The volume of end-of-life lithium-ion batteries (LIBs) is expected to rapidly increase over the coming decade. Consequently, it is imperative to develop an environmentally sustainable LIB recycling technology to manage hazardous, albeit valuable, wastes from the disposal of the end-of-life LIBs. Recent efforts on LIB recycling research have focused on recycling and reuse of cathode constituents due to their high value in LIBs. While the traditional LIB recycling processes including pyrometallurgical and hydrometallurgical methods convert the valuable cathode material into lower-valued metal constituents, the direct recycling process aims to regenerate the cathode without destroying its original functional structure, which could potentially maximize the return value from end-of-life LIBs.

This work fundamentally investigates how the spent cathodes at the different state of health (SOH) conditions are regenerated to regain their original electrochemical performance through the direct recycling approach. To simulate the cathode materials at different levels of degradation, the pristine lithium cobalt oxide (LCO) cathode is chemically delithiated using the oxidizer, NO₂BF₄. The two model LCOs with different degrees of delithiation (Li_0.8CoO₂ and Li_0.6CoO₂) are characterized, and their electrochemical characteristics are systematically compared with those of the pristine LCO. These lithium deficient LCO samples analogous to the degraded LCOs from spent LIBs are regenerated using a hydrothermal-based direct recycling method. The regenerated LCOs are compared with the pristine and delithiated LCOs in terms of morphology, crystallinity, and electrochemical performance. Our results suggest that the direct recycling is effective in regenerating spent cathodes at different degrees of degradation. The use of chemically delithiated samples provides the opportunity to fundamentally examine how the disordered, lithium-deficient LCO cathode from used LIBs is regenerated while preventing the complications associated with other cathode degradation mechanisms, including surface layer formation and particle cracking.