Preparation And Electrochemical Properties Of ZnFe₂O₄ Anode Materials For High Performance Lithium-ion Batteries

Due to the gradual depletion of fossil fuels and increased energy demand,the development of new energy storage technology is imminent.Lithium ion batteries have been widely used in electric vehicles,power grids,and portable devices due to their high energy density,long cycle life and large rate capability.However,typical conventional graphite anodes have low theoretical capacity(372 m Ah g^-1)and fast capacity decay,which limits their application in high performance lithium ion batteries.Therefore,new anode materials with larger capacity and higher stability should be developed.As a new anode material,mixed transition metal oxides with two metal cations have attracted widespread attention.The synergistic effect of many metals makes them have a higher theoretical capacity than graphite anodes.As a typical spinel mixed transition metal oxide,ZnFe₂O₄ stands out among candidate anode materials due to its low cost,low toxicity of zinc and iron,and ease of processing.In particular,the lithium ion storage mechanism of ZnFe₂O₄ involves transformation and alloying reactions,so it has a high theoretical capacity of 1072 m Ah g^-1.However,ZnFe₂O₄anodes face the problems of rapid capacity decay and poor rate performance during charging and discharging,which is due to the large volume expansion during cycling,particle agglomeration,and the inherent low conductivity of metal oxides.In order to overcome these problems,this article modified the ZnFe₂O₄ material through a reasonable strategy of coating and structural design to improve the conductivity of the material,reduce the degree of collapse,increase the cycle stability of the material,and improve the overall electrochemical performance of ZnFe₂O₄ as a negative electrode material for lithium ion batteries.The research contents of this thesis are as follows:（1）The ZnFe₂（C₂O₄）₃ precursor was prepared by co-precipitation method using steel waste pickling liquor as raw material,and then it was sintered in air at 800°C to obtain ZnFe₂O₄ material.PEDOT was coated on the surface by in-situ polymerization.The hierarchical bead chain ZnFe₂O₄-PEDOT composites with network of macroporous channels was finally successfully obtained,and the porosity of ZnFe₂O₄-PEDOT composites is as high as 86.86%.As a negative electrode material for lithium-ion batteries,this special composite structure has high reversible capacity and excellent cycle stability.Electrochemical measurements showed that the 15 wt%PEDOT coated ZnFe₂O₄ composites（ZFPE-15）delivered a discharge capacity of1510.5 m Ah g^-1 at 100 m A g^-1 after 200 cycles,which was much larger than that of pure ZnFe₂O₄(1055.9 m Ah g^-1),10 wt%PEDOT coated ZnFe₂O₄ composites(ZFPE-10,1264.2 m Ah g^-1),and 20 wt%coated ZnFe₂O₄ composites(ZFPE-20,900.1 m Ah g^-1).In addition,even after 200 cycles at a high current density of 1 A g^-1,the ZFPE-15composite material exhibited a high reversible capacity of 1004.7 m Ah g^-1.These more desirable electrochemical properties can be attributed to the interconnected macroporous channels in the hierarchical bead chain skeleton,which can buffer volume expansion during circulation and inhibit particle agglomeration.Furthermore,the PEDOT nanoparticle coating produces a composite with greater mechanical strength and higher electrical conductivity,which provides higher electron transfer during the charge/discharge processes.（2）By calcining a single bimetallic metal organic framework（MOF）precursor（ZIF-ZnFe,molar ratio of 3:1）,a two-component Zn O/ZnFe₂O₄/C microsphere（ZZFO-C）with hierarchical hollow structure was obtained.The bicomponent-active hetero structure can facilitate charge transport to improve the lithium storage performance of electrode materials because of its internal electric field at hetero-interfaces.Observed through an electron microscope photograph,the obtained material was porous and was composed of numerous nanoparticles（with a size within 20 nm）in an orderly manner.For comparison,porous Zn O/ZnFe₂O₄（ZZFO）microspheres were obtained by tuning the annealing conditions.ZZFO-C composite material shows excellent lithium storage performance as a lithium ion battery anode.At a current density of 200 m A g^-1,after 100 charge/discharge cycles,the discharge capacity can still be maintained at 1000 m Ah g^-1,while the discharge capacity of the ZZFO anode material under the same conditions can only be maintained at 650 m Ah g^-1.The excellent performance of ZZFO-C arises from its unique hierarchical hollow structure and the synergy between the two active components and the N-doped carbon matrix.