Structural Modifications Induced by the Second Discharge Process in the Ni(OH)2/NiOOH Redox System

Electrochemical behavior and related structural modifications of the nickel hydroxide/oxyhydroxide electrode are investigated as a function of the reduction state achieved on discharge. Reduction proceeds at two successive potential steps, with recovering of the initial structural properties only after the second discharge process. Active material cycled in the positive potential range (vs. Hg/HgO), including only the first discharge step, is non: stoichiometric and characterized by high lattice defect content. Whatever the oxidation state, the presence of Ni(3+) defects and protonic delocalization are observed. The 2nd discharge process restores O-H covalent bonds and a better defined Ni(II) state, similar to the precursor nickel hydroxide chemically obtained. Electrochemical cycling in the positive potential range is characterized by the retention of the same nano-crystallite dimension and a high fragmented morphology, while the 2nd discharge induces crystallite agglomeration.