Study On The Effect Of Biomass Carbon Source On The Performance Of Na₃V₂（PO₄）₃/C Cathode Materials For Sodium Ion Batteries

At present,sodium ion battery is considered as the most promising electrochemical energy storage equipment in the field of large-scale energy storage power grid and electric vehicles because of its advantages of abundant precursor reserves,wide sources,low price and environmental protection,and has attracted more and more attention.Polyanionic materials have open sodium ion channel（NASICON）,high thermal stability and good structural stability,and have great application prospects in cathode materials of sodium ion batteries.Sodium vanadium phosphate（Na₃V₂（PO₄）₃）has NASICON structure with three-dimensional stable framework,high theoretical specific capacity(117.6 m Ah·g^-1)and excellent ionic conductivity,which has become a representative of polyanionic cathode materials with great application and development prospects.However,the electronic conductivity of such material is extremely poor.It is proved that carbon materials can effectively improve the defects of electronic conductivity.In this paper,to solve the problem of low electronic conductivity,Na₃V₂（PO₄）₃/C composites were prepared by impregnation method and sol-gel method,and their structure,morphology and electrochemical properties were studied.The conclusions are as follows:（1）The Na₃V₂（PO₄）₃/C composites with three-dimensional（3D）porous structure were prepared by impregnation method and subsequent heat treatment with cotton soft towel as biomass carbon source.The cotton soft towel calcined at high temperature provides crystallizing point for Na₃V₂（PO₄）₃ particles,inhibits the growth and agglomeration of particles,and forms a unique 3D interconnected porous morphology.This unique morphology improves the specific surface area of the material in contact with electrolyte,expands the transmission path of Na⁺and enhances the electronic conductivity of the material.It is found that the optimum addition amount of cotton soft towel is 6 wt.%（theoretical carbon content）,and the material has a large number of mesopores with carbon content of 13%,and has a high discharge specific capacity of 108.4 m Ah·g^-1 at 0.1 C in the voltage range from 2.0 V to 4.0 V;At the rate of 2 C,the discharge capacity in the first week is as high as 87.1m Ah·g^-1,and after 2000 cycles,the capacity retention rate still exceeds 65%;Even after 1000 cycles,the reversible capacity is still 63.2 m Ah·g^-1 at a high rate of 5 C,and the capacity retention rate is as high as 75%.（2）Using tea as the biomass carbon source,Na₃V₂（PO₄）₃/C composite material with honeycomb structure was successfully prepared by the sol-gel method.The results show that the addition of carbon does not change the crystal structure of Na₃V₂（PO₄）₃,but obviously changes the surface morphology of Na₃V₂（PO₄）₃/C composites,which makes the Na₃V₂（PO₄）₃/C composites have unique honeycomb structure.Biomass tea leaves become porous carbon skeleton after heat treatment,which not only helps to prevent the volume collapse caused by Na⁺intercalation/deintercalation,but also provides growth sites for Na₃V₂（PO₄）₃nanoparticles and refines the grains.The electrochemical performance test showed that the addition of biomass carbon improved the electrochemical performance of Na₃V₂（PO₄）₃/C composites.The best addition amount of tea is 7 wt.%.At this time,the sample has excellent electrochemical performance:it still has a high discharge specific capacity of 110.6 m Ah·g^-1 at 0.1 C in the voltage range of 2.0～4.0 V.After10 cycles,the discharge capacity is still as high as 100.9 m Ah·g^-1,and even after 50cycles,the capacity remains as high as 85%.