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 alpha- and beta>(*) over bar * (II)-nickel hydroxides during ageing tests in 8 M KOH and during redox reactions is investigated. During ageing tests, the beta>(*) over bar * (II)-type crystallites grow via an Oswald ripening process, while the alpha -phase is transformed into a beta>(*) over bar * (II)-phase via a dissolution, nucleation and growth process. For both materials, zinc is partially lost during the ageing tests. Soft oxidation of the beta>(*) over bar * (II)-phase leads to a beta>(*) over bar * (II)-type one; subsequent reduction yields a beta>(*) over bar * (II)-phase very similar to the starting material but exhibiting a mosaic texture. Strong oxidation of the beta>(*) over bar * (II)-phase leads to a well-crystallised gamma -oxyhydroxide which is reduced to an or-phase with an interslab distance of 7.3 Angstrom. Strong oxidation of the alpha -phase leads to a poorly crystallised interstratified gamma -type phase containing carbonate ions together with a small amount of the starting alpha -phase. Subsequent reduction yields an alpha -phase with a structure very similar to that of the starting material (interslab distance close to 8.4 Angstrom). Repeated strong oxidation and reduction reactions of the or-phase progressively lead to a beta>(*) over bar * (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 beta>(*) over bar * (II)- and alpha -phases. Since a large amount of zinc is lost during strong oxidation reactions, octahedral vacancies must remain within the slabs of the resulting beta -phase. (C) 2000 Elsevier Science BN. All rights reserved.