An update on the reactivity of nanoparticles Co-based compounds towards Li

In our comprehensive understanding of reactivity of Coo towards Li, we studied the effect of Coo electrode weight and composition (carbon-free or not) on the cycling performances of CoO/Li half-cells. Capacity and lifetime were measured as a function of the cycling rate and temperature. The lightest electrodes (2 mg/cm(2)) were shown to behave the best, with sustained capacities as high as 600 mAh/g up to about 250 cycles at 20 degreesC, and with capacities that peaked up to 1700 mAh/g when cycling was performed at 75 degreesC. Searching for the origin of this huge "extra capacity" over the normal conversion process (Co2+O --> Co-0, 715 mAh/g), we dissociated phenomena by testing carbon-loaded and carbon-free CoO-based electrodes in CoO/Li half cells. We unravelled. a temperature-driven capacity rate increase similar for both cells, although the initial reversible capacity was quite different. The carbon-free CoO/Li half cell showed limited initial reversible capacity due to the poor efficiency of the conversion process for non-conducting electrodes. This increase appears to be nested in the reversible growth of a polymeric gel-like film resulting from kinetically activated electrolyte degradation. Polymeric layers were also shown to form from the Li electrochemical reduction of numerous binary phases, differentiating either by the nature of the 3d-metal (Cu, Ni, Fe instead of Co) or that of the anion (S, F, N instead of O). Finally, purely coincidental or not, from comparative studies of Coo, CoSb3, and CoSb2 Li half-cells, we found that, to a certain extent, the capacity amplitude associated with the growth of the polymer film scales with the surface developed by Co nano-particles. This fact would imply a possible catalytic role of 3d metals in assisting the electrolyte decomposition. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.