The recovery of high purity iron phosphate from the spent lithium extraction slag by a simple phosphoric acid pickling

Recycling metal resources from spent lithium-ion batteries is promising for resource reuse and environmental conservationis, but there are challenges in obtaining high purity recovered materials. The alkaline solutions (NaOH) is employed to precipitate FePO4 from spent LiFePO4, however, the introduction of sodium leads to low purity and lattice defects in the recovered FePO4. Here, we propose an ultrasonic-centrifugal H3PO4 pickling method able to regenerate battery-grade FePO4 in the low Na/Fe environment. In a weak acid environment, the potential of NaH2PO4 increases and the NaH2PO4 attached to the outer layer of FePO4 is converted to FePO4 by H3PO4 pickling. When Na/Fe ≤ 0.8, sodium exists in the form of NaH2PO4, which can be removed by H3PO4 pickling to obtain perfectly crystalline FePO4. When Na/Fe ≥ 2, sodium co-exists in the form of NaH2PO4, NaFe3P3O12 and NaFeP2O7. The latter two impurities are too stable to be removed by pickling, resulting in lattice distortion of FePO4. In addition, the electrochemical performance of regenerated 0.8Na-FePO4 reached the level of commercial FePO4 (C-FePO4), meanwhile the cost of 0.8Na-FePO4 was calculated to be $2.08/kg. This is about 15 % lower than the cost of C-FePO4 ($2.45/kg). This work proposes a high value recovery method of FePO4 with considerable economic advantages and environmental benefits for engineering applications.