| Sodium ion batteries have attracted much attention due to sodium abundant reserves and low cost.Cathode materials have become a research hotspot because they seriously affect the electrochemical properties of sodium ion batteries.Na3V2(PO4)3 is a promising cathode material for sodium ion batteries because of its typical NASICON structure and an open skeleton structure,moderate voltage platform,high theoretical energy density and good thermal stability at room temperature.In this paper,cathode materials Na3V2(PO4)3 were synthesized by combining solution method with sintering method based on the comprehensive analysis of the research progress of cathode materials for sodium ion batteries,then Na3V2(PO4)3 cathode materials were modified by thermal decomposition of glucose and chemical vapor deposition of ethylene glycol respectively.The carbon content,crystal structure and micro-morphology of the cathode materials as prepared were characterized by carbon and sulfur analyzer,X-ray diffractometer and field emission scanning electron microscopy respectively,and the electrochemical properties of the cathode materials synthesized were tested by LAND test system and electrochemical workstation.The effects of sintering temperature and holding time on the structure and electrochemical properties of cathode materials Na3V2(PO4)3 synthesized by the combination of solution method and sintering method were studied.The main phase of cathode materials as synthesized was Na3V2(PO4)3,and the sintering temperature and holding time had little effect on the structure of cathode material Na3V2(PO4)3.Electrochemical properties of Na3V2(PO4)3 synthesized at 700? for 8h were the best.Particle of the sample with partially agglomeration was micron-sized.It exhibited the initial discharge specific capacity of 95.6mAh/g.The agglomeration of the particles,the weakened redox and the sluggish dynamic behavior after 50 cycles result in decay of the capacity seriously.Cathode materials Na3V2(PO4)3 synthesized had low specific capacity and poor cycle stability because of its low electrical conductivity.Na3V2(PO4)3 had been modified by the following two methods as for the defects itself.On the one hand,residual carbon from the thermal decomposition of organic glucose is introduced to improve the electrochemical properties of Na3V2(PO4)3.The doped cathode materials Na3V2(PO4)3/C were synthesized under different glucose content(4.5%,6.5%,8.5%).The main phase of the synthesized cathode materials was Na3V2(PO4)3,and no second phase was observed.The capacity,cycling stability and kinetics of cathode materials Na3V2(PO4)3/C were significantly improved compared with undoped carbon Na3V2(PO4)3.Among them,Na3V2(PO4)3/C cathode material with 6.5%glucose content exhibited specific capacity of 102.4 mAh/g and capacity retention rate of 99.4%after 50 cycles.Particle size of the sample was mainly micron scale,but not uniform,and even the lamellar material appeared.Discharge specific capacity of the samples was 97.1,87.9,76.3,61.9 and 44.5mAh/g respectively at 0.2,0.5,1,2 and 5C.On the other hand,the other cathode material Na3V2(PO4)3/C was synthesized by carbon coated on the surface of Na3V2(PO4)3 with ethylene glycol as carbon source by simple chemical vapor deposition technique to improve electrochemical properties.The prepared sample was micron-sized particles with uniform distribution.It delivered the discharge specific capacity of 111.9mAh/g at 0.1 C,which was higher than bare Na3V2(PO4)3 Of 95.6mAh/g.At the same time,the coated sample Na3V2(PO4)3/C exhibited smaller polarization voltage and better kinetic properties than bare Na3V2(PO4)3. |
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