Preparation And Modification Study Of Na₃V₂?PO₄?₃/C As Cathode Materials For Sodium Ion Batteries

Due to the abundant resources,the wide distribution,the low cost and the environmental friendliness of sodium resources,sodium ion batteries are regarded as the most potential secondary batteries which would replace the lithium-ion batteries.In 2016,the price of battery-level sodium carbonate has been approximately triple.Under this condition,the advantage of the low cost of sodium resources is clearer,especially in the large-scale energy storage equipment.However,the large ions radius and heavy mass of sodium ions make the electrochemical performance of SIBs worse than LIBs.So,it is very hard to find suitable electrode materials with excellent performance.When it comes to the cathode materials,Na3V2(PO4)3 owns the unique NASICON structure and good thermal stability,which is a potential industrial material.The research subject of this paper is Na3V2(PO4)3 and the purpose of these researches are to improve the electrochemical performance of Na3V2(PO4)3.Thus,there are several modified materials prepared and the contents of this paper are as follow:(1)The simple freeze-drying-assisted thermal treatment method is introduced to prepare the porous Na3V2(PO4)3@C nanoparticles enwrapped in three-dimensional graphene.The microstructure and electrochemical performance are investigated.According to the research results,the specific surface of these porous Na3V2(PO4)3 nanoparticles is relatively high(143.7 m2 g-1)which is caused by the freeze-drying technology,facilitating the contact between Na3V2(PO4)3 nanoparticles and electrolyte.The double restrictions of graphene matrix and carbon coating on the surface contribute to the improved electrochemical performance,constructing a stable 3D structure.During the calcination process,the size of synthesized Na3V2(PO4)3 nanoparticles was well controlled by the double carbon protection.As the results,the electrochemical performance of this material is very impressive.When the current rate was changed from 0.2 C to 60 C,a high capacity retention of 67.9% was obtained.At current rate of 10 C,the initial capacity is 102.3 mAh g-1.After 1000 cycles,about 95.0 % of the specific capacity is retained.At current rate of 40 C,a reversible specific capacity of 91.6 mAh g-1 can be achieved.Even after 1500 cycles,a high capacity retention of 82.0 % can be obtained.(2)Mn doped porous Na3V2-xMnx(PO4)3/C composites(x=0.0,0.1,0.2,0.3)were prepared using a simple freeze-drying method.The microstructure and electrochemical performance influenced by the Mn substitution are investigated.Through the XRD Rietveld refinement result of Mn doped sample,the Mn subsititution increases the crystal volume and facilitates the transportation of sodium ions.Meanwhile,accroding to the EIS and CV results,the Mn doping improves the electron/ion conductivity.Based on the above analysis,Na3V1.8Mn0.2(PO4)3/C delivered 77.8 m A h g-1 at a high current rate of 30 C.Even more impressive,a high capacity retention of 82.0 % was obtained after 10 000 cycles at 30 C(corresponds to ultralow 0.0018% capacity decay per cycle).