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 25μm-diameter, tin-coated copper wire; in situ lithiation produced a Li x Sn (x ∼ 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°C showed that the positive electrode impedance increase was the main contributor to cell impedance rise, especially during the first month of aging.