Year

2013

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

The lithium-ion battery is currently the most widely used electrochemical storage system on the market, with applications ranging from portable electronics to electric vehicles, to aerospace. In order to satisfy the growing demand for higher-energy and higher-power-density batteries, dramatic improvements are required. In this doctoral work, a strategy of using facile, scalable, and low-cost methods to synthesize nanostructured electrode materials was applied. The electrode materials that were investigated are of high capacity, and they include germanium, germanium oxide, tin-antimony, molybdenum dioxide, and vanadium pentoxide. Carbon allotropes such as amorphous carbon, graphene, and carbon nanotubes were also introduced into the electrode materials to form composites. The as-synthesized composite electrode materials were characterised in terms of their physical properties and their electrochemical lithium storage performance. The effects of the nanostructured composite materials towards improvement of lithium storage properties were also investigated.

A facile synthesis method was used to prepare germanium/carbon composite. Simple hydrolysis was used to prepare the precursor germanium dioxide nanoparticles, and then simultaneous carbon coating and the thermal reduction method were applied to form self-assembled clustered germanium/carbon composite. The clustered Ge/C nanostructure displayed good cycling stability at the 0.2 C rate (0.32 A/g) for over 50 cycles and at the 1 C rate (1.6 A/g) for over 120 cycles. Surprisingly, the clustered Ge/C shows exceptionally high rate capability up to the 40 C rate (64 A/g). The relationship between the morphology of the nanostructure and the electrochemical properties was studied. When the germanium dioxide nanoparticles were partially reduced, GeO2/Ge/C was formed. The GeO2/Ge/C composite showed high capacity of up to 1860 mAh/g and 1680 mAh/g at the 1 C (2.1 A/g) and 10 C rates, respectively. This good electrochemical performance is related to the fact that the elemental germanium nanoparticles present in the composite increase the reversibility of the conversion reaction of GeO2. These factors have been found through investigating and comparing GeO2/Ge/C, GeO2/C, nanosized GeO2, and bulk GeO2.

SnSb/graphene porous three-dimensional composite with dual buffering capability was prepared by an in situ chemical reduction of SnCl2, SbCl3, and graphene oxide prepared using a modified Hummers’ method. Field emission scanning electron microscope and transmission electron microscope images show that the SnSb nanoparticles are distributed homogenously across the surface of the graphene sheets, and some are also found to be trapped in the corrugated graphene structure. The SnSb/graphene composite delivered 688 mAh/g at the 2nd cycle (compared to a calculated theoretical value of 768 mAh/g) and showed good capacity retention of 420 mAh/g after 100 cycles. A reaction model to explain the dual buffering effects of SnSb/graphene composite as anode material for lithium insertion and extraction has been proposed. Graphene/molybdenum dioxide composites in several ratios were also prepared through a facile synthesis method. Depending on the ratio, the assynthesized composites consist of either 2-dimensional graphene sheets with MoO2 particles anchored to them or a clustered agglomerate morphology. The sample with highest amount of MoO2 (78 wt%) displayed the most promising lithium storage properties, with stable cycling performance at 0.2 A/g that shows negligible capacity loss over 50 cycles, retaining a capacity of 640 mAh/g. The rate capabilities were also tested and showed a capacity of 380 mAh/g at 2.0 A/g, which is comparable to the theoretical capacity of graphite and previously reported work on similar materials.

Free-standing and flexible V2O5 films have been prepared by filtration of ultra-long nanowires synthesised via the hydrothermal technique. In order to improve the conductivity of the films, multiwalled carbon nanotubes (MWCNTs) were added to the V2O5 nanowires to form an integrated web-like structure. The free-standing and flexible film electrodes exhibited good rate capability and excellent cycling performance, with capacity of 140 mAh/g, even after 50 cycles at 1.7 C in the voltage range of 2.5-4.0 V. The superior reversible lithium storage capability can be attributed to the fully reversible phase transition of α-V2O5 through to δ-LiV2O5, good lithium diffusion in V2O5, and increased electronic conductivity and electrolyte diffusion from the incorporated MWCNT web. Layer-structured V2O5·nH2O xerogel was synthesized via a simple, environmentally friendly hydrothermal technique by dissolving commercial V2O5 powder in de-ionized water and hydrogen peroxide. Graphene-V2O5·nH2O xerogel composites were then prepared by mixing and filtration of as-prepared V2O5·nH2O xerogel and graphene in the desired ratio. Increasing the graphene content in the composites resulted in better cycling stability. The initial and the 50th discharge capacities of the composite with 39.6% graphene were 212 and 190 mAh g-1 in the voltage range of 1.5- 4.0 V, and the capacities were 143 and 163 mAh g-1 when cycled between 2.0 and 4.0 V, respectively. The outstanding electrochemical performance could be attributed to the unique structure and morphology induced by the graphene.

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