Carbon nitride based mesoporous materials as cathode matrix for high performance lithium–sulfur batteries

Abstract

A facile silica template nanocasting method was adopted to effectively synthesize mesoporous carbon nitride (MCN) based materials as cathode matrixes for advanced lithium/sulfur (Li/S) batteries. The materials were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. The MCN/S composite cathode displays excellent electrochemical performance with initial discharge capacities of 1284.5 and 1107.1 mA h g⁻¹ for 66.7 wt% active material at 0.1 and 0.5C, respectively. And it presents good rate performance and stability with remaining discharge capacity of 828.4 mA h g⁻¹ after 100 cycles at 0.5C. The excellent performances are mainly attributed to the crosslink mesoporous structure and the strong chemical interaction between sulfur and carbon–nitrogen framework, which not only can provide polysulfides reservoirs and transport channels for the transportation of ions and electrons within MCN/S, but also is beneficial to restrain polysulfides migration.

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1. Introduction

With the continually increasing requirements of high-performance portable electronics, electric vehicles and large-scale energy-storage systems, advanced rechargeable batteries with high power, high energy density, and long-term lifespan are attracting more attention.^1–3 Among various types of rechargeable batteries, the lithium sulfur (Li/S) battery is one of the most promising candidates to meet these needs, which could be mainly attributed to the extremely energy density of 2500 W h kg⁻¹ and high theoretical capacity of 1675 mA h g⁻¹ from S to Li₂S higher than the commercial secondary lithium batteries.^4–7 Furthermore, sulfur has other advantages such as low cost, non-toxic characteristic, abundance of raw material and wide operating temperature range. Therefore, Li/S batteries show the great potential for the next generation rechargeable batteries. Despite these significant advantages in Li/S batteries, there are still tremendous obstacles impeding the commercialization of Li/S batteries for recent decades. On the one hand, insulating and insoluble sulfur and discharge products (Li₂S/Li₂S₂) are highly resistive in both electron and ion transportation, leading to low utilization of sulfur and reducing the merit of energy density, thereby the excessive amount of conductive additive are needed to enhance the conductivity of sulfur cathode for Li/S batteries.^8–10 On the other hand, the high solubility of lithium polysulfides in the liquid electrolytes can cause the shuttle effect during charge/discharge process, which not only results in low coulombic efficiency but also causes the serious lithium anode corrosion and irreversible loss of active material.^11–13 Those undesirable factors aggravating capacity fading are reflected during extended cycles.

In recent years, various approaches have been proposed to settle these issues mentioned above, which mainly concentrated upon the combination of a conductive materials as cathode matrix with sulfur to form a highly conductive nanocomposite. Various carbon matrixes, for example, mesoporous carbons,^14,15 graphene,¹⁶ carbon tube¹⁷ and carbon nanofiber,^2,11 have been employed as conductive matrixes to provide good electron transport and uniform dispersion of sulfur within the carbon networks. In addition, the carbon hosts effectively suppress the shuttle effect and hinder volume variation of the active material during extended cycling. To further improve the discharge capacity, attain prolonged cycle life and suppress the shuttled polysulfides, doping in carbon materials has been used to modify the properties of carbon matrixes and obtained advanced carbon–sulfur composite. In general, doping of a heteroatom, e.g. nitrogen¹⁸ or boron,¹⁹ alters the chemical bond and generates an activation region for suppressing the shuttle phenomenon and promoting cations migration. It was also reported that nitrogen doping in CNTs made tunnels in the tube wall for Li ion transportation.¹⁷ Additionally, the interactions between Li ions and the nitrite are beneficial to enhancing Li storage capability of the material and attaining high polysulfide retention.²⁰

Due to the incorporation of nitrogen atoms in the carbon framework can improve the mechanical, field-emission, conductivity and energy-storage properties, carbon nitride (CN) has attracted more and more attention.^21–26 In this work, we adopt a facile silica template nanocasting method to effectively synthesize mesoporous carbon nitride (MCN) based materials as cathode matrixes for to further effectively anchor sulfur, retard polysulfides shuttling and improve the electrochemical performance of cathode in Li/S batteries. MCN not only can provide a conductive framework for electron transport and lithium ions, but also an ideal network-like structure that forms a stable structure for trapping polysulfides during the charge–discharge process. And nitrogen inside the carbon framework can produce more active sites and increases the carbon framework activity, therefore, effectively induce chemical adsorption of sulfur on the conductive carbon framework. Thus, the special structure and nitrogen–carbon framework play an important role for obtaining excellent electrochemical performance of Li/S batteries.

2. Experimental

2.1 Synthesis of MCN and MCN/S composite

The synthesis of MCN involves evaporation-induced co-assembly of bi-component precursor and template to form mesoporous nanostructure composite, followed by pyrolysis and removal of the template. In Scheme 1, we projected the effective procedure to MCN. A typical synthesis of MCN, 1.0 g SBA-15 (pore diameter: 7–9 nm, BET: 550–600 m² g⁻¹, Nanjing XFNANO Materials Technology Co., Ltd.) was added to a mixture of 2.03 g ethylenediamine (Aladdin reagent, AR) and 4.5 g carbon tetrachloride (Aladdin reagent, AR). The obtained mixture was refluxed and stirred at 90 °C for 6 h. Then, the obtained solid sample was dried at 100 °C for 12 h, then ground into fine powder. The powder was heat treated with a heating rate of 3.0 °C min⁻¹ and kept at 600 °C in a nitrogen atmosphere for 5 h. MCN were obtained after removal by the silica templates in 10 wt% HF (Aladdin reagent, AR) (aq), filtration, washing several times with ethanol and drying at 100 °C.

Scheme 1 Schematic illustration of the synthesis process for the MCN and MCN/S composite.

The MCN/S composite was prepared by a simple melt-diffusion strategy as shown in Scheme 1. The sublimed sulfur (Aladdin reagent) was mixed with as-prepared MCN at a mass ratio of 2 : 1. Then, the mixture was thermally treated under N₂ at 155 °C for 24 h, and no significant mass change was observed for before the mixture and after thermal treatment.

2.2 Structure characterization

The crystallographic information of the samples was obtained from powder X-ray diffraction (XRD, Rigaku D/max-IIB) using Cu-Kα radiation. Field emission scanning electron microscopy (SEM, JEOL-JSM5610) and transmission electron microscopy (TEM, JEOL-JEM2010F) were applied to characterize the morphology of samples. The preparation of the samples for SEM and TEM analysis involved sonication in ethanol for 10 min and deposition on a copper grid.

Nitrogen adsorption and desorption isotherms measurements were used to estimate the specific surface area and pore size of MCN. Before measurement, the samples were degassed at 150 °C on a vacuum line following a standard protocol. The special surface area was calculated by means of the Brunauer–Emmett–Teller (BET) equation. The total pore volumes were calculated from the amount adsorbed at a relative pressure (P/P₀) of 0.99. The pore size distribution (PSD) was derived from the adsorption branch of the isotherms using the Barrett–Joyner–Halenda (BJH) model.

X-ray photoelectron spectroscopy (XPS) measurements equipped with a high resolution ESCALAB250Xi analyzer were carried out in an ultrahigh vacuum environment. Survey and multi-region spectra were recorded at C 1s and N 1s and S 2p photoelectron peaks. Each spectral region of photoelectron interest was scanned several times to obtain a good signal-to-noise ratio.

2.3 Cell assemble and characterization

The MCN/S composite was used for the cathodes of Li/S batteries. The cathode consists of MCN composite powder, conductive carbon black and polyvinylidene fluoride binder (PVDF) (Aladdin reagent) with a mass ratio of 8 : 1 : 1. The N-methyl-pyrrolidone (Aladdin reagent) was slowly dropped into the material to prepare the slurry of the mixtures. Then, the slurry of mixtures was well distributed on the thin aluminum current collector, and dried under vacuum at 60 °C for 6 h.

All of the coin cells were assembled in an argon-filled glove box and used for testing electrochemical performance. In these cells, lithium foil and Celgard 2400 polypropylene (PP) film were used as anode and separator, respectively. The electrolyte solution was 1 M lithium bis(trifluoromethanesulfonyl)imide (Aladdin reagent, AR), bis(trifluoromethane) sulfonamide lithium salt (Sigma Aldrich) and 0.1 M lithium nitrate (Aladdin reagent) dissolved in the mixture of 1,2-dimethoxyethane (Aladdin reagent) and 1,3-dioxolane (Aladdin reagent) in a volume ratio of 1 : 1. The ratio of electrolytic solution to sulfur is around 0.025 L g⁻¹.

The cyclic voltammetry (CV) was performed on by CHI 660B electrochemical workstation at a scan rate of 0.1 mV s⁻¹ from 1.5 to 3.0 V versus Li⁺/Li. The galvanostatic discharge–charge test was conducted using a Land CT2001A testing system (Wuhan Jinnuo Electronics Co., Ltd. China). And the rest period between half-cycles in the galvanostatic cycling experiments is 5 min. These cells were cycled at different current rates in the voltage of 1.8–3.0 V. Capacity values were calculated on the basis of sulfur mass. Electrochemical impedance spectra (EIS) of cells were measured by using CHI 660B electrochemical workstation over a frequency range of 100 mHz to 1 MHz at an amplitude of 1 mV.

3. Results and discussion

Scheme 1 presents the schematic illustration of synthesis procedure of the MCN and MCN/S composite. The mixture of ethylenediamine and carbon tetrachloride was firstly mixed in SBA-15 to obtain a homogeneous solution under ultrasonic treatment and stirring. Due to the capillarity and electrostatic interaction between the template and the mixture, the mixture can be impregnated to the channel of SBA-15. Then the polymer/silica composite was obtained by polymerization at low temperature. Then, the polymer/silica composite was pyrolyzed under inert gas atmosphere. Finally, the silica template was removed by HF, and then the MCN was obtained.

The porosity of MCN and MCN/S were measured by nitrogen adsorption/desorption isothermal analysis and corresponding pore size distribution (PSD) curve. As shown in Fig. 1a, the nitrogen adsorption/desorption isotherms of the MCN sample are of a typical type IV according to the International Union of Pure and Applied classification, which are typical classifications for structures containing enough mesopores.²⁷ It exhibits a H1-hysteresis loop with the capillary condensation at a relative pressure (P/P₀ = 0.40–0.90), which indicates the presence of abundant uniform mesoporous structures in MCN. Fig. 1b shows the PSD of MCN and MCN/S. The PSD of MCN shows a narrow pore size distribution centered at 4.05 nm that mainly comes from the mesopores formed after dissolution of the silica matrix from the template, which is expected to favor the confinement of sulfur and polysulfides. The surface areas and total pore volumes of samples were estimated using the BET and BJH methods, respectively. MCN possesses a surface area of 770.5 m² g⁻¹ and a pore volume of 1.19 cm³ g⁻¹, which is derived from the formation of interconnected mesopores. It is noted that the specific surface area and the specific pore volume of MCN/S are decreases rapidly and almost approaches to zero after loading sulfur into the pores of MCN, indicating that most of pores are occupied by the element sulfur. According to the obtained results, the MCN is expected to provide enough space for accommodating the active materials and promoting a good contact with electrolyte and active substances, which may have the ability to achieve excellent electrochemical performance for Li/S batteries.

Fig. 1 (a) N₂ adsorption/desorption isotherms and (b) pore size distribution of MCN and MCN/S.

The inset of Fig. 2 shows low-angle diffraction pattern (0–10°) of MCN. The inset of Fig. 2 exhibits two clear peaks that can be indexed to the [100] and [110] reflections of the highly ordered two dimensional hexagonal nanostructure with the space group of p6mm,²⁸ which derives from the two dimensional hexagonal channels structure of SBA-15. Fig. 2 presents the powder XRD diffraction patterns of the S, MCN and MCN/S composite. The diffraction peaks of MCN sample exhibits a single broad diffraction peak near 24.70°, the diffraction peaks of MCN sample exhibits a single broad diffraction peak near 24.70°, corresponding to the characteristic diffraction peak in carbon nitride.²⁹ This indicates the carbon and nitrogen atoms in the graphene layers of MCN. The special structure of MCN is good for sulfur uniformly dispersed inside the MCN and intimate contact with the conductive carbon walls. The sharp diffraction peaks patterns of the S (Fig. 2) present two prominent peaks at 2θ = 21° and 28°, corresponding to elemental sulfur (JCPDS: 08-0247) with an F_ddd orthorhombic structure, suggested that sulfur existed in typical orthorhombic sulfur crystalline diffraction peaks. And the diffraction peaks patterns of the MCN/S composite display that the sharp diffraction peaks of bulk crystalline sulfur disappeared entirely, indicating that the MCN framework can effectively suppress the crystal growth of sulfur particles and form a fine dispersed state sulfur inside MCN. The MCN/S composite with highly dispersed sulfur is mainly attributed to the special pore structure and the absorption capacity of conductive walls in MCN. Furthermore, the high dispersed sulfur impregnated into the nanopores of MCN can take intimate contact with conductive framework and give rise to a pathway for transport of both lithium ion and electron.

Fig. 2 XRD patterns of S, MCN, and MCN/S composite.

SEM and TEM were used to investigate the morphologies of MCN. Fig. 3a displays that the crosslink structure of MCN is comprised of ordered mesoporous carbon nitride rods, which were formed inside the tunnels of the SBA-15 template. These carbon nitride rods are rigidly interconnected by small carbon nitride segments, which are formed inside the tunnels of the SBA-15. As shown in Fig. 3b, the morphology of MCN/S composite (Fig. 3b) is preserved as the original shape of MCN without visible agglomerations of bulk sulfur on the surface of MCN, indicating that sulfur was highly dispersed in the pore of MCN by heating treatment. The TEM image (Fig. 3c) viewed down the pore axis display an ordered structure with uniform mesoporous channels, which further confirmed the results of XRD in above. It can be found the pattern of the outside and inner channels of the mesoporous carbon presents ambiguity, due to the sublimed sulfur penetrated into the channels of the MCN, which confirmed that sulfur is successively loaded in the channels of the MCN. The special structure of MCN not only can provide cross-linked framework to improve electronic conduction capacity, but also trap higher-order soluble polysulfides transferring out the MCN matrix. Fig. 3d–g exhibit the element analysis mapping and distribution of C, N and S in MCN/S sample. Fig. 3f presents the distribution of N and proves the presence of N in the MCN/S. In Fig. 3g, the sulfur mapping clearly demonstrates that sulfur is distributed homogeneously within the MCN/S composite. It is apparent that the obtained MCN with ordered channel structure are good host matrixes for encapsulating sulfur. Thus the MCN/S composite cathodes are expected to suppress the dissolution of S into electrolyte and maintain excellent cycling stability.

Fig. 3 SEM images of MCN (a) and MCN/S composite (b) TEM images of MCN/S (c). TEM images MCN/S (d) and the corresponding EDS element mapping of C (e), N (f) and S (g).

The MCN and MCN/S composite materials were further analyzed by X-ray photoelectron spectroscopy (XPS) techniques to confirm the strong chemical interaction between S and CN matrix. Fig. 4a shows XPS survey spectra of the MCN and MCN/S composites. In the XPS survey spectra of MCN, three peaks centering at 285.0, 400.0 and 512.0 eV, corresponding to C 1s, N 1s and O 1s, respectively, can be easily observed.¹⁸ For comparison, the MCN annealed under the same conditions as those of the MCN/S composite was also measured, and no S 2p signal was detected. In contrast, a strong S 2p signal was observed for the MCN/S composite. In addition, the presence of oxygen in the sample may be due to the moisture or CO₂ adsorbed on the surface of MCN. Fig. 4b displays the XPS C 1s spectra of MCN sample. The fitted curve of C 1s spectra could be deconvoluted into three peaks with binding energies of 288.7, 285.6, and 284.1 eV, which were very close to the reported binding energy values of nonporous carbon nitride samples.³⁰ The lowest energy contribution fitted for C 1s in MCN corresponds to pure graphitic sites in the amorphous CN matrix and the peak at 285.6 eV is attributed to the sp² carbon atoms bonded to N inside the aromatic ring. The highest energy contribution of 288.7 eV is assigned to the sp³-hybridized carbon in aromatic ring attached to NH₂ groups.^25,31,32 The XPS nitrogen peaks in MCN are fitted into two peaks centered at 397.9 and 400.1 eV in Fig. 4c. The peak at highest binding energy at 400.1 eV corresponds to nitrogen sp²-bonded to carbon in the C–N network, while the peak at 397.9 eV is attributed to N atom trigonally bonded to all sp² carbon. In the S 2p spectra (Fig. 4d), the two fitted peaks located at 164.8 and 161.6 eV are assigned to S 2p_1/2 and 2p_3/2 due to spin orbit coupling for sulfur atoms. Furthermore, the presence of the broad and strong binding energy peak position 168.7 eV is direct evidence of the chemical interaction between sulfur and CN matrix. Moreover, the percentage of N and C in the MCN measured by the XPS is found to be 15.1% and 76.8%, respectively.

Fig. 4 (a) Survey spectrum of the MCN and MCN/S samples, (b) C 1s spectrum of the MCN sample, (c) N 1s spectrum of the MCN sample and (d) S 2p spectrum of the MCN/S sample.

The electrochemical performances of the coin cell assembled with the MCN/S cathode and a metal lithium foil anode were evaluated by cyclic voltammetry (CV), galvanostatic charge/discharge testing, and electrochemical impedance spectroscopy (EIS). In Fig. 5a, the typical CV curves of MCN/S composite cathode with 66.7 wt% S are shown in the potential window of 1.0–3.0 V at a scan rate of 0.1 mV s⁻¹ and it presents well-defined characteristic of the oxidation and reduction of sulfur in the CV curves. In the discharge plots, two apparent reduction peaks at around 2.08 and 2.28 V are assigned to the multistep reaction mechanisms of sulfur with lithium. The weak reduction peak at higher potential position can be assigned to the reduction of orthorhombic sulfur to high order lithium polysulfides (Li₂S_n, 4 ≤ n ≤ 8). The strong cathodic peak corresponding to lower potential position can be ascribed to a strong reduction of soluble long-chain polysulfide anions to solid-state Li₂S₂/Li₂S. In the following anodic scan, there is only one sharp oxidation peak at about 2.42 V, which is related to the conversion of Li₂S or Li₂S₂ to long-chain polysulfides and sulfur in the subsequent scan cycles, except for the feeble intensity difference of these peaks due to the impotent polarization of the electrode materials in the charge processes, the oxidation and reduction peak potentials of active materials have no obvious changes. The CV results indicate that the MCN/S composite can help to effectively prevent sulfur from dissolving into the electrolyte and suppress the polysulfide shuttle during charge/discharge processes, and then to acquire good reactive reversibility and cycling stability of the MCN/S cathode.

Fig. 5 (a) Cyclic voltammogram of the cell with MCN/S composite.

Fig. 6a presents galvanostatic charge/discharge cycling of the MCN/S composite cathode for Li/S batteries, which is measured in the potentials interval from 1.8 V to 3.0 V at 0.1, 0.2 and 0.5C (1C = 1675 mA g⁻¹), respectively. The charge/discharge capacities are calculated on the basis of sulfur mass. Two typical plateaus at around 2.1 and 2.3 V for the MCN/S composite electrode with different rate are observed and can be defined as the multistep reaction of MCN/S composite in the discharge process. The one plateau observed in the charge process at about 2.4 V corresponds to the oxidation process resulting from conversion of Li₂S and polysulfides to element S. The plateaus platform corresponding to the voltage match the peaks of the MCN/S composite electrode in the CV curves. The MCN/S composite delivers high initial discharge capacities of 1284.5, 1155.8 and 1107.1 mA h g⁻¹ at 0.1, 0.2 and 0.5C, respectively. Fig. 6b shows the cycling performance and coulombic efficiency of the MCN/S composite cathode upon extended cycling at 0.5C. The MCN/S composite electrode delivers the initial discharge capacity of 1107.1 mA h g⁻¹ and coulombic efficiency of 93.8%. The MCN/S presents a good cycling stability and obtains a reversible capacity of 828.4 mA h g⁻¹ at the 100th cycle. The average coulombic efficiency reaches as high as 93.4% for 100 cycles. The excellent cycle performance and coulombic efficiency suggest that the MCN/S composite can provide reservoir to the effective utilization of sulfur and anchor polysulfide anions diffusing into the electrolyte.

Fig. 6 (a) The initial discharge/charge curves of the cell with MCN/S composite at 0.1, 0.2 and 0.5C. (b) Cycling performance and coulombic efficiency of the cell with MCN/S composite at 0.5C. (c) The charge/discharge profiles of Li–S cell with MCN/S composite cathode at 1C and 2C rate, respectively. (d) The charge/discharge profiles of Li–S cell with MCN/S composite cathode at 2C rate.

Fig. 6c and d show the charge–discharge curves of the MCN/S composite electrode at 1C and 2C and the detailed behavior for 1st, 5th, 20th, 50th and 100th cycle. By comparing the charge/discharge plateaus at 1C and 2C, the charge plateau at 2C is more than the charge plateau at 1C, and the discharge plateaus at 2C are lower than the discharge plateaus at 1C. This might be attributed to the excess precipitation of insulating lithium sulfides, which becomes electrochemically inaccessible on the electrode at a higher current density. The initial specific discharge capacity of the MCN/S composite at 1C is of 956.5 mA h g⁻¹ with a coulombic efficiency of 95.8%. The capacity retention at 1C reaches 98.4% and 95.2% for the 5th and 20th cycle, respectively, and then remains the high value of 796.6 mA h g⁻¹ after 100 cycles. The charge–discharge profile reveals a specific discharge capacity of 692.8 mA h g⁻¹ in the first cycle and still remains 571.0 mA h g⁻¹ after 100 cycles, even at a higher rate 2C. Capacity fading from the 1st to 100th cycle is gradual and smooth without any abrupt change with an average capacity fading of 1.22 mA h g⁻¹ per cycle, illustrating stable cycling performance. These results clearly demonstrate that MCN can be a promising candidate as the matrix material for Li/S batteries.

Fig. 7a displays the rate capability of MCN/S composite electrode for Li/S battery from 0.2 to 2C. After the initial discharge capacity of 1157.8 mA h g⁻¹ at 0.2C, a sharp capacity decrease is found in the first few cycles at 0.2C and then remained to stabilize at the capacity of around 1100 mA h g⁻¹. Following the charge/discharge rate to 0.5 and 1C, a reversible capacity of 924.4 mA h g⁻¹ and 792.5 mA h g⁻¹ is reached, respectively. Even at higher rate of 2C, the capacity of 632.8 mA h g⁻¹ can be delivered. When the charge/discharge rates are abruptly switched from 2 to 0.2C again, the initial capacity is largely recovered, indicating excellent stability of the cathode material.

Fig. 7 (a) Rate performance of the MCN/S composite cathode. (b) EIS spectra of the cell with MCN/S composite after cycling.

Fig. 7b shows EIS of MCN/S composite electrode after the 1st, 50th and 100th, and the fitting results are given in Table 1. Impedance plots of three electrodes display a semicircle at high frequency and a straight line at low frequency. The R_e refers to the resistance of electrolyte. The semicircle corresponds to the Warburg diffusion process, R_ct refers to the charge-transfer resistance, W_o refers to the Warburg impedance, and CPE refers to the constant phase element.^33,34 The results in Table 1 show that the total impedance is largely increased with the discharge–charge cycles. The electrolyte resistance (R_e) slightly increases, which is directly related to the concentration of dissolved polysulfides in the electrolyte. The increase of R_ct might be due to the accumulative depositions of insulating solid products such as Li₂S₂ or Li₂S on the surface of lithium metal during extended cycles. It is notable that the increase of the resistances slows down from 50th to 100th cycle, which is in consistence with the cycle performance. Thus the obtained results indicate that MCN/S composite cathode for Li/S could maintain a stable electrochemical environment in extended cycles.

Table 1 Fitting results of EIS plots in Fig. 7

	Re/Ω	Rct/Ω
1st	4.7	78.4
50th	7.3	117.5
100th	8.6	138.7

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The obtained outstanding performance of the MCN/S composite cathode for Li/S batteries could be contributed to the combination of following aspects. Firstly, the ordered mesoporous structure of MCN provides electronic conductive channels to ensure high sulfur utilization. Secondly, MCN with a high surface area and high pore volume has ability to accommodate the high sulfur loading and sulfur exists in a highly dispersed amorphous state improving the conductivity and electrochemical activity. Thirdly, the tunnels of the MCN provide polysulfides reservoirs and hinder the polysulfides transportation in the electrolyte, enhancing the cycling stability of the MCN/S composite cathode. Finally, the synergetic action of the ordered mesoporous structure with CN framework and the strong chemical interaction between S and C–N framework effectively suppress the shuttle effect.

4. Conclusions

In summary, we have successfully synthesized MCN/S composite for effectively confining sulfur. MCN is connected by carbon nitride rods “links” to form an ordered mesoporous structure for suppressing the diffusion of dissolved polysulfides and possesses a high surface area and pore volume for encapsulating a substantial amount of sulfur and MCN/S as the cathode material in Li–S batteries delivers the initial specific capacity of as high as 1284.5 mA h g⁻¹ at 0.1C. In addition, MCN/S composite cathode at a higher current rate 0.5C and 1C presents a initial specific capacity of 1107.1 and 956.5 mA h g⁻¹, respectively, and then remains at 828.4 and 796.6 mA h g⁻¹ after 100 cycles. As a result, the MCN/S composite reveals high specific capacity, and excellent cycling stability and rate capability.

Acknowledgements

This work was financially supported by the Science and Technology Project of Anhui Province (1301022077) and Provincial Natural Science Research Project of Anhui Colleges (KJ2015A224).

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