Mass Transport, Wicking, and Resistance‐Related Phenomena in the Sulfur Electrode of Sodium‐Sulfur Cells

Abstract

Operation of the sulfur electrode in sodium‐sulfur cells is limited by the transport rates of sodium ions as shown by concentration polarization, circulating currents, and the formation of isolated pockets of sulfur and sodium polysulfides. Previous attempts to increase these rates by improved wicking of sodium polysulfides through alumina addition to the carbon mat led to increases in cell resistance. Consequently, the authors' efforts were concentrated mainly on methods to introduce alumina uniformly throughout the carbon mat and on understanding the principal factors affecting mat resistance. Alumina can be introduced uniformly into carbon mat from sulfate solutions. Such additions improve sodium polysulfide wicking without affecting the mat resistance. The effects of packing density, fiber length, the presence of sulfur, and alumina loading on mat resistance were studied along with factors affecting measurement reliability. The current collector‐carbon fiber contact resistance was usually found to be substantially larger than that of the intrafiber component or the fiber‐fiber contacts. In confirmation of these findings, sodium‐sulfur cells containing alumina‐treated carbon mat displayed less concentration polarization and exhibited cell resistances comparable to control cells containing untreated carbon mat.