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

Abstract

With the large-scale application of LiFePO4 batteries in electric vehicles and energy storage, the recycling of spent LiFePO4 cathode materials is receiving increasing attention. Hydrometallurgical derived full-component separation strategy has been verified as the most capable recycling process for LiFePO4 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 FePO4·2H2O 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 FePO4·2H2O, markedly complicating the succedent separation and purification. In this investigation, a practical method for directly regenerating FePO4·2H2O from H2SO4-based full-component leaching solution is proposed. In this process, H2SO4 as leaching agent is used to digest LiFePO4 completely, and then Fe and P are premeditatedly recovered from the extremely acidic filtrate (pH < 0.8) as FePO4·2H2O 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 FePO4·2H2O 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 FePO4·2H2O or amorphous iron phosphate to increase the nucleation sites. Additionally, the primary nanosheet size of regenerated FePO4·2H2O causes a smaller primary particle size of the corresponding LiFePO4/C material, contributing to its better physical and electrochemical properties. This work provides a guidance for the direct regeneration of FePO4·2H2O from excessive acidic leachate and has potential industrial applications.