Researchers from Lanzhou University, Tohoku University and SRM University have developed a molecularly engineered covalent organic framework (COF)-graphene interface that addresses two persistent challenges in lithium-sulfur (Li-S) batteries: polysulfide shuttling and sluggish conversion kinetics.
Li–S batteries are widely regarded as a next-generation energy storage technology due to sulfur’s high theoretical capacity and natural abundance. However, their practical deployment has been hindered by the dissolution and migration of intermediate lithium polysulfides (Li₂Sₙ), which leads to active material loss, parasitic reactions and rapid capacity decay over repeated cycles. To tackle this issue, the researchers designed a new COF, termed TUS-44, constructed via Schiff-base condensation between a tetrathiafulvalene (TTF)-based tetraaniline node and a benzocrown-6-derived tetrabenzaldehyde linker. The resulting material is an imine-linked, π-conjugated framework with a two-dimensional topology, uniform micropores of approximately 0.9 and 1.2 nm, and a BET surface area of about 516 m² g⁻¹.
Unlike conventional porous carbons, TUS-44 integrates multiple chemically distinct functional sites into a single framework. These include imine nitrogen atoms, crown-ether oxygen atoms and sulfur-rich TTF units, forming a hierarchical interaction network (N > O > S). Within this architecture, imine nitrogen sites act as primary Li⁺ anchoring centers, crown-ether oxygen atoms provide auxiliary coordination and ion transport pathways, and TTF moieties contribute to charge delocalization and redox mediation.
This combination enables simultaneous control over ion coordination, polysulfide binding and electron transport within a structurally ordered and electronically active framework.
To further enhance conductivity and interfacial charge transfer, the team integrated TUS-44 with conductive graphene to form a hybrid interlayer (TUS-44@G) on a polypropylene separator. This thin, homogeneous coating functions as a chemisorptive and conversion-active interface rather than a passive barrier.
The graphene component provides rapid electron transport, while the COF introduces site-specific chemical interactions with polysulfides. Together, they establish a cooperative chemical-electronic coupling that regulates the Li₂Sₙ equilibrium, promotes reversible redox reactions and suppresses the diffusion of soluble intermediates.
As described by Saikat Das, junior associate professor at Tohoku University: "Our goal was to design an interlayer that does not simply block polysulfides, but actively manages their reaction pathway",
"By integrating crown ether and tetrathiafulvalene chemistry into an ordered COF and coupling it with graphene, we created a cooperative interface that can anchor, redistribute and convert sulfur species more efficiently."
Electrochemical testing demonstrates that this interfacial design significantly improves both capacity and stability. Cells incorporating the TUS-44@G layer deliver:
- A high reversible capacity of 1455.7 mA h g⁻¹ at 0.2 A g⁻¹
- A rate capability of 773 mA h g⁻¹ at 10 A g⁻¹
- Long-term cycling stability with only 0.034% capacity fading per cycle over 1000 cycles at 5 A g⁻¹
These results indicate that the hybrid interface not only suppresses polysulfide shuttling but also accelerates the underlying conversion reactions that govern sulfur electrochemistry.
Importantly, the approach also translates to larger-format devices. A Li-S pouch cell (5.0 × 6.5 cm²) incorporating the TUS-44@G interlayer, with a sulfur loading of 44.558 mg, achieves an initial energy density of approximately 674 Wh kg⁻¹ at 0.05 A g⁻¹.
"This study shows that reticular chemistry can be used to program battery interfaces at the molecular level," said Professor Yuichi Negishi of Tohoku University. "The TUS-44@G design offers a route toward lightweight, durable and high-rate Li-S batteries by unifying polysulfide immobilization with catalytic sulfur conversion."
Overall, the work demonstrates how rationally designed COF architectures - combined with conductive components - can transform the separator interface into an active, conversion-kinetics-promoting layer. By integrating chemical adsorption, ion coordination and electron transport within a single platform, the TUS-44@G system provides a scalable strategy for advancing high-energy, long-life Li–S batteries.
