Origin of the Semi-circles Observed in Nyquist Plots of Porous Lithium-ion Battery Composite Electrodes Obtained with a Four-electrode Cell

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The origin of the semi-circles in Nyquist plots of lithium-ion battery composite electrodes obtained using a four-electrode cell was investigated using Li₄Ti₅O₁₂ (LTO) composite electrodes. No distinct semi-circle was observed for the LTO composite electrode, even though the electrolyte within pores was present. In contrast, semi-circles assigned to the impedance related to ionic transport resistance within the composite electrode were observed for the LTO/acetylene black (AB) composite electrode. These results indicate that the presence of the electrolyte within pores is insufficient for the appearance of the semi-circles and that a continuous electronic conduction phase must be formed on the solid side. Thus, the semi-circles are assigned to an apparent R//C response originating from the coupling between ionic transport resistance in the pores and the capacitance made observable by the continuous electronic conduction phase.