Highly Swollen Layered Nickel Oxide with a Trilayer Hydrate Structure

This paper reports the synthesis of alkali-free layered nickel oxides with a highly swollen hydrate structure, derived from NaNiO2 through soft-chemical processes involving oxidation with bromine and subsequent acid treatment. Complete removal of interlayer Na+ ions and subsequent hydration yielded a single phase of HxNiO2·nH2O (x < 1, n ∼1) with an enlarged basal spacing of 9.6 Å. The materials had a monoclinic structure (C2/m); unit cell parameters for a typical composition of H0.66NiO2·0.9H2O were a = 4.8993(8) Å, b = 2.8256(4) Å, c = 9.7598(9) Å, and β = 98.88(2)°. Rietveld refinement revealed that the structure was composed of pseudohexagonal NiO2 sheets accommodating partially occupied three layers of H2O molecules and H3O+ ions in the galleries. The highly expanded layered structure is analogous to Na0.3NiO2·1.3H2O and other layer oxides such as buserite-type manganese oxide and superconducting NaxCoO2·yH2O but differs in alkali-free composition. The 9.6 Å phase underwent partial dehydration to give a basal spacing of 7 Å upon exposure to atmosphere with a relative humidity of <30%.