Universal Upcycling of Spent Cathodes Into Lithium Donors for High‐Performance and Resilient Batteries

ABSTRACT Conventional direct regeneration struggles to restore the electrochemical performance of spent cathodes, particularly long‐term cycling stability, and this challenge is amplified for unsorted mixed waste streams. Instead of restoring reversible electrochemical activity, we develop a universal reversible‐to‐sacrificial strategy that converts spent cathode materials into transition metal/Li 2 O nanocomposites as efficient sacrificial lithium sources with negligible gas evolution during initial charge. This transformation is achieved by coating spent cathode powders onto commercial separators, followed by contact lithiation with thick lithium foil at room temperature, creating a lithium‐donating separator. Unlike conventional direct‐contact prelithiation methods relying on ultrathin lithium foils that are difficult to fabricate and handle and may damage electrodes, our separator‐based approach enables the use of thick lithium foil while avoiding electrode degradation. Full cells incorporating this functional separator exhibit markedly improved electrochemical performance, especially reversible capacity and cycling stability. Moreover, the modified separator enhances cell resilience under zero‐voltage storage and over‐discharge conditions by serving as a lithium buffer, stabilizing the absolute potentials of both electrodes against detrimental deviation. This strategy is applicable to representative cathode chemistries and mixed cathode waste streams, providing a scalable route for battery recycling and lithium replenishment.