Christian Prehal1,Jean-Marc von Mentlen1,Heinz Amenitsch2,Lionel Porcar3,Stefan Freunberger4,Vanessa Wood1
ETH Zurich1,Graz University of Technology2,Institut Laue-Langevin3,ISTA (Institute of Science and Technology Austria)4
Christian Prehal1,Jean-Marc von Mentlen1,Heinz Amenitsch2,Lionel Porcar3,Stefan Freunberger4,Vanessa Wood1
ETH Zurich1,Graz University of Technology2,Institut Laue-Langevin3,ISTA (Institute of Science and Technology Austria)4
Beyond intercalation-type batteries, such as Li-sulfur (Li-S) batteries could be game-changers in many respects: a theoretical specific capacity among the highest of all batteries paired with low cost and sustainability of sulfur. However, insufficient understanding of the mechanism that reversibly converts sulfur into lithium sulfide (Li<sub>2</sub>S) via soluble polysulfides hampers the realization of high-performance Li-S cells. Developing (<i>operando</i>) experimental techniques with seamless sensitivity from atomic to sub-micrometer length scales is crucial to identifying physicochemical mechanisms and important structure-property relationships.<br/>Here we present <i>operando</i> small- and wide-angle X-ray scattering (SAXS / WAXS) and <i>operando</i> small-angle neutron scattering (SANS) to track the growth and dissolution of solid deposits from atomic to sub-micron scales during charge and discharge (<i>Prehal et al., preprint: https://doi.org/10.21203/rs.3.rs-818607/v2</i>). Stochastic modeling based on the SANS data allows quantification of the chemical phase evolution in real space (<i>Prehal et al. PNAS 118, e2021893118, 2021 & Prehal et al. Nature Communications 11, 4838, 2020</i>). Cryo-scanning transmission electron microscopy with electron energy loss spectroscopy (cryo-STEM/EELS) provides complimentary local structural information. It verifies the model input for analyzing the <i>operando</i> scattering data via stochastic modeling. We show that the deposit nanostructure in Li-S batteries consists of the known nanocrystalline Li<sub>2</sub>S and a second solid discharge product which we identify as smaller, solid Li<sub>2</sub>S<sub>2</sub> particles. Knowing this has important consequences on how to influence the reaction, discharge capacities, and rate performance. Based on the example of Li-S batteries, we demonstrate that structural information on mesoscopic length scales (1 – 1000 nm) is key to understanding complex transformations such as the electrodeposition and stripping of insulating materials in beyond-intercalation-type battery cathodes. <i>Operando</i> SAXS/SANS, cryo-STEM/EELS, and stochastic modeling combine the advantages of integral time-resolved structural information, local element-specific microscopy, and quantitative data analysis.