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 NaNiO<sub>2</sub> through soft-chemical processes involving oxidation with bromine and subsequent acid treatment. Complete removal of interlayer Na<sup>+</sup> ions and subsequent hydration yielded a single phase of H<sub><i>x</i></sub>NiO<sub>2</sub>·<i>n</i>H<sub>2</sub>O (<i>x</i> < 1, <i>n</i> ∼1) with an enlarged basal spacing of 9.6 Å. The materials had a monoclinic structure (<i>C</i>2/<i>m</i>); unit cell parameters for a typical composition of H<sub>0.66</sub>NiO<sub>2</sub>·0.9H<sub>2</sub>O were <i>a</i> = 4.8993(8) Å, <i>b</i> = 2.8256(4) Å, <i>c</i> = 9.7598(9) Å, and β = 98.88(2)°. Rietveld refinement revealed that the structure was composed of pseudohexagonal NiO<sub>2</sub> sheets accommodating partially occupied three layers of H<sub>2</sub>O molecules and H<sub>3</sub>O<sup>+</sup> ions in the galleries. The highly expanded layered structure is analogous to Na<sub>0.3</sub>NiO<sub>2</sub>·1.3H<sub>2</sub>O and other layer oxides such as buserite-type manganese oxide and superconducting Na<sub><i>x</i></sub>CoO<sub>2</sub>·<i>y</i>H<sub>2</sub>O 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%.