Preparation And Study Of Vanadate-based Nanostructured Materials For Lithium Storage

Rechargeable lithium-ion batteries (LIBs) are expected to contribute significantly to large-scale energy storage system such as electric vehicles, smart grids. Accordingly, that intensifies a trend toward the search for desirable electrode materials, particularly advanced cathode materials capable of satisfying the commensurate expansion in energy density and power density. Vanadium-based host materials have been highlighted as alternative cathodes for a promising solution to that issue due to their advantageous features, such as high lithium intercalation capacity, low cost together with easy synthesis. To date, the main focus of vanadium-based materials for Li+ intercalation research on the vanadium oxides such as V2O5, VO2 and vanadates based on the incorporation of alkali metal such as UV3O8, NaV3O8, etc. For the latter, in particular, intercalation of alkali-metal ions in V-O layered frameworks can contribute to the structure stability and show a better structural reversibility. Therefore, in this thesis, a novel two-step approach combining room temperature solid-liquid phase arc discharge (SLPAD) technique and the following hydrothermal treatment was designed to fabricate LiV3O8 and NaV6O16·3H2O. When evaluated as Li+ intercalation materials, they suffered from inferior cyclability and insufficient rate capability, which is a common problem for vanadium-based host materials and consequently remains far from matching the demand for further applications. To address that issue, we propose intercalation of divalent alkaline-earth metal ions in vanadium oxide layered frameworks can effectively resist structural instability, that is vanadates based on the incorporation of alkaline-earth metal, which has not attracted much attention of academic research. Herein, we incorporated alkaline-earth metals as interlayer materials within the vanadium oxide tunnel framework, leading to a whole family of hydrated alkaline-earth metal vanadate minerals with a general formula MV6O16·nH2O (M= Mg, Ca, Sr, Ba), which are proposed as potential Li-intercalated materials for the first time in this thesis and have never been reported in the literature. In this thesis, we firstly fully studied the two such candidate, that is CaV6O16·3H2O、MgV6O16·9H2O. The major research results were concluded as follows:showed ultra-thin nanosheets morphology with a thickness of～10nm, NaV6O16·3H2O showed super-long nanoribbons with a typical length of up to hundreds of micrometers. Based on the research results, the initial specific discharge capacity of LiV3O8 reached 313.1 m A h g-1 and only yielded a retained capacity 149.5 m A h g-1 after 30 cycles, corresponding to capacity retention 47.7%, showing poor cyclic stability. Crystal waters play an important role in Li+ intercalation/deintercalation electrochemistry for NaV6O16-3H2O. Thermal treatment could effectively improve the cycling performance, although the discharge capacity was sacrificed to some extent. For example, the capacity retention remained only 70% after 20 cycles with a initial discharge capacity 235.7 mA h g"1, but showing superior to LiV3O8, which could be due to enlarged layer spacing resulted from larger ionic radius of Na+ compared to that of Li+. After thermal treatment under 450℃, although the capacity was reduced to～200 mA h g-1, the sample exhibited more outstanding cycling performance with a high capacity retention 96.6% after 50 cycles, which can be ascribed to enhanced crystallinity as a result of complete dehydrations after heat treatment.CaV6O1643H2O and MgV6O16·9H2O displayed ultralong ribbon-like nanostructure with a length of hundreds of micrometers even up to several millimeters, whose lithium-storage properties were investigated in details and the results showed superior cyclability integrated with excellent rate capability when they were evaluated as cathode materials for LIBs. The CaV6O16·3H2O electrode can stand up to ultra-high current densities even at 6 and 10 A g-1 with considerable reversible capacity of 103,78 mA h g-1. Furthermore, the cells were cycled up to 1000 cycles under high current rates of 2,6 A g-1 and the capacity retention reached up to 83.6%,89.5%, respectively. This electrode presented here shows optimal cycling stability for vanadate-based cathode materials for LIBs ever reported. The MgV6O16·9H2O electrodes were investigated at the high current rates of 1000 and 2000 mA g"1, the discharge capacity still remained high at 132,105 mA h g-1, respectively. Moreover, to demonstrate the cycling performance, the electrodes were tested at 100、200、500、800、1000mA g-1, respectively, no capacity fading was observed over 100 cycles. Analysising the crystal structure, we concluded that, based on the superiorities of ultralong ID architecture, these vanadates based on the incorporation of alkaline-earth metal exhibited excellent long-term cyclability integrated with high-rate kinetics afforded by a synergistic effect between intercalated Ca ions and water molecules within the vanadium oxide layered framework.Divalent alkaline-earth metal ions substitution for alkali metal ions in vanadium oxide layered framework could induce a better electrochemical response. This work not only open up a new sub-group of vanadium oxide bronzes with application potential for LIBs, but also auspiciously suggests that broad classes of outstanding vanadium-based materials for lithium storage could be discovered.