Preparation And Electrochemical Performance Of V, Mo Oxide Nanoarrays For Lithium Ion Batteries

Transition metal oxide materials have recently drawn much attention because their theoretical capacities are2-3times higher than traditional theoretical capacity of carbon materials, as lithium ion specific deintercalation and transformation mechanism. However, there are disadvantages limit transition metal oxide’s commercialization process, including poor conductivity, large volume expansion coefficient, and low actual capacity. Nanostructure electrode can probably solve those problems. For instance, nanoarrays growing directly on conductive substrate have gained much attention when used as lithium ion batteries electrodes. This specific infrastructure can offer many advantages, such as tight bonding between current collectors and active materials, short transport pathway lengths for both electrons and Li+ions, and large flexibility and toughness for accommodating strain introduced by Li+insertion/extraction. Transition metal oxide materials are expected to provide foundation for the development of thin film lithium battery.The main research content of this thesis is as follows: (1) Hydrothermal methods synthesis of V2O5nanobelt arrays on conductive substrate on Ti are based on calcinated VO2. V2O5nanobelt arrays assemble into foam structure with pore size of2-3μm. The effects of reaction temperature and acidity on the V2O5nanobelt arrays are studied. By changing the acid species (hydrochloric acid, phosphoric acid, acetic acid and oxalic acid), the V2O5nanoarrays (nanorods, andnanobelts, nanoplates) with different morphologies are obtained. The different products are characterized by XRD, SEM, TEM, XPS, Raman and other instruments. A proton-delivery and vanadate-hydrolysis related mechanism was proposed to explain the formation of differentiated vanadium hydrate precursors.V2O5nanostructures with different morphology are evaluated as lithium ion battery cathodes. The nanobelt arrays showed reversible capacity as high as255mAh·g-1and very good cycling stability, which benefit from higher crystallinity. While the nanoplate arrays showed good rate capabilities. We also found that surface area and the crystallinity can influence a lot of the properties of electrochemical performance of V2O5. Such results implied that higher crystallinity would benefit high rate capability.(2) Relatively low valence VO2was chosen as the sacrificial template for construction of hierarchical MoO3nanostructures. Oriented growth of MoO3nanorods was obtained under the ambience with carefully controlled pH. Tiny MoO3nanorods were observed growing on both sides of "nanowalls" with well aligned orientation, which are roughly perpendicular to the substrate. We further investigated the growth mechanism by means of SEM, XRD, Raman, EDS characterization, that is the VO2nanoarrays gradually dissolved in an acidic environment; while MoO42-could be protonated and dehydrated to form MoO3. Hierarchical MoO3nanostructures were fabricated into anode for lithium ion batteries, which exhibited the first discharge capacity up to633mAh·g-1. However, in the following cycles, capacity faded fast suggesting that this phenomenon is caused by disorder of nanorods arrangement which lead to the increase resistance.(3) The hierarchical MoO2nanoparticles which are composed of a basic unit in the diameter of about50nm were synthesized from the hierarchical MoO3nanostructures by carbothermal reduction method. The hierarchical MoO2nanoparticles could act as a high capacity anode electrode for lithium ion batteries. The phenomenon of discharge capacity displays no decay but gradual increase, may suggest the existence of activation process. After80cycles at current density of100mA·g-1, the hierarchical MoO2nanoparticles can output retention of reversible capacity of611mAh·g-1.(4) MoS2nanosheet arrays with200nm in length and5nm in thickness were synthesized by hydrothermal method. MoO3films of500nm thick, consisting of nanoparticles of50nm diameter and20nm thicknesses, were prepared by oxidation method. Compared to the nanorod arrays, capacity of lithium ion bateries anode for MoO3films remained at650mAh·g-l, after120cycles, and the coulomb efficiency was still higher than99.9%, indicating very high cycling stability.