Vanadium-Based Nanostructured Materials Or Nanocomposites As Cathodes For Lithium Batteries

My research is focused on the systhesis of vanadium-based cathode materials with superior electrochemical performance for lithium batteries. Nano-structured or nanocomposites of vanadium-based cathode materials exhibit enhanced properties, such as specific capacity, cycling stability and rate capability.The main accompalishments of this work are:(Ⅰ) the synthesis of V2O5-carbon cryogel nanocomposites, nano-structured V2O5 particles and the evaluation of their electrochemical performance; (Ⅱ) developing nanorod-structured LiV3O8, nanosheet-structured LiV3O8 as cathode materials for lithium batteries application; (Ⅲ) the fabrications of Li3V2(PO4)3/C nanocomposite, Li3V2(PO4)3 nanobelt and their application as cathode materials for lithium ion batteries.(1) The nanocomposites have been successfully prepared by electrodepositing hydrous vanadium pentoxide onto the scaffold of carbon cryogels. As-fabricated electrodes have much higher specific discharge capacity and better cycle stability than V2O5·nH2O thin film. It delivers a specific discharge capacity of 280 mAh g-1 at the current density of 100 mA g-1. Also it exhibits excellent cycle stability at various current densities. The results demonstrate that thus-fabricated V2O5·nH2O-carbon cryogel nanocomposite has enhanced electrochemical performance.(2) V2O5 nanoparticles can be prepared by thermal decomposition of self-made VOC2O4-nH2O precursor which was obtained by reacting V2O5 with oxalic acid. As-synthesized nanorod particles display uniform morphology, and are well-separated with large space between each particle. The specific discharge capacity of nanostructured electrodes is two times higher than conventional micro-sized electrodes. The electrochemical performance of the nanostructured materials can be further optimized by adjusting the percentage of conductive carbon in the electrode and by changing the molar ratio of the starting reagents. Under the rate of C/2, it shows an initial specific discharge capacity of 270 mAh g-1 and good capacity retention only with 0.32% capacity fading per cycle. Even discharging at a rate of 4 C (1.17 A g-1), a specific discharge capacity of 198 mAh g-1 can be reached.(3) LiV3O8 nanorods have been synthesized by a template-free and low temperature method, the diameter of which ranges from 30 to 150 nm and stacking defects inside the rod structures are revealed by the TEM technology. It exhibits the specific discharge capacity of 320 mAh g-1 and 239 mAh g-1 at the current densities of 100 mA g-1 and 1 A g-1, respectively. It also shows good capacity retention, only with 0.23% capacity fading per cycle. The specific discharge capacity is much higher than that of traditional method prepared LiV3O8 electrode. This electrode shows the best rate performance among the results ever published, which indicates our thermal decomposition method prepared electrode not only cost-effective, but also has superior electrochemical properties.(4) The LiV3O8 with nanosheet-structured was first time synthesized by adding polyethylene glycol (PEG) as structural modifier to the liquid solution, followed by calcinations at various temperatures. Thus-fabricated LiV3O8 crystalline have excellent cycle stability and high specific discharge capacity. A specific discharge capacity of 260 mAh g-1 can be obtained at 100 mA g-1. No capacity fading has been detected within 100 cycles. The excellent electrochemical behavior has been attributed to the novel nanosheet structures. The results suggest the morphology of LiV3O8 plays a key role in the final products performance.(5) The nanocomposite was successfully synthesized by incorporating Li3V2(PO4)3 liquid precursor solution into the mesoporous carbon and calcinating at high temperature under the protection of inert atmosphere. The diameter of as-synthesized Li3V2(PO4)3 nanoparticle is less than 50 nm and the particles are well-dispersed within the mesoporous carbon substrate. A reversible specific discharge capacity of 122 mAh g-1 is obtained at 1C rate between 3 and 4.3 V vs. Li/Li+. Even at 32 C, it still delivers a specific discharge capacity of 83 mAh g-1. As far as we know, this is the best rate performance for Li3V2(PO4)3 materials ever reported. Because the growth of Li3V2(PO4)3 within the mesoprous carbon has been effectively limited during the high temperature calcinations process and the active material has good attachment with the carbon matrix, the nanocomposite shows superior behavior.(6) Combining the advantages of solid state reaction and self-assembling, Li3V2(PO4)3 particles with unique nanobelt structure are first time synthesized in molten organic media through one-step solid reaction. The thickness of the nanobelt is around 50 nm and the broadness of the nanobelt is about 200 nm. The nanobelt-structured Li3V2(PO4)3 electrode exhibits a specific discharge capacity of 131 mAh g-1 at 1 C, which is very close to the theoretical one. It also has superior cycle stability. The good electrochemical performance is attributed to the following reasons: (a) reduced lithium ion diffusion and electron transportation distance because of the nanoscale particle size; (b) mesoprous structure facilitates the electrolyte penetration and increases the interfacial surface area between electrode and electrolyte; (c) the residual carbon produced from the decomposition of carbon increase the conductivity of the materials.