A lattice-gas model of an intercalation compound is discussed in which the latter is described as an effective one-component medium of equally charged intercalated ions forming a lattice of interstitial sites with short-range nearest-neighbor repulsive interactions. For sufficiently strong interactions a second order phase transition has been found accompanied by the formation of a fine structure of intercalated ions at certain critical concentrations. As a result an open circuit voltage (OCV) shows a step-like behavior with nonanalytic features at the critical concentrations. The phase diagram and OCV are studied theoretically, within the framework of the mean field theory and the Bethe approximation, and by means of Monte Cru lo simulations. Theoretical predictions are in perfect agreement with simulations for the OCV and in a fair agreement with simulations for the phase diagram.