Direct upcycling of spent layered oxide cathodes via a dual-functional eutectic salt for sodium-ion batteries

Abstract

With the impending retirement of sodium-ion batteries, developing efficient recycling methods proves crucial to minimize resource waste and environmental impact. Direct regeneration has gained prominence owing to its dual environmental and economic merits. Herein, we propose a novel strategy of K⁺-synergistic sodium layer vacancy restoration for direct upcycling of spent NaFe_1/3Mn_1/3Ni_1/3O₂ cathodes. Employing the Na₂CO₃–K₂CO₃ eutectic mixture as a dual-functional repair agent enables simultaneous resodiation and trace K⁺-doping modification, along with complete reconstruction of surface degraded phases through strengthened reactant interfacial contact. Critically, the incorporation of K⁺ into sodium sites expands interlayer spacing, facilitating bulk Na⁺ replenishment and efficient regeneration, while accelerating de/intercalation kinetics. The optimally K⁺-doped regenerated material with a well-ordered crystalline structure achieves a high recovery capacity of 135.12 mA h g⁻¹ and enhanced rate capability (80.24 mA h g⁻¹ at 5C), surpassing its commercial counterpart (67.5 mA h g⁻¹). Moreover, it exhibits excellent cycling stability with 82.5% capacity retention after 100 cycles at 1C. This work establishes a sustainable and economically viable approach for value-added direct recycling of spent sodium-ion batteries.