Abstract Herein, the concept of constructing binder‐ and carbon additive‐free organosulfur cathode was proved based on thiol‐containing conducting polymer poly(4‐(thiophene‐3‐yl) benzenethiol) (PTBT). The PTBT featured the polythiophene‐structure main chain as a highly conducting framework and the benzenethiol side chain to copolymerize with sulfur and form a crosslinked organosulfur polymer (namely S/PTBT). Meanwhile, it could be in‐situ deposited on the current collector by electro‐polymerization, making it a binder‐free and free‐standing cathode for Li‐S batteries. The S/PTBT cathode exhibited a reversible capacity of around 870 mAh g−1 at 0.1 C and improved cycling performance compared to the physically mixed cathode (namely S&PTBT). This multifunction cathode eliminated the influence of the additives (carbon/binder), making it suitable to be applied as a model electrode for operando analysis. Operando X‐ray imaging revealed the remarkable effect in the suppression of polysulfides shuttle via introducing covalent bonds, paving the way for the study of the intrinsic mechanisms in Li‐S batteries.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/cssc.202200434

Constructing Binder‐ and Carbon Additive‐Free Organosulfur Cathodes Based on Conducting Thiol‐Polymers through Electropolymerization for Lithium‐Sulfur Batteries

Solvation Structure of Conjugated Organosulfur Polymers for Lithium–Sulfur Battery Cathodes

Conductive sulfur/carbon copolymers with a high reversible capacity are promising alternative cathode materials for lithium-sulfur batteries. Here, the focus is set on nitrogen-terminated zigzag graphene nanoribbons (N-GNRs) resembling intermediate product of electrospun polyacrylonitrile (PAN)-based carbon nanofibers. In particular, the possibility of combining strong nitrogen-sulfur interactions, which has been recently shown to lead to a shuttle-free discharge mechanism, and superior electrical conductivity of graphene nanoribbons is explored in S/N-GNR copolymers. Structural and electronic properties of ${\mathrm{S}}_{x}/$N-GNR structures, $x=1,\dots{},8$, prior and during the discharge are studied using density-functional theory calculations along with ab initio molecular dynamics simulations. It is found that the GNR backbone assumes a rippled structures in all S/N-GNR structures considered here. The most favorable sulfur structures in S/N-GNR copolymers are found to consist of short sulfur chains with $x\ensuremath{\sim}4,5$. It is also observed that, similar to N-GNRs, S/N-GNR copolymers show a metallic behavior which is brought about by the conductive backbone. In addition, consecutive lithiation reactions are studied and product structures are obtained. It is demonstrated that a shuttle-free, solid-solid transformation could also be expected during the discharge of S/N-GNR copolymers, whereas the lithiated structures remain electrically conductive, irrespective of the discharge state. Therefore, the current study promotes the use of S/N-GNR copolymers as an alternative to other poorly conductive PAN-based S/C copolymer cathodes for lithium-sulfur batteries (such as S/cPAN), while largely mitigating lithium polysulfide formation. 

Sulfur/Polyacrylonitrile-Based N-Terminated Graphene Nanoribbon Cathodes for Li-S Batteries

For lithium–sulfur (Li–S) batteries to become competitive, they require high stability and energy density. Organosulfur polymer-based cathodes have recently shown promising performance due to their ability to overcome common limitations of Li–S batteries, such as the insulating nature of sulfur. In this study, we use a multiscale modeling approach to explore the influence of the regiochemistry of a conjugated poly(4-(thiophene-3-yl)benzenethiol) (PTBT) polymer on its aggregation behavior and charge transport. Classical molecular dynamics simulations of the self-assembly of polymer chains with different regioregularity show that a head-to-tail/head-to-tail regularity can form a well-ordered crystalline phase of planar chains allowing for fast charge transport. Our X-ray diffraction measurements, in conjunction with our predicted crystal structure, confirm the presence of crystalline phases in the electropolymerized PTBT polymer. We quantitatively describe the charge transport in the crystalline phase in a band-like regime. Our results give detailed insights into the interplay between microstructural and electrical properties of conjugated polymer cathode materials, highlighting the effect of polymer chain regioregularity on its charge transport properties.

How Regiochemistry Influences Aggregation Behavior and Charge Transport in Conjugated Organosulfur Polymer Cathodes for Lithium–Sulfur Batteries

Exploring Inverse Vulcanization in Lithium-sulfur Batteries

Polymer-based batteries that utilize organic electrode materials are considered viable candidates to overcome the common drawbacks of lithium-sulfur (Li-S) batteries. A promising cathode can be developed using a conductive, flexible, and free-standing polymer, poly(4-thiophen-3-yl)benzenethiol) (PTBT), as the sulfur host material. By a vulcanization process, sulfur is embedded into this polymer. Here, we present a combination of electronic structure theory and statistical mechanics to characterize the structure of the initial state of the charged cathode on an atomic level. We perform a stability analysis of differently sulfurized TBT dimers as the basic polymer unit calculated within density-functional theory (DFT) and combine this with a statistical binding model for the binding probability distributions of the vulcanization process. From this, we deduce sulfur chain length ("rank") distributions and calculate the average sulfur rank depending on the sulfur concentration and temperature. This multi-scale approach allows us to bridge the gap between the local description of the covalent bonding process and the derivation of the macroscopic properties of the cathode. Our calculations show that the main reaction of the vulcanization process leads to high-probability states of sulfur chains cross-linking TBT units belonging to different polymer backbones, with a dominant rank around n = 5. In contrast, the connection of adjacent TBT units of the same polymer backbone by a sulfur chain is the side reaction. These results are experimentally supported by Raman spectroscopy.

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Combined first-principles statistical mechanics approach to sulfur structure in organic cathode hosts for polymer based lithium-sulfur (Li-S) batteries.

Jiaoyi Ning

Papers

Combined first-principles statistical mechanics approach to sulfur structure in organic cathode hosts for polymer based lithium-sulfur (Li-S) batteries.