Direct Integration of Spent LiMn ₂ O ₄ with High Voltage Aqueous Zinc‐Manganese Redox Flow Batteries as a Practical Upcycling Process

Abstract With the explosive growth of lithium‐ion batteries (LIBs), research on the recycling of spent batteries is widely conducted. However, conventional processes involve complex procedures, high costs, and environmental issues. This study introduces the electrochemical upcycling of spent LiMn ₂ O ₄ (LMO) cathode material, incorporating pre‐filtration (PF) and pre‐reduction (PR) processes to enable its direct application in redox flow batteries (RFBs). Moreover, a double membrane system is applied to address the low operating voltage and energy density by balancing the different pH levels of the zinc anode and manganese cathode. The aqueous zinc‐manganese RFB, derived from spent LMO pouch full cells, achieves coulombic efficiency of 90% and energy efficiency (EE) exceeding 70% over 250 cycles. The LMO‐containing electrolyte is further treated to precipitate the manganese ions by a simple pH adjustment, enabling 100% separation of lithium, thereby enhancing the sustainability of rare metal resources. This work presents a remarkable advancement in the upcycling method of LIB cathode materials and contributes to establishing a circular system for battery materials.