Sustainable production of high-purity crystalline FePO₄ from sludge for battery electrodes

A growing demand for phosphorus to produce lithium iron phosphate (LiFePO₄) battery electrodes has raised concerns about further depletion of limited resources of mined phosphorous. Primary sludge from wastewater treatment plants could provide a highly sustainable source of P for battery electrodes but this material has not been successfully used for P recovery to make LiFePO₄ electrodes. The major obstacles for recovering P from typical waste sludges have been the difficulty and low efficiency in P extraction, and obtaining FePO₄ of sufficient quality (purity) and durability for use in battery electrodes and avoiding contamination by other metals. To demonstrate the feasibility using sludge to produce high quality LiFePO₄, we developed a new approach based on upfront P capture using Fe-enhanced primary sedimentation, followed by P extraction from the P-rich primary sludge using acidogenic fermentation, and then selective precipitation of P and Fe using acidic and highly oxidative conditions (pH=2, with H₂O₂ addition). Chemical analysis of the precipitate showed negligible metal impurities. The solids were sintered to produce crystalline FePO₄ with a purity of 99.3 %, which surpassed that of typical industrial sources (93.8 %). The performance and durability of the recovered FePO₄ was assessed for making LiFePO₄/C cathodes. The battery performance based on energy discharge capacity after 50 cycles of 143±1 mAh/g was not significantly different (p = 0.14) than that of the control prepared with standard industrial FePO₄ (140±2 mAh/g). An economic assessment showed that production of FePO₄ for batteries had good economic advantages compared to P recovery for use in fertilizers. These findings demonstrate for the first time that battery-grade FePO₄ can be economically produced from the phosphorus captured in Fe-enhanced primary sludge, and that the performance of electrodes made using this wastewater-derived material was comparable to that using conventional materials.