Study On The Mechanism Of Na₃V₂（PO₄）₂F₃ Cathode Material Modified By Natural Biomass Carbon Source

At present,sodium ion batteries are considered one of the most promising electrochemical energy storage devices in the fields of new energy vehicles and large-scale energy storage equipment due to their advantages such as low extraction cost,environmental friendliness,abundant sodium resource,and wide electrochemical window.Among the cathode materials of sodium ion batteries,polyanion compounds play an important role in the development of sodium ion batteries because of their good structural stability and open sodium ion transport channels.In polyanionic compounds,sodium vanadate fluorophosphate（Na₃V₂（PO₄）₂F₃）has a stable three-dimensional NASICON structure.It also has defects such as low intrinsic conductivity and large capacity attenuation in the later stage of cycling.The research shows that carbon materials can effectively improve the conductivity and cycling stability of Na₃V₂（PO₄）₂F₃ material.This work takes Na₃V₂（PO₄）₂F₃ as the research object and optimizes its structure and electrochemical performance by changing the drying method during the preparation process.Based on this,two biomass（porous pomelo peel and tubular ramie fiber）with different morphologies are used as carbon sources to modify the Na₃V₂（PO₄）₂F₃ cathode material.At the same time,the structure,morphology,and electrochemical performance of the composite cathode materials are studied.The specific research is as follows:（1）The Na₃V₂（PO₄）₂F₃ cathode material was optimized and prepared using low-temperature freeze-drying method.The crystal structure,microstructure,electrochemical characteristics,and reaction kinetics of the material are explored.It is found that low-temperature（-60°C）freeze-drying can affect the nucleation and growth rate of grains.The prepared cathode material is dispersed with small particle size.At a rate of 0.2 C,the first discharge specific capacity of Na₃V₂（PO₄）₂F₃ prepared by freeze-drying method is 96.85 m Ah g^-1.After 100 cycles,it exhibits a reversible specific capacity of 70.38 m Ah g^-1 with a capacity retention of 72.67%,which is significantly higher than the reversible specific capacity(59.38 m Ah g^-1)and capacity retention（67.58%）of the material obtained under ordinary drying（80 ^oC）.The particle size of Na₃V₂（PO₄）₂F₃ materials prepared by freeze-drying is small,which is conducive to increasing the reaction active site of materials,expanding the contact area between materials and electrolyte,and improving the transmission rate of sodium ions.（2）Using pomelo peel as a biomass carbon source to composite with Na₃V₂（PO₄）₂F₃ material,a three-dimensional porous spatial carbon conductive network was established,allowing nanoscale materials to adhere and grow in the carbon pores,which significantly improves the cycling stability of the material.When the mass fraction of carbon derived from pomelo peel in the composite material is 7.5%,the first discharge specific capacity of the sample increases to 110.77 m Ah g^-1 at a rate of 0.2C.After 200 cycles,its reversible specific capacity can still remain stable at 93.52 m Ah g^-1 with a capacity retention of 84.43%.The carbon network formed after the calcination of pomelo peel has a skeleton supporting effect,which can slow down the attenuation phenomenon of the material in the later stage of cycling.The carbon pores limit the particle size of the material,making the active particles fully contact with the electrolyte,providing more active sites for electrochemical reaction.In addition,the carbon network structure has a protective effect on the material,which can suppress the occurrence of side reactions and the volume expansion of the material during cycling.（3）Using ramie as a biomass carbon source to composite with Na₃V₂（PO₄）₂F₃material,a tubular carbon conductive fiber network structure was constructed.The material presents inside and outside the carbon tube wall and the particle size is maintained at the nanometer level,which significantly improves the specific capacity and high rate performance of the material.When the mass fraction of carbon derived from ramie in the composite material is 6%,the first discharge specific capacity of the sample at a rate of 0.2 C increases to 112.87 m Ah g^-1 with the first Coulombic efficiency of 75.31%.After 200 cycles,the reversible specific capacity can be stabilized at 86.76m Ah g^-1,and the capacity retention reaches 76.90%.The specific capacity of the sample can reach 91.46 m Ah g^-1 at a rate of 10 C.After 5000 cycles,the specific capacity of the material remains at 45.75 m Ah g^-1 with a capacity retention of 50.02%.The excellent electrochemical performance of the composite benefits from the natural fiber tubular structure of ramie,which effectively reduces the particle size of active particles,increases the active reaction site of the material,improves the electron transmission efficiency,and protects the material from the corrosion of electrolyte.