Electrochemical impedance spectroscopy (EIS) of self-assembled lithium-iodine batteries was performed in both two and three-electrode configurations. The in situ EIS spectrum of the cell transitioned from that associated with an initial ideal ionic conductor for the LiI-based composite in the unformed state to that of three depressed semicircles as the cell formed. The assignments of these spectral features were made empirically using various techniques. Three-electrode measurements using a Ag vertical bar AgI reference electrode and dynamic impedance measurements were used to confirm these assignments. The charge-transfer process at the positive electrode is the main kinetically limiting factor, especially toward the end of the lithiation cycle. In situ Raman spectroscopy confirmed the formation of the polyiodide anions I-3(-) and I-5(-) in the positive electrode region during cell formation. (C) 2008 The Electrochemical Society.