Lithium-sulphur batteries are promising next-generation energy-storage technologies thanks to their relatively low cost and high theoretical energy density of 2600 Wh/kg. However, these batteries use solid-state sulphur cathodes that progressively produce unwanted polysulphides that then shuttle between the cathode and anode in a device. This shuttling significantly reduces the energy capacity of the cathode and severely limits battery life.

Researchers led by Haoshen Zhou of AIST in Japan and Nanjing University in China have now developed a copper-based MOF-graphene oxide (MOF@GO) composite as a separator for Li-S batteries that reduces shuttling of these sulphur species as the battery operates. The composite, which is, in fact, often used as a coating for cathodes, acts as an ionic sieve that selectively separates out Li+ ions while stopping polysulphides migrating to the anode.

MOF separates out polysulphides

MOFs are a recently developed class of ordered solids made of inorganic sub-units connected by organic linkers. They have a large surface area and highly ordered pores, explains Zhou, and their porosity can be tuned. “Our MOF membrane separates out polysulphides based on their size and shape,” he tells nanotechweb.org. “We chose a copper MOF that possesses a 3D structure with micropores that have narrow accessible size windows of around 9 Å. This is smaller than the diameter of lithium polysulphides (that are between 4 and 8 Å in diameter) and so the pores effectively block the sulphides.”

The researchers, reporting their work in Nature Energy doi:10.1038/nenergy.2016.94, found that the MOFs do not degrade even after 200 cycles of battery charge/discharge, and no polysulphides pass through the membrane for 48 hours.

Christian Serre of the Université de Versailles in France writes in a related Nature News & Views article that it would be now “of great interest to gain an understanding at the microscopic scale of the physical interaction between the polysulphides and the copper metal sites of the MOF that leads to the formation of electroactive Cu-S bonds. As it has been shown elsewhere that Cu-S species introduced into MOFs can lead to dramatic enhancement of electrical activity, one could even envisage that electroactive MOFs could not only block polysulphide diffusion but also enhance the electrochemical performance.”