Bismuth-Enhanced Electrochemical Stability of Cobalt Hydroxide Used as an Additive in Ni/Cd and Ni/Metal Hydride Batteries

Abstract

For performance purposes, the electrochemically active positive electrode in Ni‐based rechargeable alkaline batteries consists of particles coated with CoOOH. The positive attributes of the CoOOH coating, however, vanish upon cycling because it transforms into or even through dissolution‐crystallization processes. The benefit of adding Bi‐based salts or bismuth oxide to prevent these phase transformations is presented. From chemical and electrochemical studies, coupled with potentiometric titration and ultraviolet‐visible spectroscopy, we demonstrate that the positive effect of Bi additives is twofold. First, Bi adsorption onto the Co surface modifies the CoOOH chemical reactivity and thereby its dissolution. Second, since bismuth forms Bi‐Co‐oxohydroxo complexes in solution, as experimentally proved, it delays precipitation of the and phases during cycling. Finally, an implementation of these findings toward the most efficient use of nickel positive electrodes containing Co‐based additives is shown. © 2000 The Electrochemical Society. All rights reserved.