Synthesis And Modification Research Of Sodium Vanadium Titanium Phosphate Cathode Materials For Sodium-ion Batteries

As a new and efficient energy storage methods,the sodium-ion batteries（SIBs）have attracted tremendous attention due to their excellent performances,rich resources and low cost of sodium（Na）.So far,many research have done on the cathode materials of SIBs.As the most representative polyanionic material,NASICON-type polyanion compounds attracted extension attention for its threedimensional open structure and providing a sodium ion transmission channel to ensure a high ion diffusion rate.In addition,the strong P-O covalent bond results to high structural stability and thermal stability,which is expected to become a very important cathode materials.However,the polyanion group suffer from extremely low intrinsic conductivity,limiting the practical application of such materials.Therefore,a series of effective measures need to be taken to improve the electrochemical performance.In this study,based on the new Na₂VTi（PO₄）₃（NVTP）material,we carried out a variety of strategies to improve its electrochemical performance on both synthesis process and further modification.The modified sol-gel method is used to synthesize the carbon-coated NVTP/C material,while the sodium source adjustment and the calcining temperature are optimized.After detailed comparison,the optimum process conditions are as followed: CH3 COONa is used as the sodium source,and the calcining temperature is 750-800oC to prepare the target product.After testing,it is found that CH3 COONa as a sodium source can reduce the particle size,and the higher temperature would cause better crystallinity of the material,but it will increase the particle size significantly,which is not conducive to charge and discharge performance.Taking everything into account,the material has the highest actual specific capacity when the temperature was 800oC,it can deliver a highest capacity of 149.4 m Ah g-1 at 0.1C,while the material has a considerable cycle performance at 750oC.The specific capacity of the materials after 500 cycles reach a capacity retention of 79.4% and the stability is quite impressive.By selecting different carbon sources to perform the double carbon layer coating measures for NVTP materials.It is found that when using cetyltrimethylammonium bromide（CTAB）and polyvinylpyrrolidone（PVP）as carbon sources,the material exhibits the best sodium storage performance.The results show that when the CTAB is coated on the surface of the material,it has a high reversible capacity of 150.5 m Ah g-1 at 0.1C,and the rate performance is also quite excellent.When using PVP as a carbon source,the capacity of 133.8 m Ah g-1 can be achieved at 0.1C and the ultra-high capacity retention rate of the materials after 100 cycles at 0.2C rate is 94.3%.In addition,a template method is used to investigate nano-materials of NVTP.The results show that the particle size of the prepared material is greatly reduced.However,it is difficult to completely remove the non-electrochemically active silica template with etching treatment,which greatly reduces the conductivity and discharge capacity of the material.Nevertheless,the material prepared by the template method still has a specific capacity of 132.2 m Ah g-1 at low current density,exceeding the pristine material,indicating that reducing the particle size can achieve higher discharge capacity,which providing a new approach for the further research.To improve the physical and chemical properties of NVTP materials,a systematic study on the doping and modification of materials is also carried out.Ndoped and S-doped are carried out on the materials.The results show that the particle size of the material is significantly reduced after adding S and N,and it also generates more active sites and defect sites inside the carbon layer,accelerating the diffusion rate of sodium ions.After testing,the material shows an ultra-high actual specific capacity of 164.8 m Ah g-1.It can also show a capacity of 152.4 m Ah g-1 and the capacity retention rate is 82.8% after cycling 100 cycles at 0.2C rate,which has great development potential.In addition,the effect of fluoride ion introduction on the properties of the material was also explored.When the doping amount of F is 0.03,the structural stability can be greatly enhanced,and the specific capacity reaches 116.8 m Ah g-1 at 0.5C with a ultra-high capacity retention of 96.5%,exhibiting the excellent rate performance.Finally,we attempt to prepare fluorophosphate through substitution of fluorine element.After characterization,it is found that the effect of the strong electronegative element F makes the high-voltage region V4+/V5+ redox couples participating in the reaction,and provides a specific discharge capacity of 147.5 m Ah g-1.However,the reversibility and cycle stability are limited,which need further improvement.