Elucidating the Structure of Lithium Polysulfides by Complementary X-ray and Neutron Total Scattering Techniques

Lithium-sulfur (Li-S) batteries are an emerging class of rechargeable electrochemical cells with applications in speciality electric vehicles, including aviation, and grid-scale energy storage. The high theoretical specific capacity (1675 Ah kg–1), energy density (~2500 Wh kg–1) and the potentially low cost of Li-S batteries have led to significant scientific and industrial interest. During discharge, the sulfur (S8) in the cathode is reduced to soluble lithium polysulfide species (Li2Sn, where n = 2–8). These species are then progressively reduced via a poorly understood mechanism to insoluble lithium sulfide (Li2S). Being highly mobile and difficult to control, these Li2Sn species can participate in a process known as the polysulfide shuttle effect, during which they diffuse through the electrolyte towards the Li metal anode and react to form a solid-electrolyte interphase increasing the impedance of the cell, reducing the S and Li inventory. By investigating these solutions, we aim to better understand the structural and chemical influences that the electrolyte system has on the Li2Sn species. These insights will have fundamental importance and enable detailed in-situ/operando studies by providing key Li2Sn signatures that can be monitored during cycling.