Dithiine linkage formation via a dynamic and self-correcting nucleophilic aromatic substitution reaction enables the de novo synthesis of a porous thianthrene-based two-dimensional covalent organic framework (COF). For the first time, this organo-sulfur moiety is integrated as a structural building block into a crystalline layered COF. The structure of the new material deviates from the typical planar interlayer π-stacking of the COF to form undulated layers caused by bending along the C–S–C bridge, without loss of aromaticity and crystallinity of the overall COF structure. Comprehensive experimental and theoretical investigations of the COF and a model compound, featuring the thianthrene moiety, suggest partial delocalization of sulfur lone pair electrons over the aromatic backbone of the COF decreasing the band gap and promoting redox activity. Postsynthetic sulfurization allows for direct covalent attachment of polysulfides to the carbon backbone of the framework to afford a molecular-designed cathode material for lithium–sulfur (Li–S) batteries with a minimized polysulfide shuttle. The fabricated coin cell delivers nearly 77% of the initial capacity even after 500 charge–discharge cycles at 500 mA/g current density. This novel sulfur linkage in COF chemistry is an ideal structural motif for designing model materials for studying advanced electrode materials for Li–S batteries on a molecular level.

compared to conventional lithium-ion batteries. [3] However, their widespread commercialization is hampered by the problem of capacity degradation during cycling, even in the state-ofthe-art ether-based electrolyte: Lithium bis(trifluoromethanesulfonyl)imide) (LiTFSI) in 1,2-dimethoxyethane/1,3dioxolane (DME/DOL) with LiNO 3 additive. These degradation processes have various origins and are caused, for example, by the so-called polysulfide shuttle effect. [4][5][6] The diffusion of soluble polysulfides from the cathode side to the anode lowers the coulombic efficiency and may lead to irreversible chemical reactions. This gradual consumption of active material and electrolyte and the corrosion of the metallic lithium anode lead to battery degradation. Another major challenge of Li/S batteries on their way to commercialization is the electrically insulating properties of the two end products of the cathode after charging (elemental sulfur) and discharging (lithium sulfide, Li 2 S). To still achieve the highest possible energy density, carbon is used as the cathode host. Carbon is exceptionally lightweight and offers many different synthesis routes being able to selectively adjust material properties such as porosity, surface functionalization, wettability, and conductivity. [7][8][9][10][11] Many improvements in Li/S battery performance have been achieved in recent years through materials science In recent years, the technology readiness level of next-generation lithium-sulfur (Li/S) batteries has shifted from coin cell to pouch cell dimensions. Promising optimizations of the electrodes, electrolytes, active materials, and additives lead to improved performance and cycling stability. However, new challenges arise with the pouch cell design and engineering (including electrode stacking and electrolyte filling), which influence the mechanistic processes of the cell. This study presents an unprecedented multimodal operando investigation of Li/S batteries on a pouch cell level and provides an inside view of material transformations during battery cycling, using X-ray radiography, electrochemical impedance spectroscopy, and spatially resolved temperature monitoring. With the comparison of two different electrolytes, new experimental details about sulfur and lithium sulfide deposition and dissolution processes are revealed and related to electrolyte and temperature distribution. Operando impedance measurements on monolayer pouch cells yield a clear correlation of electrochemical and macroscopic radiographic observations. Understanding the monolayer cells' behavior represents an optimal foundation for further studies on multilayer pouch cell prototypes and demonstrators with the developed operando setup. Herein the proof of principle for correlated measurement methods on pouch cell level is shown, and the experimental proof of concept for sulfur crystal suppression in sparingly solvating electrolyte is visualized.

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