Revealing role of oxidation in recycling spent lithium iron phosphate through acid leaching

Abstract

The efficient recycling of spent lithium iron phosphate (LiFePO₄, also referred to as LFP) should convert Fe (II) to Fe (III), which is key to the extraction of Li and separation of Fe and is not well understood. Herein, we systematically study the oxidation of LiFePO₄ in the air and in the solution containing oxidants such as H₂O₂ and the effect of oxidation on the leaching behaviors of LFP. In the air, O₂ breaks down the LFP olivine structure at 550 °C for 1 h by oxidizing Fe (II) to Fe (III) in terms of converting LFP to Li₃Fe₂(PO₄)₃ and Fe₂O₃. After that, Li is leached in 0.5 M sulfuric acid solution and is further recycled as Li₃PO₄ with a Li recovery efficiency of 97.48%. Meanwhile, Fe is recovered as FePO₄ and Fe₂O₃. Compared with H₂SO₄–H₂O₂, the air oxidation saves H₂O₂ but increases the leaching efficiency of Fe and H₂SO₄ consumption. The discrepancy of Fe leaching efficiency can be attributed to the different leaching mechanisms involving the solid-to-solid and solid-to-liquid-to-solid conversions. Furthermore, the results of the Everbatt model analysis show that the air roasting-H₂SO₄ leaching method has low emission and potentially high income, which is simple and safe. Overall, this work will deepen the understanding of acid leaching of LFP and favorably stimulate the maturation of the LFP recycling technique.

Graphical abstract