Structural and electrochemical properties of the proton / γ-MnO2 system

This paper presents a structural description of the electrochemically active forms of manganese dioxide, known as γ- and ε-MnO 2 and used in Leclanché and alkaline batteries, in relation to an investigation of their electrochemical properties. These manganese dioxides, either natural (NMD) or prepared chemically (CMD) or electrochemically (EMD), have long been known to be an intergrowth of pyrolusite and ramsdellite units (a defect for which we coined the name of De Wolff disorder), but we show that another kind of structural defect, identified as microtwinning, is responsible for the poor quality of X-ray powder diffraction patterns of most γ-MnO 2 and for their difficult characterization. The model we introduce enables the reproduction of details of the diffraction patterns of these materials and, conversely, the quantitative determination of the structural disorder present. Comparison with experimental diffraction data shows that all synthetic samples of γ-MD contain both microtwinning and De Wolff disorder. It is found that the amount of microtwinning is linked to the method of preparation of EMDs and that CMDs and EMDs exhibit different quantities of De Wolff disorder. With regard to the structure, a main conclusion of this work is that γ- and ε-MnO 2 are similar materials. Both forms derive from the ramsdellite structure, and differ only by the quantity of structural defects present, the so-called ε-MnO 2 exhibiting more microtwinning than γ-MnO 2 samples. Practical methods to analyse X-ray powder diffraction patterns and calculate the amount of structural faults in real materials are proposed and lead to a new classification of γ-MDs. It appears that only a small number of the possible members of the γ-MnO 2 structural family have been prepared up to now. The electrochemical behaviour of several MDs has been studied in alkaline electrolyte, using step potential electrochemical intercalation spectroscopy (SPECS) and in situ neutron powder diffraction. The successive steps of the first reduction of γ-MDs, reduction of surface states, reduction of the ramsdellite units with proton intercalation and reduction of pyrolusite units, are analysed in detail. Correlations with the structural analysis of the pristine materials evidence the crucial role of disorder and defects in the electrochemical behaviour, and enables us to propose detailed mechanisms for the reduction of γ-Mn 2 up to one electron per Mn. A notable result is the fact that reduction proceeds along different paths according to how far from equilibrium it is carried out. The consequences of these results are discussed in term of practical applications.