Advanced transmission electron microscopy of garnet type solid electrolytes for high-performance solid-state batteries

Li-ion batteries (LiBs) are a key technology for a variety of emerging technologies, ranging from portable devices to electro-mobility, considered to have a huge impact on modern society. Unfortunatly, the energy density of LiBs is approaching its theoretical limit (1). Therefore, solid-state batteries (SSB) came in the spot-light of battery research, potentially enabling a doubling in energy density compared to conventional LiBs and, at the same time, avoiding drawbacks of liquid electrolytes, such as its flammability and toxicity (1). Some of the most promising candidates to be used as solid electrolyte in SSBs is Li7La3Zr2O12 (LLZO), exhibiting a room-temperature Li-ion conductivity rivaling that of liquid electrolytes (1, 2). Despite the significant progress, a throughout understanding of structure, defect formation and interface morphology, on the atomic level, is still lacking. This lack in understanding is related to the susceptibility of these materials to air and humidity as well as their sensitivity to ion- and electron irradiation, posing significant challenges to conventional TEM analysis techniques. Therefore, we develop low-dose (scanning) transmission electron microscopy methods including electron energy loss spectrometry (EELS) and energy dispersive X-ray spectrometry (EDX), paired with advanced sample preparation and transfer techniques. Herein, e.g., we demonstrate that lattice resolution at a LiCoO2 (LCO)|LLZO interface can be achieved and crystalline quality can be assessed in the STEM mode. Most interestingly, EELS spectrometry reveals that not only Co diffuses into LCO during high temperature processing; also La diffuses into LCO which potential significant impact on the electrode performance.