Search

Abstract: The validity of impedance response measurements within the plateau region of galvanostatic charge–discharge curve of LiFePO4 cathode material was studied in detail. Based on (i) measurement of dependence of impedance response on the mode (voltage, current) and amplitude of excitation, (ii) analysis of higher harmonics of the response signal, and (iii) comparison of impedance response with time-domain measurements, we conclude that above a certain critical frequency that lies in the range 1–100mHz, the impedance response of LiFePO4 satisfies the criteria of linearity and time invariance (LTI). At lower frequencies the impedance response becomes scattered and does not satisfy the LTI criteria anymore. The critical frequency at which the transition from LTI to non-LTI behaviour occurs, depends on the amplitude of current excitation signal. [Copyright &y& Elsevier]

Abstract: Based on careful analysis of nine papers by different research groups, we show, for the first time, that in LiFePO4-based cathode materials the electrode resistance depends solely on the mean particle size. The effect of carbon coating is marginal, it suffices that each LiFePO4 particle is point-contacted with a reasonable number of carbon black particles usually added in the course of electrode preparation. We present a simple but general theoretical model that consistently explains this unexpected result. The main reason for the relatively small importance of carbon coatings is the fact that the ionic conductivity (ca. 10−11–10−10 Scm−1 at RT) is much smaller than the electronic (>10−9 Scm−1 at RT). The present finding could be of significant importance not only for further optimization of LiFePO4 cathodes, but also for preparation of other cathode materials in which the ionic conductivity is much lower than the electronic. [Copyright &y& Elsevier]