Improving Lithium-Ion Conductivity by Co-Doping Al/Ta to Li₇La₃Zr₂O₁₂ using Molten Salt Synthesis Method

Solid state batteries are attracting attention as promising materials for next-generation energy storage systems due to their high energy density and safety. The sulfide solid electrolyte has excellent lithium-ion conductivity, but sulfur electrolyte easily reacts with moisture in the air to release sulfide. On the other hand, the oxide solid electrolyte does not exhibit the above toxic gas when decomposed. This research selected Li₇La₃Zr₂O₁₂ (LLZO), a garnet-type cubic phase, in light of chemical stability and introduced the molten salt synthesis (MSS) method and doped Al/Ta co-doped to improve lithium-ion conductivity. In particular, this method can overcome the disadvantage of MSS that forms an impurity phase during multi-component synthesis. In addition, using molten salt-based LiCl-KCl can form the cubic phase at a lower temperature in a shorter time than the traditional solid phase synthesis method. Al/Ta co-doping may induce more vacancies occupied by Li ions than single doping. In particular, Al single doping can inhibit 96h site substitution that interferes with lithium ion conductivity, and Al/Ta co-doping induces displacement of Li vacancy to 24d site, extending the path of Li⁺. Such a mechanism increases ion conductivity because the atoms of 24d having a small Li vacancy are located close to each other, thereby shortening the movement path of Li⁺. Preparation of pure cubic LLZO phase was performed in the range of 900°C to 1000°C with a calcination process of less than 4 hours. The synthesized powder was pelletized and subjected to a heat treatment process to increase the density at 900°C for 6 hours. For the measurement of Li⁺ conductivity, electrochemical impedance spectroscopy (EIS) was used in the frequency range of 10 Hz~1 MHz. As a result, small particles of 0.8~1.3 μm were obtained compared to the single-doped LLZO by a similar method, and the ion conductivity was 2.398×10^-4 S cm^-1, and a small amount of Al/Ta doping was sufficient to impart stability to the garnet structure of LLZO. In conclusion, the Al/Ta co-doped MSS method suggested that the LiCl-KCl molten salt serves to remove impurities, which is advantageous in synthesizing high-purity materials.