Hybrid cathodes improve energy density of lithium-sulfur batteries

Thanks to their better energy density and light weight, lithium-sulfur batteries hold a lot of promise as a potential successor to the time-honored lithium-ion batteries. But there have been growing pains, and the emerging technology has its own problems to overcome. Now, engineers have developed a new type of hybrid cathode for a lithium-sulfur battery that seems to boast better energy density than existing versions of both lithium-ion and lithium-sulfur batteries.

Normally, the sulfur in a lithium-sulfur battery is in the cathode, and these are built in one of two ways. Unfortunately though, there's a trade-off to be made – they can either have a high energy density in terms of weight (gravimetric), or in terms of volume (volumetric), but not both. In the most basic sense, that means your battery is either small but heavy, or light but large.

And that's the problem targeted by the new study, involving researchers from MIT, the Samsung Advanced Institute of Technology, Tongji University, the Chinese Academy of Sciences and the Songshan Lake Materials Laboratory.

The researchers built their new hybrid sulfur cathode in an attempt to get the best of both worlds. The solid cathode is made of a combination of particles of pure sulfur and Chevrel-phase molybdenum sulfide, compressed together. Because the end result is highly electrically conductive, it doesn't need as much carbon – it only has about 10 percent, compared to the 20 or 30 percent needed in other sulfur cathodes.

Using the new cathode, the team built a three-layer pouch battery with a 1,000-mAh capacity, and says it already outperforms existing battery technology in some ways, even before it's been optimized. Generally, the energy density of commercial lithium-ion batteries tops out at 265 watt-hours per kilogram and 700 watt-hours per liter. Current lithium-sulfur batteries, meanwhile, can reach 400 watt-hours in both regards.

The new lithium-sulfur battery can currently reach over 360 watt-hours per kg and 581 per L, meaning it already has a better gravimetric energy density than lithium-ion, and a better volumetric density than other lithium-sulfur devices. The team believes with more work the new design should hit 400 watt-hours per kg and 700 per L.

But the new lithium-sulfur cathode has one major disadvantage: lifespan. That's a fairly common problem with these batteries, and the researchers say they're working on improving the overall cell design to overcome the issue. Other teams are tackling it by improving the other end – the lithium anode.

The research was published in the journal Nature Energy.

Source: MIT