Phase Stability of Nickel Hydroxides and Oxyhydroxides

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