APB/SISF/APB defect structure in Co-based superalloys at the low-temperature condition, similar to the defect structure observed in CoNi, in spite of containing no Ni. At 982 $$^\circ $$ C, there is no evidence of faults and precipitates instead contain antiphase boundaries. The role of composition and temperature-dependent fault energies in the deformation process is addressed.

Spinel intercalation hosts are well known to to facilitate high rate capability and high voltage Li-ion batteries. A recent experimental study has shown that Mg can reversibly intercalate in spinel TiS2, demonstrating the viability of Li intercalation host crystal structures for Mg-ion batteries [Sun, X.; Energy Environ. Sci. 2016, 9, 2273−2277]. We report on a first-principles statistical mechanics study of Mg insertion into spinel TiS2, accounting for occupancy on both octahedrally and tetrahedrally coordinated interstitial sites. In contrast to Li-containing spinels, we predict mixed octahedral and tetrahedral site occupancy at nondilute Mg concentrations consistent with the recent experimental study of Sun et al. The onset of mixed occupancy is correlated with an increase in the spinel volume upon Mg insertion, which is more pronounced in MgxTiS2 than in its Li counterpart. The results in this study suggest that the degree of mixed occupancy could be controlled through the volume of the host with addi...