Microstructural and Electrochemical Properties of Al- and Ga-Doped Li7La3Zr2O12 Garnet Solid Electrolytes

The Garnet-type solid electrolyte Li7La3Zr2O12 (LLZO) showing high ionic conductivity and a wide electrochemical potential window is considered as one of the most promising candidates for solid-state batteries. Among various doping derivatives, the Al- and Ga-doped LLZO electrolytes are intensively studied because of their ability to enhance the stability of the cubic structure and to promote good sinter ability as well. Despite showing great similarities in site preference and sintering behavior, the Al and Ga doping derivatives differ by 1 order of magnitude in total ionic conductivity. Therefore, a comparative study on the doping characteristics of Al and Ga with respect to ionic conductivity and sintering behavior is necessary. Herein, we simultaneously introduced Al and Ga into Li sites (AlxGa0.25–x-LLZO) to study their influences on the microstructure and electrochemical properties of LLZO. The results show that Ga doping enables a higher conductivity than Al doping and largely promotes sinter ability at the same time. Compared to Al single doping (Al0.25), Ga-contained compositions (x < 0.25) show significant grain growth. Moreover, a slight inclusion of Ga (Al0.20Ga0.05) not only modifies the sintering behavior that results in a microstructure transition from fine grains of Al0.25 (5–20 μm) to abnormally large grains (several hundred micrometers) but also greatly enhances the conductivity, yielding a value that is 3 times higher. However, further increases in Ga ratio in AlxGa0.25–x results in marginal increases in conductivity. The total conductivity reaches a maximum of 1.19 × 10–3 S cm–1 for Ga single doping (Ga0.25) at room temperature. In addition, Al0.25 also shows fast ion conducting behavior along the grain boundaries with a conductivity of 8.30 × 10–4 S cm–1. Consequently, this study sheds lights on the different doping characteristics between Al and Ga, providing guidance for fine composition engineering of these two promising dopants.