Computational modelling and EXAFS studies of electrolytic manganese dioxide

Abstract

Electrolytic Manganese Dioxide (EMD) is an important material for manufacturing alkaline electrolyte in commercial batteries. The structure of this material, which tends to be an intergrowth of pyrolusite and ramsdellite polymorphs, is very complex. The current study combines the computational modelling and EXAFS (used to investigate the local structure) studies to gain a better understanding of the structure. We have studied structural and electronic properties of pyrolusite and ramsdellite using the density functional technique (DFT) where pseudopotential plane wave methods have been invoked. In particular, the equations of states (EOS) are determined and bulk moduli predicted. The partial density of states (PDOS) and charge deformations of pyrolusite and ramsdellite, provide information on the bonding at different pressures. Atomistic simulation techniques, based on interatomic potentials, are used to investigate the surface structures, stability and reactivity of pyrolusite and ramsdellite polymorphs. The flat surfaces {001}, {010}, {011}, {100}, {101}, {110} and {111} were modelled for both pyrolusite and ramsdellite using the shell and the rigid ion models. For pyrolusite, {110}a surface is found to be the most stable surface with the relaxed surface energies 2.54 and 2.07 J.m-2 for the shell model and the rigid ion model respectively. For ramsdellite, the rigid ion model predicted {111}a surface to be the most stable with surface energy of 1.51 Jm-2. Molecular dynamics (MD) study on the effect of temperature on pyrolusite and ramsdellite structures was carried out. The bulk structures of pyrolusite and ramsdellite and the low index surfaces of pyrolusite were described using the Radial Distribution Functions (RDFs). The structures show that as the temperature is increased the height of peaks (pair distribution function) is decreased and the peaks become broader. An Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is used to investigate the local structure of EMD, pyrolusite and ramsdellite. The local structure is described using the RDFs and compared with the results obtained using the MD technique. Equivalent surface energies of pyrolusite and ramsdellite structures provided guidance in building various intergrowths, which tend to occur in natural EMD. Changes of RDF peak widths and positions with temperature were studied by the MD method. Amorphization and recrystallization simulation technique has been used to generate pyrolusite-ramsdellite interface models. The evolutionary method was applied to study large simulation cells of pyrolusite and ramsdellite interfaces. For pyrolusite MnO2/MnO2(001) interface +10% and +13% lattice misfit were applied to the thin film whereas for ramsdellite MnO2/MnO2(100) interface –6% lattice misfit was applied. Generated ramsdellite interface yielded a model composed of pyrolusite (1 x 1) single chains and ramsdellite (2 x 1) double chains after partial amorphization and recrystallization. Structural descriptions of the models, in particular RDFs, show an excellent correlation with our experimental and literature results. Furthermore, the methodology generated models, which reveal the atomic structure and give information on the defects (vacancies and clustering), grain boundary structures and epitaxial relationships. A simulated amorphization and recrystallization methodology has also been used to generate atomistic models of MnO2 nanoparticles. The morphologies of the resulting nanoparticles are spherical for both pyrolusite and ramsdellite. The morphologies exhibit {110}, {100} and {010} faces for pyrolusite and {110}, {100} and {001} for ramsdellite. This shows that the surface properties have an influence on the structural morphology of the system.