Doctor of Philosophy
Institute for Superconducting & Electronic Materials - Faculty of Engineering
Chen, Yao, Investigation on advanced active materials for lithium-ion batteries, PhD thesis, Institute for Superconducting & Electronic Materials, University of Wollongong, 2006. http://ro.uow.edu.au/theses/709
The objectives of the Ph.D project were: (1) to prepare and study advanced active materials with high capacity and long cycle life for rechargeable lithium ion batteries; (2)to understand the relationship between the structural, physical, and lectrochemical properties of the electrode materials; (3) to identify the main parameters which are responsible for the differences in electrochemical behaviour. In this study, several methods have been used to improve the performance of electrodes, including element substitution, synthesis methods, carbon-enriching, etc.
A starting point for the study was a survey of the literature pertaining to the cathode and anode materials for lithium ion batteries.
Many different electrode materials are described. The thesis provides a detailed description of the status of current research and development in electrode materials for lithium ion batteries. A variety of cathode materials have been investigated. Some of them have shown good electrochemical performance when used in lithium ion cells. Electroactive LiCoxMnyNi1- x-yO2 powders were prepared from lithium compounds and CoxMnyNi1-x-y(OH)2 precursor by heating at 850-900 °C.
High density spherical LiCo0.25Ni0.75O2 compounds were synthesized from lithium compounds and spherical Co0.25Ni0.75 (OH)2 precursor by heating in oxygen at 750°C and 800°C for 12 hrs. The structural characteristics of the compounds were determined by x-ray diffraction. The best sintering temperature was found to be 800 °C, in terms of stabilising the layered structure and improving the cycle life. It was found that the spherical LiCo0.25Ni0.75O2 compounds sintered at 800 °C have a highly ordered layered structure with reduced cation mixing.
Carbon-enriched nanocrystalline LiFePO4 compounds were prepared by the sol-gel method and the R/F carbon gel method, respectively. A layer of carbon was laid down as a coating on the surface of the lithium crystals, which dramatically enhanced the electronic conductivity of the LiFePO4 compounds. A high capacity of 160 mAh/g at the C/5 rate has been demonstrated for the LiFePO4 electrode. These electrodes also exhibited good cyclability at different charge/discharge rates.
A variety of anode materials have also been investigated to search out alternative anode materials to replace graphite for lithium ion batteries. Five types of cobalt oxides were prepared by high temperature decomposition, low temperature decomposition, high energy ball-milling, and the chemical decomposition of cobalt octacarbonyl in toluene, respectively. The LT-Co3O4, the CoO, and the nanosize Co3O4 powders prepared by decomposing organo-cobalt compounds at low temperature show good capacity retention on charge/discharge cycling, which is promising for their use as anode materials in Liion cells. It was found that the electrochemical properties of cobalt oxides are sensitive to the crystallinity, morphology, and particle size.
Nanosize Ag powders were prepared via a reverse micelle approach. The size of Ag nanoparticles can be influenced by the concentration of the reduction agent. In addition, SnO2 nanopowders were synthesized by using the same method. 1D structured SnO2 nanorods were observed in the SnO2 nanopowders. The SnO2 nanopowder electrodes show high electrochemical reactivity toward lithium in Li-ion cells.
The synthesized materials were characterized by x-ray diffraction, SEM and TEM, through which the phase composition and microstructure were observed. The capacity and cycle life of the cathode materials were obtained from charge/discharge cycling tests. The kinetic characteristics and kinetic parameters of lithium ion insertion and extraction within the electrode materials were determined by a.c. impedance spectroscopy and cyclic voltammograms.
In summary, the investigations in this project have produced several types of electrode materials with high capacity and long cycle life. In particular, the study focuses on the preparation of cathode materials with high density and ordered structure, and anode materials with ultrafine particles. Analysis of the electrochemical processes in lithium ion cells was conducted by various techniques. All of these studies provide a fundamental basis for the development of high energy density electrode materials.