Phase Evolution and Reaction Mechanism of LiFePO₄ During NaHSO₄·H₂O-Assisted Roasting Process

In this study, we described a novel process for separating Li and Fe from spent LiFePO₄ power battery cathode materials. The phase evolution and the reaction mechanism of the LiFePO₄-NaHSO₄·H₂O system during the roasting and water leaching process were studied by TG-DSC, XRD, SEM, thermodynamics, and ICP. At 25°C–600°C, the LiFePO₄-NaHSO₄·H₂O system exhibited significant weight loss and endothermic phenomena. The results show that the phase transition of LiFePO₄-NaHSO₄·H₂O system with the increase of NaHSO₄·H₂O added as follows: LiFePO₄, NaHSO₄·H₂O → Li₃Fe₂(PO₄)₃, Fe₂O₃, LiNaSO₄ → Li₃Fe₂(PO₄)₃, Fe₂(SO₄)₃, LiNaSO₄ → LiNaSO₄, Fe₂(SO₄)₃, NaFeP₂O₇. The surface of the roasted product particles became flat and dense, and plate-like crystal precipitation occurred. The reaction mechanism of the roasting process may be as follows: the production of SO₃(g) promoted the transformation of Li₃Fe₂(PO₄)₃ and Fe₂O₃ into LiNaSO₄ and Fe₂(SO₄)₃, Li₃PO₄ is converted into metallic sulfates prior to FePO₄, and \({{P}_{2}O}_{7}^{4-}\) ${P_{2}O}_7^{4-}$ was more stable than \({PO}_{4}^{3-}\) ${\mathit{PO}}_4^{3-}$ at higher temperatures.