Phase Stability of Nickel Hydroxides and Oxyhydroxides

Abstract

We investigate phase stability of several nickel hydroxides from first-principles. Hydrogen removal from β‐Ni(OH)2β‐Ni(OH)2 is predicted to occur through a biphasic reaction to β‐NiOOHβ‐NiOOH involving a change in the stacking sequence from T1 [for β‐Ni(OH)2β‐Ni(OH)2 ] to P3 (for β‐NiOOHβ‐NiOOH ). Further topotactic removal of hydrogen from β‐NiOOHβ‐NiOOH can only occur after a step in a range between 0.4 and 0.9V0.9V is surpassed. We also propose an energetically stable crystal structure for stoichiometric γ‐NiOOHγ‐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 γ‐NiOOHγ‐NiOOH .