High ion conductivity in garnet-type F-doped Li₇La₃Zr₂O₁₂

A novel garnet-type solid electrolyte, F-doped Li₇La₃Zr₂O₁₂ (LLZO), was prepared via conventional solid-state reaction. Certain fluoride compounds were known to form the pure cubic garnet structure after continuous calcination at 900℃ and 1125℃, seperately. Fluorine ions doped in LLZO contributed to the stabilization of high ion conductivity garnet phase as well as the enhancement of sintering activity. The effects of fluorine ions were studied using the dopants LiF and CaF₂. The 1.0wt% LiF-LLZO samples exhibited a total conductivity of 5×10^-4 S/cm close to the bulk conductivity value of un-doped LLZO, while its activation energy was 0.26 eV, lower than that of other cation-doped LLZO samples. The strong intensity of the peaks indexed to (321), (400), (642), and (800) in XRD patterns indicated the growth of ceramic grains of F-doped LLZO in certain favorable crystallographic orientations during sintering process. In addition, a unique microstructure was revealed in the novel garnet-type oxide pellets, showing the grain boundary almost removed and closed pores formed, which resulted in the negligible grain boundary and the high total ion conductivity.