On the electrochemical reactivity mechanism of CoSb3 vs. lithium

The electrochemical reactivity of CoSb3 vs. lithium has been studied. This phase reacts with more than 9.5 lithium in a two-step process, consisting of the uptake of 9 Li at a constant voltage close to 0.6 V, and of about one lithium over the final voltage decay to 0.01 V. Upon recharge, only 8 lithium can be extracted. From in situ X-ray diffraction, microscopy, and magnetic measurements, we provide evidence that the constant voltage process is rooted in the decomposition of CoSb3, leading to the formation of a composite made of Co and Li3Sb nanograins. We also illustrate that the mechanism by which the internal nanostructured (Co + Li3Sb) electrode, formed during reduction, converts back to CoSb3, is quite unusual. It involves, concomitant with the Li3Sb --> Li2Sb --> Sb dealloying reaction, a chemical reaction between Co and Sb nanograins. The extra capacity, measured at low potential, appears to be nested in a decomposition-type reaction catalyzed by the cobalt nanoparticles, in a manner similar to that previously reported for CoO. Although these materials can reversibly uptake about 8 lithium, they are of negligible value, since their capacity rapidly decays with cycling, independent of the electrode processing. (C) 2003 The Electrochemical Society.