Defects as Assets for Upcycling Spent Lithium-Ion Battery Cathode Materials

The rapid deployment of lithium-ion batteries has intensified the accumulation of end-of-life cells, posing both environmental risks and resource challenges. Conventional recycling routes largely rely on energy- and chemical-intensive metallurgical processes that downcycle electrode materials into low-value products. More recently, direct regeneration strategies have emerged to restore degraded electrodes; however, their ability to meet the performance demands of next-generation batteries remains limited. In this review, we present a defect-centric perspective on the upcycling of spent lithium-ion battery materials, reframing degradation-induced defects from liabilities into functional design assets. We analyze the hierarchical defect landscapes in aged electrodes, spanning atomic-scale vacancies, surface and subsurface lattice distortions, and particle-scale cracking and pulverization, and discuss how these intrinsic defects can be harnessed to enable targeted reconstruction through doping, compositional tuning, surface modification, and crystal-structure reconfiguration. By correlating defect characteristics with reconstruction pathways and electrochemical outcomes, we highlight emerging strategies that transform low-value waste materials into high-performance electrodes. Finally, we discuss key challenges associated with scalability, compositional control and performance consistency, and outline future directions for defect-driven upcycling toward a sustainable and circular battery economy.