Comprehensive Study on Unusual Polarization Behaviors of the LiFePO4 Electrode Using Mathematical Modeling

Abstract

LiFePO4 has been commercialized as a cathode material for high-power lithium-ion batteries for power-tools and hybrid-vehicles because of its low cost, high power, and thermal stability. The behavior of LiFePO4 electrodes, consisting of fine nanoparticles (30-150 nm), are influenced by the interactions between the constituent particles. In addition, the non-monotone potential profile of LiFePO4 single particles, which is characteristic of a many-particle model and originates from two-phase reactions, complicates the polarization behavior of the electrode's charge/discharge profile. One of the unique polarization behaviors of the LiFePO4 electrode is path-dependence[1], wherein the polarization during charge and discharge is influenced by the previous charge and discharge. In addition, the so-called memory-effect was recently reported as an unique polarization behavior of LiFePO4 electrode[2]. Here, an extra polarization bump appears in the (dis)charging profiles at around the SOC at which the previous partial (dis)charge was stopped. These unique polarization behaviors have been investigated in the past using mathematical simulations[3]. However, the detailed cause for the polarization has not been clarified yet. In this study, we investigated the polarization behavior of a LiFePO4 electrode by mathematical simulations using a many-particle model and deciphered the behavior using an active population[4]. It was clarified that the polarization of the memory-effect was caused by the reduction of the active population by relaxation during a previous rest. Through this mechanism, we suggested a new polarization behavior, so-called relaxation-induced-polarization (RIP), which affects the polarization in the (dis)charging profile after the rest. The RIP can be triggered by only the last rest, whereas the appearance of the memory-effect needs specific processes with at least three steps. We also investigated path-dependence behavior, which is a well-known but unusual polarization behavior of LiFePO4 electrodes, and suggested a mechanism for this behavior. Unlike the former two polarizations, the path-dependence was considered to be caused by kinetically inhomogeneous reactions for each particle. Using mathematical simulations incorporating a many-particle model, we further predicted that narrowing the particle size distribution was effective to reduce this polarization, but it cannot be fully erased. The comprehensive understanding of the three polarization behaviors by our model can give us more accurate estimation of the state-of-charge of Li-ion batteries with the LiFePO4 electrode.