Bismuth-enhanced electrochemical stability of cobalt hydroxide used as an additive in Ni/Cd and Ni/metal hydride batteries

For performance purposes, the electrochemically active positive electrode i n Ni-based rechargeable alkaline batteries consists of Ni(OH)(2) particles coated with CoOOH. The positive attributes of the CoOOH coating, however, v anish upon cycling because it transforms into Co3O4 Or even Co(OH)(2) throu gh dissolution-crystallization processes. The benefit of adding Bi-based sa lts or bismuth oxide to prevent these phase transformations is presented. F rom chemical and electrochemical studies, coupled with potentiometric titra tion and ultraviolet-visible spectroscopy, we demonstrate that the positive effect of Bi additives is twofold. First, Bi adsorption onto the Co surfac e modifies the CoOOH chemical reactivity and thereby its dissolution Second , since bismuth forms Bi-Co-oxohydroxo complexes in solution, as experiment ally proved, it delays precipitation of the Co3O4 and Co(OH)(2) phases duri ng cycling. Finally an implementation of these findings toward the most eff icient use of nickel positive electrodes containing Co; based additives is shown. (C) 2000 The Electrochemical Society. S0013-4651(99)11-103-0. All ri ghts reserved.