Synthesis And Ionic Conductivity Study Of Zr Site Doped Li₇La₃Zr₂O₁₂ Solid Electrolytes

In recent years,lithium-ion batteries have made rapid progress,especially in the application of various electronic products and new energy vehicles.But there are many safety problems in organic liquid electrolyte of lithium ion batteries.Electrolyte is a necessary condition for lithium-ion batteries to obtain high voltage,high cycle performance and other advantages.However,these organic solvents are extremely flammable.Once the battery is heated or sparked due to internal short circuit,the electrolyte will be ignited in an instant and cause the whole battery to explode.Flammable liquid electrolyte can be converted into solid electrolyte to reduce the safety risk caused by flammability.However,the conductivity of solid electrolyte is lower than that of liquid electrolyte.How to improve its electrochemical performance has become a research hotspot.Li₇La₃Zr₂O₁₂,a lithium ion conductor,was synthesized from LiOH·H₂O,La₂O₃ and ZrO₂.Nd³⁺and Ta⁵⁺doped Li₇La₃Zr₂O₁₂ have been prepared by high temperature solid-phase synthesis.X-ray diffraction（XRD）analysis,scanning electron microscope（SEM）,and electrochemical impedance spectroscopy（EIS）have been used to characterize the phase composition,microstructures and ionic conductivity.Li₇La₃Zr₂O₁₂ tetragonal powders were synthesized by calcination at temperature 900℃for6 h.The effects of sintering temperature and holding time on the synthesis of LLZO were investigated.The results show that the solid electrolyte LLZO material sintered at 1175℃for 12 h has relatively high density and stable cubic phase.But the grain sizes of the samples are different,and there are a lot of holes in the samples.The ionic conductivity is 4.97×10^-5S/cm and the total activation energy of lithium ion migration is 0.41 eV.Lithium ion solid-state electrolyte with the formula Li_7+x+x La₃Zr_2-xNd_xO₁₂（x=0.1、0.2、0.3、0.4、0.5）have been synthesized by solid-phase method with LiOH·H₂O,La₂O₃ ZrO₂ and Nd₂O₃ as raw materials.The diffraction peaks of Nd-doped ceramics change from tetragonal phase to cubic phase at 1175℃.Nd doping reduces the number of pores in the sample and increases the density of the ceramics.With the increase of doping amount,the conductivity at room temperature also increases.But its conductivity is lower than that of undoped LLZO.The Ta-LLZO ceramics were synthesized by solid-state method,spark plasma sintering（SPS）and SPS combined with solid-state method（two-step method）with LiOH·H₂O,La₂O₃,ZrO₂and Ta₂O₅ as raw materials.The samples obtained by three synthetic methods are all cubic phase structures.However,Ta-LLZO samples with solid-state method have different particle sizes and porous holes.The Ta-LLZO samples sintered by SPS have fewer internal voids and smaller particles.The two-step method not only retains the high compactness of SPS samples,but also increases the grain size.The results of SEM show that the electrical properties of Ta-LLZO sintered by two-step method is the best.Its total ionic conductivity is2.9×10^-4 S/cm,and activation energy is 0.38 eV.