Preparation And Energy Storage Mechanism Of Vanadium Nitride Anode Materials

Supercapacitor is an electrochemical energy storage device with high power density,high charge and discharge rate,and good cycle stability,and the development of advanced electrode materials is the main means to improve the electrochemical performance of supercapacitors.As a very important anode material for supercapacitors,vanadium nitride?VN?has been widely used for its wide potential window?-1.2?0 V under alkaline aqueous condition?and high specific capacitance(1,340 F g^-1).However,some shortcomings of VN electrode material also limit its further development.For example,under alkaline aqueous conditions,the advantages of vanadium nitride as a supercapacitor electrode material can be best demonstrated,but its cycle stability is very poor;vanadium nitride needs to be combined with carbon materials to improve its stability and specific capacitance,but it brings the problem of voltage drop;the most intrinsic charge storage process of vanadium nitride is still in the guessing stage,and no clear mechanism has been proposed so far.This dissertation focuses on the above problems and mainly includes the following four parts:?i?The electrochemical stability of vanadium nitride is poor,and serious capacitance degradation occurs under a short charge-discharge cycle.Carbon material coating is an effective method to improve the cycle stability of electrode active materials.Chitosan is a widely used natural biomass material,which can be assembled with surfactant F-127 into a special structure system.Using biological sources formed from chitosan,F-127 and ammonium metavanadate to prepare metal-organic nanomaterials as precursors,to prepare carbon-coated vanadium nitride/carbon composite electrode materials to improve their electrochemical performance,and especially the cycle stability.The results show that the specific capacitance of vanadium nitride prepared by this strategy is 300.4 F g^-1,and the specific capacitance can maintain 71%of the initial specific capacity after 5000 cycles which is better than the traditional VN materials.?ii?The oxide layer on the surface of vanadium nitride also has a great influence on its cycle performance,because some valence states of vanadium(the most obvious is the oxide containing V⁵⁺)will generate soluble salts under alkaline conditions and destroy the cycle stability.The imidazole-vanadium-metal organic framework structure was synthesized and used as the precursor to prepare vanadium nitride electrode materials;by controlling the synthesis conditions,a single oxide VO₂ layer was constructed on the surface of vanadium nitride.The results show that vanadium nitride with a single VO₂ oxide layer has a capacitance retention rate of 85%after being cycled 1000 times under the condition of 1 A g^-1.More importantly,on the basis of significantly improving cycle stability,the potential window of the vanadium nitride electrode material can still be maintained at 1.2 V.?iii?Using pectin as an adsorbent for heavy metal ions,a series of vanadium-containing precursors with different water content are prepared,and they are heat-treated to obtain a composite of VN and carbon.By studying these composites,we found that increasing the amount of water can effectively increase the oxygen content of the final composite,thereby improve the wettability or hydrophilicity of the composite material.More importantly,it is found that the voltage drop of the composite material is related to the hydrophilicity of the material.When the surface of the material is rich in chemisorbed oxygen,the material's voltage drop will be further improved by improving the hydrophilicity of the material,thereby increasing the specific capacitance of the material.?ix?In order to further explore the energy storage mechanism of VN materials,two vanadium-based metal organic framework structures with different anionic ligands as precursors are selected to further prepare materials with different impurity atoms that can increase the interplanar spacing.The prepared VN material has a higher specific capacitance,and through ex-situ XRD and in-situ Raman studies,it is found that the charge storage of VN is contributed by two processes.The first process is provided by the insertion and extraction of K⁺VN crystals,and the other is provided by the redox reaction of OH^-ions adsorbed on the surface of the VN material.According to the first-principles calculation,it is revealed that the entry of impurity atoms into the VN crystal can increase the interplanar spacing of the VN crystal,thereby improve the process of K⁺intercalation and extraction,and enhance the specific capacitance of the material.Through the energy storage mechanism,it is found from the mechanism that gel and thin-layer carbon coating can improve the cycle stability of VN materials.The reason is that VN has intercalating pseudocapacitance.The occurrence of the pseudocapacitance will cause the collapse of the VN material structure.When the outer surface is coated with a thin layer of carbon,this process can be effectively alleviated or even prevented.On the other hand,when the OH^-ion on the surface of VN undergoes a redox reaction with it,the oxide of V⁵⁺will form a soluble salt,and it can be seen through in-situ Raman that the oxide containing V⁴⁺can occurs reversible redox reactions stably.