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 1-100 mHz, 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. (c) 2007 Elsevier B.V. All rights reserved.