The Sodium Vanadium Phosphate/carbon Composite Is Used As A Cathode Material For Sodium Ion Batteries

The working principle of sodium-ion batteries is the same as that of lithium-ion batteries.Because of the abundant reserves and low cost,sodium-ion batteries have been considered as the most promising candidate for large-scale energy storage applications.The performance of sodium ion batteries mainly depends on the cathode materials.However,poor cycling stability and inferior rate capability of existing cathode materials for sodium-ion batteries restrict future developments.NASICON sodium vanadium phosphate has an open three-dimensional network framework,but its biggest drawback is poor conductivity.Studies have shown that the problem of poor conductivity can be mitigated by reducing particle size,coating carbon,and designing structure.Aiming at its poor conductivity,this paper uses different methods to study and improve the electrochemical performance of sodium vanadium phosphate.The details are as follows:1.A sodium vanadium phosphate/reduced graphene oxide-carbon nanotubes composite having a double conductive three-dimensional framework was prepared by freeze drying and subsequent annealing treatment.To be specific,sodium phosphate precursor solution and the suspension of reduced graphene oxide and carbon nanotubes were ultrasound mixed at the first place,then they were freeze-dried for 3 days and annealed..During this period,sodium vanadium phosphate particles were anchored in a three-dimensional,double conductive network of reduced graphene oxide and carbon nanotubes,exhibiting high initial coulombic efficiency(>93%),reversible capacity,as well as excellent cycle stability(?105 mAh g-1 after 500 cycles at 1 C).2.The sodium vanadium phosphate/carbon composite material is obtained by ball-milling different carbon materials with sodium vanadium phosphate.The carbon nanomaterial such as expanded graphite,carbon nanotubes and carbon black were subjected to high-energy ball milling with the sodium vanadium phosphate precursor obtained by freeze-drying method to obtain a composite material of sodium vanadium phosphate coated with three different carbon substrates.Studies have shown that the sodium vanadium phosphate coated with expanded graphite is more complete.Sodium vanadium phosphate/expanded graphite composite has the largest specific surface area,which is conducive to the immersion of electrolyte and the transmission of electrons.A high reversible capacity of 111.4 mAh g-1 at 1 C,excellent rate performance(105 mAh g-1 at 50 C)and long cycle life(48 mAh g-1 after 20,000 cycles at 50 C)were obtained.3.Hollow sodium vanadium phosphate/reduced graphene oxide microspheres are prepared by spray drying and subsequent annealing treatment.Firstly,the sodium vanadium phosphate precursor solution was mixed with graphene oxide and ultrasonically dispersed,then the solution was spray-dried to obtain sodium vanadium phosphate precursor/graphene oxide microspheres.Finally,the hollow vanadium phosphate/reduced graphite oxide microspheres were yielded by annealing treatment.Benefiting from the high conductivity of graphene and stable hollow microsphere structure,the microspheres has a high initial coulombic efficiency(97.6%),excellent rate performance(reversible capacity of 99.1 mAh g-1 at 20 C)and excellent cycle stability(capacity retention of 90.8%after 400 cycles).