Structure Design And Surface Control Of Iron?Bismuth Compounds/Graphene Composites As Anodes For Sodium Ion Batteries

Lithium ion battery is widely used in electronic commerce,transportation and other fields due to its high energy density,environmental friendliness,portability and so on.However,the shortage of lithium resources greatly restricts the development of lithium ion batteries.Sodium possesses the similar physical and chemical properties to lithium,but has more abundant reserves than lithium.So,sodium ion battery has a broad promising application in sustainable energy development.Metal oxides and selenides,as the anodes of sodium ion batteries,have attracted tremendous interest due to their high theoretical capacity.However,the electrical conductivity of metallic compound is poor,and it also suffers from large volume change during cycle,which would cause fast capacity decay due to the structure failure and instability of SEI film.In order to give full play to the advantages of metallic compounds,it is necessary to synthesize them with elaborate design.Introducing carbon materials,designing amorphous structure,building strong interfacial interaction and controlling surface are main design ideas.Based on these,we successfully synthesized amorphous Fe2O3/graphene,nanorod FeSe2/graphene,single-crystalline and polycrystalline Bi2Se3/graphene composites.The morphology,structure and sodium-ion storage performances of these composites are characterized,and the sodium-ion storage mechanisms are investigated in detail.(1)Amorphous Fe2O3/graphene composite(Fe2O3@GNS)was synthesized.It reveals that the amorphous Fe2O3 nanoparticles with an average diameter of 5 nm were uniformly anchored on the surface of graphene by the strong C-O-Fe oxygen-bridge bond.Compared to well-crystalline Fe2O3,amorphous Fe2O3@GNS exhibits superior sodium storage properties such as high electrochemical activity,high initial Coulombic efficiency of 81.2%and good rate performance.At a current density of 100 mA/g,amorphous Fe2O3@GNS composite shows a specific capacity of 440 mAh/g,which is obviously higher than that of crystalline Fe2O3@GNS-500(284 mAh/g).Even at a high current density of 2 A/g,amorphous Fe2O3@GNS composite still exhibits a specific capacity as high as 219 mAh/g.Compared with crystalline Fe2O3,the volume expansion of amorphous Fe2O3 is smaller and more homogeneous.The addition of graphene and the stronger interfacial interaction between Fe2O3 and graphene construct an efficient electronic transmission network,which can greatly promote electronic transfer and effectively buffer the volume change in host material.(2)Because the sodium storage performance depends largely on the rate of sodium ion diffusion across the electrode surface,the surface feature is a crucial factor that determines the energy storage properties of electrode materials.In this work,the influence of surface oxides on Na-ion storage behavior of FeSe2 was investigated by designing FeSe2 with varying oxide content.It is found that surface oxide has an inhibitory effect on the electrochemical activity of FeSe2.The oxide can be converted to a sodiated shell with high mechanical strength and poor conductivity,which generates phase-transformation resistance to suppress the sodiation of FeSe2 core,blocks the transfer of Na-ions and electrons in subsequent sodiation processes.Small-sized FeSe2 on graphene with higher surface oxide content exhibits obviously inferior performance compared to large-sized FeSe2 with lower oxide content.By controlling oxide content,the prepared FeSe2 nanorod/graphene composite exhibits a high capacity of 459 mAh/g at 0.1 A/g and superior rate performance.Only 10%capacity decrease occurs with the increase in current density from 0.1 to 5 A/g.Even at 25 A/g(=50 C),it delivers a capacity of 227 mAh/g with almost no decay after 800 cycles.(3)Polycrystalline Bi2Se3/graphene composite was synthesized by a selenization reaction.In the composite,Bi2Se3 particles with the average diameter of 100 nm are uniformly dispersed onto graphene with strong interfacial interaction(C-O-Bi),the pressing density of Bi2Se3/graphene composite can reach a high value of 2.07 g/cm3.The Na-ion storage mechanism of Bi2Se3 should be attributed to a combined conversion-alloying process by a series of ex situ measurements.The composite can deliver a high gravimetric specific capacity of 346 mAh/g and volumetric specific capacity of 716 mAh/cm3 at a current density of 0.1 A/g.Even at 10 A/g(?30 C),the composite still possess a gravimetric capacity of 183 mAh/g and volumetric capacity of 379 mAh/cm3 with ultrastable cyclability up to 1000 cycles.(4)Except for the surface oxide,the exposure of facets is also a crucial factor to determine the diffusion rate of Na-ions.Herein,preferable oriented growth along(00l)facets in single-crystalline Bi2Se3 was realized by using bismuth-based metal-organic framework(MOF)as precursor,and the influence of electrode surface's crystal orientation on Na-ion storage behavior was investigated through controlling the heating rate of carbonization process.It is found that the oriented growth along(00l)facets in Bi2Se3 possesses open two-dimensional interlayer structure with a layer spacing of 9.80 A,which can provide a large amount of electrochemical active area and kinetically favorable channels,accelerate the diffusion and insertion/extraction of Na-ion among the host material.In addition,the strong oxygen bridges of C-O-Bi provide a highly improved electronic transfer network in the composite.Remarkably,the obtained Bi2Se3-C/G-1 composite exhibits a high reversible capacity of 377 mAh/g at 0.1 A/g and extraordinary rate performance with a small capacity decay of 12%with the increase in current density from 0.2 to 10 A/g.The developed two-dimensional oriented-arranged interlayer structure and highly exposed edges effectively accelerate the diffusion of sodium ions,reduce the phase-transformation resistance of redox reactions,and improve the electrochemical reactivity of electrode materials.