Nanostructured vanadium-based cathodes for rechargeable lithium ion batteries

• Main Author: Rui, Xianhong
• Other Authors: Alex Yan Qingyu
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Chemical engineering::Industrial electrochemistry; DRNTU::Science::Chemistry::Inorganic chemistry::Synthesis
• Online Access: https://hdl.handle.net/10356/58890
• Institution: Nanyang Technological University

Rechargeable lithium ion batteries (LIBs) with high specific volumetric and gravimetric energy densities are seen to be the most promising electrochemical energy storage systems for wide applications in portable electronic devices, electric vehicles (EVs), and hybrid electric vehicles (HEVs). However, with increasing demands in electronic industry, it is highly desirable to design LIBs capable of higher energy storage, faster lithium charge/discharge, and lighter weight. Cathode materials, which are the limiting factor in improving the capacities of LIBs, are strongly in need of further research study. The host materials with a layered or open structure are favorable for fast and reversible lithium intercalation/deintercalation. Amongst them, vanadium-based compounds (e.g., VO2(B), V2O5, Li3V2(PO4)3) with oxidation states ranging from V5+ to V3+ have attracted much attention recently. Unfortunately, their electronic and ionic conductivities are quite low, limiting the power capabilities of LIBs. The charge transfer kinetics can be largely improved in nanoscale electrodes, which offer a range of unique advantages over their bulk counterparts including shorter lithium ion transport distance, larger contact area between the electrode and electrolyte, and facile strain relaxation upon repeated lithium uptake and removal. Firstly, uniform carbon-coated VO2(B) nanobelts are synthesized by a simple hydrothermal reduction method using sucrose as both the reducing agent and carbon source. The one-dimensional nanobelt growth is attributed to the anisotropic structure of VO2(B) with fast growth along the [010] direction. On the other hand, the thickness of carbon shell can be tuned from 3.0 to 6.9 nm by changing the amount of sucrose in the reaction. Although carbon can enhance the electrical conductivity of the VO2(B) nanobelts, too much carbon would block Li+ diffusion. The electrochemical tests indicate that the VO2(B) nanobelts with a carbon content of 6.6 wt% (carbon thickness: 4.3 nm) exhibit highest capacities, and best rate capabilities (e.g., 100 mAhg-1 at 12.4 C). To further improve the reversible specific capacity and working potential, V2O5 active materials are extensively investigated. Hydrated V2O5 (V2O5•0.44H2O) nanobelts are initially prepared by a facile hydrothermal method with the assistance of ammonium dihydrogen phosphate (NH4H2PO4). These nanobelts are highly flexible with lengths of up to several hundred micrometers. Binder-free bulky paper electrodes are then fabricated by oven drying of these flexible nanobelts. The flexible hydrated V2O5 cathodes deliver high reversible capacities (e.g., 163 mAhg-1 at 6.8 C). Afterwards, ultrathin V2O5 nanosheets and reduced graphene oxide supported porous V2O5 spheres consisting of nanoparticles (V2O5/rGO) are synthesized to further improve the charge transfer kinetics of V2O5. Ultrathin V2O5 nanosheets with thicknesses of 2.1-3.8 nm are prepared by a liquid-phase exfoliation method.The thickness of the V2O5 nanosheets allow fast Li+ diffusion and electron transportation, resulting in remarkable power and energy densities (e.g., 117 mAhg-1 at 50 C). V2O5/rGO (rGO content: 46 wt%) hybrids, prepared by a facile solvothermal approach followed by heat treatment exhibit a reversible capacity of 128 mAhg-1 at 13 C. Furthermore, high-voltage Li3V2(PO4)3 cathodes are also investigated. Li3V2(PO4)3 nanocrystals (5-8 nm) embedded in a nanoporous carbon matrix attached onto rGO sheets were fabricated by a sol-gel process followed by heat treatment. Such unique nanostructure allows fast Li+ and electron transfer, allowing the electrode to display a capacity of 109 mAhg-1 at a high rate of 30 C when cycled between 3.0 and 4.8 V.