These batteries could last much longer: the secret is in a layer just a few micrometers thick

Lithium-sulfur (Li-S) batteries have long been considered one of the most promising alternatives to traditional lithium-ion batteries due to the high theoretical capacity of sulfur, its natural abundance, and the low costs of the raw material. Despite these advantages, their commercial diffusion has so far been hindered by a well-known technological limitation: the so-called "shuttle effect of polysulfides," which is the main cause of degradation in lithium-sulfur batteries.

A research group led by Tohoku University, along with other institutions, now claims to have developed a solution capable of directly addressing this issue through a molecular-level designed interface. The results, published in the scientific journal Small, show experimental cells able to maintain high performance for over 1,000 charge and discharge cycles.

An interface that captures polysulfides instead of just limiting their passage

During the operation of lithium-sulfur batteries, soluble lithium polysulfides form in the electrolyte. These compounds can migrate between the electrodes, triggering unwanted reactions that gradually reduce the battery's capacity and compromise its efficiency. Instead of relying on a simple physical barrier, the researchers designed a functional layer capable of chemically interacting with these polysulfides, retaining them while simultaneously promoting their participation in the normal electrochemical reactions of the battery.

The new material, named TUS-44@G, combines a covalent organic framework (COF) based on tetrathiafulvalene and crown ether with a conductive graphene layer. The result is a lightweight interface that serves a dual function: it retains lithium polysulfides, preventing their migration within the battery, and accelerates electron transport during the operation of the cell.

High performance even at high currents

In laboratory tests, Li-S cells equipped with the TUS-44@G interface achieved a reversible capacity of 1,455.7 mAh/g at a current density of 0.2 A/g. Even significantly increasing the current to 10 A/g, the capacity remained at 773 mAh/g, while durability tests showed an extremely contained capacity loss: just 0.034% per cycle over 1,000 cycles performed at 5 A/g. The research group also built a pouch battery using the same technology. In this case, the cell reached an initial energy density of about 674 Wh/kg, a figure that highlights the potential of the solution even for practical applications.

Molecular design as the key for future Li-S batteries

According to the authors of the study, the main innovative element lies in designing the interface at a molecular level instead of relying on traditional porous carbon materials, which interact only weakly with polysulfides. Although the results have been obtained in a laboratory environment and further developments are needed before potential large-scale production, this work represents an important step forward towards a technology that could offer energy densities superior to those of current lithium-ion batteries.