Upcycling of Degraded Low-Ni Cathode to Single-Crystalline Ultrahigh-Ni Cathode via Surface-to-Bulk Engineering

The rapid iteration and imminent retiring tide of lithium-ion batteries (LIBs) call for the sustainable upcycling of degraded low-Ni cathodes into ultrahigh-Ni cathodes, meanwhile addressing their inherent interfacial and bulk structural instability to meet the ever-increasing pursuit for next-generation LIBs. Herein, an upcycling strategy based on surface-to-bulk engineering is demonstrated to upgrade spent low-Ni cathodes (LiNi0.33Co0.33Mn0.33O2, NCM111) into single-crystalline ultrahigh-Ni cathodes (LiNi0.9Co0.05Mn0.05O2, NCM90) featuring Hf/B homogeneous bulk doping and Li2HfO3 coating (denoted as U-NCM90HB). Notably, B doping enables to reinforce the stability of the oxygen framework, while Hf doping with its larger ionic radius triggers the "pinning effect" within the layered structure and inhibits planar gliding and cracks. Further, the in situ formed Li2HfO3 coating reduces residual lithium compounds on the surface, facilitates the rapid diffusion of Li+, and alleviates side reactions at the cathode interface. As expected, the upcycled cathode exhibits significantly improved performance as compared to commercial counterparts, sustaining stable cycling with 89.3% retention after 100 cycles at 0.5 C and delivering a high capacity of 174 mAh g-1 at 5 C. The upcycling method presented here can inspire a broad set of engineering potentials for the sustainable and value-added upcycling of spent ternary layered oxide cathodes.