In Situ 4D STEM of LiNiO₂ Particles Heated in an Oxygen Atmosphere: Toward Investigation of Solid-State Batteries...

Cathode active material (CAM) particles and solid electrolyte (SE) - CAM composites for solid-state batteries (SSBs) are often subjected to elevated temperatures during annealing or co-sintering. This thermal treatment can affect the material's structure and induce degradation processes, particularly at the SE - CAM interface. To better understand these phenomena and improve material stability and performance, investigations by (scanning) transmission electron microscopy ((S)TEM) under realistic processing conditions, i.e., in an oxygen atmosphere, are desirable. However, preparing electron-transparent TEM lamellae of SE - CAM composites with intact interfaces is highly challenging. Therefore, an in situ heating methodology is first established using LiNiO₂ (LNO) particles as a model system. In this study, the morphological and structural evolution of thinned LNO particles during heating in an oxygen atmosphere is investigated, employing in situ 4D nanobeam STEM. The in situ observations are complemented with postmortem electron diffraction and spectroscopy measurements. These findings indicate that LNO undergoes structural degradation at temperatures ≈350 °C, transitioning from the layered ( $R\overline{3}m$ ) structure to a NiO-type rock-salt phase ( $Fm\overline{3}m$ ). This onset temperature is significantly higher than that observed in comparable in situ heating experiments conducted in vacuum, highlighting the importance of an oxygen atmosphere for replicating real-world processing conditions.