Modification Of Vanadium Oxyphosphate Nanosheets As Cathode Material For Sodium-ion Batteries And Electrochemical Property Study

With lower price and stronger safety performance than lithium-ion batteries,sodium-ion batteries have been widely concerned by researchers.However,owing to larger radius of Na⁺ion?1.02??,de/-intercalation of sodium ion is slow in kinetic,and would cause the collapse of crystal of the layered oxides.It is necessary to exploit new suitable layered oxide materials to be used as cathode in sodium-ion batteries.Classic two-dimensional layered materials with unique properties and high specific surface area have been widely used in lithium-ion batteries,sodium-ion batteries,supercapacitors,catalysis and chemical and biological sensors.Among them,?-VOPO₄ is a kind of similar two-dimensional layer material.A part of oxygen atoms in the structure are replaced by polyanion PO₄^3-,thus the structure would be stabilized during insertion and extraction of sodium ions.Owing to expanded interlayer spacing,and nanoscale diffusion distance,it will possess excellent sodium-storing performance.It is one of the most potential cathode materials for sodium-ion batteries.In this study,main focuses upon modifying its performance include the following three aspects:?1?expanding the interlayer spacing;?2?stabilizing the structure by doping of sodium ion;?3?enhancing its conductivity by compositing with reduced graphene oxide?rGO?.?1?Layered material?-VOPO₄ was exfoliated by an ultrasonic method using some organic molecules.The organic molecules were inserted into the interlayers to expand interlayer spacing.The effect of different organic molecules on the electrochemical properties of the resultant VOPO₄ material were studied.It is found that the sample is composed of ultrathin nanosheets,and the interlayer spacing is significantly expanded.The electrochemical test results show that over the voltage range of 2.5-4.3 V,the as-prepared sample using isopropyl alcohol?IPA?as inserted agent can deliver an initial capacity of 117 mAh/g under the 0.1 C rate,and retain82%of the initial capacity after 200 cycles.At the rates of 0.2 C,1 C,2 C and 5 C,the specific capacities are 96,76,56 and 40 mAh/g,respectively.Through cyclic voltammetry analysis of at various scanning rates,sodium-ion diffusion coefficient is calculated to be 4.53×10^-13 cm²·s^-1,exhibiting a pseudocapacitive behavior during the processes of charge and discharge.?2?The layered Na_xVOPO₄ nanosheets doped with sodium ions were prepared by solvent hydrothermal method assisted by ultrasonic stripping method.The effect of doping amount of sodium ion on the electrochemical properties were investigated.The results suggest that Na_0.33VOPO₄ has the better electrochemical properties compared with other samples.It can deliver an initial capacity of 164 mAh/g at the0.1 C rate over the voltage range of 2.5-4.3 V.After 200 cycles,the capacity retention is 85%of the initial capacity,displaying a good cycliability.At the rates of 0.2 C,1 C,2 C and 5 C,the discharge capacity are 139,119,98 and 68 mAh/g,respectively.After charging and discharging at various rates,the capacity returns to 148 mAh/g at 0.1 C,displaying an excellent rate capability.After doping of sodium ions,Na-diffusion coefficient increases to 2.12×10^-1212 cm²·s^-1.The results indicate that the electrochemical properties the VOPO₄ cathode can be improved significantly after interlayer doping of sodium ions because of the expanded interlayer spacing and the stable structure.?3?Ultrathin Na0.33VOPO4 nanosheets were mixed with graphene oxide by reflux method and chemically reduced to get the NVOP/rGO nanocomposite materials.From the SEM images,it is found that with the increasing content of rGO,the primary nanosheets are aggregated to form flower-like morphology.The electrochemical test results show that the NVOP/3.8%rGO sample can deliver an initial capacity of 166 mAh/g at 0.1 C rate over the voltage range of 2.5-4.3 V,and retains 97%of the initial capacity after 200 cycles.At the rates of 0.2 C,1 C,2 C and5 C,the discharge capacities are 144,125,105 and 78 mAh/g,respectively.When the rate returns to 0.1 C,NVOP/3.8%rGO can deliver a capacity of 160 mAh/g.It has a Na-diffusion coefficient increases up to 3.24×10^-1212 cm²·s^-1.The results suggest that synergistic effect of interlayer doping and compositing with conductive rGO improves significantly the electrochemical properties of the VOPO4 when used as cathode material of sodium-ion batteries.