Direct Observation of Li Distribution in Bulk-Type All-Solid-State Batteries by Operando Electron Microscopy

All-solid-state batteries (ASSBs) with solid electrolytes are expected to be promising energy storage devices. However, the large charge-transfer resistance at the electrode/electrode homo-interface and/or electrode/solid-electrolyte hetero-interface needs to be solved for practical applications. To overcome this problem, it is necessary to understand how the ions are transferred during battery operation. Here, we used operando electron energy-loss spectroscopy (EELS) using a scanning transmission electron microscope (STEM) to directly visualize the Li-ion movement in the cathode materials in bulk-type ASSBs. We show that the ion movement is strongly related to the crystal orientations of the cathode particles, and that Li-ions move even in the open-circuit conditions of the batteries.