Proton and lithium insertion into heat-treated synthetic manganese dioxides

Jones, Jason (1996) Proton and lithium insertion into heat-treated synthetic manganese dioxides. PhD thesis, Middlesex University. [Thesis]

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Abstract

Two electrodeposited and two chemical manganese dioxides were heat-treated in an air or argon environment for 24 hours at temperatures from 120-450°C and the products were
characterised by chemical analysis, XRD (X-Ray Diffraction) and FTIR (Fourier Transform Infra-Red) spectroscopy. It was found that the rate of water evolution was identical in air or argon but the weight loss was greater in argon. This was explained on the basis that the x value in MnOx increased in air but not in argon. Powder X-ray diffraction spectra of the heat-treated materials showed that there was little change in structure from 25-300°C and between 300-400°C the structure changed from a γ-phase to a β-like phase. Application of the latest structural model of γ-MnO2 in terms of de Wolff disorder and microtwinning was applied to the XRD patterns of Faradiser WSLi. It was found that there was decrease in microtwinning from 25-300°C but the amount of de Wolff disorder was essentially unchanged. Simulation of XRD patterns by taking appropriate amounts of two end-members and comparing them with one obtained experimentally showed that the transformation from γ to β like in the temperature range 300-400°C was heterogeneous. FTIR spectra of the same materials showed a progressive shift of the peaks associated with the MnO6 octahedral framework. Comparison of the peak positions with the published data of ramsdellite (γ-MnO2) and pyrolusite (β-MnO2) suggested a ramsdellite phase in the unheated material and a pyrolusite phase at 400°C.
An electrodeposited MnO2 was heat-treated at 200, 300 and 400°C and then H was chemically inserted using propan-2-ol or hydrazine hydrate. XRD spectra showed that a homogeneous reduction occurred with the material that had been heat-treated at 200°C and reduced with propan-2-ol while a heterogeneous reduction occurred with the material heat treated at 400°C and reduced with hydrazine hydrate. Within these extremes of behaviour different regions of homogeneity/heterogeneity were observed. With hydrazine hydrate heterogeneity set in at a lower level of H insertion than with propan-2-ol reduction. This was because the rate of reduction was faster with hydrazine hydrate and the critical value of r = 0.8 in MnOOHr was reached earlier at the surface of the MnO2 particles. FTIR spectra of the chemically reduced materials were interpreted in terms of OH bond formation which was revealed by the appearance of peaks centred at 2050 and 2650 cm-1. With the homogeneous reduction there was no evidence of OH bond formation until MnOOH0.8, in the heterogeneous reduction OH bond formation was observed at low levels of H insertion. The onset of OH bond formation was thought to be due to the localisation of the inserted H+ and eO.
H insertion into a range of chemical manganese dioxides heat-treated analogously to EMD was carried out. The non heat-treated CMD material exhibited the largest region of
homogeneous H insertion while H insertion for the material heat-treated at 400°C was entirely heterogeneous. Between these extremes of behaviour intermediate levels of
homogeneity/heterogeneity were observed. The crystal structure of the most reduced materials formed a similar range of intergrowth structures as was found for the EMD materials. This was δ-MnOOH for the material heat-treated at 200°C and manganite for that heat-treated at 400°C. The boundaries of the homogeneous/heterogeneous behaviour of EMD and CMD were compared and it was found that heterogeneity occurred at a higher level of H insertion for the CMD materials.
A range of chemically Li-inserted MnOOLix (0<_x<_0.8) compounds were prepared from an EMD and CMD that had been heat-treated at 200, 300 and 400°C and then reduced in an argon-filled glove box using n-butyllithium. Powder X-ray diffraction revealed that the first stage of the reaction (0<_x<_0.3) was homogeneous and for x>0.3 (EMD) and x>0.35 (CMD) a new phase was formed via a heterogeneous reaction. The X-ray diffraction patterns of the new phase could be indexed to an orthorhombic unit cell and, in contrast to H insertion, the crystal structures of the most reduced Li-inserted materials seemed to be the same regardless of the initial heat-treatment temperature.
A novel applied linear regression analysis was developed to determine possible peak movement in a spectral dataset. The analysis used the coefficient of determination R2 as a
parameter for peak movement. In the second part of the analysis it was found that a low value of R2 indicated peak movement. In the XRD patterns of EMD heat-treated at 200°C and reduced with propan-2-ol the low values of R2 correlated with peak movement. Conversely with EMD heat-treated at 400°C and reduced with hydrazine hydrate there was little or no peak movement and the values of R2 were close to 1.

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