AC impedance analysis for Li+ insertion of a Pt/lambda-MnO2 electrode in an aqueous phase

The kinetics of lithium electroinsertion into a Pt/lambda-MnO2 thin-film electrode in a 10 mmol/dm(3) LiCl solution was studied by ac impedance spectroscopy between 10(-4) and 10(2) Hz at 25 degrees C. The interfacial charge-transfer process was found to be independent of the composition x in LixMn2O4, and the exchange current density was 1.3 x 10(-6) A/cm(2) irrespective of x, whereas the chemical diffusion coefficient <(D)over tilde(Li)>, and the partial conductivity sigma(Li), of lithium depended greatly on x because of the effect of the thermodynamic enhancement factor, W. The <(D)over tilde(Li)> value varied between 1.9 x 10(-12) and 3.1 x 10(-11) cm(2)/s, reaching a minimum when x = 0.2 approximately. The sigma(Li), value was in the range from 2.8 x 10(-6) to 1.2 x 10(-5) S/cm. This low sigma(Li), value indicates the predominant contribution of electronic conductivity to the total conductivity of LixMn2O4, which is in the order of 10(-4) S/cm. The results show that solid-state diffusion of Li+ ions is a rate-determining process in the system.