In situ conversion of delithiated residues into Na4Fe3(PO4)2P2O7 towards upcycling of spent lithium-ion batteries

Abstract

With the intensive application of LiFePO₄ (LFP) lithium-ion batteries in EVs, the recycling of retired batteries has attracted significant attention owing to their environmental concerns and resource value. However, the recycling process remains a significant challenge due to the impurities generated in leaching residues during delithiation. Herein, olivine-structured FePO₄ coated with residual carbon remaining after delithiation was used as feed materials for its in situ conversion into Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode materials with single-crystal morphology. Guided by density functional theory (DFT) simulations, it was discovered that olivine-structured FePO₄ exhibits low formation energy, which facilitates its conversion reaction. A solid-phase mixing process was used for the preparation of regenerated Na₄Fe₃(PO₄)₂P₂O₇ (R-NFPP) with single-crystal morphology. A reduced particle size and aligned crystal orientation led to lower lattice distortion during the insertion and extraction of sodium ions, achieving excellent electrochemical performance with a capacity of 107.84 mAh g⁻¹ at 0.1C. In addition, R-NFPP demonstrated a superior capacity of 35.84 mAh g⁻¹ under high-rate conditions (100C), compared with Na₄Fe₃(PO₄)₂P₂O₇ (C-NFPP), which lost efficacy at this rate; R-NFPP also exhibited outstanding charge and discharge performance—55.94 mAh g⁻¹ even at −40 °C. This work utilizes the olivine-structured FePO₄ and its carbon impurities for their in situ conversion into single-crystal NFPP with a highly conductive coating, thereby enabling the upcycling of spent LFP batteries.