Dual-loop upcycling of spent LiFePO₄: defect inheritance enables durable and fast-charging sodium-ion batteries

The growing accumulation of spent lithium-ion batteries (LIBs) presents pressing environmental and societal challenges, highlighting the urgent need to reimagine them as sustainable energy resources. Traditionally, the formation of Fe vacancies ([Formula: see text]) in LiFePO₄ (LFP) cathodes during extended cycling has been regarded as the chief culprit contributing to capacity degradation. However, this study uncovers their functional potential as beneficial structural defects for sodium-ion batteries, repurposing [Formula: see text] from spent LFP batteries to engineer high-performance Na-Fe-P-O series cathode materials. These pre-existing vacancies trigger a self-adaptive lattice breathing mechanism that dynamically accommodates volume changes during rapid Na⁺ ion de-/intercalation, achieving 80% state-of-charge within 6 min and retaining 82.9% capacity after 4000 cycles at a high rate of 10 C. The proposed dual-loop upcycling model further enhances economic returns by 65% and reduces environmental footprint by 29%. This work pioneers a sustainable paradigm that transforms degradation mechanisms of LIBs into foundational design strategies for next-generation batteries.