This publication deals with the spatially resolved simulation of a sodium–iodine secondary battery. The anode compartment consists of molten sodium and the cathode compartment contains a high‐conductivity metal disc as electrode and an aqueous catholyte. The latter comprises iodide, triiodide, dissolved iodine, and sodium ions. A finite volume approach is proposed to model the transport processes and electrochemical reactions focusing on the positive half‐cell. The study investigates the influences of cathode length, C‐rate, electric conductivity, and molar concentrations on cell performance. It considers solubility limits and predicts diffusion limitation as the major constraint for the operating window. The presented investigations are confined to a simple cathode geometry. However, the results demonstrate the capability of the model to design sodium–iodine half‐cells.