Donald J. Siegel

Lithium Peroxide Surfaces Are Metallic, While Lithium Oxide Surfaces Are Not

The thermodynamic stability and electronic structure of 40 surfaces of lithium peroxide (Li(2)O(2)) and lithium oxide (Li(2)O) were characterized using first-principles calculations. As these compounds constitute potential discharge products in Li-oxygen batteries, their surface properties are expected to play a key role in understanding electrochemical behavior in these systems. Stable surfaces were identified by comparing 23 distinct Li(2)O(2) surfaces and 17 unique Li(2)O surfaces; crystallite areal fractions were determined through application of the Wulff construction. Accounting for the oxygen overbinding error in density functional theory results in the identification of several new Li(2)O(2) oxygen-rich {0001} and {1 ̅100} terminations that are more stable than those previously reported. Although oxygen-rich facets predominate in Li(2)O(2), in Li(2)O stoichiometric surfaces are preferred, consistent with prior studies. Surprisingly, surface-state analyses reveal that the stable surfaces of Li(2)O(2) are half-metallic, despite the fact that Li(2)O(2) is a bulk insulator. Surface oxygens in these facets are ferromagnetic with magnetic moments ranging from 0.2 to 0.5 μ(B). In contrast, the stable surfaces of Li(2)O are insulating and nonmagnetic. The distinct surface properties of these compounds may explain observations of electrochemical reversibility for systems in which Li(2)O(2) is the discharge product and the irreversibility of systems that discharge to Li(2)O. Moreover, the presence of conductive surface pathways in Li(2)O(2) could offset capacity limitations expected to arise from limited electron transport through the bulk.

Adhesion, stability, and bonding at metal/metal-carbide interfaces: Al/WC

We examine the relative stability and adhesion of the polar Al(1 1 1)/WC(0 0 0 1) interface using density functional theory. Relaxed atomic geometries and the ideal work of adhesion were calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. The interfacial electronic structure was analyzed to determine the nature of metal/carbide bonding. Based on the surface and interfacial free energies, we find that both the clean WC(0 0 0 1) surface and the optimal interface geometry are W-terminated. Although both terminations yield substantial adhesion energies in the range 4–6 J/m 2 , bonding at the optimal C-terminated structure is nearly 2 J/m 2 stronger, consistent with an argument based on surface reactivity. In addition, we examine the effects of Li and Mg alloying elements at the interface, and find that they result in a strain-induced reduction of metal–ceramic adhesion. 2001 Elsevier Science B.V. All rights reserved.

Charge transport in lithium peroxide: relevance for rechargeable metal–air batteries

The mechanisms and efficiency of charge transport in lithium peroxide (Li2O2) are key factors in understanding the performance of non-aqueous Li–air batteries. Towards revealing these mechanisms, here we use first-principles calculations to predict the concentrations and mobilities of charge carriers and intrinsic defects in Li2O2 as a function of cell voltage. Our calculations reveal that changes in the charge state of O2 dimers controls the defect chemistry and conductivity of Li2O2. Negative lithium vacancies (missing Li+) and small hole polarons are identified as the dominant charge carriers. The electronic conductivity associated with polaron hopping (5 × 10−20 S cm−1) is comparable to the ionic conductivity arising from the migration of Li-ions (4 × 10−19 S cm−1), suggesting that charge transport in Li2O2 occurs through a mixture of ionic and polaronic contributions. These data indicate that the bulk regions of crystalline Li2O2 are insulating, with appreciable charge transport occurring only at moderately high charging potentials that drive partial delithiation. The implications of limited charge transport on discharge and recharge mechanisms are discussed, and a two-stage charging process linking charge transport, discharge product morphology, and overpotentials is described. We conclude that achieving both high discharge capacities and efficient charging will depend upon access to alternative mechanisms that bypass bulk charge transport. More generally, we describe how the presence of a species that can change charge state – e.g., O2 dimers in alkaline metal-based peroxides – may impact rechargeability in metal–air batteries.

Generalized stacking fault energies, ductilities, and twinnabilities of Ni and selected Ni alloys

The generalized stacking fault energies, Rice-criterion ductilities, and twinnabilities of selected Ni-x solid-solution alloys (x=Nb, W, Mn, Fe, Cu) are calculated using density functional theory to elucidate how alloying alters the mechanical properties of pure Ni. Relative to Ni, the alloys have smaller stacking fault energies (γsf), similar ductilities, and a greater tendency to undergo deformation twinning. The results are compared to experimental studies of the mechanical properties of nanocrystalline (nc) Ni alloys from the literature, and it is suggested that the higher strain-hardening rate recently reported for nc-Ni-Cu (relative to nc-Ni-Fe) does not arise from differences in γsf, but from a higher dislocation density caused by more facile dislocation nucleation.

First-principles study of metal-carbide/nitride adhesion: Al/VC vs. Al/VN

Abstract We have performed density functional calculations to investigate the adhesion and electronic structure at interfaces between Al and the refractory transition metal nitrides/carbides VN and VC in order to understand the significance of the ceramics metalloid component upon interfacial properties. We find that for both systems the preferred bonding site places the metal interfacial atoms above the ceramics metalloid atoms, and that adhesion energies are comparable to those found for other metals (Ti, Ag) on MgO. The differences in magnitude and rank-ordering of the adhesion energies for the two interfaces are rationalized in terms of the the surface energies of the ceramics. Analysis of the charge density and density of states reveals that covalent Al–C/N bonds constitute the dominant metal–ceramic interaction.

Reaction energetics and crystal structure of Li4 BN3 H10 from first principles

Using density functional theory we examine the crystal structure and the finite-temperature thermodynamics of formation and dehydrogenation for the quaternary hydride

Development of glue-type potentials for the Al-Pb system: Phase diagram calculation

{\mathrm{Li}}_{4}{\mathrm{BN}}_{3}{\mathrm{H}}_{10}

Electrochemistry of Magnesium Electrolytes in Ionic Liquids for Secondary Batteries

. Two recent studies based on x-ray and neutron diffraction have reported three bcc crystal structures for this phase. While these structures possess identical space groups and similar lattice constants, internal coordinate differences result in bond length discrepancies as large as

Kinetic Stability of MOF-5 in Humid Environments: Impact of Powder Densification, Humidity Level, and Exposure Time

0.2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}

Impact of Space-Charge Layers on Sudden Death in Li/O2 Batteries

. Geometry optimization calculations on the experimental structures reveal that the apparent discrepancies are an artifact of x-ray interactions with strong bond polarization; the relaxed structures are essentially identical. Regarding reaction energetics, the present calculations predict that the formation reaction