Preparation And Doping Modification On Garnet-type Li₇La₃Zr₂O₁₂ By A Solidstate Reaction

Garnet solid electrolyte Li₇La₃Zr₂O₁₂（LLZO）has a good prospect in the application of lithium ion batteries.Compared with organic liquid electrolytes,it has the advantages of thermal stability,wide electrochemical window and high safety,but the ionic conductivity is still lower than the liquid electrolyte and remains to be improved.In this paper,LLZO was selected as the research object.It is expected that LLZO ceramics with high density and excellent ionic conductivity can be achieved through the element doping and the improvement of sintering process.The main research contents were as follows:First,the preparation process of LLZO ceramics was explored.The preparation method of powder used traditional solid-state method.The best powder preparation process was determined by analyzing the phase and morphology of powder.Moreover,the effects of sintering temperature and holding time on the density and properties of LLZO ceramics were also studied,the optimum preparation process was determined:LLZO powder was prepared by ordinary ball milling,then it calcined at 900℃for 7h,after that 5%PVA was added.Next the powder was compressed into a ceramic piece and then sintered at 1230℃for 15 h to obtain the LLZO ceramic.The density of the LLZO ceramic was 90.4%,and the ionic conductivity was 1.08×10^-4S·cm^-1.In order to improve the performance of LLZO,the ion doping method was used in this study.Al₂O₃ was added to the LLZO,the Al³⁺was substituted for lithium ions to produce Li vacancies,so that the ceramic can stabilize in a highly conductive cubic phase.The relative density of 0.2Al-LLZO ceramics was 92.5%by sintering at1220°C for 4h,and the ionic conductivity was the best:σ=2.3×10^-4S·cm^-1.There is also a Zr replacement for the doping of LLZO.In this study,Nb₂O₅ was selected as the dopant,and Nb⁵⁺replaced zirconium ions.The effect of Nb⁵⁺doping on ceramic properties was similar to that of Al³⁺doping.When the Nb⁵⁺doping amount was 0.2 mol,the ceramics obtained by sintering at 1220°C for 8 h had the best density and ionic conductivity,which are 91.5%and 2.64×10^-4S·cm^-1,respectively.The doped Al³⁺occupies the position of Li⁺,which will generate Li vacancies,so that it can stabilize in the cubic phase.At the same time,Al³⁺will hinder the transmission channel of Li⁺due to occupy the Li site.This will have an adverse effect on its ionic conductivity.Therefore,the processing of x Al-y Nb-Li_7-3x-yLa₃Zr_2-y-y O₁₂ceramics was investigated.The results showed that when the doping amount of Al was 0.05 mol and correspondingly the doping amount of Nb was 0.45 mol,the 0.05Al-0.45Nb-Li_6.4La₃Zr_1.55O₁₂ solid electrolyte was obtained at 1220℃for 4h,its density was 93.5%,and the ionic conductivity was the best:σ=3.33×10^-4S·cm^-1.The dense microstructure can reduce the grain boundary resistance,So some low-melting substances can be added to form a liquid phase during the sintering process to promote the densification process of the ceramic;what?s more,it can also create a lower-resistance grain boundary by changing the grain boundary composition.In this experiment,Li₄SiO₄ and LiBO₃ were selected to investigate the effect of additive doping amount on material properties.When the addition amount of Li₄SiO₄was 1wt.%,the density and ionic conductivity of ceramics were increased and reached the maximum,which were 95.6%and 4.48×10^-4S·cm^-1,respectively.The addition of LiBO₃ did not increase the ionic conductivity of the 0.05Al-0.45Nb-Li_6.4La₃Zr_1.55O₁₂ceramic because the formation of the glass phase at the grain boundary increases the grain boundary resistance,.In addition,this experiment studied the density of 0.05Al-0.45Nb-Li_6.4La₃Zr_1.55O₁₂ ceramics by hot-pressed,and observed the change of ion conductivity.The results showed that the density of ceramics obtained by hot pressing sintering was 97.7%,and the ionic conductivity reached 7.43×10^-4S·cm^-1.It can be seen that the increase of ceramic density improved the ionic conductivity significantly.