The effect of sodium iodide and sulfur additives on the performance of Na/beta''-alumina/NaAlCl4/NiCl2/Ni cells was investigated in quasi-sealed laboratory research cells (0.5-1.0 Ah capacity) and in sealed full-size cells (4 Ah capacity). It was found that sodium iodide additive especially in combination with sulfur in Na/NiCl2 cells significantly increases the usable capacity and reduces the impedance of the Na/NiCl2 cells. It is proposed that the use of sodium iodide enhances the energy and power performance of the NiCl2 electrode by two different mechanisms. The first mechanism, iodide ion doping of the anodically formed solid NiCl2, is dominant at potentials lower than that of iodine evolution. The doping effect of the iodide ions produces a higher-capacity, lower-impedance NiCl2 layer on the positive electrode. The second mechanism, anodic formation of very reactive iodine species, is effective when the cell is cycled through the iodine evolution potential range (2.8-3.1 V vs Na). During this process, the dissolved iodine species improve electrode kinetics through liquid-phase mass transport. Use of the sodium iodide additive is safe in sealed cells, causing no over-pressurizing problems. A maximum pressure increase of only 10 kPa was detected by a pressure sensor during severe overcharge tests.