On the nanoscale structural evolution of solid discharge products in lithium-sulfur batteries using operando scattering
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.
The example on Li-S batteries demonstrates that structural information on nanoscopic length scales is key to understanding complex transformations in beyond-intercalation-type battery cathodes. Operando SAXS/SANS has unique access to these length scales and is sensitive to the morphology of both crystalline and amorphous phases under practical operando conditions. Combined with stochastic modelling and machine learning, SAXS and SANS will be of great relevance to quantify reaction and storage mechanisms, growth processes, and phase transformations in many sorts of future electrochemical energy storage.
The results were published in Nature communications