Effect of interlayer K ordering on water intercalation behavior in δ-type layered manganese dioxide

Abstract

Layered manganese dioxide (birnessite) that contains K cations and water molecules in between the MnO2 layers exhibits reversible heat storage properties via a water-intercalation mechanism (0.5 mol H2O per K0.33MnO2). However, a certain amount of an irreversible capacity of water intercalation is observed after the initial thermal cycle, which limits the reversibly available energy density of the material for the subsequent cycles. In this study, we analyzed crystal structures of the K-containing birnessite, K0.33MnO2, before and after heat treatment, by electron diffraction and atomic-resolution scanning transmission electron microscopy, to elucidate the origin of the initial irreversible capacity (0.83 mol per K0.33MnO2). The K-containing birnessite synthesized by thermal decomposition of KMnO4 was found to basically possess a superstructure of tripled periodicity along the in-plane direction due to an ordered arrangement of the interlayer K cations, while the stacking periodicity of the interlayer K cations along the out-of-plane direction was partially lost. The K cations preferentially occupy the 2c site (space group P63/mmc), which is the most stable position as confirmed by ab initio calculations. The K cations occupy every third 2c site in almost all the interlayers after heat treatment, although the K cations in the as-synthesized state are randomly arranged in some of the interlayers because excess water molecules prevent the interlayer K cations from occupying the most stable 2c site. Thus, the ordering of the K cations once occurring without the water molecules during heat treatment hinders the excess water initially included to re-intercalate in the subsequent cycle process.