Modifications Of Cation Doping And Carbon Coating Upon Vanadium Penoxide And The Electrochemical Studies

In the past decades,vanadium oxides have been considered one of the most promising candidates for lithium-ion batteries with potential application value.Among them,the V₂O₅ with layered structure and the vanadium bronze with 3D tunnel structure have received much attention.The vanadium bronze(?-Na_0.33V₂O₅)possesses 3D tunnel structure,which is prepared by doping a higher amount of metal ions such as Ag+,Li+,Na+ and K+ into the V₂O₅ to improve its structure stability and electronic conductivity.It could deliver the theoretical specific capacities of 294 mAh g-1 and 245 mAh g-1 in the voltage range of 1.5-4.0 V and 2.0-4.0 V,respectively,corresponding to 2 and 1.67 electrons reaction.While V₂O₅ could achieve the theoretical specific capacities of 441 mAh g-1 and 294 mAh g-1 in the voltage range of 1.5-4.0 V and 2.0-4.0 V,respectively,corresponding to 3 and 2 electrons reaction.However,lower electronic conductivity and ion diffusion coefficient renders V₂O₅ not satisfactory to meet the demand of fast charge and discharge.Therefore,to resole these problems,we carried out the following work in this work:?1?doping metallic cations into the V₂O₅ material,which will stabilize its crystal structure during lithium insertion/extraction,and the formation of high-energy V-O-M bond can partially replace the V-O-V bond to achieve stable lattice.?2?preparation of nanomaterials,which can shorten the Li+ diffusion distance,provide more sites for Li+ ion intercalation,and buffer the change of crystals during charge and discharge.?3?surface coating with carbon layer,which could physically separate active materials from the electrolyte to decrease their dissolution,and accelerate electrons' transfference.The main contents were listed as following:?1?The vanadium bronze(?-Na_0.33V₂O₅)was prepared by a feasible hydrothermal method.The effect of calcination temperature on the properties of the prepared materials was investigated in this work.Among three samples,the sample that prepared by annealing the precursor at 400 ? displayed the best electrochemical properties.The sample with spherical morphology has a diameter of about 30 ?m,which is stacked by diamond pieces.It could deliver an initial capacity of 235 mAh g-1 at 0.1 C rate within the potential range of 2.0-4.0 V,and retained 81.8% of its own initial capacity after 50 cycles.The average capacity decay per cycle is 0.3%,0.36%,0.5% and 0.7% at the rates of 0.05 C,0.1 C,0.5 C and 1 C,respectively.The lithium ion diffusion coefficient calculated from the AC impedance is 8.43×10-12 cm2 s-1.What's more,it also could deliver a capacity of 283 mAh g-1 at 0.1 C rate within the potential range of 1.5-4.0 V,and the capacity retention was about 95% after 50 cycles.?2?V₂O₅ nanoparticles that obtained by gel-sol method were coated by carbon layer,which is formed by pyrolysis of the PMAA,to prepare V₂O₅@C nanorods.The PMAA was prepared by free radical polymerization on the surface of V₂O₅ nanoparticles.It is found that the as-prepared uniform nanorods display about 20-40 nm in thickness,100-150 nm in width and 200-300 nm in length.Especially,impurities are scarcely observed,implying the coating layer sticks to the nanorods firmly.The electrochemical properties of the as-prepared V₂O₅@C nanocomposite were investigated in the hybrid aqueous electrolyte of 1M CoSO₄/saturated LiNO₃.The initial discharge specific capacities are 122,118,115 and 113 mAh g-1 at the rates of 10 C,20 C,30 C and 40 C,respectively.After 1000 cycles,the corresponding capacity retentions are about 104%,96%,82% and 79% of the first discharge capacity.The results indicated that Co²⁺ in the electrolyte are doped into the interlayer of V₂O₅ during the charging/discharging process,which is proved by the XPS and EDX analyses.It is due to interlayer doping of Co²⁺ ions into V₂O₅ and surface coating of carbon,the electrochemical performances of the obtained sample are improved dramatically.Meanwhile,the interlayer doped Co²⁺ stabilized the layer structure of V₂O₅ thus prevented the collapse of V₂O₅ layered structure effectively.In addition,surface-coated carbon layer alleviated effectively the dissolution of the V₂O₅ nanorods into the aqueous electrolyte.?3?By using the PMAA microgel which has unique three-dimensional network spaces and negatively-COO-groups,as the template,the precursor VO²⁺ with positive charge was introduced into the PMAA microspheres by electrostatic attraction.After calcined at a high temperature,the PMAA was carbonized to form a matrix,which packed the generated V₂O₅ nanoparticles,and finally get the V₂O₅@C porous microspheres.In order to further stabilize the structure of V₂O₅,Mg²⁺ ions were chosen to dope the V₂O₅ because their radius?0.86 ??is similar to that of Li+ ions?0.76 ??,and finally MgxV₂O₅@C microspheres were prepared.Among them,Mg_0.1V₂O₅@C microspheres have the best electrochemical performances.The microspheres have a particle size of about 10 ?m,and are build up with submicron-sized particles with an average particle size of 200 nm.The submicron-sized particles are composed of uniform primary nanoparticles?10 nm?,which embedded in the carbon matrix.The Mg_0.1V₂O₅@C microspheres possess a specific surface area of 23.5 m2 g-1,and could deliver the initial specific capacities of 261 and 166 mAh g-1 at the current densities of 0.03 A g-1 and 6 A g-1 in the voltage range of 2.0-4.0 V,respectively.This sample could retain 94% of the initial capacity after 600 cycles at the 1.5 A g-1 current density.In addition,within the voltage range of 1.5-4.0 V?vs.Li⁺/Li?,its cycling performance and rate capability are still satisfied.The results suggested that combination of carbon coating and cationic doping can effectively improve the electrochemical performances of the V₂O₅ cathode material.