Single-Atom Coated Separator for Robust Lithium–Sulfur Batteries

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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

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Single-Atom Coated Separator for Robust Lithium−Sulfur Batteries Kun Zhang,†,∇ Zhongxin Chen,‡,∇ Ruiqi Ning,† Shibo Xi,§ Wei Tang,∥ Yonghua Du,§ Cuibo Liu,‡ Zengying Ren,† Xiao Chi,‡ Maohui Bai,† Chao Shen,† Xing Li,‡ Xiaowei Wang,‡ Xiaoxu Zhao,⊥ Kai Leng,‡ Stephen J. Pennycook,⊥ Hongping Li,# Hui Xu,*,# Kian Ping Loh,*,‡ and Keyu Xie*,†

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†

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, Northwestern Polytechnical University, Xi’an 710072, China ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore § Agency for Science, Technology and Research (A*STAR), Institute of Chemical and Engineering Sciences, 1 Pesek Road, Singapore 627833, Jurong Island, Singapore ∥ School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shaan Xi 710049, P. R. China ⊥ Department of Materials Science and Engineering, National University of Singapore, Singapore 117575 # Institute for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China S Supporting Information *

ABSTRACT: Lithium−sulfur (Li−S) batteries are strong contenders among lithium batteries due to superior capacity and energy density, but the polysulfide shuttling effect limits the cycle life and reduces energy efficiency due to a voltage gap between charge and discharge. Here, we demonstrate that graphene foam impregnated with single-atom catalysts (SACs) can be coated on a commercial polypropylene separator to catalyze polysulfide conversion, leading to a reduced voltage gap and a much improved cycle life. Also, among Fe/Co/Ni SACs, Fe SACs may be a better option to be used in Li−S systems. By deploying SACs in the battery separator, cycling stability improves hugely, especially considering relatively high sulfur loading and ultralow SAC contents. Even at a metal loading of ∼2 μg in the whole cell, an Fe SAC-modified separator delivers superior Li−S battery performance even at high sulfur loading (891.6 mAh g−1, 83.7% retention after 750 cycles at 0.5C). Our work further enriches and expands the application of SACs catalyzing polysulfide blocking and conversion and improving round trip efficiencies in batteries, without side effects such as electrolyte and electrode decomposition. KEYWORDS: single-atom catalyst, lithium−sulfur battery, battery separator, in situ Raman measurement, polysulfide shuttling effect

1. INTRODUCTION

immobilizing polysulfides so as to prevent the escape of soluble polysulfides from the cathode, however, these only met with partial success.8−13 In principle, the introduction of a separator between two electrodes would be an effective strategy to suppress polysulfide crossover for robust Li−S battery with high sulfur loading in the cathode.14−20 However, the commercial polypropylene (PP) separator is mainly used as an electronic insulator, which neither influences the Li+ ion transportation

The rapid revolution of electrified transport and smart grids causes an ever-increasing demand for new energy storage systems that can deliver high energy density and long service life.1,2 Lithium−sulfur (Li−S) batteries have attracted great interest due to their exceptional energy density (2600 Wh kg−1). The natural abundance of sulfur is another advantage for replacing conventional lithium-ion batteries.3,4 However, Li−S batteries suffer from rapid capacity fading, low Coulombic efficiency, and short cycle life due to the complex polysulfide conversion and shuttling effects, which severely impedes their practical applications.5−7 Tremendous effort has been devoted to the chemistry of porous cathode materials capable of © 2019 American Chemical Society

Received: March 30, 2019 Accepted: June 14, 2019 Published: June 14, 2019 25147

DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

Research Article

ACS Applied Materials & Interfaces

Figure 1. (A) Schematic of the preparation of the M1/NG-modified separator (M = Fe, Co or Ni). Inset: digital photo of commercial PP and Fe1/ NG-modified separators. (B−D) Atomic resolution high-angle annular dark-field imaging-scanning transmission electron microscopy (HAADFSTEM) images of Fe1/NG and (E) the corresponding line profile showing the presence of Fe atoms. (F) Fourier transform magnitudes of the experimental Fe K-edge extended X-ray absorption fine structure (EXAFS) spectra and (G) normalized Fe K-edge X-ray absorption near-edge structure (XANES) spectra of Fe foil, FePc, FeCl3, Fe3O4, Fe2O3, and Fe1/NG samples. (H) Fe L-edge XAS spectrum of Fe1/NG. (I) Crosssectional SEM image of the Fe1/NG-modified separator showing the ultrathin Fe1/NG film on the PP membrane. Scale bar: (B) 500 nm; (C) 5 nm; (D) 1 nm; (I) 50 μm.

in Li−S batteries.26 Hollow carbon nanospheres containing Co single atoms and clusters have also shown enhanced sulfur reactivity for the room temperature operation of Na−S batteries.27 However, the blend of single-atom and atom clusters makes it difficult to indicate the actual active site, and the use of SACs in the cathode typically requires a high mass loading of SACs in the active materials (>25 wt %, i.e., ∼2 mg cm−2 of catalyst at high sulfur loading), which decrease the utilization efficiency of SACs. In contrast, the incorporation of SACs on the separator is a better option because the catalyst loading can be reduced due to the more planar architecture of the separator. Furthermore, reported work is only using one type of SAC, ignoring the difference between different SACs. Herein, we developed a multifunctional battery separator by coating the commercial PP separator with N-doped graphene foam, the latter is impregnated with SACs (M1/NG, M = Fe, Co or Ni). The modified separator blocks the crossover of polysulfides effectively and improves their redox kinetics. The presence of SACs on N-doped graphene not only increases the binding energy of polysulfide species but also promotes the polysulfide conversion during cycling, which is evidenced by in situ Raman measurements. Through theoretical calculation and electrochemical testing, we found that Fe SACs may be a better option to improve the performance of Li−S batteries. As a result, the Li−S battery using the Fe1/NG-modified separator, with an extremely low metal loading of ∼2 μg in

nor the polysulfide crossover. The modification of PP separator by ceramics,18 metal sulfides,19 graphene derivatives,16 and metal organic frameworks12 can significantly improve the cycling stability by blocking the polysulfide diffusion, which is often achieved at the cost of Li+ ion conductivity and rate capability. Meanwhile, electrocatalysts such as metals and metal sulfides can be incorporated into the separator to promote the sluggish reaction of long-chain polysulfides.20−23 For instance, indium nitride nanowires on the separator can effectively restrict the dissolution of the polysulfides via chemical interaction and promote the conversion of the intermediates to give a 73.4% capacity retention after 1000 cycles. Among these functional materials, single-atom catalysts (SACs) are promising in terms of the maximized atom utilization efficiency, remarkable catalytic performances, and unique catalytic selectivity for many energy-related reactions, with the distinct advantage of ultrahigh performance-to-metal ratios. Previous studies have demonstrated the utility of SACs as the cathode materials to promote polysulfide anchoring and conversion in Li2S/Li and Li2S6/Li batteries, respectively, despite the moderate capacity and relatively low sulfur loading.24,25 Although Du et al. reported cobalt SACs in Nitrogen-doped graphene as a single-atom catalyst for high sulfur content Li−S batteries, researchers still lack understanding about the catalytic activity of different types of SACs 25148

DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

Research Article

ACS Applied Materials & Interfaces

Figure 2. (A) Electrochemical titrations of the Li2S6 adsorption on NG and M1/NG in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME). Inset: digital photo of the Li2S6 solution after 12 h. (B) Density functional theory (DFT) calculations of the Li2S6 adsorption on NG and Fe1/NG. (C, D) Polysulfide permeation tests for PP and Fe1/NG-modified separators.

robustness under extreme conditions and is mechanically durable enough to withstand stress induced by bending and wrinkling, evidencing strong adhesion. The single-atom Fe on graphene is confirmed by scanning transmission electron microscopy in the annular dark field mode (STEM-ADF), as shown in Figure 1B,C. Owing to the abundance of N dopants (∼7.5 atom %) and defective sites, Fe single atoms can be readily anchored on N-doped graphene by the impregnation method. It can be seen that the majority of Fe atoms is uniformly distributed on graphene in the subangstrom resolution HAADF-STEM images as bright spots. The intensity along the line in Figure 1D corroborates the presence of isolated Fe atoms as the signal intensity is approximately proportional to the square of the atomic number. The atomic percentage of Fe in Fe1/NG is determined to be ∼0.57 atom % from X-ray photoelectron spectroscopy (XPS). Co and Ni can be atomically dispersed on graphene using the same method as shown in Figure S1. Specific element contents of samples are listed in Table S1. The corresponding energy-dispersive X-ray spectroscopy mapping shown in Figure S2 further confirms the existence of metal single atoms on N-doped graphene. Meanwhile, all M1/NG catalysts exhibit the typical Raman peaks and XRD patterns of few-layer graphene in Figures S3−S5.28,29 The specific surface area of Fe1/NG is determined as ∼176.2 m2 g−1. The electronic structure and coordination environment of the SACs were examined by the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near-edge structure (XANES). The Fourier-transformed k3-weighted EXAFS spectrum of Fe1/NG in Figure 1F displays a dominant peak at 1.47 Å from the first coordination shell of Fe−N. The Fe−Cl peak at 1.78 Å completely vanishes after the reduction in NH3. There is also no Fe−Fe coordination peak at 2.17 Å from the metallic Fe clusters, which suggests the atomic

the whole cell, retains 83.7% capacity after 750 cycles at 0.5C, even at a high sulfur loading of ∼4.5 mg cm−2. The battery also exhibits a remarkably improved rate capability (a discharge capacity of 673 mAh g−1 at 5C) with a reduced voltage gap of 0.24 V compared to 0.48 V of the commercial PP separator.

2. RESULTS 2.1. Preparation of the SAC-Modified Separator. As illustrated in Figure 1A, the synthetic procedure for the SACmodified separator includes the anchoring of SACs on graphene foams by liquid-phase impregnation, followed by subsequent catalyst coating on the commercial polypropylene separator (PP, Celgard 2400) by standard thin-film casting techniques. Briefly, the precursor solution containing FeCl3, CoCl2, or NiCl2 was sonicated with GO dispersion and then freeze-dried to give a GO foam. As-prepared GO foams were then treated at 750 °C under Ar/NH3 to reduce GO and metal ions. The morphology of freestanding foam has been displayed through digital photos and scanning electron microscopy (SEM) images in Figure S1. Values of the mass density and pore volume fraction of foam are calculated on the basis of reported work, ∼1.83 mg cm−3 and ∼99.1% respectively, demonstrating a highly porous structure of the foam. All samples show excellent electrical conductivity (Table S4), ensuring strong electron transmission. Next, they were mixed with super P and poly(vinylidene fluoride) (PVDF) and then pasted onto the separator at a mass loading of 0.1 mg cm−2 (0.7 μgmetal cm−2). This allows an extremely low loading of inactive materials in comparison to the state-of-the-art ceramic separators (Al2O3 or SiO2) or the SAC-modified cathodes for high-performance Li−S batteries. As shown by the crosssection SEM image in Figure 1I, the Fe1/NG film is ultrathin (∼7 μm); its homogeneous and dense coverage on the PP separator can be observed in Figure S6. As is shown in Figure S4, a single-atom coated separator retains high stability and 25149

DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

Research Article

ACS Applied Materials & Interfaces

Figure 3. (A) CV profiles of the Li−S batteries with unmodified PP, the NG, or M1/NG-modified separators at 0.1 mV s−1. (B) Charge−discharge curves of the Li−S batteries at 0.5C. (C) Voltage gaps of the Li−S batteries with various separators at 600 mAh g−1. (D) CV profiles of the Li−S cell with the PP separator or the Fe1/NG-modified separator. Inset: digital photo of the in situ Raman cell. (E) In situ Raman spectra of the Li−S cell with the Fe1/NG-modified separator at different voltages as indicated in (D).

also evidenced by XPS studies before/after the absorption tests as shown in Figure S4. To ensure that only chemically bonded Li2S6 was measured, a copious quantity of solvent was used to wash away physically absorbed Li2S6. Irrespective of the metals, all of the peaks shift toward lower binding energy upon contact with Li2S614,30 due to the chemical bonding between SACs and Li2S6. Density functional theory (DFT) calculations were performed to gain an in-depth understanding of the adsorption behavior of SACs. As shown in Figures 2B and S8, the adsorption for Li2S6 on the metal sites in Fe1/NG is much stronger than that on the N dopants in NG (−2.36 vs −1.11 eV) owing to the strong electrostatic affinity between metal and nonmetal atoms. This is evidenced by the shortened distances of Fe···S (2.248 Å) and N···Li (2.135 Å), as well as the rotation of the Li2S6 molecule to face the Fe1/NG surface. The adsorption energy (Eads) follows the order of Fe > Co > Ni observed for the uptake of Li2S6 as measured by UV−vis and electrometric titration, indicating that the uptake scales with the adsorption energy. Beyond Li2S6, we also calculated the adsorption energy of Li2S4 to exclude the possible influence of polysulfide species on the strong binding of M1/NG, similar results were obtained as shown in Figure S8. Impermeability to soluble polysulfides is a prerequisite for a separator in the Li−S battery. The permeation experiments were conducted using H-type electrolytic cell devices under similar conditions, where soluble polysulfide in the left chamber slowly diffuses into the right chamber when a commercial PP separator was used. In the case of the Fe1/NGmodified separator, no diffusion of polysulfides across the separator was detected after 48 h in Figure 2D, showing effective blocking effects. Hence, the Fe1/NG-modified separator can minimize the shuttling effect in Li−S batteries compared to the PP separator. 2.3. Electrochemical Measurements on Polysulfide Redox Chemistry. The M1/NG-modified separator not only

dispersion of Fe in Fe1/NG. This is further evidenced by the pre-edge area of the Fe K-edge XANES profile. As shown in Figure 1G, the shoulder peak at ∼7.12 keV in FePc can be attributed to the square planar coordination of Fe, which is missing in the case of Fe1/NG.33 This suggests a nonplanar structure of the Fe−N complex in Fe1/NG. From the whiteline intensity and the location of peaks, Fe1/NG has a relatively high oxidation state (Fe3+) in comparison to FePc (Fe2+) and Fe foil. The high oxidation state of Fe SACs is also confirmed by the Fe L-edge XAS spectrum as shown in Figure 1H, which consists of the spin−orbit doublets Fe 2p3/2 and Fe 2p1/2 at ∼710 and 723 eV, respectively. Both binding energies are higher than those of metallic Fe(0) and should be attributed to the Fe3+ species, despite a small portion of Fe2+ species in the Fe1/NG sample. The Fe3+ species also exhibit a low spin state from the experimental branching ratios (Robs = 0.70) in the XAS spectrum, suggesting the strong coordination between Fe and the adjacent N atoms.29−31 2.2. Adsorption and Permeation Experiments. The strong binding between hosts and polysulfides is one of the factors contributing to a rapid polysulfide redox process.32,33 To verify the capacity for polysulfide immobilization, different metal SACs, adjusted to the same weight, were added into the solution containing the same amount of Li2S6. As shown in the inset of Figure 2A, the color of the Li2S6 solution with Fe1/NG is much lighter than that of Co1/NG and Ni1/NG, indicating stronger adsorption of Fe1/NG with polysulfide. In the absence of metal SAC, the color of the solution does not change after the addition of NG, indicating that the metal atoms are the main binding sites for the polysulfides. Quantitative evidence is obtained from the electrometric titration of the elemental sulfur in the supernatant as shown in Figure 2A and the UV− vis measurements of the Li2S6 solution as shown in Figure S7. The adsorbed quantities of Li2S6 by Fe1/NG, Co1/NG, Ni1/ NG, and NG are determined to be 4.10, 3.05, 2.97, and 1.53 μmol m−2, respectively.34 The metal-polysulfide interaction is 25150

DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a) Comparison of the catalyst and sulfur loading for various Li−S batteries. Catalyst loading on a cathode (open symbol) or a separator (closed symbol). (b) Comparison of the cycling stability and capacity retention for various Li−S batteries. (c) Electrochemical impedance spectroscopy (EIS) curves of the Li−S batteries with the PP separator or the NG- or M1/NG-modified separator. (d) Rate capabilities of the Li−S batteries and (e) charge−discharge curves of the Li−S batteries with the Fe1/NG-modified separator at various rates. (f) Cycling performance of the Li−S batteries and (g) the corresponding capacity retention after 750 cycles.

separator. Our observations on promoted polysulfide redox kinetics on M1/NG catalysts are further confirmed by the galvanostatic charge−discharge (GCD) curves shown in Figure 3B, where the voltage gap between the second discharge plateau and the charge plateau decreased significantly from 0.48 to 0.24 V by replacing the commercial PP separator with the Fe1/NG-modified separator. Fe SAC affords the best electrochemical activity among the transition metals from the above electrochemical measurements, which is reasonable in view of the stronger interaction with polysulfides as shown in Figure 2B. The metal loading also has a critical influence on the CV curves shown in Figure S9. To probe the influence of SACs on the polysulfide conversion chemistry, in situ Raman spectroscopy was performed as a function of discharge/charge voltage. The voltage profiles of the cells with the Fe1/NG-modified separator and the unmodified PP separator at a voltage scan rate of 0.1 mV s−1 are displayed in Figures 3D and S10, in which the corresponding polysulfide species at nine different collection points (A1−A9) are highlighted by the dashed lines. At the initial potential of 2.8 V (A1), Raman signals related to the asymmetric bending, symmetric bending, and symmetric stretching of S8 species at 150.0, 219.2, and 475.0 cm−1, respectively, can be observed for both cells. The transformation of S8 species into S82− (453.8 cm−1), S62− (397.0 cm−1), and S42− (202.1 and 445.3 cm−1) occurs even at a discharged potential of 2.42 V (A2) for the cell with the Fe1/ NG-modified separator, reflecting a promoted kinetics for the conversion of S8 to polysulfide on Fe1/NG SACs. The S8 signals completely vanish at 2.24 V (A3) for Fe1/NG, which is in sharp contrast to that of the cell with the commercial PP

suppresses the polysulfide shuttle effect, the SACs on it catalyzes polysulfide conversion. We first evaluated the polysulfide redox chemistry by cyclic voltammetry (CV). It is known that sulfur is first reduced to soluble lithium polysulfides (Li2Sn, where 4 ≤ n ≤ 8) and then to solid Li2S2 or Li2S during the discharge process. The reversibility of the process in terms of oxidizing the sulfides back to elemental sulfur is typically poor in Li−S batteries.35−40 Two cathodic peaks at 2.20−2.40 and 1.80−2.10 V in Figure 3A are assignable to the reduction of S8 to high-order soluble Li2Sn (4 ≤ n ≤ 8) and their further reduction to insoluble Li2S2/Li2S, respectively. Subsequent oxidative scan in CV results in the individual anodic peaks at 2.25−2.60 V due to the complex oxidation process from Li2S to elemental sulfur. The introduction of M1/NG electrocatalysts on the separator was observed to reduce the voltage gap and led to an increased current density, indicating promoted reaction kinetics for polysulfide reactions. Rather than the monoanodic peak in the unmodified PP or NG-modified separator, all M1/NGmodified separators exhibit two anodic peaks at 2.36 and 2.41 V, which demonstrates better reversibility of redox reactions. No superfluous peak related to side reaction in the electrolyte can be found in CV scans. We also analyzed the variation onset potentials based on the CV results (Figures S14−S19). As listed in Figure 3B, all of the M/NG coupled battery systems show higher onset potentials of cathodic peaks and lower onset potentials of the anodic peaks, compared to those of the batteries coupled with the PP separator or the NG-modified separator. The difference in peak positions and onset potentials is vital evidence to demonstrate the facilitating kinetics of Li−S batteries integrated with the M/NG-modified 25151

DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

Research Article

ACS Applied Materials & Interfaces separator. The Li2S peak at ∼375.0 cm−1 becomes dominant during the second reduction process (polysulfides to Li2S) from 2.10 V (A4) to 1.80 V (A5). Similarly, the signals of S62− and S42− residues could be observed for the PP separator battery, due to the incomplete reduction of sulfur in the cathode at the end-point. The complete sulfur conversion in the cell with the Fe1/NG-modified separator guarantees the full utilization of active materials, which is important for both capacity and cycling performance. In the subsequent charging process from 1.72 V (A6) to 2.60 V (A9), the Raman signals of Li2S, S42−, S62−, S82−, and S8 re-appear. At a fully-charged potential of 2.60 V, S82− species are still visible on the cathode of the PP battery, while such polysulfide signals completely vanish in the Fe1/NG-modified battery. A dual site mechanism may be operational, leading to synergetic interactions: Sn− species prefer to sit at Fe sites while Li+ in the electrolyte is attracted by the N dopants in Fe1/NG, thus offering a higher possibility to form Li−S bonding.34 The fact that Fe SACs are effective catalysts for polysulfide conversion should lead to improved cycle life in the Li−S battery. 2.4. Lithium−Sulfur Battery Performance. Li−S batteries were assembled using standard coin cell techniques with a high sulfur loading of ∼4.5 mg cm−2 at the cathode. Due to the high activity of SACs, an ultralow metal loading on a separator (∼2 μgmetal in each cell) is sufficient to ensure robust Li−S battery operation. Batteries using the Fe1/NG-modified separator outperforms SAC-modified cathodes and benchmark ceramic separators as shown in Figure 4a, and displays higher capacity retention versus cycling as shown in Figure 4b. Lithium-ion transference across the separator is another key parameter that affects battery performance, which is measured using chronoamperometry and electrochemical impedance spectroscopy (EIS) in the symmetrical Li−Li cells. The Li+ transference number of the Fe1/NG-modified separator is almost the same as that of the commercial PP separator while their charge transfer resistances (RCT) are also identical to the Nyquist plot shown in Figure 4c, proving that the lithium ion diffusion across the separator is not impeded by a thin electrocatalyst coating. All of the remaining catalysts show inferior lithium ion transport as shown in Figure S13 and Table S2, which can be the reason for their lower battery performance than the cell with the Fe1/NG-modified separator. Detailed investigations on the scan rates, the Li+ diffusion coefficient, and the polarization profiles in the symmetrical Li2S6-Li2S6 systems are provided in Figures S11 and S12. Owing to the highly efficient polysulfide conversion, the SAC-modified batteries demonstrate superior rate performances as shown in Figure 4d. At a low current density of 0.2C (1C = 1675 mAh g−1), all of the batteries show a high capacity of ∼1200 mAh g−1. In contrast, the battery with the PP separator shows a serious capacity loss at a higher current density. The Fe1/NG-modified battery shows satisfactory capacity retention (∼673 mAh g−1) even at 5C; when the current density is reduced back to 0.2C, a discharge capacity of 1028 mAh g−1 can be recovered, demonstrating highly reversible electrochemical performance at varying current densities. This is further proved by the GCD curves at various rates from 0.2C to 5C. Even at a high current density of 5C, two relatively flat potential plateaus are maintained and the voltage gap only increases slightly, showing the stable electrode reaction in the rapid charge−discharge process.

The long-term cycling performance of the Li−S batteries using the SAC-modified separator was evaluated at a current rate of 0.5C. As displayed in Figure 4f, all of the batteries show an initial specific capacity of ∼1000 mAh g−1. The reversible specific capacities of batteries using separators modified with Fe1/NG, Co1/NG, and Ni1/NG electrocatalysts are 892, 776, and 736 mAh g−1 after 750 cycles, with the corresponding capacity retention of 83.7, 74.8, and 72.1%, respectively (Figure 4g). In contrast, the capacity retentions are much lower for the NG-modified separator (58.3%) and the commercial PP separator (20.4%). The excellent cycling performance can be attributed to the reduced voltage gap, enhanced redox kinetics, and robust catalytic activity during the charge−discharge process. Pictures of the Fe1/NGmodified separator and the PP separator after 20 cycles at 1C has been provided to further verify the catalyst effects. The color of the PP separator turned yellow after cycling, while the Fe1/NG-modified separator still remains tidy. As is shown in SEM images of the Fe1/NG-modified separator after cycling, obvious sulfur particles deposited on the surface of the separator, demonstrating a fully concerting process.

3. CONCLUSIONS Transition-metal SACs were prepared by liquid-phase impregnation on N-doped graphene and coated onto the commercial battery separator for application in high-performance Li−S batteries. The SAC-modified separator plays a dual role in mitigating the polysulfide shuttling effect, and also serves as highly efficient catalysts for the polysulfide redox reaction. Using SAC-modified separator allows highly reversible Li−S battery even at an extremely low metal loading of ∼2 μg in the whole cell. The Li−S battery with the Fe1/NGmodified separator retains 83.7% capacity after 750 cycles at 0.5C, at a high sulfur loading of ∼4.5 mg cm−2, it exhibits an excellent rate performance with a discharge capacity of 673 mAh g −1 at 5C. This study demonstrates that with optimization selection and rational design, there is wide utility space for SACs in electrochemical energy storage devices. 4. EXPERIMENTAL SECTION 4.1. Preparation of the M1/NG-Modified Separator. 200 mg of GO was first dispersed in 400 mL of deionized water under stirring and sonication to achieve a homogeneous dispersion. An aqueous solution of 0.0675 M FeCl3 was added into the GO solution followed by sonication. The mixture was lyophilized to form a threedimensional GO foam with uniformly distributed Fe ions and then annealed at 750 °C under Ar (100 sccm) and NH3 (30 sccm) at room pressure for simultaneous N doping and GO reduction to obtain Fe1/ NG powders. To avoid aggregation of metal atoms, during the fabrication process, graphene- and Fe-containing salts were added into the solution and treated with mechanical stirring and ultrasonication to ensure monodispersion. Second, according to the reported work, further reduction treatment by NH3 at 750 °C provide plenty of nitrogen dopants into the graphene lattice that may serve as the effective binding sites to form Fe−N−C species, which will be an important factor to avoid large aggregated Fe particles. Besides, Ar was used to avoid the oxidation of Fe atoms, which may also cause a reunification process.37,38 After this, a certain amount of Fe1/NG, super P, and poly(vinylidene fluoride) (PVDF) were mixed at a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone to form a uniform slurry and then pasted on a polypropylene (PP) separator (Celgard 2400). The Fe1/NG-modified separator was dried at 50 °C under vacuum and cut into the electrode with a diameter of 1.9 cm. The mass loading of Fe1/NG in the separator is ∼0.1 mg cm−2. Co1/NG 25152

DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

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ACS Applied Materials & Interfaces and Ni1/NG-modified separators were prepared by replacing FeCl3 with CoCl2 or NiCl2, respectively. 4.2. Li−S Battery. 2032-type Li−S batteries were assembled using Li metals as the counter/reference electrode, the Li2S6 catholyte as the active material, carbon nanofiber paper as the cathode collector, and the PP or modified separator, where the sulfur loading is based on the sulfur in Li2S6 solution. For ∼4.5 mg cm−2 sulfur loading (5 mg sulfur in cathode, 1.2 cm diameter) in this paper, 26 μL of Li2S6 solution was used in every battery. Then we add 10 μL of electrolyte in the cathode (sulfur) side and 15 μL in the anode (lithium) side. Thus, the ratio of electrolyte and sulfur is ca. 10:1. The electrolyte was prepared by 1 M lithium bistrifluoromethane-sulfonylimide in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1, v/v) with 0.1 M LiNO3 additive. GCD was measured on a CT2001A cell tester (LAND BT2013A) at different current densities from 1.7 to 2.8 V (vs Li+/Li). CV was collected on an electrochemical workstation (Solartron analytical 1400E) at different scan rates from 1.7 to 2.8 V. EIS was measured in the frequency range of 100 kHz to 0.01 Hz with an AC perturbation voltage of 5 mV. 4.3. In Situ Raman Measurements. The Raman spectra were collected on an inVia confocal Raman microscope using a 532 nm laser under ambient conditions in a button cell with a central hole sealed by a piece of glass in the cathode side. The cell was first charged to 2.8 V. Every 2 min during the discharging−charging cycling of the Li−S cell, the Raman patterns were recorded until the end of the cycle. The microscope objective was of 50× magnification. 4.4. Material Characterization. TEM (Talox-F200X), SEM (FEI Tecnai G2 F30), HAADF-STEM (the aberration-corrected JEOL ARM-200F system equipped with a cold field emission gun and an ASCOR probe corrector at 60 kV), XRD (X’Pert PRO, Cu Kα), and XPS (ESCALAB 250xi). XANES and EXAFS were performed at the XAFCA beamline of the Singapore Synchrotron Light Source (SSLS) in the transmission mode.31 100 mg of sample was first ground into fine powder using a mortar and pestle before being pressed into a 10 mm pellet. Data analysis were carried out on Athena (version 0.9.23).33

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Zhongxin Chen: 0000-0001-6153-5381 Xing Li: 0000-0002-5470-1043 Xiaoxu Zhao: 0000-0001-9746-3770 Kai Leng: 0000-0003-3408-5033 Kian Ping Loh: 0000-0002-1491-743X Author Contributions ∇

K.Z., Z.C., and K.X. conceived the research and wrote the draft. K.Z. and R.N. synthesized the materials and tested the battery reactions. Z.C., X.Z., C.L., and S.J.P. conducted STEM characterization and data analysis. S.X., Y.D., X.C., and Z.C. performed the EXAFS, XANES, XAS measurements and analysis. H.L. and H.X. conducted the DFT calculations. Z.R., C.S., W.T., X.W., and K. L. assisted with materials characterization and data analysis. H.X, K.P.L., and K.X. supervised the research. All authors discussed and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.P.L. thanks the National Research Foundation, Singapore for the NRF Investigator Award: “Graphene oxide a new class of catalytic, ionic, and molecular sieving materials, award number: NRF-NRF12015-01”. K.X. acknowledges the financial support from the National Natural Science Foundation of China (51674202), the Outstanding Young Scholars of Shaanxi (2019JC-12), the Fundamental Research Funds for the Central Universities (19GH020302), and the TOP International University Visiting Program for Outstanding Young Scholars of Northwestern Polytechnical University.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05628.

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K.Z. and Z.C. contributed equally to this work.

Author Contributions

REFERENCES

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HAADF-STEM images and corresponding structural characterization of M1/NG, high-resolution Fe2p, Co2p, and Ni2p XPS spectra of M1/NG before and after the adsorption test, Brunauer−Emmett−Teller areas and pore size of M1/NG, SEM images of separators and cathodes, UV−vis spectra after Li2S6 adsorption, adsorption structures and adsorption energies of Li2S6 and Li2S4 on M1/NG, CV profiles of Li−S batteries with different Fe loadings, in situ Raman spectra of the Li−S cell with PP separator, CV of Li−S batteries at different scan rates, CV curves of symmetrical Li2S6−Li2S6 cells at different scan rates, chronoamperometry curves of Li−Li batteries, elemental composition of NG and M1/NG, comparison of Li+ diffusion coefficient values, comparison of Li + transference number, references of comparison paper in Figure 4a,b (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.X.). *E-mail: [email protected] (K.P.L.). *E-mail: [email protected] (K.X.). ORCID

Kun Zhang: 0000-0002-6877-8068 25153

DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.9b05628 ACS Appl. Mater. Interfaces 2019, 11, 25147−25154