Aerospace, Defence & Marine
Lithium Sulfur Battery Lasts 1,000 Cycles With COF Fix
Tohoku University's graphene-COF interlayer keeps a lithium sulfur battery stable for 1,000 cycles at 674 Wh/kg, solving the polysulfide shuttle problem.
Key points
- Tohoku University developed a covalent organic framework (TUS-44) integrated with graphene to create an interlayer for lithium sulfur batteries
- The interlayer traps polysulfides while letting lithium ions pass through, solving the main durability problem in Li-S cells
- Cells retained capacity over 1,000 charge-discharge cycles with an initial energy density of 674 Wh/kg
- Li-S batteries theoretically offer 5x the energy density of conventional lithium-ion at lower cost using abundant sulfur
Lithium-ion batteries are found in almost all portable devices, from smartphones to electric cars. However, they are starting to hit their limits. The best commercial batteries can store about 250 to 300 watt-hours per kilogram. In theory, lithium-sulfur batteries could store up to five times more. The main challenge has always been making them last. Now, researchers at Tohoku University in Japan have created a molecular interlayer that allows a lithium-sulfur battery to run for over 1,000 charge and discharge cycles without losing much capacity.
This research, published in the journal Small on June 16, 2026, presents a new covalent organic framework called TUS-44. The team combined it with graphene to create a hybrid material named TUS-44@G. This interlayer is coated onto a standard polypropylene separator and placed between the battery’s cathode and anode. Its main purpose is to tackle the biggest challenge in lithium-sulfur batteries: polysulfide shuttling.
When the battery discharges, sulfur at the cathode reacts with lithium ions to create lithium polysulfides. These compounds dissolve in the battery’s liquid electrolyte and move toward the anode, where they react with the lithium metal. This movement, known as the polysulfide shuttle, causes two main problems: it uses up active sulfur, which lowers the battery’s capacity over time, and it corrodes the lithium anode, eventually ruining the cell. Every lithium-sulfur battery prototype has struggled with this issue, and most lose a lot of capacity after just a few hundred cycles.
TUS-44@G tackles the polysulfide shuttle problem by working at the molecular level. The covalent organic framework is a crystalline, porous polymer with three types of chemical groups, each serving a different purpose. Imine nitrogen atoms act as docking sites for lithium ions, holding them in place. Crown-ether oxygen atoms help move lithium ions through the structure, keeping the battery conductive. Tetrathiafulvalene groups help spread electrical charge across the material, making electron transport more efficient.
Performance numbers
The batteries made by the Tohoku team showed an initial energy density of about 674 watt-hours per kilogram, which is twice as much as the best commercial lithium-ion batteries. Even more impressive, these cells kept their capacity after more than 1,000 charge and discharge cycles. For comparison, most lithium-sulfur battery prototypes start losing capacity after 200 to 300 cycles. A typical smartphone battery lasts for about 500 cycles, while electric vehicle batteries are designed for 1,000 or more.
Sulfur has several important advantages as a battery material. It is one of the most common elements in the Earth’s crust, inexpensive, and often comes as a by-product from industry. Unlike cobalt, which is used in many lithium-ion batteries, sulfur is non-toxic. If researchers can solve the durability issue, lithium-sulfur batteries could provide higher energy density at a lower cost and with fewer ethical concerns about sourcing materials.
Engineering deep dive
Trapping polysulfides
To see why TUS-44@G is effective, it helps to understand covalent organic frameworks, or COFs. These are crystalline polymers made from organic building blocks connected by strong covalent bonds. Unlike metal-organic frameworks, which use metal ions, COFs are made only from light elements like carbon, nitrogen, oxygen, hydrogen, and sometimes sulfur. This makes them both lightweight and easy to adjust chemically.
TUS-44 is made from three different molecular building blocks. The first one creates imine linkages, which are carbon-nitrogen double bonds that make up the framework's backbone. These nitrogen atoms have lone pairs of electrons that bond tightly with lithium ions. When a lithium polysulfide molecule moves toward the separator, its lithium end attaches to these nitrogen sites, trapping the polysulfide and stopping it from reaching the anode.
The second building block adds crown-ether units, which are ring-shaped molecules with several oxygen atoms facing inward. These are well-known in chemistry for their ability to selectively bind alkali metal ions. In TUS-44, the crown-ether oxygens hold onto lithium ions and help them move through the framework's pores. This is important because while the battery needs to trap polysulfides, it must still let lithium ions flow freely between the cathode and anode. Crown ethers make this possible by blocking polysulfide anions but allowing lithium cations to pass.
The third building block is tetrathiafulvalene (TTF), a sulfur-rich organic molecule that is good at donating electrons. TTF units in the framework help electrons move easily across the structure instead of getting stuck in one place. This electronic conductivity is further improved by combining the COF with graphene. Graphene is a single layer of carbon atoms arranged in a hexagonal pattern and is one of the best electrical conductors. The TUS-44@G hybrid brings together the COF's chemical selectivity and graphene's excellent conductivity.
To combine the materials, TUS-44 is grown directly on graphene sheets. During synthesis, the COF forms a thin, even layer on the graphene surface. This close contact allows electrons to move efficiently between the two materials. When this hybrid is coated onto a polypropylene separator, it creates a membrane that acts as a barrier to polysulfides, conducts lithium ions, and also conducts electrons.
The initial energy density of 674 Wh/kg is impressive when put in context. The theoretical maximum for lithium-sulfur batteries is about 2,600 Wh/kg, but real batteries never reach this because of the extra weight from the electrolyte, separator, and packaging. Still, 674 Wh/kg is much higher than most lab-made lithium-sulfur batteries, which usually range from 400 to 600 Wh/kg. Even more important, this energy density stays stable over 1,000 cycles, while many high-energy lithium-sulfur cells lose capacity quickly after the first few cycles.
Links to the curriculum: A level
Today’s electric vehicles use batteries that weigh between 400 and 600 kilograms. If lithium-sulfur battery technology can be produced at even half its theoretical energy density, cars could get the same driving range with a battery pack weighing just 150 kilograms. This would have a huge impact on vehicle weight, design, material use, and cost. While the TUS-44@G interlayer is still a lab result and not ready for production, it shows something researchers have hoped to see for decades: a lithium-sulfur battery that lasts.
Credit via interestingengineering.com
