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 ∼0.2 Å. 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 (1 1 1) crystalline planes by ∼0.05 Å. Manganese–oxygen distance changes induced by Mn(III)/Mn(IV) redox reactions are assigned to such a mechanism. Structural characterizations are focused on the (0 0 2), (2 1 1), (1 1 0) and (2 0 0) planes for MnO2 cryptomelane, while (0 0 2) and (2 0 0) 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.

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► Four MnO2 allotropes investigated as supercapacitor electrode materials.
► Charge storage mechanisms studied by in situ synchrotron X-ray diffraction.
► Specific breathing of MnO2 structures upon charge–discharge cycling.