|dc.description.abstract||Energy problem has become one of the most important problems in today's world. With the depletion of fossil fuels and the change of climate, the research on the conversion and application of new clean energy has entered a critical stage. With the deepening of research, more and more technologies and products have been commercialized and changed our daily life, such as electric vehicles (EVs). EVs refer vehicles which are powered by electric energy (lithium ion batteries). Compared with traditional fuel cars, electric vehicles (EVs) have many advantages: (i) High energy efficiency: The energy conversion efficiency of the fuel engine is only about 10%-15%, while the efficiency of the battery engine can be as high as 80%-90%; (ii) Environmentally friendly: The emissions of petrol cars include solid particles, carbon oxides, nitrogen oxides, sulphur oxides, etc, while the batteries almost have no pollution emissions; (iii) Economy: The average cost of EVs is about 4.2AUD per 100km, while the cost of petrol vehicles is about 18.7AUD per 100km. According to these obvious advantages, many countries have already announced plans to increase the uptake the of electric vehicles.
These applications demand an increased performance from the lithium-ion battery (LIB). Although this technology is quite mature after years of development, there are still many problems today. For cathode materials, the main problem is that the specific capacity is relatively low and not suited to high energy applications. For anode materials, although the specific capacity is quite good, the stability is a big problem. In addition, the safety and the high cost are also problems demanding prompt solutions. At the same time, the lithium mineral on earth is being exhausted, finding other alkaline metals, like sodium or potassium, to replace lithium is also a major direction of energy storage research.
This thesis presents four research works during my doctoral study which are mainly about the electrode materials of four different battery systems. The purposes of these works are to improve the problems existing in the traditional materials through the way of composition design and morphology control.
The first two chapters of my work are about the symmetric battery system. The symmetric batteries with an electrode material possessing dual cathodic and anodic properties have been regarded as an ideal battery configuration because of their distinctive advantages over the asymmetric batteries in terms of fabrication process, cost and safety concerns. However, the development of good performance in symmetric batteries is highly challenging due to the very limited availability of suitable symmetric electrode materials with such duplex properties of high reversible capacity. Chapter 2 introduces a triple-hollow-shell structured V2O5 (THS-V2O5) based high performance symmetric electrode material with a reversible capacity of >400mAh/g between 1.5V to 4.0V and >600mAh/g between 0.1V to 3.0V, respectively, when used as the cathode and anode. This single electrode based symmetric full lithium ion battery (LIB) constructed with THS-V2O5 exhibits a reversible capacity of about 290mAh/g between 2.0V to 4.0V, which is the best performance in symmetric energy storage systems reported to date. In Chapter 3, we report a novel NASICON-type K3V2(PO4)3 which was prepared and first employed for the symmetric KIBs. The reversible capacity of the full symmetric KIBs is about 90mAh/g between 0.01–3.0V at 25mA/g, corresponding to an initial coulombic efficiency of 91.7%. Additionally, a potential of about 2.3V was obtained in this work, which is the largest reported working potential and will benefit the output energy of this symmetric energy storage system.
The other two chapters of my work are about anode materials of lithium-ion batteries (LIBs). In Chapter 4, we reported a new yolk-shell structured high tap density composite made of a carbon-coated rigid SiO2 outer shell to confine multiple Si nanoparticles (NPs) (yolks) and carbon nanotubes (CNTs) with embedded Fe2O3 NPs. The achieved high tap density and superior conductivity can be attributed to the efficiently utilised inner void by multiple Si yolks, Fe2O3 NPs and CNTs Li+ storage materials, and the bridged spaces between the inner Si yolks and outer shell through a conductive CNTs. In Chapter 5, we present a controllable synthesis method of single to quadruple hollow NiO multi-shelled microspheres and studied the electrochemical properties. Furthermore, we made a modification on the basis of the triple-shelled structure, the hollow triple-shelled α-Fe2O3/NiFe2O4@NiO (TS-NFO) microspheres were simply synthesized by a secondary absorption method. Due to the effect of synergistically interactive, the TS-NFO microspheres exhibited an initial capacity of 2474mAh·g-1 and excellent reversible capacity of 869mAh·g-1, 2114mAh·g-1, 2061mAh·g-1 after 100, 500 and 800 cycles at 0.5A·g-1 in the electrochemistry property tests. The outstanding energy storage performance can be ascribed to the unique hollow hybrid metal oxides core@shelled structure, which can relieve the volume extension to a great extent and greatly improve the reversible specific capacity by the synergistically interactive effect.
In summary, this thesis introduces four kinds of electrode materials, which are applied to two kinds of energy storage systems. All the research works were trying to improve the performance of the batteries by composition design and morphology control, which may provide new ideas for the study of functional electrode materials development for energy storage systems.||