Capacitors are devices that store electric charge. Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge thanks to the double layer formed at an electrolyte–electrode interface when a voltage is applied.

Supercapacitors are often used to bridge the gap between conventional supercapacitors and rechargeable batteries, and are usually made from stacked layers of carbon materials or transition metal oxides. Although they have high power densities and long-term cyclability, they can be expensive, so the hunt is on for alternative materials from which to make them.

Happily, researchers have already succeeded in making electrodes from stacked graphene nanosheets that have volumetric supercapacitances as high as 300 F/cm3. Such high values mean that these structures can be used as power sources in portable electronics.

In addition to graphene (which is a layer of carbon just one atom thick), researchers have been looking into using 2D nanosheets of transition metal dichalcogenides (TMDCs) for energy storage. TMDCs have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te). The problem is, however, that although 2D TMDCs can store lots of charge, they become structurally unstable when stacked into sheets. Another drawback is that they are semiconducting (or semi-insulating), so they are not immediately an obvious choice for making supercapacitor electrodes, which need to be highly conducting.

Researchers at Rutgers University, led by Manish Chhowalla, said that they may now have overcome these challenges by showing that they can exfoliate these layered materials into single nanosheets and then restack them. The resulting “paper” appears to store lots of electrical charge while maintaining its lattice structure.

“Our restacked paper is what we call ‘compliant’ and can expand to a great extent without deforming,” explained Chhowalla. “In fact, we have shown that that it is possible to make the so-called metallic 1T phase in the MoS2 nanosheets and then use these metallic sheets to make electrodes that are sufficiently conducting.”

The researchers employed a well-known lithium intercalation technique to separate and stack their MoS2 nanosheets. The technique makes use of butyl lithium ions as the intercalants. “When we add a lot of butyl lithium between the MoS2 sheets, they ‘exfoliate’ and the lattice expands to the point where the sheets separate,” Chhowalla told “The method also produces the metallic 1T phase in MoS2 thanks to charge transfer from the butyl lithium to the MoS2. Indeed, the MoS2 undergoes a phase transition from its stable 2H phase to a metastable 1T phase to accommodate this additional charge,” he explained.

The 1T MoS2 phase is hydrophilic and 107 times more conducting than the semiconducting 2H phase, he added. This means that it could make an attractive electrode material for both aqueous and organic supercapacitor devices.

And that's not all: since most 2D layered TMDCs can transform between 1T and 2H phases, all of the materials in this family might be potentially good for making supercapacitor electrodes.

Spurred on by its initial results, the Rutgers team is now busy trying to increase the energy and power densities of its MoS2 electrodes, while looking into the possibility of using other metallic TMDCs in energy storage applications.

The work is detailed in Nature Nanotechnology doi:10.1038/nnano.2015.40.