Selection of Surface Coating Materials for Ni-Rich NCM in All-Solid-State Batteries through Electronic Band Structure Analysis Using UPS/LEIPS

In recent years, all-solid-state batteries (ASSBs) have garnered significant interest as next-generation batteries for electric vehicles and renewable energy storage. Among the various cathode materials explored for ASSBs, Ni-rich-layered oxides, primarily composed of Ni, Co, and Mn (NCMs), have emerged as promising candidates, especially when combined with sulfide-based solid electrolytes (SEs). However, as the Ni content in NCMs increases, their capacity rises, while their cycling stability decreases. Moreover, the side interactions between NCMs and sulfide-based SEs lead to interface instability, which impedes the efficient transport of Li ions. Researchers have explored surface coatings, such as LiNbO3, as a protective layer on NCMs, aiming to mitigate undesirable interactions. Despite the observed improvements in battery performance, the mechanisms underlying the enhancement via surface coating remain unclear. In this study, we investigated the electronic band structures of the NCMs and Li10GeP2S12 (LGPS), a sulfide-based SE, to predict side reactions occurring at the interface upon their junction. Based on experimental results, we will discuss the solid–solid interactions between them and the properties required for surface coatings to suppress side reactions.