- Title
- Improving energy efficiency in the manganese dioxide cathode
- Creator
- Bailey, Mark Robert
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- In this thesis the relationship between the physico-chemical properties of manganese dioxide and its electrochemical behaviour has been investigated with the aim to improve the rechargeability of the alkaline Zn/MnO2 system. The project work was carried out in two broad areas, the first focusing on fundamental electrochemical and structural studies of γ-MnO2, and the second investigating the role of Ti(IV) and Ba(II) additives in improving cathode cyclability. The electrochemical and structural studies revealed that regardless of initial material properties, the capacity loss in γ-MnO2 during cycling can be primarily assigned to the lower potential (pyrolusite) region. To understand the underlying cause of this capacity loss, heat treatment of γ-MnO2 was used to prepare a structural series of material with increasing pyrolusite content. These studies revealed that heat treatment resulted in materials less able to accommodate proton insertion, with these materials expanding to a greater degree upon reduction and thus being more susceptible to mechanical breakdown. Electrochemical cycling revealed that as the pyrolusite content increases beyond 0.5, these changes act to destabilise the entire structure and result in material failure. The first section was brought to conclusion through the electrochemical characterisation of proton diffusion, calculated as A√D values, during the discharge and cycling of a standard γ-MnO2 material. This was achieved by the application of an appropriate model to the electrode response during step potential electrochemical spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS) experiments. By comparing the penetration depth of protons during these experiments with the nanostructure of the material, imaged via high-resolution transmission electron microscopy, it was shown that A√D values determined via SPECS represent the movement of protons through the bulk of the material, whereas EIS values focus on diffusion localised at the surface (penetrating <1 nm into the material). In this manner, each set of values provided key insights into different aspects of the material: the SPECS results suggestive of a reasonable degree of stability in the bulk of the material, and the EIS results indicative of significant degradation of the surface resulting from dissolution processes. The thesis then shifts its focus to understanding the role of Ti(IV) and Ba(II) additives in improving cathode cyclability. Electrochemical studies revealed that the inclusion of titanium dioxide improves electrode performance by lowering the resistance to charge transfer at the γ-MnO2 particle surface. A consideration of proton diffusivity in the presence of titanium dioxide indicated that Ti(IV) also becomes incorporated into the bulk γ-MnO2 structure upon cycling. The association of Ti(IV) with the particle surface and its incorporation into the bulk structure were confirmed through via imaging and depth profiling carried out using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The role of Ba(II) additives was examined on the basis of the changes in discharge behaviour of electrodes containing Ba(OH)2 and detection of the extent of Mn(III) dissolution during cycling. These studies showed that the interaction of Ba(II) species with the particle surface suppresses the dissolution of Mn(III) during the latter stages of discharge, enhancing particle stability and hence rechargeable performance. In the case of both additive species, it was demonstrated that it is the interaction with the γ-MnO2 particle surface, altering either charge-transfer or dissolution processes, which provides the pathway for improving rechargeable performance.
- Subject
- electrochemistry; manganese dioxide; battery materials
- Identifier
- http://hdl.handle.net/1959.13/1044991
- Identifier
- uon:14399
- Rights
- Copyright 2014 Mark Robert Bailey
- Language
- eng
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| View Details Download | ATTACHMENT02 | Thesis | 10 MB | Adobe Acrobat PDF | View Details Download |