Owing to its high energy density, high theoretical capacity, low cost, and nontoxicity of elemental Sulfur, Lithium-Sulfur (Li-S) batteries have been considered as a promising alternative for Li-ion batteries. In order to make Li-S practical, several challenges have to be overcome such as (i) the low electronic conductivity of sulfur and sulfur discharge product (Li₂S, Li₂S₂). (ii) active species expansion during cycling. (iii) and more importantly diffusion of soluble LiPSs during discharge/charge (shuttle effect) has to be suppressed^1–2.

Herein, single Fe and Co atoms dispersed in N-containing Graphene framework (FeNC/G, CoNC/G) were proposed, synthesized, and utilized as the sulfur host materials for Li-S batteries. Transmission Electronic Microscopy (TEM) was performed to detect the presence of any unwanted Co or Fe-derived particles or crystals in the carbon support. High resolution high-angle annular dark-field scanning TEM (HAADF STEM) were carried out to confirm the presence of atomically dispersed Fe and Co and to unveil the distribution of Fe and Co single on the
graphene framework.

The electrocatalytic behavior of two prepared samples in a three-electrodes configuration in a 10 mM Li₂S₆ solution was studied. The constant rate (k°) for the LiPSs conversion using Nicolson’s equation at the presence of two electrocatalysts separately were determined, k°= 9×10^-4 cm/s, and k°= 3×10^-4 cm/s for CoNC/G, and FeNC/G respectively were calculated. CoNC/G exhibits the best electrocatalyst behavior for the LiPSs conversion. On the other hand, it has been revealed that FeNC/G which benefits from the presence of Fe with the extreme abundance in the earth’s crust can be also a promising electrocatalyst for LiPSs conversion. Consequently, Sulfur was impregnated into the two prepared host materials to obtain FeNC/G@S and CoNC/G@S composite with very high S content i.e. 83%.

The two as-prepared composites were utilized as the positive electrode for Li-S battery. The electrochemical performances of the FeNC/G@S, CoNC/G@S electrodes were evaluated by Cyclic Voltametry (CV) and Galvanostatic Charge Discharge (GCPL) with different S loading. CoNC/G@S and FeNC/G@S electrodes with high S loading (2.1-2.7 mg/cm²) and low amount of electrolyte (Electrolyte/Sulfur equal to 13 µL/mg_s) can deliver a capacity close to 800 mAh/g at 0.1C during the first cycles and good capacity retention with capacities higher than 500 mAh/g after more than 60 cycles at 0.1C.

Figure 1: (A) SEM images of the carbon structures after extracting the NaCl cubes. (B) TEM image of the graphene
without detecting any metal-derived particles or crystals. (C) HR-HAADF images demonstrating the presences of
metals atoms (white spots) and their distribution. (D) GCPL carried out at 0.1C for the FeNC/G@S, CoNC/G@S . (E)
cycling performance of the FeNC/G@S, CoNC/G@S at 0.1C.

(1) Li, Y.-J. A Novel Synergistic Composite with Multi-Functional Effects for High-Performance Li-S Batteries. Energy Environ. Sci. 2016, 9, 1998–2004.

(2) Jianing Wang. Metal-Organic-Framework-Derived N-C-Co Film as a Shuttle-Suppressing Interlayer for Lithium Sulfur Battery. Chem. Eng. J. 2018, 334, 2356–2362.