Conducting Polymer Coated Exfoliated Graphene Sheet Electrodes for Lithium Rechargeable Cells

Lithium-sulfur batteries are attractive as next-generation energy storage devices for future energy conversion and all-electric vehicles, owing to the extremely high theoretical specific capacity (1675 mAh g^-1) and energy density (2600 Wh kg^-1). In addition, sulfur is low-cost, abundant resources and environmental friendless ^[1].

Despite these diverse advantages, it is difficult to commercialize Li-S battery by several problems.  Firstly, sulfur and polysulfides are non-conductive. So, the low electrical conductivity of sulfur necessitates the use of a large amount of conductive carbon to improve utilization of the active materials. And conductive carbon should been mixed with sulfur to obtain the necessary electronic conductivity for lithium-sulfur cell operation. Secondly, the lithium-sulfur battery operates by reversible conversion of sulfur to diverse lithium polysulfide products (Li₂S_x, 1≤x≤8) via internal redox reaction. However, the dissolution of lithium polysulfides can also lead to low utilization of active material from the cathode, resulting in the loss of active material.

To solve these problems, cathode material must be well combined with high conductive and strong adsorbing agent. Conducting polymer such as polyaniline, poly(diallyldimethylammonium chloride) (PDDA) and polypyrrole have been studied recently for Li-S batteries to improve electrochemical performance ^[2].

In this present work, we introduced a PDDA coated on graphene oxide/sulfur (PDDA-GO/S) composites for lithium-sulfur battery and demonstrate that conducting polymer coated on graphene oxide sheets can effectively induce chemical interaction of sulfur on the surface carbon framework and serves as a good electron conductive framework ^[3-4].

FT-IR spectroscopy was employed to investigate the functionalized and structural properties. The IR spectrum of GO shows peaks at 1055 cm^-1 (C-O), 1230 cm^-1 (C-O-C), 1730 cm^-1 (C=O), and 1415 cm^-1 (C-OH). After treatment with NaBH₄, the peak at 1730 cm^-1 (C=O) is disappeared and the peak at 1055 cm^-1 (C-O) is attenuated. In addition to the mentioned peak, there are additional absorption peak attributed as follows: 2923 cm^-1 (C-H), 1630 cm^-1 (C=C), and 1463 cm^-1 (C=C) for PDDA-GO. These latter peaks correspond to the characteristic peaks of PDDA ^[3], indicating the functionalization of graphene oxide with PDDA ^[4]. The relating electrochemical activity of PDDA coated on graphene oxide/sulfur (PDDA-GO/S) composites will be also discussed.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning, Korea (Grant No.: NRF-2011-0009007).

References

[1] B. O. Jeong, S. W. Kwon, T. J. Kim, E. H. Lee, S. H. Jeong  and Y. Jung,  J. Nanosci. Nanotech., 13 (2013) 7870.

[2] S X. L. Ji, K. T. Lee, and L. F. Nazar, Nature Materials, 8 (2009) 500.

[3] J.Y. Park, S.J. Park, S. Kim, J. Electrochem. Soc., 161(5) (2014) F641.

[4] Y. C. Zhao, L. Zhan, J. N. Tian, S. L. Nie, and Z. Ning. Electrochim. Acta 56 (2011) 1967.