Kinetic study of Li-ion extraction and insertion at LiMn2O4 single particle electrodes using potential step and impedance methods

The kinetics of Li-ion extraction and insertion at LiMn2O4 single particles (8-21 mum diam) were investigated by cyclic voltammetry, potential step chronoamperometry (PSCA), and electrochemical impedance spectroscopy (EIS) methods using a microelectrode technique. The EIS measurements in a frequency range from 110 kHz to 11 mHz were conducted successfully on a LiMn2O4 single particle resulting in the magnitude of MOmega orders. The impedance spectra exhibited (i) a single semicircle in the high frequency region, (ii) a Warburg impedance in the low frequency region, and (iii) a limiting capacitance in the very low frequency region. The EIS spectra were fitted to a modified Randles-Ershler circuit, so that the reaction kinetics could be evaluated precisely. The dependences of the charge transfer resistance (R-ct) and the apparent diffusion coefficient of Li within the particle on the electrode potential were evaluated. Obtained values for D-app were in the range of 10(-10) to 10(-6) cm(2)/s from EIS measurements, in fair agreement with those from PSCA results. Finally, the apparent chemical diffusion coefficients of Li-ion in the single crystal, thin-film, and single particle of LiMn2O4 are compared. (C) 2003 The Electrochemical Society.