Résumé
For performance purposes, the electrochemically active positive electrode in Ni-based rechargeable alkaline batteries consists of Ni(OH)2 particles coated with CoOOH. We have shown that during the first charge, a non-stoichiometric Co4\Co3+1-xOOH1-x phase having a mosaic texture is formed, with enhanced electronic conductivity due to the presence of Co4+ ions. But, a deep cut-off voltage in the following discharge leads to Co(OH)2 and Co0. These two reactions are both associated with dissolution of cobalt (Il) species, followed by a migration of cobalt towards the current collector with the overall results being electrode degradation. Moreover, a floating or a temperature enhancement leads to the formation of the electrochemically inactive Co304 phase via a solution process. Thus, the positive attributes of the CoOOH coating, however, vanish upon cycling because it transforms into Co3Ü4 or even Co(OH)2 through dissolution-crystallization processes. When cycling in a limited potential window (1.45-1 V), by TEM and using potentiodynamic cycling, we observe a double dissolution process from cobalt oxyhydroxide and nickel hydroxide leading to the formation of a mixed Ni(l-x)Cox(OH)z(C03)y(H20)2 The benefit of adding Bi-based salts or bismuth oxide to prevent these phase transformations toward the solution is presented. From chemical and electrochemical studies, coupled with potentiometric titration and UV-visible spectroscopy, we demonstrated that the positive effect of Bi additives is twofold. Firstly, Bi adsorption onto the Co-surface modifies the CoOOH chemical reactivity and thereby its dissolution. Secondly, since bismuth forms Bi-Co-oxohydroxo complexes in solution, as experimentally proved, it delays precipitation of the Co304 and Co(OH)2 phases during cycling. Finally, an implementation of these findings towards the most efficient use of nickel positive electrodes containing Co-based additives is shown.