Synthesis and Characterization of Garnet-Type Li7La3Zr2O12 Solid Electrolyte Materials

Abstract

Garnet-type Li7La3Zr2O12 (LLZO) has attracted much attention because of its promising properties for all-solid-state lithium-ion batteries, such as high ionic conductivity of 3×10-4 S/cm at 25℃, good chemical stability against Li metal, and good mechanical strength. In this work, using doping elements of Al and Ta through a co-precipitation method, the cubic phase garnet structure has been stabilized from the tetragonal phase structure, which has a lower ionic conductivity than that of cubic phase structure. Both Al-doped and Ta-doped LLZO pellets were porous (relative density of ~60%), thereby leading to low room temperature Li-ion conductivities (4.5×10-6 S/cm for Al-doped LLZO and 1.9×10-5 S/cm for Ta-doped LLZO). A dense microstructure is needed to obtain a high Li-ion conductivity. Li6.75La3Zr1.75Ta0.25O12 (LLZTO) with a relative density of 92% was prepared by a simple co-precipitation method with cold uniaxial pressing and sintering in a lithium-rich air atmosphere, which was created by the addition of LiOH to the LLZTO cover powder. This LiOH-containing cover powder (15Li+LiOH) has been shown to improve densification and increase the room temperature Li-ion conductivity to 4.6×10-4 S/cm. This 15Li+LiOH cover powder results in the formation of an Al-containing interparticle phase, which facilitates the densification of LLZTO. The aluminum concentration increases during sintering and is located in the grain boundary. As the lithium content increases, the portion of lithium occupying the 96h octahedral site increases, which due to the higher mobility of these sites, and leads to an increase in the Li-ion conductivity. Chlorine, which was detected in the interparticle phase for LLZTO with a short sintering time of 1 or 2 hours, is not necessary for the densification, as LLZTO prepared with no Cl-containing salt also densified under the same experiment settings. However, LLZTO prepared with no Cl-containing salt of Zr(OH)4 showed a little lower relative density, which resulted in a slightly lower Li-ion conductivity than that prepared with ZrOCl2*8H2O (2.3×10-4 S/cm vs 3.4×10-4 S/cm, all sintered at 1100°C for 16 hours). Dense LLZO-PEO (LiClO4) composites with continuous LLZO ceramic body prepared in this work have low Li-ion conductivities due to the exposure to the air in both the sample preparation and conductivity test. Composite prepared by the one-time dip process has a larger total conductivity than that prepared by multi-time dip process (1.15×10-6 S/cm vs 7.2×10-7 S/cm).