Pure rocksalt (c-CoO) and wurtzite (h-CoO) phases of cobaltous oxide (CoO) have been selectively prepared through thermodynamically and kinetically controlled reactions of a single molecular precursor Co(acac)3 (acac: acetylacetonate), respectively. Changing thermal decomposition conditions of the precursor produces different phases and distinct morphologies of the cobaltous oxides. Hexagonal pyramidal shaped h-CoO nanocrystals have been formed by a flash heating of the reaction mixture (185 °C for 2 h, kinetic control condition), whereas cube shaped c-CoO nanocrystals have been produced by a prolonged heating at a relatively low temperature (130 °C for 12 h, thermodynamic control condition). Addition of o-Dichlorobenzene (o-DCB) to the reaction mixture alters the reaction condition to the thermodynamic control regime by slowing down the decomposition rate of the precursor. Further increase of the concentration of o-DCB in the reaction mixture changes the morphology of product from h-CoO hexagonal pyramids to h-CoO nanorods with various aspect ratios and finally to c-CoO nanocrystals. Air oxidation at 240 °C for 5 h of either h-CoO or c-CoO nanocrystals yields spinel Co3O4 nanocrystals with retention of the original crystal morphology. During the oxidation process, the h-CoO phase has been converted into Co3O4 via formation of the c-CoO phase, but the c-CoO phase has been directly oxidized to Co3O4. The electrochemical properties of the h-CoO, c-CoO, and spinel Co3O4 nanocrystals toward lithium exhibit characteristic features reflecting their Gibbs free energies. This work allows understanding of the detailed mechanism and energetics of selective formation, phase transformation, morphology control, and electrochemical properties in the closely related nanostructured cobalt oxides.

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Nanowires can serve as three-dimensional platforms at the nanometer scale for highly efficient chemical energy storage and conversion vehicles, such as fuel cells and secondary batteries. Here we report a coin-type Si nanowire (NW) half-cell Li-ion battery showing the Li capacity of approximately 4000 mAh/g, which nearly approaches the theoretical limit of 4200 mAh/g, with very high Coulombic efficiency of up to 98%. Concomitantly, we provide direct evidence of reversible phase transitions in the Si NW anodes at the full electrochemical cycles, varying from pure Si to Li22Si5 phase, which has been known empirically inaccessible in the bulk limit.

A chelating-agent-assisted Na2/3Fe1/2Mn1/2O2 material showed enhanced electrochemical performance due to the formation of a thin and stable solid-electrolyte interface layer.

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