Researchers from AIST in Japan and Nanjing University in China have developed a metal-organic framework-based graphene oxide composite as a separator for Li-S batteries, that could help solve the “polysulphide shuttling problem” in these batteries. The composite acts as an ionic sieve that selectively separates out Li+ ions while stopping polysulphides migrating to the anode and reportedly helps reduce capacity decay rates down to just 0.019% per cycle over 1500 cycles.
MOFs are ordered solids made of inorganic sub-units connected by organic linkers. They have a large surface area and highly ordered pores, and their porosity can be tuned. According to the researchers, the MOF membrane in this work separates out polysulphides based on their size and shape; The team 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 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.
Lithium-sulphur batteries are promising next-generation energy-storage devices, that face a challenge that arises from their use of 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.