A Comparison of Relithiation Methods for the Direct Recycling of End-of-Life NMC622 Cathode Materials

Direct recycling of lithium-ion battery (LIB) materials is a low-cost option for recovering critical materials due to the savings associated with the retention of the spent cathode’s crystal structure. In practice, end-of-life (EOL) recovered battery cathodes are usually isolated as a lithium deficient phase. The lithium deficiency was created by a variety of mechanisms, including electronic isolation of lithiated particles within the electrode, parasitic side reactions at low states of charge with the electrolyte, and solid electrolyte interphase (SEI) formation at the anode. In this overview, various lithium content restoration, or relithiation, methods will be discussed that have been used to return the lithium content and performance of EOL LiNi0.6Mn0.2Co0.2O2 (NMC622) cathodes to pristine NMC 622 conditions. The methods evaluated include solid-state thermal relithiation, hydrothermal relithiation, and ionothermal molten-salt relithiation. These processes were used to restore lithium content in 50 g batches of EOL NMC622 materials. After relithiation with each method, samples were annealed at elevated temperatures. The electrochemical performance of the restored NMC 622 material after each relithiation process compared favorably with the electrochemical performance of a commercial pristine NMC 622 material with ionothermal molten-salt relithiation demonstrating the highest specific capacity delivered with stable long-term cycling. SEM and XRD analysis showed no visible changes to the recycled material morphology and full recovery of the layered NMC 622 structure.