Graphite regenerating from retired (LFP) lithium-ion battery: Phase transformation mechanism of impurities in low-temperature sulfation roasting process

Regenerating spent graphite from retired lithium-ion batteries (LIBs) makes a great contribution to alleviate the shortage of plumbago and protect the ecological environment. In this study, low temperature sulfation roasting-acid leaching integrated with high heat treatment was applied on regenerating spent graphite. Firstly, the results of EDS combined with XRD analysis show that spent graphite contains the phases of FePO4, FePO4·2H2O and elemental Al. Next, based on the thermodynamic calculations and SEM-EDS analysis before and after sulfation roasting process, the phase transformation mechanism of impurities was proposed, namely, these impurities contain Fe and P, as well as elemental Al can be converted into corresponding water-soluble sulfate or phosphate. Afterwards, XRD and Raman analysis is conducted on these specimens, including the graphite before and after regeneration as well as commercialized graphite (CG). The result shows that regenerated graphite (RG) has a good degree of crystallinity and graphitization, which is very similar to that of CG. At last, RG exhibits an outstanding electrochemical property with an initial charge capacity of 353.4 mA h/g and a cyclic capacity stability of 99.4% in 50 cycles at 0.1 A/g, which is roughly equal to that of CG (355.6 mA h/g, 99.6%). Overall, this work clarifies phase transformation mechanism of impurities in low temperature sulfation roasting process, and also establishes a basic theory for graphite regeneration.