Effect of sodium iodide additive on the electrochemical performance of sodium/nickel chloride cells

The effect of sodium iodide and sulfur additives on the performance of Na/b eta''-alumina/NaAlCl4/NiCl2/Ni cells was investigated in quasi-sealed labor atory 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 combi nation with sulfur in Na/NiCl2 cells significantly increases the usable cap acity 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 N iCl2 electrode by two different mechanisms. The first mechanism, iodide ion doping of the anodically formed solid NiCl2, is dominant at potentials low er than that of iodine evolution. The doping effect of the iodide ions prod uces a higher-capacity, lower-impedance NiCl2 layer on the positive electro de. The second mechanism, anodic formation of very reactive iodine species, is effective when the cell is cycled through the iodine evolution potentia l range (2.8-3.1 V vs Na). During this process, the dissolved iodine specie s improve electrode kinetics through liquid-phase mass transport. Use of th e sodium iodide additive is safe in sealed cells, causing no over-pressuriz ing problems. A maximum pressure increase of only 10 kPa was detected by a pressure sensor during severe overcharge tests.