Synthesis Of Nanosheet-Assembled ZnFe₂O₄ Yolk-shell Microspheres For Lithium-ion Batteries

At present,transition metal oxides,as excellent alternative anodes materials for lithium-ion battery,have been widely studied.In the transition metal oxide system,Zn Fe₂O₄ has the advantages of higher theoretical specific capacity(1000.5 mAh g^-1),being inexpensive and abundant,being non-toxic and friendly to the environment,which is expected to be developed and applied to new anode materials for lithium-ion batteries.However,Zn Fe₂O₄ has the disadvantages of poor conductivity and large volume expansion when lithium-ions are inserted and extracted,which ultimately leads to poor cycle stability and rate performance.For this common problem of transition metal oxides in the battery field,from the view of material,the current main strategies are focused on reducing the size,constructing specific structures such as hollow/core shells,doping and compounding to improve the electrochemical performance of ZnFe₂O₄.In this dissertation,nanosheet-assembled ZnFe₂O₄ yolk-shell microspheres were designed,which have larger porosity and higher specific surface area.And it can provide more active sites and promote contact between the electrode material and the electrolyte,improving the electrochemical performance.When lithium-ions are inserted and extracted in the electrochemical process,the voids between the ZnFe₂O₄nanosheets and the core-shell structure can provide an effective buffer for volume expansion and contraction,contributing to improving the electrochemical stablity.At the same time,in order to further improve the electrochemical performance of yolk-shell ZnFe₂O₄ electrode materials,PPY?polypyrrole?-coated ZnFe₂O₄?Zn Fe₂O₄@PPY?or carbon-coated ZnFe₂O₄?Zn Fe₂O₄@C?yolk-shell structure were designed.Polypyrrole-coated or carbon-coated can provide a certain supporting for the material,buffering the volume expansion caused by the insertion and extraction of lithium-ions in the electrochemical process,and improving the conductivity of the material.Our work is as follows:In this dissertation,taking the novel binary transition metal oxide ZnFe₂O₄ as the research object,and nanosheet-assembled ZnFe₂O₄ yolk-shell microspheres have been obtained by solvothermal reaction for 8 h and annealing in the air at 400°C for 2 h.And exploring the effect of solvothermal synthesis time,synthesis temperature and annealing temperature on the morphology and structure of the material.At a current density of 0.2 A g^-1,the reversible specific capacity of ZnFe₂O₄ yolk-shell microspheres remained 780 mAh g^-1 after cycling 10 cycles.At a current density of 2A g^-1,the reversible specific capacity of ZnFe₂O₄ remained 249 mAh g^-1 after cycling200 cycles.Under the high current density of 10 A g^-1,the specific capacity of Zn Fe₂O₄ is 120 mAh g^-1.ZnFe₂O₄@C yolk-shell microspheres were synthesized by stirring at room temperature and post-nitrogen annealing.At a current density of 2 A g^-1,the reversible specific capacity of ZnFe₂O₄@C yolk-shell microspheres remained 317 m Ah g^-1 after cycling 200 cycles.Under the high current density of 10 A g^-1,the reversible specific capacity of ZnFe₂O₄@C yolk-shell microspheres is 150 mAh g^-1.ZnFe₂O₄@PPY yolk-shell microspheres were synthesized by liquid phase reaction at low temperature.At a current density of 2 A g^-1,the reversible specific capacity of Zn Fe₂O₄@PPY yolk-shell microspheres remained 533 mAh g^-1 after cycling 200 cycles.Under the high current density of 10 A g^-1,the reversible specific capacity of ZnFe₂O₄@PPY yolk-shell microspheres is 205 mAh g^-1.