Phase stability of nickel hydroxides and oxyhydroxides

We investigate phase stability of several nickel hydroxides from first-principles. Hydrogen removal from beta-Ni(OH)(2) is predicted to occur through a biphasic reaction to beta-NiOOH involving a change in the stacking sequence from T1 [for beta-Ni(OH)(2)] to P3 (for beta-NiOOH). Further topotactic removal of hydrogen from beta-NiOOH can only occur after a step in a range between 0.4 and 0.9 V is surpassed. We also propose an energetically stable crystal structure for stoichiometric gamma-NiOOH, which offers an explanation for the oxidation limit of 3.66 for Ni. In this structure, potassium in the intercalation layer resides halfway between adjacent trigonal prismatic sites. We conclude with a discussion of the role of the electrolyte in determining phase stability as well as the voltage profile of gamma-NiOOH. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2138572] All rights reserved.