UC Irvine

Garnet-type Li7La3Zr2O12 (LLZO) solid-state electrolytes have excellent ion conductivity and chemical stability and show immense potential for the advancement of next-generation all-solid-state batteries. Gaining an in-depth understanding of the structure-property relationship of LLZO is essentially important to optimize the electrochemical performance of LLZO and enhance its practicality for real-world applications. Transmission electron microscopy (TEM) is a powerful tool for the characterization of the structure, composition, and functional properties; however, it is challenging for LLZO because LLZO is susceptible to electron irradiation and also sensitive to the ambient air. In this study, advanced TEM techniques combined with in situ heating, biasing, and cryogenic cooling setups are employed as an effective approach to enable detailed characterization of the structure and transport properties of LLZO solid-state electrolytes, aiming to solve fundamental scientific questions regarding the phase transformations, grain boundary (GB) structures and compositions, and ionic conduction behaviors of LLZO. The investigation begins with a comparative study on the phase transformation during both sol-gel and solid-state syntheses of LLZO using in situ heating TEM, which identifies distinct phase transition pathways and clarifies the origin of surface amorphization. Subsequently, after careful control of sample preparation and imaging conditions to effectively mitigate electron-beam damage to LLZO, the examination of structures and compositions across the GBs of LLZO is conducted to reveal their impacts on ionic and electronic conductivities. It is found that a slight decrease in oxygen content consistently exists in the LLZO GB region, leading to the accumulation of positive charge affecting the local electrical field around the GB. Furthermore, an innovative technical advancement is achieved by the successful implementation of in situ TEM with operando electrochemical impedance spectroscopy measurements under air-free environments. This approach enables the direct measurement of the ionic conductivity of LLZO with different microstructures and further correlates it to the ion conduction and Li dendrite penetration failure at the Li/LLZO interfaces. Our findings offer valuable insights into the understanding of the structure-property relationship, elucidating the impact of microstructural features on ion transport and electrochemical performance. This work highlights the importance for microstructural engineering and compositional optimization of LLZO solid electrolytes, providing promising implications to the development of solid-state battery technologies.