AC Impedance Analysis of Polycrystalline Insertion Electrodes: Application to Li1 − x CoO2

It is argued that a porous insertion electrode can be modeled, for purposes of ac impedance analysis, by a modified Randies equivalent circuit containing a generalized constant‐phase‐angle impedance in series with the double‐layer capacitance. The impedance spectrum of the disintercalation system showed two time‐dependent semicircles, indicating the need for a further modification of the equivalent circuit by a physical process that could be represented by a resistor/capacitor combination. An adsorption process and a Li+‐ion‐electrolyte surface layer are each shown to refine to identically low factors and to be represented by equivalent circuits that transform one into the other, each having circuit values with reasonable values for the physical processes represented. Time‐dependent experiments could eliminate the adsorption models, and electron microscopy gave direct evidence for the surface‐layer model; further indirect evidence came from constant‐voltage experiments and the preparation of electrodes with greater electrode/electrolyte interface area. Although propylene carbonate appears to be kinetically stable in contact with at potentials of up to 4.5V vs. lithium, mixtures of propylene carbonate with oxide particles having produce thick polymer films.