Structural Observation of Cathode/Sulfide Electrolyte Interface in All-Solid-State Batteries by <i>In Situ</i> Thin-film X-ray Diffraction

A model cathode interface for sulfide all-solid-state lithium-ion batteries consisting of a LiCoO2(104) epitaxial film, LiNbO3 buffer layer, and Li3PS4 film was fabricated using physical vapor deposition. In situ thin-film X-ray diffraction were applied to observe the crystal structure changes in the bulk and surface regions of LiCoO2 during the initial lithium (de)intercalation. The Li3PS4/LiNbO3/LiCoO2 interface exhibited an irreversible capacity during the first charge-discharge cycle, followed by reversible lithium (de)intercalation in the second cycle. The bulk and surface structures of LiCoO2 showed reversible structural changes during charging and discharging without significant degradation, suggesting that the initial irreversible capacity was due to the oxidation side reactions in the LiNbO3 and/or Li3PS4 layers. The crystal structure of the LiCoO2 surface differs from that of the bulk region and undergoes greater structural change than the bulk region. These results indicate that the surface structure of LiCoO2 depends on structural changes at the electrolyte-side interface, where side reactions occur. Direct observation of the crystal structure changes is crucial for achieving a deeper understanding of the reactions occurring at the oxide cathode/sulfide electrolyte interface.