Defect-to-Phase Transduction and Interface Reconstruction Enable Regeneration of Spent LiFePO₄ Cathodes

Retired lithium-ion batteries are becoming an important material resource, yet recovering cathode value without dissolution and resynthesis remains challenging. Direct regeneration of spent LiFePO₄ is hindered by Li vacancies, Li-Fe anti-site defects, and heterogeneous Fe valences trapped within the olivine lattice. Here we examine defect-to-phase transduction together with carbon-interface reconstruction to address this constraint. Controlled oxidation converts the detectable olivine phase into intermediates containing Li₃Fe₂(PO₄)₃ and Fe₂O₃, removing the inherited olivine site topology. Lithiation and reduction subsequently reconstruct LiFePO₄ from these intermediates. Rietveld refinement estimates a decrease in Fe-on-Li site occupancy from 5.72% to 0.68%, with a further estimated decrease to 0.35% in the guanidine-assisted sample and a narrower measured electrochemical response. The regenerated cathode delivers 167 mAh g^-1 at 0.1 C and 123 mAh g^-1 at 5 C, retaining 99.2% capacity after 500 cycles at 1 C. Process analysis estimates a profit of US$2.28 kg^-1 of spent cell recycled with lower energy use and greenhouse gas emissions than hydrometallurgical recycling. These results link phase reconstruction and carbon-interface regulation with structural recovery and improved electrode response in spent LiFePO₄.