Functionalized N-Doped Porous Carbon Nanofiber Webs for a Lithium

Jan 10, 2014 - A free-standing sulfur-doped microporous carbon interlayer derived from luffa sponge for high performance lithium–sulfur batteries. J...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Functionalized N‑Doped Porous Carbon Nanofiber Webs for a Lithium−Sulfur Battery with High Capacity and Rate Performance Juan Yang, Jing Xie, Xiangyang Zhou,* Youlan Zou, Jingjing Tang, Songcan Wang, Feng Chen, and Luyu Wang School of Metallurgy and Environment, Central South University, Changsha, 410083, China S Supporting Information *

ABSTRACT: Functionalized N-doped porous carbon nanofiber webs/sulfur (N-PCNF/S) composites are first proposed as the cathode materials for an advanced lithium−sulfur battery. The functionalized N-doped porous carbon nanofiber webs (N-PCNF) with an appropriate N doping (4.32 wt %) are synthesized by a facile approach, which consists of pyrolyzation of polypyrrole nanofiber and a subsequent KOH activation. Instrumental analysis shows that N-PCNF possesses a large specific surface area (2642 m2 g−1) and a high inner pore volume (1.31 cm3 g−1). When evaluating its electrochemical properties in a lithium−sulfur battery, the N-PCNF/S composite with 77.01 wt % sulfur content displays an excellent electrochemical performance. The specific discharge capacity still reaches 749.8 mAh g−1 after 180 cycles at 0.2 C. At a higher rate of 1 C, the capacity stabilizes at 666.0 mAh g−1 after 200 cycles. This work demonstrates that combining the favorable aspects of N doping modification and one-dimensional nanostructure in the carbon matrix design is an effective way to improve the electrochemical performance of the carbon/sulfur cathodes. polarity, and conductivity.36,37 However, there are only a few studies focused on the application of nitrogen doping carbon in a lithium−sulfur battery. Typical studies were reported by Sun et al.38 and Sun et al.;39 they demonstrated that nitrogen doping could assist mesoporous carbons to suppress the diffusion of polysulfide species and to improve the electrochemical performance of the sulfur/carbon cathodes. In addition, according to recent studies by Qie37 and our group,36 the one-dimensional porous carbon, such as carbon nanofibers, can shorten the transport length of Li+ and provide large electrode−electrolyte interfaces for the charge transfer reaction. This inspires us to combine N-doping modification with one-dimensional nanostructure in the carbon matrix design and to investigate their effects on the electrochemical performance of sulfur/carbon cathodes. In this work, we first proposed functionalized N-doped porous carbon nanofiber webs/sulfur (N-PCNF/S) composites as the cathode materials for an advanced lithium−sulfur battery. The functionalized N-doped porous carbon nanofiber webs (NPCNF) with an appropriate N doping were synthesized by a facile approach, which consisted of pyrolyzation of polypyrrole nanofiber and a subsequent KOH activation. Due to the unique one-dimensional porous nanostructure, N-PCNF is able to allow fast and long-distance electron transport, and possesses largely exposed surface area and good electrical conductiv-

1. INTRODUCTION The rechargeable lithium−sulfur battery is a very promising system for the next generation of a high-energy energy storage device due to its high theoretical energy density of 2600 Wh kg−1, which is much higher than that of the current lithium ion batteries.1−3 In addition, the low cost, natural abundance, and environmental benignity of element sulfur make it attractive for large-scale practical applications.4,5 Despite these attractive advantages, the lithium−sulfur battery suffers several issues: sulfur has a low intrinsic conductivity (5 × 10−30 s cm−1 at 25 °C), leading to the low utilization of active material within the electrodes.6 Highly undesirable dissolution and redeposition of polysulfides (Li2Sx, 2 < x ≤ 8) will lead to the loss of active materials, capacity fade, corrosion of the lithium anode, and self-discharge.5,7 Moreover, using lithium metal as the anode may bring safety problems in practical applications due to the growth of lithium dendrite.8 To address these issues, various strategies have been developed, such as embedding sulfur within conducting polymers,3,9−12 impregnating sulfur into various carbon matrixes,3,7,8,13−26 developing new lithium− sulfur battery configurations,5,27 using oxides as additional polysulfide adsorbents,28,29 and designing a novel electrolyte.30−34 Recently, several studies have shown that doping carbonaceous materials with heteroatoms such as N and B is a promising method to improve the electronic conductivity and lithium electroactivity.35−37 Especially, nitrogen is the most attractive doping element that could substantially improve the carbon wettability, basic property, adsorptive ability, surface © 2014 American Chemical Society

Received: October 20, 2013 Revised: January 9, 2014 Published: January 10, 2014 1800

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807

The Journal of Physical Chemistry C

Article

ity,36,37 which can act as an appealing host for a high-rate lithium−sulfur battery. Meanwhile, a large number of nanopores in N-PCNF can encapsulate a large amount of sulfur and can also play a role as a strong absorbent to retain polysulfides. In addition, with an enhanced surface adsorption, nitrogen doping could further assist to suppress the diffusion of polysulfide species.39 As a result, the N-PCNF/S composites exhibit an excellent electrochemical performance.

based on the thermogravimetric analysis (TGA, SDTQ600) data. 2.5. Electrochemical Measurement. Electrochemical characterization was carried out by galvanostatically cycled in a 2025 coin-type cell. The electrode consisted of 80 wt % of the as-prepared N-PCNF/S composites, 10 wt % PVDF binder, and 10 wt % conductive carbon black. To form the electrode, the materials were mixed, dispersed in N-methyl-2-pyrrolidinone (NMP), and coated on an aluminum foil. Finally, the electrode was dried at 60 °C overnight. Test cells were assembled in a glovebox filled with Ar gas, using Li foil as the counter collector (Φ = 1.54 cm) and polyethylene film as the separator. The electrolyte was 1.0 M LiN(CF3SO2)2 (LiTFSI) and 0.1 M LiNO3 in a mixed solvent of dimethoxyethane (DME) and 1,3-dioxolane (DOL) (V/V = 1:1), which was purchased from Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd. (Jiangsu, China). The coin cells were galvanostatically cycled between 2.8 and 1.7 V (vs Li/Li+) on a LAND CT2001A instrument (Wuhan, China) at room temperature. In this study, 1 C corresponded to a current density value of 1680 mA g−1 sulfur, and the typical sulfur mass loading on the electrode was 0.9−1.0 mg cm−2. Specific capacity was corrected based on the mass of sulfur, and it had an error margin of plus or minus 1.5−1.9 percentage points. The cyclic voltammogram (CV) experiment was performed in the range of 1.6−3.0 V at a scanning rate of 0.2 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were performed with an impedance analyzer in the 100 kHz to 10 mHz frequency range in automatic sweep mode from high to low frequency.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Polypyrrole Nanofiber (PNF). PNF was synthesized by a modified oxidative template assembly route.37 Typically, CTAB ((C16H33)N(CH3)3Br, 7.3 g) was dissolved in HCl solution (120 mL, 1 mol L−1) under an ice bath. Then APS ((NH4)2S2O8, 13.7 g) was added, and a white reactive template was formed immediately. After being magnetically stirred for 0.5 h and cooling to 0−5 °C, pyrrole monomer (8.3 mL) was added into the as-formed reactive template solution. The reaction was carried out at 0−5 °C for 24 h. A black precipitate (PNF) was obtained. The resulting black precipitate was filtered off, and washed several times with excess amounts of deionized water then absolute ethanol. Finally, the product was dried overnight at 80 °C in an oven. 2.2. Synthesis of the N-Doped Porous Carbon Nanofiber (N-PCNF). The as-synthesized PNF was heated to 600 °C at a heating rate of 5 deg min−1 and kept for 1 h under a nitrogen atmosphere to form the N-doped carbon nanofiber (N-CNF). To obtain the N-PCNF sample, N-CNF was activated with KOH. Typically, KOH and the as-prepared N-CNF with a mass ration of 3:1 were added into a mixed solution of ethanol and deionized water (V/V = 1:1), followed by 12 h of static soaking in ambient conditions. After that, the mixture was dried at 100 °C for 12 h. Finally, the mixture was heated at 700 °C for 1 h in N2 atmosphere at a heating rate 5 deg min−1 and an N2 flow rate of 100 cm3 min−1. After cooling to room temperature, the resulting sample was added into 1.0 mol L−1 HCl solution with a specific volume for more than 6 h with magnetic stirring. Afterward, the sample was washed with distilled water until free of chloride ions, and then dried in a vacuum oven at 110 °C. 2.3. Synthesis of the N-Doped Porous Carbon Nanofiber/Sulfur (N-PCNF/S) Composites. The N-PCNF/ S composites were synthesized through a melt-diffusion strategy as described in our previous reports.22 Typically, the premixed sublimed sulfur and corresponding carbon at an accurate mass ratio of 80:20 and 90:10, respectively, were separately put into two sealed Teflon containers for 12 h at 155 °C, followed by another 2 h at 250 °C under argon condition. 2.4. Characterization. The structures and morphologies of the as-prepared samples were evaluated by using scanning electron microscopy (SEM,JSM-6360LV, Japan), field emission scanning electron microscopy (NOVA NANO FESEM 230), transmission electron microscopy (TEM, JEM-2100F, Japan), and X-ray diffraction (XRD, Rigaku-TTRIII, Japan). X-ray photoelectron spectra (XPS) were recorded by using an X-ray photoelectron spectrometer (K-Alpha 1063) with a monochromatic Al Kα X-ray source. The specific surface area of NPCNF was determined according to the Brunauer−Emmett− Teller (BET) method. The pore size distribution plot was recorded from the adsorption branch of the isotherm based on the nonlocal density functional theory (NLDFT) method. The content of sulfur in the N-PCNF/S composites was calculated

3. RESULTS AND DISCUSSION The TGA results determine the actual sulfur content in the NPCNF/S composites, where the weight loss is due to the evaporation of sulfur. As shown in Figure 1, TGA curves of the

Figure 1. TGA curves of the N-PCNF/S composites under nitrogen flow.

N-PCNF/S composites are very similar to the previously reported TGA data of a hard carbon spherules−sulfur electrode,40 which display two weight loss steps, i.e. the initial one at 200 °C and the second one at 300 °C. The former associates with the loss of sulfur situated at the surface and mesopore of N-PCNF and the latter is attributed to the release of sulfur confined within the micropore of N-PCNF. According 1801

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807

The Journal of Physical Chemistry C

Article

Figure 2. XPS spectra of N-PCNF, N-PCNF/S-77, and N-PCNF/S-85 (a), high-resolution spectra of N 1s for N-PCNF (b), high-resolution spectra of S 2p for N-PCNF/S-77 (c), XRD pattern of N-PCNF (d, spectrum a), the simple mixture of N-PCNF/sulfur powder (d, spectrum b), N-PCNF/ S-85 (d, spectrum c), and N-PCNF/S-77 (d, spectrum d).

Figure 2d shows XRD patterns of N-PCNF, the simple mixture of N-PCNF/sulfur powder and the N-PCNF/S composites. In these four XRD patterns, a characteristic peak of carbon appears at 2θ = 24°, which represents the phase of N-PCNF. The sharp diffraction peaks of sublimed sulfur in XRD patterns of the N-PCNF/S composites illustrate its crystalline state.20 As the sulfur content decreases, the sharp diffraction peaks of sulfur between 30° and 80° disappear entirely in the XRD pattern of N-PCNF/S-77, which indicates a good incorporation of sulfur into N-PCNF.22,40 However, there still exists some sharp peaks of sulfur between 20° and 30°, which denotes that a portion of orthorhombic sulfur particles are crystallized on the external surface of N-PCNF.20 Especially, for N-PCNF/S85, almost all of the diffraction peaks of sulfur can be observed in its XRD pattern, indicating the saturation of the micropores. SEM and TEM analyses were used to investigate the microstructure of the as-synthesized samples. As shown in Figure 3a,b, N-CNF obtained after the carbonization of PNF at 600 °C shows homogeneous morphology of cross-linked nanofibers with a diameter of 80−100 nm. To achieve highquality pores, the as-obtained N-CNF was chemically activated with KOH at 700 °C for 1.0 h under a nitrogen atmosphere. Although N-PCNF retains the same cross-linked morphology as N-CNF (Figure 3c,d), it shows a different microstructure. As shown in TEM images (Figure 4c,d), N-PCNF displays a much rougher surface, indicating some structural defects which are favorable for Li-ion diffusion from various orientations and for the sufficient contact between active materials and electrolyte.36,37 To further investigate the porous structure of NPCNF, nitrogen adsorption−desorption isotherms were measured (Figure S1, Supporting Information). The specific BET surface area reaches as high as 2642 m2 g−1. The pore size distribution is shown in Figure S2 (Supporting Information).

to the TGA results, the weight ratios of the removed sulfur are 77.01 and 85.13 wt %, respectively. Hereinafter, the N-PCNF/S composites are denoted as N-PCNF/S-77 and N-PCNF/S-85 according to their sulfur content in wt %. Figure 2a shows XPS survey spectra of N-PCNF and the NPCNF/S composites. In the XPS survey spectra of N-PCNF, three peaks centering at 285.0, 400.0, and 532.0 eV, corresponding to C1s, N1s, and O1s, respectively, can be easily observed.39 The percentage of N in N-PCNF is around 4.32 wt %, which is within the appropriate content range suggested by Sun et al.39 The nitrogen peaks in N-PCNF are fitted into three peaks at 398.5 ± 0.3, 400.48 ± 0.3, and 402.5 ± 0.3 eV, corresponding to pyridinic N (N-6), pyrrolic N (N5), and other N (N-X), respectively (Figure 2b).36,37 N-6 atom is located at the edge of graphene layers by substituting a carbon atom on the six-membered ring, which will enhance the adsorption ability of the carbons due to a strong interatomic attraction between the basic pyridinic N and polysulfides species.39 N-5 is a nitrogen atom in a five-membered ring and provides the π system with two p-electrons, which may also be able to improve the surface adsorption even though its interaction is not as strong as that of the pyridinic N.38,41 NX at higher binding energies (402−405 eV) is related to various functional groups such as nitrogen oxide, nitro or nitroso groups, etc.36 In addition, as shown in Figure 2a, the typical peaks of sulfur (2s and 2p) can be easily observed in the survey spectrum of N-PCNF/S-77 and N-PCNF/S-85, which are assigned to S8.41 In the XPS S2p spectra of N-PCNF/S-77 (Figure 2c), two fitted peaks positioned at 164.12 and 165.38 eV are corresponding to the S 2p3/2 and S 2p1/2 components, respectively.42 The sublimed sulfur was impregnated in N-PCNF through a melt-diffusion method to fabricate the N-PCNF/S composites. 1802

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807

The Journal of Physical Chemistry C

Article

sulfur particles can be easily observed outside N-PCNF even though the sulfur content is as high as 77.01 wt %. To further investigate the electrochemical performance of the N-PCNF/S composites with different sulfur content, galvanostatic cycling performance and rate capability evaluations in 2025 coin-type cells with Li metal as the anode were carried out. Typical cyclic voltammetry (CV) curves of the N-PCNF/ S-77 cathode are given in Figure 5a. In the first scanning process, there are two remarkable reduction peaks at ∼2.0 and ∼2.25 V. The upper plateau at ∼2.25 V involves the reduction of elemental sulfur to soluble lithium polysulfide (Li2Sx, 4 ≤ x < 8), and the lower plateau at ∼2.0 V corresponds to the reduction of lithium polysulfides to insoluble Li2Sx (x = 1, 2).23,24,43 Only one quite wide oxidation peak at ∼2.55 V is observed, which is related to the conversion of Li2Sx (x = 1, 2) into high-order soluble polysulfides.20,44 Note that there are only slight changes in the CV peak positions, even after 15 scans, confirming a good electrochemical reversibility of the NPCNF/S-77 cathode and indicating that the N-doped porous carbon nanofiber webs play an important role in suppressing the diffusion of polysulfide species and minimizing the mass loss of active materials during cycles. To investigate the variation of N atoms in the N-PCNF/S composites after cycles, high-resolution XPS survey of N1s for the N-PCNF/S-77 cathode after 20 cycles was recorded (Figure S3, Supporting Information). Three fitting peaks corresponding to N-6, N-5, and N-X, respectively, can be easily detected. However, the distribution of these N species in the N-PCNF/S-77 cathode samples after cycles is slightly different from that of N-PCNF, which may be due to the interaction of sulfur atom with nitrogen-containing functional groups on the surface of Ndoped carbon. The initial discharge/charge voltage profiles of the N-PCNF/ S-77 cathode and the N-PCNF/S-85 cathode at 0.2 C are compared in Figure 5b. Two discharge voltage plateaus at ∼2.3 and ∼2.0 V are observed, which are in accordance with two cathodic peaks in the CV curves (as shown in Figure 5a). For the N-PCNF/S-77 cathode, the initial discharge capacity is as high as 1077.2 mAh g−1, with a high initial Coulombic efficiency of 98.9%. In contrast, even though the N-PCNF/S-85 cathode also achieves a high initial discharge capacity of 1010.3 mAh g−1, the initial Coulombic efficiency is only 88.8%, which

Figure 3. SEM images of N-CNF (a and b), N-PCNF (c and d), and N-PCNF/S-77 (e and f).

These results demonstrate that the total pore volume and average pore diameter of N-PCNF are 1.31 cm3 g−1 and 1.88 nm, respectively. The high specific BET surface area and the unique nanostructure provide a large sulfur/N-PCNF interfacial area, which is essential for the homogeneous distribution of sulfur in the N-PCNF carbon matrix, and this result is confirmed by SEM images (Figure 3e,f) and TEM images (Figure 4e,f) of N-PCNF/S-77. As shown in Figure 3e,f, the process of thermal treatment does not bring any structure changes to the N-PCNF carbon matrix and no discernible

Figure 4. TEM images of N-CNF (a and b), N-PCNF (c and d), and N-PCNF/S-77 (e and f). 1803

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807

The Journal of Physical Chemistry C

Article

efficiency of 95%. The excellent rate cycling performance of the N-PCNF/S-77 cathode is demonstrated in Figure 6b. During the first 20 cycles, the discharge capacity fades gradually at 0.2 and 0.5 C rate (each for 10 cycles). Further cycling at higher rates shows reversible capacities of 715.7 mAh g−1 (1 C) and 547.7 mAh g−1 (2 C). When the cell was charged/discharged at 1, 0.5, and 0.2 C again, a reversible capacity of 817.4 mAh g−1 could still be retained at 0.2 C after 100 cycles, indicating an excellent rate performance of N-PCNF/S-77. According to the above results, the superhigh capacity and stable rate cycling performance of N-PCNF/S-77 can be explained by its novel nanostructure and the appropriate content of nitrogen doping. As illustrated in Figure 6c, first, the interconnected 3D carbon framework can provide a continuous pathway for electron transport, resulting in a high conductivity of N-PCNF/S-77.37 Second, due to the one-dimensional porous nanostructure, unlike the irregular polyhedral shape carbon matrix,38,39 N-PCNF can shorten the transport length of Li+ and offer large electrode−electrolyte interfaces for the charge transfer reaction, which will contribute to achieve an outstanding rate performance.36,37 Third, with the high surfaceto-volume ratios and abundant pore volumes, sulfur can be homogeneously distributed in the N-PCNF carbon matrix, which will enable the N-PCNF/S composites to obtain a high initial specific discharge capacity. Fourth, large numbers of nanopores in N-PCNF can play an important role in adsorbing polysulfides anions, leading to the minimization of the shuttling reaction. Furthermore, compared with previous studies about the porous carbon nanofibers/sulfur composites7,20−26 (Table 1), the N heteroatoms in the N-PCNF/S composites cannot only enhance the electronic conductivity, but also assist NPCNF to suppress the diffusion of polysulfide species into the electrolyte via an enhanced interface adsorption, which additionally contributes to the exceptional electrochemical performance.38,39 However, because some lithium polysulfides formed on the external surface of N-PCNF could inevitably dissolve into the liquid electrolyte, the capacity fading with cycles still occurred. To obtain further insight into the improved electrochemical performance, electrochemical impedance spectra (EIS) of the N-PCNF/S-77 cathode fully charged after different cycles were measured, as shown in Figure 7 with the equivalent circuit diagram presented as an inset. It can be seen that the impedance plots after different cycles are composed of two semicircles in the middle-high frequency region and an inclined line in the low frequence region. The first semicircle at the high frequency region corresponds to the SEI film formed on the electrodes’ surface (Rs), the middle-frequency semicircle associates with the charge transfer resistance (Rct). The linear portion in the low frequence region relates to Li+ diffusion within the electrodes, corresponding to the Warburg element (Wo). Meanwhile, the intercept at real axis Z′ represents the combination resistance Ro, corresponding to the intrinsic resistance of the active materials, the ionic resistance of the electrolyte, and the contact resistance at the active material/ current collector interface.22,41,45−47 As shown in Table 2, the electrode resistances were obtained from the equivalent circuit fitting of experimental data. During cycling, Ro of the N-PCNF/S-77 cathode decreases slightly from 6.716 Ω (1st) to 1.52 Ω (10th), which may be due to the dissolution and redistribution of sulfur crystallized on the external surface of N-PCNF, leading to the decrease of the intrinsic resistance of the N-PCNF/S-77 cathode. While,

Figure 5. (a) Cyclic voltammograms of the N-PCNF/S-77 cathode in the potential window from 1.6 to 3.0 V (versus Li/Li+) at a scan rate of 0.2 mV s−1, (b) the initial discharge/charge curves of the N-PCNF/S77 cathode and the N-PCNF/S-85 cathode at 0.2 C, (c) cycling performances of the N-PCNF/S-77 cathode and the N-PCNF/S-85 cathode at 0.2 C.

may be due to the dissolution polysulfides formed by the sulfur crystallized on the outer surface of N-PCNF. The cycling performances of the N-PCNF/S-77 cathode and the N-PCNF/S-85 cathode at 0.2 C are presented in Figure 5c. After 180 cycles, the N-PCNF/S-77 cathode retains a capacity of 749.8 mAh g−1, retaining about 69.6% of its initial capacity. Meanwhile, the N-PCNF/S-85 cathode delivers a capacity of 594.3 mAh g−1, which is only about 58.8% of its initial capacity. Then, the N-PCNF/S-77 cathode was discharged and charged at higher rates, as shown in Figure 6a. At a rate of 0.5 and 1 C, the N-PCNF/S-77 cathode delivers high initial specific discharge capacity of 1146.1 and 941.0 mAh g−1, respectively. After 200 cycles, the specific discharge capacity still remains 714.0 and 666.0 mAh g−1, both with an average Coulombic 1804

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807

The Journal of Physical Chemistry C

Article

Figure 6. (a) Cycling performances of the N-PCNF/S-77 cathode at 0.5 and 1 C, (b) rate capability of the N-PCNF/S-77 cathode, and (c) scheme of the N-PCNF/S composites for improving the cathode performance.

Table 1. A Comparison of the Electrochemical Performance of CNF/S Composites from the Literature cycling performance sample b

G -S-CNF CNFc-S CNF-S CNF-S CNF-S HCNFd-S HCNF-S N-PCNF-77

sulfur content (wt %)

current density (Ce)

initial SCa

reversible SC

cycles

ref

∼33 42 27 70 57 unknown 65 77

1 0.05 0.1 0.1 0.2 0.5 0.2 0.2

745 1400 ∼1600 1200 928 828 1230 1077.2

∼273 1220 950 760 408 ∼662 ∼738 749.8

1500 30 50 50 80 300 200 180

7 20 21 23 24 25 26 this work

SC is the discharge specific capacity/mAh g−1. bG is graphene. cCNF is porous carbon nanofibers. dHCNF is hollow carbon nanofibers. e1 C = 1672−1680 mA g−1.

a

Table 2. Electrode Resistance Obtained from the Equivalent Circuit Fitting of Experimental Data cycle

Ro ± 3% (Ω)

Rs ± 3% (Ω)

Rct ± 3% (Ω)

1st 10th 200th

6.716 1.52 1.47

50.29 60.78 89.31

75.52 145.12 154.12

compared with Ro of the 10th cycle, it almost remains the same even after 200 cycles, demonstrating that the N-PCNF/S-77 cathode has an excellent conductivity. We notice that Rs of the N-PCNF/S-77 cathode continued to increase from 50.29 Ω (1st) and 60.78 Ω (10th) to 89.31 Ω (200th), owing to the irreversible deposition and aggregation of insulated Li2S2/Li2S on the surface of N-PCNF.45,46 However, in consideration of the long cycles, there is only 0.15 Ω per cycle increase from the 10th to 200th, which means that the diffusion of polysulfide species into the electrolyte is extremely alleviated owing to the large number of nanopores and N heteroatoms in N-PCNF. Moreover, Rct of the N-PCNF/S-77 cathode also continues to

Figure 7. EIS data of the N-PCNF/S-77 cathode fully charged after different cycles. 1805

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807

The Journal of Physical Chemistry C

Article

increase from 75.52 Ω (1st) to 145.12 Ω (10th) and then to 154.12 Ω (200th). Although Rct increases fast during the first ten cycles, Rct is almost unchanged in the following cycles, indicating that the N-PCNF/S-77 cathode is in favor of the rapid electronic/ionic transport.40 Accordingly, based on the EIS analysis above, the N-PCNF/S-77 cathode achieves a good interfacial charge-transfer process and Li+ diffusion process, resulting in the excellent capacity reversibility and outstanding rate performance.

(2) Song, M.; Cairns, E. J.; Zhang, Y. Lithium/Sulfur Batteries with High Specific Energy: Old Challenges and New Opportunities. Nanoscale 2013, 5, 2186−2204. (3) Ji, X.; Lee, K. T.; Nazar, L. F. A Highly Ordered Nano-Structured Carbon-Sulphur Cathode for Lithium−Sulphur Batteries. Nat. Mater. 2009, 8, 500−506. (4) Nelson, J.; Misra, S.; Yang, Y.; Jackson, A.; Liu, Y.; Wang, H.; Dai, H.; Andrews, J. C.; Cui, Y.; Toney, M. F. In Operando X-ray Diffraction and Transmission X-ray Microscopy of Lithium Sulfur Batteries. J. Am. Chem. Soc. 2012, 134, 6337−6343. (5) Su, Y.; Manthiram, A. Lithium-Sulphur Batteries with a Microporous Carbon Paper as a Bifunctional Interlayer. Nat. Commun. 2012, 3, 1166. (6) Yin, L.; Wang, J.; Lin, F.; Yang, J.; Nuli, Y. Polyacrylonitrile/ Graphene Composite as a Precursor to a Sulfur-Based Cathode Material for High-Rate Rechargeable Li−S Batteries. Energy Environ. Sci. 2012, 5, 6966−6972. (7) Lu, S.; Cheng, Y.; Wu, X.; Liu, J. Significantly Improved LongCycle Stability in High-Rate Li-S Batteries Enabled by Coaxial Graphene Wrapping over Sulfur-Coated Carbon Nanofibers. Nano Lett. 2013, 13, 2485−2498. (8) Xin, S.; Gu, L.; Zhao, N.; Yin, Y.; Zhou, L.; Guo, Y.; Wan, L. Smaller Sulfur Molecules Promise Better Lithium−Sulfur Batteries. J. Am. Chem. Soc. 2012, 134, 18510−18513. (9) Yang, Y.; Yu, G.; Cha, J. J.; Wu, H.; Vosgueritchian, M.; Yao, Y.; Bao, Z.; Cui, Y. Improving the Performance of Lithium-Sulfur Batteries by Conductive Polymer Coating. ACS Nano 2011, 5, 9187−9193. (10) Hassoun, J.; Scrosati, B. A High-Performance Polymer Tin Sulfur Lithium Ion Battery. Angew. Chem., Int. Ed. 2010, 49, 2371− 2374. (11) Wu, F.; Chen, J.; Li, L.; Zhao, T.; Liu, Z.; Chen, R. Polyethylene-Glycol-Doped Polypyrrole Increases the Rate Performance of the Cathode in Lithium-Sulfur Batteries. ChemSusChem 2013, 6, 1438−1444. (12) He, G.; Ji, X.; Nazar, L. High “C” Rate Li-S Cathodes: Sulfur Imbibed Bimodal Porous Carbons. Energy Environ. Sci. 2011, 4, 2878− 2883. (13) Liang, X.; Wen, Z. Y.; Liu, Y.; Zhang, H.; Huang, L. Z.; Jin, J. Highly Dispersed Sulfur in Ordered Mesoporous Carbon Sphere as a Composite Cathode for Rechargeable Polymer Li/S Battery. J. Power Sources 2011, 196, 3655−3658. (14) Chen, S. R.; Zhai, Y. P.; Xu, G. L.; Jiang, Y. X.; Zhao, D. Y.; Li, J. T.; Huang, L.; Sun, S. G. Ordered Mesoporous Carbon/Sulfur Nanocomposite of High Performances as Cathode for Lithium-Sulfur Battery. Electrochem. Acta 2011, 56, 9549−9555. (15) Li, D.; Han, F.; Wang, S.; Cheng, F.; Sun, Q.; Li, W. High Sulfur Loading Cathodes Fabricated Using Peapodlike, Large Pore Volume Mesoporous Carbon for Lithium−Sulfur Battery. ACS Appl. Mater. Interfaces 2013, 5, 2208−2213. (16) Jin, K.; Zhou, X.; Zhang, L.; Xin, X.; Wang, G.; Liu, Z. Sulfur/ Carbon Nanotube Composite Film as a Flexible Cathode for Lithium−Sulfur Batteries. J. Phys. Chem. C 2013, 117, 21112−21119. (17) Yuan, L. X.; Yuan, H. P.; Qiu, X. P.; Chen, L. Q.; Zhu, W. T. Improvement of Cycle Property of Sulfur-Coated Multi-Walled Carbon Nanotubes Composite Cathode for Lithium/Sulfur Batteries. J. Power Sources 2009, 189, 1141−1146. (18) Guo, J. C.; Xu, Y. H.; Wang, C. H. Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium-Sulfur Batteries. Nano Lett. 2011, 11, 4288−4294. (19) Wang, D.; Yu, Y.; Zhou, W.; Chen, H.; DiSalvo, F. J.; Muller, D. A.; Abruña, H. D. Infiltrating Sulfur in Hierarchical Architecture MWCNT@Meso C core-Shell Nanocomposites for Lithium-Sulfur Batteries. Phys. Chem. Chem. Phys. 2013, 15, 9051−9057. (20) Ji, L.; Rao, M.; Aloni, S.; Wang, L.; Cairns, E. J.; Zhang, Y. Porous Carbon Nanofiber-Sulfur Composite Electrodes for Lithium/ Sulfur Cells. Energy Environ. Sci. 2011, 4, 5053−5059. (21) Zhang, X.; Sun, Q.; Dong, W.; Li, D.; Lu, A.; Mu, J.; Li, W. Synthesis of Superior Carbon Nanofibers with Large Aspect Ratio and

4. CONCLUSIONS In summary, the N-PCNF/S composites have been successfully fabricated. The preparation process is facile, operable, and comparatively fast. N-PCNF as the sulfur host shows a unique interconnected 3D framework formed by the one-dimensional porous carbon, allowing a fast and long-distance electron transport, and a large specific surface area (2642 m2 g−1) and a high inner pore volume (1.31 cm3 g−1), ensuring that it can retain polysulfides efficiently. Furthermore, nitrogen doping could further assist N-PCNF to suppress the diffusion of polysulfide species via an enhanced surface adsorption, which can additionally alleviate the polysulfides shuttle. In terms of these advantages, the N-PCNF/S-77 cathode presents an excellent capacity reversibility and outstanding cyclability. The specific discharge capacity is as high as 749.8 mAh g−1 at 0.2 C after 180 cycles. Most importantly, this work demonstrates that combining the favorable aspects of N doping modification and one-dimensional nanostructure in the carbon matrix design is an effective way to improve the electrochemical performance of the carbon/sulfur cathode.

■

ASSOCIATED CONTENT

S Supporting Information *

N2 adsorption−desorption isotherms and the corresponding pore size distribution of N-PCNF and high-resolution spectra of N 1s for the N-PCNF/S-77 cathode after 20 cycles. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

* Tel/Fax: +86 073188836329. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors; Jing Xie conceived the research and performed the experiments. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was financially supported by the National Nature Science Foundation of China (grant nos. 51274240 and 51204209) and the Free Exploration Projects for Undergraduates of Central South University (grant no. 2282013bks139). The authors thank Mr Hongzhuan Liu for providing technical support with the experiment and Dr. Kang Wen for assistance with the English service.

■

REFERENCES

(1) Yang, Y.; Zheng, G.; Cui, Y. Nanostructured Sulfur Cathodes. Chem. Soc. Rev. 2013, 42, 3018−3032. 1806

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807

The Journal of Physical Chemistry C

Article

Tunable Porosity for Electrochemical Energy Storage. J. Mater. Chem. A 2013, 1, 9449−9455. (22) Zhou, X.; Xie, J.; Yang, J.; Zou, Y.; Tang, J.; Wang, S.; Ma, L.; Liao, Q. Improving the Performance of Lithium-Sulfur Batteries by Graphene Coating. J. Power Sources 2013, 243, 993−1000. (23) Rao, M.; Geng, X.; Li, X.; Hu, S.; Li, W. Lithium-Sulfur Cell with Combining Carbon Nanofibers-Sulfur Cathode and Gel Polymer Electrolyte. J. Power Sources 2012, 212, 179−185. (24) Deng, Z.; Zhang, Z.; Lai, Y.; Liu, J.; Liu, Y.; Li, J. A SulfurCarbon Composite for Lithium/Sulfur Battery Based on Activated Vapor-Grown Carbon Fiber. Solid State Ionics 2013, 238, 44−49. (25) Zheng, G.; Zhang, Q.; Cha, J. J.; Yang, Y.; Li, W.; She, Z. W.; Cui, Y. Amphiphilic Surface Modification of Hollow Carbon Nanofibers for Improved Cycle Life of Lithium Sulfur Batteries. Nano Lett. 2013, 13, 1265−1270. (26) Yao, H.; Zheng, G.; Li, W.; McDowell, M. T.; Seh, Z.; Liu, N.; Lu, Z.; Cui, Y. Crab Shells as Sustainable Templates from Nature for Nanostructured Battery Electrodes. Nano Lett. 2013, 13, 3385−3390. (27) Hu, Y.; Manthiram, A. A New Approach to Improve Cycle Performance of Rechargeable Lithium-Sulfur Batteries by Inserting a Free-Standing MWCNT Interlayer. Chem. Commun. 2012, 48, 8817− 8819. (28) Ji, X. L.; Evers, S.; Black, R.; Nazar, L. F. Stabilizing LithiumSulphur Cathodes using Polysulphide Reservoirs. Nat. Commun. 2011, 2, 325. (29) Evers, S.; Yim, T.; Nazar, L. F. Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li-S Battery. J. Phys. Chem. C 2012, 116, 19653−19658. (30) Suo, L.; Hu, Y.; Li, H.; Armand, M.; Chen, L. A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481. (31) Shin, J. H.; Cairns, E. J. N-Methyl-(n-butyl)pyrrolidinium Bis(trifluoromethanesulfonyl)imide-LiTFSI-poly(ethylene glycol) Dimethyl Ether Mixture as a Li/S Cell Electrolyte. J. Power Sources 2008, 177, 537−545. (32) Lin, Z.; Liu, Z.; Fu, W.; Dudney, N. J.; Liang, C. Phosphorous Pentasulfide as a Novel Additive for High-Performance Lithium-Sulfur Batteries. Adv. Funct. Mater. 2013, 23, 1064−1069. (33) Xu, R.; Belharouak, I.; Li, J. C. M.; Zhang, X.; Bloom, I.; Bareño, J. Role of Polysulfides in Self-Healing Lithium-Sulfur Batteries. Adv. Energy Mater. 2013, 3, 833−838. (34) Zhang, S. Role of LiNO3 in Rechargeable Lithium/Sulfur Battery. Electrochem. Acta 2012, 70, 344−348. (35) Rodríguez, E.; Camean, I.; Garcia, R.; Garcia, A. B. Graphitized Boron-Doped Carbon Foams: Performance as Anodes in Lithium-Ion Batteries. Electrochim. Acta 2011, 56, 5090−5094. (36) Zhou, X.; Tang, J.; Yang, J.; Xie, J.; Huang, B. Seaweed-Like Porous Carbon from The Decomposition of Polypyrrole Nanowires for Application in Lithium Ion Batteries. J. Mater. Chem. A 2013, 1, 5037−5044. (37) Qie, L.; Chen, W. M.; Wang, Z. H.; Shao, Q. G.; Li, X.; Yuan, L. X.; Hu, X. L.; Zhang, W. X.; Huang, Y. H. Nitrogen-Doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047− 2050. (38) Sun, X.; Wang, X.; Mayes, R. T.; Dai, S. Lithium-Sulfur Batteries Based on Nitrogen-Doped Carbon and an Ionic-Liquid Electrolyte. ChemSusChem 2013, 5, 2079−2085. (39) Sun, F.; Wang, J.; Chen, H.; Li, W.; Qiao, W.; Long, D.; Ling, L. High Efficiency Immobilization of Sulfur on Nitrogen-Enriched Mesoporous Carbons for Li-S Batteries. ACS Appl. Mater. Interfaces 2013, 5, 5630−5638. (40) Kim, J.; Lee, D.; Jung, H.; Sun, Y.; Hassoun, J.; Scrosati, B. An Advanced Lithium-Sulfur Battery. Adv. Funct. Mater. 2013, 23, 1076− 1080. (41) Wang, Y.; Huang, L.; Sun, L.; Xie, S.; Xu, G.; Chen, S.; Xu, Y.; Li, J.; Chou, S.; Dou, S.; Sun, S. Facile Synthesis of a Interleaved Expanded Graphite-Embedded Sulphur Nanocomposite as Cathode of

Li-S Batteries with Excellent Lithium Storage Performance. J. Mater. Chem. 2012, 22, 4744−4750. (42) Zu, C.; Manthiram, A. Hydroxylated Graphene-Sulfur Nanocomposites for High-Rate Lithium-Sulfur Batteries. Adv. Energy Mater. 2013, 3, 1008−1012. (43) Li, J.; Ding, B.; Xu, G.; Hou, L.; Zhang, X.; Yuan, C. Enhanced Cycling Performance and Electrochemical Reversibility of Novel Sulfur-Impregnated Mesoporous Hollow TiO2 Spheres Cathode for Advanced Li-S Batteries. Nanoscale 2013, 5, 5734−5746. (44) Zhang, K.; Qin, F.; Fang, J.; Li, Q.; Jia, M.; Lai, Y.; Zhang, Z.; Li, J. Nickel Foam as Interlayer to Improve the Performance of LithiumSulfur Battery. J. Solid State Electrochem. 2013, DOI: 10.1007/s10008013-2351-5. (45) He, X.; Wang, L.; Li, J.; Gao, J.; Fang, M.; Tian, G.; Wang, J.; Fan, S. Graphene-Coated Plastic Film as Current Collector for Lithium/Sulfur Batteries. J. Power Sources 2013, 239, 623−627. (46) Xie, J.; Yang, J.; Zhou, X.; Zou, Y.; Tang, J.; Wang, S.; Chen, F. Preparation of Three-Dimensional Hybrid Nanostructure-Encapsulated Sulfur Cathode for High-Rate Lithium Sulfur Batteries. J. Power Sources 2014, 253, 55−63. (47) Ding, B.; Yuan, C.; Shen, L.; Xu, G.; Nie, P.; Lai, Q.; Zhang, X. G. Chemically Tailoring the Nanostructure of Graphene Nanosheets to Confine Sulfur for High-Performance Lithium-Sulfur Batteries. J. Mater. Chem. A. 2013, 1, 1096−1101.

1807

dx.doi.org/10.1021/jp410385s | J. Phys. Chem. C 2014, 118, 1800−1807