Polyoxometalate-based nanocarbon composite materials as lithium ion battery electrodes

Abstract

Nowadays, a dependence on coal, oil and natural gas has created severe air pollution and global warming with alarming consequences. Solar energy, wind power, nuclear energy, as well as geothermal energy represent sustainable energy sources which are however “uncontrollable” in dimensions of time and space. The electricity grid, which is powerful in accumulating and re-distributing all the power generated from the above ways, requires high financial investments and infrastructural built-up. Therefore, the whole society’s concerns are concentrating attention once again on portable chemical energy storage systems. The lithium ion battery provides the portability of chemical energy and at the same time delivers this energy as electricity with high conversion efficiency and without pollution. With the fast- increasing development of electric vehicles, the research and development of low-cost, safe, highly efficient rechargeable Li batteries is becoming a priority on the agenda of material scientists. As such, the large-scale fundamental implementation of lithium batteries (or other rechargeable batteries) can not only benefit the stationary storage of electricity connected with the grid, but also provide convenient power usage for portable electric products including vehicles. Polyoxometalate (POM) chemistry, features a large group of earth-abundant, redox active metal oxide clusters and provides material scientists with infinite possibilities to develop POM-based novel battery materials. POMs are ideal components for molecular cluster batteries (MCBs) which exhibit excellent features including multi-electron storage, where the transition metals provide empty d orbitals that can host excess electrons in order to realize the Li-interaction within molecular structure. The nanocarbon family, including various dimensional carbon allotropes, offers great conductive pathways and architectural building blocks which can incorporate POMs to construct a wide range of POM/nanocarbon hybrid/composite supramolecular structures. I In this PhD thesis, a state-of-the-art of the POM/nanocarbon energy materials is summarized and the research works that have been contributed by the author during his PhD studies are discussed. The linkage of POMs and various dimensional nanocarbon allotropes can be generally classified into covalent binding, non-covalent attachment and electrostatic assembly. On the other hand, the architectural built-up can be summarized by the topological characteristics of nanocarbon substrates, including 0D graphene quantum dots, 1D carbon nanotubes, 2D graphene and graphene oxide as well as 3D porous carbon matrix. In this context, according to the author and his collaborators’ own contribution, four different types of POM based superstructures are critically investigated which are: 1) Sonication-driven-growth of POM nanocrystals on 1D single-walled carbon nanotubes; 2) Self-assembly of POM clusters and 0D graphene quantum dots into nanorod-like composites; 3) Immobilization of POM clusters on 3D hierarchical porous polymers; 4) Incorporation of polyoxovanadates into 3D metal organic framework followed by pyrolysis to form alloyed cobalt vanadium oxide/carbon hierarchical composites.