Surface Gelation on Disulfide Electrocatalysts in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are deemed as future energy storage devices due to ultrahigh theoretical energy density. Cathodic polysulfide electrocatalysts have been widely investigated to promote sluggish sulfur redox kinetics. Probing the surface structure of electrocatalysts is vital to understanding the mechanism of polysulfide electrocatalysis. In this work, we for the first time identify surface gelation on disulfide electrocatalysts. Concretely, the Lewis acid sites on disulfides trigger the ring-opening polymerization of the dioxolane solvent to generate a surface gel layer, covering disulfides and reducing the electrocatalytic activity. Accordingly, a Lewis base triethylamine (TEA) is introduced as a competitive inhibitor. Consequently, Li-S batteries with disulfide electrocatalysts and TEA afford high specific capacity and improved rate responses. This work affords new insights on the actual surface structure of electrocatalysts in Li-S batteries.

Cutting COF-like C4N into Colloidal Quantum Dots toward Optical Encryption and Bidirectional Sulfur Chemistry via Functional Group and Edge Effects

As a newly-emerged two-dimensional (2D) layered polymer, C 4 N has aroused increasing interest. Yet, the inferior solubility of bulk C 4 N constrains its application scope. Nanostructing bulk C 4 N into quantum dots (QDs) can enable enhanced or entirely-new properties, but the C 4 NQDs study remains unavailable. Here, starting from predesigned COF-like C 4 N, we report the first synthesis of colloidal C 4 NQDs and their functional composites, and explore their optical activities for dual-mode information encryption and edge-selective adsorption-catalytic ability toward boosted sulfur chemistry in Li-S cells. Colloidal C 4 NQDs with ultrasmall size of ~2.2 nm bear rich carbonyl groups and edges, allowing good solution processability and facile assembly with other moieties for creating intriguing functionalities by exploiting functional group and edge effects of QDs. While C 4 NQDs show normal fluorescence (FL), the QD/poly (vinyl alcohol) (PVA) composites attain color-tunable afterglow and FL/room-temperature - phosphorescence (RTP) dual-mode emission, enabling the corresponding solution as a new encryption ink. The QDs anchored onto carbon nanotubes can be used as a robust barrier layer to decorate commercial separators and afford superior polysulfide adsorption-catalysis ability, endowing a Li-S cell with excellent cycling stability, high rate capability and large areal capacity of 5.8 mAh cm -2 at high sulfur loading of 7.2 mg cm -2 . Computation and experiment studies unveil that edge sites in C 4 N favor polysulfide adsorption and catalysis relative to in-plane sites and the synergy of enriched edges and carbonyl groups in QDs expedites bidirectional catalytic conversion of sulfur species.