Stacking Fault Formation in LiNi_0.6Co_0.2Mn_0.2O₂ during Cycling: Fundamental Insights into the Direct Recycling of...

As the global marketplace for lithium-ion batteries (LIBs) proliferates, technologies for efficient and environmentally friendly recycling, i.e., direct recycling, of spent LIBs are urgently required. In this contribution, we elucidated the mechanisms underlying the degradation that occurs during the cycling of a Li/LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) cell. The results provided fundamental insights into the optimum procedures for direct recycling using a recently developed, state-of-the-art positive electrode material. Capacity fade in NCM622 was induced by cycling at high voltages above 4.6 V vs Li⁺/Li, during which the rhombohedral symmetry approached cubic symmetry. The selective line broadening and peak shifts that appeared in the X-ray diffraction patterns after cycling indicated the formation of stacking faults along the c_h-axis. In addition, high-resolution transmission electron microscopy clarified that rock-salt domains were located on the NCM622 surface before and after cycling. These structural analyses confirmed that the NCM622 particles degrade not at their surfaces but rather in the bulk, contradicting previous reports where degradation during cycling is mainly caused by rock-salt domains on the surface. Material regeneration processes involving the restoration of the original stacking sequence are essential for effective direct recycling.