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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. The service life of rechargeable lithium-ion batteries has certainly improved from the early days 1,2 by using more robust materials and better manufacturing methods and has resulted in the wide-spread adoption of these batteries. While many aging parameters can affect life, such as depth-of-discharge, discharge or charge rate, and stateof-charge (SOC), increased temperature has been shown to greatly accelerate the aging processes and is very often used to expediently obtain aging information.3-5 Aging is generally characterized by measurements of coulombic capacity and power (and/or resistance) at specified reference conditions during temporary interruptions of a certain aging regime. Proposed degradation mechanisms by Liu et al. for capacity loss 6 focus on buildup of a solid-electrolyte interphase (SEI) 7 on the negative electrode and for resistance rise focus on decrepitation (micro-cracking, structural disordering, and dissolution) of the active material of the positive electrode.8-10 Although the cell voltage (i.e., positive vs. negative electrode) response is typically measured during aging studies, the employment of a reference electrode allows changes in the cell voltage to be ascribable to contributions from the individual electrodes.c The use of a reference electrode can therefore add crucial information to the understanding of the degradation mechanisms that lead to cell aging and is the primary focus of this work. The development of an in situ lithium-metal reference electrode is described, whereby the electrode is placed within the cell through a perforation and provides direct contact with the liquid electro...