Tailored Carbon Nanotubes for High-Performance Lithium-Sulfur Batteries

Significant effort has been invested into addressing critical problems of lithium-sulfur batteries since they are considered as the next generation energy storage material due to their high specific energy density and low cost.¹ Critical problems still remain including poor cycle life and rate capability originated from the insulating nature of sulfur and polysulfide intermediate dissolution into the electrolyte.² Despite the considerable success in alleviating the problems such as drastic capacity fading and poor rate capability, commercial application still has a long way to go, especially at high current rate conditions.

 Here, we report a noble carbon matrix for lithium-sulfur batteries based on a partially unzipped multi-walled carbon nanotube(MWCNT) with favorable properties for the improved lithium-sulfur batteries.  Partially unzipped MWCNT has additional benefit in that it was prepared by facile and scalable way without use of toxic or expensive materials. In contrast to MWCNT and fully unzipped nanoribbon, partially unzipped MWCNT clearly exhibited a unique structure of coexistence of carbon nanotube and nanoribbon in one tube as illustrated in Figure 1. The experimental optimization results were fully discussed with TEM, SAED patterns, EDS mapping, RAMAN, XPS, BET isotherms and EIS characterizations.

 Partially unzipped MWCNT showed increased surface area and pore volume with preserved electron conductive pathway owing to opened inner pores of MWCNT. Newly developed the pores of partially unzipped MWCNT were decorated with oxygen containing functional groups and acted as a stable polysulfide reservoir as illustrated in Figure 2.  The synergistic effect of unique pore structure and oxygen containing functional groups led to the improved electrochemical performance at a high current rate of 8375 mA g^-1 (5 C). Partially unzipped MWCNT-sulfur composite delivered 688.5 mAh g^-1 at the initial discharge and retained 544.2 mAh g^-1 after 100 cycles at the high current rate of 5 C as given in Figure 3.

 Furthermore, to understand the mechanism of the improved electrochemical performance of partially unzipped MWCNT, Partially unzipped MWCNT-sulfur was studied utilizing the Monte Carlo (MC) simulations. This modelling explains why the polysulfide dissolution into the bulk electrolyte was alleviated thus led to the improved electrochemical performance.

 We believe that our work can provide insight to many researchers in that partially unzipped MWCNT opens up a new concept of exploiting MWCNT inner pores.

Reference

1. Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A. et al. The use of elemental sulfur as an alternative feedstock for polymeric materials. Nat. Chem. 2013, 5, 518-524.

2. Manthiram, A.; Fu, Y.;Su, Y.-S. Challenges and Prospects of Lithium-Sulfur Batteries. Acc. Chem. Res. 2012, 46, 1125-1134.