Controllable Synthesis,Modification And Electrochemical Performance Of Micro-nano Structured Na₃V₂?PO₄?₂F₃ As Cathode Materials For Sodium Ion Battery

Sodium-ion battery,a new energy storage technology,has been rapidly developing due to many advantages of sodium resources such as abundant reserve,wide distribution and low price.However,the larger radius and relatively high potential of sodium ion compared with lithium ion,so it is a challenge to the requirement of electrode materials including structure stability and energy density for sodium ion batteries.The cathode materials play a crucial role in the improvement of energy density.At present,the research of positive materials mainly refers to oxides,polyanionic compounds and Prussian blue analogues.However,limited capacity and low voltage plateaus hinder improvement of energy density,power density and practical application of sodium ion batteries.Compared with other types of materials,polyanionic materials have strong structural stability due to strong X-O bonds of?XOm?n-?X = P,S,Si?,and the extremely strong induction effect of F,which makes them have a higher operating voltage.Meanwhile,compounds with NASICON structure not only possess the advantage of polyanion,but also have strong sodium ion mobility.Therefore,this material is potential cathode material for sodium ion batteries.Recently,Na₃V₂?PO₄?₂F₃ with NASICON structure has attracted much interest in virtue of stable structure,high average voltage of 3.95 V,high theoretical capacity and theoretical energy density(128 mA h g^-1 and 507 W h kg^-1).Also it has high ion conductivity.However,the poor electronic conductivity of Na₃V₂?PO₄?₂F₃ limits the increase of rate performance.In order to solve these problems,Na₃V₂?PO₄?₂F₃ micro-nano structure has been synthesized by hydrothermal method followed metal ion doping and carbon coating which alter the intrinsic structure and increase the electronic conductivity of electrode materials respectively.To reveal the growth mechanism of the crystal,the structure and morphology has been characterized by various techniques including X-ray diffraction,transmission electron microscopy,scanning electron microscopy,X-ray photoelectron spectroscopy,Raman spectroscopy and thermogravimetric analysis.The corresponding electrochemical performance has been investigated by charge-discharge cycling,cyclic voltammetry and electrochemical impedance spectroscopy.Therefore,we have stablished effective relationship between structure and electrochemical performance.The specific works are as follows:?1?Hierarchical Na₃V₂?PO₄?₂F₃ hollow microspheres consisting of nanoparticles and nanoplates have been synthesised by one-step polyol-assisted hydrothermal method.The effect of solvent on the morphology of Na₃V₂?PO₄?₂F₃ hollow microspheres was investigated by tuning the concentration of tetraethylene glycol and the relative content of citric acid.The results show that when the volume ratio of water to TEG is 1:4 and citric acid is added,a hollow microsphere structure is formed.In addition,we also have investigated the effect mechanism of hierarchical Na₃V₂?PO₄?₂F₃ hollow microspheres based on time envolution reaction.Furthermore,the behavior of sodium storage of hierarchical Na₃V₂?PO₄?₂F₃ hollow microspheres was explored.The results indicate that the electrochemical properties of the hierarchical hollow microspheres were superior to that of bulk and nanoparticles.?2?To improve the problems of poor electronic conductivity of hierarchical Na₃V₂?PO₄?₂F₃ hollow microspheres,the dissertation has investigated different dose of Mn doping into V site of Na₃V₂?PO₄?₂F₃ based on the hierarchical Na₃V₂?PO₄?₂F₃ hollow microspheres.The result indicates that the rate performance is the best when the content of Mn2+ is 0.05 for Na3V2-xMnx?PO4?2F3?x = 0,0.01,0.05,0.07?compounds.And then the surface coating was performed by chemical vapour deposition?CVD?technique.Finally,we have synthesized hierarchical Na3Vi.95Mn0.05?PO4?2F3@C hollow microspheres with uniforml conductive carbon layer of 5 nm.The discharge capacity of Na3Vi.95Mn0.05?PO4?2F3@C is 122.9 mAh g^-1.After 500 cycles,the capacity remains at 109 mA h g^-1 and the coulombic efficiency is 99.1%.We further calculated the sodium ions diffusion coefficient in the solid phase by EIS combined with simulated equivalent circuit.The results indicate that Na3V1.95Mn0.05?PO4?2F3@C has lower charge transfer resistance and higher sodium ion diffusion coefficient than those of Na₃V₂?PO₄?₂F₃.?3?The micro-nano structure Na₃V₂?PO₄?₂F₃ with different morphology has been synthesized by hydrothermal method combining concentrated nitric acid.After that,the electrochemical performance also has been investigated.The results indicate when the content of concentrated nitric acid is 4 mL,the microflower structure Na₃V₂?PO₄?₂F₃ composed of micro-sheets is obtained.The initial discharge capacity is 116.5 mA h g^-1 and maintains at 94.6 mA h g^-1 after 200 cycles.