In situ crystallographic investigations of charge storage mechanisms in MnO2-based electrochemical capacitors

Structural behaviors of four MnO2 allotropes are investigated by in situ synchrotron X-ray diffraction analyses upon charge/discharge cycling in LiCl and KCl aqueous media. During the anodic/cathodic scans of the 2D birnessite electrode, the interlayer spacing respectively extend/contract along the z direction by similar to 0.2 angstrom. This behavior is associated to electrostatic attractive forces between charged framework layers and hydrated electrolyte cations. In contrast, the oxidation/reduction of the 3D spinel electrode lead to the contraction/swelling of (111) crystalline planes by similar to 0.05 angstrom. Manganese-oxygen distance changes induced by Mn(III)/Mn(IV) redox reactions are assigned to such a mechanism. Structural characterizations are focused on the (002),(211), (110) and (200) planes for MnO2 cryptomelane, while (002) and (200) planes for MnO2 OMS-5, The 1D cryptomelane and OMS-5 electrodes also exhibit contraction/swelling processes of their structures upon discharge/charge cycling associated to deintercalation/intercalation of hydrated electrolyte cations in their channels, respectively, driven by size/steric constraints. (C) 2012 Elsevier B.V. All rights reserved.