(565c) Phase Transformation Kinetics of LiFePO4 Cathodes

Recently, it has been reported that phase transformation materials (such as LiFePO4, Li4Ti5O12) have superior power densities compared to the traditional LiCoO2 cathodes and are the promising electrode materials for high power Li-ion batteries. However the mechanism of the exceptional rate performance is still a controversial issue undergoing debates, because there is no accurate electroanalytical technique to probe ion transport phenomena in the phase transformation region. All current electroanalytical methods, including cyclic voltammetry (CV), potentiostatic intermittent titration (PITT), galvanostatic intermittent titration (GITT) and electrochemical impedance spectroscopy (EIS), can only be used to analyze the ion transport in solid solution electrode, because these techniques do not consider the effect of inter-phase boundary movement on the ion transport in phase transformation materials. We developed novel GITT and PITT techniques to study the phase transformation electrode materials by integrating our mixed-control phase transformation theory into traditional GITT and PITT techniques. Using these phase transformation GITT/PITT techniques, Li diffusion coefficients of LiFePO4 in a single phase and two phase regions were precisely determined. The dependence of Li diffusion coefficients on the Li content in LiFePO4 is demonstrated in Figure 1. The true Li diffusion coefficient in alpha phase measured using novel GITT and PITT is stable in two phase region and it is slightly higher than the diffusion coefficient in beta phase. This is consistent with the fact that the charge rate capability of LiFePO4 is better than the discharge rate capability. During Li insertion, the Li diffusion coefficient in beta phase gradually decreases with the increasing Li concentration due to the increases of accommodation energy, while the interface mobility (M) of LiFePO4 gradually increases, reaching the maximum value at 50% state of discharge, and then decreases (Figure 2). The details of these novel PITT/GITT techniques will be presented in the meeting.