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

Crystalline lithium disulfide (Li2S2) 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 Li2S2 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 Li2S2 exists in the non-equilibrium regime. The formation of crystalline Li2S2 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. (C) 2016 Elsevier B.V. All rights reserved.