Mechanisms of the Ammonium Sulfate Roasting of Spent Lithium‐Ion Batteries

Ammonium sulfate ((NH4)2SO4) assisted roasting has been proven to be an effective way to convert spent lithium-ion battery cathodes to water-soluble salts. Herein, thermogravimetric (TG) experiments are performed to analyze the mechanism of the sulfation conversion process. First, the reaction activation energies of the sulfate-assisted roasting are 88.87 and 95.27 kJ mol-1, which are calculated by Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods, respectively. Then, nucleation and growth are determined and verified as the sulfation reaction model by the Šatava-Šesták method. Finally, sub-reactions of the sulfation process are investigated and reaction controlling mechanisms are determined by the contribution of sub-reaction. Based on the thermogravimetric analysis, the phase boundary reaction is found to dominate in the initial step of the roasting process (α < 0.6) while the nucleation reaction controlls the following step (α > 0.6), agreeing well with changing trend of activation energy. Overall, thermogravimetric analysis is a general way to study the mechanism of the various roasting processes.