Structural and textural evolution of zinc-substituted nickel hydroxide electrode materials upon ageing in KOH and upon redox cycling

The structural and textural evolution of zinc-substituted α- and β(II)-nickel hydroxides during ageing tests in 8 M KOH and during redox reactions is investigated. During ageing tests, the β(II)-type crystallites grow via an Oswald ripening process, while the α-phase is transformed into a β(II)-phase via a dissolution, nucleation and growth process. For both materials, zinc is partially lost during the ageing tests. Soft oxidation of the β(II)-phase leads to a β(III)-type one; subsequent reduction yields a β(II)-phase very similar to the starting material but exhibiting a mosaı̈c texture. Strong oxidation of the β(II)-phase leads to a well-crystallised γ-oxyhydroxide which is reduced to an α-phase with an interslab distance of 7.3 Å. Strong oxidation of the α-phase leads to a poorly crystallised interstratified γ-type phase containing carbonate ions together with a small amount of the starting α-phase. Subsequent reduction yields an α-phase with a structure very similar to that of the starting material (interslab distance close to 8.4 Å). Repeated strong oxidation and reduction reactions of the α-phase progressively lead to a β(II)-phase via the formation of interstratified materials. A SEM study shows that all redox reactions occur in the solid state. A partial loss of zinc is evidenced during strong oxidation of β(II)- and α-phases. Since a large amount of zinc is lost during strong oxidation reactions, octahedral vacancies must remain within the slabs of the resulting β-phase.