Regeneration behavior of FePO4·2H2O from spent LiFePO4 under extremely acidic condition (pH < 0.8): Mechanism study and the properties of regenerated LiFePO4

With the large-scale application of LiFePO 4 batteries in electric vehicles and energy storage, the recycling of spent LiFePO 4 cathode materials is receiving increasing attention. Hydrometallurgical derived full-component separation strategy has been verified as the most capable recycling process for LiFePO 4 cathode. During which, complete elements (i.e., Li, Fe and P) are dissolved into a solution of excessive acidic leachate, and subsequently separated to obtain battery-grade FePO 4 ·2H 2 O and lithium salt, enabling high element recovery rates of up to 100 %. However, the use of excessive leaching agent leads to high acidity of the leaching solution (pH < 0.8), which brings a necessary of pH regulation prior to the regeneration of FePO 4 ·2H 2 O, markedly complicating the succedent separation and purification. In this investigation, a practical method for directly regenerating FePO 4 ·2H 2 O from H 2 SO 4 -based full-component leaching solution is proposed. In this process, H 2 SO 4 as leaching agent is used to digest LiFePO 4 completely, and then Fe and P are premeditatedly recovered from the extremely acidic filtrate (pH < 0.8) as FePO 4 ·2H 2 O via direct precipitation without any pH regulator. Consequently, there are no extra impurities affecting the recovery of Fe and P during regeneration. The regeneration behavior of FePO 4 ·2H 2 O under extremely acidic condition are investigated and the growth mechanism is also clarified by monitoring the overall microstructure and phase evolutions. The results demonstrate that the above regeneration is found to be an exponential growth process driven by the quantity of nucleation site, and meanwhile influenced by the dissolution-reprecipitation equilibrium of amorphous iron phosphate. Specifically, the process can be accelerated by introducing FePO 4 ·2H 2 O or amorphous iron phosphate to increase the nucleation sites. Additionally, the primary nanosheet size of regenerated FePO 4 ·2H 2 O causes a smaller primary particle size of the corresponding LiFePO 4 /C material, contributing to its better physical and electrochemical properties. This work provides a guidance for the direct regeneration of FePO 4 ·2H 2 O from excessive acidic leachate and has potential industrial applications.