Li-S cells promise a theoretical capacity much greater than existing Li-ion technology, but the problem of polysulfide shuttling, which leads to self-discharge and loss of active material, remains one of the largest barriers to commercial adoption. We report a spectro-electrochemical cell designed for in operando measurements of the order and concentration of lithium polysulfides (Li₂S_n, where n is the order). The design is a major improvement on existing cells, which typically require significant modifications from commercial cell geometries and require low charge/discharge rates during operation. The optically accessible Li-S cell resembles a CR2032 battery, and is constructed on a metallic disc with a diamond crystal at its center.

Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy is used for in-operando measurements of soluble polysulfides, which are present in the electrolyte-saturated pores of the sulfur electrode. The diagnostic was calibrated using ATR spectra of lithium polysulfide standards, prepared from direct reaction of lithium sulfide and sulfur. The order and concentration of lithium polysulfides in a sample are determined from the position and area of a characteristic IR absorption band. The diagnostic routine can measure polysulfide order and concentration to within 5% and 0.1 M, respectively. Polysulfides smaller than Li₂S₄ are insoluble and are not present in ATR spectra.

Li-S cells were discharged at C rates up to 2 with simultaneous measurement of lithium polysulfide order and concentration. The obtained voltage profiles are consistent with those reported in literature. During discharge, polysulfide order steadily decreases while polysulfide concentration initially increases followed by a steady decrease. The initial increase in concentration is attributed to: i) reduction from elemental sulfur to Li₂S₈, and ii) reduction of Li₂S₈ to smaller polysulfides. The subsequent decrease in concentration is attributed to formation of insoluble polysulfides, which are not measured by the diagnostic. Charging the cell follows the opposite trend, except that elemental sulfur is not recovered.