Oxalate-functionalized ternary hydroxides reconstructed from spent NCM cathodes for advanced aqueous energy storage

Upcycling spent lithium-ion battery cathodes into high-performance electrode materials for aqueous energy storage offers a sustainable route to value-added battery recycling, yet simultaneously achieving efficient structural reconstruction and high electrochemical activity remains challenging. Here, spent NCM523 was converted into ternary NiCoMn hydroxides with retained oxalate-derived ligands through a deep -eutectic-solvent-assisted reconstruction process. Combined electrochemical analyses, in situ/ex situ characterizations, and density functional theory (DFT) calculations indicate that the retained oxalate ligands may serve as functional interfacial regulators. Specifically, they tune the electronic structure and local interfacial microenvironment, strengthen OH^- adsorption, lower the deprotonation energy barrier, and facilitate reaction kinetics. Benefiting from these features, the optimized SNCM-OH-2 electrode delivers a high specific capacity of 172.53 mAh g^-1 at 1 A g^-1. Moreover, an asymmetric solid-state NiBi full battery assembled with a BiO_x/Bi anode exhibits excellent durability, retaining 91.68% of its capacity after 20,000 cycles, and delivers an energy density of 109.132 Wh kg^-1 at a power density of 850.38 W kg^-1. This work highlights the role of retained oxalate ligands in regulating electronic and interfacial processes in transition-metal hydroxides and provides a bridge from spent cathodes to advanced aqueous energy-storage materials.