A reference electrode based on a lithium-tin alloy was used to study electrode potential change during hybrid pulse power characterization (HPPC) and electrochemical impedance spectroscopy (EIS) measurements on lithium-ion cells. The cells contained a LiNi0.8Co0.15Al0.05O2-based positive electrode, a graphite-based negative electrode, and a LiPF6-bearing ethylene carbonate: ethyl methyl carbonate (EC:EMC) electrolyte. The cells were prepared and tested in an argon-atmosphere glove box to minimize the impact of moisture and oxygen. The reference electrode was prepared from a 25 mum-diameter, tin-coated copper wire; in situ lithiation produced a LixSn (x similar to 4.4) alloy that displayed a relatively stable voltage at room temperature. The impedance data obtained from these measurements were reproducible and self-consistent. Data from cells aged at 55 degreesC showed that the positive electrode impedance increase was the main contributor to cell impedance rise, especially during the first month of aging. (C) 2004 Elsevier Ltd. All rights reserved.