Modification Of FeNb₁₁O₂₉ And Its Electrochemical Performances

M-Nb-O anode materials has attracted more and more attentions as electrode for lithium-ion batteries because of its high safety,high theoretical capacity and good cycle performance.Among them,FeNb₁₁O₂₉ has a high theoretical capacity of 400mAhg^-1 due to the existence of Fe²⁺/Fe³⁺,Nb³⁺/Nb⁴⁺and Nb⁴⁺/Nb⁵⁺redox couples,which is even larger than that of graphite(372mAhg^-1).Moreover,Nb³⁺/Nb⁴⁺and Nb⁴⁺/Nb⁵⁺redox couples are within acceptable safe voltage ranges of 1.0-2.0 V,guaranteeing the security of material.In addition,FeNb₁₁O₂₉ belongs to Amma space group with a Re O₃ orthogonal shear structure,the structure consists of edge-horn shared octahedral and a small amount of tetrahedron,the octahedral and tetrahedron,forming a layered structure of A-B-A model.This unique structure makes FeNb₁₁O₂₉ material more open and better lithium ion diffusion property,as well the better structural stability.Thus,FeNb₁₁O₂₉ also has good cyclic stability.However,the actual capacites of FeNb₁₁O₂₉ material are reported to be only 168-273mAhg^-1.Therefore,in order to meet the practical application demand of FeNb₁₁O₂₉ material in lithium-ion battery anode materials,it is very necessary to modify FeNb₁₁O₂₉ material accordingly to improve its electrochemical performances.（1）In this thesis,Ga was doped into FeNb₁₁O₂₉ material by a simple one-step solid-state reaction method,and the electrochemical performances of FeNb₁₁O₂₉ were successfully improved.XRD results show that Ga-doping does not change the orthogonal shear Re O₃crystal structure of FeNb₁₁O₂₉.Scanning electron microscopy and specific surface area results indicate that the morphology and grain size of FeNb₁₁O₂₉ do not change significantly.The Ga-doping increases the electrical conductivity of FeNb₁₁O₂₉ from 6.86×10^-8 S cm^-1 to3.04×10^-6 S cm^-1.The Ga_0.2Fe_0.8Nb₁₁O₂₉ material prepared with appropriate Ga-doping amount exhibits excellent electrochemical performances.At 0.1C,the specific capacity of Ga_0.2Fe_0.8Nb₁₁O₂₉ reaches 290mAhg^-1,and the capacity can still maintain 145mAhg^-1when the current density reaches 5C.Moreover,Ga_0.2Fe_0.8Nb₁₁O₂₉ has good cycling stability with a capacity retention rate of 91.0%after 1000 cycles at 5C,while the capacity retention rate of undoped FeNb₁₁O₂₉ is only 107mAhg^-1 with a capacity retention rate of 55.9%after1000 cycles at 5C.Therefore,the Ga-doping can significantly enhance the electrochemical performances,making FeNb₁₁O₂₉ material a broad application prospect using as electrode for lithium-ion batterries.（2）A simple and effective solid-state reaction method was demonstrated to successfully dope Al into FeNb₁₁O₂₉ material.Compared with the pristine FeNb₁₁O₂₉,the crystal structure,morphology,particle size of the material were not significantly changed when reasonable amount of Al was doped.However,the structure stability of Al_0.2Fe_0.8Nb₁₁O₂₉ material is better than that of pristine FeNb₁₁O₂₉ because the bond strength of Al-O is stronger than that of Fe-O,which enhances the structural stability and therefore the electrochemical performances of FeNb₁₁O₂₉.At 0.1C,the first-cycle Coulomb efficiency of Al_0.2Fe_0.8Nb₁₁O₂₉reaches 95.0%,and the reversible capacity is further enhanced to 318mAhg^-1.When the current density increases to 10C,the Al_0.2Fe_0.8Nb₁₁O₂₉ still keeps a high reversible capacity of 142mAhg^-1 and capacity retention rate of 92.9%after 1000 cycles.Therefore,Al-doping is a promising method to improve the electrochemical performances of FeNb₁₁O₂₉.In addition,Li PFe O₄//Al_0.2Fe_0.8Nb₁₁O₂₉ full cells were assembled,which also showed good electrochemical performances.At 0.1C,the discharge capacity of full cell is 206mAhg^-1.The retention rate reaches 89.8%after 1000 cycles at 5C.These research results indicate that Al-doping technology can enhance the electrochemical properties of FeNb₁₁O₂₉ material significantly,which make FeNb₁₁O₂₉ material have a broad application prospect in the field of lithium-ion batteries.