Synthesis and characterization of vanadium-based cathode materials for lithium ion batteries

Abstract

This study aims to develop facile synthesis of vanadium-based nanomaterials for improved lithium storage properties. The morphologies, crystal structures, compositions and crystallinity of the products were evaluated by a series of advanced characterization techniques. In the Ag-V-O system, multi-scale AgVO3 particles were synthesized using a facile sol-gel method. Quantitative X-ray diffraction (XRD) technique was first applied to quantify the amount of amorphous phase in AgVO3. Under the synergistic effect of amorphous content and particle size, the products obtained at 500 oC with quicker heating rate exhibited the optimal capacity and cycling stability. In addition, hydrothermal method was utilized to prepare the silver vanadates, including ultralong Ag0.33V2O5 nanoribbons, the Ag0.33V2O5@Grapheen oxide (GO) nanoribbon composite, Ag2V4O11 consisting of nanoribbons and micro-rings, and Ag/Ag2V4O11 hybrid. The systematic investigation of the formation process of silver vanadates reveals an initial nanofiber morphology in this hydrothermal reaction, which is different from the general splitting mechanism from nanosheets. Moreover, the Ag0.33V2O5@GO electrode possessed the superior cycling and rate performance due to the unique GO distribution in the Ag0.33V2O5@GO composite. Both template-assistant and template-free methods were employed to synthesize the V2O5 electrodes with hierarchical structures. With the assistance of carbon sphere templates, the V2O5 micro-flowers assembled from nanorods were successfully fabricated. The V2O5-400 micro-flowers with more oxygen vacancies exhibited better electrochemical performance than the V2O5-450 electrode, which is also confirmed by electrochemical impedance spectroscopy (EIS) results. Two types of hierarchical hollow V2O5 microspheres, V2O5-IPA and V2O5-EG, were synthesized via a solvothermal method followed by heat treatment. As cathodes for LIBs, V2O5-IPA showed highly stable lithium storage properties. It delivered a maximum discharge capacity of 128 mAh/g at 1 A/g, with a capacity retention of 92.2% even after 500 cycles. The capacity fading of the V2O5-EG electrode was severe due to the collapse of the hollow microspheres, especially at high current density. To further exploit the useable capacity in V2O5, the Al-doped V2O5 with mesoporous structure and the VO2/V2O5 composites were fabricated. The XRD results confirm the expansion of the unit cell caused by Al-doping. The contents of VO2 and V2O5 in the composites were calculated via the quantitative XRD technique. Overall, the Al0.1V2O5 exhibited a more stable cycling performance resulting from the addition of Al ions. Although VO2/V2O5-3 suffered from a relatively large fading problem at low current density, it could achieve high capacities throughout the cycles and high cycling stability at high current density. Two new alkaline earth metal vanadium oxides: Ba0.2V2O5 and Sr0.15V2O5, were successfully prepared via a sol-gel method for the first time. By ab initio structural determination, the crystal structure of Ba0.2V2O5 and Sr0.15V2O5 were solved. Sr0.15V2O5 possesses a framework of VO6 and VO5 polyhedrons. However, Ba0.2V2O5 shows a unique down-up VO5 arrangement, which can effectively support the VO6 layers. The electrochemical results confirm the superior cycling and rate performance of the Ba0.2V2O5 cathode. For Sr0.15V2O5, although it could achieve high initial capacities in the first several cycles, it suffered from the severe capacity fading problem due to the structural deterioration during cycling.