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.