Impact and removal of fluorine impurity in the comprehensive recovery of spent LiFePO4/C

Abstract

The comprehensive recovery of spent lithium iron phosphate powder (LFP/C) remains challenging in industry due to the difficulty in impurity removal. Specifically, the impact of fluorine impurity on the recovery process is unclear. In this work, the specific effects of fluorine on the removal of aluminum impurities and the subsequent recovery of FePO₄·2H₂O from spent LFP/C were investigated, and an acid-assisted pyrolysis process was proposed to transfer fluorine species into the gas phase for fluorine removal. The results indicate that due to the coordination reactions between F^- with Al³⁺ and Fe²⁺/Fe³⁺, the presence of F^- in increased the difficulty of aluminum removal and reduced the precipitation efficiency of FePO₄·2H₂O. Additionally, F^- accelerated the aging of LFP/C cathode materials, increasing resistance to lithium-ion migration, which ultimately resulted in an irreversible decline in electrochemical performance. The acid-assisted pyrolysis process achieved a fluorine removal rate of approximately 98 % under the optimal condition (pyrolysis temperature 600-700°C, reaction time 4.0h, H₃PO₄ dosage 1.2 times of theoretic amount, and solid/liquid ratio 4.0), reducing the fluorine content from 1.69 wt% to 0.05 wt%. This work presents a potential strategy for fluorine removal, contributing to the comprehensive recovery of valuable elements from spent LFP/C.