Preparation Of Li₇La₃Zr₂O₁₂ And Doping Modification

Organic liquid electrolytes make traditional lithium-ion batteries have potential security risks such as leakage and combustion,which limit their wide application.In recent years,the rapid development of solid electrolytes has improved the safety of lithium ion batteries to a great extent.There are many kinds of solid electrolytes,among which the lithium lanthanum zirconium oxide Li₇La₃Zr₂O₁₂?LLZO?inorganic solid electrolyte with garnet structure has attracted much attention due to its good chemical stability,relatively wide electrochemical window and high mechanical strength.LLZO usually has two different phases of tetragonal phase and cubic phase.The cubic phase structure LLZO has relatively high ionic conductivity,but its structural stability is poor.Therefore,how to stabilize the cubic phase structure of materials and improve the room temperature ionic conductivity of materials have become a research hotspot.In view of above problems,LLZO and doped M-LLZO?M=Al,Ta,Nb?materials are prepared by high-temperature solid-phase method through two-step calcination process.In addition,the effect of different calcination and doping modification processes on the related properties of materials are investigated respectively.Detail contents are as follows:In the process of preparing LLZO materials,the effects of calcination temperature and calcination time on the phase structure transformation and micromorphology of the precursor materials are investigated respectively.Through comparing the test characterization results,the optimized precursor calcination process is received.Based on the pre-burning process optimized in the first step,the influences of calcination temperature and calcination time in the second calcination process on the cubic phase structure stability,micromorphology and room temperature ionic conductivity of the target product?LLZO material?are investigated.X-ray diffraction?XRD?,scanning electron microscopy?SEM?and electrochemical impedance spectroscopy?EIS?are used to study the phase transformation,micromorphology and room temperature ionic conductivity of LLZO materials by different calcination processes.According to the test results,the preferred preparation process is obtained,that is,the precursor material obtained after calcinating at 950°C for 6 h exhibits a good microscopic morphology and has a tendency to change to the cubic phase structure.After the precursor material are treated by 1150°C for 12 h,the obtained LLZO material has a cubic phase structure and has a relatively high density?79.50%?and a relatively high room temperature ionic conductivity(3.21×10^-6 S·cm^-1).In order to further improve the cubic phase structure stability and increase the density and room temperature ionic conductivity of LLZO materials,the effects of different calcination temperatures and doping amounts of elements x on the structural stability,micromorphology,room temperature ionic conductivity and other properties for the cubic phase of M-LLZO materials based on optimized calcination process are investigated.In this paper,different elements?Al,Ta,Nb?are used to study the doping modification of the material.Al is used as a low-valent doping element to replace the lithium in the material,which affects the lithium vacancy concentration of the material;Ta,Nb as a high-valence doping elements mainly replace the zirconium position in the material,and change the framework structure of the material to produce a certain concentration of lithium vacancies;the generation of lithium vacancies and the change of the material structure framework can stabilize the cubic phase structure of material to some extent and increase the ionic conductivity of the material.The phase structure,morphology and electrochemical properties of LLZO materials are characterized by XRD,SEM and EIS.The effects of Al doping on the above properties of LLZO materials are analyzed according to the test results.When the Al doping amount is x=0.3 with calcination temperature is 1100°C for 12 h,the experimental results show that the Al-LLZO material exhibits good cubic phase structure stability,high relative density?89.00%?and high room temperature ionic conductivity(9.71×10^-6 S·cm^-1).Subsequently,the effects of the doping modification process of Ta and Nb elements on the related properties of LLZO materials are investigated.The effects of different calcination temperatures and different doping amounts on the phase structure transition,microstructure and room temperature ionic conductivity of M-LLZO?M=Ta,Nb?materials are also tested by XRD,SEM and EIS.The experimental results show that the M-LLZO material prepared when the doping amount of Ta and Nb elements is x=0.3and calcined at 1100°C for 12 h can exhibit stable cubic phase structure and relatively high density.?Ta-LLZO:89.16%;Nb-LLZO:83.95%?and high room temperature ionic conductivity(Ta-LLZO:2.58×10^-4 S·cm^-1;Nb-LLZO:7.47×10^-6 S·cm^-1).