Synthesis And Air-Stability Study Of Garnet-Type Lithium Ion Electrolyte

Because of the high conductivity(¹0^-4Scm^-1),good thermal and chemical stabilities,garnet-type lithium ion electrolyte Li₇La₃Zr₂O₁₂?LLZO?is an promising alternative to traditional organic liquid electrolyte for Li ion batteries?LIBs?.In this article,the Al-doped Li₇La₃Zr₂O₁₂ with cubic garnet phase was successfully synthesized using a solid-state reaction route and the synthesis conditions including sintering temperature,Al doping level and crucibles were systematically studied.The optimal Al dopant concentration was determined to be 0.2⁰.3 mol to form cubic garnet phases at 1100°C.Besides,I synthesized 0.25Al doped Li₇La₃Zr₂O₁₂ pellets in Pt crucibles and alumina crucibles,respectively.As a result,the composition and microstructure of the pellets were different,which made great difference to the ionic conductivity,relative density and stability in the air of LLZO.The 0.25Al-LLZO pellets sintered in Pt crucibles were well-sintered and dense,which render a high relative density?96%?and high ionic conductivity(4.48×10^-4 Scm^-1).I found that the negligible grain boundaries in pellets from Pt crucibles facilitated the improvement of ionic conductivity,and air-satbility.However,the ionic conductivity of pellets from Al₂O₃ crucibles was 1.81×10^-4 Scm^-1 and dropped to 2.39×10^-5 Scm^-13 months later,due to the formation of Li₂CO₃ on the pellets.We found that the stability of garnet depended significantly on its relative density.In order to study the formation of Li₂CO₃ on LLZO,0.5Ta doped Li₇La₃Zr₂O₁₂?0.5Ta-LLZO?pellets were exposed in dry?humidity⁵%?and moist air?humidity⁸0%?for 6 weeks,respectively.After 6 weeks moist air exposure,the ionic conductivity of 0.5Ta-LLZO dropped from6.4×10^-4 Scm^-1 to 3.6×10^-44 Scm^-1.However,for 0.5Ta-LLZO exposed in dry air,there was no Li₂CO₃ formed on the surface and the ionic conductivity nearly no changed.So,the direct reaction between CO₂ and LLZO is difficult.Then,I decoupled the reaction between LLZO pellets with water and CO₂ by immersing LLZO pellets in distilled-water for a week and then exposed them to ambient air for another week.After 0.5Ta-LLZO was immersed in distilled-water,Li⁺/H⁺exchange occurred and LiOH·H₂O formed on the surface,which is the necessary mediate and a required precursor of the CO₂ reaction.After being exposed in ambient air,LiOH·H₂O reacted with CO₂ from ambient air to form a layer of Li₂CO₃on the surface of these pellets.Then,Li⁺extract from the inside of LLZO and slowly diffuse through the shell of Li₂CO₃ to react with CO₂ from air.