Abstract
The study of the electrochemical intercalation of sodium in the cobalt bronze Na x CoO 2− y has shown a strong dependence of the discharge curves on the conditions of preparation (temperature and oxygen pressure) of the starting phase, i.e. on the oxygen content. In order to explain this behavior a general study has been carried out by thermogravimetric analysis and electrical measurements. The values of the oxygen departure from stoichiometry ( y) versus synthesis conditions have been determined by thermogravimetric analysis. Conductivity and thermoelectric power measurements have been carried out on samples either quenched or under oxygen equilibrium conditions ( T>673K). The results show that the original electron holes arising from sodium nonstoichiometry are partly compensated by electrons resulting from the oxygen nonstoichiometry, α=K(P O 2 ) + 1 20 . This compensation effect is emphasized by the starting open circuit voltage of Na/liquid electrolyte/Na 0.70CoO 2−y cells and by their discharge curves.
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