Preparation,Modification And Sodium Storage Performance Of Na₃V₂（PO₄）₃ Cathode

Sodium has the characteristics of abundant reserves,low cost and wide distribution,so sodium ion batteries（SIBs）have received extensive attention from researchers in recent years.As one of the potential substitutes for lithium ion batteries（LIBs）,it has great potential in large-scale energy storage applications.However,sodium ion batteries are still in the research stage,mainly because the cathode materials of sodium ion batteries have low energy density and power density,and poor cycle stability,which hinders the further development of sodium ion batteries in the field of energy storage in the future.Therefore,it is particularly important to design and prepare sodium ion battery cathode materials with excellent performance.Polyanionic sodium vanadium phosphate（Na₃V₂（PO₄）₃,NVP）has become a sodium ion battery due to its three-dimensional open framework structure,high structural stability,high energy density,stable voltage platform and high thermodynamic stability.The most concerned cathode material.However,the electronic conductivity of Na₃V₂（PO₄）₃,is low and the energy density is still lower than that of LIBs.Therefore,this paper uses Na₃V₂（PO₄）₃,as the basis to study the preparation process of sodium ion batteries,and uses the electronegative anions O^2-and F^-to partially replace the PO₄ of Na₃V₂（PO₄）₃,to increase the specific capacity and platform voltage,and use Graphene encapsulates it to enhance its energy storage performance.The specific research content is as follows:（1）First,the random Na₃V₂（PO₄）₃ electrode material with an average particle size of only a few microns was prepared by hydrothermal-heat treatment,and the influence of heat treatment on its morphology,size and electrochemical performance was studied.The size of Na₃V₂（PO₄）₃ particles after heat treatment at 800℃for 8h is about 200 nm～3μm,and the phenomenon of large,medium and small particle size distribution is uniform,and the initial discharge specific capacity reaches97.6 m Ah·g^-1.In addition,the electrochemical performance of self-purchased and self-purchased Na₃V₂（PO₄）₃,the influence of self-purchased and self-purchased electrolyte on the electrochemical performance of Na₃V₂（PO₄）₃ are also compared,which is a high electronegative anion replacement for subsequent Na₃V₂（PO₄）₃ and graphite The performance research of ene-coated modified electrode materials lays the foundation.（2）Na₃V₂（PO₄）₃ was replaced by electronegative anions by a one-step hydrothermal method,and Na₃V₂（PO₄）₂O₂F electrode material was prepared.Its morphology is regular and uniform in submicron and micron quadrangular prisms.The length of the quadrangular prism is generally 1～3μm,and the width is 300 nm～1μm.The discharge platform of Na₃V₂（PO₄）₂O₂F is 3.8V,the first discharge specific capacity at 0.2C rate rises to 125 m Ah·g-1,after 100 cycles the specific capacity is still 105.8 m Ah·g^-1,the capacity remains The rate is 86%.Under 2C rate,the first discharge specific capacity of Na₃V₂（PO₄）₂O₂F drops to 68m Ah·g^-1,after 100 cycles there is still 60 m Ah·g^-1,the coulombic efficiency is greater than 98%,and the capacity retention rate is also 88%.the above.In addition,at 0℃,the first discharge specific capacity of Na₃V₂（PO₄）₂O₂F at 0.2C,0.5C and1C is 83.4,71.7 and 57.3 m Ah·g^-1,and the discharge platform is significantly lower than the room temperature discharge platform.In addition,after Na₃V₂（PO₄）₂O₂F is cycled 50 times at 0℃and 1C rate,the discharge specific capacity drops to 39.7 m Ah·g^-1.Although the coulombic efficiency basically remains above 98%,the cycle retention rate is only 69.3%.（3）Na₃V₂（PO₄）₂O₂F@rGO coated with rGO was prepared by a two-step hydrothermal method.Compared with Na₃V₂（PO₄）₂O₂F,its appearance and size are almost unchanged.Most of Na₃V₂（PO₄）₂O₂F The particles are covered by rGO flakes and connected by rGO flakes to form a3D conductive network structure.The first discharge specific capacity of Na₃V₂（PO₄）₂O₂F@rGO at0.5C and 2C are 124.4 m Ah·g^-1 and 88.4 m Ah·g^-1,respectively,and the first charge-discharge coulombic efficiency reached 90.3%and 89.4%,respectively.Under 2C,the discharge specific capacity of Na₃V₂（PO₄）₂O₂F@rGO after 200 cycles is still 78.7 m Ah·g^-1,the capacity retention rate is as high as 89%,and the coulombic efficiency is always maintained at about 99%,showing an excellent high Rate ratio capacity and cycle performance.In addition,the first discharge specific capacity of Na₃V₂（PO₄）₂O₂F@rGO at 0℃and 0.1C reached 101.4 m Ah·g^-1,even at a rate of 2C,there was still 57.3 m Ah·g-1,and the discharge capacity after 100 cycles The specific capacity is still 51.7 m Ah·g^-1,and the coulombic efficiency and capacity retention rate are as high as 89.7%and 89%,respectively,showing excellent low-temperature high-rate and long-cycle stability.