Synthesis,Characterization And Li-storage Performance Of Carbon-coated ZnFe₂O₄-based Anode Materials For Li-ion Batteries

As anodes for lithium ion batteries,carbon has good electrical conductivity and excellent cycle performance.However,its low theoretical specific capacity has been unable to meet people’s needs.Therefore,to invent a new anode with a larger capacity,longer cycle life,better rate,lower cost and more secure has become the current research hotspot.ZnFe2O4 is one of the ideal anode for lithium ion batteries because of its large specific capacity,low cost and no harm to environment.But there still were some defects in ZnFe2O4 anode materials,such as low electronic conductivity,large volume changing during repeated charge-discharge progress.In order to overcome those defects,we explored efficient methods to improve the electrochemical properties of ZnFe2O4 anode via reasonable structure and composition design to increase the conductivity and active sites.The main research contents and results are presented as follows:1.1D ZnFe₂0₄ nanofibers and nanotubes were tunable synthesized via a electrospinning method.And ZnFe2O4@C nanofibers and nanotubes were obtained by dopamine hydrolysis in the Tris solution（pH = 8.5）and subsquent decomposition.The morphology of nanofibers and nanotubes were observed clerely in the SEM images and TEM images.The diameter of nanofibers and the outside diameter of nanotubes were both ¹10 nm.The thickness of tube wall was about 12 nm.One dimensional structure could effeciently increase the efficiency of electron transport and ion transport.Nanofibers and nanotubes were assmebled by a large number of ZnFe₂0₄ nanoparticles,many pores distrubuting in them.The thickness of carbon shell was about 3-4 nm.Not only could the existence of carbon shell enhance conductivity of nanofibers and nanotubes,but also could mitigate the change in charging-discharging progess.Electrochemical tests showed the cycling performnace of nanotubes were better than nanofibers after 200 cycles at 200 mA/g.Because nanotubes had larger specific surface area,which could provide more electroactive sites.And the structure of nanotubes was also an effective channel for Li⁺ transmission.Carbon coated method improved the performance of Li storage and the stability of cycling performance.2.Two dimensional ZnO/ZnFe2O4 nanosheets were prepared by a method of synthesized MOF.And ZnO/ZnFe2O4@C nanosheets were obtained by dopamine hydrolysis in the Tris solution（pH = 8.5）and subsquent decomposition.The XRD test indicated that the molar ratio of ZnFe₂0₄ to ZnO is about 1 : 3.2 in ZFOZ nanosheets.ZFOZ nanosheets were observed in the SEM and TEM images.The thickness of ZFOZ NSs was about 20-30 nm and had a rich pore structure.A carbon layer with 10 nm thickness could also be observed after carbon coated via dopamine hydrolysis in the Tris solution（pH = 8.5）and subsequent carbonization.When applied as anodes for Li-ion batteries,electrochemical tests showed that ZFOZ@C NSs had excellent performances,especially the cycling stability at large current density.Before carbon coated,cycling performance of ZFOZ NSs had an obvious decay at the first 30 cycles.But it could still deliver a high reversible capacity of 450 mAh/g after 200 cycles at 500 mA/g.3.Zn₃[Fe（CN）₆]₂ spheres were successfully developed by changing the ratio of the solvent components in the above experiment.And core-shell ZnO/ZnFe2O4 spheres were obtained via decomposition of the precursors.The same method synthesized carbon coated core-shell ZnO/ZnFe₂0₄@C.Physicochemical characterizations revealed that the molar ratio of ZnFe2O4 to ZnO was 1 : 2.5.Core-shell ZnO/ZnFe2O4@C was spherical structure of 100-450 nm,making it has a high tap density.The thickness of carbon layer was 10 nm.And carbon infiltrates into interior of the spherical structure at the same time,greatly enhancing the conductivity of core-shell ZnO/ZnFe₂0₄@C.This structure had a rich mesoporous structure.The specific surface area was up to 80 m2/g.Electrochemical tests demonstrated that it had excellent performances and stability when core-shell ZZFO@C was used as anodes for Li-ion batteries.After 60 cycles,it delivered a reversible capacity of 893 mAh/g at 500 mA/g.It could still deliver a capacity of 718 mAh/g at 1000 mA/g,which showed a greater performance at large current density.And the spherical structure had not changed significantly,indicating the great stability and excellent cycling performance.