Lithium-sulphur (Li-S) batteries are promising for future energy-storage applications thanks to their extremely high theoretical energy density of 2600 Wh/kg, which is three to five-fold higher than that of state-of-the-art lithium-ion (Li-ion) batteries. However, they do suffer from several major problems. The first is that the discharge products of sulphur electrodes, namely S8 and Li2S and LiS2, are insulating. To counteract this, large amounts of conductive additives must be added, which results in less active sulphur-containing material in the electrode.

Second, the cycling intermediates produced, lithium polysulphides (LiPS), are highly soluble in liquid electrolytes and unfortunately act as internal redox shuttles. These lead to a low Coulombic efficiency in the cell and rapid loss in capacity. What is more, sulphur electrodes undergo huge volume changes during battery cycling and can expand by up to 180% when the denser S8 electrochemically reduces to Li2S during discharge.

Infiltrating nanostructured conductors into sulphur

To overcome these challenges, researchers have mainly focused on infiltrating nanostructured conductors into the sulphur to improve both its electronic and ionic conductivity. These sulphur-nanocomposites also contain some empty space to accommodate the volume change of the sulphur. Much progress has been made in this direction, but there are some inherent drawbacks to this approach.

One of these is that, although porous nanoconductors are needed to infiltrate the sulphur, this very porosity means that LiPS species then remain inside the electrodes during cycling and degrade the electrochemical cell. Second, active sulphur species may not efficiently make use of the empty space in the composites. Finally, most nanostructured sulphur electrodes are “fluffy” and it is difficult to fabricate dense and thick electrodes from these materials. All in all, the result is a disappointing low mass loading of just 2 mg/cm2 of the active sulphur material and relatively low electrode capacity.

Li2S@graphene nanocapsules have superior electrochemical properties

A team led by Khalil Amine at Argonne has now succeeded in making a new type of Li-S electrode architecture from crystalline Li2S nanoparticle encapsulated in few-layer graphene. The Li2S@graphene nanocapsules formed can accommodate the maximum amount of active sulphur species (as much as 10 mg of Li2S/cm2) and boast superior electrochemical properties. Indeed, the electrodes have a high reversible capacity of 1160 mAh/g and area capacity of 8.1 mAh/cm2.

Amine and colleagues synthesized their Li2S@graphene nanocomposites in a one-step reaction in which they reacted lithium metal foils with CS2 vapour carried by argon gas at 650°C. Li2S nanocrystals and the tight wrapper of few-layer graphene are spontaneously generated, thus forming the Li2@graphene nanocapsules. The Li2 nanoparticles are between 50 and 80 nm in size and are uniformly and seamlessly encapsulated in about 10–20 graphene layers. This significantly reduces the charge-transfer resistance between the two materials and greatly improves the electric conductivity of Li2.

Other advantages of the design

In their experiments, the researchers found that the Li2 is redox-active and oxidizes to sulphur during electrode charging and reduces back during discharging. During this electrochemical conversion, the graphene capsules effectively retain the active sulphur species so the electrode does not expand in volume.

“Other advantages of the design are that Li2 nanoparticles only form in the presence of co-formed graphene layers, so the graphene framework functions as an overall electrical conduit,” explains team member Jun Lu, also at Argonne. “This means that nearly all of the Li2 nanoparticles in the electrode are electrochemically active. The compact graphene shells also have good physical properties and preserve the structural integrity of the composites.”

And that is not all: the graphene shells also reduce the amount of polysulphides diffusing into the electrolyte during cycling, he adds.

New electrode is now “potentially viable for commercialization”

“Our new work overcomes the major constraints associated with traditional sulphur electrodes and previously reported Li2S composites,” he tells nanotechweb.org. “The simple and scalable fabrication process we have developed means that this electrode is now potentially viable for commercialization in Li-S batteries for automotive and grid storage applications, among others.”

The team, reporting its work in Nature Energy doi:10.1038/nenergy.2017.90, says that it is now busy further optimizing the lithiothermic reaction in its electrode to improve the long-term cycle life of Li2@graphene, especially at high charge–discharge rates. “We will also be looking into the effects of different electrolytes on the performance of Li-S electrochemical cells using our nanocomposites,” says Lu.