Fe3C with enhanced electrochemical kinetics for high-performance lithium-sulfur batteries

Carbon 155 (2019) 353e360

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Freestanding cellulose paper-derived carbon/Fe/Fe3C with enhanced electrochemical kinetics for high-performance lithium-sulfur batteries Yu-Jiao Zhang a, b, Jin Qu a, **, Qiu-Yu Ji a, Ting-Ting Zhang a, Wei Chang a, Shu-Meng Hao a, Zhong-Zhen Yu a, b, * a State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China b Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of Chemical Technology, Beijing, 100029, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 June 2019 Received in revised form 22 August 2019 Accepted 24 August 2019 Available online 26 August 2019

The loss and sluggish kinetics of intermediate polysulfides in electrochemical processes seriously reduce electrochemical activity of sulfur in lithium-sulfur batteries (LSBs). Herein, we design and synthesize freestanding graphitized carbon interlayers decorated with Fe/Fe3C nanocatalysts by thermal treatment of cellulose paper with adsorbed ferric nitrate at 1000
C, during which the freestanding carbon network derives from the cellulose paper while the Fe results from the reduction of Fe3þ with a carbothermal reduction process; simultaneously, the in situ formed Fe boosts graphitization of carbon to enhance electrical conductivity of the interlayer, and the carbon network effectively helps confine and accommodate Fe/Fe3C nanoparticles. Therefore, the well dispersed Fe/Fe3C nanoparticles catalytically accelerate electrochemical conversion of polysulfides, while the enhanced conductive network of carbon benefits the electron transfer during the electrocatalytic process. The cell with an optimal carbon/Fe/Fe3C interlayer delivers excellent cyclability and rate performances. At current densities of 0.2C and 2C, the specific capacities are close to 1000 and 735 mA h g1, respectively. Even after 200 cycles at 1C, the reversible specific capacity is still 772 mA h g1. Such a synergistic catalytic interlayer with an enhanced conversion kinetic towards polysulfides provides a new approach for improving electrochemical performances of LSBs. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction With entering the green energy society, a constantly increasing energy demand for transportation and electrical energy-storage systems desires for renewable energy sources [1,2]. Lithiumsulfur batteries (LSBs) attract increasing attention and become one of the most prospective next-generation rechargeable batteries [3e5]. As a cathode, sulfur offers a high theoretical specific capacity of 1675 mA h g1 based on the multi-electron electrochemistry conversion reaction [6]. However, the intermediate lithium

* Corresponding author. State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. ** Corresponding author. E-mail addresses: [email protected] (J. Qu), [email protected] (Z.-Z. Yu). https://doi.org/10.1016/j.carbon.2019.08.065 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

polysulfide could be dissolved in liquid electrolytes during the conversion reaction, causing the shuttle effect [7e9]. In addition, the insulating nature of both sulfur and Li2S/Li2S2 for lithium ions/ electrons directly limits the reversible electrochemical reactions and causes less effective utilizations of sulfur species [10]. Over the past decade, many attempts have been made in stabilizing polysulfides by physical immobilization [11e16] and chemical trapping [17e20]. However, only physical or chemical adsorption processes could not improve the conversion reaction of polysulfides, especially at high current densities. A promising idea to suppress the shuttle of polysulfides in LSBs is to accelerate the redox kinetics of polysulfides via electrocatalytic processes [21e27]. Zhang et al. reported that TiC could facilitate both the liquid-liquid transformation of polysulfide and liquid-solid nucleation/growth of Li2S [28]. They also found that CoS2, InN nanowires and black phosphorus quantum dots could catalyze polysulfide conversion by enhancing its redox kinetic [29e31]. Wang et al. utilized MoP as a

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catalyst to achieve fast electrochemical reaction kinetics [32]. Jin et al. certified that carbon nanotube (CNT)/CoS-NS could enhance the redox kinetics of polysulfide and suppress the shuttle effect [33]. Electrochemical performances of sodium-sulfur batteries (NSBs) and lithium ion batteries (LIBs) could also be improved by electrocatalytic reactions. Chou et al. reported that Co kinetically catalyzed the reduction of polysulfide in NSBs [34]. Zhang et al. found that in situ formed Fe nanoparticles efficiently catalyzed reversible formation/decomposition of a solid electrolyte interface (SEI) film, even boosted the oxidization of sulfur-containing intermediate phase to sulfur during the electrochemical reactions in LIBs [35,36]. Additionally, Fe could help graphitize carbon nanomaterials for improving their electrical conductivities during thermal treatments. Both active Fe nanocatalysts and conductive carbon substrate [37,38] would be beneficial for high-performance LSBs. Therefore, the abundant, cost-effective and eco-friendly Fe could be a promising catalyst for boosting polysulfide transformation during electrochemical reactions, which has not been reported before. Herein, by taking advantages of the electrocatalysis of Fe/Fe3C to enhance the redox reaction kinetics and suppress the polysulfide shuttle in LSBs, we design and fabricate freestanding cellulose paper-derived graphitized carbon interlayer decorated with Fe/ Fe3C nanoparticles by a carbothermal reduction process at 1000
C, during which the porous carbon network derived from the cellulose paper helps not only reduce iron nitrate to Fe but also confine and accommodate the in situ formed Fe/Fe3C nanoparticles, while the in situ formed Fe boosts the graphitization of carbon to enhance the electrical conductivity of the carbon substrate. The combination of the Fe/Fe3C nanoparticles with the graphitized carbon network highly decreases the contact resistance and boosts the redox kinetics of polysulfide by facilitating the transfer of electrons. Furthermore, the well dispersed Fe/Fe3C nanocatalysts not only efficiently contact soluble intermediate polysulfides by the porous structure of the carbon network to promote the utilization of sulfur, but also accelerate electrochemical kinetics of the sulfur-containing species during the electrocatalytic process. The freestanding graphitized carbon/Fe/Fe3C (CF) catalytic interlayers could effectively suppress the shuttle of polysulfide and accelerate the redox kinetics in LSBs. As a result, the cells with the catalytic interlayers deliver satisfactory cyclic performances and rate performances. At a current density of 0.2C, the initial discharge and charge capacities of a cell with an optimal interlayer are 1151 and 1119 mA h g1, respectively. The specific capacity is still close to 1000 mA h g1 after 50 cycles. At 1C, the reversible specific capacity still reaches 772 mA h g1 after 200 cycles. Even at 2C, the reversible capacity retains at 735 mA h g1. Such CF interlayers indeed improve the electrochemical performances of LSBs. 2. Experimental

carbon/Fe/Fe3C interlayers. The amounts of Fe(NO3)3$9H2O used were 0, 100, 200 and 500 mg, and the corresponding CF interlayers were designated as CF0, CF1, CF2 and CF5, respectively. The mass ratios between Fe(NO3)3$9H2O and cellulose paper in CF1, CF2 and CF5 interlayers are 0.7, 1.4 and 3.4, respectively. 2.3. Characterization A Zeiss Supra55 scanning electron microscope (SEM) and a FEI Tecnai G2 F20 transmission electron microscope (TEM) were used to observe microstructures and morphologies of the CF interlayers. All interlayers were characterized with a Rigaku D/Max 2500 X-ray diffractometer (XRD), a SDT Q600 thermogravimetric analyzer (TGA), a Nicolet IS5 Fourier-transform infrared spectrometer (FTIR), a Renishaw Raman spectroscopy, and a Thermo ESCALAB 250XI X-ray photoelectron spectroscopy (XPS). 2.4. Electrochemical performance measurements The working electrode was composed of 60 wt% S, 30 wt% superP, and 10 wt% LA132. The loading mass of active materials was 1.5e1.8 mg cm2. Half cells were assembled in an argon-filled glove box using lithium foil as the counter electrode, CF as the interlayer, and a Celgard 2400 polypropylene (PP) as the separator. The electrolyte was 0.6 M LiTFSI dissolved in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1) with 0.4 M of LiNO3. Electrochemical performances were recorded on a Land CT 2001A electrochemical workstation. Cyclic voltammograms (CV) in the voltage range of 1.5e3.0 V (vs. Liþ/Li) and electrochemical impedance spectra (EIS) in frequency ranges of 0.01 Hze100 kHz were measured on a CHI 660E electrochemical workstation. 2.5. Kinetics measurements For liquid-liquid kinetics, symmetric Li2S6 cells were constructed using two identical CF0 or CF2 interlayers as electrodes, a Celgard 2400 polypropylene (PP) as the separator, and 40.0 mL of Li2S6/tetraglyme (2.5 mol L1 in sulfur) as the electrolyte. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for symmetric cells were conducted on the CHI 660E electrochemical workstation. CV was measured at a scan rate of 10 mV s1 between 0.8 and 0.8 V; and EIS was performed in a frequency range from 0.1 Hz to 104 Hz. 2.6. Visualized adsorption of Li2S4 Li2S4 solution was prepared by adding a mixture of sulfur and lithium sulfide with a molar ratio of 3/1 into DOL/DME (1/1) followed by vigorous magnetic stirring. The CF interlayer with the same surface area of 0.785 cm2 was immersed in 4 mL of Li2S4 solution for 24 h.

2.1. Materials 3. Results and discussion Ferric nitrate nonahydrate (Fe(NO3)3$9H2O) and sublimed sulfur were bought from Aladdin Chemicals (China). Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium sulfide (Li2S) and tetraethylene glycol dimethyl ether (TME) were provided by Songjing New-Energy Technol. (China). 2.2. Syntheses of carbon/Fe/Fe3C interlayers Fe(NO3)3$9H2O was dissolved in 10 mL of deionized water to form an aqueous solution of Fe(NO3)3, which was dripped onto cellulose papers. The cellulose papers adsorbed with Fe(NO3)3 were dried at 60
C, and calcined at 1000
C in an argon flow to generate

Fig. 1 shows the microstructures and morphologies of CF interlayers. The CF0 interlayer is made up of interconnected cellulosederived carbon fibers (Fig. 1a), and could be easily prepared as freestanding membranes for assembly of coin-type cells (Fig. 1a inset). The cross-staggered cavities in the interlayer allow the electrolyte to infiltrate through the interlayer. The CF1, CF2 and CF5 interlayers exhibit similar microstructures and morphologies to those of CF0 (Fig. 1aed, insets). However, the surface of CF0 interlayer is very smooth (Fig. 1e); while those of CF1, CF2 and CF5 are rough because of the in situ formed Fe/Fe3C nanoparticles. The Fe/ Fe3C nanoparticles on the surface of CF1 (Fig. 1f), CF2 (Fig. 1g), and

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Fig. 1. SEM images of (a, e) CF0, (b, f) CF1, (c, g) CF2, and (d, h) CF5 interlayers with insets of corresponding optical images. (A colour version of this figure can be viewed online.)

CF5 (Fig. 1h) are approximately 250, 125 and 600 nm, respectively. With increasing the amount of Fe(NO3)3$9H2O, more and more Fe/ Fe3C nanoparticles are embedded in the graphitized carbon matrix. In fact, the in situ formation of Fe/Fe3C nanoparticles and the carbonization of the cellulose filter papers occur at the same time. The cellulose filter papers are carbonized by the heating treatment, accompanying by the release of gaseous byproducts such as CH4, H2, CO and CO2. At the same time, the adsorbed Fe(NO3)3$9H2O is decomposed to Fe2O3, which is then reduced to be Fe nanoparticles in situ by the carbon or reducing gases resulted from the cellulose filter papers. Meanwhile, the Fe nanoparticles tend to be embedded in the carbon matrix, and serve as catalysts to improve graphitization of the cellulose paper-derived carbon. Furthermore, some of Fe nanoparticles react with carbon to form Fe3C. Therefore, the end products of the whole process are Fe/Fe3C. The whole process can be described with the following equations. 2Fe(NO3)3$9H2O / Fe2O3 þ 6HNO3 þ 15H2O

(1)

Fe2O3 þ 3CO / 2Fe þ 3CO2

(2)

Fe2O3 þ 3H2 / 2Fe þ 3H2O

(3)

2Fe2O3 þ 3C / 4Fe þ 3CO2

(4)

Fe

CðamorphousÞ!CðgraphitizationÞ

(5)

Fe þ 3C / Fe3C

(6)

Therefore, the size of Fe/Fe3C nanoparticles could be restricted by the carbon matrix [39,40], making the size of Fe/Fe3C nanoparticles in CF2 interlayer is smaller than that in CF1. It is noted that the carbon matrix cannot hold too many Fe/Fe3C nanoparticles, thus leading to larger Fe/Fe3C nanoparticles in CF5. Different from the surface observation, the high-resolution SEM images of fracture surfaces of CF2 and CF5 interlayers show that nanoparticles embedded inside the carbon fibers are smaller than those on the surface of carbon fibers (Figs. S1 and S2), further indicating that Fe/ Fe3C nanoparticles could be restricted by the carbon matrix. Based on Scherrer formula, the average sizes of Fe/Fe3C nanoparticles in CF2 and CF5 interlayers are 75 and 183 nm, respectively, which are

also smaller than the surface observation results. It is also seen that the Fe/Fe3C nanoparticles are well dispersed in the interlayers, implying that porous carbon network prevents aggregation of these Fe/Fe3C nanoparticles during the thermal synthesis process (Fig. S1). The small and uniformly dispersed Fe/ Fe3C nanoparticles in CF2 could expose more active sites and improve the contact efficiency between the CF2 interlayer and sulfur-containing species, leading to an enhanced catalytic efficiency. The fracture surfaces of CF0 (Fig. S3a), CF1 (Fig. S3b), CF2 (Fig. S3c), and CF5 (Fig. S3d) confirm the thickness and microstructure of a few fibers with cross-staggered cavities. The compositions of the CF0, CF1, CF2, and CF5 interlayers are evaluated with XRD patterns (Fig. 2a). CF0 has a broad peak at ~26
after graphitized at 1000
C. Compared to CF0, the peaks at ~26
for CF1 and CF2 become sharper, indicating the metallic Fe could promote graphitization of the carbon interlayer. A satisfactory conductive interlayer would help the transfer of electrons during the electrocatalytic process. Besides, there are three main diffraction peaks, which are consistent with the representative diffraction peaks of Fe (JCPDS no 89e7194). A little characteristic diffraction peak of Fe3C is observed, indicating that a small amount of metal Fe reacts with carbon at the high temperature. Elemental analyses demonstrate the distribution of Fe and C in a Fe/Fe3C nanoparticle. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image clearly shows a Fe/Fe3C nanoparticle (Fig. 2b); and its linear scan further indicates the content of C in Fe/Fe3C nanoparticles is very little (Fig. 2c). Corresponding elemental mappings of the Fe/Fe3C nanoparticles exhibit uniform distribution of Fe element (Fig. 2e). The content of C in the Fe nanoparticle should come from both Fe3C and carbon matrix, because Fe/Fe3C nanoparticles are embedded in the carbon matrix. Therefore, the content of C element in Fe/Fe3C nanoparticles should be smaller than observed (Fig. 2c and d), implying the content of Fe3C in Fe/Fe3C nanoparticles is small. TEM image of CF2 exhibits the embedded Fe/Fe3C nanoparticles (Fig. 2f). The spacing of the fringes in the inset is 2.03 Å, corresponding to the (110) plane of Fe. These results confirm in situ generated Fe/ Fe3C nanoparticles and the enhanced graphitization of carbon substrate. FTIR spectra are used to further confirm the graphitization of the carbon interlayer. The absorption peak of C]C stretching vibration shifts to 1629 cm1 in CF1, CF2 and CF5 interlayers as compared to

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Fig. 2. (a) XRD patterns of CF0, CF1, CF2 and CF5. (b) HAADF-STEM image of a Fe/Fe3C nanoparticle, and (c) linear elemental distribution of the nanoparticle marked in (b). EDX elemental distributions of (d) C and (e) Fe. (f) TEM and HRTEM images of CF2 interlayer. (A colour version of this figure can be viewed online.)

that of CF0, confirming that Fe/Fe3C promotes the graphitization of the carbon network (Fig. 3a). The Raman spectra also indicate the interaction between the Fe/Fe3C and carbon substrate (Fig. 3b). Compared to the ID/IG ratio of CF0 (1.13), those of CF1, CF2 and CF5 become smaller and smaller with the order: CF1 (0.86) > CF2 (0.51) > CF5 (0.28), which is ascribed to the enhanced graphitization of carbon by the catalytic effect of the in situ generated Fe/Fe3C nanoparticle [41,42]. The graphitization extent of carbon interlayers increases with the Fe/Fe3C content. The Fe/Fe3C contents in CF1, CF2

and CF5 are determined based on their TGA curves (Fig. 3c) and the XRD pattern of CF2 calcined in air (Fig. S4). The content of Fe/Fe3C nanoparticles increases with the dosage of Fe(NO3)3$9H2O. XPS spectra prove the presences of C, O and Fe elements in CF1, CF2 and CF5; whereas, only C and O elements are present in CF0 (Fig. 3d). However, the O1s XPS of CF5 is relatively sharp, indicating a high contribution of oxygen in the CF5, which should come mainly from Fe2O3 intermediate phase rather than the carbon matrix, because the CeO bond in the C1s spectrum is very low (Fig. 3f). An excessive

Fig. 3. (a) FTIR spectra and (b) Raman spectra of CF0, CF1, CF2 and CF5 interlayers. (c) TGA curves of CF1, CF2 and CF5 interlayers. (d) Survey scans spectra of CF0, CF1, CF2 and CF5 interlayers. (e) Fe 2p XPS spectra of CF1, CF2 and CF5. (f) C 1s XPS spectra of CF0, CF1, CF2 and CF5. (A colour version of this figure can be viewed online.)

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amount of Fe(NO3)3 could result in partial carbothermal reduction, forming a small amount of Fe2O3 intermediate phase. The peak at 706.4 eV corresponds to Fe0 (Fig. 3e), proving directly the presence of metal Fe. In the C 1s spectra of CF1, CF2 and CF5, the new peak corresponds to CeFe bonding (Fig. 3f). It is difficult to verify the carbon content in the form of Fe3C and carbon matrix. The CeFe bond should originate from both Fe3C and the interaction between Fe/Fe3C nanoparticles and carbon material. However, the CeFe bond should mainly originate from the interfacial CeFe bond rather than Fe3C, because the content of Fe3C in Fe/Fe3C nanoparticles is small. The ratio of CeFe bond in the C 1s spectra increases first and decreases later (Table S1). CF2 has the highest ratio of 39.5%, followed by CF1 (38.8%) and CF5 (32.1%). The interfacial CeFe bond indicates the efficient contact between Fe and carbon by covalent bonding, resulting in rapid transports of ions and electrons for electrocatalytic reaction [43e46]. It means that the CF2 is more suitable as the catalytic interlayer, boosting the redox kinetics of polysulfides and suppressing the shuttle effect. These results show that the in situ formed Fe/Fe3C nanoparticles not only promote the graphitization of the cellulose filter paper-derived carbon, but also generate CeFe bonds to facilitate the electrocatalytic reaction. The electrocatalytic effects of Fe/Fe3C nanoparticles are investigated using symmetrical Li2S6eLi2S6 cells with polarization curves and Tafel curves, which are widely used to study the kinetics of liquid-liquid interface conversion reactions in LSBs. As shown in Fig. 4a, the current response of the cell with CF2 interlayer at a voltage bias of 0.8 V significantly increases as compared to CF0, indicating that the Fe/Fe3C nanoparticles dynamically accelerate the electrochemical reactions of polysulfides. In other words, when the soluble polysulfide diffuses through the Fe/Fe3C nanoparticles, it could be rapidly reduced to low-order polysulfide by the electrocatalysis of the Fe/Fe3C nanoparticles to mitigate the shuttle effect. Tafel plots directly demonstrate the responsive current density to the voltage change, revealing the kinetic conversion reactions during the redox process [47]. The Tafel curves for initial lithiation of symmetrical Li2S6eLi2S6 cells with CF0 and CF2 are shown in Fig. 4b. The symmetrical cell with CF2 exhibits higher exchange current and lower polarization potential than those with CF0, demonstrating a more facile polysulfide redox reaction.

Fig. 4. (a) Polarization curves and (b) corresponding Tafel curves for the initial lithiation of symmetrical Li2S6eLi2S6 cells with CF0 and CF2 interlayers. Electrochemical impedance plots of symmetrical Li2S6eLi2S6 cells with (c) CF0 and (d) CF2 interlayers. (A colour version of this figure can be viewed online.)

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Besides, the electrochemical impedance plots also verify the electrocatalysis of the Fe/Fe3C nanoparticles by prompting the charge transfer rate. The charge transfer resistance (Rct) of the cell with CF2 remarkably shrinks from 48 U of CF0 (Fig. 4c) to 1 U (Fig. 4d). Therefore, the charge transfer at the Fe/Fe3C-polysulfide interface is much faster than at the carbon-polysulfide interface, indicating the greatly enhanced redox kinetics of polysulfide by the in situ formed Fe/Fe3C nanoparticles. Coin-type asymmetrical LieS cells are used to investigate the liquid-solid interface reactions with the Fe/Fe3C nanocatalysts. The intensities of the redox reaction peaks of the cell with the CF2 interlayer (Fig. 5b) are much higher than those of the cell with CF0 (Fig. 5a). For comparison, the cell without an interlayer is also examined with cyclic voltammetry (CV), and its broad redox peaks show severe polarization (Fig. S5). These results indicate that the carbon substrate could improve the electrochemical reactions, and the in situ formed Fe/Fe3C nanocatalyst could efficiently utilize sulfur-containing species and deepen the redox reactions. In addition to one anodic peak, the CV curves of the cells have two cathodic peaks corresponding to liquid-solid conversion reactions of S8 4 Li2S8 and Li2S4 4 Li2S2/Li2S [48,49]. In the first CV cycle, there are two cathodic peaks at 2.25 V (I) and 2.00 V (II) and one anodic peak at 2.46 V (III) for the cell with the CF2 interlayer (Fig. 5c). Compared to the cell with the CF0, the presence of Fe/Fe3C nanoparticles virtually mitigates the polarization by raising the cathodic peaks and reducing the anodic peak. Furthermore, the second-cycle CV curve overlaps well with the third-cycle curve for the cell with the CF2, indicating that the Fe/Fe3C nanoparticles suppress the electrochemical polarization and enhance reversibility of the redox reaction. More importantly, the cathodic peak (II) changes remarkably, indicating that the Fe/Fe3C catalyst is more likely to accelerate the reduction of high-order polysulfides to loworder polysulfides, thus suppressing the shuttle effect effectively. The onset potential changes of the three redox peaks could validate the electrocatalytic effects of the Fe/Fe3C nanoparticles on redox conversion reactions of the sulfur-containing species. In the differential CV curves of the cells (Fig. 5d and e), the baseline voltage and current are defined as the values before the redox peaks, where the variation on current is the smallest (dI/dV ¼ 0). The orange and blue dotted lines represent the onset potentials of the cathodic peaks and the anodic peaks, respectively. Compared to the onset potentials of the cell with CF0, the presence of Fe/Fe3C nanoparticles increases the onset potentials of the cathodic peaks (I and II) and decreases that of the anodic peak (III) (Fig. 5f), demonstrating that the Fe/Fe3C nanocatalyst has an electrocatalytic effect on enhancing the polysulfides redox kinetics. Similar to the peak voltage change of the cell with CF2, the onset potential of the cathodic peak (II) increases remarkably, confirming the accelerated reduction of high-order polysulfides to low-order polysulfides because of the suppression of shuttle effect by the Fe/Fe3C nanocatalyst. In addition, the colors of the Li2S4 solution with interlayers and without interlayer are almost the same after adsorption for 24 h (Fig. S6), further revealing that the good electrochemical performances are mainly attributed to the electrocatalytic effect of Fe/Fe3C, rather than the adsorption effect. In addition to the electrocatalytic effect of Fe/Fe3C nanoparticles, the conductivity of the carbon substrate and the size of Fe/Fe3C nanoparticles are also crucial for electrochemical performances of LSBs. As shown in Fig. 6a, the conductivity of CF interlayers increases with increasing the content of Fe/Fe3C, and CF5 has the highest electrical conductivity. Therefore, the EIS curves of the cells with CF interlayers (Fig. 6b) have much smaller semicircle diameters and higher slopes than those of the cell without interlayer (Fig. S7). It implies that the introduction of CF interlayer reduces the charge-transfer resistance and enhances the lithium-ion diffusion

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Fig. 5. CV curves of cells with (a) CF0 and (b) CF2 interlayers; and (c) corresponding peak voltages of cells with CF0 and CF2 interlayers. Differential CV curves of cells with (d) CF0 and (e) CF2 interlayers, and (f) corresponding onset potentials of cells with CF0 and CF2 interlayers. (A colour version of this figure can be viewed online.)

Fig. 6. (a) Electrical conductivities of CF interlayers. (b) EIS curves of cells with CF interlayers. Comparisons of (c) peak voltages and (d) onset potentials of cells with and without CF interlayers (lines between points are guide to the eye). (A colour version of this figure can be viewed online.)

rate. The CV curves of the cell with CF1 (Fig. S8) are roughly the same as those with CF0, while the CV curves of the cell with CF5 (Fig. S9) are nearly the same as those with CF2. Their peak voltages (Fig. 6c) and onset potentials (Fig. 6d) also show obvious relationships with the conductivities of the CF interlayers. The change trend of the cathodic peaks (I and II) almost has a positive relationship with the conductivity of the CF interlayers, while the change of the anodic peak (III) shows almost a negative trend. It indicates that the improved conductivity not only promotes the electrochemical redox kinetics of sulfur-containing species, but also deepens the

reaction depth [50,51]. However, the extremum of every curve comes from the cell with CF2, although CF2 has not the best conductivity. In other words, CF2 interlayer exhibits the best electrocatalytic effect and the fastest polysulfides redox kinetics. Besides the improved conductivity, another reason is the efficient contact between Fe/Fe3C nanocatalysts and sulfur-containing species. The smallest size of Fe/Fe3C nanoparticles in CF2 makes themselves expose more active sites, facilitating the contact between Fe/Fe3C nanocatalysts and sulfur-containing species. In addition, the interfacial CeFe bonds (Fig. 3f, Table S1) also benefit the charge transfer in the electrocatalytic process. As a result, the cell with the CF2 interlayer exhibits the best electrochemical reaction kinetics and satisfactory electrochemical performances, because of the synergistic effects of high conductivity, high contact efficiency, and the electrocatalytic effect of Fe/Fe3C nanoparticles. The lithium storage performances of the cells with different interlayers are measured between 1.7 and 3.0 V. The galvanostatic discharge/charge profiles of the cells with CF0, CF1, CF2 and CF5 at 0.1C and 2C are shown in Fig. 7a and b. There are two discharge plateaus at ~2.30 and ~2.05 V and one charge plateau at ~2.40 V, corresponding to the reduction of long-chain polysulfides to insoluble lithium sulfide and their reverse process, respectively. As proved above, the Fe/Fe3C nanoparticles decorated on graphitized carbon interlayers could significantly enhance the utilization of sulfur-containing species by blocking and recycling intermediate polysulfides, because of the fast charge transfer at the interfaces of intermediate polysulfides and Fe/Fe3C nanoparticles. Therefore, the specific capacities of the cells with CF interlayers are higher than that of the cell without the interlayer. Interestingly, the cell with CF2 interlayer has the highest discharge/charge capacities, and it also exhibits a much lower polarization at both low current density of 0.1C and high current density of 2C. The lower polarization indicates the accelerated redox kinetics of polysulfides by the Fe/Fe3C catalysts. The cyclic performances of the cells with and without CF interlayers are shown in Fig. 7c. As expected, the cell with CF2

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electron transfer and enhanced redox kinetics by the catalysis of Fe/ Fe3C nanoparticles. 4. Conclusion Freestanding cellulose paper-derived graphitized carbon/Fe/ Fe3C catalytic interlayers are designed and synthesized to mitigate the irreversible loss of soluble polysulfides and improve the utilization of sulfur. The Fe nanoparticles result from the carbothermal reduction of iron ions by the cellulose paper-derived carbon, while the electrical conductivity of the carbon network is promoted by graphitization effect of the Fe nanoparticles during the calcination treatment. The freestanding graphitized carbon network confines and accommodates the in situ formed Fe/Fe3C nanoparticles for enhancing the contact efficiency of the Fe/Fe3C nanoparticles with sulfur-containing species. Therefore, the fast electron transfer and the enhanced redox kinetics at the interfaces between sulfurcontaining species and the Fe/Fe3C nanocatalysts suppress the shuttle effect and increase the utilization of sulfur. The cell with CF2 exhibits excellent cyclic and rate performances. The reversible specific capacity is still ~1000 mA h g1 at 0.2C after 50 cycles, and especially, the reversible capacity retains at 735 mA h g1 at 2C. Even after 200 cycles at the current density of 1C, the reversible capacity of the cell with CF2 stably maintains at 772 mA h g1. This work demonstrates a facile and efficient approach to construct conductive carbon interlayer decorated with metal nanocatalysts for applications in secondary batteries, supercapacitors, and flexible energy storage devices. Fig. 7. Galvanostatic discharge/charge profiles of cells without and with CF interlayers at (a) 0.1C and (b) 2C. (c) Cyclic performances and (d) rate performances of cells without and with the CF interlayers. (e) Long cyclic performances of the cell with CF2 at 1C. (A colour version of this figure can be viewed online.)

exhibits excellent electrochemical properties as compared to the cells with CF1 and CF5. It delivers a charging capacity of 1556 mA h g1 at 0.1C, and its reversible specific capacity after 50 cycles is still close to 1000 mA h g1 at 0.2C. Although bare carbon interlayers could raise cyclic capacities of the cell, the presence of Fe/Fe3C nanoparticles improves the reversible specific capacity and cyclability more obviously. Indeed, the electrocatalytic effect of Fe/ Fe3C nanocatalysts and the conductive carbon substrate help suppress the shuttle effect, benefiting the superior specific capacity. The Coulombic efficiency further confirms the excellent electrochemical performances of the cells with CF interlayers (Fig. S10). The Coulombic efficiency of the cell without any interlayer is more than 100% with cycles and much higher than those of the cells with CF interlayers. However, the Coulombic efficiencies of the cells with CF interlayers are basically maintained at 100%, which adequately manifests the mitigation of the shuttle effect due to the Fe/Fe3C catalysis. The rate performances of the cells with CF0, CF1, CF2 and CF5 interlayers are evaluated at different current rates (Fig. 7d). At current densities of 0.1, 0.2, 0.5, 1 and 2C, The cell with CF2 exhibits the highest specific capacities of 1170, 1085, 1005, 875 and 735 mA h g1, respectively. Remarkably, when the current density returns to the initial 0.1C, the reversible specific capacity quickly retains at 1150 mA h g1 after 60 cycles. In contrast, the cells with other interlayers deliver much lower specific capacities, especially at high current densities. Moreover, the reversible specific capacity of the cell with CF2 stably maintains at 772 mA h g1 at 1C after 200 cycles (Fig. 7e), exhibiting an excellent long cycle performance as compared to those reported in the literature (Table S2) [19,49,52e56]. Such exceedingly high-capacity and high-rate lithium storage performances are mainly ascribed to rapid

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