AC impedance experiments have been performed on three-electrode plastic LiCoO2/graphite batteries using a thin reference electrode. Various shapes, positions, and types of reference electrodes have been tested to select the most reliable and accurate configuration, Once biphasic lithium insertion materials, such as Li4Ti5O12, were identified as the optimum material for the reference electrode, the evolution of the Nyquist plots for each electrode of the battery was followed as a function of the cycle number or storage time. An analysis of the impedance diagrams for each electrode allowed us to determine the interface responsible for the capacity loss during cycling or storage in plastic Li-ion cells.