Aniline-Modified Ordered Mesoporous Carbon As an Insoluble Discharge Product Stabilizer for High Performance Li-S Batteries

As the demand for sustainable energy has continuously grown, energy storage system with high energy density has received great attention. However, current cathode system of Li-ion batteries based on an intercalation reaction has insufficient theoretical capacity for electric vehicles or grid energy storage. Sulfur is one of the most promising cathode materials because of its high theoretical capacity of 1672 mAh g^-1, which is around 5 times higher than that of existing cathode system [1]. Lithium sulfur batteries operate based on a redox reaction near 2.2 V (vs. Li/Li⁺), providing a high specific energy of 2600 Wh kg^-1. Furthermore, sulfur has another attractive features, low cost, environmental friendliness and natural abundance. Despite these advantages, sulfur cathode is still a long way from the commercial application due to low sulfur utilization and poor cycle stability. Low sulfur utilization comes from the insulating nature of sulfur and its intermediates. Thus, various carbon materials and conducting polymers have been employed to increase sulfur utilization. Poor cycle stability is a more complicate issue since it is caused by diffusion of the intermediate soluble polysulfides and aggregation of insoluble discharge products. To date, considerable researches have been conducted to prevent diffusion of soluble polysulfides by various strategies [2, 3]. However, issues from insoluble discharge products have not been addressed compared to that from soluble polysulfides. To manage the insoluble discharge products during cycling for high performance Li-S batteries, there are some challenges: i) a smaller functional group which can interact with insoluble discharge product is suitable for homogeneous functionalization and efficient sulfur infiltration and ii) milder functionalization method is need to maintain original microstructures of carbon matrix.

In this work, we employ simple diazotization method to introduce aniline into the pore wall of ordered mesoporous carbon (CMK-3). In the LPG refinement process, the amine-type functional groups have been widely used because it can interact with H₂S through a strong Lewis acid-base interaction [4]. It can be assumed that the aniline functional group can interact strongly with insoluble discharge products in a similar manner. The aniline functional groups provide effective interaction sites for a homogeneous distribution of the insoluble discharge products during cycling as schematically described in Figure 1. Also, it is demonstrated that the ordered microstructure was maintained after diazotization resulting in retaining inherent encapsulating ability of ordered mesoporous carbon. As shown in Figure 2, the aniline-modified CMK-S composite exhibit stable cycling performance (1026 mAh g^-1 at 0.1 C after 50 cycles and 920 mAh g^-1 at 0.2 C after 100 cycles) and good rate capability (814 mAh g^-1 at 1 C) when evaluated as a cathode material for Li-S batteries.

References

[1]         Y.-X. Yin, S. Xin, Y.-G. Guo, L.-J. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186.

[2]         X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500.

[3]         G.-C. Li, G.-R. Li, S.-H. Ye, X.-P. Gao, Adv. Energy Mater. 2012, 2, 1238.

[4]         P. Gilli, L. Pretto, V. Bertolasi, G. Gilli, Acc. Chem. Res. 2008, 42, 33.