Lithium-ion cells for hybrid electric vehicle applications must deliver and accept current pulses at relatively high rates and over a range of temperatures. We examined the effect of test temperature from room temperature to 55 C on the performance of three electrode cells (LiNi0.8Co0.15Al0.05O2-based positive electrode, Li4/3Ti5/3O4-based negative electrode, and Li-Sn alloy reference electrode) by galvanostatic cycling and electrochemical impedance spectroscopy (EIS) measurements. Increasing the test temperature reduced cell impedance, which improved its power delivery capability. The higher test temperatures reduced the width of the midfrequency impedance arc in the positive-electrode EIS data but did not affect the low-frequency Warburg impedance portion. We discuss the impedance reductions and capacity gains from the higher temperature measurements in terms of the electrochemical processes occurring at the electrode/electrolyte interfaces. (c) 2006 The Electrochemical Society.