Nitrogen-Doped Porous Carbon Networks with Active FeâNx Sites to

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Nitrogen-doped Porous Carbon Networks with Active Fe-Nx Sites to Enhance Catalytic Conversion of Polysulfides in Lithium-Sulfur Batteries He Yang, Yanan Yang, Xu Zhang, Yongpeng Li, Naeem Akhtar Qaisrani, Fengxiang Zhang, and Ce Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08962 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Nitrogen-doped Porous Carbon Networks with Active Fe-Nx Sites to Enhance Catalytic Conversion of Polysulfides in Lithium-Sulfur Batteries He Yang, Yanan Yang, Xu Zhang,* Yongpeng Li, Naeem Akhtar Qaisrani, Fengxiang Zhang* and Ce Hao State Key Laboratory of Fine Chemicals, School of Petroleum & Chemical Engineering, Dalian University of Technology, Panjin 124221, China * Correspondence: [email protected]，[email protected]

ABSTRACT: The practical development of lithium-sulfur (Li-S) batteries is largely obstructed by their poor cycling stability due to the shuttle effect of soluble polysulfides. To address this issue, we herein report an interconnected porous N-doped carbon network (NPCN) incorporating Fe3C nanoparticles and Fe-Nx moieties, which is used for separator modification. The NPCN can facilitate lithium ion and electron transport, and localize polysulfides within the separator’s cathode side due to a strong chemisorption; the Fe3C/Fe-Nx species also provides chemical adsorption to trap polysulfides, and Fe3C catalyzes the redox conversion of polysulfides. More importantly, the catalysis effect of Fe3C is promoted by the presence of Fe-Nx coordination sites as indicated by the enhanced redox current in cyclic voltammetry. Due to the above synergistic effects, the battery with the Fe3C/Fe-Nx@NPCN modified separator exhibits high capacity and good cycling performance: at the current density of 0.1 C , it yields a high capacity of 1517 mA h g-1 with 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2 mg cm-2 sulfur loading and only experiences a capacity decay rate of 0.034% per cycle after 500 cycles at 1 C; it also delivers a good capacity of 683 mA h g-1 at 0.1 C with a high sulfur loading of 5.0 mg cm-2; after 200 cycles, the battery capacity can still reach 596 mAh g-1, corresponding to 87% capacity retention. Our work provides a new and effective strategy to achieve catalytic conversion of polysulfide and is beneficial for the development of rechargeable Li-S batteries. Keywords: Nitrogen-doped porous carbon network; Fe3C/Fe-Nx; enhanced electrocatalysis; polysulfide conversion; separator modification

1. Introduction Owing to high theoretical energy density, natural abundance and non-toxicity of sulfur as the active substance at cathode, the lithium-sulfur (Li-S) batteries are considered one of the most promising next generation secondary batteries.1-2 However, a few technical hurdles must be overcome to realize practical application and ultimate commercialization of Li-S batteries. For example, insulation and insolubility of the charged and discharged products (S and Li2S) result in sluggish redox kinetics;3-4 the intermediates formed during battery discharge and charge (Li2Sn,8≥n≥3) are electrolyte soluble and thus can migrate to the lithium metal anode, leading to low Coulombic efficiency and rapid capacity decay.5 2 ACS Paragon Plus Environment

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In order to address these challenges, tremendous efforts have been made to date, such as using novel binders,6 electrolyte additives7-8, porous carbon cathodes.9-11 and separators.12-13 Separator, as an indispensable and pivotal component in batteries, prevents the contact between cathode and anode and serves as an ion sieve for Li+ transport in the electrochemical process.14 Traditional polyolefin separators, however, feature a large and open pore structure, thus leading to low sulfur utilization and free migration of polysulfides. Various strategies have been developed for separator modification. A typical example is the carbon-coated separators that can produce a prolonged lifetime of the assembled battery.15 Carbon nanotubes,16-17 porous carbon18-19 and carbon hybrids20-22 have been used for separator modification, but the weak physical interaction between carbon and polar polysulfides is insufficient to mitigate polysulfides shuttling over long-term cycling. Metal compounds such as Al2O3,23 Li4Ti5O12,13 BaTiO3,24 TiO225 have also been employed; they can act as chemical trappers for polysulfides due to their strong polarity. Although effective for binding polysulfides, most of the conventional physical and chemical adsorption based methods can hardly accelerate the redox reactions of polysulfides.26-29 Polar conductors such as Ir,30 TiO2/TiN,31 MoO3,32 CoSe2,33 can provide adsorption sites and high electrical conductivity for polysulfide 3 ACS Paragon Plus Environment


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redox reactions. Yang et al.31 developed a twinborn TiO2-TiN modified separator that can enable smooth trapping-diffusion-conversion of polysulfides, thus effectively alleviating the shuttle effect, and promoting the redox kinetics of Li-S batteries. Zhang et al.32 modified the separator with a molybdenum trioxide nanobelt catalytic layer, which shows strong chemical interaction with polysulfides and also accelerates polysulfides conversion effectively. Hence, apart from allowing for Li+ transport, an ideal separator should also have a physical/chemical adsorption capability for anchoring polysulfides and superb electrocatalysis for polysulfide conversion. Fe-based materials, especially Fe3C, can provide sufficient adsorption sites for inhibiting the shuttle effect.22,

34-36

However, the low catalytic

activity of Fe3C compared with other metal carbides greatly impedes its further application in Li-S batteries. It is well known that N coordinated Fe atoms (Fe-Nx) on carbon skeletons are efficient electrocatalysts for oxygen reduction reaction.37-39 In addition, Fe-Nx on the carbon skeletons may present a polar surface for polysulfide adsorption. Therefore, combining the Fe-Nx moieties with the Fe3C composites may be very effective to facilitate the redox reaction of polysulfides and increase the sulfur utilization for Li-S batteries. Based on the above understanding, we herein design a Fe3C/Fe-Nx incorporated, N-doped porous carbon network (Fe3C/Fe-Nx@NPCN) for 4 ACS Paragon Plus Environment

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separator modification. The N dopants and the Fe3C provide abundant exposed sites for intimate adsorption of polysulfides, and the Fe-Nx moieties can promote polysulfides conversion. Moreover, the NPCN provides efficient pathways for Li+ transport and exposes the catalytic Fe-Nx sites for polysulfide conversion. Due to the above synergistic effect, the battery based on the Fe3C/Fe-Nx@NPCN modified separator shows an outstanding capacity of 1517 mA h g-1 with 1.2 mg cm-2 sulfur loading at 0.1 C and a high capacity retention of ca. 91% after 150 cycles at 0.5 C. Excellent cycling stability (87% capacity retention at 0.1 C) can also be achieved at a high sulfur loading of 5.0 mg cm−2.

2. Experimental 2.1 Synthesis of Fe3C/Fe-Nx@NPCN, Fe3C@C and NPC Graphene oxides (GO) were prepared according to a previous report.17, 40-41

0.52 g iron nitrate nonahydrate (Fe(NO3)3·9H2O) and 10 mL GO (5

mg mL-1) solution were added to 40 mL deionized water, ultrasonically treated 2 h and then vigorously stirred 12 h to form a uniform suspension. Next, 1.05 g D-glucose was added to above suspension, and stirred for 8 h. The mixture was then transferred into a Teflon-lined autoclave and heated at 180℃ for 15 h. After cooling to room temperature, the precipitate was filtered, washed with deionized water three times, and dried at 60℃ for 12 h to get carbon coated Fe2O3 supported on GO 5 ACS Paragon Plus Environment


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(C@Fe2O3-GO). The C@Fe2O3-GO was mixed with melamine at a mass ratio of 1:10 by grinding; the mixture was heated in a quartz tube to 900°C at a heating rate of 5°C min-1 under a flow of argon, and kept at 900°C for 2 h. After cooling to room temperature, Fe3C/Fe-Nx@NPCN was obtained. For comparison purposes, N-doped porous carbon (NPC) and carbon loaded Fe3C (Fe3C@C) were also prepared by the same method as above except for the use of Fe(NO3)3·9H2O and melamine, respectively. 2.2 Separator modification Fe3C/Fe-Nx@NPCN modified separator was fabricated by direct coating of the slurry containing Fe3C/Fe-Nx@NPCN and PVDF onto Celgard 2325 polypropylene (PP) separator. The slurry was prepared by mixing Fe3C/Fe-Nx@NPCN and PVDF in NMP with a weight ratio of 9:1 and stirred for 12 h. The modified separator was punched into discs with a diameter of 16 mm for further usage in batteries. The coating layer on each separator was controlled to have same mass loading (0.25 mg cm2). NPC and Fe3C@C modified separator were prepared by the same method. 2.3 Fabrication of 0.2 M Li2S8 solution 0.200 g Li2S and 0.945 g sublime sulfur were dissolved in 21 mL mixed solvent (DOL: DME = 1:1, V/V) with LiNO3 (2 wt%) as an additive, and stirred in an Ar-filled glove box to yield a 0.2 M Li2S8 6 ACS Paragon Plus Environment

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solution. 2.4 Assembly of symmetric batteries and evaluation of polysulfides conversion Fe3C/Fe-Nx@NPCN and PVDF with a weight ratio of 9:1, were mixted with NMP and stirred for 12 h to form a uniform slurry, which was coated on an aluminum foil and dried at 80℃ for 12 h. The resultant Fe3C/Fe-Nx@NPCN coated foil was punched into disc electrodes of 13.0 mm diameter with ~0.19 mg cm-2 Fe3C/Fe-Nx@NPCN loading. NPC and Fe3C@C coated electrodes were prepared by the same method. Two Fe3C/Fe-Nx@NPCN electrodes were used to assemble CR2016 coin-type symmetric battery with a Celgard 2325 PP separator and each electrode side had 20 µL of 0.2 M Li2S8 electrolyte. Cyclic voltammetry (CV) was performed on the symmetric battery between -1 and 1 V at different scan rates. 2.5 Li2S nucleation tests Using the fabricated Fe3C/Fe-Nx@NPCN electrode as cathode, Li foil as anode and Celgard 2325 PP membrane separator, a CR2016 coin-type battery was assembled, where 20 µL electrolyte without Li2S8 was added to the anode side and 20 µL 0.2M Li2S8 electrolyte to the cathode side of the membrane. The batteries were galvanostatically discharged to 2.06 V at 0.112 mA and maintained potentiostatically at 2.05 V until the current was below 10-5 A for the nucleation and growth of Li2S. 7 ACS Paragon Plus Environment


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2.6 Density functional theory calculations All density functional theoretical (DFT) calculations were performed with the hybrid exchange-correlation functional CAM-B3LYP and LANL2DZ basis sets in the Gaussian 09 program.42 Basis set superposition error (BSSE) was corrected by using a counterpoise method. The binding energy is calculated using the equation: 𝐸𝑎𝑑𝑠 = 𝐸𝑡𝑜𝑙 ― 𝐸𝐿𝑆 ― 𝐸𝑠𝑢𝑟 + 𝐸𝐵𝑆𝑆𝐸 Where Etol refers to the total energy of Li2Sx molecule adsorbed at the Fe-N4 moieties on N-doped porous carbon networks; Esur and ELS are the energies of Fe-N4 moieties on N-doped porous carbon networks and Li2Sx, respectively. 2.7 Electrochemical measurements The cathode material was prepared by mixing elemental sulfur and Super-P carbon with a mass ratio of 7:3 or 8:2; the mixture was heated at 155°C and kept for 12 h. The resultant composite, Super-P, and PVDF (with a weight ratio of 80:10:10) were dispersed in N-methyl pyrrolidone (NMP), and the slurry obtained was spread on an aluminum foil and dried at 80 ℃ overnight. The dried, coated foil was punched into disc electrodes with 1.2 -5 mg cm-2 sulfur loading. The resultant C/S electrode, Li foil and modified PP membrane were assembled into CR2016 coin-type batteries as the cathode, anode and separator, respectively. The electrolyte volume was ca. 20 μL mg-1 sulfur. CHI760e electrochemical 8 ACS Paragon Plus Environment

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workstation was used to record the Cyclic voltammetry (CV) of the batteries. EIS tests were measured in the frequency range of 10-2-105 Hz at open circuit voltage. CV measurements were performed between 1.7 and 2.8 V at a scan rate of 0.1 mV-1. LAND CT2001A battery test system was used to measure the electrochemical performance of these batteries at 25 ℃. 2.8 Material characterization TGA Mettler-Toledo was conducted to measure the sulfur contents. The Brunauer-Emmett-Teller (BET) surface area was measured at 77K on Autosorb-iQ-C instrument. The surface information of the samples were collected by the XPS, ESCALAB250Xi. X-ray diffraction (XRD) patterns were recorded to examine the crystal forms of the samples using a Shimadzu X-ray diffractometer with Cu Ka radiation. The morphologies of the samples were investigated on FEI Nova NanoSEM 450 and FEI Tecnai G2F30 microscopes.17, 19

3. Results and Discussion The fabrication of N-doped porous carbon network carrying Fe3C/Fe-Nx (Fe3C/Fe-Nx@NPCN) is illustrated in Figure 1. Graphene oxide (GO) with abundant oxygen containing functional groups adsorbs Fe3+ and D-glucose, which underwent a hydrothermal reaction to produce carbon coated Fe2O3 supported on GO (C@Fe2O3-GO) as indicated by 9 ACS Paragon Plus Environment


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the X-ray diffraction peaks of Fe2O3 in Figure S1. The resultant C@Fe2O3-GO powder was then grinded with melamine and pyrolyzed at 900℃, during which Fe2O3-GO got reduced by carbon, forming metallic Fe atoms that are chemically active and can react with N and C atoms to give Fe3C and Fe-Nx.39, 43 Fe-Nx means coordination between Fe and N atoms. Actually, due to the harsh formation conditions of iron nitride (FeNx),44-47 FeNx cannot be formed under the synthetic conditions adopted in this work. Meanwhile, the ammonia gas generated from melamine can corrode the carbon substrate at 900℃, forming a porous carbon network, which can facilitate Li+ transport, and improve the reaction kinetics of the Li-S battery; it also provides a three-dimensional conductive network, which can accelerate electron transport to improve the utilization of sulfur. Moreover, the highly porous structure leaves a large number of Fe3C and Fe-Nx sites well exposed and available for polysulfides adsorption and conversion. 48-49

Figure 1. Schematic illustration of Fe3C/Fe-Nx@NPCN fabrication.

Figures 2 and S2 show the field-emission scanning electron microscopy (FESEM) images of different samples. Apparently, Fe3C@C 10 ACS Paragon Plus Environment

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has a dense surface (Figures 2a-b and 2b), which is similar with that of the C@Fe2O3-GO (Figure S2a), and may hinder Li+ transport and weaken its polysulfide adsorption ability. By contrast, a well interconnected porous structure is observed in Fe3C/Fe-Nx@NPCN (Figures 2c-d and S2c) and NPC (Figures 2e-f and S2d). Meanwhile, the elemental mapping (Figure S3) shows a uniform distribution of Fe, C and N elements in the Fe3C/Fe-Nx@NPCN composite.

Figure 2. SEM images of (a,b) Fe3C@C, (c,d) Fe3C/Fe-Nx@NPCN, and (e,f) NPC at different magnifications.

Figure 3 shows the X-ray diffraction (XRD) patterns of different samples. The broad peak at 20-30° indicates the presence of amorphous carbon in NPC; apart from this signal, peaks at 35.6°, 37.8°, 44.9°, 49.5°, 51.9° and 74.8 ° are observed in the Fe3C/Fe-Nx@NPCN and Fe3C@C samples, corresponding to the (0 2 0), (1 1 2), (0 2 2), (1 2 2), (2 1 2) and (3 2 1) planes of Fe3C (PDF#35-0772);34, 50 the peaks at 44.6°, 65.2° and 82.5 ° are attributed to the (1 1 0), (2 0 0) and (2 1 1) planes of Fe 11 ACS Paragon Plus Environment


(PDF#06-0696). It is interesting to note that the intensities of Fe3C peaks decreased in Fe3C/Fe-Nx@NPCN relative to those in the other samples, which may be due to Fe3C consumption during Fe-Nx formation. The Fe-Nx characteristic peaks were not found, implying that the iron atoms might have been incorporated into the carbon framework in a disordered manner.

Fe3C Fe

NPC (1 1 0) (0 2 2)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fe3C@C (2 1 2)

(0 2 0)

(2 0 0)

(3 2 1)

(2 1 1)

Fe3C/Fe-Nx@NPCN (1 1 2)

10

20

30

40

(1 2 2)

50

60

70

80

90

2-theta (Degrees)

Figure 3. XRD patterns of Fe3C/Fe-Nx@NPCN, Fe3C@C and NPC.

The morphologies of different samples were also investigated by the transmission electron microcopy (TEM). Seen from Figure S4a, the carbon layer on the surface of Fe3C@C is thick and dense, which may mask the Fe based active sites and hinder polysulfide adsorption. The NPC sample displays a porous structure (Figure S4b), which is consistent with that observed from the SEM images (Figures 2e-f). The 12 ACS Paragon Plus Environment

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TEM images in Figures 4a and S5 reveal an interconnected porous structure of Fe3C/Fe-Nx@NPCNs, and also the presence of Fe based nanoparticles of ca. 15-20 nm; the nanoparticle is further confirmed by high-resolution TEM image (Figure 4b), where the lattice spacing of 0.2484 nm corresponds to the (220) plane. The presence and uniform distribution of C, N, and Fe elements on the Fe3C/Fe-Nx@NPCN composite can be found clearly from the scanning TEM (STEM) and the corresponding elemental mapping images (Figure 4c-f).

Figure 4. (a) TEM image of Fe3C/Fe-Nx@NPCN. (b) High-magnification HRTEM image of Fe3C nanoparticles (inset: lattice images of Fe3C). (c) STEM image of Fe3C/Fe-Nx@NPCN, and corresponding elemental mapping of (d) C, (e) N, (f) Fe.

X-ray photoelectron spectroscopy (XPS) study was performed to analyze the composition of Fe3C/Fe-Nx@NPCN. As shown in Figure S6, peaks of C 1s, N 1s, O 1s and Fe 2p are identified at 285 eV, 400 eV, 531 13 ACS Paragon Plus Environment


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eV and 708 eV, respectively, consistent with the elements in Fe3C/Fe-Nx@NPCN. The deconvoluted N 1s XPS spectrum in Figure 5a shows the characteristic peaks at 398.2 eV, 400.8 eV, 401.9 eV and 405.4 eV, which can be ascribed to pyridinic, pyrrolic, graphitic and oxidized pyridinic N species, respectively;51-52 in particular, there was an obvious peak at 399.7 eV attributable to the Fe-N bond formed in the pyrolysis process.53 The nitrogen species is expected to improve the adsorption of polysulfides, thereby enhancing the electrochemical performance of Li-S batteries.54 Figure 5b shows the high-resolution deconvoluted Fe 2p spectrum, where the peaks at 706.9 and 720.0 eV are indexed to Fe0 coming from Fe3C and Fe,55-56 which demonstrates the existence of Fe3C; the peaks at 712.2 and 723.5 eV are indexed to ferric state (Fe3+), and the detection of ferric state species can be confirmed the presence of Fe-N bond in the samples.56-59 The peak at 709.2 eV corresponds to the Fe-N coordination bond, again demonstrating the existence of chemical interaction between the Fe and N atoms.58, 60 The Fe3C and Fe-Nx species in the sample will provide abundant catalytic sites to promote polysulfides conversion.

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Figure 5. High resolution (a) N 1s and (b) Fe 2p XPS spectra of Fe3C/Fe-Nx@NPCN.

Surface area is an important parameter determining performance of the fabricated Fe3C/Fe-Nx@NPCN composite when used as the separator modification layer. According to the nitrogen adsorption-desorption measurements (Figure S7), the surface area of Fe3C/Fe-Nx@NPCN is 266 m2 g-1, higher than that of NPC (224 m2 g-1) and Fe3C@C (238 m2 g-1), which may benefit the adsorption of polysulfides and subsequent conversion. This can be seen from the following adsorption results. As shown in Figure S8, the Li2S8 solution soaking Fe3C/Fe-Nx@NPCN changed from yellow to colorless in 30 min while the solution with Fe3C@C and that with NPC exhibited incomplete discoloration, evidencing that Fe3C/Fe-Nx@NPCN does adsorb polysulfides better than the other two materials. Such a difference is definitely also related to the synergistic effect of N-doped carbon network, Fe3C and the Fe-Nx species.



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The above adsorption behavior was further studied by Density Functional Theory (DFT) calculation. As shown in Figure S9, models of Fe-N4 (according to ref. 52 and ref. 53)52-53 on NPCN was considered in our simulation. Different polysulfides such as Li2S, Li2S2, Li2S4 and Li2S6 were selected as the representative prototype molecules for modelling. Figure 6 demonstrates the optimized geometric structure and bonding states when different polysulfides are adsorbed on the Fe-N4 moieties; the calculated adsorption energies of Li2S, Li2S2, Li2S4, and Li2S6 are -2.39 eV, -1.96 eV, -1.85 eV, and -1.41 eV, respectively, indicating that Fe-N4 moieties possesses excellent adsorption ability for liquid or solid polysulfides.

Figure 6. Optimized geometries of (a) Li2S; (b) Li2S2; (c) Li2S4 and (d) Li2S6 on Fe-N4 moieties on N-doped porous carbon network.


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The catalytic effect of Fe3C/Fe-Nx@NPCN in comparison with NPC and Fe3C@C on polysulfide redox reactions was investigated by cyclic voltammograms (CV) using symmetric batteries. As can be seen from Figure 7a, the CV curve of the NPC symmetric battery shows only one pair of peaks with a low redox charge storage, indicating a low catalytic activity for polysulfide redox reactions; by contrast, both the Fe3C@C and Fe3C/Fe-Nx@NPCN batteries exhibit more significant redox peaks or higher

current

densities

than

NPC.

More

importantly,

the

Fe3C/Fe-Nx@NPCN battery exhibits the highest current density among different samples, implying the significantly enhanced redox kinetics between liquid and solid phase polysulfides; this is probably because the Fe-Nx species in Fe3C/Fe-Nx@NPCN can act as additional catalytic sites to accelerate polysulfides conversion. Figure 7b shows the first five CV scans of the Fe3C/Fe-Nx@NPCN symmetric battery, where the nearly perfect overlapping peaks suggest good stability of the battery. CV curves of the Fe3C/Fe-Nx@NPCN symmetric battery were also recorded at different scanning rates as shown in Figure 7c, where the reduction and oxidation peaks still remain with increased scanning rate, suggesting fast catalytic redox reactions of polysulfides. Figure 7d displays the electrochemical impedance spectra of the Fe3C/Fe-Nx@NPCN, Fe3C@C and NPC symmetric batteries, 17 ACS Paragon Plus Environment


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where the Fe3C/Fe-Nx@NPCN battery shows a lower charge transfer resistance than the other two, indicating good electrochemical performance of the material.

Figure 7. (a) CV profiles of symmetric batteries with identical electrodes of Fe3C@C, Fe3C/Fe-Nx@NPCN and NPC symmetric batteries in electrolytes with 0.2 M Li2S8 at 3 mV s-1. (b) Multi-cycle CV profiles of the Fe3C/Fe-Nx@NPCN symmetric battery at 3 mV s-1. (c) CV profiles of the Fe3C/Fe-Nx@NPCN symmetric battery at different scan rates. (d) Electrochemical impedance spectra of Fe3C@C, Fe3C/Fe-Nx@NPCN and NPC symmetric batteries.

The catalytic effect of Fe3C/Fe-Nx@NPCN was further investigated by monitoring the nucleation behavior of Li2S produced in the polysulfide redox reaction. A series of potentiostatic discharge experiments were conducted for this purpose. As observed in Figure 8a-b, the Li2S 18 ACS Paragon Plus Environment

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nucleation occurred earlier on Fe3C/Fe-Nx@NPCN than on Fe3C@C and NPC. This is because nitrogen doping in Fe3C/Fe-Nx@NPCN promotes polysulfide adsorption, the porous structure facilitates Li+ transport, and the Fe3C and Fe-Nx species can provide more catalytic activity sites. In other words, the synergistic effect of Fe3C, Fe-Nx and NPCN catalyzes polysulfide conversion and makes the occurrence of Li2S nucleation earlier on Fe3C/Fe-Nx@NPCN than on Fe3C@C and NPC. Moreover, Fe3C/Fe-Nx@NPCN produces a higher current density than the other two electrodes; its current density decreased to a lower value in 20 min (Figure 8c), demonstrating its effectiveness to suppress polysulfides shuttle. These results clearly confirm that Fe3C/Fe-Nx@NPCN has superior electrocatalytic activity than Fe3C@C and NPC.

Figure 8. (a) Potentiostatic discharge profiles at 2.05 V on different batteries. (b) Magnified view between 0-4 min. (c) Magnified view between 15-20 min.



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The Fe3C/Fe-Nx@NRPC composite was employed to modify the commercial polypropylene (PP) separator. As shown in Figure S10, the pristine PP separator (detailed parameters provided in Table S1) has a smooth and open pore surface. By contrast, the modified separator, Fe3C/Fe-Nx@NRPC-PP (Figure 9a-b), is rougher and denser. The optimal thickness of the modification layer was controlled to be ~18 μm based on a series of experiments. As shown in Figure S11, too thick coating resulted in low discharge capacity of the battery due to hindered Li+ transfer; if the coating is too thin, the battery also gave a low capacity probably because of insufficient polysulfide adsorption capability of the modified separator.

Figure 9. SEM images of (a) typical front-view images and (b) cross-sections of a Fe3C / Fe-Nx@NPCN-PP.

To evaluate the performances of the separators, the cells were firstly assembled with a carbon/sulfur composite containing 70% sulfur (based on TGA, Figure S12) as the cathode material and Li foil as the anode. Figure S13a shows the CV curves of batteries assembled with different 20 ACS Paragon Plus Environment

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separators. The Fe3C/Fe-Nx@NPCN-PP battery shows an oxidation peak between 2.26 V and 2.60 V, and two reduction peaks at 2.02 V and 2.31V; its reduction peaks shift to a higher potential and the oxidation peaks shift to a lower one compared with Fe3C@C-PP, NPC-PP and PP batteries, suggesting that the redox reaction kinetics and the reversibility are greatly enhanced with the incorporation of the Fe-Nx species. Moreover, one can hardly find significant changes on the curves in the four CV cycles, indicating a good cycling stability of Fe3C/Fe-Nx@NPCN-PP battery. The Fe3C/Fe-Nx@NPCN-PP battery shows good electrochemical performances. As seen from Figure 10a, its capacity is 1571 mAh g-1 at 0.1 C and 851 mAh g-1 at 2 C, much higher than those of the Fe3C@C-PP, NPC-PP and PP batteries. Figures S14a-d are the discharge platforms of the batteries assembled with different separators; it is seen that all the modified separators have led to significant improvement in both discharge capacity and voltage compared with the battery using pure PP separator, and in particular, the Fe3C/Fe-Nx@NPCN-PP battery displays two obvious discharge platforms even at the high rate of 1 C, which is consistent with the rate performance shown in Figure 10a. The above results demonstrate that Fe3C/Fe-Nx@NPCN-PP can improve the utilization rate of sulfur, and promote polysulfides adsorption and conversion.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Discharge Capacity (mAh/g)


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1800

Fe3C/Fe-Nx@NPCN-PP

1600

Fe3C@C-PP

0.1 C

0.1 C

0.2 C

1400

PP

NPC-PP

0.5 C

1200

1C 2C

1000 800 600 400 200 0

0

5

10

15

Cycles

20

25

30

Figure 10. The rate performance of Li-S batteries assembled with Fe3C/Fe-Nx@NPCN-PP, Fe3C @C-PP, NPC-PP and PP separators.

The cycle stability of the batteries assembled with different separators are shown in Figure 11. It can be found in Figure 11a that the initial capacity of the Fe3C/Fe-Nx@NPCN-PP battery with a sulfur loading of 1.2 mg cm-2 is 1268 mAh g-1 at a current density of 0.2 C; after 100 cycles, the capacity remained at 1028 mAh g-1 (81% retention) and the average Coulombic efficiency is 98%. By contrast, capacities of 870 mAh g-1, 810 mAh g-1, and 437 mAh g-1 are retained after 100 cycles for Fe3C@C-PP, NPC-PP and PP batteries, respectively, corresponding to capacity retentions of 71%, 70% and 43%, respectively. After increasing the

current

density

to

0.5

C,

the

initial

capacity

of

the

Fe3C/Fe-Nx@NPCN-PP battery is 1094 mAh g-1; the capacity is maintained by 91% after 150 cycles, suggesting a good cycling stability, 22 ACS Paragon Plus Environment

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while the capacity retention of Fe3C@C-PP, NPC-PP and PP batteries are only 71%, 64% and 60% (Figure 11b), respectively. Figure 11c shows the long-term cycle performance of the Fe3C/Fe-Nx@NPCN-PP battery at the current density of 1 C. After the activation, the battery yields a discharge capacity of 1045 mA h g-1 and maintains 83% of the initial capacity after 500 cycles (0.034% per cycle). When the sulfur loading is increased to 1.9 mg cm-2, the Fe3C/Fe-Nx@NPCN-PP battery still exhibits 76% capacity retention with an average Coulomb efficiency of 98%.

Figure 11. (a, b) Cycling performance of Fe3C/Fe-Nx@NPCN-PP, Fe3C@C-PP, NPC-PP and PP batteries at a current density of 0.2 C and 0.5 C. (c) The long-term cycling performance of the Fe3C/Fe-Nx@NPCN-PP battery with different sulfur loadings (1.2 and 1.9 mg cm-2) at a high current density of 1 C.

To investigate the internal resistances of Fe3C/Fe-Nx@NPCN-PP, Fe3C @C-PP, NPC-PP and PP based batteries, electrochemical impedance spectroscopy (EIS) tests were performed. All the batteries have a sulfur 23 ACS Paragon Plus Environment


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loading of 1.2 mg cm-2. Their Nyquist impedance plots all exhibit a semicircle at high frequency and a straight line at low frequency (Figure S15), where the Rs refers to the resistance of electrolyte, and the semicircle corresponds to a parallel element, the charge transfer resistance (Rct) and the constant phase capacitance (Cdl).61 The battery with the Fe3C/Fe-Nx@NPCN-PP separator shows the smallest Rct as indicated by the smallest diameter of the semicircle at low frequencies, confirming the Fe species and N-doping can improve conductivity of the carbon network. After cycling 50 times at 0.5 C, all the batteries show a reduced Rct, which may be due to the redistribution of sulfur and its better contact with the electrolyte. In

order

to

further

investigate

the cycle

stability

of

the

Fe3C/Fe-Nx@NPCN-PP battery, its sulfur content in the C/S cathode material was increased to 80 wt.% (Figure S12). As shown in Figure 12, the battery with 3.5 mg cm-2 sulfur loading yields a high discharge capacity of 980 mA h g-1 at 0.1 C; its capacity can remain at 700 mAh g-1 after 200 cycles. Even at a high sulfur loading of 5 mg cm-2, it can still yield a capacity of 683 mAh g-1, and maintains 596 mA h g-1 (87% capacity retention) after 200 cycles. The capacity increase in the first several cycles arose from the chemical activation and redistribution of sulfur containing species, enabling their unitization in the charge and discharge reactions.62-63 All these results indicate that the synergistic 24 ACS Paragon Plus Environment

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chemisorption and electrocatalysis effects can suppress the shuttle effect and improve the cycle stability of the battery even at a high sulfur

120

1400

100

1200 1000

0.1 C

800

60

600

40

400 S= 5 mg cm-2

200 0

80

0

50

20

S= 3.5 mg cm-2

100

150

0 200

Coulombic Efficiency (%)

loading.

Specific Capacity (mAh/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cycles

Figure 12. Cycling performance of Fe3C/Fe-Nx@NPCN-PP battery with different sulfur loadings (3.5 mg cm-2 and 5 mg cm-2) at a current density of 0.1 C.

4. Conclusion In

summary,

we

have

designed

and

fabricated

a

novel

Fe3C/Fe-Nx@NPCN composite via hydrothermal method followed by a carbonation process with melamine. It was further employed for PP separator modification. The modified separator, Fe3C/Fe-Nx@NPCN-PP, is advantageous over conventional separator because the N-doped porous carbon networks facilitate Li+ transport, the N atoms and Fe based species can provide chemisorption sites for trapping polysulfides, and the Fe-Nx species can accelerate the redox reaction of polysulfides. Due to the above synergistic effects, the Fe3C/Fe-Nx@NPCN-PP based battery 25 ACS Paragon Plus Environment


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delivers a high discharge capacity of 1517 mA h g-1 at 0.1 C with 1.2 mg cm-2 sulfur loading; it yields a discharge capacity of 1045 mA h g-1 after the activation at 1 C and experiences a small capacity decay rate of 0.034% per cycle after 500 cycles. The Fe3C/Fe-Nx@NPCN-PP battery also exhibits a good capacity of 683 mA h g-1 at 0.1 C with a high sulfur loading of 5.0 mg cm-2; after 200 cycles, its capacity can still maintain at 596 mAh g-1 (87% capacity retention), showing an excellent cycle stability. The present work provides an innovative strategy to prepare separators

combining

strong

polysulfide

chemisorption

and

electrocatalytic conversion for high performance Li-S batteries.

Author information Corresponding Author *E-mail: [email protected]；[email protected];

Conflicts of interest The authors declare no competing financial interest.

Acknowledgements This work is supported by the Science and Technology Innovation Fund of Dalian (2018J12GX052), National Natural Science Foundation of China (Grant No. 21776042), the China Postdoctoral Science Foundation (No. 2019M651118), the Doctoral Start-up Foundation of Liaoning Province (20170520263), and the Fundamental Research Funds for the Central Universities (DUT18RC(4)059).


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Table of Contents

 N-doped porous carbon networks loaded with Fe3C/Fe-Nx (Fe3C/Fe-Nx@NPCN) for separator modification, the Fe3C provide abundant exposed sites for intimate adsorption of polysulfides and Fe-Nx can promote the fast conversion of polysulfides.


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