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Electrochemical impedance spectra (EIS) of Li+ insertion in spinel LixMn2O4 (0 ≤ x ≤ 1) were obtained by using a powder microelectrode. A new equivalent circuit, distinguishing the kinetic properties of Li+ insertion in LixMn2O4 at a lithium-rich state (0.5 ≤ x ≤ 1) from a lithium-depleted state (0 ≤ x < 0.5), is proposed to simulate the experimental EIS. The fitting results are in good agreement with the experimental results, and parameters for the kinetic process of Li+ insertion in LixMn2O4 at different Li+ inserted states can be obtained with the proposed equivalent circuits as well as the modified Voigt−FMG equivalent circuit proposed by Aurbach et al. At the lithium-depleted state, Li+ ions diffuse rapidly and then occupy the available Li+ insertion sites in the LixMn2O4 lattice. Thus, the diffusion process and occupation process occur successively at the lithium-depleted state, and this process can be well-simulated with the modified Voigt−FMG equivalent circuit, in which Warburg impedance and occupation capacitance are in series. At the lithium-rich state, however, the diffusion speed of the Li+ ions decreases due to the repulsive effect from the inserted Li+ ions. The diffusion of Li+ ions in the lattice takes place at the same time of the occupation of Li+ ions because the inserted Li+ ions have to hop and occupy their nearest neighbor vacant sites and vacate their sites for the incoming Li+ ions. Thus, the diffusion process and occupation process occur simultaneously, and Warburg impedance and occupation capacitance should be in parallel. The change of kinetic parameters of Li+ insertion in LixMn2O4 with potential and the influence of immersion time for LixMn2O4 in the electrolyte on the kinetic parameters are discussed in detail.
Lu et al. (Tue,) studied this question.