Electrochemical characteristics of advanced lithium-ion batteries (ALBs) have been investigated using a three-electrode system. The reference electrode is a piece of lithium inserted between the anode and cathode. Electrochemical impedance spectroscopy shows that both the solid electrolyte interface (SEI) and charge-transfer resistances depend on the battery's state of charge (SOC). In a fully charged ALB, the main contribution to the resistance comes from the SEI of the anode; whereas, in a fully discharged state, the contribution comes from the charge-transfer resistance of the cathode. The discharge capacity under high currents is limited by the diffusion of lithium ions within the cathode; the effects of the anode are small. The charge-transfer resistance of the cathode increased most, by approximately 200%, after 200 cycles at SOC = 50%, and thus this value has the largest effect on the cycle life of the ALB. During low-temperature discharge, the anode shows the largest cell resistance of 2.8 Omega, but near the completion of discharge, the cathode contribution starts to dominate (0.4- 2.7 Omega), and the anode resistance decreases to 1.3 Omega. (C) 2004 The Electrochemical Society.