Résumé
All‐Solid‐State Batteries (ASSBs) are promising emerging devices for meeting high‐energy demands and an in‐depth understanding of the reaction mechanisms occuring during their operation will help in their design for better performance. In this context, neutrons, with their high penetration depth and sensitivity to light elements such as lithium, provide a powerful tool for investigating the structural mechanisms occurring in bulk ASSBs, while the electrochemical operation of large batteries (required for neutron diffraction) remains a challenge. In this study, we demonstrate the reversible electrochemical Li + extraction/insertion within a 2.5 mm thick ASSB system comprising 140 mg of LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) as the positive electrode material (238 mWh energy density), Li 5.4 PS 4.4 BrCl 0.6 (LPSClBr) as the solid electrolyte and Li 0.5 In as the negative electrode. Thanks to the use of the newly‐designed ILLBAT#5 electrochemical cell, we were able to perform operando neutron powder diffraction (NPD) of the system, which coupled with ex situ diffraction, allowed us to gain valuable insights into the structural evolution of NMC622 within the ASSB as well as to probe the structural stability of the Argyrodite solid electrolyte throughout the initial cycle. Herein, we report on the formation and the co‐existence of H1‐H2 phases in NMC622, attributed to system inhomogeneity.