Preparation And Lithium/Sodium Ion Storage Performances Of Iron/Tin-Based Oxide/Sulfide Composites

Iron/tin oxygen/sulfur compounds are extremely promising electrode materials in Li-ion batteries due to their high theoretical specific capacity(～1000 m Ah g^-1)and abundant natural resources.However,the poor electrical conductivity(～10^-4 S cm^-1)of iron tin oxide/sulfide will lead to poor rate and cycling performance of the material,and the dramatic volume change during Li/Na ion insertion/extraction causes the severe pulverization of the material and the electrode,which deteriorates the rate and cycle performance of the material,hindering its further commercial application.Therefore,in this thesis,the composite materials of iron/tin oxygen/sulfur compounds with different structures and morphologies were synthesized by different methods.The morphological characteristics,structural characteristics and electrochemical properties were analyzed by various characterization methods,and the energy storage mechanism of the material was studied.The details are as follows:In order to improve the conductivity of iron oxide,Fe₂O₃/RGO（GFe₂O₃）composites were prepared by in-situ thermal decomposition using the reduced RGO at high temperature as a matrix.The tight combination of Fe₂O₃ nanoparticle and RGO enhances transport rate of electrolyte ions and electrochemical reaction kinetics,thereby improving the lithium/sodium storage performance of the material.The effect of RGO content on the morphology,structure and lithium/sodium storage performance of GFe₂O₃ composites was investigated.The material has the optimal electrochemical performance when the mass fraction of RGO reaches 5%.As for Li-ion storage,it has a specific capacity of 1175 m Ah g^-1 at 0.2 A g^-1 and.The full cell assembled with commercial cathode material of Li Fe PO₄ shows an energy density of 356 Wh kg^-1.As for sodium ion storage,the GFe₂O₃ composite exhibits a specific capacity of 701 m Ah g^-1 at 0.1 A g^-1,and a reversible specific capacity of 210 m Ah g^-1 at 1 A g^-1 after 300 cycles.Compared with composite RGO,the coating of amorphous carbon can not only improve the electrical conductivity of the material but also effectively suppress the volume expansion.Porous Fe₂O₃/Fe₃O₄/carbon（PC@Fe O_x）composites were synthesized using the Fe₂O₃nanocubes as precursors,dopamine as carbon source and Si O₂ as hard template.The effects of porous structure and PDA-derived carbon content on the electrochemical performances of PC@Fe O_x composites were investigated.The PC@Fe O_x composite has the optimal lithium/sodium ion storage performance when the dopamine content is 20 mg.When the PC@Fe O_x composite is used as an anode for Li-ion batteries,it exhibits a high discharge specific capacity of 1149 m Ah g^–1 at 0.1 A g^–1.When the PC@Fe O_x composite is used as the anode of Na-ion batteries,it exhibits a discharge specific capacity of 406 m Ah g^–1 at 0.1 A g^–1.Besides composite conductive carbon materials,heteroatom doping is an effective method to improve the electrical conductivity of materials.NC@MXene/Co-doped Fe₃O₄ composites（NCMCFO）were prepared by co-precipitation method at room temperature using carbon-coated MXene nanosheets（NC@MXene）as the matrix.The Co-Fe3O4 particles with a diameter of～7 nm are uniformly distributed on the surface of the NC@MXene nanosheets,which effectively restrain the stacking of the NC@MXene layer and the self-aggregation of the Co-Fe3O4 particles,and enhance the diffusion and transport rate of electrons and ions.The effects of Co heteroatom and NC@MXene nanosheet content on the lithium ion storage performance of NCMCFO composites were investigated.When both Co heteroatom and NC@MXene content are 15%（NCMCFO-15）,NCMCFO composite has the optimal lithium ion storage performance.NCMCFO-15 composite shows a specific capacity of 1214 m Ah g^-1 at 0.1 A g^-1,and a specific capacity of 1220 m Ah g^-1 at 1 A g^-1 after 500 cycles.The maximum energy density and power density of the NCMCFO-15//AC LICs assembled with the activated carbon（AC）cathode are 112 Wh kg^-1 and 17 k W kg^-1,respectively.Compared with single metal oxides,double metal oxides have higher electrical conductivity and electrochemical activity.The Zn Fe（OH）_x/NC@MXene precursor was synthesized by microwave reaction using iron nitrate,zinc nitrate and NC@MXene as raw materials,and then Zn Fe₂O₄/NC@MXene composites（NCMZFO）was prepared by high temperature calcination under nitrogen atmosphere.NC@MXene effectively suppress the agglomeration of Zn Fe₂O₄ nanoparticles,enhances the electronic conductivity of the material.The effects of calcination temperature and NC@MXene content on the lithium storage performances of the composite were investigated.The NCMZFO composite has best lithium storage performance when the calcination temperature is 500°C and the NC@MXene content is 10%（NCMZFO-10）.The NCMZFO-10 composite shows the specific capacity of 1075 m Ah g^-1 at 0.1 A g^-1 and 789 m Ah g^-1 at 10 A g^-1.The maximum energy density and power density of the NCMZFO-10//AC LICs assembled with the activated carbon（AC）cathode are 124 Wh kg^-1 and 18 k W kg^-1,respectively.Compared with metal oxides with poor electronic conductivity,metal-sulfur bonds and narrow band gaps of metal sulfides are more favorable for electron conduction and electrochemical reactions.The N doped r GO/Sn Sx composite（Sn Sx/N-r GO）were prepared by a hydrothermal method.The Sn S_x nanoparticles with abundant heterostructures are uniformly dispersed on the surface of N-r GO nanosheets,which can effectively accelerate the charge transport and ion diffusion.The effects of N-r GO content on the lithium/sodium ion storage performances of the composite was investigated.The Sn S_x/N-r GO composite has best lithium/sodium ion storage performance when the N-r GO content is 30%.The Sn Sx/N-r GO-30composite exhibits a reversible capacity of 1273 m Ah g^-1 at 0.1 A g^-1 as anode for lithum ion batteries.The Sn Sx/N-r GO-30 composite shows a discharge specific capacity of 640 m Ah g^-1at 0.1 A g^-1 as anode material for sodium ion batteries.The maximum energy density and power density of the AC//Sn Sx/N-r GO-30 LICs assembled with the activated carbon（AC）cathode are108 Wh kg^-1 and 17 k W kg^-1,respectively.Compared with single metallic sulfides,double metallic sulfides exhibit higher electrical conductivity and ion diffusion kinetics.MXene/NC@Fe Sn S_x composites were prepared by co-precipitation-gas-phase vulcanization-electrostatic assembly methods.The presence of NC@Fe Sn S_x effectively prevents the self-stacking of MXene nanosheets.The tight encapsulation of MXene suppresses the agglomeration of NC@Fe Sn S_x nanocubes,enhances the electrical conductivity of the material,facilitates the diffusion of ion.The effects of gas-phase vulcanization temperature and MXene content on the lithium/sodium ion storage performance of the composite were investigated.The composite has best electrochemistry performance when the vulcanization temperature was 350°C and the MXene content was 5%.As for lithium ion storage,MXene/NC@Fe Sn S_x-5 exhibits specific capacity of 1080 m Ah g^-1at 0.1 A g^-1 and a specific capacity of 725 m Ah g^-1 at 10 A g^-1.For sodium ion storage,the MXene/NC@Fe Sn S_x-5 composite shows a specific capacity of 688 m Ah g^–1 at 0.1 A g^–1.