An analytical expression for the impedance response of an insertion cathode/ separator/ foil anode cell sandwich is presented. The analytical expression includes the impedance contributions from interfacial kinetics, double- layer adsorption, and solution-phase and solid- phase diffusion processes. The validity of the analytical solution is ascertained by comparison with the numerical solution obtained for a LiCoO2/ polypropylene/ lithium metal cell. The flexibility of the analytical solution is utilized to analyze various limiting conditions. An expression to estimate solid- phase diffusion coefficient of insertion species in a porous electrode influenced by the solution- phase diffusion process is also derived. (c) 2006 The Electrochemical Society.