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

Abstract

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.