# In Situ 4D STEM of LiNiO2 Particles Heated in an Oxygen Atmosphere: Toward Investigation of Solid‐State Batteries Under Realistic Processing Conditions

## TL;DR

This study uses in situ electron microscopy to observe how LiNiO2 particles degrade when heated in oxygen, relevant for improving solid-state battery materials.

## Contribution

A new in situ heating protocol for studying cathode materials under realistic oxygen-rich conditions is developed and applied to LiNiO2.

## Key findings

- LiNiO2 degrades at ~350°C in oxygen, transitioning to a NiO-type rock-salt phase.
- Degradation onset is higher in oxygen than in vacuum, showing the importance of atmosphere in processing.
- A sample preparation and experimental setup protocol is established for in situ heating in oxygen.

## Abstract

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 (R3¯m) structure to a NiO‐type rock‐salt phase (Fm3¯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.

An experimental protocol, including sample preparation and experimental setup, for heating large, thinned cathode active material particles in situ in an oxygen atmosphere is presented. Using 4D scanning transmission electron microscopy nanobeam diffraction and ex situ measurements, the structural evolution of LiNiO2 during heating in oxygen is investigated. The oxygen atmosphere affects the onset temperature of degradation, differing from similar experiments conducted in a vacuum.