Variation in surface energy and reduction drive of a metal oxide lithium-ion anode with Stoichiometry: A DFT study of lithium titanate spinel surfaces

J. Mater. Chem. A In Press

Li4Ti5O12 is a “zero-strain” lithium-ion anode material that shows excellent stability over repeated lithium insertion–extraction cycles. Although lithium (de)intercalation in the bulk material has been well characterised, our understanding of surface atomic-scale–structure and the relationship with electrochemical behaviour is incomplete. To address this, we have modelled the Li4Ti5O12 (111) , Li7Ti5O12 (111) and α-Li2TiO3 (100), (110), and (111) α-Li2TiO3 surfaces using Hubbard-corrected density-functional theory (GGAU ), screening more than 600 stoichiometric Li4Ti5O12and Li7Ti5O12 (111) surfaces. For Li4Ti5O12 and Li7Ti5O12 we find Li-terminated surfaces are more stable than mixed Li/Ti-terminated surfaces, which typically reconstruct. For α-Li2TiO3, the (100) surface energy is significantly lower than for the (110) and (111) surfaces, and is competitive with the pristine Li7Ti5O12 (111) surface. Using these stoichiometric surfaces as reference, we also model variation in Li surface coverage as a function of lithium chemical potential. For Li4Ti5O12, the stoichiometric surface is most stable across the full chemical potential range of thermodymamic stability, whereas for Li7Ti5O12, Li deficient surfaces are stablised at low Li chemical potentials. The highest occupied electronic state for Li7Ti5O12 (111) is 2.56 eV below the vacuum energy. This is 0.3 eV smaller than the work function for metallic lithium, indicating an extreme thermodynamic drive for reduction. In contrast, the highest occupied state for the α-Li2TiO3 (100) surface is 4.71 eV below the vacuum level, indicating a substantially lower reduction drive. This result demonstrates how stoichiometry can strongly affect the thermodynamic drive for reduction at metal-oxide–electrode surfaces. In this context, we conclude by discussing the design of highly-reducible metal-oxide electrode coatings, with the potential for controlled solid-electrolyte–interphase formation via equilibrium chemistry, by electrode wetting in the absence of any applied bias.

Stability of the M2 phase of vanadium dioxide induced by coherent epitaxial strain

Tensile strain along the cR axis in epitaxial VO2 films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the TiO2 substrate and identify the phases of VO2 as a function of temperature in epitaxial VO2/TiO2 thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the cR axis is insufficient for describing the transition pathway of VO2 epitaxial films and that a strain induced increase of electron correlation effects must be considered.

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Influence of Rotational Distortions on Li+- and Na+-Intercalation in Anti-NASICON Fe2(MoO4)3

Anti-NASICON Fe2(MoO4)3 (P21/c) shows significant structural and electrochemical differences in the intercalation of Li+ and Na+ ions. To understand the origin of this behavior, we have used a combination of in situ X-ray and high-resolution neutron diffraction, total scattering, electrochemical measurements, density functional theory calculations, and symmetry-mode analysis. We find that for Li+-intercalation, which proceeds via a two-phase monoclinic-to-orthorhombic (Pbcn) phase transition, the host lattice undergoes a concerted rotation of rigid polyhedral subunits driven by strong interactions with the Li+ ions, leading to an ordered lithium arrangement. Na+-intercalation, which proceeds via a two-stage solid solution insertion into the monoclinic structure, similarly produces rotations of the lattice polyhedral subunits. However, using a combination of total neutron scattering data and density functional theory calculations, we find that while these rotational distortions upon Na+-intercalation are fundamentally the same as for Li+-intercalation, they result in a far less coherent final structure, with this difference attributed to the substantial difference between the ionic radii of the two alkali metals.

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Sparse cyclic excitations explain the low ionic conductivity of stoichiometric Li7La3Zr2O12

We have performed long time scale molecular dynamics simulations of the cubic and tetragonal phases of the solid lithium-ion electrolyte Li7La3Zr2O12 (LLZO), using a first-principles parametrized interatomic potential. Collective lithium transport was analyzed by identifying dynamical excitations: persistent ion displacements over distances comparable to the separation between lithium sites, and stringlike clusters of ions that undergo cooperative motion. We find that dynamical excitations in c-LLZO (cubic) are frequent, with participating lithium numbers following an exponential distribution, mirroring the dynamics of fragile glasses. In contrast, excitations in t-LLZO (tetragonal) are both temporally and spatially sparse, consisting preferentially of highly concerted lithium motion around closed loops. This qualitative difference is explained as a consequence of lithium ordering in t-LLZO and provides a mechanistic basis for the much lower ionic conductivity of t-LLZO compared to c-LLZO.

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Lithium-ion conductivity in Li6Y(BO3)3: a thermally and electrochemically robust solid electrolyte

The development of new frameworks for solid electrolytes that exhibit fast Li-ion diffusion is critical for enabling new energy storage technologies. Here, we present a combined experimental and computational investigation into the ionic conductivity of Li6Y(BO3)3, a new class of solid electrolytes with a pseudo-layered structure. Temperature-dependent impedance spectroscopy shows the pristine material exhibits an ionic conductivity of 2.2×10-3 S/cm􀀀 around 400°C, while density functional theory calculations point to multiple remarkably low-energy diffusion pathways. Our calculations indicate small energy barriers for lithium interstitials to diffuse along one-dimensional channels oriented in the c-direction, and also for lithium vacancies diffusing within ac planes. This coexistence of diffusion mechanisms indicates that Li6Y(BO3)3 is an extremely versatile host for exploring and understanding mechanisms for lithium-ion conductivity. We find no evidence for reactivity with moisture in the atmosphere and that the material is electrochemically stable when in direct contact with metallic lithium. This robust stability, alongside ionic conductivity that can be manipulated through appropriate aliovalent substitution, make Li6Y(BO3)3 an exceptionally promising new class of solid electrolyte.

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Density functional theory screening of gas-treatment strategies for stabilization of high energy-density lithium metal anodes

To explore the potential of molecular gas treatment of freshly cut lithium foils in non-electrolyte-based passivation of high-energy-density Li anodes, density functional theory (DFT) has been used to study the decomposition of molecular gases on metallic lithium surfaces. By combining DFT geometry optimization and Molecular Dynamics, the effects of atmospheric (N2, O2, CO2) and hazardous (F2, SO2) gas decomposition on Li(bcc) (100), (110), and (111) surfaces on relative surface energies, work functions, and emerging electronic and elastic properties are investigated.

The simulations suggest that exposure to different molecular gases can be used to induce and control reconstructions of the metal Li surface and substantial changes (up to over 1 eV) in the work function of the passivated system. Contrary to the other considered gases, which form metallic adlayers, SO2 treatment emerges as the most effective in creating an insulating passivation layer for dosages 1 monolayer. The substantial Li/adsorbate charge transfer and adlayer relaxation produce marked elastic stiffening of the interface, with the smallest change shown by nitrogen-treated adlayers.

Relationships between atomic diffusion mechanisms and ensemble transport coefficients in crystalline polymorphs

Ionic transport in conventional ionic solids is generally considered to proceed via independent diffusion events or “hops”. This assumption leads to well-known Arrhenius expressions for transport coefficients, and is equivalent to assuming diffusion is a Poisson process. Using molecular dynamics simulations of the low-temperature B1, B3, and B4 AgI polymorphs, we have compared rates of ion-hopping with corresponding Poisson distributions to test the assumption of independent hopping in these common structure-types. In all cases diffusion is a non-Poisson process, and hopping is strongly correlated in time. In B1 the diffusion coefficient can be approximated by an Arrhenius expression, though the physical significance of the parameters differs from that commonly assumed. In low temperature B3 and B4 diffusion is characterised by concerted motion of multiple ions in short closed loops. Diffusion coefficients can not be expressed in a simple Arrhenius form dependent on single-ion free-energies, and intrinsic diffusion must be considered a many-body process.

Understanding the electronic structure of IrO2 using hard-x-ray photoelectron spectroscopy and density-functional theory

The electronic structure of IrO2 has been investigated using hard X-ray photoelectron spectroscopy (HAXPES) and density functional theory (DFT). Excellent agreement is observed between theory and experiment. We show that the electronic structure of IrO2 involves crystal field splitting of the iridium 5d orbitals in a distorted octahedral field. The behaviour of IrO2 closely follows the theoretical predictions of Goodenough for conductive rutile-structured oxides. 1 Strong satellites associated with the core lines are ascribed to final state screening effects. A simple plasmon model for the satellites applicable to many other metallic oxides appears to be not valid for IrO2.

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Molecular dynamics simulation of coherent interfaces in fluorite heterostructures

The standard model of enhanced ionic conductivities in solid electrolyte heterostructures follows from a continuum mean-field description of defect distributions that makes no reference to crystalline structure. To examine ionic transport and defect distributions while explicitly accounting for ion—ion correlations and lattice effects, we have performed molecular dynamics simulations of a model coherent fluorite heterostructure without any extrinsic defects, with a difference in standard chemical potentials of mobile fluoride ions between phases induced by an external potential. Increasing the offset in fluoride ion standard chemical potentials across the internal interfaces decreases the activation energies for ionic conductivity and diffusion and strongly enhances fluoride ion mobilities and defect concentrations near the heterostructure interfaces. Non-charge-neutral “space-charge” regions, however, extend only a few atomic spacings from the interface, suggesting a continuum model may be inappropriate. Defect distributions are qualitatively inconsistent with the predictions of the continuum mean-field model, and indicate strong lattice-mediated defect–defect interactions. We identify an atomic-scale “Frenkel polarisation” mechanism for the interfacial enhancement in ionic mobility, where preferentially oriented associated Frenkel pairs form at the interface and promote local ion mobility via concerted diffusion processes.

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The electronic structure of silver orthophosphate: experiment and theory

Since the original discovery of the water-splitting activity of silver orthophosphate (Ag3PO4), considerable effort has been devoted to improving its photocatalytic activity and stability through morphology control and the design of multi-component electrode systems. Relatively little attention, however, has been paid to understanding the fundamental electronic properties of this material. Using X-ray photoelectron spectroscopy and hybrid density functional theory (DFT) calculations, we have studied the electronic structure of Ag3PO4. Our results indicate that hybrid DFT calculations closely reproduce the structural, electronic, and optical properties of Ag3PO4. From further analysis of the experimental and theoretical electronic structure data we have constructed a revised molecular orbital diagram for Ag3PO4 that highlights the strong covalent interactions formed in the tetrahedral PO4 structural units.

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