Kinetics, Mechanism, and Optimization Modeling of a Green LFP Delithiation Process Developed for Direct Recycling of Lithium-Ion Batteries

Orthorhombic LiFePO4 (LFP) offers highly reversible redox reactions, making it an attractive cathodic material for lithium-ion batteries. This electrochemical property was exploited to develop an environmentally benign selective lithium extraction process based on CO2 and hydrogen peroxide that can be applied to direct LFP recycling. The proof of concept of this green delithiation process was demonstrated in a previously published paper, while the process optimization and the establishment of the reaction kinetic mechanism are addressed in the current paper. First, the effects of solid to liquid ratio (S/L), temperature, CO2 pressure, and initial H2O2 to LFP molar ratio were studied through an orthogonal design of experiments. In the range of conditions studied and considering the objective of maximizing the S/L ratio, the optimal conditions are a temperature of 20 °C, a CO2 pressure of 2 atm, and a H2O2 to LFP molar ratio of 1.25. In addition, reaction kinetic models were used to determine the reaction mechanism. The activation energies obtained based on rate constants from shrinking core and Avrami models are 15.7 and 13.9 kJ mol–1, respectively. While these values reveal a mixed or diffusion-controlled heterogeneous reaction, the analysis of half-delithiated LFP particles under scanning–transmission electron microscopy revealed the reaction being controlled by nucleation rather than diffusion. In this context, the Avrami model that accounts for nucleation and growth in solid-state reactions proved the most appropriate. Further, the reaction mechanism is concluded to be limited by nucleation of FP phase within the body of LFP during the early reaction stage and to sequentially shift to the one-dimensional diffusion-limited crystallite growth regime. Finally, it is shown that CO2 acts as a buffering agent by neutralizing the LiOH formed by Fenton-like reactions between H2O2 and ferric iron in LFP.