To meet the needs of emerging technologies such as all-electric vehicles with a long drive range, there are now strong motivations for the development of rechargeable batteries with high specific energy, so called “beyond Li-ion batteries.” Among these beyond Li-ion batteries, lithium–air (Li–O₂) batteries have received much attention owing to their five times higher theoretic energy density (ca. 3500 W h/kg) than that of lithium-ion batteries. However, Li–O₂ batteries still suffer from several challenges that hinder their practical applications, including low energy efficiency, short cycle life, and poor rate capability. These problems are mainly caused by the sluggish oxygen reduction/evolution reaction (ORR/OER), low stability of electrolyte against O₂^−•, and the poor conductivity of discharge product (Li₂O₂).¹ To improve the performance of Li–O₂ batteries, much effort has been devoted to searching for new catalytic cathode materials to promote ORR/OER kinetics, which is crucial to lowering overpotential and improving the reversibility.²

Metal–organic frameworks (MOFs), a novel type of crystalline porous materials, are built with metal nodes or clusters (secondary building units) and organic linkers. For the last two decades, MOFs have received considerable attentions for gas storage, catalysis, and energy storage, due to their high surface area and chemically unsaturated metal sites.³ It has been also demonstrated that bulk MOFs could be good electrocatalysts for Li–O₂ batteries.⁴ However, most of MOFs have poor electrical conductivity, which limits their applications in electrocatalysis.

Herein, we report nanocrystalline MOFs/CNTs composites as catalytic cathode materials for Li–O₂ batteries to further enhance the catalytic performance of MOFs. The direct growth of MOFs on CNTs (MOFs@CNTs) can not only enhance the electronic conductivity but also mitigate the agglomeration issues of MOFs nanoparticles, making them suitable in the applications of electrocatalysis. The as-prepared MOF-74@CNTs hybrid cathodes showed the improved cycling performance of Li-O₂ batteries. We have also investigated the effects of chemically unsaturated and various redox-active metal sites on the catalytic performance in Li–O₂ batteries. The fundamental mechanism of how MOFs enhance the performance of Li-O₂ batteries will also be discussed.

Reference

1 Aurbach, D., McCloskey, B. D., Nazar, L. F. & Bruce, P. G. Advances in understanding mechanisms underpinning lithium–air batteries. Nature Energy 1, 16128, doi:10.1038/nenergy.2016.128 (2016).

2 Ma, Z. et al. A review of cathode materials and structures for rechargeable lithium–air batteries. Energy & Environmental Science 8, 2144-2198 (2015).

3 Furukawa, H., Cordova, K. E., O’Keeffe, M. & Yaghi, O. M. The chemistry and applications of metal-organic frameworks. Science 341, 1230444 (2013).

4 Wu, D. et al. Metal–organic frameworks as cathode materials for Li–O₂ batteries. Adv Mater 26, 3258-3262, doi:10.1002/adma.201305492 (2014).