Doctor of Philosophy
Faculty of Engineering
Guo, Z. P., Investigation on cathode materials for lithium-ion batteries, Doctor of Philosophy thesis, Faculty of Engineering, University of Wollongong, 2003. http://ro.uow.edu.au/theses/1831
This Ph.D project was motivated by a desire to prepare and study advanced cathode materials with high capacity and long cycle life for rechargeable lithium ion batteries. The structural, physical and electrochemical properties of the cathode materials determine the performance of cathodes and thereby affect the whole performance of lithium ion batteries. In this study, several methods have been used to improve the performance of electrodes, including doping, synthesis methods, etc.
A starting point for the study was a survey of the literature pertaining to the cathode materials for lithium ion batteries. Many different cathode materials are described. It provides a detailed description of the status of current research and development in cathode materials for lithium ion batteries.
A variety of cathode materials have been investigated. Some of them have shown good electrochemical performance when used as cathode materials in lithium ion cells. The fine-particle o-LiMn02 materials were prepared using either a one-step intermediate temperature solid state reaction or sol-gel methods.
LiMn1.xMx02 compounds doped with different metal ions were prepared by the solid-state route or by the Pechini method. The structural characteristics of the compounds were determined by x-ray diffraction. The best dopant effects were found with Cr3+ ions, in terms of stabilising the layered structure and improving the cycle life. Pechini preparation can further improve the electrochemical performance (specific capacity and rate capability) of LiMn1_xCrx02 by synthesizing powders with smaller particle size and good homogeneity.
A Li[Lio.3Cro.1Mno.6]O2 compound with a hexagonal structure in the pristine state was studied using a synchrotron based in situ x-ray diffraction technique during charge/discharge cycles. A reversible phase transformation between hexagonal phases HI and H2 has been identified. The integrity of the crystal structure is preserved during cycling since the phase transition takes place within the hexagonal symmetry.
LiMn204 and LiFeP04 cathode materials were synthesised by a modified solid-state reaction method. The compounds obtained have many advantages over comparable samples prepared by conventional solid state reaction, including higher specific capacity and good rate capability.
The innovative layered structure compounds Li[Co1/3Mn1/3Ni1/3]02 and Li(Co1_xLix/3Mn2x/3)02 were also synthesised to explore the feasibility of utilising cheaper, safer, but higher capacity materials than LiCo02 as cathode materials for lithium ion batteries.
In general, fine powders can deliver high capacity and good rate capability, but in some cases, they increase the capacity fading during cycling. However, for LiFeP04, the cycle life has also been improved, as the slow lithium ion diffusion in LiFeP04 is the main cause of the poor electrochemical performance. Dopant effects were found to have a positive impact on the structural and electrochemical stability, as well as the rechargeability of the cathode materials.
The synthesized materials were characterized by x-ray diffraction, S E M and T E M, 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 cathode materials were determined by a.c. impedance spectroscopy and cyclic voltammograms.
In summary, the investigations in this project have produced several types of cathode materials with high capacity, long cycle life and good rate capability. In particular, the study focuses on the preparation of cathode materials by practical methods (suitable to be used in high volume production). Analysis of the electrochemical process in lithium ion cells were conducted by various techniques. All of these studies provide a fundamental basis for the development of high energy density cathode materials