Upcycling of degraded NCM cathode materials for prolonged high-rate stability: simultaneous dual modification from surface to bulk

Abstract

The upcycling of spent ternary cathodes has attracted much interest owing to the enhanced lithium storage performance. However, most reports have focused on replenishing active lithium and repairing cracks while ignoring the inevitably formed NiO rock-salt phase on the surface. This rock-salt phase can trigger an unstable electrode/electrolyte interface and continuous capacity decay, especially at high rates or high cut-off voltages. Herein, motivated by the undesirable rock-salt phases on the surface and lattice vacancies in the bulk, an upcycling strategy is rationally designed with simultaneous dual modification of bulk Sb⁵⁺-doping into and in situ Li₃Ni₂SbO₆ surface coating onto the spent NCM622 cathode materials, endowing the refreshed NCM cathodes with high capacity retentions of 97.7% and 81.6% after 100 cycles and 300 cycles, respectively, at a high rate of 5 C, high cut-off voltage of 4.6 V, and high loading exceeding 20 mg cm⁻². Such remarkable stability is attributed to the synergistic effect of the broadened lattice and robust Sb–O bonds with high oxygen vacancy formation energy from bulk Sb-doping and physical barrier from electrolytic corrosion and promoted charge transfer from the in situ formed Li₃Ni₂SbO₆. More encouragingly, this unique synchronous upcycling strategy could be extended to various systems, such as Nb-based and Te-based bulk/surface upcycling, as well as for NCM111 and Ni83 systems. Thus, this work paves a new pathway to upcycle degraded NCM into high-performance ones with prolonged high-rate stability.