Synthesis,Characterization And Li-storage Performance Of One-Dimensional ZnFe₂O₄ Composite Materials

Nowadays,the anode materials of commercial lithium ion batteries are mainly carbon materials with excellent conductivity and stable structure,but the theoretical specific capacity of carbon materials is relatively low,and it is far from meeting human needs for portable,high-capacity energy storage.The anode material with a high theoretical capacity has become a subject of research.The theoretical capacity of the Zn Fe₂O₄ reaches 1072 mAh g^-1.Due to the abundant reserves of zinc and iron,the preparation process has no pollution to the environment,and the cost is low,which has attracted widespread attention.However,the poor conductivity of ZnFe₂O₄ and the serious volume effect during charge and discharge process lead to low cycle stability,fast capacity decay,and poor rate performance,which limit the application of this material.In this paper,through reasonable structure design,coating and other means,to improve the structural stability of the material,reduce the volume effect of the material,increase its conductivity,increase its electrochemical active sites,reduce the polarization effect of the material to make the material has a stable charging and discharging platform,improves the overall electrochemical performance of the Zn Fe₂O₄anode material.The study is as follows:（1）One-dimensional single crystal ZnO nano-fibers were synthesized by a hydrothermal method.Below the 0°C,one-dimensional ZnO nano-fibers with diameter less than 100 nm and loaded with ZnFe₂O₄ nanoparticles were obtained by the ion exchange method with the different reaction time.The specific surface,average pore size and pore volume of the material are significantly increased,and the active sites of the material are increased.As the number of active points increases,the conductivity increases.At a current density of 0.5 A g^-1,after 250 cycles of charging and discharging,its capacity reached 490.7mAh g^-1,which reaches 49.8%of the theoretical capacity.In the rate performance test,when the current density reaches 2 A g^-1,the capacity is 288 mAh g^-1.（2）ZnFe₂O₄ nano-fibers were prepared by a simple electro-spinning process and pyrolysis to obtain carbon-coated ZnFe₂O₄.As the results shown,the 20 nm diameter nanoparticles were loose and deposited into ZnFe₂O₄ nano-fibers with a diameter of about70 nm.Through the vapor deposition process,a layer of conductive polypyrrole is coated on the surface of ZnFe₂O₄ nano-fibers.The test results show that the polypyrrole is evenly coated on the surface of the nanoparticles;the coating layer is about 1.5 nm,and the coating makes the loose grain become denser.As the electrochemical performance shown,the polypyrrole coating significantly improves the electrical conductivity and the cycle stability of the material.At a high current density of 1 A g^-1,after 220 cycles,its capacity is maintained at 881.1 mAh g^-1.In the rate performance test,when the current density reaches 10 A g^-1,its capacity still maintaines at 286.87 mAh g^-1.The microstructure of the electrode material after the cycle was observed,and it was found that although the fiber size was slightly increased,its morphology remained intact.Through in-situ XRD technology,the energy storage mechanism of ZnFe₂O₄ during charge and discharge was studied.In the initial stage of intercalation of lithium,Li⁺is only embedded in the lattice gap of ZnFe₂O₄,and did not change the type of crystal system and space group.During the intercalation of lithium,the material was oxidized to form Zn_xFe_yO phase.At the later stage of lithium intercalation,the crystalline material was transformed into an amorphous structure.Finally,the amorphous structure continues to charge and discharge.（3）Based on the above investigation,ZnFe₂O₄ nano-fibers were also synthesized by electro-spinning technology.The difference is that the polymer PAN is used as a carbon source,and after pre-oxidation,a thermally stable,trapezoidal stable structure of ZnFe₂O₄nano-fibers is formed.In this experiment,a tubular furnace was used to carbonize the material to produce stable,carbon-coated,low-valence Zn and Fe.The material was then calcined in a muffle furnace to allow the low-valence states Zn and Fe to be oxidized to high valences.Finally,carbon coated ZnFe₂O₄ nano-fibers were obtained.It was found that after calcination at 400°C,the low-valence states Zn and Fe were all oxidized to the highest valence state;at the same time,carbon-coated ZnFe₂O₄ was obtained.It can be known from the structural tests that the carbon layer completely encapsulates the Zn and Fe oxide grains.At a high current density of 1 A g-1,after 500 cycles,its capacity can still be maintained at1237.6 mAh g^-1.In the rate performance test,when the current density reaches 10 A g-1,its capacity can still maintain 260.3 mAh g^-1.The preparation process successfully improves the conductivity of the over-plated metal oxide,reduces the volume effect of the material,saves the production cost,and increases the yield.