Enhancing the electrochemical performance of lithium-sulfur batteries requires improved fundamental understanding of the reduction and oxidation of the soluble lithium polysulfide species. To this end, we have designed a ‘donut-shaped’ cell to enable direct optical observation of phase transformations of a liquid polysulfide catholyte to solid lithium sulfide during electrochemical cycling. We use this technique to image the spatio-temporal distribution of the solid lithium sulfide as it deposits on a carbon matrix at different charging rates. These experiments indicate that the reduction and oxidation of polysulfide catholyte occurs as a thin film deposition and growth process during both lithiation and delithiation. We then investigate the ramifications of these morpological changes in terms of mechanical stability by measuring the evolution of stress during discharge of the polysulfide catholyte. The stress measurements indicate that the average stress during discharging decreases with increasing the charging rate, which we attribute to less dense deposition of lithium sulfide at high discharge rates.