Study On Preparation And Modification Of Sodium Vanadium Phosphate As NASICON Cathode Material

In recent years,in addition to the boom in solar,wind,hydropower and other renewable and clean energy sources,rechargeable batteries with reliable and efficient energy storage systems have shown great potential.Among them,lithium-ion batteries occupy a dominant position in electronic products and equipment such as large power grids,plug-in hybrid vehicles,and electric vehicles with the advantages of high operating voltage,high energy density,wide operating temperature range and long cycle life.However,lithium-ion batteries also have problems such as high cost and uneven distribution.Based on the above reasons,the researchers turned their attention to sodium in the same main group as lithium,which is abundant,widely distributed,and low in cost on the earth,has similar chemical properties and reaction mechanisms to lithium,and has good application prospects in large-scale power grid energy storage.In the cathode material system of sodium-ion batteries,Na₃V₂（PO₄）₃ material with three-dimensional structure of sodium superion conductor type has attracted much attention because of its high-voltage platform and good cycle stability,however,its poor intrinsic conductivity seriously limits its practical application.In this paper,different modification methods are used to modify its electrochemical performance,and the specific research content is as follows:（1）Firstly,different carbon sources were selected and the Na₃V₂（PO₄）₃/C cathode material was synthesized by ball milling-assisted solid phase method,and it was found that the material coated with citric acid showed the best electrochemical performance:at 1 C,the initial discharge specific capacity of the material was 99.1 m Ah g^-1.At 5 C,the initial discharge specific capacity is 94.6 m Ah g^-1,and after 500 cycles,the discharge specific capacity is 72.5 m Ah g^-1 and the capacity retention rate is 75.5%.（2）In order to improve the electrochemical performance of Na₃V₂（PO₄）₃/C cathode materials,thiourea was used as a nitrogen source and sulfur source to dope on its carbon layer,and the effects of different doping amounts on the morphology and electrochemical properties of Na₃V₂（PO₄）₃/C cathode materials were explored.Among them,when the molar ratio of thiourea:citric acid is 0.2:2,the electrochemical performance of the electrode material is the best.the initial discharge specific capacity of the material is 110.0 m Ah g^-1 at 5 C,and after 500 cycles,the discharge specific capacity is 107.5 m Ah g^-1,and the capacity retention rate is 97.8%,and the synergistic effect of this nitrogen-sulfur co-doped carbon layer improves the reaction kinetics of the material.（3）Na₃V_2-xRE_x（PO₄）₃/C（x=0,0.01,0.03,0.05）were synthesized by introducing La³⁺,Gd³⁺,Lu³⁺rare earth metal ions instead of some V³⁺,respectively,into the Na₃V_1.97La_0.03（PO₄）₃/C cathode material.,among which,NVP/C-La0.03 showed the best cycle stability and rate performance.At 1 C,the initial discharge specific capacity was 110.0 m Ah g^-1.At 5 C,the initial discharge specific capacity of the material was105.8 m Ah g^-1,and after 500 cycles,the discharge specific capacity was 99.8 m Ah g^-1,the capacity retention rate was 94.3%,indicating that the doping of La³⁺rare earth metal ions could improve the electronic conductivity and sodium ion diffusion coefficient of Na3V2（PO4）3/C cathode materials.