The dominance of electrospinning in fabricating nanofibres for research is no surprise given the control that it affords over fibre properties and structures. Despite these advantages, however, some important drawbacks prevent the commercial adoption of the technique. The low yield (and therefore high cost) of the method is the main barrier, but the necessary combination of flammable solvents and high voltages is also a significant deterrent.

Centrifugal spinning is not a new idea: melted sugar was first spun this way to make candy floss in the nineteenth century, and the technique is an established way to produce fibreglass. Neither of these uses involves nanofibres, but as Xiangwu Zhang of North Carolina State University’s College of Textiles explained, centrifugal spinning allows fibres with microscale or even nanoscale diameters to be formed simply by varying the spinneret’s rate of rotation.

At a spin rate that results in the formation of nanofibres, a spinneret with a single outlet can produce 50 grams per hour – approximately 500 times the output of a typical electrospinning set-up. Multi-outlet spinnerets are also possible, which would allow even greater yields. The technique can be applied to many different materials and composites, the texture and properties of which can be controlled by changing the spin rate and the solvent used in the process.

Spin-off applications

Zhang and colleagues used the technique to produce carbon nanofibres incorporating nanoparticles of silicon. The exceptional mechanical properties of carbon nanofibres make them a durable anode material in lithium-ion batteries, while the superior electronic properties of silicon resulted in a high-capacity battery with excellent cyclability.

The process was also used to fabricate a cathode from a composite of lithium fluoride, iron and carbon (LiF/Fe/C). As a thin film this material is too brittle to use, but centrifugal spinning allowed Zhang’s team to create a robust, high-capacity nanofibrous electrode with a cyclability to match their carbon–silicon anode.

Next, the researchers applied their technique to a problem that affects lithium–sulphur batteries. This is an attractive battery chemistry because such devices would be lightweight and cheap and have high energy densities. Unfortunately, intermediate polysulphides produced during cycling can be dissolved in the electrolyte, resulting in the deposition of lithium sulphide (Li2S) on the anode. A dual-layer cathode that includes a simple carbon barrier can prevent this but at the expense of increased overall mass and reduced energy density.

The solution arrived at by Zhang and his team was to centrifugally spin nanofibres in which carbon is incorporated with the sulphur. The resulting anode material has a high conductivity and a gradated sulphur content that naturally inhibits the migration of polysulphides.

The scalability of the centrifugal spinning technique, and the value of the products derived from it, mean that Zhang expects to see commercial applications of the technology appear within the next few years.