Restoration of Li+ pathways in the [010] direction during direct regeneration for spent LiFePO4†

Abstract

LiFePO₄ (LFP) cathodes primarily degrade due to Li⁺ depletion and Fe(III) phase formation, while preserving their crystal structure, rendering them ideal candidates for direct regeneration. In spent LFP, however, the Li⁺ transport pathways are obstructed by Fe²⁺ ions, which occupy the LiO₆ octahedra and distortions in the O1–O2–O3–O3 tetrahedra, presenting significant challenges for direct regeneration. This study overcomes these challenges through tartaric acid (TA)-based hydrothermal treatment followed by brief annealing, enabling the successful regeneration of LFP by facilitating Li⁺ reinsertion along the [010] direction of the crystal structure. The regenerated LFP exhibits excellent electrochemical performance, delivering a discharge capacity of 150.5 mA h g⁻¹ at 0.5C, retaining 94.9% of its capacity after 500 cycles. Neutron pair distribution function (NPDF), neutron powder diffraction (NPD) and theoretical calculations are employed to elucidate the underlying mechanisms of the improved performances. The results reveal that the performance enhancement is attributed to restoring Li⁺ diffusion pathways, including the eliminated Fe–Li anti-site defects and the expanded Li-conducting O1–O2–O3–O3 tetrahedra. Furthermore, this approach demonstrates broad applicability, enabling the regeneration of spent LFP at varying degradation levels while facilitating efficient, non-destructive cathode stripping.