Study on Electrode Kinetics of Li+ Insertion in LixMn2O4 (0 ≤ x ≤ 1) by Electrochemical Impedance Spectroscopy

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.