Heteroatom dopings and hierarchical pores of graphene for synergistic improvement of lithium–sulfur battery performance

Jiahui Lia, Caining Xuea, Baojuan Xi*a, Hongzhi Maoa, Yitai Qian*ab and Shenglin Xionga
aKey Laboratory of the Colloid and Interface Chemistry, Ministry of Education, and School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, PR China. E-mail: baojuanxi@sdu.edu.cn; qianyt@sdu.edu.cn
bHefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, PR China

Received 21st February 2018 , Accepted 12th March 2018

First published on 12th March 2018


Lithium–sulfur batteries have attracted wide research interest due to their high specific capacity (1675 mA h g−1) and high specific energy (2500 W h kg−1). Herein, the two-dimensional nitrogen/sulfur synchronous-doped few-layer graphene (N,S-FLG) was prepared by a simple calcination route at 900 °C under an argon atmosphere with g-C3N4 as a structural template. The calcination temperature favorably adjusted the doping levels and surface texture of the as-obtained samples. Relying on the synergistic interaction of the structural traits, as the reservoir of sulfur, NS-FLG-900 (calcination temperature 900 °C) can maintain a reversible capacity of 602 mA h g−1 after 300 cycles at a current density of 0.8 A g−1 and 506.4 mA h g−1 after 500 cycles at a current density of 1.6 A g−1. The outstanding performance is obtained due to the synergistic effect of the introduction of N and S into the carbon lattice, the high pore volume (1.31 cm3 g−1) and large specific surface area (460.9 m2 g−1).


1. Introduction

Nowadays, the advancement of energy storage systems has become increasingly important to achieve the generation and transportation of power. In terms of secondary batteries, lithium-ion batteries (LIBs) have been attracting huge interest for the application in electronic devices owing to their high performance.1–3 However, the limited energy density of commercial LIBs has shifted the focus of the researchers on the more prospective lithium–sulfur (Li–S) batteries, which possess a high theoretical energy density of 2600 W h kg−1 (more than 5 times that of LIBs). In addition, elemental sulfur has the advantages of low cost, non-toxicity, eco-friendliness and so forth.3–9 All these virtues enable Li–S batteries to be a potential contender for the next generation of energy storage devices. Despite these strengths, several challenges related to the cathodes that inhibit it from practical applications3,8 remain to be overcome: (i) a pretty low utilization of the active material caused by the inherent insulation of sulfur (5 × 10−30 S cm−1) and its discharge products (Li2S2 or Li2S); (ii) the irreversible loss of capacity and poor Coulombic efficiency resulting from the notorious shuttle effect of soluble polysulfides; (iii) the tendancy of the electrode structure to break down because of the distinct volumetric change (∼80%) during the repeated process of charging or discharging.3,10–12

Various ways have been devised to solve the above-mentioned issues, such as passivation of the metallic Li anode,13,14 replacing liquid electrolytes with solid electrolytes,15,16 and placing an interlayer between the separator and the cathode to prevent the shuttling of polysulfide.17–19 However, as demonstrated before, the structure and property of electrodes exert a significant and vital influence on the performance of the assembled batteries. Hence, great efforts have been made to engineer the structural configuration of the sulfur cathode. Combining sulfur with a carbonaceous matrix is one of the most effective routes due to its low density, excellent electrical conductivity, abundant pore structure and high chemical stability.20 In recent years, various carbon structures have been extensively studied as sulfur host, which can increase the conductivity and stability of a sulfur cathode to optimize the battery performance.3,21–23 For example, porous and hollow carbon materials24–30 can suppress the migration of polysulfide intermediates and can also accommodate more electrolyte to allow lithium ions to pass quickly. However, just relying on the nonpolar physical adsorption of carbon materials is not efficient enough to prevent the polysulfide from shuttling. It is a well-established fact that heteroatom doping with elements including sulfur, nitrogen and fluorine can effectively improve the polarity of carbon materials. This strengthens the chemical interaction between the sulfur and polysulfide, thus enhancing the usage of sulfur.3,12,31–33

Moreover, in case of carbonaceous materials, two-dimensional (2D) graphene has brought forward new opportunities and served as a greatly promising conductive supporter due to its remarkable electrical conductivity, larger specific surface area and outstanding structure flexibility originating from the single layer of carbon atoms.34–37 Hence, the use of integrated heteroatom-doped graphene opens up effective possibilities to enable the optimization of the sulfur cathode performance. However, till now, few studies3 have reported the application of N/S-codoped graphene as a sulfur matrix.

Based on the above considerations, herein, we prepared in situ nitrogen and sulfur codoped few-layer graphene (denoted as N,S-FLG) by a two-step calcination method. The graphitic C3N4 as the template was prefabricated for the synthesis of porous few-layer graphene, during which N and S atoms were simultaneously doped with dithiooxamide as a doping reagent. Moreover, the calcination temperature was found to exert a vital influence on the specific surface area and pore volume of the final graphene. In comparison with the original graphene, the as-prepared N,S-FLG exhibited better cycling capability and higher capacity. Due to the introduction of N and S heteroatoms into the carbonaceous materials, the adsorption effect on the polysulfide and the stability of the entire electrode is effectively improved. Simultaneously, the coexistence of micro/mesopores, on the one hand, can rivet sulfur and on the other can relieve the volume change of active material during the process of charging and discharging. This synergistic effect ensures that the material exhibits an outstanding electrochemical performance.

2. Experimental section

2.1 Materials

All the chemicals were purchased from Sinopharm Chemical Reagent and used without any further purification.

2.2 Preparation of g-C3N4 template

The template g-C3N4 was prepared according to the previous report38 by pyrolyzing the moderate melamine at 550 °C for 3 h with a heating ramp of 4 °C min−1 in the static air atmosphere.

2.3 Preparation of nitrogen and sulfur co-doped few-layer graphene (denoted as N,S-FLG)

C3N4 template (2.0 g), glucose (0.375 g) and dithiooxamide (DTO, 0.375 g) were poured in an agate jar and ethyl alcohol (2 mL) was added into the above mixture, followed by ball-milling for several hours to mix evenly. The obtained product was dried in a vacuum oven at 60 °C for 5 h and then calcined under an argon atmosphere at 700 °C for 3 h with a heating ramp of 3 °C min−1, followed by increasing the temperature to 800–1000 °C and maintaining for 3 h with a heat ramp of 4 °C min−1. Finally, N/S-codoped graphene was obtained and named as N,S-FLG800, N,S-FLG900 and N,S-FLG1000.

2.4 Preparation of N,S-FLG/S composite

The composite with sulfur was synthesized by a typical melting-diffusion strategy. Briefly, after the carbon matrix was mixed homogeneously with a certain amount of sulfur, the mixture was sealed in a container and heated at 155 °C for 12 h under an argon atmosphere.

2.5 Material characterization

The advanced testing techniques were used to analyze the morphology and microstructures of the as-obtained samples. The X-ray powder diffraction (XRD) patterns were obtained using a Bruker D8 advanced X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Field-emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) images were collected on Gemini-500 (Zeiss) and JEM-1011 (JEOL), respectively. The percentage of sulfur in the carbon matrix was evaluated by TGA (a Mettler Toledo TGA/SDTA851) with a heating rate of 10 °C min−1 from room temperature to 800 °C under an N2 atmosphere. The nitrogen adsorption–desorption isotherms were measured at 77 K with a Micromeritics ASAP-2020HD88 analyzer. Raman spectra were gained using a NEMUS670 spectrometer. The elemental composition and valence state of the samples were characterized on an X-ray photoelectron spectrometer (ESCALAB 250 spectrometer, PerkinElmer).

2.6 Electrochemical measurement

The CR2016 coin cells were assembled in the glove-box filled with argon to estimate the electrochemical property of the cathode materials for the Li–S batteries. To prepare the cathode slurry, 70 wt% of N,S-FLG/S composite, 12 wt% acetylene black, 10 wt% of carbon nanotube and 8 wt% polyvinylidene difluoride (PVDF) were mixed evenly in an N-methylpyrrolidone (NMP) solvent dispersant. The ready slurry was coated onto aluminum foil (purchased from Shenzhen BiYuan Electronics) with the thickness of 200 μm without calendaring and then dried at 60 °C for 10 h in a vacuum oven. The areal density of sulfur in the electrode was more than 1 mg cm−2. The electrolyte was made of 1 mol L−1 Li bistrifluoromethanesulphonylimide in the mixed solvent of 1,3-dioxolane and 1,2-dimethoxyethane (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) with 2 wt% LiNO3 as an additive. Pure lithium foil acted as the reference/counter electrode and Celgard 2400 as a separator. The galvanostatic charge–discharge tests were recorded using a LAND CT2001A battery cycler at a temperature of 30 °C. The CV curves were measured using a CHI760E electrochemical workstation (Shanghai Chenhua) in the range of 1.7–2.8 V at a scan rate of 0.1 mV s−1.

3. Results and discussion

3.1 Morphology and characterization of N,S-FLG and N,S-FLG/S

The fabrication of N,S-FLG was carried out stepwise as illustrated in Scheme 1. First, the g-C3N4 template was fabricated by thermal polycondensation of melamine39,40 (step I). Melamine molecules condensed into melem, which subsequently polymerized into a g-C3N4 network. The graphitic carbon nitride is a type of organic semiconductor consisting of triazine units with a high sp2-hybridized nitrogen content of 57 at%.41 Due to its outstanding structural features and rich nitrogen, g-C3N4 is applied as a template or doping nitrogen source for effective synthesis of lateral materials or N-doped structures.40 The as-obtained g-C3N4 was characterized by transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) techniques to monitor the morphology and phase traits. Fig. S1A–C (ESI) clearly demonstrate the sheet-like structure of g-C3N4. The corresponding XRD pattern is present in Fig. S1D (ESI), in which two dominant peaks at 13.3° and 27.5° result from (100) and (002) crystal planes of g-C3N4. After balling with glucose as a carbon source and dithiooxamide as a sulfur-doping reagent, followed by calcination, N,S-FLG was successfully prepared with in situ-doping of nitrogen and sulfur (step II). At a calcination temperature of 900 °C, the sample of N,S-FLG was obtained (N,S-FLG900). The light contrast can be definitely observed in Fig. 1A and B, demonstrating the thin layer of graphene. Furthermore, the remarkable wrinkle features of graphene are observable in both TEM and FESEM images in Fig. 1A–D. The yield of N,S-FLG900 is calculated to be about 50% based on the mass of melamine and the corresponding digital photograph is inserted in Fig. 1D, from which we can learn that it has a fluffy appearance and infer that it has a relatively large specific surface area, which can be subsequently confirmed by the BET measurements.
image file: c8qi00160j-s1.tif
Scheme 1 Fabrication of nitrogen and sulfur dual-doped few-layer-graphene (N,S-FLG) starting from melamine reagent. (Carbon: gray, nitrogen: blue, sulfur: yellow.)

image file: c8qi00160j-f1.tif
Fig. 1 Morphology and microstructure of N,S-FLG900: (A,B) TEM images and (C,D) FESEM images (digital photograph is inset in Fig. 2D). Scale bar: (A) 250 nm, (B,C) 200 nm, (D) 100 nm.

It was found that the calcination temperature played a significant role in the structure and property of the as-obtained carbon materials. The resultant sample obtained on decreasing the temperature to 800 °C is revealed in Fig. S2 (ESI). From the TEM images (Fig. S2A and B, ESI), the carbon nanosheets are observed to be very thin, i.e., several layers of graphene are observed (named as N,S-FLG800). The FESEM images (Fig. S2C and D) display the high-area porous structure of N,S-FLG800, which is similar to N,S-FLG900.

Another sample N,S-FLG1000 was fabricated at 1000 °C and characterized by FESEM and TEM techniques. Similar to N,S-FLG800, the porous and wrinkled structure characteristic of graphene is apparently observed (Fig. S3, ESI). XRD was used to determine the composition and phase of the as-obtained products. Fig. 2A shows that N,S-FLG samples obtained at 800, 900 and 1000 °C display similar XRD profiles. Typically, the feature of the broad (002) peak and weak (101) peak indicate the low-degree graphitic structure.26,29 Raman spectra were applied to determine the quality of three samples. As shown in Fig. 2B, we can observe remarkable D-band and G-band peaks at ∼1330 and ∼1580 cm−1, respectively. The ratio (ID/IG) of the intensity of the D band to G band is indicative of the graphitization degree of the carbon materials.29,31 From the Raman curves, a high intensity ratio (ID/IG) for both samples is obtained and suggests ample defects in the N,S-FLG, which are assigned to the introduction of N and S atoms in the carbonaceous matrix. With an increase in calcination temperature, the graphitization degree of carbon improved and less doping level was enabled, rendering better graphitization quality. The Raman data confirmed that the ID/IG values were 1.244, 1.185 and 1.097 for N,S-FLG800, N,S-FLG900, and N,S-FLG1000 respectively. Moreover, the investigation over the porous texture of N,S-FLG prepared at various temperatures was carried out via N2 adsorption/desorption experiments at 77 K. The adsorption–desorption isotherms over N,S-FLG900 are displayed in Fig. 2C. An intense adsorption curve appears at low relative pressure, which is typical of the presence of micropores and the hysteresis of the H3 type is associated with capillary condensation in mesoporous structures, implying the presence of hierarchical pores. The corresponding pore size distribution plot (inset of Fig. 2C) confirms the above analyses and the pores are in the range of 0.5–1.7 nm (micropores) and 3.5–35 nm (mesopores). Fig. S5A and B (ESI) also show the N2 adsorption–desorption isotherms of the other two samples of N,S-FLG800 and N,S-FLG1000. By comparison, N,S-FLG900 possesses a high Brunauer–Emmett–Teller (BET) surface area of 460.9 m2 g−1 and a total pore volume of 1.31 cm3 g−1. In case of N,S-FLG800 and N,S-FLG1000, the BET surface area and total pore volume are 135.1 m2 g−1/0.66 cm3 g−1 and 595.3 m2 g−1/1.29 cm3 g−1, respectively. The large specific surface area and pore volume of N,S-FLG900 is helpful for the even dispersion of sulfur, relieving the volume change of active material during the process of charging or discharging.


image file: c8qi00160j-f2.tif
Fig. 2 Structural characterizations of N,S-FLG and N,S-FLG/S composite: (A) XRD patterns of N,S-FLG800, N,S-FLG900 and N,S-FLG1000. (B) Raman spectra of N,S-FLG800 and N,S-FLG900. (C) N2 adsorption–desorption isotherms and the corresponding pore-size distribution of N,S-FLG900 and N,S-FLG900/S. (D) TGA curve of N,S-FLG-900/S composite.

After combination with sulfur via the melting-diffusion route, the N,S-FLG900/S composite can be readily used as the cathode material of the Li–S batteries. As presented in Fig. 3, the sheet structure resembles that before sulfur anchoring. However, the TEM image contrast grows much darker owing to the sulfur impregnation and no large sulfur aggregates are found on the N,S-FLG900 surface. The analytical results indicate that sulfur particles are uniformly distributed in the carbonaceous materials. In order to testify the infiltration of sulfur into pores of carbon matrix, N2 adsorption–desorption isotherms of N,S-FLG900/S were also recorded (Fig. 2C). As clarified earlier, a prominent hysteresis loop is observed in the nitrogen adsorption/desorption isotherms of N,S-FLG900, while it almost vanishes after sulfur loading. Considering the pore size distribution of N,S-FLG900, the mesopores of N,S-FLG900/S nearly disappear and the volume of micropores also drastically reduces. Based on the calculation, the BET surface and pore volume decrease significantly to 53.4 m2 g−1 and 0.24 cm3 g−1, firmly demonstrating that most of the active material is allowed into the pores of the carbon holder during the combination process.30,40 In the TGA curve of N,S-FLG900/S (Fig. 2D), it is worth noting that two steps of sulfur removal are present. The second slow-slope stage occurs at a higher temperature (from 260 to 400 °C) than the first stage (from 150 to 260 °C), implying that more energy is required for the sulfur volatilization. This phenomenon accounts for partial sulfur existing in the smaller micropores,42 which is consistent with the abovementioned N2 sorption results. The sulfur loading is calculated to be about 65%. After sulfur loading, N,S-FLG800/S and N,S-FLG1000/S were obtained, which exhibited the morphology similar to their counterparts (see Fig. S2E–F and S4, ESI). In comparison, the TGA curves of N,S-FLG800/S and N,S-FLG1000/S are shown in Fig. S6 (ESI), displaying the similar sulfur loading of about 65%.


image file: c8qi00160j-f3.tif
Fig. 3 Morphology and microstructure of N,S-FLG900/S: (A,B) TEM and (C,D) FESEM images, (E,F) FESEM and the corresponding elemental mappings: carbon, nitrogen and sulfur. Scale bars: (A,C,E) 500 nm, (B,D,F) 200 nm.

X-ray photoelectron spectroscopy (XPS) characterization was conducted to explore the elemental composition and valence state of the as-prepared samples. In the survey spectrum of N,S-FLG900 (Fig. 4A), five distinct peaks at 531.73, 401.08, 284.8, 228.3 and 163.92 eV correspond to O 1s, N 1s, C 1s, S 2s and S 2p, respectively, confirming that there is no impurity. In case of the C 1s spectrum in Fig. 4B, three bands at 284.8, 285.6 and 286.7 eV are ascribed to C–C, C–S and C–N–C, respectively. It is verified that elemental S and N were introduced into the lattice structure of FLG materials. The high-resolution N 1s spectrum (Fig. 4C) can be well-fitted into three peaks, i.e., pyridinic N, pyrrolic N and quaternary N with binding energies of 398.3, 399.5 and 401 eV, respectively. It is reported that pyridinic and pyrrolic N can supply abundant active sites for capturing and fixing polysulfides, while quaternary N can greatly enhance the electrical conductivity of carbon materials.43 In the S 2p photoelectron spectrum (Fig. 4D), two peaks at 163.8 and 165.0 eV are derived from thiophene-type S 2p3/2 and S 2p1/2, respectively.44 Another peak at 168.6 eV is typical of higher-oxidation-state sulfur.31 The nitrogen and sulfur doping contents of N,S-FLG800, N,S-FLG900 and N,S-FLG1000 are calculated based on the XPS results and placed in Table 1. After combination with sulfur, N,S-FLG900/S has an analogous XPS spectra to N,S-FLG900 as shown in Fig. S7 (ESI).


image file: c8qi00160j-f4.tif
Fig. 4 XPS spectra of N,S-FLG900 material: (A) survey spectrum, (B) C 1s, (C) N 1s, (D) S 2p.
Table 1 The characteristic traits of N,S-FLG obtained at different temperatures
T (°C) BET surface area (m2 g−1) Pore volume (cm3 g−1) Dominant porosity (nm) Doped N (atom%) Doped S (atom%)
800 135.1 0.66 Micropores and mesopores 9.41% 1.06%
900 460.9 1.31 Micropores and mesopores 7.27% 0.88%
1000 595.3 1.29 Micropores and mesopores 5.68% 1.44%


3.2 Electrochemical performance of the N,S-FLG/S cathode materials

Fig. 5A displays the cyclic voltammogram (CV) charts of N,S-FLG900/S in a voltage window of 1.7–2.8 V versus Li+/Li with a sweep rate of 0.1 mV s−1. In the first cathodic scan process, two major broad peaks dominate, which are associated with the two-step sulfur reduction of cyclo-S8 to Li2S. One cathodic peak is at about 2.22 V, corresponding to the sulfur reduction to higher-order polysulfide (Li2S4-Li2S8).45 The other cathodic peak at about 1.94 V is related to the further reduction of higher-order polysulfides from Li2S2 to Li2S.45 In the anodic process, only one oxidation peak is present at about 2.49 V, which is considered to be attributed to the oxidation reaction of Li2S to sulfur. In the second loop, the CV curves almost overlap, suggesting that the sulfur was well-fixed in the N,S-FLG900 matrix and protected from escaping into the electrolyte.30,46 In order to evaluate the superiority of these samples, the N,S-FLG800 and N,S-FLG1000 were also combined with sulfur to construct the cathode for Li–S batteries. The corresponding CV curves of N,S-FLG800/S and N,S-FLG1000/S are present in Fig. S8 (ESI). They show the similar CV profile to N,S-FLG900/S except the distinct current decrease with cycling, implying the more severe polarization of the two samples.
image file: c8qi00160j-f5.tif
Fig. 5 (A) CV curves of N,S-FLG900/S measured between 1.7–2.8 V versus Li+/Li at a sweep rate of 0.1 mV s−1; (B) rate capability of the N,S-FLG800/S, N,S-FLG900/S and N,S-FLG1000/S at different current densities (discharge capacity); (C) galvanostatic charge–discharge profiles of N,S-FLG900/S hybrid at different rates from 0.3 to 1.6 A g−1 in the voltage window of 1.7–2.8 V versus Li+/Li; (D) Nyquist plots of the fresh N,S-FLG900/S and after 5 and 20 cycles at 800 mA g−1 from 1 MHz to 100 mHz at room temperature.

The rate performance of the three electrodes is given in Fig. 5B. For N,S-FLG900/S, the discharge capacities are 906, 804.3, 748.5, and 653.1 mA h g−1 at the current densities of 0.3, 0.5, 0.8, and 1.6 A g−1, respectively. When the current density returned to 0.3 A g−1, the capacity can return to 859.7 mA h g−1. However, N,S-FLG800/S and N,S-FLG1000/S offer similar capacity in respective rates, which are much lower than that of the N,S-FLG900/S. Fig. 5C describes the galvanostatic charge–discharge voltage profiles of N,S-FLG900/S at different current densities. At the higher current rate, the voltage gap between discharge and charge plateaus becomes larger, resulting from the polarization. Fig. 5D shows the Nyquist plots of the N,S-FLG900/S electrodes, which were measured before and after different cycles. We can clearly see that each curve consists of a semicircle in the high frequency region and a slope line in the low frequency region associated with the resistance of charge transfer (Rct) and the diffusion of lithium ions, respectively.47,48 The Rct notably decreases with cycle proceeding, demonstrating that the initial activation process favours the redistribution of sulfur within the electrode and enhances the electronic conductivity. The slope of the line changes slightly, suggesting the unchanged ionic diffusion resistance during the cycling, resulting from the viscosity of the electrolyte. Moreover, the intermediate products, i.e., polysulfides, are well restricted within the cathode instead of escaping into the electrolyte. The viscosity of the electrolyte remained constant during the cycling from the unchanged ionic diffusion.

As shown in Fig. 6A, an initial capacity of 1148.5 mA h g−1 was obtained with a high coulombic efficiency of 97% at a current density of 0.8 A g−1. At the initial electrochemical stage, the active sulfur presented mainly on the surface of the carbon matrix as crystals and distributed unevenly. During the activation process of the first several cycles, sulfur and its corresponding discharge products would redistribute throughout by the repeated electrochemical reactions with the lithium ions. Moreover, a small amount of sulfur and long-chain polysulfides were unavoidably escaping into the electrolyte and peeling from the carbon scaffolds, causing the loss of specific capacity in the first several cycles. Subsequently, the Coulombic efficiency was maintained at almost 100% until 300 cycles and a stable reversible capacity of ∼602 mA h g−1 can be maintained at the end. In comparison, the capacity of the N,S-FLG800/S and N,S-FLG1000/S composite decreased from an initial value of 1046 to 480 mA h g−1 and 1087 to 495 mA h g−1, respectively. The long-term cycling further shows the effective confinement of polysulfides in the N,S-FLG900/S composite cathode, resulting from the stronger interaction of sulphur and the discharge products with N,S-FLG900. The morphology of N,S-FLG900/S after cycling for 50 cycles was detected by the technique of FESEM (Fig. S9, ESI), giving us an impression of the good retention of the graphene structure. Fig. 6B shows the cyclability of N,S-FLG900/S at a current density of 1.6 A g−1. After the initial activation at 0.5 A g−1 for 10 cycles, a reversible capacity of 784 mA h g−1 was reached upon abruptly increasing to 1.6 A g−1. After 500 cycles, the reversible capacity as high as 506.4 mA h g−1 was retained with a capacity decay rate of only 0.072% with respect to the initial capacity at the current density of 1.6 A g−1. Fig. 6C and D display the charge–discharge voltage profiles of N,S-FLG900/S at a current density of 0.8 and 1.6 A g−1, respectively. At each current density, the profiles at different cycles can overlap significantly, which is a token of the excellent reversibility of the electrochemical reactions. Due to the higher current density, the polarization becomes more severe, demonstrated by the larger voltage gap in Fig. 6D than that in Fig. 6C.


image file: c8qi00160j-f6.tif
Fig. 6 (A) Cycle performance of N,S-FLG800/S, N,S-FLG900/S and N,S-FLG1000/S composite at a current density of 0.8 A g−1. (B) Cycling performance of N,S-FLG900/S at a current density of 1.6 A g−1. (C,D) Galvanostatic charge–discharge voltage profiles of N,S-FLG900/S at a current density of 0.8 and 1.6 A g−1, respectively.

The above analysis results verified that the N,S-FLG900/S offered superior rate capability and cycling stability than N,S-FLG800/S and N,S-FLG1000/S, which is attributed to its advantageous structural features. As established, N and S doping into carbon lattice can effectively improve the electronic conductivity and furnish much more active sites for immobilizing polysulfides, which is an effective route to improve the activity of carbon host in cathodes of Li–S batteries. N,S-FLG900 possessed higher surface area and pore volume, advantageously rendering the uniform dispersion of active sulfur within the matrix. During the charge/discharge processes, the electrolyte can readily infiltrate and lithium ions can be transported with smaller resistance. Although the surface area of N,S-FLG1000 is higher than N,S-FLG900, the doping content is lower, unfavorably sequestrating the soluble polysulfides. As a result, the synergy of heteroatom dopings, surface area and hierarchical pores of graphene contributes to the improvement of Li–S battery performance.

4. Conclusion

In summary, we prepared a two-dimensional nitrogen/sulfur dual-doped carbonaceous material with g-C3N4 as the template by a stepwise method. Nitrogen and sulfur atoms were doped in situ in a carbon matrix. Through controlling the calcination temperature, the as-obtained N,S-FLG can be tuned in terms of surface profile and doping levels. From the comparative results, the larger specific surface area, the suitable pore distribution and the higher doping level synergistically make N,S-FLG900 serve as an excellent supporter of sulfur for Li–S batteries. Abundant pores can offer space of volume expansion in the process of charging and discharging, benefiting the structure stability of the entire electrode. A doped carbon matrix supplies ample sites to chemically confine polysulfides, improving the sulfur utilization. N,S-FLG900/S can deliver a reversible capacity of ∼506.4 mA h g−1 at the current density of 1.6 A g−1 after 500 cycles. The two-dimensional N,S-FLG with a favorable configuration possibly exhibits potential for applications of other energy storage devices including secondary batteries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial supports provided by the National Natural Science Fund of China (No. 21601108), Young Scholars Program of Shandong University (2017WLJH15), the Fundamental Research Funds of Shandong University (No. 2016JC033, 2016GN010), and the Taishan Scholar Project of Shandong Province (No. ts201511004).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qi00160j

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