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

Upcycling of recycled LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) cathodes offers an economical route to produce cathode materials with increased energy density (i.e., LiNi_0.8Mn_0.1Co_0.1O₂, 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 LiNiO₂ (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.