Synthesis And Performance Of Doped-Li₇La₃Zr₂O₁₂ Solid Electrolytes Using In All Solid State Batteries

There are two polymorphs existing in the garnet-type structure:cubic structure and tetragonal structure.The ionic conductivity of the former is nearly two orders of magnitude higher than that of the latter(?10^-6 S·cm^-1).Co MParing with liquid electrolytes,solid-state electrolytes have a wide electrochemical potential and a great long-term cycling performance.Therefore,it can assembly with more anode and cathode materials.However,there are few researches on the assemblability between solid electrolytes with anode and cathode materials.Also,a major shortcoming of lithium garnet electrolytes is that side reactions take place when exposing to air and/or water.Traditional solid-state reaction procedure was adopted for the synthesis of Li₇La₃Zr₂O₁₂.The effect of sintering temperature,method and holding time on the structures and densities of Li₇La₃Zr₂O₁₂ has been investigated.The mixture powders were calcined at an optimized temperature of 850°C in zirconia crucibles.The solid electrolyte pellets were covered with mother powders and sintered at an optimized temperature of 1100°C for 6 h in air.Pressureless sintering covered with mother powders method was adopted for the synthesis of Li_7-xLa₃Zr_2-xM_xO₁₂?M=Nb,Ta?and Li_6+yLa_3-yA_yZr Nb O₁₂?A=Sr,Ba?.The effect of Nb and Ta,Sr and Ba on the structures,ionic conductivities and activation energies of Li_7-xLa₃Zr_2-xM_xO₁₂?M=Nb,Ta?and Li_6+yLa_3-yA_yZr Nb O₁₂?A=Sr,Ba?has been investigated.Among the Nb-Ta-codoped garnets,Li_6.5La₃Zr_1.5Nb_0.4Ta_0.1O₁₂?x=0.4?pellets have the highest relative density and finest grains.The lithium ionic conductivity and activation energy were reached to 1.04×10^-3 S·cm^-1 and 0.45 e V.Among the Sr-Ba-codoped garnets,Li_6.5La_2.5Sr_0.4Ba_0.1Zr Nb O₁₂?y=0.4?pellets have the highest relative density and finest grains.The lithium ionic conductivity and activation energy were reached to 7.23×10^-4 S·cm^-1 and 0.37 e V.In addition,due to lithium garnet electrolytes are easily to have side reactions when exposing to air and/or water.The stability of the lithium garnet against environment has been investigated and the degradation mechanism has been attempted to clarify.The cubic garnet structure remains stable when the relative humidity under 20%.When the relative humidity between 20%and 70%,the moisture in air reacts with garnet oxides via a H⁺/Li⁺exchange mechanism and leads to the formation of Li OH?H₂O as a surface layer which further reacts with CO₂ in air to form Li₂CO₃.However,the carbonation reaction can not happen when the relative humidity higher than 70%.Spark plasma sintering method was adopted for the synthesis of Li_6.5La₃Zr_1.5Nb_xTa_0.5-xO₁₂ and Li_6.5La_2.5Sr_yBa_0.5-yZr Nb O₁₂.The effect of Nb and Ta on the structures,ionic conductivities and electrochemical performances of Li/Li_6.5La₃Zr_1.5Nb_0.4Ta_0.1O₁₂/Li Co O₂ cells has been investigated.After 100 cycles,the charge and discharge specific capacity were 63.49 and 59.20 m Ah/g,respectively and the coulombic efficiency was 98.26%.In addition,the effect of Sr and Ba on the structures,ionicconductivitiesandelectrochemicalperformancesof Li/Li_6.5La_2.5Sr_0.4Ba_0.1Zr Nb O₁₂/C cells has been investigated.After 50 cycles,the charge and discharge specific capacity were 359.80 and 357.20 m Ah/g,respectively and the coulombic efficiency was 99.27%.