THE EFFECT OF COBALT ON THE CHEMICAL AND ELECTROCHEMICAL-BEHAVIOR OF THE NICKEL-HYDROXIDE ELECTRODE

New hydrated cobalt-substituted nickel hydroxides were prepared and characterized. These materials were obtained either by precipitation (alpha) or by the "chimie douce" technique (alpha* and alpha'). Cobalt generally stays in the trivalent state. In contrast with the alpha(Co2+) phases, the alpha(Co3+) phases are stable in KOH if at least 20% cobalt is substituted for nickel. In order to compensate for the excess of positive charge resulting from the presence of trivalent ions in the brucite-type layers, anions (sulphate or carbonate) are inserted between the Nil-xCox(OH)2 slabs. In the case of well-crystallized alpha* phases, an interstratified packing is obtained for low cobalt contents (x < 0.20). As shown by a study of the physical properties, the electronic conductivity of the oxidized phases (gamma(Co)) increases with the amount of cobalt. These phases were used as the positive electrode material of Ni/Cd cells. At the beginning of cycling, 1.3e- per (Ni + Co) atom can be reversibly exchanged between the gamma and alpha-phases. During long-range cycling tests, a slow evolution from the gamma/alpha-system to the beta(III)/beta(II) system is observed. This point is discussed in relation to the amount of cobalt.