Preparation And Lithium Storage Performance Of Freestanding CoFe₂O₄@C Nanofiber As Anode Materials For Lithium Ion Batteries

With the rapid development of portable wearable electronic products,flexible lithium-ion batteries with high energy density are more and more widely used in practice.The key element of flexible lithium-ion batteries is to use flexible electrodes with high conductivity to realize rapid charge and discharge.As a new anode material,spinel-type ferrite has the advantages of high specific capacity,environmental protection and abundant raw materials.However,due to the poor electrical conductivity of such materials,it is easy to cause a huge change in volume during the embedding/exiting process of lithium ions,which results in poor cycling performance.Carbon nanofiber is a kind of widely used carbon material,which has the advantages of light weight,large aspect ratio,high strength,good electrical conductivity,corrosion resistance and low production cost.In this thesis,CoFe₂O₄ particles were composite with carbon fiber,and the carbon nanofiber composite film embedded with CoFe₂O₄ particles was formed in situ by electrospinning technology.The effects of temperature,salt content and porous structure on the microstructure,phase structure and electrochemical properties of the composite nanofiber film were studied.Benefit from the high theoretical specific capacity of CoFe₂O₄ material with one-dimensional nanostructure and spinel structure and the high electrical conductivity of carbon fiber,the composite nanofiber films show excellent electrochemical performance.Specific research results are as follows:1.The flexible CoFe₂O₄ carbon nanocomposite fiber(CoFe₂O₄@CNFs)film was prepared by electrospinning technology combined with heat treatment.The film has good flexibility and self-supporting characteristics,and can be directly use as a binder-free anode electrode.At the same time,CoFe₂O₄ particles are evenly distributed in the carbon fiber matrix,which greatly shortens the lithium ion transmission distance.In addition,carbon nanofibers are cross-linked and stacked to form a three-dimensional conductive network,and electrons can be transported in the conductive network through migration,jumping and tunneling.With the increase of the carbonization temperature,the specific capacity of the CoFe₂O₄@CNFs film electrode increases first and then decreases;at the carbonization temperature of 600?,with the increase of the metal salt content in the solution,the specific capacity of the CoFe₂O₄@CNFs film electrode Increase first and then decrease.When the carbonization temperature is 600?and the salt content in the solution is 3%,the prepared composite nanofiber film electrode has the best electrochemical performance.When charged/discharged at a current density of 100 m A g^-1,the initial discharge specific capacity reaches 1179.2 m A g^-1,after 100 cycles of charge and discharge,the specific capacity can still be maintained at 611.4 m Ah g^-1,when the current density increases to 2 A g^-1,the specific capacity remains at 349.1 m Ah g^-1.2.The flexible CoFe₂O₄@CNFs film was subjected to aperture treatment to observe the effect of the pore structure on the composite nanofiber film.As the content of PMMA increases,the specific surface area increases,and the corresponding specific capacity first increases and then decreases;when PAN When the ratio to PMMA is 8:2,the porous composite nanofiber membrane electrode has the best electrochemical performance.It is charged and discharged at a current density of 100 m A g^-1,and the initial discharge specific capacity reaches 1158.1 m Ah g^-1,after 100 cycles of charging After the discharge cycle,the specific capacity remains at 755.8 m Ah g^-1,and at a high current density of 2 A g^-1,the specific capacity can still be maintained at 476.5 m Ah g^-1.It can be seen that the porous structure further improves the electrochemical performance of the composite nanofiber membrane electrode.