On the nanoscale structural evolution of solid discharge products in lithium-sulfur batteries using operando...

The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li₂S) via soluble polysulfides (PSs) formation impedes the development of high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li₂S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li₂S₂ precipitates from the solution and then is partially converted via solid-state electroreduction to Li₂S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells.