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Etude de phases spinelle cobaltée et d'oxydes lamellaires dérivés de Na0,6CoO2 employés comme additifs conducteurs dans les accumulateurs Ni-MH

Abstract : In Ni-MH batteries, current technology of pasted positive electrodes needs to use conductive cobalt additive, since the Ni(OH)2 electrochemical active material exhibits a poor electronic conductivity. The research for new cobalt compounds is a critical aim to promote these batteries in high power applications. In this context, two potential conductive additives were studied in this work.
First, the study was focused on a highly conductive HxLiyCo3-δO4 spinel phase, synthesized by electrochemical oxidation of CoO powder. A thermal treatment of such phases was shown to have an important influence on the electronic conductivity properties of the material. In situ X-ray diffraction,
TGA-MS, NMR analysis and electronic conductivity measurements allowed to demonstrate a cationic redistribution within the spinel framework, which leads to an increase of the Co4+/Co3+ ratio in the [Co2O4] network, without any change in the average oxidation state of cobalt. Such behavior leads to an increase of the electronic conductivity by 3 orders of magnitude.
Second, the electrochemical behavior of the Na0.6CoO2 phase was studied. The mechanisms involved during the cycling of the phase were investigated. The reactions of topotactic exchange/insertion of alkaline ions, which occur during oxidation and reduction of the phase, were examined, and a mechanism was proposed. The γ-hydrated cobalt oxyhydroxide, formed by oxidation of Na0.6CoO2, exhibits very good performances during battery tests. Formation of an intermediate interstratified phase, with a slow reduction kinetics, leads to stabilize the conductive additive at low potential, and consequently to maintain the integrity of the conductive network.
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Myriam Douin. Etude de phases spinelle cobaltée et d'oxydes lamellaires dérivés de Na0,6CoO2 employés comme additifs conducteurs dans les accumulateurs Ni-MH. Matériaux. Université Sciences et Technologies - Bordeaux I, 2008. Français. ⟨tel-00373899⟩

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