The development and implementation of an in situ lithium-metal reference electrode for use in lithium-ion cells is described. The reference electrode is inserted into cylindrical, commercially available 1.2 Ah lithium-ion cells via perforation of the cell's outer metallic casing. The reference electrode is used to investigate voltage characteristics of the individual electrodes throughout calendar (i.e., zero duty) aging and duty cycle aging regimes. The response of the electrode voltages (vs. reference) to a given current pulse prior to and during interruptions of the aging regime indicates that the resistance of the positive-electrode is much larger than that of the negative electrode. Prior to aging, the ratio of the positive electrode resistance to that of the negative electrode is approximately two. After aging, the resistance of the positive electrode nearly triples, resulting in a ratio of approximately six, with the resistance of the negative electrode remaining approximately unchanged for both calendar-and cycle-aging regimes. The changes in the cell's coulombic capacity at various discharge rates are also characterized and indicate the additional contribution of cycle aging over the calendar aging alone to the capacity loss of cells. Furthermore, both "charge depletion" and "charge sustaining" cycle aging were investigated and indicates the added stress of charge depletion operation. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.