Preparation And Electrochemical Performance Of Novel Nanostructural Vanadium Oxide Electrode Materials

Due to the successive consumption of fossil fuels and followed by severe environment pollution,it is becoming a consensus that developing clean new energies as power systems to replace fossil fuels.Electrochemical energy storage systems with high energy density,high power density,long span life and friendly environment has been a promising research point that replacing fossil fuels.Among them,lithium ions batteries are one of important representatives.The electrochemical performance of lithium ions batteries depends more on the properties of electrode materials used.Therefore,in order to apply the lithium ions batteries in energy storage systems,exploring and developing electrode materials with excellent performance is necessary.Vanadium oxide,as a transition metal oxide,processes advantages of abundant resources,low price,high theoretical capacity,power density,and energy density,especially,the unique layered structure can imbed more Li⁺to improve capacity of electrode.It is considered as one of the most potential electrode materials for LIBs.But vanadium oxide has certain defects,such as low conductivity,poor Li⁺diffusion coefficient and dissolving of electrode materials in electrolyte during charging and discharging,which would limit its commercial application.In order to improve the diffusion dynamics of Li⁺and prevent the structure collapse of vanadium oxide electrode materials in the process of charging and discharging,unique porous yolk-shell V₂O₅ microspheres have been prepared via a facile hydro/solvothermal method with subsequent annealing treatment.The formation mechanism of microstructure has been studied.It is found that the porous structure on shell of yolk-shell V₂O₅ microspheres would be beneficial to the penetration of electrolyte,and large specific surface area can increase the contact area between electrode material and electrolyte,shortening the diffusion distance of Li⁺,and improving the kinetics of Li⁺diffusion.Meanwhile,the vacant space between core and shell can accommodate the volume change during lithiation/delithiation,reduce structure stress and improve the stability of electrochemical performance.Compared with commercial V₂O₅ powders,as-prepared porous yolk-shell V₂O₅microspheres deliver excellent rate capability as cathode material for LIBs.Porous yolk-shell V₂O₅ microspheres can exhibit 225.7 mA h g^-11 at the current density of 50mA g^-11 over 50 cycles.Meanwhile,hierarchical flower-like yolk-shell amorphous V₂O₃ microspheres?a-V₂O₃?have been successfully synthesized through one-step hydro/solvothermal approach.The hierarchical shell of a-V₂O₃ is assembled by numerous ultrathin cross-connecting nanosheets,like honeycombed structure.The hierarchical urchin-like crystallized V₂O₃ microspheres?c-V₂O₃?have been obtained after calcining a-V₂O₃ in N₂ atmosphere.The hierarchical urchin-like structure on shell of c-V₂O₃ is formed by lots of nanoparticles.The specific surface areas of a-V₂O₃ and c-V₂O₃ are 137.35 and 73.97 m² g^-1,respectively.In addition,the formation process and mechanism of a-V₂O₃ have been systematic studied during hydro/solvothermal reaction.Initially,organic alcohol functional groups?-OH?may interact with vanadium oxide chemical bonds?V=O and O-V-O?to fabricate curved surface of low surface energy,which would facilitate nucleation into nanospheres.Then nanospheres transform into solid microspheres via aggregating to hold a stable state of system.Finally,solid microspheres gradually develop into the hierarchical flower-like yolk-shell amorphous a-V₂O₃ microspheres through Ostwald-ripening process from inside to outside.When a-V₂O₃ and c-V₂O₃ have been served as anode material for LIBs,respectively,the initial discharge specific capacity of a-V₂O₃ can reveal 881.5mA h g^-11 at a current density of 1000 mA g^-1,and 870.1 mA h g^-11 after 250 cycles.Thus,a-V₂O₃ is a promising anode material for LIBs.In order to improve conductivity and prevent dissolution of vanadium oxide electrode materials,in situ carbon-coated vanadium oxides microspheres have been fabricated,using PANI as carbon source,via a facile hydro/solvothermal approach combined with subsequent carbonization process.Then the structure,properties and electrochemical performance of carbon-coated vanadium oxides microspheres with different time of heat treatment have been characterized and studied.It is showed by investigating that core-shell V₂O₅/C-30 is obtained by calcining in air atmosphere for30 min,exhibiting outstanding electrochemical properties as cathode material for LIBs.For example,core-shell V₂O₅/C-30 electrode could deliver 209.1 mA h g^-11 at a current density of 50 mA g^-11 after 50 cycles,and capacity retention rate is 71.2%.Even,discharge specific capacity could exhibit 123.2 mA h g^-11 at 1000 mA g^-11 over1000 cycles.