- Nano Express
- Open Access
A Free-Standing Sulfur/Nitrogen-Doped Carbon Nanotube Electrode for High-Performance Lithium/Sulfur Batteries
© Zhao et al. 2015
- Received: 19 September 2015
- Accepted: 11 November 2015
- Published: 19 November 2015
A free-standing sulfur/nitrogen-doped carbon nanotube (S/N-CNT) composite prepared via a simple solution method was first studied as a cathode material for lithium/sulfur batteries. By taking advantage of the self-weaving behavior of N-CNT, binders and current collectors are rendered unnecessary in the cathode, thereby simplifying its manufacturing and increasing the sulfur weight ratio in the electrode. Transmission electronic microscopy showed the formation of a highly developed core-shell tubular structure consisting of S/N-CNT composite with uniform sulfur coating on the surface of N-CNT. As a core in the composite, the N-CNT with N functionalization provides a highly conductive and mechanically flexible framework, enhancing the electronic conductivity and consequently the rate capability of the material.
- Lithium/sulfur battery
- Sulfur/nitrogen-doped carbon nanotube composite cathode
- Free-standing electrode
Lithium/sulfur (Li/S) batteries possess great potential as advanced rechargeable batteries for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their large theoretical capacity at 1672 mAh g−1 and high theoretical energy density of 2600 Wh kg−1 [1, 2]. Furthermore, as a cathode material, sulfur has the advantages of natural abundance, low cost, and environmental friendliness . However, the commercialization of Li/S batteries faces several challenges related to insulating the nature of sulfur, solubility of polysulfides as discharge products in the electrolyte, and volume change of sulfur cathode during lithiation/delithiation [4, 5].
To circumvent the problem, various efforts have been made and various types of conductive carbon materials and conductive polymers have been used to composite with sulfur in order to enhance the electric conductivity of the sulfur composite and hinder the dissolution of the polysulfides into the electrolyte [6–14]. Among them, carbon nanotubes (CNTs), with their high electrical conductivity and a unique tubular structure, are widely used as a flexible matrix to form composite cathodes for Li/S batteries [11–13]. Notably, it was reported that nitrogen-doped carbon nanotubes (N-CNTs) have a significantly improved electronic conductivity due to the nitrogen atoms providing additional free electrons for the conduction band [15, 16]. Furthermore, Sun et al. demonstrated that compositing the nitrogen-doped mesoporous carbon with sulfur leads to easy and enhanced sulfur reduction activities .
In this work, for the first time, we introduced the binder-free sulfur/nitrogen-doped carbon nanotube (S/N-CNT) composite prepared by a simple solution mixing method as a cathode material for Li/S batteries. It was demonstrated that utilization of N-CNT has led to the high electrochemical performance, suggesting the great potential of N-CNT as a cathode additive for high-performance lithium/sulfur batteries.
The surface morphology and microstructure of the composite were examined by field emission scanning electron microscopy (SEM, JSM-6490, JEOL) and high-resolution transmission electron microscopy (HRTEM, JEM-2800, JEOL) with energy dispersive spectroscopy (EDX) mapping. The S content in the S/N-CNT composite was determined using chemical analysis (CHNS, Vario Micro Cube, Elementar). The electrochemical performance of the S/N-CNT composite cathode materials was investigated using coin-type cells (CR2032). The cell was composed of lithium metal anode and S/N-CNT cathode separated by a microporous polypropylene separator soaked in 1 M lithium bis (trifluoromethanesulfonate) (Aldrich) in tetraethyleneglycol dimethyl ether (Aldrich) electrolyte. The resulting cathode film was used to prepare the cathodes by punching circular disks with 1 cm in diameter. The coin cells were assembled in an Ar (99.9995 %)-filled glove box (MBraun) and tested galvanostatically on a multichannel battery tester (BTS-5V5mA, Neware). The cyclic voltammetry tests were performed using VMP3 potentiostat/galvanostat (Bio-Logic Science Instrument Co.). Applied currents and specific capacities were calculated on the basis of the weight of S in each cathode.
The chemical analysis of the composite confirmed a high sulfur content of 61 wt%, which was possible due to the formation of self-standing film and avoiding the needs of using a polymer binder.
The electrochemical performance of the S/N-CNT composite as a cathode material in Li/S batteries was further investigated by galvanostatic discharge/charge tests, and the results are displayed in Fig. 5b. The first plateau at about 2.4 V is related to the formation of higher-order lithium polysulfides (Li2Sn, n ≥ 4), which are soluble in the liquid electrolyte. The following electrochemical transition of these polysulfides into lithium sulfide Li2S is associated to a prolonged plateau around 2.0 V, which are well-corresponded with the CV data. Figure 5c presents the cycling performance of the S/N-CNT composite at 0.2 C. The S/N-CNT composite exhibited a stable cycling behavior with small capacity loss even after 100 cycles. A reversible capacity of 1098 mAh g−1 was obtained by the S/N-CNT composite in the second cycle, and the cell retained about 73.5 % of its initial reversible discharge capacity after 100 cycles; the coulombic efficiency was maintained above 93 %. Based on the above phenomenon, we can conclude that the N-CNT could diminish the polysulfide dissolution in a physical and chemical way, thereby stabilize the capacity significantly. Furthermore, the free-standing S/N-CNT composite film possesses a robust and flexible structure, which could accommodate the solubilization/precipitation of sulfur during the cycles .
The rate capability results, as depicted in Fig. 5d, reveal excellent performance of the S/N-CNT composite at various current densities from 0.5 to 2 C. At the initial cycle at 0.5 C current, the composite achieves a discharge capacity of 1016 mAh g−1. There is a gradual capacity reduction with the increase in the current rate, although 298 mAh g−1 reversible capacity was sustained even at 2 C rate. More importantly, the composite regained the most of its reversible capacity (742 mAh g−1) when the discharge rate was modulated back to 0.5 C, which shows a high abuse tolerance of the S/N-CNT composite. This superb rate performance can be attributed to the excellent high-rate discharge capability of the composite sulfur cathode due to the good electrical conductivity of N-CNT and existence of good lithium ion transport path in the composite structure.
Literature comparison between the electrochemical performances of S/CNT and S/N-CNT composite cathodes for Li/S batteries
1st discharge capacity (mAh g−1)
nth cycle reversible capacity (mAh g−1)
Sulfur-porous carbon nanotubes
Sulfur-multiwalled carbon nanotubes
Aligned carbon nanotube/sulfur
Multiwalled carbon nanotubes-sulfur
200 mA g−1
In this work, the free-standing S/N-CNT composite was prepared by simply mixing nanosized sulfur particle suspension and nitrogen-doped carbon nanotube suspension followed by vacuum filtration. In the composite, N-CNT serves as carbon skeleton for the S/N-CNT composite, forming a stable interconnected network structure. The S/N-CNT composite cathode exhibits good cyclability and rate capability in rechargeable lithium/sulfur battery. Moreover, the formation of the free-standing and flexible film of the S/N-CNT composite allowed for increasing the active cathode material content (S), and it makes this approach to be one of the possible ways to prepare flexible and/or bendable next-generation Li/S batteries.
This research was supported by the Technology Commercialization Project of the World Bank and the Government of Kazakhstan (group 157) and partially by the grant from the Ministry of Education and Science of Kazakhstan “High energy density polymer lithium-sulfur battery for renewable energy, electric transport and electronics” (the 2015–2017 call). The authors are grateful for the financial support by the National Natural Science Foundation of China (Grant No. 21406052) and financial support by Program for the Outstanding Young Talents of Hebei Province (Grant No. BJ2014010). H. Xie is grateful for the financial support by the Youth Foundation of Hebei Educational Committee (Grant No. QN2014094).
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