Hydrothermal Preparation And Electrochemical Properties Of Vanadate Compounds

Lithium ion batteries have been studied intensively due to their high energy density, high voltage, long cycle life and environmental advantages. In the first chapter of this thesis, we reviewed the development of the cathode materials for primary- and rechargeable lithium batteries. We realized that vanadate compounds are promising cathode materials due to their very high energy densities. Therefore, in this work we prepared some typical vanadate compounds using hydrothermal synthesis. The structural and electrochemical properties of the materials were investigated as cathode materials for rechargeable lithium batteries.Firstly, phase pure highly crystallizedα'-NaV₂O₅ nanorods were prepared by hydrothermal process. Scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) confirmed theseα'-NaV₂O₅ nanorods were single crystals, fabricating along the <010> axis with the width of 200 nm and the length of 10μm. ICP and EDX analysis showed that the Na:V molar ratio of the material was close to 1:2. X-ray photoelectron spectroscopy (XPS) showed that the oxidation state of vanadium inα'-NaV₂O₅ was +4.5. The structural properties of the material were also studied by Raman scattering. The formation mechanism of theα'-NaV₂O₅ nanorods was proposed. It is showed that the presence of F- ions was crucial for the formation ofα'-NaV₂O₅ nanorods. Without F- or in the presence of other haloid anions such as Cl- and Br-, onlyα'-NaV₂O₅ flakes could be obtained. α'-NaV₂O₅ nanorods exhibited a discharge capacity of 118 mAhg-1 in the potential window of 1.0-3.5 V.α'-NaV₂O₅ flakes had a smaller discharge capacity, but showed much better capacity retention ability. Comparing to those of transition metal vanadate materials, such as CuV₂O₆ and Ag₂V₄O₁₁, the discharge capacity ofα'-NaV₂O₅ was much smaller because of its relatively higher vanadium oxidation state of +4.5. But, the material exhibited excellent capacity retention. This was attributed to its good strcutrural stability during charge/discharge cycling.α-CuV₂O₆ nanowires were prepared by hydrothermal process under 210oC. TEM showed that the material was in nanometer scale, with the width about 100 nm and the length of 5μm. HRTEM and FFT revealed that theα-CuV₂O₆ nanowires were highly crystallized, growing along the <020> axis. The local structural properties of the material were further investigated by Raman scattering and FTIR. In addition, the oxidation state of different elements in the material was confirmed by XPS.Electrochemical study showed that theα-CuV₂O₆ nanowires had a high discharge capacity of 425 mAhg-1 in the potential window of 1.5-4.0 V with the current density of 60mAg-1, corresponding to 4.2 mol of Li+ intercalated into the material lattice. However, the material exhibited poor capacity retention, which only showed a capacity of 150mAhg-1 after 20 cycles. CV analysis showed the Cu²⁺/Cu⁺, V⁵⁺/V⁴⁺ and Cu⁺/Cu, and V4+/V3+ redox couples in the first cycle. However, only V5+/V4+ redox couple was observed in the second cycle. The chemical diffusion coefficients of material were as small as 10-12 cm²s^-1 based on PITT and EIS analysis. In addition, EIS also showed the formation of SEI film on the material particle surface after the first cycle. Based on these studies, the poor capacity retention of theα-CuV₂O₆ nanowires was attributed to the following three reasons: the absence of Cu2+/Cu+ and V4+/V3+ redox couples with cycling; the formation of SEI film; and the low chemical diffusion coefficient of the material.In the last chapter, we prepared Li_0.86V_0.8O₂ crystals by hydrothermal synthesis at 200\oC. XRD analysis showed that the quenching process is necessary for the preparation of Li_0.86V_0.8O₂. SEM photograph showed that the material had an octahedral morphology with particle size about 300 nm. The material had an layered rock salt structure. However, the (003) diffraction of the material was very weak, due to the Li/V site disorder. The Li/V site disorder was also confirmed by FTIR and magnatic experiements. In addition, XPS analysis showed that most of the vanadium ions in Li_0.86V_0.8O₂ were V₄⁺, only with a slight amount of V³⁺ ions.Li_0.86V_0.8O₂ showed a discharge capacity about 113 mAhg-1 in the first cycle, which is much better than that of previously reported LiVO2 and Li_0.86V_0.8O₂. But, the capacity retention of the material was still unsatisfying, which was only 80 mAhg-1 after 20 cycles. CV analysis showed irreversible electrochemical process in the intial stage of cycling. In addition, an irreversible phase transformation was observed by XRD after several cycles. Based on these observations, the bad cycling performance of Li_0.86V_0.8O₂ was attributed to the continuous Li/V site disorder and the irreversible phase transformation during charge/discharge cycling.The poor capacity retention ability is a major obstacle to the practical applications of vanadate cathode materials. The new findings presented in this thesis are useful for further applications of vanadate compounds in rechargeable lithium batteries.