CO2-assisted pH regulation drives synergistic lithium recovery from spent LiFePO4 batteries and high-value FePO4 waste residue utilization

Recycling of spent lithium iron phosphate batteries (SLFP) can avert resource wastage and lessen environmental pollution. Compared to traditional strong acid leaching systems, the mild CO2+H2O2 selective lithium extraction system has significant advantages due to its ability to effectively reduce the generation of amorphous FePO4 waste residue. However, it is hindered by low lithium leaching concentrations and reduced product purity during mother liquor recirculation. Therefore, this paper proposes a novel synergistic SLFP recycling strategy using CO2-assisted pH regulation to drive lithium recovery and preserve the crystal structure for high-value FePO4 residue utilization. Concretely, regulating the pH to 4.09–6.53 facilitates a stable phase transition from LiFePO4 to FePO4 while maintaining an intact olivine framework, yielding a 99.74% lithium leaching with negligible Fe (0.17%) and P (0.42%) dissolution. Crucially, this pH window ensures that 83.36-99.23% of phosphorus exists as H2PO4-, suppressing Li3PO4 precipitation and enriching the mother liquor Li+ to 5.47 g·L-1 (significantly outperforming the pH unregulated 5.04 g·L-1) after three cycles. Benefiting from the high lithium concentration and low impurity content, the resulting Li-rich LiHCO3 solution economical yields 99.90% battery-grade Li2CO3 via simple purification, while pyrolysis-derived CO2 can be recycled to the leaching stage. Beyond Li recovery, the structurally intact FePO4 residue serves directly as an efficient lithium extraction electrode, maintaining 85.61% capacity over 50 cycles. Ultimately, this process achieves highly efficient Li recovery and maximizes the economic value of spent LFP residues, increasing the profitability by two orders of magnitude compared to traditional hydrometallurgy (380.36 $ vs.2.8$ per kg).