MnO₂-based nanomaterials for energy conversion and storage applications

Abstract

Manganese dioxide has been extensively investigated in the areas of catalysis and energy storage due to the merits of high electrochemical activity and mixed-valence properties. Recently, MnO₂ with various morphologies and tunnel structures have been synthesized, which can be used as precursors to prepare cathode materials for lithium ion batteries. However, it is still possible to further optimize the electrochemical capacitive performance of MnO₂-based nanomaterials and explore the new applications in the area of energy storage and conversion. In this dissertation, the energy storage and conversion capabilities of MnO₂-based nanomaterials with controllable morphologies and tunnel structures are studied in detail. One-dimensional nanowires, amorphous MnO₂ nanoparticles and two-dimensional nanosheets have been synthesized by controlling synthetic parameters. The electrochemical capacitive performances were investigated. The enhanced capacitive characteristics of MnO₂ nanocomposites electrodes are attributed to morphology complementation of MnO₂ nanowires and amorphous MnO₂ nanoparticles with the unique properties of MnO₂ nanowires that act as not only conductive networks, but also agents to resist the aggregation of amorphous MnO₂ nanoparticles. It is also demonstrated that the capacitance of MnO₂ nanosheets and amorphous MnO₂ nanoparticles are improved by introduction of graphene oxide with an increase of effective reaction interface. The One-dimensional nanowires and two-dimensional nanosheets were used as structural and morphological direction-agents to prepare spinel-type LiMn₂O₄ nanosheets and facet-dominant LiMn₂O₄ that exhibit excellent rate capacity performance as cathode materials in rechargeable lithium ion batteries. Layered LiNi₁/₃Co₁/₃Mn₁/₃O₂ nanorods were prepared using MnO₂ nanowires and metallic nitrates. Rate discharge performances and cycling stabilities were improved via coating of La₂O₃ on the surface of LiNi₁/₃Co₁/₃Mn₁/₃O₂ nanorods. Investigation of the thermoelectric performance of flexible manganese dioxide nanowire paper-like sheets was also conducted. The electrical conductivity is increased by mixed with conductive RuO₂. With the low thermal conductivity due to an increase in phonon scattering by nanowire, enhanced thermoelectric properties are achieved. The reduced graphene oxide was also incorporated into the MnO₂ nanowires sheet by directly mixing the graphene oxide and MnO₂ nanowires followed by annealing process to increase the mechanical properties. However, the increased thermal conductivity and the decreased electrical conductivity are unfavorable for the thermoelectric performances.