Insights into the Mechanisms Behind Structural Repair of Spent Layered Cathode Materials for Lithium-Ion Batteries

Structural repair is a vital step in the direct recycling of spent LiNi_xCo_yMn_zO₂ lithium-ion batteries, yet its underlying mechanisms remain insufficiently clear. Herein, the thermal solid-state structural repair of spent LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) layered cathode material is systematically investigated. Through multiscale techniques combining XRD, XAS, and ⁶Li solid-state NMR, we identify the structural degradation in spent NCM622 and monitor both long- and short-range structural evolution during repair. Our findings reveal that degradation predominantly occurs through Ni migration into Li octahedral sites, while Co and Mn demonstrate relatively lower occupancies in the Li layer. Such occupancies are primarily responsible for structural disorder and cubic-symmetry domain formation within the spent material. The repair process is demonstrated to involve re-lithiation, oxygen capture, increased transition metal (TM) oxidation states, and the migration of TM ions from the Li layer back to the TM layer, followed by cation diffusion. Both temperature and lithium compensation ratio are identified as critical factors promoting these processes. Capacity recovery studies show a strong correlation between reduced TM occupancy in the Li layer and improved electrochemical performances. These insights allow us to move beyond conventional phase-transition perspectives, offering an atomic-level understanding of structural degradation and repair mechanisms in spent layered cathode materials.