We measured the internal resistance of high-power lithium-ion secondary batteries for next-generation electric vehicles. We succeeded in separating the two elements of the electrode (positive and negative)/electrolyte interfacial resistances (R-int) by measuring the ac impedance spectrum at a low state of charge and low temperature. In addition, R-int took a value close to the capacity ratio between small-(280 mAh) and large-capacity (7 Ah) cells when the internal resistances of the two cells calculated from the ac impedance spectrum were compared. These results indicate that interfacial degradation, which is one of the main factors causing battery degradation, can be estimated by analyzing R-int using a smaller cell. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3525277] All rights reserved.