Effective upcycling of NMC 111 to NMC 622 cathodes by hydrothermal relithiation and Ni-enriching annealing

It is imperative that a sustainable approach to the recycling of lithium-ion batteries (LIBs)—in particular the spent NMC cathodes—which reach their end-of-life (EOL) is realized as 11 million metric tonnes are expected to reach EOL by 2030. The current recycling processes based on pyrometallurgy and hydrometallurgy are not fully sustainable options as they recover only the value metals. By contrast direct recycling that aims in regenerating EOL LIB cathodes without breaking down the active compound's crystal structure offers the most sustainable option. In this paper the direct recycling of NMC cathodes is investigated in combination with their upcycling. Upcycling is going to be in growing demand since the first generation NMC 111 cathode chemistries evolve to higher energy/nickel-rich formulations. In this work, the baseline is established for direct recycling of low and high nickel NMC cathodes by analyzing the three key steps of chemical delithiation of pristine NMC cathode material, hydrothermal relithiation (4M LiOH for 4 h at 220 °C), and annealing (4 h at 850 °C) in order to set the ground for investigating the upcycling of NMC 111 to NMC 622. Upcycling is affected via the co-addition of pre-calculated excess NiSO4 and Li2CO3 salts during annealing, following the hydrothermal relithiation step. Use of NiSO4 that is commonly used as p-CAM provides a lower cost alternative to Ni(OH)2 as Ni source. Characterization revealed the upcycled material to have been endowed with the typical α-NaFeO2 layered structure and have surface morphology and composition similar to pristine NMC material. The upcycled NMC 622 cathode yielded good cycling stability (91.5% retention after 100 cycles) and >99% Coulombic efficiency albeit with certain polarization loss justifying further optimization studies.