English

• 0.   Introduction

  Since the commercialization in the 1990s, the application of lithium-ion batteries (LIBs) caused great changes in relation to the realm of energy storage and conversion^[1]. With the rapid development of electric vehicles, and the continuously increasing public demands, the energy density of current LIBs is approaching the limit. Seeking batteries with higher energy density and power density, as well as prolonged lifespans is extremely important. Compared with LIBs, lithium-sulfur (Li-S) batteries with the configuration of metallic Li anode and sulfur cathode have a high theoretical capacity (1 675 mAh·g^-1) and energy density (2 567 Wh·kg^-1) and are widely regarded as one of the most promising electrochemical energy storage devices^[2-3]. For Li-S battery systems, it is generally accepted that the reversible reactions between S₈ molecules and the metallic Li anode enable the steady charge/ discharge process. However, the product of lithium polysulfides (LiPSs) derived from the discharge shows superior solubility in common organic electrolytes, the formed soluble long-chain LiPS species will shuttle from the cathode to the anode, and the further reduced short-chain LiPSs (Li₂S₂ and Li₂S) will be deposited^[4-5]. The so-called"shuttle effect"is adverse to the fixation of sulfur and capacity retention. Additionally, the inherent insulative of sulfur and sulfides, as well as the non-negligible volume expansion (ca. 78%) during the charging-discharging process, severely hinder the realworld application^[6].

  To address the aforementioned issues, one of the most effective strategies is to develop advanced sulfur host materials that simultaneous acting as sulfur carriers and promoters for the accelerated electrocatalytic conversion of LiPSs^[7-13]. For an applicable host material, the following material design principles should be included: First, the host material should possess excellent electronic conductivity, which can enable the rapid migration of electrons, thereby improving the rate capabilities; Second, the superior physical confinement, the chemical affinity, as well as the effective catalytic conversion of LiPSs are combined in a competent host material for sulfur fixation; Third, the large specific surface area or abundant inner cavities of the host material can accommodate more elemental sulfur and alleviate the volume expansion, as well as enable the exposure of more catalytically active sites to boosting the overall performances of Li-S batteries.

  Pure carbon as a sulfur host can limit the dissolution of LiPSs by simple physical adsorption. However, the electroneutrality of pure carbon materials is generally non-polar, and intermolecular interactions with polar intermediates during charge and discharge are weak. The good cycling and rate performance of Li-S cells are largely dependent on good contact between carbon and sulfur and the excellent wettability of the electrolyte. In addition, the abundant polysulfide traps of sulfur at the interface mitigate the dissolution of polysulfides during the electrochemical process, avoiding shuttle effects, and improving the utilization of polysulfides. The number of active sites will be determined by the surface properties of the carbon materials at the interface, which has an impact on polysulfide adsorption. Therefore, various strategies have been developed to optimize interfaces with fast electron/ion transfer and sufficient active sites. For example, functional groups can improve the polarity of non-polar carbon-based materials, which can help to improve the interfacial immersion of carbon/sulfur electrodes in the electrolyte and enable close contact through the formation of bonds between carbon and sulfur (i.e., C—S and S—O). In addition, polar functional groups can provide chemically active sites for polysulfides through strong polar interactions and effectively limit the shuttling of polysulfides from the anode to the cathode. Therefore, the rational design of optimized interfaces in nanostructured carbon/sulfur materials would provide a new way to improve the performance of Li-S batteries. Thus, heteroatom-doped (e.g., oxygen (O), nitrogen (N), and boron (B)) carbon materials are chemically fabricated or derived from the residues of carbonaceous precursors^[14-18]. For instance, nitrogen-doped carbon materials are used to improve the polarity and conductivity of the carbon materials interface in Li-S cells through their different chemical environments with pyridine N, graphite N, and pyrrole N^[19-21]. Compared to pure carbon materials, the carbon with N-functional groups exhibited a high discharge capacity of 1 100 mAh·g^-1 (0.35 mA·cm-2) and relative cycling stability at a multiplicity of 0.70 mA·cm^{-2 [22]}. It is believed that the electroactive N-functional groups in the carbon composites can effectively trap LiPSs by forming chemical bonds with lithium ions during the electrochemical process.

  In this work, we design and synthesize two-dimensional (2D) metal-free, N and S co-doped porous carbon (NSPC) nanosheets. The petrochemical-derived pitch, melamine, and porous MgO were utilized as the carbon skeleton, the nitrogen source, and the hard template, respectively. An N, S co-doped 2D carbon nano-porous sheet with high specific surface area and micropores can be obtained through a simple calcination treatment. The formed unique structure can accommodate more elemental sulfur and relieve the volume expansion during the charge and discharge process. Concurrently, the micropores that existed on the surface of the carbon nanosheets can improve the wettability of the electrolyte and improve the cycle performance and the introduction of N and S atoms are beneficial for the fixation of LiPSs. Compared with traditional carbon-sulfur composite materials, 2D NSPC nanosheets showed better electrochemical performance and cycle stability when used as lithium-sulfur battery cathodes.

  1.   Experimental

  1.1   Synthesis of porous MgO template

  Typically, purchased MgO powder was mixed with deionized water with ultrasonic agitation. The formed mixture was boiled for 24 h in a reflux apparatus. The obtained material was calcined at 500 ℃ for 30 min after filtration and drying to afford porous MgO.

  1.2   Synthesis of 2D NSPC

  In a typical synthesis, the pitch powder, porous MgO template, and melamine were uniformly mixed in ethanol in the proportion of 1:1:1 in weight. After continuous stirring and drying in an oven at 80 ℃, a grey powder mixture was obtained. The powder was then crushed and calcined in a quartz tube under 700 ℃ for 30 min with argon protection. The obtained black powder was treated with a hydrochloric acid solution (1 mol·L^-1). After washing and drying, the hierarchical porous carbon material doped with N and S named NSPC was finally obtained. For comparison, S-doped porous carbon (SPC) was prepared by treating the mixture of MgO and pitch (1:1 in weight) as above.

  1.3   Synthesis of metal-free NSPC/S

  Typically, 60 mg of NSPC and 240 mg of sulfur were ground in the agate mortar for 30 min. The mixture was sealed in a pasteurized glass tube and heated at 155 ℃ for 10 h in an oven. The obtained product was marked as NSPC/S. For comparison, SPC/S with the same weight ratio of sulfur was also prepared by the same method using SPC.

  1.4   Polysulfide adsorption test

  Li₂S₆ solution was prepared by mixing sublimed sulfur and Li₂S with a molar ratio of 5:1 in 1, 2-dimethoxyethane (DME) and 1, 3-dioxolane (DOL) (1:1 in volume) solution. The stirring was then done at 60 ℃ for 12 h until the powders were completely dissolved and the solution turned yellow. A Li₂S₆ solution with a concentration of 3 mmol·L^-1 was used. 20 mg of NSPC and SPC were added to 4 mL of the Li₂S₆ solution, separately. Then, the mixture was vigorously stirred to obtain thorough adsorption, and a glass vial filled with the bare Li₂S₆ solution served as the blank sample.

  1.5   Characterizations

  Powder X-ray diffraction (PXRD) was performed on an X′Pert3 Powder instrument (PANalytical, Netherland) using a Cu Kα radiation (λ=0.154 16 nm) at an operating voltage of 40 kV and current of 40 mA with scanning range of 5°-90°. Scanning electron microscopy (SEM) was conducted on a Nova NanoSEM 450 field-emission scanning electron microscope at acceleration voltages of 15 kV. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) was carried out using a JEM-1400-plus instrument (Japan) at acceleration voltages of 200 kV. Atomic force microscopy (AFM) was conducted using a SPI3800/SPA400 instrument (Seiko Inc., Japan). Raman spectroscopy (InVia-Reflex, Renishaw) was performed with a laser source of 633 nm wavelength. X-ray photo-electron spectroscopy (XPS) data were collected with an ESCALAB 250Xi (Thermo Fisher Scientific). Nitrogen adsorption-desorption tests were carried out at 77 K using Micrometrics ASAP 2020 analyzer after vacuum degassing of the samples at 120 ℃ for 8 h. The thermogravimetric analysis (TGA) was conducted on a STA499f5 analyzer at a heating rate of 10 ℃ ·min^-1. The ultraviolet-visible (UV-Vis) spectra were obtained on a Hitachi U-3900H UV-Vis spectrophotometer.

  1.6   Electrochemical measurements

  Coin cells (CR2025) were employed to evaluate the electrochemical performance of the obtained NSPC/ S product. The assembly process was performed in an Ar gas-filled glove box with both H2O and O2 lower than 10-7. The active material, conductive graphite, and polyvinylidene fluoride (PVDF) binder was fixed at a mass ratio of 8:1:1. The formed slurry was stirred in deionized water and blade-coated on an Al foil with a coarse surface. After drying at 50 ℃ in a vacuum for 10 h, the coated Al foil was cut into disks 12 mm in diameter. The electrolyte consisted of a solution of 1.0 mol·L^-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1.0% LiNO₃ in a mixture of DME/DOL with a volume ratio of 1:1, and a Celgard 2400 film was used as the separator. The counter electrode was pure Li slices. The charge-discharge tests were performed on a Neware battery testing system (Shenzhen, China) at the constant current mode in the voltage range of 1.7-2.8 V. The specific capacities were calculated based on the total weight of active materials. The loading weight of active material was fixed at ca. 1.0 mg·cm^-2. For comparison, SPC/S electrodes were also prepared in the same way. The electrochemical impedance spectroscopy (EIS) was recorded on an electrochemical workstation (CHI660E, Chenhua, CHN) with a range of 100 kHz to 0.01 Hz over the entire frequency range.

  1.7   Theoretical calculations

  Theoretical calculations based on density functional theory (DFT) were performed in this work by using the Vienna ab initio simulation package (VASP). The projector-augmented-wave (PAW) potentials were adopted while the generalized gradient approximations (GGA) of Perdew-Burke-Ernzerhof (PBE) pseudopotentials were applied for the exchange-correlation functional^[23-24]. The Li₂S_n (n=1, 2, 4, 6, 8) adhesion binding energy (E_b) was calculated using the formula of E_b=E_NSPC/S-E_NSPC-E_Li2Sn, where E_NSPC/S is the total energy of the adsorbed system, E_NSPC is the total energy of the surface, and E_Li2Sn is the total energy of the isolated Li₂S_n polysulfide. Herein, more negative values demonstrate the possibility of adsorbate-surface interactions due to the energy advantage.

  2.   Results and discussion

  As shown in Fig. 1a, NSPC/S was synthesized via a facile two-step method. Firstly, the pitch, melamine, and porous MgO were uniform mixed, with facile annealing and pickling, and NSPC was attained. Fig. S1a and S1b (Supporting information) show the morphology of NSPC, which exhibits the typical hierarchical features. NSPC is composed of closely assembled nanoplatelets. The EDS analysis (Fig. S1c) of NSPC also displays the atomic fractions of doped N and S were 1.06% and 0.68%, respectively. Fig. 1b displays the elemental mapping results of NSPC, which show the presence of C, N, and S elements. Herein, as shown in Fig.S2a, the MgO powders used in this work exhibited porous sheet-like structures, and regular lattice fringes with a d-spacing of 0.152 nm were further confirmed with high-resolution transmission electron microscopy (HRTEM). As shown in Fig.S2b, the insert Fourier transform diffraction explicitly demonstrates the exposure of the (220) lattice plane of MgO^[12]. Secondly, NSPC/S was prepared through a typical meltdiffusion strategy with elemental sulfur. As illustrated, NSPC/S in Fig. S1d maintained the initial morphology of NSPC/S, and no aggregated sulfur particle was observed on the surface. The elemental mapping results (Fig. S1d) show the presence of C and S elements, demonstrating the homogeneous distribution of sulfur in NSPC. The thickness of the ultrathin structures was investigated by AFM measurements, which are presented in Fig. S3. The average thickness of the obtained NSPC was calculated as 1-3 nm, implying that a large lateral-to-thickness ratio is very beneficial for electron mobility.

  Figure 1

  

  Figure 1.  (a) Schematic illustration showing the preparation of NSPC and NSPC/S; (b) SEM image and mappings of NSPC

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  PXRD analysis was employed to investigate the crystalline phase of the prepared samples. As illustrated in Fig. 2a, the PXRD patterns of both NSPC and graphite showed a sharp peak at approximately 25° and another weak peak at approximately 44°, which can be assigned to the typical (002) and (100) planes of carbon, respectively. Moreover, compared with graphite, it should be noted that the peak location of the (002) plane for NSPC was slightly lower, signifying enlarged d-spacing^[25], which is consistent with the TEM results. At the same time, the broadened peak width of NSPC also confirms the very thin structures. The Raman spectrum of NSPC is presented in Fig. 2b, showing the D band at 1 340 cm^-1 (defects) and the G band at 1 580 cm-1 (crystalline graphite)^[26]. The peak intensity ratio of the D to G band (I_D/I_G) was 3.2, implying the presence of abundant defects in NSPC. In addition, the XPS technique was employed to investigate the chemical states and elemental ratios of N and S. The survey XPS spectrum of NSPC is shown in Fig. 2c. Only elements of C, O, N, and S can be detected, and the atomic fractions of N and S were 9.5% and 1.7%, respectively. The C1s XPS spectrum of NSPC is shown in Fig. 2d. The deconvolution can be mainly divided into two peaks: a sharp peak at 284.7 eV (C—C bond) and a broad peak at approximately 286.0 eV (C—O, C—N, etc.) ^[27]. The higher N content compared to the EDS results implies that the doped N is mostly adsorbed on the surface of NSPC. Fig. 2e shows the N1s XPS spectrum of the NSPC nanosheets, and the peaks located at 398.6, 400.0, and 401.0 eV are attributed to pyridinic N, pyrrolic N, and graphitic N, respectively^[28]. The ratio of pyridinic N, pyrrolic N, and graphitic N was estimated to be 1∶0.74∶0.84. Concurrently, graphitic N with three sp² carbon atoms plays an important role in improving the conductivity of graphitic carbon^[29-30]. Besides, the pyridinic N and pyrrolic N promote the catalytic conversion of LiPSs effectively^[16-18]. Therefore, in this work, the contents of different types of N can achieve a good balance while considering the capacity and rate performance of the NSPC electrodes. Fig. 2f exhibits the S2p XPS spectrum of NSPC, and the peaks at 164.0 and 165.1 eV can be assigned to S2p_3/2 and 2p_1/2, indicating that most of the S are integrated into the graphite matrix^[31]. Additionally, the S2p XPS spectrum of NSPC contained a peak at 168.5 eV, which is attributed to oxidized S^[32].

  Figure 2

  

  Figure 2.  (a) XRD patterns of graphite and NSPC; (b) Raman spectra and (c) survey, (d) C1s, (e) N1s, and (f) S2p XPS spectra of NSPC

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  Moreover, the adsorption-desorption isotherms and pore size distributions of NSPC and SPC are shown in Fig. 3a, 3b, and Fig. S4a, S4b, respectively. Fig. 3a and Fig. S4a show that the sorption isotherm can be indexed to type Ⅳ with an H1 hysteresis loop and Brunauer-Emmett-Teller (BET) surface area of 339 and 522 m²·g^-1 for NSPC and SPC, respectively; these results indicate the presence of mesopores in the structure. The H1 hysteresis loop indicates that both NSPC and SPC have clearly narrow pore size distributions, as illustrated in Fig. 3b and Fig.S4b. The pore size distributions of approximately 6.5 and 8 nm for NSPC and SPC were observed using the Barrett-Joyner-Halenda (BJH) model for analysis. The narrow pore size distribution is likely linked to the porous MgO template. Meanwhile, the reduced BET surface area and pore size for NSPC are mainly ascribed to the introduction of melamine. Therefore, the large specific surface area and suitable pore size facilitate the efficient storage of more sulfur.

  Figure 3

  

  Figure 3.  (a) N₂ adsorption-desorption isotherm, and (b) pore size distribution of NSPC

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  In addition, N elemental doping effectively improves the adsorption capacity of the NSPC/S electrode to LiPSs, which can be seen from the polysulfides adsorption test in Fig. 4a and 4b. Besides, the accurate sulfur content in NSPC/S was determined by the TGA result under N₂ atmosphere from room temperature to 700 ℃. As presented in Fig. 4c, owing to a large number of pores and high pore volume of NSPC/S, the sulfur content (mass fraction) in NSPC/S can be as high as ca. 68%. When employed as sulfur cathode for Li-S batteries, NSPC/S with porous nanostructures, highly electrical conductive channels, and strong chemisorption capability with sulfur species can effectively alleviate the issues caused by the low electrical conductivity of sulfur and the severe shuttle effect of polysulfides.

  Figure 4

  

  Figure 4.  (a) Optical photo of the adsorption effect of NSPC and SPC on Li₂S₆; (b) UV-Vis spectra of NSPC and SPC; (c) TGA of pure S, NSPC, and NSPC/S; (d) EIS curves of NSPC and SPC; (e) CV curves of NSPC/S from 3.0 to 1.5 V (vs Li/Li⁺) at a scan rate of 0.2 mV·s^-1; (f) Voltage profiles of NSPC/S electrodes at various current densities; (g) Rate performance of NSPC/S electrodes; (h) Cycling performance of NSPC/S electrodes at high current densities; (i) Schematic diagram of the catalytic conversion of LiPSs

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  To investigate the electrochemical behaviors of the synthesized sample, a CR2025 coin-like half-cell was employed with a pure Li plate as a counter electrode to evaluate the electrochemical performance. For comparisons, the control sample of SPC/S was also examined under the same experimental conditions. The detailed cell assembly process is presented in the Experimental Section. The first three cycles of the cyclic voltammetry (CV) curves of the NSPC/S electrode at a scan rate of 0.2 mV·s^-1 in the voltage range of 1.5 and 3.0 V (vs Li/Li⁺) are shown in Fig. 4e. In the first cathodic scan, the reduction peaks at 2.2 and 1.7 V correspond to Li₂S₆ or Li₂S₄ and insoluble Li₂S₂ and Li₂S products, respectively^[33]. In addition, in the first anode scan, the oxidation peak at 2.5 V should be ascribed to the oxidation of Li₂S to S₈^[34]. In the next two scans, the positions of the oxidation and reduction peaks shifted, indicating that the redox reaction of the battery was improved and the full contact between electrolyte and electrode.

  Fig. 4f presents the last discharge-charge voltage profiles of the NSPC/S electrode at different current densities. As the current increased, the voltage hystere sis (ΔE) of the NSPC/S electrode increased gradually. And the ΔE of NSPC/S (Fig. 4f) was smaller than that of SPC/S at 0.1C (Fig.S5), implying the reaction kinetics rate was improved, which accounts for the better electrochemical performance at high current densities.

  Furthermore, the rate performance of the NSPC/S electrode is shown in Fig. 4g. Notably, the electrode was activated at a low current density for several cycles before testing. The NSPC/S electrode delivered high specific capacities of 1 176, 1 032, 761, 626, 415, and 153 mAh·g^-1 at current densities of 0.1C, 0.2C, 0.6C, 1C, 2C, and 4C, respectively. Importantly, the NSPC/S electrode exhibited good reversibility, with the specific discharge capacity recovering to the initial value while the current density returns to 0.1C. For comparison, the SPC/S electrode without doped N only delivered 1 099, 442, 243, 163, 114, and 63 mAh·g^-1 at the same current densities listed above. Hence, the doped N plays an important role in enhancing the rate performance of NSPC/S electrodes. As shown in Fig. 4d, the NSPC/S electrode exhibited a lower internal resistance than that of SPC/S, indicating that doped graphitic N enables higher conductivity. The illustration in Fig. 4d is the equivalent circuit diagram of the EIS, R₁ stands for solution resistance, R₂ stands for charge transfer resistance, Z_W stands for diffusion resistance, and CPE1 stands for capacitive reactance.

  The long-term cycling stability of NSPC/S and SPC/S electrodes is shown in Fig. 4h. At the little current density of 0.1C (Fig.S6), the NSPC/S cathode also provided 1 185 mAh·g^-1 after 50 cycles. And at the high current density of 2C, the NSPC/S cathode retained a reversible capacity of 463 mAh·g^-1 after 200 cycles (Fig.S7), which is significantly higher than that of SPC/S (157 mAh·g^-1). In addition, the NSPC/S cathode delivered an outstanding cycling performance, which provided 762 mAh·g^-1 at 0.6C after 200 cycles. Moreover, a stable Coulombic efficiency (CE) of >99% was achieved throughout the long-term cycling, confirming the high S utilization, excellent LiPSs confinement, and good reversibility of LiPSs conversion of NSPC/S. Compared with the SPC/S electrode material without N doping, the electrode capacity attenuates faster, and the shuttle effect of LiPSs affects the longterm cycle stability of the battery.

  The catalytic conversion of LiPSs with pyridinic N, pyrrolic N, S¹, and S² (different S substitution positions in the graphene host, as shown in Fig. S8) are shown in Fig. 4i. To shed more light on the functional mechanism, the interactions between the series of NSPC and polysulfides Li₂S_n (2≤n≤8) were investigated by first-principles DFT calculations. For pyridinic N, pyrrolic N, S¹, and S², the relevant calculation results are shown in Fig. 5 and Fig.S8. It was concluded from the calculations that pyridinic N has the highest adsorption energy to Li₂S_n (2≤n≤8). Hence, the electrochemical performance of SPC without nitrogen doping is much lower than NSPC.

  Figure 5

  

  Figure 5.  Adsorbability of pyrrolic N, pyridinic N, and S¹ and S² of Li₂S_n (n=1, 2, 4, 6, and 8)

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  3.   Conclusions

  In summary, we propose a facile synthesis method for the production of N, S co-doped carbon nanomaterial, which features 2D nanostructures. After sulfur fixation, it still retains the previous morphologies. The doped N can effectively improve the electrical conductivity of the material and the adsorption capacity of LiPSs. The electrochemical performance of NSPC/S has been greatly improved when compared with pure graphite, which is mainly ascribed to the doped N and S and also confirmed by the theoretical calculation. The NSPC/S electrode delivered a superior rate capability (415 mAh·g^-1 at 2C) and an outstanding cell performance (762 mAh·g-1 at 0.6C after 200 cycles, 463 mAh·g^-1 at 2C after 200 cycles). The promotion of polysulfides transformation with NSPC demonstrates that synergizing strong polysulfides adsorption and superior electronic conductivity is critical for the rational design of cathode materials in Li-S batteries. Furthermore, this work provides a novel strategy to synthesize 2D N doped porous carbon nanomaterial for realizing advanced Li-S batteries.

  Conflicts of interest: There are no conflicts to declare.

  Supporting information is available at http://www.wjhxxb.cn

  
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  Figure 1  (a) Schematic illustration showing the preparation of NSPC and NSPC/S; (b) SEM image and mappings of NSPC

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  Figure 2  (a) XRD patterns of graphite and NSPC; (b) Raman spectra and (c) survey, (d) C1s, (e) N1s, and (f) S2p XPS spectra of NSPC

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  Figure 3  (a) N₂ adsorption-desorption isotherm, and (b) pore size distribution of NSPC

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  Figure 4  (a) Optical photo of the adsorption effect of NSPC and SPC on Li₂S₆; (b) UV-Vis spectra of NSPC and SPC; (c) TGA of pure S, NSPC, and NSPC/S; (d) EIS curves of NSPC and SPC; (e) CV curves of NSPC/S from 3.0 to 1.5 V (vs Li/Li⁺) at a scan rate of 0.2 mV·s^-1; (f) Voltage profiles of NSPC/S electrodes at various current densities; (g) Rate performance of NSPC/S electrodes; (h) Cycling performance of NSPC/S electrodes at high current densities; (i) Schematic diagram of the catalytic conversion of LiPSs

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  Figure 5  Adsorbability of pyrrolic N, pyridinic N, and S¹ and S² of Li₂S_n (n=1, 2, 4, 6, and 8)

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