Impedance spectroscopy of Li electrodes. 4. A general simple model of the Li-solution interphase in polar aprotic systems

In this work Li electrodes were studied in a variety of electrolye systems using impedance spectroscopy. The Li electrodes were all prepared in situ by shearing the surfaces in the same solutions in which the experiments were performed. The electrolyte systems included propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), tetrahydrofuran (THF), 2Me-THF, and 1,3-dioxolane (DN) as the solvents, LiAsF₆, LiClO₄, LiBF₄, and LiPF₆ as the salts, and trace water up to a concentration of 200 ppm. The electrochemical behavior of these systems is controlled by films formed spontaneously on the Li surfaces in solutions through which Li ions migrate under an electrical field, and therefore, Li⁺ migration in fact determines the interfacial resistance measured. It is assumed that these surface films have a multilayer structure to which a "Voigt" type analog of RC circuits in series can be fitted. All the spectra could be modeled by five RC circuits in series (one of which also contains a "Warburg" type element). From the R and C values, the thickness and resistivity of the various layers comprising the Li-solution interphase formed in all the above electrolyte systems could be calculated. The above model was examined using four different experimental routes: (1) studying the influence of the solution composition (solvent, salt, and the presence of H₂O) and of storage time; (2) studying the effect of temperature; (3) experiments in which the solutions were changed during storage; (4) current passage. The significance of the model was explored by following the changes in parameters such as the layers' resistivity, thickness, and activation energy for Li⁺ migration as a function of the experimental conditions (e.g. solution composition, storage time, temperature, and current passage).