Although cobalt hydroxide is currently added to Ni(OH ) 2 paste to prepare nickel composite electrodes used in Nibased rechargeable alkaline batteries, its redox chemistry in alkaline media is still poorly documented. The Co(OH ) 2 ACoOOH oxidation reaction in KOH media was investigated, and found to be dependent upon the experimental conditions, namely, temperature, oxidizing agent and reaction time. In addition, this reaction was shown, as determined by means of X-ray diffraction, electronic microscopy and atomic absorption measurements, to occur through a two step mechanism process involving first a dissolution process followed by a solid state reaction. This dissolution step enables preparation, by adjusting the cycling conditions, of cobalt oxyhydroxide with well defined morphology and texture, thereby providing an opportunity to optimize its efficiency as an additive in nickel electrodes.understand how CoOOH forms in electrochemical cells. In

As an attempt to understand better how cobalt hydroxide additives improve the nickel electrode performance, the Co ( OH ) 2 / CoOOH redox system has been investigated through electrochemical cycling starting from a commercial Co ( OH ) 2 sample. A study of the influence of texture and morphology as well as cycling parameters was performed. For charge rates greater than C/5, relative to the amount of Co ( OH ) 2 , the electrochemical oxidation was found to be a solid‐state process. This process led to a nonstoichiometric Co x 4 + Co 1 − x 3 + OOH 1 − x phase having a mosaic texture with enhanced electronic conductivity due to the presence of Co 4 + ions. For lower charge rates (C/100), the reaction rate is slower, and Co 2 + can dissolve in the electrolyte, leading to a less conductive phase having a stoichiometric composition (CoOOH) and a monolithic texture. When present, the Co 4 + ions are reduced to Co 3 + , at 1.05 V while other reductions Co 3 + → Co 2 + and Co 2 + → Co ° take place at a lower potential, 0.67 and 0.0 V, respectively. These two reactions are both associated with a dissolution of Co(II) species, followed by a migration of cobalt toward the current collector, with the overall result being an electrode degradation. © 2000 The Electrochemical Society. All rights reserved.

Replacement of nickel by aluminium in the brucite-type Ni(OH), layers leads to layered double hydroxides (LDH), which can be used as active materials for nickel hydroxide electrodes. Ni/AI LDH compounds [O< AI/(Ni + Al) 6 0.251 were synthesized by precipitation with ammonia from mixed Ni/AI nitrate solutions and characterized by X-ray powder diffraction, infrared spectroscopy, chemical analysis and transmission electron microscopy. X-Ray powder diffractograms of fresh precipitates are characteristic of turbostratic layered double hydroxides. A structural model describing the turbostratic layered double hydroxides is proposed on the basis of the experimental results. The brucite-type layers are non-stoichiometric and present hydroxide vacancies: their chemical composition can be written as [Ni:?, Ali+ (OH-), -,,+,,-x,] and the interlamellar layers as [(NO,), (CO;-);nH,O] with xGO.25. The interlamellar anions (NO; or COZ-) play two roles: those in DJh symmetry, compensate for the positive-charge excess of A13+ ions, while those in C,, symmetry, compensate for that of hydroxide vacancies. When aged, mixed turbostratic Ni/Al hydroxides with the higher aluminium content (x>0,18) change into synthetic takovites (crystallized Ni-AI-CO, double hydroxides) through a dissolution-recrystallization process.

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