Carbon Cloth as Bifunctional Electrode for Effective

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Tungsten Nitride/Carbon Cloth as Bifunctional Electrode for Effective Polysulfide Recycling Yange Wang, Rongjie Luo, Yingge Zhang, Yan Guo, Yang Lu, Xianming Liu, Jang-Kyo Kim, and Yongsong Luo ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00165 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Energy Materials

Tungsten Nitride/Carbon Cloth as Bifunctional Electrode for Effective Polysulfide Recycling Yange Wang,a, b Rongjie Luo,a, b Yingge Zhang,a, b Yan Guo,a, b Yang Lu,a, b Xianming Liu, c Jang-Kyo Kimd and Yongsong Luoa, b, *1 a School

b Key

of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, P. R. China.

Laboratory of Microelectronics and Energy of Henan Province, Xinyang Normal University, Xinyang 464000,

P. R. China.

c College

d

of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471934, P. R. China.

Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear

Water Bay, Kowloon, Hong Kong, P. R. China.

Abstract The inevitably dissolution, diffusion and migration of polysulfides cause an irreversible loss of active material leading to poor cyclic performance in lithium sulfur batteries. Herein, a freestanding tungsten nitride nanorod/carbon cloth (WN/CC) interlayer is prepared by hydrothermal growth to function as both current collector and physicochemical barrier to soluble lithium polysulfides (Li2Sx). The cells containing a dual-functional interlayer deliver a significantly improved initial discharge capacity of 1337 mAh g-1 with a reversible capacity of 814.2 mAh g-1 after 500 cycles at 100 mA g-1. The enhanced electrochemical performance is attributed to the highly adsorptive WN nanorods grown on conductive CC to entrap polysulfides, leading to effective 

To whom correspondence should be addressed: Tel./fax: +86 0376 6390801, E-mail: [email protected] (Y. S.

Luo).

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recycling of active materials. The density functional theory (DFT) calculations prove important roles of the WN (200) surface in entrapping polysulfides through their strong adsorption energies (3.21-4.67 eV) with Li2Sx and S8. The potential of the dualfunctional WN/CC composite interlayer in improving the electrochemical performance of lithium sulfur batteries has not been reported previously.

Keywords: dual-functional interlayer; DFT calculations; lithium sulfur battery; tungsten nitride; polysulfide adsorption; interlayer.

1. Introduction Li-sulfur battery (LSBs) has attracted much attention thanks to its exceptionally high theoretical capacity of 1675 mAh g-1 1, 2 while sulfur cathode material is inexpensive, nontoxic, ubiquitous and environmentally friendly. However, the practical application of sulfur cathodes has been severely hindered because of the inevitable migration of polysulfides, the so-called shutting effect, leading to anode pollution and the irreversible loss of active materials,3 main reasons for poor cyclic stability of LSBs.4-6 Many attempts have been made to encapsulate sulfur into different host materials to mitigate the shuttling effect, such as carbon materials (carbon nanospheres@carbon,7 nitrogen-doped porous carbon8), metal oxide (C@TiO2@C,9 NiFe2O4/N-doped graphene10), metal nitrides (VN,11 TiN12,

13

and WN14) and metal sulfides (MoS2,15

Co3S416 and Co9S8@CNT17). However, this strategy has not been quite successful partly because the polysulfides cannot be completely restricted in the cathode at a high sulfur loading. The use of modified separators capable of strong physicochemical adsorption with polysulfides has also been suggested as a viable approach.4,


18, 19

Functional

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separators or interlayer/separators prepared using different materials have been developed, such as multiwall carbon nanotube/N-doped carbon quantum dots (MWCNT/NCQD),20 niobium carbide,21 VS4/graphene,22 MoO3@CNTs,23 functional carbon nanofibers,24 TiO2 nanotubes/graphene oxide,19 MoS2,25 Ni3FeP26 and Sb2S3 nanosheet/CNTs.27 These modified separators were able to confine the polysulfides within the cathode. Our team studied the application of titanium nitride (TiN) with different pore contents as a cathode material in LSBs, and found that the TiN with honeycomb structure could provide excellent performance of 650.4 mAhg-1 after 500 cycles at 100 mA g-1.28 Therefore, we extend our research to different transition metal nitrides and used them as interlayers for LSBs to better understand the role of morphology and composition in improving the electrochemical properties. This work is dedicated to developing a freestanding composite interlayer consisting of tungsten nitride nanorods hydrothermally grown on carbon cloth (WN/CC) for improved electrochemical performance of lithium sulfur cells for the first time. The sulfur slurry was coated onto one side of the WN/CC interlayer, which not only functioned as the cathode current collector, but also as an efficient polysulfide immobilizer to facilitate recycling of sulfur via combined chemical adsorption and physical barrier. The ultrahigh metallic conductivity of WN nanorods and carbon cloth enabled fast Li ion diffusion and electron transport ability. The WN/CC cells with a higher loading of S (~7.42 mg cm-2) exhibited an excellent initial discharge capacity of 1337 mAh g-1 and superior cycle stability of 814.2 mAh g-1 after 500 cycles at 100 mA g-1.



2. Experimental 2.1 Preparation of WN/CC The WN/CC composites were prepared via a hydrothermal reaction similar to the previous report on hydrogen evolution cathode.29 Prior to the growth process, the carbon cloth of 4 cm × 4 cm square was ultrasonically cleaned in HCl, deionized (DI) water and alcohol in sequence. The H2WO4 solution was prepared by mixing 4.123 g Na2WO4·2H2O and 4.4 g H2C2O4 with 107 ml HCl (0.2 mM) solution at room temperature, which was then diluted to 250 ml for further use. The 80 ml H2WO4 precursor was transferred into a 100 ml Teflon-lined stainless autoclave, and 4 g (NH4)2SO4 was added to the solution. After vigorous stirring for 30 min, the carbon cloth was immersed in the solution and kept at 180 oC for 16 h. After cooling to room temperature, the carbon cloth was taken out and rinsed several times with DI water to remove loose materials on the product surface, and dried at 80 oC. Finally, the asprepared sample was calcined at 450 oC for 1 h in Ar and at 600 oC for 2 h in a NH3 atmosphere. 2.2 Characterization The crystalline structure of the products were identified by X-ray diffraction (XRD) analysis (a D8 Advance (Bruker) automated X-ray diffractometer system with Cu-Kα (λ = 1.5418 Å) over 2θ ranging from 10o to 80o). Raman spectra of the materials were measured on a Renishaw INVIA system. The microstructure was examined by field emission scanning electron microscope and transmission electron microscope (FESEM and TEM, JEOL S-4800 and JEM-2010). The X-ray photoelectron spectroscopy (XPS)


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was performed on a PerkinElmer PHI 5600 XPS system. The thermogravimetric analysis (TGA, TA Instruments, SDT-Q600) was conducted at a heating rate of 10 oC min-1 from room temperature to 800 oC under flowing air. 2.3 Electrochemical measurements The sulfur cathode material was prepared by mixing sulfur, carbon black, and polyvinylidene fluoride (PVDF) with a weight ratio of 7:2:1 in N-methyl-2pyrrolidinone (NMP). The slurry was coated onto one side of the WN/CC composite which was dried at 60 oC for 12 h under vacuum. The mass loading of the active materials (S) on WN/CC interlayer is approximately 7.42 mg cm-2. For interlayer-free electrodes, scrape the active material onto the Al foil. 2032-type coin cells were assembled in an Ar-filled glove box using a polypropylene microporous separator (Celgard 2400). 1.0 M bis (triﬂuoromethane) sulfonimide lithium (3M, USA) in dimethoxyethane and 1, 3-dioxolane solvents (1:1 in volume, Alfa Aesar, China) with 0.1 M LiNO3 additive was used as the electrolyte. The assembled coin cells were kept at 25 oC for 8 h and charge/discharge cycled between 1.7 and 2.8 V on Neware battery testing system. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were measured using a CHI 660E electrochemical workstation (Chenhua, Shanghai, China) over a frequency range of 100 kHz to 0.1 kHz with an AC oscillation of 5 mV. All measurements were made at room temperature. The Li+ diffusion coefficients for the different cells were determined using a series of cyclic voltammograms based on the Randles-Sevick equation given below:24, 25 I = 269000 × n1.5 × A × D0.5 × C ×v0.5


(1)


1

𝐼

𝐷0.5 = 269000 × 𝑛1.5 × 𝐴 × 𝐶 × 𝑣0.5

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(2)

in which I represents the peak current (in A); n is the number of electrons (= 2 for Li-S cells); A refers to the electrode area (= 1.54 cm2), D refers to Li+ diffusion coefficient (in cm2 s-1), C represents the Li+ concentration (= 0.55 mol L-1) and v is the scan rate (in V s-1). D was determined directly from the slope of I vs v0.5 when n, A and C remained unchanged. To study the beneficial role of WN/CC interlayer, the polysulfides diffusion tests were conducted where different interlayers were separately added in a polysulfide solution and the color changes of the solutions were recorded using a digital camera. The polysulfide solution was synthesized after reaction between the stoichiometric amounts of S8 and Li2S (Sigma) in component solvent (DME and DOL of 1:1 in volume ratio), and vigorous magnetic stirring at 60 oC for 24 h. All the procedures were performed in an Ar-filled glovebox. The blank electrolyte after 16 h of osmosis was characterized (UV-1901 spectrophotometer, Beijing puxi), and the blank solvent was used as a reference. The density functional theory calculation was completed in the Materials Studio 8.0 software, and the adsorption energy between WN (200) and carbon cloth surface and Li2Sx/S8 were simulated by generalized gradient approximation (GGA) with Perdew-Burke-Ernzerh (PBE) exchange-correlation functional in the CASTEP module.30 The k-point sampling were used for WN (200) and carbon cloth surface, and the cut-off energy was set at 500 eV. In addition, in order to avoid continuous slabs


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interaction, a vacuum thickness (larger than 10 Å) was added on the WN (200) and carbon cloth surface. In the geometry optimization process, the atoms in WN (200) and carbon cloth slabs were fixed at the cut-off position. The adsorption energy (Ea) was calculated by Eq. (3) Ea = E(Li2Sx) + E(surface) - E(Li2Sx/surface)

(3)

Where E(Li2Sx), E(surface) and E(Li2Sx/surface) represent the energy of Li2Sx in vacuum, the energy of WN (200) and carbon cloth surface and the total energy of absorbed system, respectively.

3. Results and discussion

Figure 1. (a) Schematic representation of the fabrication procedure of WN/CC composites. Schematic illustration of (b) fast kinetics of WN nanorod arrays on CC layer (WN/CC), and (c) three cathode structures: (i) S, (ii) CC and (iii) WN/CC.



The preparation procedure of the dual-functional WN/CC electrode is schematically shown in Figure 1a. First, WN nanorods were grown on CC by a two-step process, and in view of color change of W-CC from blue-purple to black after nitridation (Figure S1). Finally, sulfur was coated on one side of WN/CC interlayer. Figure 1b shows how the WN/CC cathode framework works during cell operation. The WN/CC interlayer functioned not only as a current collector for fast electron transfer, but also as a barrier to polysulfide migration between the sulfur cathode and separator. Finally, three different cathode structures: (i) S, (ii) CC and (iii) WN/CC were designed, as shown in Figure 1c, which were tested to evaluate the ameliorating roles of WN nanorod arrays and CC layer in Li-S cells.

Figure 2. SEM images of (a) CC, (b) WO3/CC, (c) WN/CC. (d) TEM and (e) HRTEM image of WN nanorods with the corresponding fast Fourier transform (FFT) pattern in inset. (f) XRD pattern of WN/CC. The insets of (b) and (c) are the low-magnification SEM images of WO3/CC and WN/CC, respectively.

The SEM image of Figure 2b shows that the sub-micrometer WO3 with a typical


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needle- or rod-shape morphology was uniformly grown on CC (Figure 2a), and the nanorod structure was maintained after nitridation (Figure 2c). The TEM image of a WN nanorods (Figure 2d) taken off from WN/CC reveals its length of about 630 nm and a diameter of 35-65 nm. The HRTEM image in Figure 2e presents a WN crystalline structure with a lattice spacing of 0.206 nm, corresponding to the (200) plane29 according to the fast Fourier transform (FFT) pattern. The XRD pattern of the WN/CC composite obtained after nitridation is displayed in Figure 2f. The relatively broad peak at ~26.4° is assigned to carbon (JCPDS No. 75-1621), whereas the peaks at 37.7, 43.5, 63.6 and 76.4 are indexed to the (111), (200), (220) and (311) planes of WN (JCPDS No. 65-2898), respectively.31 There were no redundant peaks, suggesting the transformation of WO3 to WN. After nitridation, the intensity ratio, ID/IG, obtained from the Raman spectra (Figure S2) marginally increased from 0.92 to 0.99, signifying Ndoping of CC.32 And according to the TGA analysis (Figure S3), the mass content of W element was 48.5 wt% (61.2*183.84/(183.84+16*3) = 48.5 wt%), and the carbon content is 37.4 wt%. In addition, the slight weight loss before 300 oC can be attributed to the remove of adsorbed water on WN/CC composite.

The elemental composition and valence of WN/CC were investigated by X-ray photoelectron spectroscopy (XPS). The survey XPS spectrum in Figure S4a exhibits C, N, W and O peaks and these peaks were deconvolued in Figure S4b-d. The oxygen arose mainly from the formation of an oxide or oxynitride passivation layer on the surface of WN.33, 34 The deconvoluted C 1s spectrum (Figure S4b) had five peaks at 284.5 eV (C-C, C=C and C-H bonds), 285.5 eV (C-N bond), 286.8 eV (C-O bond),



288.6 eV (C=O bond) and 291.3 eV (O=C-O).35 The two peaks located at 32.4 and 34.2 eV in the deconvoluted spectrum of W4f (Figure S4c) can be assigned to the W-N bonds. The two peaks at 35.4 and 37.6 eV are attributed to the W-O bonds, likely resulting from the unavoidable surface oxidation of WN upon exposure to air.36 In the deconvoluted N 1s peak (Figure S4d), the main peak located at 397.3 is attributed to the W-N bond,33,

35

while the other peaks at 398.2, 400 and 401.9 eV represent

pyridinic-N, pyrrolic-N and graphitic-N, respectively, a reflection of N-doping of carbon.35-37


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Figure 3. Polysulfide diffusion tests. (a) Diffusion test apparatus consisting of interlayer, separator, polysulfide solution and fresh electrolyte. The polysulfide bottle was dipped into the blank electrolyte and the concentrations of polysulfides that penetrated through the interlayer/separator layers were monitored visually and by UVvis spectroscopy. (b) Photographs of polysulfide diffusion across the separator with/without an interlayer taken after testing times of 0, 4, 8 and 16 h. (c) UV-vis absorption spectra of polysulfide solutions after 16 h. (d) Molecular structures of Li2Sx and S8 used in DFT calculations. (e) Optimized configurations for binding of Li2Sx and



S8 with the WN (200) plane and the corresponding adsorption energies.

To reveal the role of WN/CC composite interlayer in hindering the shuttling effect, the polysulfide diffusion tests were conducted, as shown in Figure 3a and b. Figure 3a is a schematic diagram of the diffusion test apparatus prepared according to the previous report18 where Li2S6 solution was used to simulate the shuttle effect. The results in Figure 3b indicates that with increasing diffusion time, the electrolyte color was gradually changed from transparent to yellow. The darker yellow color of the electrolyte means more polysulfide diffusion and thus less entrapment of polysulfides by the interlayer.22, 24 It can be concluded that the existence of an interlayer obviously mitigated the shutting effect, and the WN/CC interlayer was far more effective than the neat CC interlayer. Besides the direct visual comparison, the polysulfide solutions obtained after 16 h of diffusion was subjected to UV-Vis spectroscopy (Figure 3c). It is clearly seen that the absorbance rate was increases successively in the order of the samples with WN/CC and CC interlayers and without interlayer. The low absorbance rate further proves the effective polysulfide entrapment by the WN/CC interlayer. The sharp peak located at ~315 nm for the sample without interlayer is attributed to the S62or S42- species.38 The foregoing experimental findings were further verified by the adsorption energies between different polysulfides and WN based on the DFT calculations. The molecular structures of Li2Sx (x = 4, 6 and 8) and S8 are shown in Figure 3d, and the lowest energy adsorption geometric configurations of these sulfur species on the WN (200) plane and carbon cloth surface are presented in Figure 3e and Figure S5a. In


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Figure 3e, the favorable binding sites of the Li atoms were the N atoms, while the sulfur tended to bond with the W and N atoms. The adsorption energies of Li2Sx on WN (200) surfaces were 3.21, 3.68, 4.67 and 4.59 eV, respectively. On the other hand, the corresponding adsorption energies of Li2Sx (x=4, 6, 8) and S8 on carbon cloth surfaces are 0.62, 0.71, 1.33, 0.95 eV (Figure S5). The higher binding energy indicates the stronger chemical anchoring of soluble Li2Sx. Unlike 2D materials which prefer a moderately low adsorption energy for polysulfide entrapment,27, 39 the strong bonding energies of WN nanorods with polysulfide species indicate that the WN effectively adsorbs Li2Sx/S8 species by forming S-W-N bond.14, 40, 41 The anchoring of polysulfides facilitates recycling of active materials and thus contributing significantly to enhanced battery performance.

Figure 4. (a-c) Schematic illustrations of the electron transport pathways in different



hierarchical structured electrodes. CV curves at various voltage scan rates and corresponding linear fits of the peak currents of Li-S cells without interlayer (d, g), with CC (e, h) and WN/CC interlayers (f, i).

Figure 4d-f shows the CV curves measured at different scanning rates to study the Li+ diffusion coefficients. All CV curves had two cathode peaks and two closely spaced anode peaks, consistent with the galvanostatic charge/discharge profiles (Figure 5a-c). The Li+ diffusion rates, D, were taken directly from the slope of the peak current (I) vs v0.5 plots (Figure 4g-i), assuming constant n, A and C values. Compared with the cells without and with a CC interlayer, the cell with a WN/CC interlayer had consistently much higher slopes for all peaks, A (0.72), B (0.35) and C (0.31), and according to Eq. (2), the Li+ diffusion rate of WN/CC cell is much higher than that of the other two cells. As illustrated in Figure 4a-c, this can be ascribed to WN provides more active sites, contact area and conductivity. The good properties of Li+ diffusion in the cell can effectively improve the rate performance of the battery.


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Figure 5. Charge/discharge profiles of the cells (a) without an interlayer, with (b) CC and (c) WN/CC interlayers measured at 100 mA g-1. (d) Upper plateau discharge capacity (QH) at 100 mA g-1 of the cells with CC and WN/CC interlayers. (e) Rate capabilities and (f) cyclic stability of the cells with different interlayers at 100 mA g-1.

The galvanostatic charge/discharge profiles of the cells without and with different interlayers measured at 100 mA g-1 are presented in Figure 5a-c. The discharge profiles can be divided into two regions, namely the upper and lower plateau. The upper



discharge plateau corresponds to the conversion of sulfur to Li2Sx (4 < x ≤ 8), and the lower plateau represents the further conversion to the final product, Li2S2/Li2S.42 The interlayer-free cells had a low discharge capacity of 320 mAh g-1 (Figure 5a) owing to the irreversible loss of active materials on the cathode.43 In contrast, the cells with a WN/CC interlayer had a much higher discharge capacity of ~1050 mAh g-1 (Figure 5c), as a result of the WN/CC interlayer effectively mitigating the loss of soluble polysulfides in the cathode. The introduction of an interlayer in general reduced the electrochemical polarization or voltage hysteresis (ΔE, the overpotential between the oxidation and reduction plateaus in Figure 5a-c)44, and greatly enhanced the reaction kinetics in LSBs (Figure 4g-i).34, 45 The higher polarization (ΔE) of the cells without an interlayer or with a CC interlayer is attributed to the loss of active materials and the insulating Li2S2/Li2S layer deposited on the electrode surface.46 The lower ΔE ranging (0.2-0.22 V) and overlapped charge/discharge curves obtained in the 2nd cycle and onwards for the WN/CC interlayer are a reflection of improved reversibility and stability performance of the Li-S cells.20, 46, 47 As a consequence, the upper discharge capacity (QH) of the WN/CC cell was the highest among the three and was almost constant at 358 mAh g-1, as shown in Figure 5d. Figure 5e shows the rate capabilities of Li-S cells with different types of interlayers. The discharge capacities of the cell with a WN/CC interlayer were 1337, 971.7, 877.1 and 738.1 mAh g-1 at 100, 200, 500 and 1000 mA g-1, respectively, which are generally higher than those of the cells without an interlayer or with a CC interlayer. Remarkably, upon reverting the current density to 100 mA g-1, the electrode delivered a reversible


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capacity of 1050.9 mAh g-1, confirming excellent electronic and ionic transport properties and reaction kinetics.48 The cyclic stability measured of the cells with different interlayers (Figure 5f) exhibits significantly enhance discharge capacity and capacity retention of the cells with a CC and WN/CC interlayer, compared with the cells without, indicating beneficial role of the CC interlayer in mitigating shutting of polysulfides. The reversible discharge capacity of the cells with a WN/CC interlayer was much better than the other two cells with a remarkable remaining capacity of 814.2 mAh g-1 even after 500 cycles and a low capacity decay rate of 0.078% at 100 mA g-1. Figure S6 displays the cycling performance of the cells with WN/CC interlayer at high rates of 500 mA g-1 and 1000 mA g-1. At 500 mA g-1, a large discharge capacity of 1018.4 mAh g-1 is obtained in the initial cycle, and with a high Coulombic efficiency of over 98.4% after 100 cycles. Remarkably, at a higher rate of 1000 mA g-1, the WN/CC cell still can maintain a high specific capacity of 540.8 mAh g-1 after 100 cycles, which is benefited from the effective confinement of polysulfides. Table S1 presents a comparison of electrochemical performance of Li-S batteries with different types of separators/interlayers, indicating superior or competitive performance of the current WN/CC interlayer in terms of remaining capacity after long cycles and capacity decay rate. In particular, the almost 100 % Coulombic efficiency of the cell with a WN/CC interlayer signifies high sulfur utilization and reversibility of the electrode reaction.49 To determine the effect of interlayer on interface resistance, the EIS of different cells were studied before and after cycles and the results are shown in Figure S7a-d.



The equivalent circuits (Figure S7e) were used to determine three important impedance parameters which was summarized in Table S2. The cell without an interlayer displayed two semicircles with a large diameter at high frequencies and a smaller diameters at mid frequencies, corresponding to the charge transfer resistance (Rct) and the interface contact resistance (Rsf), respectively. The resistance of electrolyte (Re) of the CC cell is marginally lower than the WN/CC cell, i.e. 3.25 vs 3.88 , indicating that the addition of WN nanorods on CC lead to a slight increase in internal resistance. This observation is consistent with the similar slopes, δ values, between the two interlayers of the plots between the real part of impedance spectra and angular frequency, Z’ vs ω-1/2 (ω=2πf),17 as shown in Figure S7b. The Warburg factor (δ) is known to be inversely proportional to the diffusion coefficient of Li+ ion.50 More importantly, the WN chemically adsorbed polysulfides, reducing both the interfacial resistance and the charge-transfer resistance.20 Consequently, the Rct value sharply decreased from 24 to 0.89  after the interdiction of a WN/CC interlayer, thanks to the enhanced surface reactions. Further, the resistance of the cell with a WN/CC interlayer remained the lowest among the three cells even after 500 cycles (Figure S7c, Table S2), confirming the enhanced redox kinetics. The effective recycling of polysulfides by the combined action of WN and CC means that the insoluble Li2S2 and Li2S deposited on the electrode surface remained relatively thin.27 It is worth noting, however, that Warburg factor δ of the cell with a WN/CC interlayer became even smaller after cycles whereas those of the other cells surged significantly after cycles, as shown in Figure S7d. This observation signifies enhanced diffusion kinetics of the former cell, in contrast to the


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much deteriorated diffusion kinetics of the other cells, after cycles.51 The battery cells were disassembled after 500 cycles and the digital and SEM images taken of their separators facing the cathode side are shown in Figure S8. It is of interest to note that a large amount of yellow substance appeared on the separator without an interlayer or with a CC interlayer, while there was no visible yellow remnant on the separator of the WN/CC cell (Figure S8a). The SEM images further confirmed abundant polysulfide particles on the separators of the former two cells, indicating no or incomplete barrier capability of the CC interlayer, whereas the same was almost absent on the separator with a WN/CC interlayer verifying its highly efficient barrier functionality and thus facilitating recycling of active materials.52, 53 The excellent electrochemical performance and cyclic stability of the cells with a WN/CC interlayer are attributed to the distinctive, dual-functional features of the cathode. First, the WN/CC interlayer was able to effectively entrap and confine the soluble polysulfides in the cathode side by its combined actions of physical barrier and chemical adsorption, facilitating active material recycling and thus excellent cyclic stability. Second, the highly conductive WN/CC interlayer much reduced the interface resistance allowing fast electron and ion transfer.

4. Conclusions In summary, the dual-functional WN/CC interlayer has been successfully synthesized using the hydrothermal growth of WN nanorod arrays on CC. The interlayer greatly reduced the irreversible loss of polysulfides by combined actions of physical barrier and chemisorption, facilitating active material recycling within the cathode side thus



significantly improving the electrochemical performance of the Li-S cells. The cells containing a WN/CC interlayer delivered an ultra-high rate performance of 1337 mAh

g-1 at 100 mA g-1 and 738.1 mAh g-1 at 1000 mA g-1 with a remarkable remaining capacity of 814.2 mAh g-1 after 500 cycles at 100 mA g-1. The concept of dualfunctional interlayers made from a new WN nanorod structure may open new insight into designing other types of electrode material for high-performance rechargeable batteries.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 61574122 and 61874093), Zhongyuan Thousand Talents Plan - Science & Technology Innovation Leading Talents Project (No. 194200510009) and Xinyang Normal University Analysis & Testing Center.

ASSOCIATED CONTENT Supporting Information Additional information as noted in the text.

ORCID Yongsong Luo: 0000-0002-8000-3126

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Graphical Abstract (For Table of Contents Only)

The multifunctional WN/CC interlayer acts as an efficient polysulfide immobilizer and current collector, leading to improved sulfur utilization by physical barrier to and chemisorption of polysulfides.


Carbon Cloth as Bifunctional Electrode for Effective

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