Synthesis,Modification And Electrochemical Performance Of Na₃V₂（PO₄）₃ Cathode Material For Sodium Ion Batteries

Sodium-ion batteries have the characteristics of abundant resources and low cost,which are the research focus in the field of energy storage in the future.NASICON-type Na₃V₂（PO₄）₃has the advantages of high voltage platform,high ion diffusion rate and stable crystal structure,and is a candidate material for future commercial sodium ion batteries.However,the low intrinsic electronic conductivity,high price and toxicity of Na₃V₂（PO₄）₃ limit its industrial development.In view of the above problems,this paper uses surface modification and bulk doping strategies for modification,and designs a low-vanadium polyanion cathode material to promote the commercial application of Na₃V₂（PO₄）₃.The specific research contents of this paper are as follows:（1）A nitrogen-doped carbon layer-coated Na₃V₂（PO₄）₃/NC composite was designed and prepared.The carbon coating can stabilize the crystal structure of Na₃V₂（PO₄）₃/NC and improve its electronic conductivity.In addition,the doping of N atoms forms a large number of C-N bonds with carbon atoms,and the presence of C-N bonds can introduce defects in the carbon layer,generating a large number of active sites and accelerating the electron migration.The discharge specific capacity of Na₃V₂（PO₄）₃/NC is 109.18 m Ah g^-1at 1 C,even at 50 C,the discharge specific capacity is still 88.30 m Ah g^-1.In addition,the capacity retention rate of Na₃V₂（PO₄）₃/NC is 72.87%after 8000 cycles at 50 C,and the capacity decay rate per cycle is 0.0034%.（2）Ti-doped Na₃V_2-xTi_x（PO₄）₃/C composites were synthesized by ball milling assisted solid-state method.The partial substitution of Ti⁴⁺for V³⁺increases the Na vacancy and improves the electronic conductivity of Na₃V₂（PO₄）₃.At the same time,Ti⁴⁺doping reduces the unit cell volume of Na₃V₂（PO₄）₃ and improves the stability of the crystal structure.The surface-coated carbon layer and the abundant Na⁺diffusion channels inside solve the internal and external electron conduction problems of Na₃V₂（PO₄）₃.The synergistic effect of two additional redox pairs(V⁴⁺/V⁵⁺and Ti³⁺/Ti⁴⁺)makes the capacity of Na₃V_1.9Ti_0.1（PO₄）₃/C exceed the theoretical capacity of Na₃V₂（PO₄）₃.The discharge capacity of Na₃V_1.9Ti_0.1（PO₄）₃/C is 123.3 m Ah g^-1 at 0.1 C and 89.5 m Ah g^-1 at 30 C.The capacity retention is 62.3%after 8000 cycles at 20 C.（3）A low vanadium polyanion cathode material Na_3+xV_2-xMn_x（PO₄）₃/C was designed and synthesized.The substitution of Mn for V reduces the cost and toxicity of Na₃V₂（PO₄）₃.The introduction of Mn²⁺can effectively improve the voltage platform（3.5 V）of Na₃V₂（PO₄）₃ and broaden the diffusion channel of Na⁺.Under the synergistic effect of V³⁺/V⁴⁺,V⁴⁺/V⁵⁺and Mn²⁺/Mn³⁺redox pairs,the discharge specific capacity of Na_3.5V_1.5Mn_0.5（PO₄）₃/C at 0.1 C is as high as 136.2 m Ah g^-1.When the current density is 20C,the discharge specific capacity of Na_3.5V_1.5Mn_0.5（PO₄）₃/C is still 96.4 m Ah g^-1,and the capacity retention rate is 64.7%after 8000 cycles.