Decoupling Electrochemical Kinetics of Li-Rich and Li-Poor Phases in LiFePO₄ Cathodes Using Single-Particle Electrochemical Impedance Spectroscopy

LiFePO₄ (LFP) undergoes a two-phase transformation during lithium insertion or extraction, forming lithium-rich and lithium-poor phases. Determining the kinetic parameters of these phases is crucial for electrochemical models but remains challenging. In this study, we decouple the reaction and diffusion kinetics of the Li-rich and Li-poor phases in LFP cathodes using single-particle electrochemical impedance spectroscopy (EIS). LFP agglomerates comprising primary particles are fabricated into single-particle microelectrodes. EIS measurements are conducted on single LFP particles at various insertion ratios. A physics-based impedance model is developed for phase-transformation electrodes, and the evolution of the exchange current density (<i>i</i>₀) and diffusion coefficient (<i>D</i>_Li) for both phases is extracted. In the single-phase region, the Li-poor phase exhibits a steeper change in <i>i</i>₀ with varying insertion ratios compared with the Li-rich phase. In the two-phase coexistence region, the Li-poor phase shows a higher <i>i</i>₀ than the Li-rich phase. Additionally, <i>D</i>_Li for the Li-poor phase is higher than that for the Li-rich phase in both the single-phase and two-phase coexistence regions. We also compare the kinetic parameters of the Li-rich and Li-poor phases in LFP agglomerates of varying particle sizes to clarify the impact of particle size on electrochemical kinetics. The proposed impedance-based approach decouples the electrochemical kinetics of Li-rich and Li-poor phases in LFP cathodes, and the extracted kinetic parameters serve as the basis for developing models considering phase transformation.