As concern of depletion of fossil fuels and destruction of the environment, rechargeable lithium-ion batteries (LIBs) are catching on as green energy technologies. Especially, Li-S battery is in the spotlight as next generation battery which demands for mid-large scale battery such as the electrical vehicles (EV) and the energy-storage system (ESS).^[1] Because Li-S battery has not only a high theoretical capacity (1675 mAh g^-1) and energy density (2600 Wh kg^-1) but also non-toxicity and abundance of sulfur.^[2] Despite these advantages, there are several drawbacks to commercialize Li-S battery system. First, elemental sulfur and Li₂S (reduction product at end of discharge step) has a low electronic conductivity. It is difficult that increasing sulfur content or loading level per unit area within electrode due to increase of resistance of battery.^[3] Second, polysulfide (intermediate products Li₂S_x, X ≥ 4) dissolves into the electrolyte during cycling. Once polysulfide is dissolved into the electrolyte, they migrate to anode side during charge-discharge step. This phenomenon causes shuttle reaction and loss of active material.^[4] Finally, large volume change (about 80%) during lithiation/delithiation of the active material, result in destruction of electrode and eventually degradation of the cell performance.^[5] To overcome these problems, many research groups have been investigated through the various strategies. For example, sulfur-carbon (multiwall carbon nanotubes, graphene, carbon nanofibers) composite is used to improve electronic conductivity of active material. Some researchers used metal oxide with high polysulfide absorption ability such as TiO₂, Al₂O₃, Mg_0.4Ni_0.6O, encapsulated sulfur in hollow or porous carbon structure, or functionalized carbon because of the strong chemical bonding between sulfur and functional group such as nitrogen to suppress polysulfide dissolution.^[6]

Herein, we report a free-standing bilayer carbon-sulfur (FBCS) cathode with superior electrochemical performance at high sulfur loading level (3 mg cm^-2). Top part of the FBCS cathode is composed of interlacing multiwall carbon nanotube (MWCNT) and bottom part is made up of mixed layer, MWCNT including sulfur and N-doped porous carbon (NPC). MWCNT layer (top part of FBCS cathode) blocks polysulfide migration from cathode to anode, and NPC in bottom part of FBCS cathode not only provides spacious active site but also absorbs polysulfide by the functional group of nitrogen. The designed novel FBCS cathode delivered a high initial discharge capacity of 964 and 900 mAh g^-1at 0.5 and 1 C, respectively. Moreover, it displays excellent cycle retention of 83.1% at 0.5 C and 83.4% at 1 C after 300 cycles.

References

[1]. B. Scrosati, J. Hassoun, Y.-K. Sun, Energy Environ. Sci. 2011, 4, 3287.

[2]. D. Bresser, S. Passerini, B. Scrosati, Chem. Commun. 2013, 49, 10545.

[3]. Y. Wu, M. Gao, X. Li, Y. Liu, H. Pan, J. Alloys and Compounds. 2014, 608, 220.

[4]. F. Wu, J. Chen, R. Chen, S. Wu, L. Li, S. Chen, T. Zhao, J. Phys. Chem. C 2011, 115, 6057.

[5]. G. Zheng, Y. Yang, J. J. Cha, S. S. Hong, Y. Cui, Nano Lett. 2011, 11, 4462.

[6]. A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Chem. Rev. 2014, 114, 11751.