Anais da 43ª Reunião Anual Virtual da SBQ

The concerns about the depletion of fossil fuels, global warming caused by the increase of greenhouse gases like CO2, have increased at last years. The science focus consists on the development of new technologies to renew the energy matrix, using renewable energy to decrease the problems. One way is the increase of specific capacity of energy storage materials, specifically the positive electrode (sulfur) in Lithium-Sulfur Batteries (Li-S). The advantages of sulfur as positive electrode are its high theoretical specific capacity (1675 mA h g-1), low cost and abundance. On
the other hand its low electric conductivity, shuttle effect caused by dissolution of the discharge/charge intermediates (polysulfides, PS) in organic electrolytes and the volumetric expansion are the challenges to be overcome. The sulfur encapsulation in ZIF8 - zeolitic imidazole framework consist in a good alternative since this kind of MOF are well known as a class of porous materials that can offer diverse geometries and exceptionally high surface area; therefore, they are widely used in the storage, separation and release of guest molecules [1]. Then the sulfur is confined to the ZIF8 pores where charge/discharge processes will take place, avoiding the shuttle and volumetric expansion effects. Here, we report a green synthesis method of ZIF8 by mechanochemistry following by the encapsulation of S8 via different ways to optimize the best condition: (i) By a new synthetic method “one plot”; (ii) physical mixing and (iii) sulfur-melting. The different phase purities were confirmed by XRD, and the morphologies and sulfur distribution by
TEM/EDS. The sulfur amount into the MOF was confirmed by TGA curves and the specific capacities were determined considering this sulfur mass. The cyclic voltammetry shows the stability of the composite, where the two reduction peaks are referred to the lithiation process by PS formation and the oxidation peak consists to the PS return to the elemental sulfur form. It was observed that melting incorporation is the best way to incorporate sulfur, showing a especific capacity of 700 mA h g-1. This works shows that MOFs are good approaches to applied in Li-S batteries.

[1] ZHENG, Y. et al., Journal of Materials Chemistry A, 7 (2019) 3469-3491