Direct Visualization of Metastable Charged States in Positive Electrode Materials by Cryo-STEM

LiNiO2 is regarded as an ideal positive electrode material for Li-ion battery applications, offering a large reversible capacity, >200 mA h g–1, with a lower cutoff voltage, 4.3 V. The fully charged state, Li1–xNiO2 (x ≈ 0.9), is a metastable phase obtained by electrochemical oxidation in Li cells. Due to its instability, atomic-scale imaging of this phase by using scanning transmission electron microscopy (STEM) is particularly challenging, as it readily decomposes under electron beam exposure. In this study, the effect of cryogenic conditions during STEM observation is examined. While as-prepared LiNiO2 shows strong resistance to beam damage, Li1–xNiO2 (x ≈ 0.9) undergoes structural changes under room-temperature beam exposure. In contrast, beam-induced damage is substantially suppressed under cryogenic conditions (−150 °C), enabling successful acquisition of atomic-resolution STEM images. These findings underscore the importance of cryo-STEM techniques for imaging metastable battery materials.