Anomalous voltage-discharge behavior observed in topotactic intercalation materials is explained in terms of a lattice-gas model for interstitial sites with short-range nearest-neighbor repulsive interactions. For sufficiently strong interactions a second order phase transition results in the formation of fine structure of intercalated ions at certain critical concentrations. As a result a voltage-discharge curve shows a step-like behavior with nonanalytic features at the critical concentrations. (C) 2000 Elsevier Science B.V. All rights reserved.