| dc.description.abstract | This dissertation contains six chapters. Introduction and conclusion are presented in Chapter 1 and Chapter 6, respectively. All results and discussion are divided into the rest of chapters. In Chapter 2, we used urea as the fuel for the solution combustion synthesis of nanoparticulate LiMn2O4 from metal nitrates to use as a cathode for 4 V lithium batteries. Lithium–manganese oxides have received more attention in recent years as high-capacity intercalation cathodes for rechargeable lithium-ion batteries and nanostructured electrodes have been shown to enhance cell cyclability. The optimal synthesis protocol was 10 h calcination at 700 oC, which produced a product that could sustain 229 cycles between 3.0 and 4.3 V at a charge–discharge rate of 0.1 C before reaching an 80% charge retention cut-off value. X-ray diffraction and electron diffraction pattern investigations demonstrated that all the LiMn2O4 products are a spinel phase crystal. TEM micrographs show the prepared products are highly crystalline with an average particle size of 20–50 nm. Cyclic voltammetry shows the absence of phase transitions in the samples ensures negligible strain, resulting in a longer cycle life. This work shows the feasibility of the solution combustion method for obtaining manganese oxides with nano-architecture and high cyclability, and suggests it is a promising method for providing short diffusion pathways which improve lithium-ion intercalation kinetics and minimize surface distortions during cycling.
In Chapter 3, we attempted to synthesize different wt. % 3LaAlO3:Al2O3-coated LiCoO2 cathode materials by an in-situ sol-gel process and study their electrochemical performance at higher charging voltages. The LiCoO2 particles were coated with various wt. % of lanthanum aluminum garnets (3LaAlO3:Al2O3) by an in-situ sol-gel process, followed by calcination at 1123 K for 12 h in air. X-ray diffraction (XRD) patterns confirm the formation of a single-phase hexagonal ?-NaFeO2-type structure of the core material without any modification. Scanning electron microscope (SEM) images reveal surface modification of the cathode particles. Transmission electron microscope (TEM) images indicate that the surface of the core material is coated with a uniform compact layer of 3LaAlO3:Al2O3, with an average thickness of 40 nm. Galvanostatic cycling studies demonstrated that the 1.0 wt. % 3LaAlO3:Al2O3 coated LiCoO2 cathode showed excellent cycle stability of 182 cycles, which was much more than the 38 cycles sustained by the pristine LiCoO2 cathode material when charged at 4.4 V.
In Chapter 4, we adopted a solid state method to obtain fine LiFePO4 powders and mixed them with small amounts of a high surface area carbon precursor to prepare the carbon-coated LiFePO4 cathode material. The LiFePO4 particles were embedded in amorphous carbon and the carbonaceous materials were synthesized by the pyrolyzing peanut shells under argon in a two-step process that occurred between 573 and 873 K. The shells were also treated with a proprietary porogenic agent in order to alter the pore structure and surface area of the pyrolysis products. The carbon coating can significantly enhance the electronic conductivity of LiFePO4. The electrochemical properties of the as-prepared LiFePO4/C composite cathode materials were systematically characterized by X-ray diffraction, scanning electron microscope, element mapping, energy dispersive spectroscopy, Raman spectroscopy, and TOC analysis. The specific capacity, cycle property and rate capability were impressive compared to the pure olivine LiFePO4 material. The carbon-coated LiFePO4 cathode demonstrated high capacity and stable cyclability.
Chapter 5 presents a novel concept of synthesizing the LiFePO4 by a solid state method using acidic surfactant treated polystyrene spheres that were a uniformly dispersed source, and characterizes its electrochemical behavior. The resultant carbon was entirely coated on LiFePO4 particles as a thin layer of about 2 nm. The LiFePO4/C composite delivered a first discharge capacity of 147 mAhg-1 at a 0.2 C-rate between 4.0 to 2.8 V, and the capacity remains 100% after 180 cycles. The electrochemical behavior and the four-point probe conductivity measurements revealed that the capacity and the conductivity of LiFePO4/C both improved as increased levels of carbon were added. However, too much carbon coating could reduce the capacity of LiFePO4/C because it lowers the ratio of active material in the composite and raises the resistance of lithium ion diffusion on the surface of the material. Adding an optimum amount of carbon increases the utilization of the active material and the electrical conductivity of electrode. | en_US |