Recycling of spent LiFePO4 batteries by oxidizing roasting: Kinetic analysis and thermal conversion mechanism

The recycling of spent lithium-ion batteries has gained increasing attention due to its economic and environmental benefits. The oxidizing roasting has been preliminarily proven to be a feasible method to pre-treat and recycle spent LiFePO4 battery. However, the kinetics and thermal conversion mechanism of spent LiFePO4 battery during oxidizing roasting process remains indefinable and needs to be elucidated. Therefore, the kinetic analysis and thermal conversion mechanism of the spent LiFePO4 battery during the oxidizing roasting process were investigated in this study. The results indicated that the oxidizing roasting process of the electrode materials (LiFePO4 and C) could be divided into four weight loss stages, and the maximum weight loss took place in the third weight loss stage (500–800 °C). The model-free methods (Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose models) could well describe the oxidizing roasting process, and the corresponding Eα obtained were 257.38 and 255.35 kJ/mol respectively. The model-fitting methods showed that the three-dimension diffusion model was the most suitable kinetic reaction model for the electrode materials oxidizing roasting process. The graphite in the electrode materials could be easily oxidized to CO2 (the major gaseous product). Besides, the LiFePO4 was firstly oxidized and converted into Li3Fe2(PO4)3 and Fe2O3. Then the Li3Fe2(PO4)3 was decomposed into Li3PO4 and P2O5. Finally, P2O5 could react with Fe2O3 to form FePO4. The physical state of oxidizing roasting products changed from powder to molten state from 600 to 800 °C because of the lower melting point temperature of Li3Fe2(PO4)3, and the oxidizing roasting temperature was suggested to be controlled below 800 °C. The final products obtained from the oxidizing roasting process of the spent LiFePO4 battery electrode materials were Li3Fe2(PO4)3, FePO4 and Li3PO4. This study provides a comprehensive understanding of the kinetics and thermal conversion mechanism during spent LiFePO4 battery oxidizing roasting recovery process.