Synthesis And Modification Of Na₃V₂（PO₄）₃ As Cathode Material For Sodium-ion Battery

As a sodium ion cathode material,Na3V2(PO4)3 with NASICON structure has a three-dimensional channel,which not only facilitates the rapid migration of sodium ions,but also relieves volume deformation.However,due to its own structure,the electronic conductivity is limited.In this paper,the electrochemical performance was improved by optimizing the synthesis process,changing the morphology and doping.The structure,microstructure and electrochemical properties of the improved Na3V2(PO4)3 cathode materials were systematically research.The Na3V2(PO4)3/C cathode material was synthesized by hydrothermal assisted sol-gel method.The process parameters(reaction temperature of 190℃,reaction time of 40 h,NVP:C=1:1.75,NVP:NaOH=1:1.2)were determined as the best process parameter.The hydrothermal method is helpful to refine the particles and increase the specific surface area of the material.Under this condition,the capacity of the Na3V2(PO4)3 sample at 0.2 C was 105.6 mAh/g,showing excellent cycle performance.A porous structure of Na3V2(PO4)3/C cathode material was prepared for the first time using polystyrene microspheres.The effects of addition order and addition amount on the structure,morphology and electrochemical performance were of NVP were studied.It was found that sample(VNVP solution:VPS=1:3)had a discharge specific capacity of 92 mAh/g,and the capacity retention rate was 94.48%after 500 cycles.Appropriate amount of templating agent increases the contact between the active material particles and the active material and the electrolyte,uniformly coating the surface of the positive active material,improving the electrical conductivity of the material.Na3-xMxV2(PO4)3/C(M=Li,K)composites were investigated.K ion acts as a functional supporting ion,broadens the diffusion path of Na ions,and can alleviate the deformation pressure during deintercalation,Na2.9K0.1V2(PO4)3/C sample has the best electrochemical performance and structural stability.The initial discharge specific capacity was 107.7 mAh/g at 0.2 C.The proper Li ion doping helps to reduce particle size and activatesthe Na(2)site activity.The initial discharge specific capacity of Na2.8Li0.2V2(PO4)3/C electrode at 0.2 C was 106.9 mAh/g,and the capacity retention rate is 99.82%after 100 cycles.Even at 20 C,the discharge specific capacity was still maintained at 102.0 mAh/g.Na3V2(PO4)3/CN-x cathode material was prepared by hydrothermal assisted sol-gel method.The proper amount of N doping can increase the defects of C layer,reduce the charge transfer resistance,and improve the electronic conductivity.When nC3N6H6:nNVP is 2,the sample exhibited an initial discharge specific capacity of 111.4 mAh/g at 0.2 C.After 70 cycles of different discharge rate(0.2 C-20 C),the discharge capacity can be restored to 111.2 mAh/g at 0.2 C.