Preparation And Ionic Conductivity Of Ga/Al Doped Li₇La₃Zr₂O₁₂ Solid State Electrolyte By Solid Phase Method

With the rapid development of lithium-ion-battery technology today,people begin to devote themselves to the research of solid state electrolyte because the liquid electrolyte system widely used in the market has serious safety risks.Among this,the garnet Li₇La₃Zr₂O₁₂?LLZO?material,a kind of solid state electrolyte with relatively high ionic conductivity,has the advantage of high security.However,it still has lower ionic conductivity than liquid electrolyte and higher interface impedance between the electrode and electrolyte.Cubic LLZO?c-LLZO?tends to own better ionic conductivity than tetragonal LLZO?t-LLZO?due to Li disorder arrangement,but it is stable at high temperature.In this paper,undoped LLZO and Ga/Al-doped LLZO solid state electrolyte were prepared via the solid phase reaction method.The influence of sintering and doping modification processes on the cubic phase structure and ionic conductivity of LLZO electrolyte were also discussed.The specific research contents are as follows:The undoped LLZO electrolyte was prepared by ball-milling method and the pure tetragonal presintering powder could be obtained under the calcination condition of 900oC-6h.When the sintering temperature is higher than 1160 oC,all the ceramics exhibit cubic phase,and grains are more tightly bound with relatively density over 90% at higher than 1220 oC.A small amount of tetragonal phase would remain after too short insulation time.With the extension to 15 h of thermal insulation time,the grain boundary porosity decreased gradually,and the density reached up to 94.9%.As the increase of sintering temperature,the total resistance of the ceramic decreased.Under the optimal sintering condition of 1230oC-15 h,the highest ionic conductivity was 1.14×10-4 S/cm.For the Li_7-3xGa_xLa₃Zr₂O₁₂?Ga-LLZO?,the main phase of the unreacted interphase Ga-LLZO presintering powder obtained by low temperature calcination coexisted with c-LLZO+ LZO.Despite the cubic phase structure of the ceramics,the sintering characteristics have not been improved.The temperature range of ceramic densification is relatively narrow.At 1240 oC,the density was about 90%,the grain size was 20?40?m,and EDS energy spectrum analysis showed that Ga was mainly distributed at the grain boundary but non-enriched,the electrical conductivity performance was slightly improved.Under the sintering condition of 1240oC-15 h,the Ga0.15 ceramic exhibited ?r = 92%,? = 3.86×10-4 S/cm,and Ea = 0.27 e V.By contrast,the presintering powders calcinated at high temperature showed garnet phase structure and pure cubic LLZO when x = 0.15.The sintering characteristics of ceramics were significantly improved,with wide densification temperature range.The densities were over 93% at higher sintering temperature than 1060 oC,which was about 150 oC lower than that of undoped LLZO electrolyte.The Ga-LLZO ceramics show pure cubic phase.Under the sintering temperature of 1080 oC,the grains size reduced to under 10?m for 10 h after insulation,but too long insulation time or too high sintering temperature led to abnormal grain growth.EDS energy spectrum showed that Ga obviously concentrates at the grain boundary to promote sintering densification.When the components of x = 0.15?0.25 were sintered for 10?15h at 1080 oC or x = 0.1?0.25 were sintered for 10 h at 1100 oC,the ionic conductivity of electrolyte could reach?7?10?×10-4 S/cm,which was nearly an order of magnitude higher than that of undoped LLZO.The lowest Li+ conduction activation energy of Ga0.25 ceramics sintered at 1100oC-10 h was Ea = 0.25 e V.For the Li_7-3yAl_yLa₃Zr₂O₁₂?Al-LLZO?electrolyte,the presintering powder after planetary ball milling showed pure cubic phase with y = 0.25,and the ceramics showed pure cubic with y ? 0.25.With the increase of Al content,the density increased gradually and the ionic conductivity increased first and then decreased.Among them,the Al0.25 ceramic showed ?r = 90.7% and the highest ? = 2.08×10-4 S/cm under 1200oC-15 h.After roller ball milling,the presintering powder of y = 0.2 was pure cubic,and the ceramics with y = 0.2?0.3 were pure cubic phase at 1160oC?1180oC but the abnormal peak strength existed at 1200oC?1220oC.As the Al content increased,the densities and ionic conductivities first increased and then decreased.Under the sintering condition of 1220oC-15 h,the Al0.2 ceramic showed ?r = 91.6% and the highest ? = 1.89×10-4 S/cm,comparable to the ceramics prepared by planetary ball milling.