Direct observation of lithium polysulfides in lithium–sulfur batteries using operando X-ray diffraction

In the on going quest towards lithium-battery chemistries beyond the lithium-ion technology, the lithium–sulfur system is emerging as one of the most promising candidates. The major outstanding challenge on the route to commercialization is controlling the so-called polysulfide shuttle, which is responsible for the poor cycling efficiency of the current generation of lithium–sulfur batteries. However, the mechanistic understanding of the reactions underlying the polysulfide shuttle is still incomplete. Here we report the direct observation of lithium polysulfides in a lithium–sulfur cell during operation by means of operando X-ray diffraction. We identify signatures of polysulfides adsorbed on the surface of a glass-fibre separator and monitor their evolution during cycling. Furthermore, we demonstrate that the adsorption of the polysulfides onto SiO2 can be harnessed for buffering the polysulfide redox shuttle. The use of fumed silica as an electrolyte additive therefore significantly improves the specific charge and Coulombic efficiency of lithium–sulfur batteries. The presence of polysulfides in Li–S batteries is highly relevant to the battery performance, but their formation and evolution during battery operation are not well understood. Here the authors design an operando X-ray diffraction experiment to reveal their reaction mechanisms.