The burgeoning accumulation of spent lithium-ion batteries (LIBs), a byproduct of the widespread adoption of portable electronics and electric vehicles, underscores the imperative for developing efficient recycling strategies. Direct recycling of LIBs represents a promising approach for maximizing the value of waste and reducing harmful environmental outcomes. However, current large-scale direct recycling efforts face challenges, including extreme reaction conditions (high temperature, high pressure) with safety concerns, heterophase residues (e.g., Li₂CO₃, LiOH) in the recycled products, uncontrolled interfacial instability, and limitations imposed by the physicochemical composition and structure of the intrinsic degraded cathode materials. In this thesis, different recycling and upcycling strategies are studied for spent LiNixCoyMnzO2 (0 < x,y,z <1, x + y + z = 1, or NCM) cathode materials. Firstly, a safe and energy efficient direct regeneration process based on low-temperature hydrothermal relithiation (LTHR) at low pressure was demonstrated for spent NCM cathode materials. A low concentration of low-cost redox mediator is employed to improve the relithiation kinetics of spent NCM materials, enabling full relithiation temperature to be reduced from 220 oC to 100 oC or below. Correspondingly, the pressure incurred in the relithiation process can be reduced from ~25 bar to 1 bar, significantly improving operational safety. Specifically, three NCM materials, including chemically delithiated LiNi0.33Co0.33Mn0.33O2 (NCM111), cycled (degraded) NCM111, and cycled LiNi0.6Co0.2Mn0.2O2 (NCM622), were successfully regenerated with complete recovery of composition, crystal structure, and electrochemical performance, achieving the same effectiveness as that achieved at high temperature process. Meanwhile, the total energy consumption of spent cell recycling and the greenhouse gas emission are also reduced. This work provides a facile and scalable way to more sustainable LIB recycling with high economic return, high operation safety and low cost. Secondly, a refined direct recycling process was proposed to improve cathode interface stability by leveraging in-situ reaction between surface residual lithium species and a weak inorganic acid (H3BO3) to form a conformal Li+ conductive coating (LiBO2) that stabilizes the regenerated Ni-rich layered cathodes with significantly reduced water footprint. Our findings reveal that the conductive coating also prevents direct contact between contaminants (e.g., moisture from air) and the cathode surface, thus improving the ambient storage stability. Since the typical method of an extensive washing step is no longer necessary to remove residuals on the cathode surface, this intensified direct recycling process significantly reduces water consumption. This work holds the potential for transitioning direct recycling from laboratory to industrial-scale applications with improved product quality and environmental sustainability.
Lastly, a thermally driven selective upcycling process was proposed, which selectively extracts lithium from spent polycrystalline NCM111 using NiSO4, converting residues to single-crystal NCM622 with minimal input of nickel precursors. Notably, sulfur remains in the form of SO42- throughout the closed-loop process, avoiding contamination. We demonstrate the upgrading of composition, structure, and electrochemical performance of both delithiated NCM111 and spent NCM111 black mass, to levels equivalent to pristine NCM622. This work provides a feasible pathway toward affordable and efficient upcycling in the sustainable development of NCM cells, paving the way for next-generation selective recovery and upcycling for LIBs.
