Multiscale observations on mechanisms for direct regeneration of degraded NCM cathode materials

The direct regeneration of degraded cathode materials in spent lithium-ion batteries (LIBs) is an environmentally sustainable and cost-effective strategy to "make waste be wealth". However, the microscopic regeneration mechanisms and kinetics for this process are almost elusive. In this paper, we attempt to fully extract the regeneration mechanisms during direct regeneration of degraded NCM 523 (LiNi0.5Co0.2Mn0.3O2) materials in a multiscale manner. A series of in situ (TEM, SEM, synchrotron XPS) and ex situ (TEM & EDS tomography, electrochemical measurements) characterization techniques are used for establishing the complete physicochemical picture of NCM 523 regeneration. Important dynamical details across atomic-scale, nanoscale, microscale, and single-particle level including reverse transformation from rock salt to layered phase, healing of defects (pores and cracks), and valence transformations of metal ions are followed and discussed, which is further supplemented by systematical theoretical analysis as well as full-cell electrochemical tests. This study provides an in-depth understanding toward the direct regeneration of degraded cathode materials and may shed light on the future development and optimization of direct regeneration strategies.