Role of Conductive Carbon in Porous Li-Ion Battery Electrodes Revealed by Electrochemical Impedance Spectroscopy Using a Symmetric Cell

To clarify the role of conducting carbon in porous electrodes for lithium-ion batteries on internal resistance, the dependence of internal resistance on the conductive carbon ratio in positive electrodes was systematically investigated by applying electrochemical impedance spectroscopy with symmetric cells. Based on an assessment of the loading-weight dependence of the porous electrode internal resistance at each conductive carbon ratio, the dependence of ionic resistance in the pores (Rion), the charge-transfer resistance (Rct), and the tortuosity factor (τ) was compared. As the conductive carbon ratio increased, the slope of linearity through the origin of Rion with respect to the loading weight increased due to an increase in the ion transport path distance as τ increases in the porous electrodes. This tendency continues until the pores inside the porous electrode are sufficiently filled with conductive carbon particles. On the other hand, all Rct values for the different conductive carbon ratios followed the same inverse proportionality because Rct depends on the reaction area of the active material and not on the electrode structure, which is affected by the conductive carbon ratio. This research will contribute to the porous electrode design with low resistance and high energy densities for state-of-the-art lithium-ion batteries.