Ion Pairing and Diffusion in Magnesium Electrolytes Based on Magnesium Borohydride.

One obstacle to realizing a practical, rechargeable magnesium-ion battery is the development of efficient Mg electrolytes. Electrolytes based on simple Mg(BH4)2 salts suffer from poor salt solubility and/or low conductivity, presumably due to strong ion pairing. Understanding the molecular-scale processes occurring in these electrolytes would aid in overcoming these performance limitations. Toward this goal, the present study examines the solvation, agglomeration, and transport properties of a family of Mg electrolytes based on the Mg(BH4)2 salt using classical molecular dynamics. These properties were examined across five different solvents (tetrahydrofuran and the glymes G1-G4) and at four salt concentrations ranging from the dilute limit up to 0.4 M. Significant and irreversible salt agglomeration was observed in all solvents at all nondilute Mg(BH4)2 concentrations. The degree of clustering observed in these divalent Mg systems is much larger than that reported for electrolytes containing monovalent cations, such as Li. The salt agglomeration rate and diffusivity of Mg2+ were both observed to correlate with solvent self-diffusivity: electrolytes using longer- (shorter-) chain solvents had the lowest (highest) Mg2+ diffusivity and agglomeration rates. Incorporation of Mg2+ into Mg2+-BH4- clusters significantly reduces the diffusivity of Mg2+ by restricting displacements to localized motion within largely immobile agglomerates. Consequently, diffusion is increasingly impeded with increasing Mg(BH4)2 concentration. These data are consistent with the solubility limitations observed experimentally for Mg(BH4)2-based electrolytes and highlight the need for strategies that minimize salt agglomeration in electrolytes containing divalent cations.

Role of Crystal Symmetry in the Reversibility of Stacking-Sequence Changes in Layered Intercalation Electrodes.

The performance of many technologies, such as Li- and Na-ion batteries as well as some two-dimensional (2D) electronics, is dependent upon the reversibility of stacking-sequence-change phase transformations. However, the mechanisms by which such transformations lead to degradation are not well understood. This study explores lattice-invariant shear as a source of irreversibility in stacking-sequence changes, and through an analysis of crystal symmetry shows that common electrode materials (graphitic carbon, layered oxides, and layered sulfides) are generally susceptible to lattice-invariant shear. The resulting irreversible changes to microstructure upon cycling ("electrochemical creep") could contribute to the degradation of the electrode and capacity fade.

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

The performance of Li/O2 batteries is thought to be limited by charge transport through the solid Li2O2 discharge product. Prior studies suggest that electron tunneling is the main transport mechanism through thin, compact Li2O2 deposits. The present study employs a new continuum transport model to explore an alternative scenario, in which charge transport is mediated by polaron hopping. Unlike earlier models, which assume a uniform carrier concentration or local electroneutrality, the possibility of nonuniform space charge is accounted for at the Li2O2/electrolyte and Li2O2/electrode interfaces, providing a more realistic picture of transport in Li2O2 films. The temperature and current-density dependences of the discharge curves predicted by the model are in good agreement with flat-electrode experiments over a wide range of rates, supporting the hypothesis that polaron hopping contributes significantly to charge transport. Exercising the model suggests that this mechanism could explain the observed enhancement in cell performance at elevated temperature and that performance could be further improved by tuning the interfacial orientation of Li2O2 crystallites.

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.