Study On Modification Of Sodium Vanadium Phosphate Cathode Materials For Sodium Ion Batteries

This article focuses on the synthesis and modification of sodium vanadium phosphate（Na₃V₂（PO₄）₃）cathode materials,and the performance of Na₃V₂（PO₄）₃ is improved by carbon coating and metal ion doping.The synthesis,composition,structure and electrochemical properties of Na₃V₂（PO₄）₃,Na_3+xV_2-xSn_x（PO₄）₃ and Na_3-xLi_xV₂（PO₄）₃ were mainly explored.The details are as follows:A carbon-coated Na₃V₂（PO₄）₃ cathode material was prepared by sol-gel method using oxalic acid chelating agent,glucose and corn straw as carbon sources.The influences of different calcination temperatures,different calcination times,and different carbon sources on the composition,structure,and electrochemical performance of Na₃V₂（PO₄）₃ were investigated.The results showed that the calcination temperature（700℃-850℃）,calcination time（6h-10h）and the addition of corn straw did not change the phase of Na₃V₂（PO₄）₃,but only affected the lattice parameters.The straw-added sample showed a porous"sponge-like"micromorphology.The results of electrochemical tests show that the calcination temperature,calcination time and carbon source have a positive effect on the electrochemical performance.The specific capacity of the sample calcined at 800℃for 8 hours at 20C was 75.6 m Ah g^-1 with glucose as the carbon source;while under the same calcination condition,the specific capacity reached 83.9 m Ah g^-1 of the sample at 20C with glucose and straw as the carbon source.A series of tin-doped samples Na_3+xV_2-xSn_x（PO₄）₃ were synthesized using electrochemical performance were explored.The best doping amount Na_3.25V_1.75Sn_0.25（PO₄）₃ was found through a wide range of screening and then gradually refinement method.The test results indicate that the main crystal phase of the synthetic material is still sodium vanadium phosphate,indicating that the doping improves the electronic conductivity of Na₃V₂（PO₄）₃,the specific discharge undoped sample at 20C is only 76.3 m Ah g^-1,which has the best electrochemical performance.A series of sample Na_3-xLi_xV₂（PO₄）₃ were synthesized using lithium acetate（CH₃COOLi·2H₂O）as the lithium source.The effect of different lithium ion doping amount on the composition,structure and electrochemical performance of Na₃V₂（PO₄）₃were investigated.At the same time,the synthesized Na_3-xLi_xV₂（PO₄）₃ sample was used as cathode,and the sintered peanut shell was used as anode to assemble the full battery.Results show that the introduction of lithium ions does not change the main crystal phase of Na₃V₂（PO₄）₃,but because the radius of lithium ions is smaller than that of sodium ions,it will cause the reduction of the unit cell volume.The effect of different lithium doping amounts on the morphology of Na₃V₂（PO₄）₃ is not obvious.And samples showed a porous honeycomb shape with pore sizes between 200-500 nm,and the pores spread almost throughout the sample.After doping lithium ions,the Na1site of Na₃V₂（PO₄）₃ is activated,which promotes the electrochemical performance.The sample Na_2.96Li_0.04V₂（PO₄）₃ has the best performance.The specific discharge capacity at 30C is 104.9 m Ah g^-1 while the specific discharge capacity of undoped sample at the same rate is only 68.5 m Ah g^-1.At the same time,the charge transfer resistance（Rct）of Na_2.96Li_0.04V₂（PO₄）₃ is 130Ω,which is the smallest resistance among all samples.In addition,the discharge capacities of the full battery Na_2.96Li_0.04V₂（PO₄）₃//HC at 1C,5C and 10C are 116.4 m Ah g^-1,112.4 m Ah g^-1,105.6m Ah g^-1.This full battery has a discharge capacity of 80.2 m Ah g^-1 at 10C after 100cycles.