In-situ conversion of de-lithiation residues into Na4Fe3(PO4)2P2O7 towards upcycling of spent lithi-um-ion batteries

Abstract

As the intensive application of LiFePO4 (LFP) lithium-ion batteries in EVs, the recycling of these retired batteries has attracted significant concerns due to their environmental and resource nature. However, the recycling process remains a significant challenge, as the impurities generated during the delithiation processes. Herein, Olivine-structured FePO4 coated with remained carbon after delithiation are used as feeds towards its in-situ conversion into single-crystal morphology cathode materials of Na4Fe3(PO4)2P2O7. Guided by Density Functional Theory (DFT) simulations, it is found that Olivine-structured FePO4 exhibits a lower formation energy, which facilitates its reaction. A solid phase mixing process is used to synthesize regenerated Na4Fe3(PO4)2P2O7 (R-NFPP) with single-crystal morphology, reduced particle size, and uniform crystal orientation, resulting in lower lattice distortion during the insertion and extraction of sodium ions. It achieves excellent electrochemical performance, characterized by 107.84 mAh g-1 at 0.1 C, superior capacity 35.84 mAh g-1 compared to criteria Na4Fe3(PO4)2P2O7 (C-NFPP) losing efficacy at 100 C, and outstanding charge and discharge performance 55.94 mAh g-1 even at -40 °C. This work utilizes the olivine structure FePO4 and its carbon impurities for their in-situ conversion into single-crystal Na4Fe3(PO4)2P2O7 with highly conductive coating, while can attribute to the up-cycling of spent LFP batteries.