Synergistic Effect of Molecular-Type Electrocatalysts with Ultrahigh

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Synergistic Effect of Molecular-Type Electrocatalysts with Ultrahigh Pore Volume Carbon Microspheres for Lithium-Sulfur Batteries Won-Gwang Lim, Yeongdong Mun, Ara Cho, Changshin Jo, Seonggyu Lee, Jeong Woo Han, and Jinwoo Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02258 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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Synergistic Effect of Molecular-Type Electrocatalysts with Ultrahigh Pore Volume Carbon Microspheres for Lithium-Sulfur Batteries

Won-Gwang Lim, Yeongdong Mun, Ara Cho, Changshin Jo, Seonggyu Lee, Jeong Woo Han, and Jinwoo Lee*

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang 37673, Gyeongbuk, Republic of Korea

KEYWORDS: Molecular-type electrocatalysts, carbon microsphere, enhanced kinetics, high electrode density, lithium-sulfur batteries

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ABSTRACT

Lithium-sulfur (Li-S) batteries are regarded as potential high-energy storage devices due to their outstanding energy density. However, the low electrical conductivity of sulfur, dissolution of the active material, and sluggish reaction kinetics cause poor cycle stability and rate performance. A variety of approaches have been attempted to resolve the above issues and achieve enhanced electrochemical performance. However, inexpensive multifunctional host materials which can accommodate large quantities of sulfur and exhibit high electrode density are not widely available, which hinders the commercialization of Li-S batteries. Herein, mesoporous carbon microspheres with ultrahigh pore volume are synthesized, followed by the incorporation of FeN-C molecular catalysts into the mesopores, which can act as sulfur hosts. The ultrahigh pore volume of the prepared host material can accommodate up to ~87 wt.% sulfur while the uniformly controlled spherical morphology and particle size of the carbon microspheres enable high areal/volumetric capacity with high electrode density. Furthermore, the uniform distribution of Fe-N-C (only 0.33 wt.%) enhances the redox kinetics of conversion reaction of sulfur and efficiently capture the soluble intermediates. The resulting electrode with 5.2 mg sulfur per cm2 shows excellent cycle stability and 84% retention of the initial capacity even after 500 cycles at a 3 C rate.


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Sulfur is considered a promising cathode material due to its low cost, high theoretical capacity (1,675 mA h g-1), and non-toxicity.1,2 When sulfur is paired with metallic lithium, the theoretical energy density reaches values as high as 2,500 W h kg-1.3 Therefore, lithium-sulfur (Li-S) batteries have received worldwide attention as next-generation batteries. Despite their beneficial features, the low electrical conductivities of sulfur and lithium sulfides (Li2S or Li2S2), dissolution of lithium polysulfides (Li2Sx, 2 < x ≤ 8) (the so-called shuttle effect), and slow redox kinetics of the active materials impedes the practical use of Li-S batteries.4-6 Even though porous carbons have been extensively used as host materials for sulfur,5, 7-16 the weak binding of nonpolar carbon surfaces with polar LiPS limits the further improvement in the electrochemical performance of the developed batteries. To overcome this issue, a variety of polar LiPS adsorbents, such as metal oxides (TiO2, MnO2, NiFe2O4, SiO2, Ti4O7)17-24, metal sulfides and nitrides (NiS, WS2, TiS2, C3N4)18, 25-29 are composited with carbon-based materials to reduce the shuttle effect. Moreover, very recently, it was reported that catalytic materials enhance the redox kinetics of LiPS, thus resulting in a decrease in the charge transfer resistance and an increase in the reversible capacity. Hence, a large number of efforts have been devoted to developing effective catalysts and combining them with carbonaceous host materials.30-32 However, in spite of the extensive research on composite catalysts with carbonaceous materials in Li-S batteries, they are limited by insufficient view for practical use in Li-S batteries.33 For example, Arava and co-workers reported that Pt nanoparticle-decorated graphene led to 40% enhancement in the specific capacity over bare graphene due to the electrocatalytic effect of Pt in converting short-chain LiPS to long-chain LiPS.31 However, Pt is a high-cost noble metal and sulfur species could be adsorbed on the surface of the metallic catalyst,34 resulting in a loss of active sites. In another instance, Donghui Long and co-workers used Nb2O5


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nanocrystal-decorated mesoporous carbon as the host material and found that Nb2O5 accelerated the reduction of soluble LiPS into insoluble lithium sulfides.30 Despite improved capacity retention at high current densities, insufficient pore volume (~ 2.9 cm3 g-1) limited the amount of sulfur that could be loaded to ~ 60 wt.%. Moreover, the high content of Nb2O5 (~ 9.5 wt.%) is electrochemically inactive in the potential range of Li-S batteries, and thus cannot contribute to capacity enhancement. In most of the previous studies on composites of catalysts with carbon hosts, the content of the catalytic material was generally high; different contents of catalysts – 9.3 wt.%,35 11.3 wt.%,36 and 38.6 wt.%.37 – have been reported. When the electrode configuration is considered, such catalysts significantly decrease in both volumetric and gravimetric energy densities. Therefore, it is desirable to minimize the content of the catalytic material in the composite, and at the same time develop highly effective low-cost catalysts for Li-S batteries. In this context, a clue can be found in industrially important reactions. In the fuel cell field, a number of studies were conducted to replace expensive Pt with low cost non-noble metal catalysts to improve the kinetics of the oxygen reduction reaction (ORR).38 The incorporation of metal into heteroatom-doped carbon led to a large enhancement in the ORR activity.39 In particular, Fe-N-C sites, in which a single Fe ion is ligated to the N functionalities at the edge of the carbon basal plane, have attracted much attention and are considered as promising catalysts for ORR due to their high performance and low cost.40,41 Furthermore, an outstanding ORR activity was achieved at extremely low loadings of Fe and N in a moleculartype Fe-N-C site.42 Herein, we report the electrocatalytic effect of a molecular-type Fe-N-C site which has been extensively studied in ORR, on the conversion reaction of LiPS in Li-S batteries. Ultrahigh pore volume mesocellular carbon foam with a spherical morphology (S-MCF) is prepared and the Fe-


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N-C site is incorporated into S-MCF via a simple synthetic route (Fe-N-C/S-MCF). The assynthesized Fe-N-C/S-MCF is utilized as the host material for sulfur owing to which the following changes could be observed in Li-S batteries. i) In addition to the high electrical conductivity of S-MCF, its high surface area (1,267 m2 g-1) increases the number of available FeN-C active sites while the ultrahigh pore volume (3.5 cm3 g-1) allows a theoretical loading of sulfur up to 87.9 wt.%. ii) A higher electrode density is feasible due to the spherical morphology and micron-size (5 µm) of Fe-N-C/S-MCF particles compared with that of a non-homogeneous morphological material or aggregated nanoparticles. iii) Fe is naturally abundant, non-toxic and inexpensive.43 Most importantly, Fe-N-C sites have strong chemical interaction with soluble LiPS and can enhance the redox kinetics of LiPS. The catalytic effect and high adsorption ability of Fe-N-C sites with only 0.33 wt.% decoration on the pores of S-MCF enables greater reversible capacity and rate capability compared to bare S-MCF.


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RESULTS AND DISCUSSION

The synthesis of Fe-N-C/S-MCF is schematically illustrated in Figure 1 along with its effect as a host material for sulfur. S-MCF with controlled morphology and size is synthesized from using spherical mesocellular siliceous foam (S-MSF) as the template; subsequently, Fe-N-C sites are incorporated into the pores of S-MCF using a simple impregnation method. After sulfur is impregnated into Fe-N-C/S-MCF by melt-diffusion, it is directly used as a high density cathode for Li-S batteries. The morphologies of the as-prepared S-MSF and S-MCF are characterized by SEM and TEM. As shown in Figure S1a and b, S-MSF exhibits a spherical morphology and uniform particle size (~ 5 µm). A similar morphology and particle size distribution are observed in S-MCF (Figure 2a and b); further, a wormhole-like mesoporous structure is also well developed (Figure 2c). To further investigate the porous structure of S-MSF and S-MCF, N2 physisorption analysis was performed. Both S-MSF and S-MCF exhibit Type V N2 physisorption isotherms and the sudden increase in the peaks at ~0.9 P/Po indicates the presence of uniform large pores in the samples.44 The BET surface area and pore volume of S-MSF are 516 m2 g-1 and 2.2 cm3 g-1, respectively, while the mesopores are ~ 31 nm in size (Figure S1c and d). On the other hand, S-MCF exhibits a large BET surface area of 1,267 m2 g-1 with 4 nm- and 30 nmsized mesopores (Figure 2d); the small 4 nm pores are generated due to the dissolution of the silica template wall.45 Most importantly, the pore volume of S-MCF calculated at ~0.99 P/Po is as high as 3.5 cm3 g-1. The ultrahigh pore volume of S-MCF enables us to accommodate a large amount of sulfur (~87.9 wt.%, based on the density of alpha phase sulfur) into the pore.46 In order to confirm the effect of controlled spherical morphology of S-MCF, a mesocellular carbon foam with non-homogeneous sized particle and morphology (MSU-F-C) was prepared (Figure S2).45 As the tap density correlates with the electrode density, the tap densities of MSU-


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F-C and S-MCF were estimated with packed particles based on the mass of each sample (Figure 3a). The tap density of S-MCF is about two times higher than that of MSU-F-C; the high tap density of S-MCF is attributed to the uniformly controlled spherical morphology of the particles. To compare the electrode density of S-MCF and MSU-F-C, 60 wt.% of sulfur is composited with each sample and the areal density of sulfur on each electrode is unified to 1.0 ~ 1.2 mg cm-2 (Figure S3). As shown in Figure 3b and c, the electrode of MSU-F-C is ~15.2 µm thick and several cracks can be observed on it due to lack of uniformity in the size and morphology of the particles. On the contrary, the electrode of S-MCF exhibits a crack-free surface and a thickness of ~10.9 µm (Figure 3d and e). The high electrode density of S-MCF is attributed the densely packed micron-sized spherical particles; this observation is consistent with the tap density results. The gravimetric and volumetric capacities of S-MCF and MSU-F-C are evaluated by a galvanostatic test at 0.5 C rate at an areal density of ~2.5 mg cm-2. In the case of gravimetric capacity, MSU-F-C exhibits capacity of 1,128 mA h g-1 in the first cycle and 513 mA h g-1 in the 100th cycle (Figure 3f, S4a and b). However, S-MCF exhibits a charge capacity of 1,132 mA h g1

in the first cycle and a capacity of 750 mA h g-1 is retained even after 100 cycles. Moreover, the

volumetric capacity of S-MCF is approximately two times higher than that of MSU-F-C due to its high packing density (Figure 3g, S4c and d). S-MCF with its uniform morphology and particle size enables the fabrication of stable electrodes with a high electrode density (0.4458 g cm-3 of S-MCF vs 0.2945 g cm-3 of MSU-F-C); further, it leads to an enhanced cycle stability, and higher volumetric capacity. To create a synergistic effect, molecular type Fe-N-C catalytic site is incorporated in the SMCF. The weight percent of Fe in Fe-N-C/S-MCF is calculated to be 0.33 wt.% by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Scanning transmission electron


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microscopy dark-field images of composite of Fe-N-C/S-MCF with sulfur show the presence of micron-sized spherical particles in the substrate (Figure 4a). The corresponding EDS mapping images indicate the homogeneous distribution of C, N, Fe, and S elements across the entire particle and it can be seen that sulfur and Fe-N-C catalytic sites are uniformly incorporated in SMCF (Figure 4b-f). Furthermore, nitrogen functionalities of Fe-N-C/S-MCF are characterized using the N1s spectrum generated by XPS. The N1s spectrum of Fe-N-C/S-MCF is deconvoluted into pyridinic, pyrrolic, and graphitic N sites (Figure S5). Of the total quantity of nitrogen functionalities in Fe-N-C/S-MCF, 81.2% is attributed to pyridinic and pyrrolic N sites. The abundant presence of these N functionalities enables Fe ions to be coordinated as the pyridinic and pyrrolic N sites are positioned at the edges of the carbon basal plane. Additionally, two peaks observed at 711.0 and 725.0 eV in the Fe 2p region suggest the existence of coordination between Fe and N without any metallic Fe (Figure S6).47 Further, the chemical state and coordination symmetry of the Fe-N-C sites are characterized by XAFS. The X-ray absorption near-edge structure (XANES) spectra of the reference Fe foil and Fe-N-C/S-MCF are exhibited in Figure 4g. Firstly, the white-line in the spectrum of Fe-N-C/SMCF is of greater intensity and the peak position shifted to higher energy levels compared to the Fe foil. This suggests that Fe sites coordinated with the N sites in Fe-N-C/S-MCF exhibit a valence oxidation state rather than a metallic (Fe (0)) state. Secondly, a small peak at 7,114 eV is observed in the pre-edge region of Fe-N-C/S-MCF (Figure 4g, blue dotted line). This peak is attributed to the transition of 1s → 3d pre-edge features with dipole forbidden, but quadrupole allowed. This peak implies that the coordination geometry of Fe in Fe-N-C/S-MCF is a square pyramidal structure with the Fe center positioned to out-of-square planar.48 The local geometry around Fe in Fe-N-C/S-MCF is further investigated by extended X-ray absorption fine structure


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(EXAFS) analysis. Figure 4h shows the Fourier-transformed (FT) EXAFS spectra of the Fe foil and Fe-N-C/S-MCF. In the FT spectrum of the Fe foil, the main peak generated by back scattering from the first coordination shell is observed at 2.2 Å, which indicates a Fe-Fe metallic bond.49 On the other hand, the corresponding peak at 2.2 Å in the FT spectrum of Fe-N-C/SMCF, disappears and new sharp peaks appear at 1.4 Å and 2.4 Å. The first shell peak at 1.4 Å in Fe-N-C/S-MCF is attributed to the Fe-N coordination bond and the second shell peak at 2.4 Å corresponds to the bonding distance between Fe and C, located very close to N. The EXAFS results indicate that there is no aggregation of Fe metallic particles in Fe-N-C/S-MCF and Fe single ions are well coordinated with N functionalities. Moreover, X-ray diffraction (XRD) pattern of Fe-N-C/S-MCF is similar with S-MCF (Figure S7), which indicates the exclusion of formation of another crystalline phase such as iron nitride. To investigate the electrochemical performance, 60 wt.% and 80 wt.% of sulfur are composited with S-MCF and Fe-N-C/S-MCF and electrodes with different areal densities (2.5 and 5.2 mg cm-2) are prepared. The rate performance of each electrode with 2.5 mg cm-2 areal density is measured by a repetitive galvanostatic discharge/charge process at different current densities from 0.1 C rate to 5 C rate stepwise for every 5 cycles (Figure 5a); the corresponding voltage profiles are shown in Figure S8a and b. The charge capacities of Fe-N-C/S-MCF electrode in the 5th cycle in each step are 1,244, 1,128, 1,012, 948, and 798 mA h g-1 at 0.1, 0.2, 0.5, 1, and 5 C rate, respectively. The charge capacities of the S-MCF electrode are 1,037, 892, 794, 731, and 504 mA h g-1 at 0.1, 0.2, 0.5, 1, and 5 C rate, respectively. In comparison with the charge capacities at 0.1 C rate, the Fe-N-C/S-MCF electrode retains 64% of the capacity even at 5 C rate, but only 48% is retained by the bare S-MCF electrode. Especially, the degree of polarization is much more severe in S-MCF at high current densities (Figure 5b). The


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polarization potentials of S-MCF are 160 and 389 mV at 0.1 and 1 C rate, respectively. The drastic increase in the polarization potential (~240%) at 1 C rate is attributed to the slow kinetics of conversion of LiPS. On the other hand, Fe-N-C/S-MCF exhibits polarization potentials of 120 and 219 mV at 0.1 and 1 C rate, respectively. In spite of its current density being ten times higher, changes in the polarization potential in Fe-N-C/S-MCF are insignificant due to the enhancement in reaction kinetics by the Fe-N-C sites. In order to validate the electrocatalytic effect of Fe-N-C sites, the cyclic voltammograms (CVs) of the Fe-N-C/S-MCF electrode are measured at a low scan rate of 0.1 mV s-1 and the recorded current is normalized by the amount of sulfur in the electrode (Figure 5c). The CVs of both FeN-C/S-MCF and S-MCF exhibit two cathodic anodic peaks corresponding to the redox reaction of sulfur. The first cathodic peak corresponds to the reduction of sulfur to high order LiPS, while the second cathodic peak reflects further reduction of high order LiPS to low order lithium sulfides; in S-MCF, the first and second peaks are positioned at 2.25 and 2.00 V, respectively. However, the presence of Fe-N-C shifts the peaks to 2.28 and 2.02 V along with higher current delivery. A similar tendency is observed in the anodic peaks generated by the oxidation of lithium sulfides to elemental sulfur. The two anodic peaks positioned at 2.35 and 2.4 V in the CVs of the S-MCF electrode negatively shifted to 2.32 and 2.38 V in the case of Fe-N-C/S-MCF. The peak separation between the second cathodic and anodic peaks is 0.35 V in the case of SMCF, while it is 0.30 V in the case of Fe-N-C/S-MCF, which implies that polarization is improved. The positive shift in the cathodic peaks, negative shift in the anodic peaks, and increased recorded current in the Fe-N-C/S-MCF electrode reflect the catalytic activity of the FeN-C sites on the conversion reaction of sulfur. Moreover, the Tafel plots of the two electrodes are derived from their CVs by considering the ohmic drop potential, which is measured by EIS


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(Figure 5d). During the reduction reaction of LiPS, the Fe-N-C/S-MCF electrode exhibited a smaller Tafel slope and lower overpotential compared to the bare S-MCF electrode. Even though the Tafel slopes of both electrodes are similar in the case of the oxidation reaction, the overpotential in Fe-N-C/S-MCF is much lower than in S-MCF. The enhanced kinetics of Fe-N-C/S-MCF electrode is also proved by measuring the current with increasing the scan rates in CVs (Figure S9). When scan rates change from 0.2 to 2.0 mV s-1, S-MCF electrode shows the severe polarization due to sluggish reaction kinetics but Fe-N-C/SMCF electrode exhibits much lower degree of polarization. Moreover, reaction kinetics of each electrode is further characterized by below power law equation: i = avb where the i indicates measured current, v indicates the scan rate in CVs. To calculate the b-value of each electrode, the plot of log (i) versus log (v) is derived (Figure S10). During the discharge process, the b-value of Fe-N-C/S-MCF electrode is 0.81 and 0.51 in peak 1 and 2, respectively. Meanwhile, S-MCF electrode exhibits the b-value of 0.71 and 0.48 in peak 1 and 2, respectively. During the charge process (peak 3), the b-value of Fe-N-C/S-MCF and S-MCF is 0.70 and 0.51. The b-value is highly correlated to reaction rate; b=0.5 indicates the reaction is affected by semiinfinite linear diffusion, and b=1.0 indicates the reaction is affected by surface-control. Higher bvalue which is nearer to 1.0 in Fe-N-C/S-MCF electrode clearly represents that Fe-N-C sites accelerate the conversion reaction of LiPS.30 The cycle performance of Fe-N-C/S-MCF and S-MCF with an areal density of 2.5 mg cm-2 is tested at 0.5 C rate (Figure 5e). The initial discharge and charge capacities of the Fe-N-C/S-MCF electrode are 1,631 and 1,543 mA h g-1, which are extremely close to the theoretical capacity of sulfur; on the other hand, the initial discharge and charge capacities are 1,280 and 1,132 mA h g-1


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in the case of the bare S-MCF electrode. After 100 cycles, the charge capacities of Fe-N-C/SMCF and S-MCF electrodes are 1,064 and 750 mA h g-1, respectively. The Fe-N-C/S-MCF electrode exhibits 73% greater charge capacity in the first cycle and 70% higher charge capacity in the 100th cycle compared to the bare S-MCF electrode. The initial coulombic efficiency is improved to 95% in the Fe-N-C/S-MCF electrode but it is only 88% in the S-MCF electrode. The electrocatalytic effect of the Fe-N-C site during the repetitive conversion reaction of sulfur contributes to the greater participation of LiPS and increases the reversible capacity of the electrode. Especially, the polarization potential of S-MCF is 250 mV while that of Fe-N-C/SMCF is 190 mV (Figure S11a and b). The lower polarization potential of Fe-N-C/S-MCF electrode is consistent with the CVs results. The Fe-N-C sites, homogeneously distributed in FeN-C/S-MCF, promote the kinetics of the conversion reaction of sulfur and decrease the overpotential in the cathode. To further investigate the electrode surfaces of Fe-N-C/S-MCF and S-MCF, we disassembled the coin cells after 50 cycles and performed ex-situ SEM analysis (Figure S12). The spherical morphologies of both electrodes are well-preserved during cycling, but polymer-like phase formed on the surface of carbons. This phase may consist of solid state Li2S2 or Li2S, which may be attached during the further lithiation of dissolved soluble lithium polysufdies. The cycled S-MCF electrode is covered by irregular large insoluble products (1 ~ 3 µm) because sluggish kinetics of reaction and poor affinity with LiPS cause the aggregation of insulating products (Figure S12a-f). On the other hand, the cycled Fe-N-C/S-MCF electrode partially shows smaller particle (~500 nm), indicating that higher affinity with LiPS and catalytic effect of Fe-N-C site prevent the aggregation of insulating products on the outer surface. As a result, Fe-N-C/S-MCF shows enhanced utilization of LiPS (well-matched with the cycle performance) (Figure S12j-l). To analyze the high energy density and long term cycle stability of


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the Fe-N-C/S-MCF electrode at high current densities, a high amount of sulfur (80 wt.%) is loaded into a Fe-N-C/S-MCF electrode (Figure S3). The distribution of sulfur throughout the particle is investigated by linear EDS analysis (Figure S13). Even after the impregnation of huge sulfur (80 wt.%) into Fe-N-C/S-MCF, sulfur is homogeneously distributed from outer to deep inner part due to the ultrahigh pore volume of particle. The prepared electrode has a sulfur areal density of ~5.2 mg cm-2 and it was later tested at 3 C rate for 500 cycles (Figure 5f). The slight increase in capacity over dozens of cycles is attributed to the full activation of the high-loading electrode. The Fe-N-C/S-MCF electrode with high sulfur loading exhibits a specific capacity of 384 mA h g-1 and an areal capacity of 2.0 mA h cm-2. A maximum specific capacity of 483 mA h g-1 and areal capacity of 2.5 mA h cm-2 could be achieved. More notably, the charge capacity of Fe-N-C/S-MCF fades by 0.0038% per cycle. A specific capacity of 322 mA h g-1 and an areal capacity of 1.7 mA h cm-2 are retained even after 500 cycles, resulting in 84% retention of the initial capacity. Even though mass ratio of Fe in Fe-N-C/S-MCF is only 0.33 wt.% in the composite, Fe single ion is sum-nanometer, Å scale. Thus, small amount of Fe-N-C site can cover the large surface of mesoporous carbon and is fully utilized as catalytic site. It results in outstanding performance with ~5.2 mg cm-2 of sulfur. It is supposed that outstanding performance of Fe-N-C/S-MCF electrode is attributed to not only the catalytic effect of Fe-N-C sites but also the strong adsorption ability with soluble LiPS, so resultingly shuttle effect is efficiently suppressed. The improvement of adsorption strength with LiPS is investigated by theoretical calculation of binding energy between soluble LiPS (Li2S8, Li2S6, and Li2S4) and the surface of Fe-N-C sites. For the comparison, supercells of both Fe-N-C embedded graphene and bare graphene are geometrically optimized (Figure S14). As shown in Figure 6a, soluble LiPS is chemically interacted with Fe-N-C sites but not interacted


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with the bare graphene. As a result, adsorption energy (Eads) of all of LiPS on the Fe-N-C sites is higher than on the bare graphene (Figure 6b). The high Eads of LiPS on the Fe-N-C sites is attributed to chemical interaction between Li+ of LiPS and N of Fe-N-C sites, also between S- of LiPS and Fe ion of Fe-N-C sites. Especially, Eads of S- of LiPS in Fe-N-C sites is -4.18 eV but only -1.89 eV in bare graphene (Figure 6c). Furthermore, static adsorption test of Fe-N-C/SMCF and S-MCF was performed using Li2S8 solution to experimentally investigate the affinity with LiPS (Figure 6d). The resulting photographs exhibit that Li2S8 solution is decolored by FeN-C/S-MCF due to the strong interaction between LiPS and Fe-N-C site, while it is remained without any change of color by bare S-MCF. It indicates that Fe ion which is coordinated with N atom in Fe-N-C sites is strongly interacted with soluble LiPS and effectively mitigates the dissolution problem during cell operation.


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CONCLUSION

To further the development of Li-S batteries for practical applications, we developed micronsized spherical mesoporous carbon decorated with Fe-N-C sites, a molecular catalyst. The ultrahigh pore volume of Fe-N-C/S-MCF, along with the uniformly controlled particle size and morphology enables high electrode density and improved areal/volumetric capacity. Furthermore, the uniformly distributed Fe-N-C catalytic sites in Fe-N-C/S-MCF enhance the redox kinetics of LiPS and the presence of only 0.33 wt.% Fe-N-C site significantly decreases the overpotential. Additionally, Fe which is coordinated with N functionalities as a single ion has strong adsorption ability with soluble LiPS to effectively suppress the shuttle effect. The synergistic effect of the mesoporous carbon structure and molecular type Fe-N-C catalyst results in an outstanding performance. We therefore believe that a variety of catalysts used in other research fields could be potentially applied in Li-S batteries.


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EXPERIMENTAL SECTION

Materials. Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123, Mn ~5,800), tetraethyl orthosilicate (98%), ammonium fluoride (NH4F, 98%), tetraethylene glycol dimethyl ether (TEGDME, 99%), furfuryl alcohol (98%), carbon disulfide (CS2, 99.9%), and iron(II) phthalocyanine (dye content ~90%) were purchased from Sigma-Aldrich. 1,3,5,trimethyl benzene (mesitylene, 98%) was obtained from Merck Millipore and aluminum(III) chloride hexahydrate (AlCl3, 98%) was purchased from Kanto Chemical Co. All the chemicals were used as received without further purification. Synthesis of spherical mesocellular siliceous foam (S-MSF). Spherical MSF (S-MSF) was produced using a slightly modified version of a process reported earlier.50,51 In a typical synthesis procedure, 8 g of P123 and 10 g of KCl were dissolved in a mixture of 20 mL of HCl, 10 mL of ethanol, and 130 mL of deionized water. To this solution, 9.26 mL of mesitylene was added. After stirring for 2 h at 40 °C, 18.4 mL of tetraethyl orthosilicate was added to the mesitylene-added solution with stirring over 5 min; subsequently, it was kept without stirring for 20 h at 40 °C. NH4F (0.092 g) was added with stirring for 5 min and the prepared solution was aged for 24 h at 100 °C in an oven. The as-prepared product was further calcined at 550 °C for 4 h in ambient condition. Synthesis of spherical mesocellular carbon foam (S-MCF). The prepared S-MSF was used as a template for the synthesis of S-MCF. Typically, 0.21 g of AlCl3·6H2O was dissolved in ethanol to which 1 g of S-MSF was added with stirring. After evaporating all the ethanol, the aluminum-incorporated S-MSF powder was calcined at 550 °C for 4 h in air. A mixture of furfuryl alcohol and TEGDME was blended with aluminum-treated


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S-MSF and the blend was held for 1 h in static vacuum. The prepared powder was further held at 85 °C for 8 h and carbonized at 850 °C for 3 h in an argon atmosphere. Finally, S-MCF was collected after etching the template with HF. Synthesis of Fe-N-C/S-MCF and impregnation of sulfur. Fe-N-C/S-MCF was prepared by a simple impregnation method. In a typical procedure, iron (II) phthalocyanine was homogeneously mixed with S-MCF and heat-treated at 700 °C for 1 h in an Ar atmosphere. The prepared powder was added to a 1 M HCl solution and stirred for 5 h to etch the metallic compound. After filtering and washing with deionized water, the powder was heat-treated again up to 700 °C in an Ar atmosphere and held at that temperature for 20 min in an ammonia atmosphere. The as-synthesized Fe-N-C/S-MCF and S-MCF were composited with sulfur by a common melt-diffusion method at 155 °C using CS2 as the solvent. Synthesis of Li2S8 solution and static adsorption test. As a representative of soluble LiPS, Li2S8 solution was synthesized by chemically mixing the sulfur (S8) and Li2S in 1,4-dioxane (99.8%, Aldrich) in glove box. The static adsorption test was performed by adding S-MCF and Fe-N-C/S-MCF to prepared Li2S8 solution, followed by stirring for 1 h. The added amount of each host material is calculated based on the measured BET surface area. Material characterization. The structures of S-MSF, S-MCF, and Fe-N-C/S-MCF were analyzed by scanning electron microscopy (S-4200 field emission SEM, Hitachi) and transmission electron microscopy (TEM; JEM-1011, JEOL Ltd.). Nitrogen (N2) physisorption analysis was conducted at 77 K using a Micromeritics Tristar II 3020 system. X-ray photoelectron spectroscopy (XPS) data were obtained using a VG Scientific Escalab 250 (Al Kα) instrument. Energy dispersive spectroscopic


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(EDS) mapping was performed to analyze the elemental composition of the electrodes using a high-resolution

TEM

(HR-TEM;

2100F

with

Cs-corrected

STEM)

instrument.

Thermogravimetric analysis (TGA, Perkin Elmer, USA) was conducted to analyze the amount of sulfur in the composited samples. X-ray adsorption fine structure (XAFS) measurements of the Fe K-edge of Fe-N-C/S-MCF were performed on the 8C XAFS beamline at Pohang Light Source II using a top-up mode operation (3.0 GeV, 300 mA). Electrochemical measurement. To fabricate working electrodes, a slurry was made by mixing the composite of sulfur with SMCF or Fe-N-C/S-MCF, conductive carbon additive (Super P), and a polymeric binder (polyvinylidene difluoride, PVDF) in a weight ratio of 8:1:1 using N-methyl-2-pyrrolidone (NMP) as the solvent. The prepared slurry was coated on an Al foil and dried at 60 °C for 8 h. Later, the electrode was pressed and cut into the shape of a coin. The average mass loading of the active materials was controlled in the range of 1.5 mg cm-2 to 5.2 mg cm-2. The electrochemical performance of the sulfur host materials (S-MCF and Fe-N-C/S-MCF) was analyzed using a coin-type cell (CR2032) and lithium metal was used as both reference and counter electrodes. The polymeric porous membrane (polypropylene, Welcos Ltd) was used as separator. A solution containing 1.0 M bis(trifuloromethane) sulfonamide lithium salt (LiTFSI) in a mixed solvent of dimethoxyethane and 1,3-dioxolane (DME/DOL, 1:1 volume ratio, PANAX E-TEC Co., Korea) with 2 wt.% of lithium nitrate (LiNO3, 99.99% metal basis, Sigma-Aldrich) as an additive was used as the electrolyte. The amount of electrolyte was fixed at 20 µL for 1mg of sulfur in all the measurements. Galvanostatic charge-discharge analysis was performed in the potential range of 1.7 V to 3.0 V (vs. Li/Li+) at different current densities in the range of 0.1 to 5 C rate (1 C rate ~ 1,675 mA g-1). The capacity of the electrode was calculated based on the weight of sulfur in the


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electrode. Cyclic voltammetry (CV) analysis was conducted at a scan rate of 0.1 mV s-1 from 1.7 V to 3.0 V (vs. Li/Li+). Electrochemical impedance spectroscopy (EIS) was conducted using a potentiostat (Reference 600, Gamry Instruments, USA) in the frequency range of 0.05 to 105 Hz at a magnitude of 5 mV. Tafel plots were derived from the measured CV curves and EIS results. To perform ex-situ SEM analysis, the coin cells were disassembled in Ar-filled and moisturecontrolled glove box. The disassembled electrodes were characterized after being washed by dimethyl carbonate, anhydrous (DMC, 99.0%, Aldrich), followed by complete drying. Computational details. All calculations were performed using the the Vienna ab initio simulation package (VASP) with the projector augmented wave (PAW) method.52,53 The electron exchange and correlation were described by the Perdew-Burke-Enrzerhof (PBE) functional of the generalized gradient approximation (GGA).54,55 A plane-wave basis set with an energy cut off of 400 eV was adopted. The Gaussian smearing method was employed with a width of 0.05 eV. It is important to introduce the van der Waals interaction in order to describe the adsorption configurations and energies of Li2Sx (x=1~8) on substrates accurately56. We employed the DFT-D3 method in this model57. During geometrical optimization, all atoms were fully relaxed until the forces on unconstrained atoms were less than 0.03 eV Å-1. The iron-nitrogen embedded graphene (Fe-N-C) and pristine graphene that contains a (6 × 6) supercell with a vacuum region of 17 Å were used. The Brillouin zone was sampled using gamma-point centered 3 × 3 × 1 k-point meshes. For the calculation of species of lithium poly sulfides Li2Sx (x=4, 6, 8) in gas phase, the molecular geometries were optimized in a 20 Å cubic box with a single gamma point. We defined the adsorption energy (Eads) of molecules as Eads = ELi2Sx/substrate – ELi2Sx – Esubstrate, where ELi2Sx/substrate


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is the total energy of the Li2Sx (x=4, 6, 8) adsorbed on the substrate, and ELi2Sx and Esubstrate are total energy of isolated poly lithium sulfide and that of slab, respectively.


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Figure 1. A schematic representation of synthetic route for Fe-N-C/S-MCF and effect of resulting electrode.


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Figure 2. (a) and (b) SEM images, (c) TEM image of S-MCF (sample was cut out to thin section by a micro-toming). (d) N2 physisorption isotherms and pore size distribution of S-MCF.


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Figure 3. (a) Relative comparison of tap density of MSU-F-C and S-MCF. (b-e) SEM images of MSU-F-C and S-MCF electrode. (b) Top view and (c) side view of MSU-F-C electrode. (d) Top view and (e) side view of S-MCF electrode. (f) Galvanostatic performance of S-MCF and MSUF-C at 0.5 C rate. (g) Calculated volumetric capacity of S-MCF and MSU-F-C electrode at 0.5 C rate.


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Figure 4. (a) Dark-field STEM image, (b-f) EDS mapping of Fe-N-C/S-MCF and sulfur composite, (b) C, (c) N, (d) Fe, (e) S, (f) integration of elements. (g) XANES curves, (h) Fouriertransformed EXAFS curves of reference Fe foil and Fe-N-C/S-MCF.


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Figure 5. (a) Galvanostatic test at different current density from 0.1 C to 5 C rate of S-MCF and Fe-N-C/S-MCF electrodes. Capacities are based on charge process. (b) Polarization potential of Fe-N-C/S-MCF and S-MCF electrodes from 0.1 to 1 C rate. (c) CV plots, (d) Tafel plots, and (e) Cycle performance of Fe-N-C/S-MCF and S-MCF electrodes (tested with 2.5 mg cm-2 of sulfur areal density at 0.5 C rate). (f) Cycle performance of Fe-N-C/S-MCF with 5.2 mg cm-2 of sulfur areal density at 3 C rate (before cycling at 3 C rate, electrode was activated for 3 cycles at 0.1 C rate).


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Figure 6. (a) Geometrical configuration of adsorption of soluble LiPS (Li2S4, Li2S6, and Li2S8) on the Fe-N-C embedded graphene and bare graphene. (b) Calculated adsorption energy of each soluble LiPS. (c) Adsorption energy of Li and S atom in LiPS to bare graphene and to the Fe-NC sites. (d) Static adsorption test of S-MCF and Fe-N-C/S-MCF with Li2S8 solution.


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Table of Content


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the website at http://pubs.acs.org. Additional SEM images, EDS, and N2 physisorption data of samples TGA, XPS, and XRD data of samples Additional voltage profiles and CVs of prepared electrode Plot of log(current) vs log(scan rate) of prepared electrode Additional computational DFT data of Fe-N-C and graphene (PDF) The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding authors E-mail : [email protected]

ACKNOWLEDGMENT The authors gratefully acknowledge the support by the LG Chem. Ltd and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1A2B3004648).

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(27) Ye, C.; Zhang, L.; Guo, C.; Li, D.; Vasileff, A.; Wang, H.; Qiao, S. Z., A 3D Hybrid of Chemically Coupled Nickel Sulfide and Hollow Carbon Spheres for High Performance Lithium– Sulfur Batteries. Adv. Funct. Mater. 2017, 27, 1702524-1702532. (28) Xiao, Z.; Yang, Z.; Zhang, L.; Pan, H.; Wang, R., Sandwich-Type NbS2@ S@ I-Doped Graphene for High-Sulfur-Loaded, Ultrahigh-Rate, and Long-Life Lithium–Sulfur Batteries. ACS Nano 2017, 11, 8488-8498. (29) Zhang, H.; Zhao, Z.; Hou, Y.; Tang, Y.; Dong, Y.; Wang, S.; Hu, X.; Zhang, Z.; Wang, X.; Qiu, J., Nanopore Confined g-C3N4 Nanodots in N, S co-doped Hollow Porous Carbon with Boosted Capacity for Lithium-Sulfur Batteries. J. Mater. Chem. A 2018, in press, DOI: 10.1039/c8ta00529j. (30) Tao, Y.; Wei, Y.; Liu, Y.; Wang, J.; Qiao, W.; Ling, L.; Long, D., Kinetically-Enhanced Polysulfide Redox Reactions by Nb2O5 Nanocrystals for High-Rate Lithium–Sulfur Battery. Energy Environ. Sci. 2016, 9, 3230-3239. (31) Al Salem, H.; Babu, G.; V. Rao, C.; Arava, L. M. R., Electrocatalytic Polysulfide Traps for Controlling Redox Shuttle Process of Li–S Batteries. J. Am. Chem. Soc. 2015, 137, 1154211545. (32) Mi, Y.; Liu, W.; Li, X.; Zhuang, J.; Zhou, H.; Wang, H., High-Performance Li–S Battery Cathode with Catalyst-Like Carbon Nanotube-MoP Promoting Polysulfide Redox. Nano Res. 2017, 10, 3698-3705. (33) Liu, D.; Zhang, C.; Zhou, G.; Lv, W.; Ling, G.; Zhi, L.; Yang, Q. H., Catalytic Effects in Lithium–Sulfur Batteries: Promoted Sulfur Transformation and Reduced Shuttle Effect. Adv. Sci. 2017, 5, 1700270-1700281.


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Synergistic Effect of Molecular-Type Electrocatalysts with Ultrahigh

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