A Low Cost and High Specific Surface Area Leaf-Derived Microporous Carbon for High Performance Li-S Batteries

Lithium-sulfur (Li-S) batteries has recently attracted worldwide attention as sulfur possesses an extremely high theoretical capacity of 1675 mAh g^-1 in addition to the advantages of natural abundance, low cost and environmental friendless, making it as one of the most promising cathode materials for the next generation of high energy batteries.^{1, 2}  However, it’s widely reported that Li-S batteries suffer from low utilization of sulfur, poor reaction kinetics and inferior charge/discharge efficiency owing to its poor electronic and ionic conductivity, intrinsic polysulfides shuttling and Li corrosion. Therefore, construction of nanostructured sulfur cathode with fast diffusion paths for lithium ions and electrons as well as good trapping of polysulfides are the key factors to enhance its electrochemical performance.³

Nanostructured porous carbon materials such as ordered mesoporous carbon, porous carbon spheres, porous graphene, porous carbon nanofibers and etc have been widely used in Li-S batteries.⁴ Thanks to their high electronic conductivity, unique pore structure, high specific surface area and good affinity for sulfur active materials, significant improvement on the cycle life and rate capability of sulfur cathode has been achieved. However, these carbon materials are struggling to the complicated synthetic process such as difficult template removal, long synthesis period, high cost fabrication method, low yield and so on, leading to the need of further exploration for porous carbon materials with low cost, high scale-up and superior electrochemical properties.

In the current work, a leaf-derived microporous carbon (LMC) with high surface area was prepared through one-pot carbonization followed by KOH activation. The sulfur/carbon composite with sulfur loading around 60wt. % (S/LMC) was then synthesized through a melt-diffusion strategy. The electrochemical test results shown that it can keep a stable capacity around 450 mAh g^-1 in 200 cycles without obvious capacity fading at a charge/discharge rate of 0.5 C (1C=1675 mA g^-1), showing higher reversible capacity and better cycle stability than commercial multi-walled carbon nanotubes with even lower sulfur loading of 50wt. % (S/MWCNTs).

Figure 1a and the inset show the SEM image of LMC, in which micro-sized carbon aggregate with existence of numerous pores was obvious observed. The porous structure and the pore size were further confirmed by the N₂ adsorption-desorption curve in the Figure 1b and the inset, showing a surface area of 1412.0 m² g^-1 and a pore volume of 0.759 cm³ g^-1, and main pore size around 2-4 nm. Figure 1c is the TGA curves of S/LMC and S/MWCNTs from room temperature to 600 °C at argon atmosphere. A significant mass loss owing to the evaporation of sulfur in the sulfur/carbon composite was observed in the both two TGA curves.  However, it can be seen that sulfur has been already evaporated below 350 °C for S/WCNTs; while in the case of S/LMC, most of the sulfur was removed at a temperature up to 500 °C. This finding may indicate a stronger adsorption of between sulfur and LMC than that of sulfur and MWCNTs. Figure 1d gives the comparison of the cycle life of S/MWCNTs and S/LMC at a rate of 0.5C. It was shown that S/LMC can deliver a stable capacity ca. 450 mAh g^-1 in 200 cycles without any obvious capacity loss. On the contrary, S/MWCNTs exhibited a continuous capacity fading. Such difference in their electrochemical performance may originate from the high specific surface area and unique microporous structure of LMC, leading to better adsorption of polysulfides during charge/discharge.

In summary, a low cost leaf-derived microporous carbon with high specific surface area was harvested from dead leaf and can demonstrate high performance in Li-S batteries.

Acknowledgements: Research at the Argonne National Laboratory was funded by U.S. Department of Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357.

References

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4.      S. Evers and L. F. Nazar, Accounts of Chemical Research, 2013, 46, 1135-1143.