Preparation, ion-exchange, and electrochemical behavior of Cs-type manganese oxides with a novel hexagonal-like morphology

Cs-type layered manganese oxide with a novel hexagonal-like morphology (Cs–BirMO) was prepared by a solid-state reaction procedure. The Cs⁺ extraction and alkali–metal ion insertion reactions were investigated by chemical analyses, x-ray analyses, scanning electron microscopy observation, Fourier transform-infrared spectroscopy, thermogravimetric differential thermal analyses, pH titration, and distribution coefficient (K_d) measurements. A considerable percentage (88%) of Cs⁺ ions in the interlayer sites were topotactically extracted by acid treatment, accompanied by a slight change of the lattice parameters. Alkali–metal ions could be inserted into the interlayer of the acid-treated sample (H–BirMO), mainly by an ion-exchange mechanism. The pH titration curve of the H–BirMO sample showed a simple monobasic acid toward Li⁺, Rb⁺, and Cs⁺ ions, and dibasic acid behavior toward Na⁺ and K⁺ ions. The order of the apparent capacity was K⁺ > Li⁺ ≈ Na⁺ ≈ Rb⁺ ≈ Cs⁺ at pH < 6. The K_d study showed the selectivity sequence of K⁺ > Rb⁺ > Na⁺ > Li⁺ for alkali–metal ions at the range of pH <5; H–BirMO sample showed markedly high selectivity for the adsorption of K⁺ ions. Preliminary investigations of the electrochemical properties of the Li⁺-inserted sample Li–BirMO(1M, 6d) showed that the obtained samples had a relatively high discharge capacity of 115 mAh g⁻¹ and excellent layered stability.