Synthesis Of High Energy Density Electrode Materials For Li-ion Batteries And Their Capacity Fading Mechanism Investigation

Recently, many green energy technologies have been regarded as a promising approach to address both energy crisis and environmental pollution. Due to their high energy density, long cycle life and environmental friendly, Li-ion batteries have been widely used in portable devices such as telephone, digital camera and laptop. Besides, researchers are also exploring the application of Li-ion batteries in energy storage systems and electrical vehicles, which have a higher requirement for energy density. This thesis focuses on high energy density cathode materials for Li-ion batteries, including the preparation of V2O5, Fe-V oxide thin films, spinel Li0.05Mn1.95O4and LiNi0.5Mn1.5O4powders. It also involves in the doping and surface coating of LiNi0.5Mn1.5O4, the study of LiNi0.5Mn1.5O4||Li4Ti5O123V full cells and LiNi0.42Mn0.42Co0.16O2||Li4Ti5O122.5V full cells.Chapter1gives a general introduction of the working mechanism of Li-ion batteries, some typical cathode and anode materials as well as some conventional electrochemical examinations. Then a detailed introduction of high precision charger (HPC) in Dalhousie University is presented. A summary of recent researches using the HPC for Li-ion batteries is conducted as well.In Chapter2, we introduce the experimental reagents, methods and equipments used in the project of this thesis. A detailed procedure of coin-cell fabrication in Dahn lab is presented as well as some electrochemical, structure and morphology analysis.In Chapter3, Fe doped V2O5and Fe-V oxides thin films are prepared by the electrostatic spray deposition technique. Such a three-dimensional structure allows the electrolyte to soak well into the active material and facilitate the kinetics of lithium-ion transport. After introducing Fe3+into the V2O5structure, the stability of the layered structure can be improved, leading to an improved cycling and rate performance in the voltage range of2.0-4.0V. The crystalline Fe2V4O13thin film performs a structural degradation in the voltage range of1.0-4.0V while it is very stable during2.5-4.0V. The amorphous Fe2V4O12.29thin film shows good cycling performance and rate capability during1.0-4.0V due to the enhanced electronic conductivity caused by the existence of mixed valence states of Fe and V.In Chapter4, nanometer-sized Mn3O4powder is prepared in an oil-bath synthesis process with diethylene glycol as the solvent. Then micrometer sized Li1.05Mn1.g5O4powders are synthesized by the mixture of Mn3O4and LiCH3COO·2H2O. The L1.05M1.95O4||Li cells exhibit a high rate performance with a specific capacity of98.4mAh/g at5C at room temperature. A stable cycling performance is observed at55℃that90.5%of its initial capacity can be obtained after100cycles at1C. Furthermore, the Li1.05Mn1.95O4sample also shows much stable cycling performance at low temperatures with a specific capacity of84.5mAh/g at-20℃. The diffusion coefficients of lithium ion measured by CV method show a drop from10-10cm2/s at25℃to10-12cm2/s at-20℃.In Chapter5, LiNi0.5Mn1.5O4is prepared by a solid state reaction from a mixture of Li2CO3and Nio.25Mno.75(OH)2, which is obtained by a co-precipitation method. An accurate coulombic efficiency (CE) study of LiNi0.5Mn1.5O4||Li cells by HPC using different surface area electrodes cycled at different temperatures and C-rates is performed. CE is found to be dependent on temperature, specific surface area and C-rate for LiNi0.5Mn1.5O4||Li cells. The charge and discharge profile of the LiNi0.5Mn1.5O4||Li cells slips to large relative capacities with continued cycling, indicating serious parasitic reactions inside the cells. EC/DMC as the electrolyte solvent increases the CE of LiNi0.5Mn1.5O4||HLi cells compared to EC/DEC. Measurements of the CIE/(time of a cycle) show that parasitic reactions continue at the same rate independent of the cycling current.In Chapter6, a study of doping and surface coating for LiNi0.5Mn1.5O4is performed. Al, Co, Fe and Cr substituted LiNi0.5Mn1.5O4samples are successfully synthesized from a hydroxide precursor.2wt%ZnO and2wt%Al2O3coated LiNi0.5Mn1.5O4is made from a carboxymethyl cellulose containing slurry. A study of the CE of LiNi0.5Mn1.5O4||Li using the substituted LiNi0.5Mn1.5O4shows that there is no "magic" improvement in the CE of cells using the transition metal substituted samples. The charge and discharge slippage increases with cycles, indicating serious parasitic reactions occurring inside all the cells. SEM results show that ZnO or Al2O3is uniformly coated on the surface of LiNi0.5Mn1.5O4. But both electrodes show the same capacity retention and close CE versus cycle number compared to uncoated LiNi0.5Mn1.5O4electrode. The capacity retention is improved while no improvement in CE is observed for the LiNi0.5Mn1.5O4||Li cells by comparing the ordered P4332phase LiNi0.5Mn1.5O4with or without Mn3+In Chapter7, the capacity degradation mechanism of LiNi0.5Mn1.5O4||Li4Ti5O12cell is investigated by a designed "back-to-back" cell. The electrode-electrode interactions that occur in LiNi0.5Mn1.5O4||Li4Ti5O12Li-ion cells are examined. Species created at the LiNi0.5Mn1.5O4electrode by electrolyte oxidation migrate to the Li4Ti5O12electrode, cause excess charge consumption and lead to the slippage of the Li4Ti5O12electrode. Therefore, LiNi0.5Mn1.5O4-limited LiNi0.5Mn1.5O4||Li4Ti5O12cells show rapid capacity fading while Li4Ti5O12-limited cells cycle without loss for a short while before the electrolyte is exhausted. Measurements of CE of LiNi0.5Mn1.5O4-limited LiNi0.5Mn1.5O4||Li4Ti5O12cells at C/20, C/2,1C and2C show that the CE is dependent on cycling current due to a significant parasitic current determined to be close to C/2000for these cells. Besides, it revealed that LiNi0.5Mn1.5O4||Li4Ti5O12cells prefer to cycle well at2C that500cycles could be obtained with80%of its initial capacity.In Chapter8, we apply some strategies to improve the CE of LiNi0.5Mn1.5O4||Li4T15O12cells, such as Al、Co、Fe and Cr substituted LiNi0.5Mn1.5O4, ZnO or Al2O3coated LiNi0.5Mn1.5O4, different electrolyte salts and the use of electrolyte additives. There is no improvement for Al, Co, Fe and Cr substituted LiNi0.5Mn1.5O4from the aspect of CE and charge/discharge slippage. ZnO or Al2O3coated LiNi0.5Mn1.5O4can effectively decrease the interactions between LiNi0.5Mn1.5O4and Li4Ti5O12electrodes and increase the CE of the LiNi0.5Mn1.5O4||Li4Ti5O12cells. Compared to LiClO4LiBOB and LiBF4, LiPF6gives the best CE and the least slippage for LiNi0.5Mn1.5O4||Li4Ti5O12cells. LiNi0.5Mn1.5O4||Li4Ti5O12cells with1wt%LiO-t-C4F9electrolyte additive show improved capacity retention versus cycle number compared to control cells. However their CE, capacity end point slippages and self-discharge are worse than control cells, indicating that this additive increased, not reduced, the rate of parasitic reactions occurring at the LiNi0.5Mn1.5O4electrode. Therefore, although this additive initially looks "good" it turns out to be poor when all factors are considered. By contrast,1wt%Al(HFiP)3does decrease the rate of parasitic reactions at the positive electrode based on the results of storage experiments.In Chapter9, the electrochemical performance of LiNi0.42Mn0.42Co0.16O2||Li4Ti5O12cells are studied, the electrode-electrode interactions between positive and negative electrodes are detected in such2.5V full cell. Li4Ti5O12electrode shows slippage in the voltage range of1.0-2.5V while no slippage is observed during1.0-2.27V, indicating the decreased parasitic reactions. Elevated temperature is found to accelerate the Li4Ti5O12slippage. The slippage of Li4Ti5O12electrode leads to a gradual capacity increase in the voltage range of1.0-2.5V while a constant capacity is obtained without Li4Ti5O12slippage at1.0-2.27V. Besides,2wt%Vinylene carbonate (VC) is shown to be negative for the LiNi0.42Mn0.42Co0.16O2||Li4Ti5O12cells due to the increased charge and discharge slippage from both LiNio.42Mn0.42Co0.16O2and Li4Ti5O12electrodes.In Chapter10, we give an overview of the innovation and deficiencies of this thesis. Some prospects and suggestions for the future work are presented as well.Finally, there is still some work of MnO/C as anode materials for Li-ion batteries involved in this thesis, which is given in the appendix1.