Phase Stability of Nickel Hydroxides and Oxyhydroxides

Abstract

We investigate phase stability of several nickel hydroxides from first-principles. Hydrogen removal from ?-Ni?OH?2 is predicted to occur through a biphasic reaction to ?-NiOOH involving a change in the stacking sequence from T1 ?for ?-Ni?OH?2? to P3 ?for ?-NiOOH?. Further topotactic removal of hydrogen from ?-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 ?-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 ?-NiOOH. ©