Synthesis And Optimization Of LiFePO₄ As Cathode Material For Li-ion Batteries And Investigation Of Its Interface

Among the various available storage technologies, the Li-ion battery, which has conquered the portable electronic market, such sa mobile phones, laptops, cameras and other portable electronic devices, has become the prime candidate to power the next generation of ElectricVehicles (EVs) and Plug-inHybrid ElectricVehicles (PHEVs). Lithium iron phosphate has drawn a lot of attention ever since it was proposed by Goodenough in 1997 and is considered one of the most promising cathode materials of next-generation lithium-ion batteries for EVs and HEVs due to its low cost, high theoretical capacity, good electrochemical and thermal stability and environmental compatibility. In this paper, a systematic study on the synthesis and structural and electrochemical optimization of LiFePO₄ was conducted, with emphasis on interfacial properties which have been found to be crucial for the performance of nanosized LiFePO₄.In chapter 2,we briefly desctibe the experimental processes and equipment used in the research. A detailed description on the process of making a test cell is presented. Instruments and methods used for structual and electrochemical analysis are also included.In chapter 3,core-shell LiFePO₄/C nanocomposite is prepared by a sol-gel method. The mean size of the spherical core LiFePO₄ is about 30 nm, and thickness of carbon shell is about 3 nm. The bonding character on the interface of core-shell was revealed by X-ray absorption spectroscopy (XAS) collected at C and O K-edge. The as-prepared sample was characterized by XRD, Raman spectrum, TEM and HRTEM. Charge-discharge tests show the core-shell LiFePO₄/C demonstrates high rate capability (106mAhg-1 at 20C) and good cycling performance (negligible capacity loss after 250 cycles).In chapter 4,LiFePO₄/C particles with different LiFePO₄–C interface are prepared with a sol-gel method and investigated with emphasis on the interface property which has been found to be important for the power performance of LiFePO₄. Soft-x-ray-absorption (XAS) spectra collected at O and C K-edge show a distinct bonding character between carbon and LiFePO₄ in carbon coated LiFePO₄/C. Further study demonstrates that the bonding between C and O can induce Fe²⁺ in the surface layer of LiFePO₄ to become Fe³⁺. Galvanostatic change-discharge at different rates and electrochemical impedance tests demonstrate that the interface action between LiFePO₄ and carbon within LiFePO₄/C composites has a crucial effect on its electrochemical performance. The limiting factor for charge/discharge reaction of nanosized LiFePO₄ is the delivery of Li+ and electrons to the surface rather than bulk diffusion. The bonding between carbon and LiFePO₄ can lead to the formation of an amorphous layer at the interface, which can enhances delivery of Li+ to the surface of LiFePO₄ and therefore greatly improves the electrochemical performance of LiFePO₄.In chapter 5, different types of LiFePO₄/C composites with the same carbon coating thickness were prepared using a sol-gel technique, enabling direct comparison of the influence of carbon coating porosity on the performance of LiFePO₄/C cathodes. It was found that electrochemical performance of LiFePO₄/C should be strongly dependent on the carbon coating structure for core-shell nanocomposites. It is desirable electrochemical applications for electrode material to have an optimal combination of high porosity and surface area with good transport properties. A hierarchical pore system is considered to serve better as paths for fast ionic and electronic in a battery electrode. And it's more optimal to introduce a hierarchical pore system featuring dual porosity into the electrode materials to deliver both good capacity and achieve good capability.In chapter 6, core-shell LiFePO₄/C composite was synthesized via a sol-gel method and doped by fluorine to improve its electrochemical performance, and the influence of flurine doping on the structural and electrochemical properties of LiFePO₄ is investigated. Structual characterization indicates that F-doping can reduce lattice parameters of LiFePO₄ which helps improve the crystal stability during charge and discharge. The interatomic distances of Li–O are enlongated after F-doping, which facilitates the extraction of Li from the lattice of LiFePO₄. Electrochemical characterization shows that F-doping facilitates the electrochemical reaction during cycling and enhances the Li+ diffusion in the bulk of the material, and thus improves the high-rate electrochemical performance of LiFePO₄/C.Fimlly,In chapter 7,the author summarized the originality and deficiency of this work, and put forward some prosperts and suggestions for future research.