Impedance Spectroscopy Characterization of Porous Electrodes under Different Electrode Thickness Using a Symmetric Cell for High-Performance Lithium-Ion Batteries

To understand the relationship between the specific energy and power of lithium (Li)-ion batteries, the dependence of the internal resistance of porous electrodes with high loading weight on thickness was systematically investigated. The ionic resistance in pores (Rion) and charge-transfer resistance for Li intercalation (Rct) normalized per unit electrode geometric area were assessed using a combination of electrochemical impedance spectroscopy with symmetric cells and the transmission line model for cylindrical pores. The changes of Rion and Rct and their magnitude show opposite trends with respect to electrode thickness. For thin electrodes, Rion is lower than Rct. The specific power decreases slightly as the electrodes become thicker because the total internal resistance is predominantly affected by the charge-transfer resistance, and there is no delay of the response in the depth direction. In contrast, for thick electrodes, Rion is higher than or approximately equal to Rct, so there is a delay of the reaction in the depth direction. As a result, the power of the battery is dramatically reduced because the total internal resistance is strongly influenced by both Rion and Rct.