Synthesis And Electrochemical Properties Of Vanadium-based Compounds And Their Carbonous Composites

Today,people become more and more dependent on energy,and electrochemical devices used for energy storage play an increasingly important role in making up for the depletion of non-renewable energy.And supercapacitors are widely used in transportation,military and aerospace fields.Electrode materials are one of the key factors for the performance of electrochemical energy storage devices,Therefore,the development of electrode materials with high specific capacity,high stability and high power density is still a research hotspot in recent years.Vanadium oxide and ammonium vanadates are a type of electrode material that has been widely studied.There are many advantages of high specific capacity,wide working voltage window and so on for vanadium oxide and ammonium vanadates,which make them excellent energy storage materials.Nevertheless,vanadium oxide and ammonium vanadate electrode materials still have shortcomings in terms of electrons,ion transmission rate and cycle stability,which greatly limits their large-scale commercial applications.In this thesis,vanadium oxide and ammonium vanadate nanomaterials were synthesized and modified by cladding,compositing and other methods,in order to improve their electrochemical properties such as electrical conductivity,ion transport efficiency and stability.The main research contents are as follows:(1)B-type vanadium dioxide(defined as VO2(B))nanobelts were synthesized through using commercial ammonium metavanadate,oxalic acid via one-step hydrothermal technique.The VO2(B)nanobelts as supercapacitors electrode exhibited a large voltage window(-0.8-1.0 V)in Na2SO4 electrolyte.The measured capacitance was based on the pseudocapacitance.When the discharge current density was 0.5,1 and 10 A·g-1,the VO2(B)showed the specific capacitance of 287,246 and 222 F·g-1,respectively.(2)The method of carbon coating was used to synthesize the VO2(B)@C composites with glucose as the carbon source by hydrothermal method,and the electrochemical performance of the composites as electrode material for supercapacitor was studied in LiCl electrolyte.The carbon shell in the composite material reduced the charge transfer resistance of the composite material,but it also hinderd the rapid transmission of ions.The specific capacity of VO2(B)@C composites was 696 F·g-1 at 1 A·g-1,which was higher than that of VO2(B),and the cyclic stability was also improved compared with pure phase VO2(B).The electrochemical performance of the flexible supercapacitor device composed of composites and activated carbon was studied.In addition,the influence of electrolyte on the electrochemical performance of the device was also studied.(3)The VO2(B)/rGO composites were synthesized by hydrothermal method,graphene oxide(GO)was reduced to reduced grapheme oxide(rGO),and the electrochemical performance of the composites as electrode material for supercapacitor was studied.The addition of GO not only greatly increased the specific capacitance of the material,but also improved the stability of the cycle.The specific capacity of VO2(B)/rGO composites was 915 F·g-1 at 0.5 A·g-1,which was higher than that of VO2(B),and the cycle stability was greatly improved compared with pure phase VO2(B).The composites and activated carbon were assembled into a flexible supercapacitor device with good flexibility.After 2000 cycles,the capacity retention rate of VO2(B)/rGO was 90%.(4)(NH4)2V4O9 sheets were synthesized via one-step hydrothermal technique.(NH4)2V4O9 sheets exhibited electrical storage based on the faradaic mechanism with excellent capacity and rate capability.The voltage window of(NH4)2V4O9 sheets was-0.6?0.8 V in Na2SO4 electrolyte.Specific capacitance of(NH4)2V4O9 sheets were 249,230,221,212,190,174 and 147 F·g-1 at the current density of 0.5,1,2,3,5,10 and 20 A·g-1.(5)The(NH4)2V4O9/rGO composites were synthesized by hydrothermal method.The rGO in the composites not only reduced the charge transfer resistance of the composite material,but also increased the ion transfer rate of the material.The specific capacity of(NH4)2V4O9/rGO composites was 348 F·g-1 at 0.5 A·g-1,which was higher than that of(NH4)2V4O9,and the cycle stability was also greatly improved compared with pure phase.The composites and activated carbon were assembled into a flexible supercapacitor device with excellent stability and flexibility which exhibited a specific capacity of 516 mF·cm-2 at 0.5 mA·cm-2.