An NMR Investigation of Structural Changes in Recycled and Upcycled NMC Cathodes Prepared via Different Synthetic Routes

Recycling end of life lithium-ion battery (LiB) components is essential in developing a robust supply chain for these critical materials, particularly high-value cathodes such as LiNi_xMn_yCo_zO₂ (NMC). Recently, direct recycling methods have taken great attention as the cathode structure can be retained eliminating the need for resynthesis from the precursors. As such, various processes have been investigated to recycle as well as upcycle these NMC cathode materials, including ionothermal, hydothermal, solid-state, or redox methods. In conjunction with the recycling process development, detailed structural characterization is necessary in order to understand the mechanisms and effectiveness of various cathode recycling processes. Nuclear magnetic resonance (NMR) spectroscopy as well as high-resolution X-ray diffraction (XRD) were therefore used to characterize recycled and upcycled NMC cathodes. For our NMR methodology, we used ⁶Li and ⁷Li MAS (magic angle spinning) NMR to probe the lithium local ordering, structural changes and surface impurities that occur during each step in the various recycling and upcycling processes of cathode materials. During recycling and relithiation, we observe the reinsertion of Li into the NMC lattice and changes in cation ordering. During upcycling, where the goal is to increase the Ni content of NMC cathodes, we can observe the incorporation of Ni-rich phases into the bulk Li environment and possible nickel segregation. These studies allow us to understand the key structural changes during recycling, provide valuable information to modify recycling processes to reach targeted cathode composition and structure and correlation toward the performance of recycled materials in comparison to pristine cathodes.