Mapping Reaction Mechanism During Overcharge of a LiNiO₂/Graphite–Silicon Lithium‐Ion Battery: A Correlative <i>Operando</i> Approach by Simultaneous Gas Analysis and Synchrotron Scattering Techniques

Abstract Li‐ion battery degradation processes are multi‐scale, heterogeneous, dynamic, and depend on the battery usage. Degradation mechanisms during overcharge of LiNiO 2 are well known at the material level featuring O 2 gas release and concomitant surface reconstruction of LiNiO 2 . However, there are still debates regarding the role of the high voltage phase formation, so called O1, on gas production. Moreover, little information is available on the effect of produced gases on the cell components (anode or sensors), or the effect of overcharge on electrode level behavior. In this work, we simultaneously measure the gas evolution using operando mass spectrometry while spatially resolving nanostructure and crystallographic lattice parameter changes using operando micro small/wide angle X‐ray scattering (SAXS/WAXS) mapping during the formation and overcharge of a LiNiO 2 /Graphite─Silicon pouch cell. This new correlated operando characterization experiment allowed to (1) confirm the absence of O1 phase even with substantial gas produced at end of charge, (2) unveil the effect of gases on reference electrode and (3) show that overcharge increases in‐plane reaction heterogeneities by creating local degraded regions lagging behind the ensemble electrochemistry. These findings will be important to optimize ageing of devices based on similar chemistries, in particular Ni‐rich cathodes, while showing the strength of correlated characterization leading to more efficient and robust information on complex mechanisms.