In Situ Diffraction and Ex Situ Transmission X‐Ray Microscopy Studies of Solid‐State Upcycling for NMC Cathodes

Abstract Upcycling of recycled LiNi _0.6 Mn _0.2 Co _0.2 O ₂ (NMC622) cathodes offers an economical route to produce cathode materials with increased energy density (i.e., LiNi _0.8 Mn _0.1 Co _0.1 O ₂ , NMC811) that meet the performance needs of present‐day electric vehicles. In this work, solid‐state upcycling of NMC622 via calcination with Ni(OH) ₂ and LiOH was monitored using in situ synchrotron powder X‐ray diffraction measurements. Sequential Rietveld refinements indicate that the calcination proceeds by initially converting Ni(OH) ₂ to a rocksalt NiO phase followed by lithiation of NiO to form LiNiO ₂ (LNO), with both NMC and LNO phases present in nearly equal proportions at the calcination endpoint. Variable‐energy transmission X‐ray microscopy tomograms of upcycled samples reveal that the NMC and LNO domains are intermixed at sub‐micron length scales. Depth‐dependent analysis of multi‐elemental fitting maps matches the expected NMC811 composition at the secondary particle level and indicates that transition metal diffusion is not limited by the secondary particle size.