Structural Observation of Cathode/sulfide Electrolyte Interface in All-solid-state Batteries by In Situ Thin-film X-ray Diffraction

Abstract

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.