Textural and structural studies on nickel hydroxide electrodes: I. Crystallized nickel hydroxide materials submitted to chemical and electrochemical redox cycling

Abstract

The mechanism of operation of the crystallized nickel(II) hydroxide electrode used in the nickel-cadmium battery is a matter of considerable study and controversy. In this paper particular emphasis is placed on the textural and structural modifications of the phases involved, as a new approach to the chemical and electrochemical redox behaviour of the nickel electrode. X-ray diffraction, electron microscopy and diffraction were performed on two kinds of β(II)-nickel hydroxide materials, during chemical and electrochemical cycling, in an effort to determine the oxidation-reduction mechanism. The existence of important textural modifications was established during the first oxidation cycle. Starting from β(II)-nickel hydroxide with a monolithic texture, it consists in the formation of a mosaic texture. This mosaic texture is preserved during the first reduction and further cycling. A model based on the pseudomorphous and topotactic characteristics of the reactions involved was used to explain the formation of the mosaic texture. The reactions that take place in the solid state by oriented growth of β(III) or γ(III) on β(II) induce strains within the particles; they are due to the large differences in the Ni-Ni distances in the oxidized and reduced phases. Strain relaxation brings about the formation of a mosaic texture, which gives the particles elastic properties that minimize the further strains and make their relaxation easier. The formation of this mosaic texture can be related to the well known forming process of the electrode and its preservation during cycling can be related to the life cycle performance of the system.