Synthesis of hollow-structured electrode material for energy storage

Abstract

Due to the high specific surface area, high chemical stability and good characteristics of electrical and thermal conductivity, hollow-structured materials have attracted extensive research interest as electrode materials for energy storage applications. The high specific surface area of hollow-structured materials provides a high contact area with electrolyte, facilitating the rapid diffusion and transport of electrolyte ions. The walls of electrode materials in hollow structures are relatively thin, providing short electron/ion diffusion length. Besides, the voids in hollow structures are beneficial for accommodating the volume expansion of the anode materials in lithium-ion batteries. Thus the specific capacity and cycling stability of the energy storage devices is enhanced with increased utilization of the electrode materials. However, it still remains challenging on maintaining the stability of the delicate hollow structure during cycling. Developing a facile strategy for the large-scale synthesis of hollow-structured electrode materials is worth studying as well. The aims of this project include the design and synthesis of transition metal oxides hollow microspheres for lithium-ion battery application and highly porous carbon microspheres for supercapacitor and aluminum ion battery applications. In the aspect of transition metal oxides, double-shelled polypyrrole-zinc ferrite hollow microspheres will be fabricated and its electrochemical properties will be investigated. In the other aspect of highly porous carbon, nitrogen-doped mesopore-dominated carbon microspheres will be synthesized and its electrochemical properties will be investigated. Nitrogen-doped macroporous carbon microspheres will be fabricated and its structural parameters will be investigated. The specific achievements obtained in this thesis are listed as follows: Nanocompositing is known as a promising approach for the structural stabilization of the preformed hollow structures. In the first part, ZnFe2O4 double-shell hollow microspheres are synthesized for the accommodation of the large volume expansion during lithiation. A facile and efficient vapor-phase polymerization method is developed to coat the ZnFe2O4 hollow spheres with polypyrrole. The thin polypyrrole coating improves not only the electronic conductivity but also the structural integrity of the ZnFe2O4 hollow spheres. When evaluated as the anode in lithium-ion batteries, the polypyrrole coated ZnFe2O4 hollow spheres manifest significantly improved cycling stability and rate capability than the pristine ZnFe2O4 hollow spheres. Apart from transition metal oxide hollow spheres that used as anode in lithium-ion batteries, highly porous carbon materials show great potential on supercapacitors and other energy storage devices. Thus, in the second part, we develop a novel “spray drying-vapour deposition” method for the synthesis of the N-doped mesoporous carbon microspheres, in which, commercial Ludox silica nanoparticles are used as porogens for the surfactant-free synthesis. The resultant mesoporous carbon microspheres possess a mesopore-dominate (95%) high surface area of 1528 m2/g, an increased apparent density of 0.5 g/cm3, thin walls with a high nitrogen content of 8 At %, and consequently superior gravimetric/volumetric capacitance performance (533.6 F/g and 208.1 F/cm3) as an electrode material in EDLCs. Following the second part, the developed “spray drying-vapour deposition” strategy is extended to the synthesis of N-doped three-dimensionally ordered macroporous carbon microspheres. The hexagonal-close-packed periodicity of macropores is achieved through the spray drying method. Vapour deposition of polypyrrole followed by carbonization and etching is beneficial for the generation of carbon network with a wall thickness of 3.5 nm. The resultant three-dimensionally ordered macroporous carbon microspheres possess a hexagonal close packing structure with a high surface area of 892 m2/g, an ultrahigh pore volume of 7.5 cm3/g and a high nitrogen doping of 8 At%. As a result, the synthesized three-dimensionally ordered macroporous carbon microspheres show good reversible capacity in aluminum ion batteries. In conclusion, our innovative approaches in this thesis have shown great potentials in stabilizing the hollow structure of transition metal oxides and fabricating highly porous carbon materials, which is important for enhancing the electrochemical properties of the electrode for energy storage applications.