Battery-grade FePO₄ recovery from P-rich urine via field-induced electro-Fenton in a compartmental electrolytic cell

The recovery of ferric phosphate (FePO₄) from wastewater offers a promising application as a valuable precursor in the new energy industry due to the substantial demand for the LiFePO₄ battery. However, conventional FePO₄ recovery from the massive neutral wastewaters is constrained by pH adjustment and the formation of low-purity precipitation. In this study, a new compartmental electrolytic cell was developed to recover high-purity FePO₄ from phosphorus (P)-rich urine via an electric field-induced electro-Fenton process. The membrane-separated induced electro-Fenton (M/I-EF) system enabled the urine pH to decrease to < 3.0 within 5.0 min without pH pre-adjustment. Under the best conditions (H₂O₂ concentration, 10 mM; current density, 10 mA/cm²; initial pH, 6.0; reaction time, 40 min), the system achieved a total phosphorus (TP) recovery efficiency of 95.67%. Compared to the membrane-separated electro-Fenton (M/EF), the M/I-EF system exhibited a more rapid decrease in pH and ensured that Fe²⁺ was released from the induced Fe foam at a slow rate, resulting in the high-purity FePO₄. Elemental analysis of the recovered products confirmed that the material obtained from the M/I-EF system was FePO₄ with a purity of 98.41%, whereas the precipitate from the M/EF system exhibited a purity of only 87.72%. The effects of initial H₂O₂ concentration, current density, initial pH, and initial TP concentration were evaluated in this study. The recovered FePO₄ was employed as a precursor to synthesize LiFePO₄/C cathode material, which delivered a high specific discharge capacity of 161.7 mAh g^-1 at 0.1C and maintained 99.55% of its initial capacity after 100 cycles at 0.5C, thereby demonstrating excellent electrochemical performance and cycling stability. Ultimately, the system demonstrated a TP recovery efficiency of 95.12% when treating real urine, with the recovered product being FePO₄. The compartmental electrolytic cell offers an effective strategy for phosphorus recovery from urine, providing significant economic and environmental benefits.