Grain Size Effects in Ga-Doped Li_7La_3Zr_2O_(12) Solid Electrolyte for All Solid-State Lithium Ion Batteries

摘要

Lithium-ion batteries (LIBs) are one of the most promising energy storing devices, offering high volumetric and gravimetric energy density compared with other battery technologies. However, LIBs are suffering from safety issues due to the poor chemical stability and the flammability of organic liquid electrolytes. Solid state electrolytes are expected to solve such safety issues. Since the initial study by Murugan et al. in 2007, Li_7La_3Zr_2O_(12) (LLZO) garnet has received much attention due to its high ionic conductivity (10~(-3) to 10~(-4) S cm~(-1)) and stability against Li metal. LLZO is crystallized in both cubic (space group Ia-3d) and tetragonal (space group I4_1/acd) form. The ionic conductivity of cubic phase is two order magnitudes higher than that of tetragonal phase. However, it is difficult to obtain the cubic phase LLZO because the cubic phase is not stable at room temperature and can be obtained at high sintering temperature (> 1200°C). Therefore, super-valentcation doping, such as Al~(3+) and Ga~(3+), is introduced in order to increase the number of vacancies and results in the enhancement of stability of the high conductivity cubic phase at room temperature. Ga~(3+) is one of the most promising candidates due to its high ionic conductivity. Both grain size and density are important factors in determining the ionic conductivity, but the relationship is not clear in Ga-doped LLZO.