Quantitative Operando Visualization of Electrochemical Reactions and Li Ions in All-Solid-State Batteries by STEM-EELS with Hyperspectral Image Analyses

All-solid-state lithium-ion batteries (LIBs) are one of the promising candidates to overcome some issues of conventional LIBs with liquid electrolytes. However, high interfacial resistance of Li-ion transfer at the electrode/solid electrolyte limits their performance. Thus, it is important to clarify interfacial phenomena in a nanometer scale. Here, we present a new method to dynamically observe the Li-ion distribution and Co-ion electronic states in a LiCoO2 cathode of the all-solid-state LIB during charge and discharge reactions using operando scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). By applying a hyperspectral image analysis of non-negative matrix factorization (NMF) to the STEM-EELS, we succeeded in clearly observing the quantitative Li-ion distribution in the operando condition. We found from the operando observation with NMF that the Li ions did not uniformly extract/insert during the charge/discharge reactions, and the activity of the electrochemical reaction depended on the Li-ion concentration in a pristine state. An electrochemically inactive region was formed about 10-20 nm near the LiCoO2/Li2O-Al2O3-TiO2-P2O5-based solid electrolyte interfaces. The STEM-EELS, electron diffraction, and Raman spectroscopy experimentally showed that the inactive region was a mixture of LiCoO2 and Co3O4, leading to the higher interfacial resistance of the Li-ion transfer because Co3O4 does not have pathways of Li-ion diffusion in its crystal.