Preparation And Electrical Conductivity Of Doped Li₇La₃Zr₂O₁₂ Ceramics

Li₇La₃Zr₂O₁₂（LLZO）is a promising new electrolyte for Li ion battery with high ionic conductivity,excellent chemical stability with Li electrode a nd wide electrochemical window.However,the conductivity of LLZO is yet to be improved compared to the conductivity of liquid electrolyte.In the process of preparing LLZO by high temperature solid phase reaction,the formation of the second phase with low conductivity results from the volatilization of Li element caused by high temperature sintering,will affect the material properties.In this article,two LLZO compounds namely Li6.1–2x Znx Al0.3La3Zr2O12 and Li6.1–2xMgxAl0.3La3Zr2O12 were synthesized by high temperature solid phase reaction in the atmosphere of air.X–ray diffraction,scanning electron microscopy and electrochemical impedance spectroscopy were performed to characterize the microstructure and electrical properties of the compounds at diffe rent temperature.The effects of heating rate,milling time,purity of lithium carbonate,amount of raw material,sintering temperature,holding time,the content of Al and the content of lithium on the formation of c–LLZO and elimination of La2Zr2O7 were investigated.It has been found that Zn/Mg element doping plays an important role in the stabilization of c–LLZO as well as lowering the sintering temperature.As a result,the synthesis process was optimized with excellent powder dispersion and promoted crystallization.Garnet type of LLZOs was prepared at different sintering temperature with particle sizes at several microns.It was found that the density and crystallization can be increased by increasing the sintering temperature and the conductivity can be improved by the doping of Zn/Mg.Proven by research,with the increase of the content of doped elements,the conductivity of the material will increase first and then decreases in the system of Li6.1–2xZnxAl0.3La3Zr2O12 and Li6.1–2xMgxAl0.3La3Zr2O12.The intrinsic conductivity of the material is determined by two aspects: the transport of cations and vacancies.The increase of the crystallinity of the material will decrease the transport rate of both cations and vacancies,while the increase of densit y,which means a decrease in the porosity and a increase in the transport area,will lead to an improvement of the conductivity.More cationic vacancies can be formed by doping high valence cations,which will increase the conductivity of the materials;however,the increased ion radius and valence caused by high valence cations doping will lower the ion transport further,thus reducing the electrical properties of material.Therefore,changing of the temperature,the type and content of doped elements,all these factors will influence the conductivity of LLZO.When the sintering temperature of LLZO increased from 1000 to 1100 ℃,the conductivity of the material first increased and then decreased.The electrical conductivity of the material Li6.1–2x Znx Al0.3La3Zr2O12 reached the maximum at x=0.2 in 1050 ℃ sintering.At the temperature of 50 ℃,the conductivity of the material was 1.20×10–3m S·cm–1.