Preparation And Electrochemical Properties Of The Layered Vanadium Oxide-based Compounds

The layered vanadium oxide-based compounds have exhibited broad and potential applications in energy storage and conservation,catalysis,optical switches and sensors.The goal of this dissertation is to employ novel chemical reaction routes for preparation of hierarchical nanostructured vanadium pentoxide and various vanadium bronzes with diverse morphologies and investigate the corresponding electrochemical performance of the resultant layered vanadium oxide-based compounds.The relationship between crystal structure,morphology and performance has also been studied in this dissertation.The main details and results are summarized briefly as follows:1.Solvothermal and heat treating methods were adopted to synthesize various V₂O₅ hierarchical structures composed of nanoparticles by adjusting the solvothermal reaction duration.During the solvothermal reaction,the morphology of product changed as the treatment progress,which was believed to result from an Ostwald-ripening process.Because the hierarchical nanostructured V₂O₅ material featured excellent structural stability and could facilitate the transport kinetics,it exhibited high discharge capacity?276.3 mAh/g?,good rate capability and excellent cyclic stability.The electrode achieved 78.31%capacity retention after 80 cycles at a current density of 1200 mA/g.2.Ammonium vanadate and formic acid used as raw materials,a novel ammonium vanadium bronze with square brick-like morphology was synthesized via a simple hydrothermal treatment.The chemical formula of new phase could be expressed as?NH₄?_0.6V₂O₅ after characterization and analysis.Besides,preparation conditions and formation process of?NH₄?_0.6V₂O₅ were investigated and it could be concluded that?NH₄?_0.6V₂O₅ was more likely to be an intermediate phase between NH₄V₄O₁₀ and VO₂?D?.?NH₄?_0.6V₂O₅ square bricks were tested as cathode materials for lithium-ion batteries and the electrode showed a highly reversible lithium storage?280.2 mAh/g?inferring that Li⁺could reversibly insert into and extract from?NH₄?_0.6V₂O₅ crystalline in the organic electrolyte.3.Plate-like ammonium vanadium bronze?NH₄?_0.6V₂O₅ was prepared via a simple hydrothermal method using ammonium vanadate and LiBH₄ solution in tetrahydrofuran as reactants.Compared with the hydrothermal treatment using formic acid as reducing agent,the synthesis temperature was lower and the morphology and size of particles were smaller.Besides,as a cathode material,the plate-like?NH₄?_0.6V₂O₅ delivered a higher discharge capacity,better cycling performance and rate capability than that of the?NH₄?_0.6V₂O₅ square brick.In addition,in order to explore the additional application of novel ammonium vanadium bronze?NH₄?_0.6V₂O₅,the plate-like?NH₄?_0.6V₂O₅ was successfully transformed to pure phase VO₂?M?with small holes by adjusting vacuum degree and temperature in annealing treatment.The conversion between the two phases might be ascribed to the redox reaction from V⁵⁺to V⁴⁺by NH₃ arising from the pyrolysis of?NH₄?_0.6V₂O₅.The film made up of as-prepared flake-like VO₂?M?powder showed an excellent thermochromicity:the infrared modulation at 2500 nm was up to56.6%with a 27.89%visible transmittance.4.Square block-like?-Mg_0.25V₂O₅·H₂O was prepared by a facile hydrothermal method using vanadium pentoxide and magnesium nitrate hexahydrate as raw materials.The lithium storage property of?-Mg_0.25V₂O₅·H₂O was investigated and it exhibited high reversible capacity?294.2 mAh/g?and excellent cycling performance at high current densities,which might be attributed to the aspects that the interlayered water molecules could increase neighbor interlamellar spacing,providing enhanced diffusion channel of Li⁺,and the inserted divalent Mg²⁺bonded with oxygen atoms could generate stronger ionic bonds,better linking the neighboring layers together.The?-Mg_0.25V₂O₅·H₂O electrode exhibited an average decay of 0.053%per cycle over 400cycles at 800 mA/g.5.Nanorods of?-Ca_0.24V₂O₅·H₂O,derived from vanadium pentoxide and calcium nitrate hydrate,were fabricated by a facile hydrothermal method and could be transformed into the tunnel?geometry(?-Ca_0.24V₂O₅)through a vacuum annealing treatment.As cathode materials,both calcium vanadium bronzes exhibited high reversible capacity,good rate capability,as well as superior cyclability.Compared with the hydrated vanadium bronze,although the?-Ca_0.24V₂O₅ nanoribbons showed lower specific capacity,it depicted better cycling performance?a capacity fading of only 0.035%per cycle over 500 cycles at 500 mA/g?,which could be attributed to the structural differences of the two materials.Furthermore,the structural evolution of the two phases during lithiation/delithiation process was also investigated.The?-Ca_0.24V₂O₅·H₂O electrode converted into?phase after the first cycle,which might result from the propagation of crystallographic slip during the lithiation-delithiation process.While the?-Ca_0.24V₂O₅·H₂O electrode showed excellent structural stability during cycling,due to its rigid tunnel structure.