One step production of in situ nitrogen doped mesoporous carbon confined sulfur for lithium–sulfur batteries

Abstract

Nitrogen doped mesoporous carbon (NMC) with high specific surface area, large pore volume and stable nitrogen content has been prepared by in situ doping through one step carbonization to immobilize sulfur for lithium–sulfur batteries. The structure and composition of the prepared NMC and NMC@S composites are confirmed with nitrogen sorption, elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction patterns (XRD), high resolution transmission electron microscopy (HR-TEM) and thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) images show a homogeneous distribution of sulfur within NMC@S. In comparison with free doped mesoporous carbon (MC), the NMC@S with high sulfur content (65%) is found to exhibit a higher initial discharge capacity of 1012 mA h g⁻¹ and higher capacity retention. The electrochemical performance of Li–S batteries is improved by the nitrogen doping, which not only enhances the electric conductivity but also further confines polysufides to the mesoporous carbon from shuttle effect.

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1. Introduction

To satisfy the demand for efficient energy storage for use in smart grids, electric vehicles and renewable energies, the development of novel, low-cost, eco-friendly battery systems with remarkable specific capacity has become an important task for researchers. Compared with traditional lithium ion batteries (LIBs), lithium–sulfur (Li–S) batteries have attracted increasing attention as next-generation energy storage devices, owing to their extremely high theoretical specific capacity (1675 mA h g⁻¹) and the high energy density (2600 W h kg⁻¹) of sulfur. In addition, sulfur is low-cost, earth abundant and eco-friendly, making this system attractive for large scale applications.¹ Despite these considerable advantages, the commercialization of Li–S batteries is hampered by several technical limitations. First, the poor intrinsic conductivity of pristine sulfur results in low utilization of sulfur cathodes;² second, the dissolution and migration of polysulfide intermediates produced during electrochemical reactions leads to the loss of active mass.

Furthermore, the gradual loss of active mass from the cathode into the electrolyte and onto the Li metal anode results in ‘shuttle effection’,³ severe self-discharge, increased-resistance, low coulombic efficiency and fast capacity decay on cycling,⁴ which hinders the use of Li–S batteries in practical applications.

Hitherto, numerous efforts have been employed to address the polysulfide shuttle challenge in Li–S batteries. Porous carbon with strong conductivity, high specific surface area and large pore volume is introduced into Li–S batteries to immobilize S and its discharge product, ensuring the desired sulfur content as well as high energy density, such as mesoporous carbon,^1,5–8 microporous carbon,^9,10 carbon nanotubes^11,12 and hollow porous carbon.¹³ The nano-scaled architecture accommodating a nano-sized sulfur composite shows enhanced utilization of the active sulfur, and shortens the distance for ionic and electronic transportation. Simultaneously, sulfur and the polysulfide can be adsorbed to the porous carbon, which refrains the insulated discharge product (Li₂S or Li₂S₂) from block deposition onto the electrode, thus alleviating polarization and extending cycle life of Li–S battery.

Although porous carbon materials with diverse textural characteristics have been designed for sulfur–carbon composite cathodes and shown improved performances, the properties of carbon materials as a matrix are also affected by the surface chemistry and electrical conductivity.¹⁴ Generally, heteroatoms (such as boron,¹⁵ nitrogen,¹⁶ sulfur^17,18) doped into a carbon substrate can effectively modify its surface chemistry and electron structures. Among them, nitrogen doping could remarkably improve the wettability, basic property, adsorptive ability, surface polar and conductivity of carbon materials,¹⁹ which has been proved by both theoretical and experimental studies. Sun et al. firstly used nitrogen-doped mesoporous carbon (NC) and sulfur to prepare an NC/S composite cathode.²⁰ Comparing with an AC/S composite, the battery based on the NC/S composite exhibited a higher discharge potential and an initial capacity of 1420 mA h g⁻¹ at a current density of 84 mA g⁻¹ (C/20). However, the nitrogen doping was achieved by high temperature NH₃ treatment, with only a low sulfur content of 24%. Xu et al. synthesized porous nitrogen-doped carbon nanotubes from a tubular polypyrrole.²¹ The initial discharge capacity was 1341 mA h g⁻¹ at a current rate of 1 C and a high reversible capacity of 933 mA h g⁻¹ was retained, even after 50 cycles at the same rate. Nitrogen doped carbon can perform not only as a superior cathode for Li–S batteries, but also as an electrode material for supercapacitors that provides great pseudo capacitance and exhibits excellent performance with the help of the nitrogen functionalities.¹⁸ However, among current methods of nitrogen doping, it is common to introduce nitrogen onto carbon frameworks by a post process using ammonia or melamine;^{18,20,22–24} as mentioned above, these methods result in a dreary process and instable properties given by nitrogen because of its uncontrollable amount and inhomogeneity.

We report herein a one step process to synthesize in situ nitrogen doped mesoporous carbon (NMC) with high specific surface area and large pore volume for high capacity lithium–sulfur batteries. Industrial melamine–formaldehyde resin (MF) and magnesium citrate were employed in the one step carbonization without any activation processes or subsequent extra reactions to introduce nitrogen. The prepared NMC serves as a good electron conductive framework and sulfur stabilizer. Besides, the facile approach to introduce nitrogen onto the carbon framework can guarantee a stable nitrogen content and pore structure under the flexible control of the harsh working conditions. Further, the low-cost MF and magnesium citrate can be largely synthesized, facilitating the practical application of Li–S batteries. In comparison with free doped mesoporous carbon (MC), the NMC exhibits a higher capacity retention. The improvement of electrochemical properties of Li–S batteries could be attributed to the interaction between the nitrogen functionalities on the surface of NMC and polysulfide as well as the enhanced electronic conductivity of the carbon matrix. The advantages of the nitrogen doped mesoporous carbon were investigated in detail.

2. Experiment

2.1 Preparation of nitrogen doped mesoporous carbon

Nitrogen doped mesoporous carbon (NMC) was synthesized using melamine-formaldehyde resin (MF, TianJin Resin Factory) as the carbon precursor as well as the nitrogen source, and magnesium citrate (Tianjin Guangfu Fine Chemical Research Institute) as both the poregen and the carbon precursor. In a previous report, magnesium citrate was used as the precursor of the carbon and nano-sized MgO particles were used as the template.^25–27 The synthesis was performed as illustrated below: MF powder and magnesium citrate were mixed homogeneously before carbonization. Thereafter, the mixture was heated at 700 °C under nitrogen flow (60 mL min⁻¹) at a heating rate of 4 °C min⁻¹. The final temperature (700 °C) was held for 2 h to ensure that the precursor was completely carbonized, meanwhile obtained nitrogen doped carbon–MgO nano-composites. The NMC was recovered after removal of the MgO templates through immersing in 1 mol L⁻¹ HCL aqueous solution after filtration, washing several times with distilled water, and then drying at 120 °C. For comparison, mesoporous carbon (MC) was synthesized through the above steps without the addition of MF.

2.2 Preparation of NMC@S composites

The NMC@S composites were prepared following a melt-diffusion strategy. The sublimed sulfur and as-prepared NMCs with a weight ratio of 1 : 2 were mixed homogeneously, followed by heating in a sealed vessel at 160 °C for 5 h under N₂ atmosphere. In this melt infiltration, the melt sulfur diffused into the pores of the NMCs adequately. Then, the temperature was increased to 300 °C for 3 h to vaporize the superfluous sulfur on the outer surface. The as-prepared composites were denoted as NMC@S. The MC@S composites were prepared by the same method.

2.3 Material characterization

Scanning electron microscopy (SEM, Hitachi-S4800), energy-dispersive X-ray spectroscopy (EDX) and high resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL) at 200 kv were employed to determine the morphology and microstructure of the materials. X-ray diffraction (XRD) patterns were obtained with a D8-Focus diffractometer using CuKα radiation (40 kV, 100 mA, λ = 1.54056 Å). The weight percentage of sulfur was determined by means of thermogravimetric analysis (TGA, STA 409 PC/PG, NETZSCH) in the temperature range of 30–700 °C at a heating rate of 10 °C min⁻¹ under a N₂ flow rate of 10 mL min⁻¹. The surface area and pore structure were characterized by nitrogen sorption using an Autosorb-iQ adsorption analyzer (Quantachrome) at 77 K. The surface area was determined by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution plot was calculated by the desorption branch of the Barrett–Joyner–Halenda (BJH) model. Elemental analysis was carried out using a vario EI CUBE. The carbon (C), hydrogen (H) and nitrogen (N) contents of the NMC were determined directly by a thermal conductivity detector. The electronic resistivity of the carbon powder was measured by a Model FZ-2010 semiconductor powder resistivity meter at a pressure of 100 kg. X-ray photoelectron spectrometer (XPS) measurements were carried out with a K-Alpha XPS from Thermo Fisher Scientific.

2.4 Electrochemical tests

The electrochemical characterization was performed using a CR-2430 type coin cell with lithium as the anode and polypropylene microporous membrane as the separator. The composite NMC@S cathode was obtained by casting a well-homogenized slurry of composite powder (70 wt%), conductive carbon black (super P, 20 wt%, TIMCAL Belgium) and water soluble binder LA132 (10 wt%, Chengdu Indigo Power Sources Co., Ltd) together in water on an aluminium current sheet and then dried at 50 °C under vacuum overnight. The electrode was further dried at 50 °C for 12 h before being transferred into the argon filled glovebox for battery assembly. The electrolyte was 1 M bis-(trifluoromethane sulfonimide lithium (LiTFSI, Alfa Corp.) dissolved in a mixture of dimethoxyethane (DME, Aladdin) and 1,3-dioxolane (DOL, Aladdin) (1 : 1 by volume) with an LiNO₃ concentration of 1 wt%.

The galvanostatic charge/discharge tests were carried out in potential range of 1.7–2.8 V with a LAND CT-2001A instrument (Wuhan, China). All capacity values were calculated on the basis of sulfur mass. Cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) measurements were conducted with a CHI 660D electrochemical measurement system. The CV tests were performed at a scan rate of 0.1 mV s⁻¹ in the voltage range of 1–3 V. The EIS were measured in the frequency range from 100 kHz to 0.1 Hz at open-circuit voltage. All the electrochemical tests were conducted at room temperature.

3. Results and discussion

3.1 Materials characterization

NMCs with developed mesoporous structure of high surface area and large pore volume were synthesized by one-step carbonization without activation or complicated post-processes to introduce nitrogen, followed by removal of the nanometer sized template. A commercial poly(melamine-co-formaldehyde) resin (MF) was selected as precursor and served as the carbon and nitrogen resource. To get a high pore volume and high surface area for sulfur loading and immobilizing, MgO with appropriate size, which was derived from the carbonization of magnesium citrate, was used as a template. Compared with the surface chemical post-modification for introducing N (for example, NH₃ treatment of porous carbon at high temperature), the one step production of in situ nitrogen doped mesoporous carbon we developed here is more facile, economical and environmentally friendly, which makes large scale fabrication feasible. In order to confirm the chemical composition of NMC, elemental analysis was performed. As shown in Table 1, elemental analysis discloses that the nitrogen content is 2%.

Table 1 Elemental content and pore parameters of carbon materials

Elemental analysis				Pore parameters
Sample	C (at%)	N (at%)	O (at%)	SBET^a (m^2 g^−1)	VT^b (cm^3 g^−1)
a BET specific surface area.b Total pore volume (p/p0 = 0.993).
NMC	84.18	2.00	11.00	1599	1.53
NMC@S	—	—	—	11.25	0.0329

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To characterize the surface properties of NMC, XPS measurements were performed and the results are exhibited in Fig. 1. The peaks centered at about 285.0, 400.0 and 531.0 eV in all survey spectra correspond to the C_1s, N_1s and O_1s, respectively, thus excluding the presence of any other impurities. The high resolution N_1s peaks (Fig. 1b) can be divided into three peaks with binding energy of 398.5 ± 0.3, 400.5 ± 0.3 and 401.6 ± 0.3 eV that correspond to pyridinic N, pyrrolic N and graphitic N, respectively. Further, a schematic model of these N species in carbon frame work is shown in Fig. 1c. The pyridinic N is located at the edges of the carbon layer by substituting a carbon atom on the C₆ ring,²² which contributes one pair of lone electrons and induces basic properties to the carbon surface.²⁸ In this case, because of the strong inter-atomic interaction, the NMCs bind favourably with S and polysulfides species, hence further limiting the extent of polysulfides dissolution.²⁸ As for pyrrolic N, the nitrogen atom on a five-membered ring contributes two p-electrons to the π system, which may also improve the adsorption ability of the NMCs. What's more, the graphitic N locates inside the graphitic carbon plane by substituting the carbon atom, bonding with three sp² carbon atoms, which are regarded as electron acceptors due to the higher electronegativity of N (3.04) compared with C (2.55).²⁹ The enhanced electronegativity will introduce more electrons to the graphitic layers, promoting the electronic conductivity of the carbon.³⁰ As discussed above, introducing nitrogen into the carbon framework enhances the material's electronic conductivity, which can be confirmed through measuring the powder electrical resistivity of the NMC and MC under the same pressure of 100 kg. As shown in Fig. 2, the electronic resistivity decreases from 1.60 Ω to 0.67 Ω cm by nitrogen doping.

Fig. 1 (a) XPS survey. (b) High-resolution N_1s XPS spectrum of NMC. (c) Schematic model of three types of NMC: pyridinic, pyrrolic, and graphitic.

Fig. 2 Electronic resistivity of NMC and MC at a pressure of 100 kg.

The porous structure of NMC was analyzed by nitrogen sorption and transmission electron microscopy (TEM). The nitrogen adsorption isotherms of NMC and NMC@S are shown in Fig. 3, and the corresponding textural parameters are summarized in Table 1. According to the IUPAC classification, the sample shows type IV adsorption isotherms with a steep condensation step within a narrow relative pressure range and type H1 hysteresis loops for large cylindrical mesoporous.³¹ The BET surface area of NMC reaches 1599 m² g⁻¹, and the BJH pore volume is as high as 1.53 m³ g⁻¹. The BJH pore size distribution shows that NMC possesses pore size centered at 3–4 nm, templated by the MgO from the magnesium citrate precursor. The high surface area and large pore volume of NMC guarantee to accommodate a high loading of nano-scaled sulfur and absorb polysufides strongly, implying an excellent electrochemical performance. As shown in Table 1 and Fig. 3, after the impregnation of sulfur, the pore volume and BET surface area of NMC remarkably reduced to 0.0329 m³ g⁻¹ and 11.25 m² g⁻¹, respectively, indicating that the sulfur is impregnated into the small pores of the NMC. Fig. 4a and b show TEM images of NMC, and it can be seen that NMC has a disordered mesoporous structure, which can accommodate S and polysufides, while the thin walls of the pores not only separate the material into compartments, but also connect the pores with each other, thus constructing the entire conductive network. Moreover, the walls of the pores act as conductors of electrons and Li⁺, and the disordered structure shortens the transmission distance of Li⁺.

Fig. 3 (a) Nitrogen adsorption/desorption isotherms of NMC at 77 K. (b) BJH pore size distributions of the NMCs before and after sulfur loading.

Fig. 4 TEM images (a and b) and SEM image (c) of NMC. SEM image (d) and SEM mapping (e) of NMC@S.

The sulfur content of NMC@S was determined by TG analysis. As shown in Fig. 5, the sulfur content is as high as 65%, and the high sulfur content in the composites will ensure a high overall energy density per gram of cathode in Li–S batteries. In addition, differential thermal gravity analysis (DTG) of NMC@S and pristine S were conducted and the results are displayed in Fig. 5. Compared with pristine S, the S in the NMC@S evaporates at a significantly elevated temperature and depressed rate, indicating that the nitrogen doped carbon substrate has a stronger interaction with S, which may enhance the performance of Li–S batteries.

Fig. 5 TGA and DTG of the NMC@S and S under N₂ atmosphere at a heating rate of 10 °C min⁻¹.

XRD analysis was carried out to further investigate the configuration of S in the NMC@S; the patterns of NMC, NMC@S and pristine S are shown in Fig. 6. Similar to the NMC pattern, there are no obvious reflection peaks of bulk sulfur in the NMC@S curves, which is consistent with amorphous carbon materials, demonstrating that nano-sized S was anchored into the pores by adsorption as a result of the disruption of the S crystal structure.

Fig. 6 XRD patterns of S, NMC, and NMC@S.

In order to demonstrate the dispersed state of sulfur after sulfur loading, SEM was conducted on the NMC and NMC@S. By comparison of the SEM images before and after sulfur impregnation, no obvious agglomerations of S particles are detected on the surface of NMC@S. To further confirm the distribution of sulfur within the carbon matrix, elemental X-ray mapping was performed at the same time. In Fig. 4, the sulfur mapping shows a homogeneous distribution of sulfur within NMC@S, which is in conformity with the XRD pattern of NMC@S with no observable reflection peaks of sulfur bulk. These observations further confirmed that sulfur diffuses into the pores of the carbon during the melt-diffusion process, which is consistent with the results of BET and pore size distribution test.

3.2 Electrochemical characterization

Fig. 7a shows the CV profiles of the NMC@S nanocomposite cathode between 1–3 V vs. Li⁺/Li for the initial three cycles. Two reduction peaks are observed for the cathode. In the cathodic reduction progress, the peak at 2.3 V involves the reduction of elemental S to soluble lithium polysulfides (Li₂S_n, 4 ≤ n ≤ 8); the peak centered at 2.0 V corresponds to the further converting of lithium polysulfides to solid-state Li₂S₂/Li₂S,³² while only one oxidation peak is observed, suggesting the transformation of all the polysulfides into the intermediates, which is believed to be S₈²⁻ with the most facile oxidation kinetics.^9,11,33 Moreover, apart from the redox peaks for sulfur, no other peaks can be observed, illustrating that the nitrogen atoms in NMC@S do not participate in the charge–discharge process of the Li–S battery.

Fig. 7 (a) Cyclic voltammetry plots at a scan rate of 0.1 mV s⁻¹ for the initial three cycles of NMC@S. (b) EIS before cycling of NMC@S and MC@S.

Fig. 8 displays the discharge profiles of the NMC@S and MC@S cathode at a current density of 100 mA g⁻¹. Typically, two well-defined discharge potential plateaus, corresponding to the formation of higher-order polysulfides (Li₂S_n, 4 ≤ n < 8) at 2.3 V, and lower-order Li₂S₂/Li₂S at 2.0 V, are obviously observed for all samples in the discharge process, which is consistent with two cathodic peaks in the CV curves. The initial discharge capacities of NMC@S and MC@S were 1012 and 898 mA h g⁻¹, respectively, corresponding to 61% and 53% utilization of sulfur, respectively. The relatively high capacity in the NMC@S cathode benefits from the enhanced electronic conductivity after nitrogen doping. The prolonged plateau around 2.0 V contributes to the main discharge capacity, and the potential plateaus of the NMC@S cathode are still stable and distinct after 50 cycles, which indicate the excellent electrochemical performance of NMC@S composites. The diffusion of polysulfides should be attributed to the synergistic effect of the developed porous nanostructure, large specific area and the modified surface chemistry with nitrogen doping.

Fig. 8 Discharge profiles of (a) NMC@S and (b) MC@S electrodes at 100 mA g⁻¹.

Fig. 9 shows the cycling performance of Li–S batteries based on the NMC@S and MC@S composite electrodes under a current density of 100 mA g⁻¹. Compared to the MC@S composite electrode, the NMC@S composite electrode delivers a higher discharge capacity and has slower capacity fading. The enhanced cycling performance should be attributed to the introduction of the nitrogen onto mesoporous carbon, which improves the adsorption ability towards polysulfides and retards diffusion of polysufides away from the cathode. Owing to the strong inter-atomic interaction, the NMCs bind favourably with S and polysulfides species, hence further limiting the extent of polysulfides dissolution. In addition, the enhanced cycling performance also profits from the promoted electronic conductivity resulted from nitrogen being doped into the carbon, which is further demonstrated by the lower charge transfer resistance observed in the EIS of the NMC@S electrode compared to that of the MC@S electrode (Fig. 7b). The semicircle diameters in the high frequency region of the Nyquist plots reveal that the charge-transfer kinetics are improved by nitrogen doping, indicating the enhanced electronic conductivity. The results are also in good agreement with the results of the powder electronic resistivity (Fig. 3). Additionally, the reactions in Li–S batteries are complex and the mechanism is still under study;³⁴ the cycling stability requires improvement in our subsequent study.

Fig. 9 Cycle performance of Li–S batteries based on NMC@S and MC@S.

4. Conclusions

In summary, we have developed a novel, in situ nitrogen doped, mesoporous carbon (NMC) for the first time by using poly(melamine-co-formaldehyde) through a one-step facile template approach wherein MgO is used as the template. The NMC has a stable nitrogen content, high surface area and large pore volume. The nitrogen doping could not only improve the electronic conductivity of mesoporous carbon, but also assist the mesoporous carbon to immobilize sulfur and trap the diffusion of soluble polysulfides with the help of the improved adsorption ability. Moreover, the developed nitrogen doped mesoporous carbon can also help the cathode buffer the abuse of volume change in the charge/discharge process. As a result, the NMC@S composite with high sulfur loading of 65% exhibits an enhanced cycling stability of 388 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹, while the MC@S delivers 220 mA h g⁻¹ with free doped. Additionally, our method to prepare in situ doped mesoporous through one-step carbonization is very facile and reproducible, which makes it feasible for large-scale industrial production. The exciting results provide a promising candidate for cathode materials for Li–S batteries and other energy storage devices.

Acknowledgements

This work was supported by a grant from the National High Technology Research and Development Program of China (863 Program) (no. 2013AA050905), the National Natural Science Foundation of China (no. 50902102 and no. 51172160) and National Natural Science Foundation of Tianjin (no. 14JCQNJC07200), Tianjin High School Science & Technology Fund Planning Project (20130307).

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