Lithium-ion batteries are considered to be the next battery system for hybrid electric vehicles (HEVs) due to their high power density. However, their power is severely limited at -30 degrees C and the concern exists that lithium metal could plate on the negative electrode during regen (charge) pulses. The goal of this work is to determine the reason for this poor low-temperature performance using an in situ LiySn micro reference electrode (RE) over a wide temperature range of 30 degrees C to -30 degrees C. A variety of negative and positive electrode materials with unique morphologies was used in this work to help elucidate the dominant low-temperature mechanism. In this work, it was observed that the potential of graphite negative electrodes does dip below lithium potentials not only during charge pulses, but also under normal charging if the cell cutoff voltage is not reduced from its room-temperature setting of 4.1 V, whereas hard carbon electrodes do not because they operate further from lithium potential. The most surprising finding from this work was that a second impedance mechanism dominates below 0 degrees C that affects the positive and negative electrodes almost equally. This suggests that the responsible phenomenon is independent of the active material and is most likely a pure electrolyte-interface effect. (c) 2007 Elsevier B.V. All rights reserved.