Characterization of Aged Li-Ion Battery Electrodes for Direct Recycling Process Design

Developing a recycling protocol for lithium-ion batteries (LIBs) at their end of life is crucial to both supply chain stability and waste mitigation. Novel nondestructive recycling methods are under investigation, but lack the process engineering specifications required for full-scale operation. Specifically, the ability of end-of-life LIB components (i.e., recycling feedstock) to withstand the mechanical stresses inherent in industrial reprocessing techniques has not been established. To this end, we have mechanically characterized the electrodes of both fresh cells and “cycled-aged” (C-A) cells (cells that have been cycled and subsequently calendar-aged, and thus reflect a representative recycling input). We report the components’ performance under uniaxial tensile stress and compressive loads, and determine the effects of tensile strain rate and electrode coating on mechanical response. Further, we explore how these measured mechanical properties affect design parameters for a roll-to-roll (R2R) direct recycling process. Cycle-aging is found to significantly reduce the tensile strength of coated electrodes across all strain rates, and electrodes are found to have significantly lower elastic moduli than their respective current collectors. Positive strain rate dependence is observed for bare current collectors but not for coated electrodes, implying that the coating of active material permanently alters the current collector matrix. Compressive failure is not observed for any of the samples; however, C-A cathodes reach elastic deformation at a lower strain than do fresh cathodes. In the context of direct recycling, these findings suggest that roll tension may need to be reduced by 13-65% for C-A components relative to fresh components to avoid irreversible damage. Additionally, C-A cathodes are expected to permanently deform at a lower calendering force than fresh cathodes. Finally, we have investigated each component’s contribution to overall cell capacity loss, and have probed the degradation mechanisms of cathodes to inform recycling method development. Electrochemical analysis suggests that phase shift (layered to spinel phase) and buildup of electrolyte residues at both the primary particle and in the inter-particle pore space may significantly contribute to cathode degradation. Washing C-A cathodes with aprotic solvents is found to improve half-cell cycling performance, likely due to the removal of electrolyte residues, but does not reverse structural degradation. These electrochemical results suggest the importance of a washing step prior to chemical relithiation in direct recycling methods. Thus the present work, which combines mechanical and electrochemical analyses of cycle-aged LIB components, informs the process design of industrial-scale nondestructive LIB recycling.