Transient existence of crystalline lithium disulfide Li2S2 in a lithium-sulfur battery

Abstract Crystalline lithium disulfide (Li 2 S 2 ) is identified, for the first time, as a transient species in the lithium-sulfur cell, by using an operando X-ray diffraction (XRD) technique. The observed XRD pattern precisely matches with the predicted pattern based on the density function theory. The formation of Li 2 S 2 crystals is repetitively found in the highly concentrated (7 M Li + ) electrolyte at high voltage region (>2 V) near the end of the first charge cycle and before the end of the second discharge cycle. These conditions indicate that crystalline Li 2 S 2 exists in the non-equilibrium regime. The formation of crystalline Li 2 S 2 under only the specified conditions suggests that it is not formed as an intermediate discharge product, contrary to what is generally believed, but as a transient species by the disproportionation reaction from higher order polysulfides which is facilitated by the “solvent-in-salt” conditions.