Mg Intercalation in Layered and Spinel Host Crystal Structures for Mg Batteries

We investigate electrochemical properties of Mg in layered and spinel intercalation compounds from first-principles using TiS₂ as a model system. Our calculations predict that Mg_<i>x</i>TiS₂ in both the layered and spinel crystal structures exhibits sloping voltage profiles with steps at stoichiometric compositions due to Mg-vacancy ordering. Mg ions are predicted to occupy the octahedral sites in both layered and spinel TiS₂ with diffusion mediated by hops between octahedral sites that pass through adjacent tetrahedral sites. Predicted migration barriers are substantially higher than typical Li-migration barriers in intercalation compounds. The migration barriers are shown to be very sensitive to lattice parameters of the host crystal structure. We also discuss the possible role of rehybridization between the transition metal and the anion in affecting migration barriers

Stability of Prismatic and Octahedral Coordination in Layered Oxides and Sulfides Intercalated with Alkali and Alkaline-Earth Metals

Many layered oxide and sulfide intercalation compounds used in secondary batteries undergo stacking-sequence-change phase transformations during (de)­intercalation. However, the underlying reasons why different intercalants result in different stacking-sequence changes are not well understood. This work reports on high-throughput density functional theory calculations on the prototype systems A_<i>x</i>CoO₂ and A_<i>x</i>TiS₂ (where A = [Li, Na, K, Mg, and Ca]), which show that a few simple rules explain the relative stability among the O1, O3, and P3 stacking sequences. First, for large intercalants (Na, K, and Ca), P3 stacking is favorable at intermediate concentrations (<i>x</i> ∼ 0.5) as its intercalant site topology minimizes in-plane electrostatic repulsion. At the extreme compositions (<i>x</i> ∼ 0 and <i>x</i> ∼ 1), O1 or O3 are stable, with more ionic compounds preferring O3 and covalent ones O1. These rules explain why stacking-sequence changes are much more common in Na materials than Li ones

Simulating Charge, Spin, and Orbital Ordering: Application to Jahn–Teller Distortions in Layered Transition-Metal Oxides

The degrees of freedom associated with orbital, spin, and charge ordering can strongly affect the properties of many crystalline solids, including battery materials, high-temperature superconductors, and naturally occurring minerals. This work reports on the development of a computational framework to systematically explore the ordering of electronic degrees of freedom and presents results on orbital ordering associated with Jahn–Teller distortions in four layered oxides relevant for Li- and Na-ion batteries: LiNiO₂, NaNiO₂, LiMnO₂, and NaMnO₂. Our calculations reveal a criterion for the stability of orbital orderings in these layered materials: each oxygen atom must participate in two short and one long transition-metal/oxygen bond. The only orderings that satisfy this stability criterion are row orderings, such as the “zigzag” ordering. The near degeneracy of such row-orderings in LiNiO₂ suggests that boundaries between domains with distinct but symmetrically equivalent Jahn–Teller distortions will be relatively low in energy. Based on this result, we speculate that a microstructure consisting of nanoscale Jahn–Teller domains could be responsible for the apparent absence of a collective distortion in experiments on LiNiO₂

First-Principles Study of Spinel MgTiS₂ as a Cathode Material

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 Mg_<i>x</i>TiS₂ 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 addition of electrochemically inactive species

Phase Stability and Transport Mechanisms in Antiperovskite Li₃OCl and Li₃OBr Superionic Conductors

We investigate phase stability and ionic transport mechanisms in two recently discovered superionic conductors, Li₃OX (X = Cl, Br), from first principles. These compounds, which have an antiperovskite crystal structure, have potential applications as solid electrolytes in Li-ion batteries. We identify a low-barrier three-atom hop mechanism involving Li interstitial dumbbells. This hop mechanism is facile within the (001) crystallographic planes of the perovskite crystal structure and is evidence for the occurrence of concerted motion, similar to ionic transport in other solid electrolytes. Our first-principles analysis of phase stability predicts that antiperovskite Li₃OCl (Li₃OBr) is metastable relative to Li₂O and LiCl (LiBr) at room temperature. We also find that although the band gap of Li₃OCl exceeds 5 eV, the metastable antiperovskite becomes susceptible to decomposition into Li₂O₂, LiCl and LiClO₄ above an applied voltage of 2.5 V, suggesting that these compounds are most suited for low-voltage Li batteries provided the formation of Li₂O can be suppressed

Factors Contributing to Path Hysteresis of Displacement and Conversion Reactions in Li Ion Batteries

We investigate the thermodynamic and kinetic attributes of electrode materials that are necessary to suppress path hysteresis during displacement and conversion reactions in Li ion batteries. We focus on compounds in the Li–Cu–Sb ternary composition space, as the displacement reaction between Li_1+ϵCu_1+δSb and Li₃Sb can be cycled reversibly. A first-principles analysis of migration barriers indicates that Cu, while not as mobile as Li in the discharged phase (Li₃Sb), nevertheless should exhibit mobilities similar to that of Li in common intercalation compounds. A calculation of phase stability in the ternary Li–Cu–Sb system predicts that the intermediate phases along the reversible charge/discharge path are stable in a large Cu chemical potential window. This ensures that intermediate phases are not bypassed upon Li extraction even when large thermodynamic driving forces are needed to reinsert Cu into the discharged electrode. Our study suggests that the suppression of path hysteresis during displacement reactions requires (i) a high mobility of the displaced metal and (ii) the thermodynamic stability of intermediate phases along the reversible path in a wide metal chemical potential window. Even in the absence of path hysteresis, displacement and conversion reactions suffer from polarization needed to set up thermodynamic driving forces for metal extrusion and reinsertion. This polarization can be estimated with a Clausius–Clapeyron analysis

Kinetics of Anatase Electrodes: The Role of Ordering, Anisotropy, and Shape Memory Effects

We perform a comprehensive first-principles statistical mechanical study of the thermodynamic and kinetic properties of lithiated anatase Li_<i>x</i>TiO₂. We establish that the experimentally observed step in the voltage vs lithium composition curve between <i>x</i> = 0.5 and 0.6 is due to Li ordering. Furthermore, we predict that full lithiation of anatase TiO₂ is thermodynamically possible at positive voltages but that there is an enormous difference in Li diffusion coefficients in the dilute and fully lithiated forms of TiO₂, providing an explanation for the limited capacity in large electrode particles. We also predict that Li diffusion in the ordered phase (Li_0.5TiO₂) is strictly one-dimensional. The TiO₂ to Li_0.5TiO₂ phase transition has much in common with shape memory alloys. Crystallographically, it can support strain invariant interfaces separating TiO₂ and Li_0.5TiO₂ within the same particle. The strain invariant interfaces are parallel to the one-dimensional diffusion direction in Li_0.5TiO₂, which, we argue, has important consequences for the role of particle shape on achievable capacity, charge and discharge rates, and hysteresis

Kinetics of Anatase Electrodes: The Role of Ordering, Anisotropy, and Shape Memory Effects

We perform a comprehensive first-principles statistical mechanical study of the thermodynamic and kinetic properties of lithiated anatase Li_<i>x</i>TiO₂. We establish that the experimentally observed step in the voltage vs lithium composition curve between <i>x</i> = 0.5 and 0.6 is due to Li ordering. Furthermore, we predict that full lithiation of anatase TiO₂ is thermodynamically possible at positive voltages but that there is an enormous difference in Li diffusion coefficients in the dilute and fully lithiated forms of TiO₂, providing an explanation for the limited capacity in large electrode particles. We also predict that Li diffusion in the ordered phase (Li_0.5TiO₂) is strictly one-dimensional. The TiO₂ to Li_0.5TiO₂ phase transition has much in common with shape memory alloys. Crystallographically, it can support strain invariant interfaces separating TiO₂ and Li_0.5TiO₂ within the same particle. The strain invariant interfaces are parallel to the one-dimensional diffusion direction in Li_0.5TiO₂, which, we argue, has important consequences for the role of particle shape on achievable capacity, charge and discharge rates, and hysteresis

Financial aggregates for identifying the recessions in the financial market

В статье проведено обобщение финансовых агрегатов, идентифицирующих снижения устойчивости финансового рынка. На основе пробит-анализа выявлены финансовые агрегаты, которые наилучшим образом идентифицируют рецессию в финансовом секторе экономики.The financial aggregates identifying decrease in stability of the financial market are summarized. Based on probit analysis the financial aggregates that are the best for identifying the recession in the financial sector are extracted

Thermodynamics of Lithium in TiO₂(B) from First Principles

We use first-principles density functional theory (DFT) calculations combined with statistical mechanical techniques based on the cluster expansion method and Monte Carlo simulations to predict the lithium site occupancies, voltage curves, and phase diagram for TiO₂(B), a candidate anode material for lithium ion batteries. We find that Li intercalation is thermodynamically favorable up to a Li/Ti ratio of 1.25, higher than the theoretical maximum usually assumed for TiO₂. The calculated phase diagram at 300 K contains three first-order phase transformations corresponding to major changes in the favored intercalation sites at increasing Li concentrations. Calculations based on DFT predict the stability of a new Li site at high Li concentrations in TiO₂(B) and the occurrence of a dramatic site-inversion as Li is added to the host