(665d) Insights into over-Lithiation Mechanisms and Electrochemical Restoration during Lithium-Ion Battery Direct Recycling

Direct recycling is a promising alternative for low-cost and more environmentally friendly disposal of lithium-ion batteries. Typically, direct recycling methods involve a 'relithiation' step to address the accumulated Li loss during electrode use, followed a short annealing step to resolve remaining surface degradation. It is widely reported that excess Li is necessary during the annealing step to prevent further degradation. Our work has built on previous advancements of optimizing hydrothermal relithiation parameters, and we have demonstrated that it is not necessary to pre-determine added Li at each stage, making this approach lucrative for scaling. However, we have observed the possibility of 'over-lithiation' for various lithium nickel cobalt manganese oxide (NCM) cathode materials, where the lithium stoichiometry exceeds 1.05. This over-lithiation can lead to the broadening of cell parameters in layered transition metal oxides under certain conditions. Through a study of over-lithiation (Li <=1.2), the crystalline phases of excess Li within the particles are identified and their resulting impact on electrochemical performance is measured. Furthermore, approaches to remove excess Li to return stoichiometric balance and optimal electrochemical performance are proposed. This improved understanding of over-lithiation mechanisms and lithium diffusion enables the full restoration of pristine electrochemical capacity during direct recycling, thus paving the way for at-scale rejuvenation.