| Due to the wide abundance of sodium resources and similar chemistry between lithium and sodium,sodium ion batteries(SIBs)have recently attracted wide interest as a low cost alternative to LIBs.However,because of the large radius of sodium ion,the electrode material has a larger volume change compared with that of lithium ion battery,which affects its cycle performance,so sodium ion batteries are more demanding on the selection of electrode materials.In the study of negative electrode materials for sodium ion batteries,it is found that the carbon fibers prepared by electrospinning have good cycle performance and rate performance.But,the morphology,structure and the specific reasons for the improvement of the properties of the materials during charge and discharge are not very clear.What's more,a wide range of layered compounds have been applied in the cathode materials in sodium ion batteries because of their larger inter-planar distances.In former work,several kinds of nano-sized layered SnS2 were prepared,doped with different number of Fe,0 at%,20 at%,40 at%,60 at%,respectively(the atomic ratio of Fe to Sn is 2:8,4:6 and 6:4,respectively).The electronic tests showed that the 40%Fe-SnS2 material had the best electro-chemical property.But the role the iron element play in it remained unclear.To figure out the reason for such performance,we utilized the in-situ transmission electron microscopy(TEM),hoping to explain it thoroughly from the perspective of micro scale.By building the half sodium ion nano battery(individual nano-material as work electrode,tungsten needle as the counterpart electrode,with the PVA-NaOH-H2O gel basic compound at the tip of the tungsten needle as the electrolyte and sodium source)in TEM,in-situ research on the discharge process of the individual nano-sized materials can be realized.Based on this method,the sodium inactivation processes of the carbon fiber and the sequence of the iron doped SnS2 electrodes were studied and the results were as follows:1.The sodium intercalation mechanism in amorphous carbon fiber prepared by electrospinning was discovered to be controlled by long-range diffusion.After sodiation,the bulk expansion rate of the carbon fiber was only 44.6%,which was far lower than the most electrode materials of the sodium batteries(Sn nano-particle,420%;Ge nano-line,300%).The structure didn't change a lot and suffered little destruction,so as to giving a battery better cycling stability.2.40%Fe-SnS2 prepared by chemical vapor deposition maintained the same structure as pure SnS2.But the inter-planar distance of the 40%Fe-SnS2 increased a lot compared with the pure SnS2,providing larger room for sodium inactivation and facilitating it;According to the different reaction products of 40%Fe-SnS2 and pure SnS2 with sodium,it was inferred that the addition of iron could increase the reaction depth of the material;Before sodiation,iron elemental in 40%Fe-SnS2 distributed on the material averagely with the valence being +3.While after sodiation,the iron transformed into the elementary substance with the valence being 0,forming the conducting network and improving the conductivity,which gave the 40%Fe-SnS2 material higher velocity of the reaction at the same conditions. |
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