EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Properties and function of beyond intercalation-type batteries are not only rooted in the chemistry but at least as much in the structure all the way from atomic to sub-micrometer lengthscales. This concerns specifically complex transformations such as the electrodeposition of insulating materials in conversion-type lithium-sulfur (Li-S) or lithium-air (Li-O2) battery electrodes. Substantial performance improvements, therefore, rely on a quantitative physico-chemical understanding at all relevant length scales, which puts high demands on (operando) analytical techniques.
Here we present operando small and wide angle X-ray scattering (SAXS/WAXS) as a novel method to study the nanoscale phase evolution of solid Li2O2 during charging and discharging a custom-built in situ Li-O2 cell. For data analysis our recently developed method used for supercapacitors was adapted (C. Prehal et al., Nat. Energy 2017, 2, 16215) and synergistically combined with modelling of Li2O2 nucleation and growth in a realistic 3D carbon pore model. This allows visualizing the nanoscale product formation in real-time and distinguishing between Li2O2 formed via disproportionation (solution mechanism) or direct electroreduction (surface mechanism). Results explain capacity limitations, ways to overcome them, and overturn significant parts of the currently accepted reaction model in Li-O2 batteries (preprint: https://doi.org/10.26434/chemrxiv.11447775.v2).
Next to Li-O2 batteries, the potential of the method is demonstrated by quantifying the reversible electrodeposition of solid reaction products (Li2S) during charging and discharging a custom-built operando Li-S cell. Operando SAXS / WAXS data analysis combined with stochastic modelling (C. Prehal et al., Nat. Commun. 2020, 11, 1, 4838) indicates that Li2S forms via solution-mediated disproportionation of higher order polysulfides rather than direct electroreduction at the carbon-electrolyte interface. The found mechanism provides rational design criteria for improved capacities and less self-discharge.