Year

2010

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

Lithium-ion batteries and supercapacitors are both important energy storage systems and can make energy storage and usage more efficiently than with previous solutions. Both systems would be excellent choices for Electrical Vehicles (EVs) or Hybrid Electrical Vehicles (HEVs), and also other portable devices requiring both high power and high energy density. The key aspect for improving the performance of both kinds of energy devices is to improve the performance of the active materials. The use of nanostructured materials and conductive composite materials is designed to enhance both ion transport and electron transport by shortening the diffusion lengths of ions (such as, Li+, Na+, K+, H+, and OH-) and increasing the conductivity within the electrode materials, respectively. In this doctoral work, several nanostructured materials and conductive (carbon or conducting polymer) composites were examined and characterized for possible application as electrode materials for lithium-ion batteries or supercapacitors. For the Li-ion battery, tin dioxide (SnO2) nanotubes, carbon-coated SnO2 composite, hematite (α-Fe2O3) carbon composite, lithium iron phosphate (LiFePO4) polypyrrole composite, and vanadium pentoxide (V2O5) nanomaterials were investigated. Meanwhile, several flexible electrode materials for supercapacitors were also studied: manganese dioxide (MnO2) nanowire-carbon nanotube composite, MnO2 nanowires on stainless steel mesh (SSM), porous vanadium oxide (VOx) on SSM, and cobalt hydroxide (Co(OH)2) nanoflakes on SSM. Anode materials for Lithium-ion battery The SnO2 nanotubes were synthesized by anodic electrochemical deposition under ambient conditions without using any additional template. Controlled self-bubbling O2 acted as both the template and the oxidizing agent for obtaining SnO2 tube structures at the interface of the gas (O2) and the liquid (electrolyte). The length of the tube could be controlled by adjusting the electrochemical deposition time. Electrochemical results show that the nanotubes have higher discharge capacity and better high-rate capability than microtubes. The morphology of the hollow nanotube structures composed of ultra-fine nanoparticles could be responsible for the enhanced high-rate performance and improved cycling stability as an electrode material. Electrochemical impedance spectroscopy (EIS) measurements showed that the nanotubes had a much higher electrochemically active surface area than microtubes. From the Arrhenius plots, the apparent activation energies were calculated to be 61.9 and 85.7 kJ mol-1 for the nanotubes and microtubes, respectively, indicating the enhanced kinetics. Carbon-coated SnO2 nanoparticles were prepared by a novel facile route using commercial SnO2 nanoparticles treated with concentrated sulfuric acid in the presence of sucrose at room temperature and ambient pressure. The key features of this method are the simple procedure, low energy consumption, and inexpensive and non-toxic source materials. The electrochemical measurements showed that the carbon-coated SnO2 nanoparticles with 10 % carbon and using carboxymethyl cellulose (CMC) as a binder displayed the best electrochemical performance, with the highest specific capacity of 502 mAh g-1 after 50 cycles at a current density of 100 mA g-1. In addition, owing to the water solubility of CMC, the usage of CMC as binder makes the whole electrode fabrication process cheaper and more environmentally friendly. Hollow-structured α-Fe2O3/carbon (HIOC) composite with high surface area around 260 m2 g-1 was synthesized by one-step, in-situ, and industrially-oriented spray pyrolysis method using iron (II) lactate solution and sucrose as the precursor. The electrochemical tests show that the HIOC composite with 14.7% carbon using CMC as binder, without pressing of the electrode, shows the best electrochemical performance, in terms of the high capacity (1000 mAh g-1 at 0.1 C), good rate capability (700 mAh g-1 at 2 C), and good cycling stability (720 mAh g-1 at 2 C up to 220 cycles). The high surface area, hollow structure, selected binder, and carbon content account for the high performance with respect to lithium storage properties. Cathode materials for Lithium-ion battery Highly flexible, paper-like, free-standing polypyrrole (PPy) and polypyrrole-LiFePO4 composite films were prepared using the electropolymerization method. The films are soft, lightweight, mechanically robust, and highly electrically conductive. The electrochemical performance of the free-standing pure PPy electrode was improved by incorporating the most promising cathode material, LiFePO4, into the PPy films. The cell with PPy-LiFePO4 composite film had a higher discharge capacity beyond 50 cycles (80 mAh g-1) than that of the cell with pure PPy (60mAh g-1). The free-standing films can be used as electrode materials to satisfy the new market demand for flexible and bendable batteries that are suitable for the various types of design and power needs of soft portable electronic equipment. V2O5 nanomaterials, including nanoribbons, nanowires, and microflakes, have been synthesized by an ultrasonic assisted hydrothermal method combined with a post annealing process. A room temperature ionic liquid (RTIL) was used as the electrolyte in rechargeable lithium metal batteries along with V2O5 nanomaterials as cathode materials. The electrochemical tests show near-theoretical specific capacity, improved cycling stability, good high-rate capability, and enhanced kinetics. The thermogravimetric analysis (TGA) results show that the RTIL can prevent the dissolution of V2O5 during charge and discharge. The rechargeable lithium battery using V2O5 nanoribbons as cathode material and RTIL as the electrolyte could be the next generation lithium battery with high capacity, excellent safety, and long cycle life. Materials for Supercapacitor MnO2 nanowires were electrodeposited onto carbon nanotube (CNT) paper by a cyclic voltammetric (CV) technique. The as-prepared MnO2 nanowire/CNT composite paper (MNCCP) can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that the MNCCP electrode displayed specific capacitance as high as 167.5 F g−1 at a current density of 77 mA g-1. After 3000 cycles, the composite paper can retain more than 88% of initial capacitance, showing good cyclability. The CNT paper in the composite acts as a good conductive and active substrate for flexible electrodes in supercapacitors, and the nanowire structure of the MnO2 could facilitate the contact of the electrolyte with the active materials, and thus increase the capacitance. Flex porous Co (OH)2 nanoflake, porous vanadium oxide, and MnO2 nanowire films were synthesized by an electrochemical deposition method using stainless steel mesh as a substrate. The cyclic voltammetry (CV) results show that the MnO2 nanowire film has a specific capacitance of 195, 170, 158, and 91 Fg-1 at scan rates of 5, 10, 20, and 50 m V s-1, respectively. The porous VOX film displays specific capacitance of 152,124, 99 and 76 F g at scan rates pf 5, 10, 20, and 50 mV s- 1, respectively. The capacitance losses for the MnO2 nanowire film and the porous VOX film are only 5% and 10% for 1000 cycles, respectively. The pourus Co(OH) 2 nanoflake film shows the highest capacitance of 609.4 Fg-1, The electrochemically active specific area of the annealed porous Co(OH) 2 nanoflake film remained virtually unchanged after 3000 cycles, showing the stability of the microstructure.

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