A systematic approach to the impedance of surface layers with mixed conductivity forming on electrodes

Electrode impedance can be evaluated on the basis of the electrode reaction kinetics in many systems, even for complicated electrode reactions. However, when a surface layer is present on the electrode surface, the theoretically well-established impedance model of the electrode reaction is often completed with phenomenological equivalent circuit elements in order to achieve the number of time constants as derived from the electrode impedance spectra measured. In these cases, the meaning of the phenomenological equivalent circuit elements are often unclear, though the presence of these elements is helpful to describe the system throughout the frequency domain used for the measurement. In the present work, an attempt will be shown to separate the effect of the electronic and ionic charge transfer in a surface layer and to identify the appropriate equivalent circuits. Examples are shown from the fields of lithium-ion batteries where a solid electrolyte interface as a surface layer is present at the negative electrode and the contribution of various charge carriers may be of importance.