A sandwich-structure composite carbon layer coated on separator to trap polysulfides for high-performance lithium sulfur batteries

Journal Pre-proof A sandwich-structure composite carbon layer coated on separator to trap polysulfides for high-performance lithium sulfur batteries Yaoyao Geng, Zhipeng Ma, Li Su, Lin Sang, Fing Ding, Guangjie Shao PII:

S0925-8388(19)33435-8

DOI:

https://doi.org/10.1016/j.jallcom.2019.152189

Reference:

JALCOM 152189

To appear in:

Journal of Alloys and Compounds

Received Date: 10 June 2019 Revised Date:

31 August 2019

Accepted Date: 6 September 2019

Please cite this article as: Y. Geng, Z. Ma, L. Su, L. Sang, F. Ding, G. Shao, A sandwich-structure composite carbon layer coated on separator to trap polysulfides for high-performance lithium sulfur batteries, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152189. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

A Sandwich-Structure Composite Carbon Layer Coated on Separator to Trap Polysulfides for High-Performance Lithium Sulfur Batteries Yaoyao Gengabc, Zhipeng Mac, Li Suc, Lin Sangb, Fing Dingb, Guangjie Shaoac* a

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China

b

National Key Laboratory of Science and Technology on Power Sources, Tianjin Institute of Power Sources, 300384, China

c

Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, 066004, China

*Corresponding Author E-mail: [email protected]

Abstract: Lithium sulfur battery with a high energy density holds great potential to be the next-generation storage device. However, the extensive commercial applications of Lithium sulfur batteries still have many obstacles, such as the most serious “shuttle effect”. Here, we synthesize a novel nitrogen-doped mesoporous carbon only by simple calcination and acid treatment processes to effectively adsorb 1

the diffused polysulfides. Then, the nitrogen-doped mesoporous carbon is sandwiched between two carbon nanotube layers to form a sandwich-structure composite carbon layer on the separator, which effectively synergizes the advantages of sp3-hybridized nitrogen-doped mesoporous carbon and sp2-hybridized carbon nanotube. The results indicate that the sandwich-structure composite carbon layer not only can well block the shuttle of polysulfides, but also has good conductive and mechanical properties. Therefore, the cells with the sandwich-structure composite carbon layers exhibit excellent long cycling performance at a high rate (422.9 mAh g−1 after 400 cycles at 4C with a capacity decay of 0.081% per cycle) and outstanding rate capability (1279.4 mA h g−1 at 0.2C and 644.3 mA h g−1 at 4C). This work has certain practical value for Lithium sulfur batteries with its excellent electrochemical performances and simple synthesis processes.

Keywords: Lithium sulfur batteries, Separator, Nitrogen-doping, Porous, Synergistic effect, Polysulfides

Introduction Fossil fuels, one of the most readily available chemical sources, are being rapidly consumed. Its consumption has brought great contribution to the world, but also brought great pollution to the environment. Therefore, there is an urgent need to develop the conversion and storage device of sustainable energy[1-4]. Although the performance of lithium-ion battery (LIB) has been improved continuously in recent decades, the low theoretical energy density (400 Wh kg-1) of LIB is still not enough to 2

meet the increasing requirements of large-scale energy storage systems. The theoretical energy density of lithium sulfur battery (LSB) is up to 2600 Wh kg−1, which makes it hold great potentials to be the next-generation energy conversion and storage device[5-7]. Moreover, sulfur is more attractive in cathode materials, because of its abundant resources, cheap and pollution-free. However, the commercial application of LSBs still have many obstacles, such as the poor electrical conductivity of sulfur and discharge products (Li2S/Li2S2), serious volume expansion (80%) of sulfur cathode during the cycling, risk of safety of the lithium metal anode, and the most serious “shuttle effect” initiated by the Lithium polysulfide (Li2Sx, 4≤x≤8)[8-11]. All of these have great impacts on the electrochemical performance or safety assurance of LSBs. To overthrow the above obstacles, researchers have made many efforts to confine sulfur into the hollow or porous carbon hosts[12-15]. However, the complex manufacturing processes of sulfur cathode composites and the lower sulfur content or areal sulfur loading in the electrode are still the present challenges for extensive commercial applications of LSBs[16]. Recently, inserting an interlayer between the sulfur cathode and the separator has been perceived as a simple and promising strategy to inhibit the shuttle effect in LSBs. In the early, researchers are committed to exploring single-carbon materials that are economical and simple to prepare, such as, multi-walled carbon nanotube (MWCNT)[17-20], graphene[21, 22], carbon fibers[23, 24], carbon black[25] and porous carbons[26-28]. The conductive single-carbon layer can not only provide effectively physical adsorption for the dissolved polysulfides, 3

but also act as a second current collector to facilitate the active material for further reutilization[29]. Despite promising advances, there are still several crucial problems [30-33]: (1) The porous carbon and activated carbon possess poor conductivity, because of the sp3-hybridized C–C bonding and amorphous structure；(2) The MWCNT and graphene with sp2-hybridized C–C bonding have limited pore structure, which will affect the storage of active materials；(3) The interaction of non-polar carbon with the polar polysulfides is weak, leading the irreversible loss of active materials over the long cycling periods. Subsequently, in order to enhance the chemical adsorption capacity of the interlayer for polysulfides, elemental doping (N[16, 34-36], B[37], O[38] and S[39]) of carbon materials have attracted significant attentions. Among them, LSBs modified by N-doped carbon materials render excellent electrochemical performances. Density functional theory and first-principle calculation reveal that the N-doping significantly enhances the affinity between insulated polysulfides and carbon hosts via dipole–dipole electrostatic interaction[40-42]. Due to the substantial Li-N interactions between Li+ in polysulfides and electronegative N atoms in the carbon materials, the N-doped carbon materials show strong chemisorption ability to polysulfides[43]. Electrocatalytic applications demonstrate that the N-doping can promote the facile electron transfer[44], which accelerates the adsorbate for direct redox and reuse. Consequently, appropriate N-containing functional groups can effectively prevent the shuttle of polysulfides and eventually prolong the lifespan of LSBs. But, it is worth noting that the processes of N-doped carbon materials are usually complicated and 4

easy to produce environmental and safety issues. Hence, we have synthesized a novel N-doped mesoporous carbon (NMC) only by simple calcination and acid treatment processes to effectively trap polysulfides for high-performance LSBs. Due to the rich mesoporous structure effectively reinforces the physical constraint to polysulfides and the higher N-doping also provides more chemical coupling interactions between insulated polysulfides and carbon hosts, the NMC possesses good adsorption capacity for polysulfides. Then, the NMC is sandwiched between two carbon nanotube (CNT) layers to form a sandwich-structure composite carbon layer (SCL) on the commercial polypropylene (PP) separator for LSB. The CNT networks act as skins of the SCL, thus the NMC layer can be uniformly formed and firmly fixed on the PP separator. CNT has poor adsorption capacity for polysulfides, but it makes the SCL possess excellent conductive and mechanical properties. As the second collector, the conductive SCL not only effectively inhibits the shuttle of polysulfides, but also promotes the activation and reuse of trapped active materials. The major advantages of SCL are summarized as follows: (1) Large surface area and reasonable pore size distribution are favorable for the storage of diffused active materials and absorption of electrolyte; (2) Strong chemisorption ability for polysulfides to effectively suppress the shuttle effect; (3) Outstanding mechanical properties to accommodate the volume variation of cathode; (4) Excellent conductivity to reduce the polarization effects of electrochemical reactions; (5) Light weight (0.25 mg cm-2) to ensure the high energy density of the cell. LSBs using pure sulfur electrodes with such SCL-coated separators exhibit 5

excellent rate capability and long cycling performance at a high rate. Moreover, the serious self-discharge effect of traditional LSB is also well suppressed by the SCL.

Experiment section Synthesis of NMC material Typically，2 g of sodium citrate was first dried at 155℃ for 24 h in oven and then ball-milled for 12 h mixed with 0.25 g melamine together. Collect the composite powder into the ceramic boat and anneal in a tube furnace at 800℃ under argon gas for 3 h with a heating rate of 5℃ min-1. After cooling to room temperature, the resultant was etched with 2 mol L-1 hydrochloric acid and repeatedly washed with abundant deionized water. Finally, the target NMC was obtained after drying in oven at 80℃ overnight. The N-free porous carbon (PC) synthetized at the same process except that the carbon precursor without the melamine. Fabrication of SCL-coated separator The SCL-coated separator was fabricated by a vacuum-filtration method. The CNT and NMC were first dispersed in alcohol by intensive sonication for 2 h to form 0.2 mg mL-1 and 0.5 mg mL-1 suspension, respectively. Then, the CNT, NMC and CNT suspensions were filtered evenly on the PP separator in turn and formed the flexible SCL-coated separator. The CNT-coated separator was prepared only by filtering the CNT suspension on the PP separator. Final, the modified separators were cut into round pieces with diameter of 23 mm and dried under vacuum at 60℃ overnight. The areal weight of the SCL and CNT layers deposited on each separator are about 0.25 6

mg cm-2 and 0.24 mg cm-2, separately. Simultaneously, we had fabricated the CNT / PC / CNT composite layer (N-free SCL) at the same process. Material characterization The micro-morphology of the carbon sample and the modified separators were characterized

by

scanning

electron

microscopy

(SEM,

HITACHIS4800).

Transmission electron microscopy (TEM, JEOL-2100F) was used for micro-structure imaging of carbon samples. The pore structure of the carbon sample was identified by the Brunauer–Emmett–Teller surface area analyzer (BET, JW-BK122W). The N-elemental in sample was identified with X-ray photoelectron spectroscopy (XPS). Raman spectroscopy measurements were performed using a Horiva (Xplora Plus) at an excitation wavelength of 532 nm with 0.1 mW Raman laser power on the sample. Li2S6 adsorption and diffusion tests Li2S6 adsorption test：The 5 mM Li2S6 solution was prepared by mixing Li2S and sulfur with mole ratio of 1:5 in 1,2-dimethoxymethane (DME, purity: 99.9%, Aldrich):1,3-dioxolane (DOL, purity: 99.9%, Aldrich) (volume ratio 1:1)

and

subsequently stirring for 72 h at 60℃. Then, place 10 mg of each sample in the bottle, and pour 5 ml Li2S6 solution into each bottle. We judged the adsorption effect by color change over time. Li2S6 diffusion test：The 10 mM Li2S6 solution was prepared by the same process as above. The blocking device consists of several 20 ml and 3 ml glass bottles. Inject 3 ml Li2S6 solvent and 6 ml DOL:DME (volume ratio 1:1) into the 3 ml glass bottles 7

and 20 ml glass bottles, respectively. The modified separators were put into the hollow caps of the 3 ml glass bottles. Then, place the 3 ml glass bottles upside down in 20 ml glass bottles. We judged the diffusion effect by color change over time. Electrochemical measurements All 2430-type coin cells were assembled in a glove box full of argon. The sulfur cathode is fabricated mainly through two steps: Firstly, melt 70wt% sulfur into 30wt% super P at 155℃ under argon for 12 h. Then, 90wt% active molten materials were fused with 10wt% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). The slurry was coated on an Al foil and dried at 60℃ for 24 h in vacuum oven and eventually cut into round pieces with diameter of 16 mm. The sulfur loading of each piece was controlled at 1.35~1.55 mg cm-2. The anode material was metal Lithium foil. The electrolytes consisted of 1 mol L-1 lithium bis(triuoromethylsulfonyl)imide (LiTFSI, purity: 99%, Aldrich) dissolved in DOL:DME (volume ratio 1:1)

and

2wt% lithium nitrate (LiNO3, purity: 99%, Aldrich) as the additive. The cells were cycled at the voltage range of 1.7-2.8 V (vs. Li/Li+) at room temperature by a Land CT2001 battery tester. Cyclic voltammetry (CV) tests were carried out also at the voltage range of 1.7-2.8 V with a scan rate of 0.1 mV S-1. Electrochemical impedance spectroscopy (EIS) tests were performed at the frequency range of 1000 kHz-10 mHz with an amplitude of 5 mV. All the CV and EIS tests were achieved by a Princeton (PARSTAT 2273) electrochemical workstation.

8

Results and discussion

Figure 1. (a) Schematic of LSBs with conventional PP separator and SCL-coated separator: ⅰ. Li anode, ⅱ. pristine PP separator, ⅲ. S cathode, ⅳ. Li+, ⅴ. SCL, ⅵ. polysulfides. (b) Folded SCL-coated separator. (c) Recovered SCL-coated separator. (d) The top-view SEM images of the SCL-coated separator. (e) The cross-sectional SEM images of the SCL-coated separator. A schematic diagram of the LSBs with conventional PP separator and SCL-coated 9

separator are shown in Figure 1a. The conventional PP separator in LSB is porous and nonpolar, which is an electronic insulator and doesn’t prevent the conduction of ions. Polysulfides are polar and easily diffuse in electrolytes, so the conventional PP separator is invalid to obstruct the migration of polysulfides during electrochemical cycles. The functional SCL coated on one side of the separator can well restrict and intercept the polysulfides in the cathode side, which has enhanced the stability of the internal mechanism of LSBs. The CNT networks act as skins of the SCL, thus the NMC layer can be uniformly formed and firmly fixed on the PP separator (Figure 1d). After folding, the SCL can still adsorb well and uniformly on the surface of the PP separator (Figure 1b and c). This reveals that SCL has superior flexibility and mechanical strength on the separator, which ensures the robust stability of LSBs during long cycling. The SEM image in Figure 1e shows the cross-section of SCL with 20um in thickness. The mass loading of SCL coating is about 0.25 mg cm-2.

Figure 2. (a) SEM and (b) TEM images of the NMC. (c) Digital image of the Li2S6 solutions (0.005 mol L-1) before and after soaking with the CNT and NMC for 1h. (d) N2-sorption isotherms and (e) pore-size distribution of the NMC. (f) The XPS N1s 10

spectra of the NMC. Detailed morphology and structure of the constructive NMC sample are shown in Figure 2a and 2b by SEM and TEM images. Figure 2a shows a well-organized 3D porous architecture with uniformly distributed pores, which consists of multitudinous carbon sheets. The pore size is about hundreds of nanometers with distinct porous structure. From the TEM image of Figure 2b, we also observe the carbon sheets are thin and flat with massive mesopores on the surface. N-free PC is composed of many thin-walled carbon shells showing the morphology in Figure S1 (Supporting information). To evaluate the strong absorption ability of NMC for polysulfides, as shown in Figure 2c, we added 10mg NMC and 10mg CNT to Li2S6 solution (5 mL, 5 mM), respectively. After 1h, the color of Li2S6 solution containing NMC is visibly faded, while the solution containing CNT still remains bright yellow. As a result, the NMC possesses strong ability to capture the migrating polysulfides. The Raman spectroscopy measurement is an effective tool to detect the effect of N-doping. Figure S2 shows two typical carbon curves of known D (disordered carbon) band at 1324 cm−1 and G (graphite carbon) band at 1582 cm−1. The intensity ratio of D and G bands (ID/IG) is proportional to the degree of defects in carbon materials. The ID/IG of NMC and PC are 1.15 and 1.05, indicating that the N-doping has caused bonding disorders and vacancies in PC lattice[45, 46]. The texture of the NMC sample was characterized by N2 adsorption-desorption testing. As shown in Figure 2d, the isotherm of NMC sample exhibits a typical IV pattern in the IUPAC classification, indicating the hierarchical porous structure 11

formed by mesoporous. The derived BET specific surface area and total pore volume are 1204.7 m2 g-1 and 1.297 cm3 g-1, respectively. Figure 2e presents the pore size distribution of the NMC sample calculated by the non-local density functional theory (NLDFT). The pore size is mainly distributed at 4.3 nm, which also reveals the NMC composed of numerous mesopores. Recent reports have demonstrated that the N-doping mesoporous carbon with large surface area and reasonable pore size distribution is usually good at absorbing the polysulfides generated during cycling[26]. X-ray photoelectron spectroscopy (XPS) characterizations were performed to reveal the elemental statuses and functional N-bonding groups on the surface of NMC. The XPS spectrum clearly shows NMC is consisted of carbon (C), nitrogen (N) and oxygen (O) and the nitrogen content is 3.44% (Figure S3a). In Figure 2f, the N1s spectrum displays four peaks at 398.56, 399.96, 400.95 and 402.95 eV. The main peak at 399.96 eV is assigned to pyrrole-like N, while the peaks fitted at 398.56, 400.95

and

402.95

eV

corresponds

to

pyridine-like,

graphitic-like

and

pyridine-N-oxide-like N. The N-doping endows the NMC a better affinity with the polysulfides, thereby suppressing the “shuttle effect” of LSBs and enhancing the cycling stability[15, 47]. The C1s spectrum in Figure S3b also displays four peaks at 284.72, 285.64, 287.09 and 290.19 eV, corresponding to C=C/C-C, C-N/C-O, C=O and O-C=O. These carboxylic groups, cyclic oxygen and ketone groups also have certain ability to absorb polysulfides[48].

12

Figure 3. Discharge–charge curves at 0.5C of the LSBs with (a) the pristine separator (b) the CNT-coated separator and (c) the SCL-coated separator. (d) CV profiles of LSBs with SCL-coated separator. (e) Rate performances of LSBs with different 13

separators. (f) Cycling performance at 0.5C. (g) Long-term cycling performance at 1C, 2C and 4C of LSBs with SCL-coated separator. To investigate the superiority of SCL-coated separator in LSBs, we have assembled all kinds of separators in coin-type cells and conducted some electrochemical performance tests. The active material of the cathode only uses the mixture of sulfur and common carbon (Super P) with sulfur-loading of 70%. Figure 3a-c shows the galvanostatic charge–discharge curves of the LSBs at 0.5C (1C=1675mAh g-1) between 1.7 and 2.8V (vs. Li/Li+). During discharge, the cells with CNT and SCL-coated separator exhibit two typical voltage plateaus at ~2.35V and ~2.10V, which associate with the two conversion reactions of S8 to long-chain polysulfides (Li2Sx, 4≤x≤8) and then further to short-chain polysulfides (Li2Sx, 1
conductivity. The kinetically efficient reaction process of the LSB with SCL-coated separator is efficient and has a small potential barrier, showing the lowest polarization voltage (Figure S4). Undoubtedly, polysulfides in the LSBs with SCL-coated separators are well-retained and the utilization of active materials is increased. The CV test conducted at a sweep rate of 0.1 mV S-1 to explore whether the modified separators have an effect on the redox reaction of sulfur. As shown in Figure 3d, the CV curves of the LSB with SCL-coated separator consist of two typical reduction peaks and one typical oxidation peak, corresponding to the charge–discharge curves, which indicates the additional interlayers don’t change the oxidation-reduction mechanism of sulfur. Notably, both oxidation and reduction peaks without obvious change for voltage and current in subsequent cycles, showing excellent electrochemical reversibility. The cells with SCL and CNT modified separators have higher and narrower redox peaks than the cell with pristine separator, which proves the addition of functional interlayers have enhanced the activity of sulfur and reduced the polarization effect of electrochemical reactions in the cell (Figure S5). Figure 3e shows the rate capabilities of LSBs with SCL, CNT-coated and pristine separators within the voltage window of 1.7-2.8 V. Obviously, the performance of the cell with pristine separator is worst. The discharge capacity is only 29.6 mAh g-1 when the discharge rate is increased to 4C. The addition of functional interlayers can effectively improve the rate performances of the cells. The discharge capacity of the cell with CNT-coated separator is 1243.6 mAh g-1 at the discharge rate of 0.2C and 184.9 mAh g-1 at the discharge rate of 4C. By contrast, the cell with SCL-coated 15

separator has high discharge capacities of 1279.4, 1065.5, 995.2, 891.0 and 644.3 mAh g-1 at the discharge rates of 0.2, 0.5, 1.0, 2.0 and 4.0C, respectively, which shows most prominent capacity and rate performance. After the high-rate test, the discharge capacities can still go back to 794.7, 912.0, 942.0 and 1058.2 mAh g-1 when the discharge rates finally return to 2.0, 1.0, 0.5 and 0.2C, respectively, indicating the excellent rate stability and electrochemical reversibility. The cycling performances of the cells with SCL, CNT-coated and pristine separators were tested at the 0.5C (Figure 3f). The SCL-coated separator achieves an initial discharge capacity as high as 1294.5 mAh g-1 and a prominent residual capacity of 888.5 mAh g-1 after 200 cycles, corresponding to the capacity retention of 71.8% as against the second cycle after the capacity is stable and the high coulombic efficiency of 98%. In comparison, the cells with pristine and CNT-coated separators deliver lower initial discharge capacities of 728.7 mAh g-1 and 1117.2 mAh g-1. After 200 cycles, their capacity retention rates are only 69.8% and 62.5%, respectively. Undoubtedly, the SCL-coated separator can promote the activation and utilization of sulfur active materials more effectively than the other separators. Besides, the long cycling stability of the cell with SCL-coated separator is also better than the other separators. Due to N-free PC has weak chemical adsorption to polysulfides, the cycling stability of the cell with N-free SCL-coated separator is obviously inferior to the cell with SCL-coated separator (Figure S6). Additionally, the SCL achieves the stability of long-term cycling at 1, 2 and 4C after initial three cycles at 0.2C, as revealed in Figure 3f. The cell with SCL-coated separator delivers an initial discharge 16

capacity of 1073.0 mAh g-1 and retains reversible capacity of 700.4 mAh g–1after 400 cycles at 1C. The reversible capacity is still remained at 640 mAh g-1 after 550 cycles, with the average capacity fade rate of 0.071% per cycle as calculated from the second cycle after the capacity is stable (Figure S7). When the current rate is increased to 2C, the initial discharge capacity is 984.1 mAh g-1 and retains a reversible capacity of 622.3 mAh g-1 after 400 cycles, with the average capacity fade rate of 0.086% per cycle. Even at the high rate of 4C, the initial discharge capacity is as high as 705.4 mAh g-1. After 400 cycles, the reversible capacity is still remained at 422.9 mAh g-1, with the average capacity fade rate of 0.081% per cycle. This result indicates that the cells with SCL-coated separators have good long cycling stability even at high current rates. With further increasing sulfur loading to 3.7 mgs cm-2, the cell with SCL-coated separator still maintains good cycling stability (Figure S8). The reversible capacity is still remained at 582.9 mAh g-1 after 200 cycles at 0.2C, with the average capacity fade rate of 0.071% per cycle as calculated from the sixth cycle after the capacity is stable. Table S1 compares current work with previous reports. It can be concluded that the cycling performances of the cells with SCLs are better than the cells with most other functional interlayers, which is attributable to the synergistic effect of NMC with effective polar bond in the rich mesoporous structure and CNT with prosperous conductivity and mechanical properties. The EIS curves and equivalent circuit models of fresh and cycled cells with different separators were examined (Figure S9). Before cycling, as shown in Figure S9a, the Nyquist plots consist of an inclined line at low frequency range related to the 17

mass transfer process (Warburg impedance, W1), and one semicircle at high frequency range related to charge transfer resistance (Rct) between the interface of cathode and electrolyte. The intercept at high frequency region represents the bulk resistance (Rs), which is related to the impedance of the cell's internal components. In addition, the CPE in the equivalent circuit corresponds to the double-layer capacitance between the electrolyte and the electrode. Rs and Rct of the cells with CNT and SCL-coated separators are obviously less than the cells with pristine separators (Table S2), indicating the interlayers optimize the interface between the electrode and the electrolyte and increase the diffusion rate of Li-ion. After cycling, as shown in Figure S9b, except for an inclined line at low frequency range, there are two semicircles in the high-to-medium frequency range. The semicircle in the high frequency region corresponds to the impedance (Rf) of solid electrolyte interlayer (SEI) at the electrode interface, while the semicircle in the middle frequency region represents the charge transfer resistance (Rct). Rf and Rct of the cycled cell with pristine separator are significantly larger than others due to the diffusive polysulfides have formed a thicker SEI film on the surface of electrodes during cycling (Table S2). Therefore, the functional interlayers not only have activated the sulfur material, but also effectively suppress the shuttle of polysulfides.

18

Figure 4. Digital images for separators in cycled cells: (a) SCL-coated separator, (d) CNT-coated separator, (g) Pristine separator. SEM images for cycled Li anodes in cells with (b,c) SCL-coated separator, (e,f) CNT-coated separator, (h,i) Pristine separator. Figure 4 exhibits the digital images of the different separators and the SEM images of their corresponding Li anodes for the cells after 100 cycles at 0.5C. Previous studies showed that the reaction products from polysulfides and electrolyte additive could form a passivation layer on the surface of the cycled Li anode[51]. Obviously, pristine and CNT-coated separators have a large number of yellow polysulfide residues on the surface (toward anode side) after electrochemical cycling. Correspondingly, the surfaces of the cycled Li anodes are seriously corroded and have thick passivation layers. The SCL-coated separator of cycled cell has little yellow polysulfide residues on the surface (towards anode side). Correspondingly, the surface of the cycled Li anode is smooth and only has a thin passivation layer. These results 19

indicate again that SCL has the best ability to suppress shuttle effect.

Figure 5. Diffusion test of polysulfides with (a) Pristine separator, (b) CNT-coated separator and (c) SCL-coated separator. To further prove the ability of the SCL to inhibit polysulfides diffusion in the LSB, the diffusion processes of polysulfides through the pristine, CNT and SCL-coated separators were designed shown in Figure 5a-c. As time goes on, polysulfides penetrate the pristine separator immediately and the clean solvent in outside bottle turns yellow obviously after 300 min. The speed of polysulfides permeating through the CNT-coated separator decreases markedly, but the clean solvent in outside bottle also turns bright yellow after 300 min. In contrast, the SCL-coated separator shows a good ability to block the polysulfides diffusion and the color of the clear solvent in 20

outside bottle changes a little after 300 min. The slow diffusion rate of the polysulfides indicates that SCL-coated separator has the best trapping ability of polysulfides. In addition, in order to demonstrate which structure of the intermediate adsorbed by the separator adsorbing polysulfide, the adsorption process of the SCL separator in Li2S6 solution (3 ml, 5 mM) was as shown in Figure S10. The experimental results show that the color of the Li2S6 solution becomes lighter after 12 hours.

Figure 6. (a) Average self-discharge performances of LSBs rested for 200 h at 0.2C. Corresponding discharge–charge curves of LSBs with (b) Pristine, (c) CNT-coated separator and (d) SCL-coated separators. Self-discharge is an important criterion to assess the commercial utility of cells. Self-discharge caused by the reaction of dissolved polysulfides/sulfur and Li anode, which brings serious damage to the LSBs[52]. The suppression effects of pristine, CNT and SCL-coated separators on self-discharge are studied at 0.2C. As shown in 21

Figure 6a, these cells were first cycled for 19 cycles to fully activate sulfur cathode, then shelved for 200 h and cycled for a further 131 cycles. Corresponding charge–discharge curves at the 1st, 19th, 20th, and 21th cycles are shown in Figure 6b-d. After 200 h of shelving, the average self-discharge values of the cells with pristine, CNT and SCL-coated separators are 39.8%, 15.5% and 7.5%. Moreover, the discharge capacity recovery rates of the second cycles after rest are 84.1%, 92.7% and 97.2%, respectively. It can be seen that the SCL-coated separator not only has the best inhibitory effect on the self-discharge of LSB, but also can reuse the polysulfides generated during the self-discharge process. Therefore, the cell with SCL-coated separator still shows the best cycling stability after rest. To further prove the ability of SCL to suppress self-discharge, the open-circuit voltage (OCV) self-discharge of LSB with SCL-coated separator is also studied. After rest for 24 h, the OCV of the cell decreases only 0.2 V (Figure S11a) and the initial discharge capacity is almost no loss (Figure S11b). Obviously, SCL can effectively suppress the self-discharge, which further promotes the breakthrough of a difficult issue in traditional LSBs and accelerates the practicality of next generation LSBs.

Conclusions In summary, this work presents the synergistic advantages of sp3-hybridized NMC and sp2-hybridized CNT to trap polysulfides for high-performance LSBs. The novel NMC layer in the center of SCL possesses large surface area and reasonable pore size distribution to reinforce the physical constraints on polysulfides and appropriate N-doping to provide substantial chemisorption for polysulfides. Meanwhile, the SCL 22

with two CNT layers on the surface has outstanding mechanical properties to accommodate the volume variation of cathode and good conductivity to reduce the polarization effects of electrochemical reactions. With the SCL-coated separators, the cells present excellent cycling performance, rate capability and self-discharge suppression ability. This work has certain value for the further design and optimization of LSBs’ multifunctional hybrid separators. Corresponding Author *E-mail: [email protected] Acknowledgements This work was supported by the Foundation of Science and Technology on Power Sources Laboratory(6142808020117C03). Supporting Information. Supporting information related to this article can be found at…….

References [1] J.B. Goodenough, Electrochemical energy storage in a sustainable modern society, Energy & Environmental Science, 7 (2014) 14-18. [2] Y. Sun, N. Liu, Y. Cui, Promises and challenges of nanomaterials for lithium-based rechargeable batteries, Nature Energy, 1 (2016) 16071. [3] L. Su, L. Gao, Q. Du, L. Hou, Z. Ma, X. Qin, G. Shao, Construction of NiCo2O4@MnO2 nanosheet arrays for high-performance supercapacitor: Highly cross-linked porous heterostructure and worthy electrochemical double-layer capacitance contribution, Journal of Alloys and Compounds, 749 (2018) 900-908. [4] J. Du, L. Wang, L. Bai, S. Dang, L. Su, X. Qin, G. Shao, Datura-like Ni-HG-rGO as highly efficient electrocatalyst for hydrogen evolution reaction in alkaline conditions, Journal of Colloid and Interface Science, 535 (2019) 75-83. 23

[5] H. Zhou, F. Yu, M. Wei, Y. Su, Y. Ma, D. Wang, Q. Shen, Substituting copolymeric poly(alkylenetetrasulfide) for elemental sulfur to diminish the shuttling effect of modified intermediate polysulfides for high-performance lithium–sulfur batteries, Chemical Communications, 55 (2019) 3729-3732. [6] M. Li, R. Carter, A. Douglas, L. Oakes, C.L. Pint, Sulfur Vapor-Infiltrated 3D Carbon Nanotube Foam for Binder-Free High Areal Capacity Lithium–Sulfur Battery Composite Cathodes, ACS Nano, 11 (2017) 4877-4884. [7] D. Aurbach, Introduction to the Focus Issue on Lithium-Sulfur Batteries: Materials, Mechanisms, Modeling, and Applications, Journal of The Electrochemical Society, 165 (2018) Y1. [8] Y.V. Mikhaylik, J.R. Akridge, Polysulfide Shuttle Study in the Li/S Battery System, Journal of The Electrochemical Society, 151 (2004) A1969-A1976. [9] J. He, Y. Chen, W. Lv, K. Wen, C. Xu, W. Zhang, W. Qin, W. He, Three-Dimensional CNT/Graphene–Li2S Aerogel as Freestanding Cathode for High-Performance Li–S Batteries, ACS Energy Letters, 1 (2016) 820-826. [10] D. Liu, C. Zhang, G. Zhou, W. Lv, G. Ling, L. Zhi, Q.-H. Yang, Catalytic Effects in Lithium–Sulfur Batteries: Promoted Sulfur Transformation and Reduced Shuttle Effect, Advanced Science, 5 (2018) 1700270. [11] L. Fan, S. Chen, J. Zhu, R. Ma, S. Li, R. Podila, A.M. Rao, G. Yang, C. Wang, Q. Liu, Z. Xu, L. Yuan, Y. Huang, B. Lu, Simultaneous Suppression of the Dendrite Formation and Shuttle Effect in a Lithium–Sulfur Battery by Bilateral Solid Electrolyte Interface, Advanced Science, 5 (2018) 1700934. [12] F. Yu, Y. Li, M. Jia, T. Nan, H. Zhang, S. Zhao, Q. Shen, Elaborate construction and electrochemical properties of lignin-derived macro-/micro-porous carbon-sulfur composites for rechargeable lithium-sulfur batteries: The effect of sulfur-loading time, Journal of Alloys and Compounds, 709 (2017) 677-685. [13] F. Yu, H. Zhou, Q. Shen, Modification of cobalt-containing MOF-derived mesoporous carbon as an effective sulfur-loading host for rechargeable lithium-sulfur batteries, Journal of Alloys and Compounds, 772 (2019) 843-851. [14] Z. Zhang, L.-L. Kong, S. Liu, G.-R. Li, X.-P. Gao, A High-Efficiency 24

Sulfur/Carbon Composite Based on 3D Graphene Nanosheet@Carbon Nanotube Matrix as Cathode for Lithium–Sulfur Battery, Advanced Energy Materials, 7 (2017) 1602543. [15] K. Chen, Z. Sun, R. Fang, Y. Shi, H.-M. Cheng, F. Li, Metal–Organic Frameworks (MOFs)-Derived Nitrogen-Doped Porous Carbon Anchored on Graphene with Multifunctional Effects for Lithium–Sulfur Batteries, Advanced Functional Materials, 28 (2018) 1707592. [16] J. Song, Z. Yu, M.L. Gordin, D. Wang, Advanced Sulfur Cathode Enabled by Highly Crumpled Nitrogen-Doped Graphene Sheets for High-Energy-Density Lithium–Sulfur Batteries, Nano Letters, 16 (2016) 864-870. [17] G. Wang, Y. Lai, Z. Zhang, J. Li, Z. Zhang, Enhanced rate capability and cycle stability of lithium–sulfur batteries with a bifunctional MCNT@PEG-modified separator, Journal of Materials Chemistry A, 3 (2015) 7139-7144. [18] S.-H. Chung, A. Manthiram, High-Performance Li–S Batteries with an Ultra-lightweight MWCNT-Coated Separator, The Journal of Physical Chemistry Letters, 5 (2014) 1978-1983. [19] M.S. Kim, L. Ma, S. Choudhury, L.A. Archer, Multifunctional Separator Coatings for High-Performance Lithium–Sulfur Batteries, Advanced Materials Interfaces, 3 (2016) 1600450. [20] H.M. Kim, J.-Y. Hwang, A. Manthiram, Y.-K. Sun, High-Performance Lithium–Sulfur Batteries with a Self-Assembled Multiwall Carbon Nanotube Interlayer and a Robust Electrode–Electrolyte Interface, ACS Applied Materials & Interfaces, 8 (2016) 983-987. [21] A. Vizintin, M.U.M. Patel, B. Genorio, R. Dominko, Effective Separation of Lithium Anode and Sulfur Cathode in Lithium–Sulfur Batteries, ChemElectroChem, 1 (2014) 1040-1045. [22] P.-Y. Zhai, H.-J. Peng, X.-B. Cheng, L. Zhu, J.-Q. Huang, W. Zhu, Q. Zhang, Scaled-up fabrication of porous-graphene-modified separators for high-capacity lithium–sulfur batteries, Energy Storage Materials, 7 (2017) 56-63. [23] R. Singhal, S.-H. Chung, A. Manthiram, V. Kalra, A free-standing carbon 25

nanofiber interlayer for high-performance lithium–sulfur batteries, Journal of Materials Chemistry A, 3 (2015) 4530-4538. [24] A. Zhang, X. Fang, C. Shen, Y. Liu, I.G. Seo, Y. Ma, L. Chen, P. Cottingham, C. Zhou, Functional interlayer of PVDF-HFP and carbon nanofiber for long-life lithium-sulfur batteries, Nano Research, 11 (2018) 3340-3352. [25] S.-H. Chung, A. Manthiram, Bifunctional Separator with a Light-Weight Carbon-Coating for Dynamically and Statically Stable Lithium-Sulfur Batteries, Advanced Functional Materials, 24 (2014) 5299-5306. [26] J. Balach, T. Jaumann, M. Klose, S. Oswald, J. Eckert, L. Giebeler, Functional Mesoporous Carbon-Coated Separator for Long-Life, High-Energy Lithium–Sulfur Batteries, Advanced Functional Materials, 25 (2015) 5285-5291. [27] Z. Ma, F. Jing, Y. Fan, J. Li, Y. Zhao, G. Shao, High electrical conductivity of 3D mesporous carbon nanocage as an efficient polysulfide buffer layer for high sulfur utilization in lithium-sulfur batteries, Journal of Alloys and Compounds, 789 (2019) 71-79. [28] J. Balach, T. Jaumann, M. Klose, S. Oswald, J. Eckert, L. Giebeler, Mesoporous Carbon Interlayers with Tailored Pore Volume as Polysulfide Reservoir for High-Energy Lithium–Sulfur Batteries, The Journal of Physical Chemistry C, 119 (2015) 4580-4587. [29] Y.-S. Su, A. Manthiram, Lithium–sulphur batteries with a microporous carbon paper as a bifunctional interlayer, Nature Communications, 3 (2012) 1166. [30] Z.A. Ghazi, X. He, A.M. Khattak, N.A. Khan, B. Liang, A. Iqbal, J. Wang, H. Sin, L. Li, Z. Tang, MoS2/Celgard Separator as Efficient Polysulfide Barrier for Long-Life Lithium–Sulfur Batteries, Advanced Materials, 29 (2017) 1606817. [31] X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Nanostructured Metal Oxides and Sulfides for Lithium–Sulfur Batteries, Advanced Materials, 29 (2017) 1601759. [32] F. Wu, S. Zhao, L. Chen, Y. Lu, Y. Su, Y. Jia, L. Bao, J. Wang, S. Chen, R. Chen, Metal-organic frameworks composites threaded on the CNT knitted separator for suppressing the shuttle effect of lithium sulfur batteries, Energy Storage Materials, 14 (2018) 383-391. 26

[33] X. Zhou, Q. Liao, T. Bai, J. Yang, Rational design of graphene @ nitrogen and phosphorous dual-doped porous carbon sandwich-type layer for advanced lithium–sulfur batteries, Journal of Materials Science, 52 (2017) 7719-7732. [34] F. Pei, L. Lin, A. Fu, S. Mo, D. Ou, X. Fang, N. Zheng, A Two-Dimensional Porous Carbon-Modified Separator for High-Energy-Density Li-S Batteries, Joule, 2 (2018) 323-336. [35] X. Yuan, L. Wu, X. He, K. Zeinu, L. Huang, X. Zhu, H. Hou, B. Liu, J. Hu, J. Yang, Separator modified with N,S co-doped mesoporous carbon using egg shell as template for high performance lithium-sulfur batteries, Chemical Engineering Journal, 320 (2017) 178-188. [36] S. Niu, G. Zhou, W. Lv, H. Shi, C. Luo, Y. He, B. Li, Q.-H. Yang, F. Kang, Sulfur confined in nitrogen-doped microporous carbon used in a carbonate-based electrolyte for long-life, safe lithium-sulfur batteries, Carbon, 109 (2016) 1-6. [37] S. Yuan, J.L. Bao, L. Wang, Y. Xia, D.G. Truhlar, Y. Wang, Graphene-Supported Nitrogen and Boron Rich Carbon Layer for Improved Performance of Lithium–Sulfur Batteries Due to Enhanced Chemisorption of Lithium Polysulfides, Advanced Energy Materials, 6 (2016) 1501733. [38] Z. Li, Q. Jiang, Z. Ma, Q. Liu, Z. Wu, S. Wang, Oxygen plasma modified separator for lithium sulfur battery, RSC Advances, 5 (2015) 79473-79478. [39] Y. Dong, S. Liu, Z. Wang, Y. Liu, Z. Zhao, J. Qiu, Sulfur-infiltrated graphene-backboned mesoporous carbon nanosheets with a conductive polymer coating for long-life lithium–sulfur batteries, Nanoscale, 7 (2015) 7569-7573. [40] T.-Z. Hou, X. Chen, H.-J. Peng, J.-Q. Huang, B.-Q. Li, Q. Zhang, B. Li, Design Principles for Heteroatom-Doped Nanocarbon to Achieve Strong Anchoring of Polysulfides for Lithium–Sulfur Batteries, Small, 12 (2016) 3283-3291. [41] H.-J. Peng, T.-Z. Hou, Q. Zhang, J.-Q. Huang, X.-B. Cheng, M.-Q. Guo, Z. Yuan, L.-Y. He, F. Wei, Strongly Coupled Interfaces between a Heterogeneous Carbon Host and a Sulfur-Containing Guest for Highly Stable Lithium-Sulfur Batteries: Mechanistic Insight into Capacity Degradation, Advanced Materials Interfaces, 1 (2014) 1400227. 27

[42] J. Song, T. Xu, M.L. Gordin, P. Zhu, D. Lv, Y.-B. Jiang, Y. Chen, Y. Duan, D. Wang, Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries, Advanced Functional Materials, 24 (2014) 1243-1250. [43] J. Song, M.L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, J. Lu, Y. Ren, Y. Duan, D. Wang, Strong Lithium Polysulfide Chemisorption on Electroactive Sites of Nitrogen-Doped Carbon Composites For High-Performance Lithium–Sulfur Battery Cathodes, Angewandte Chemie International Edition, 54 (2015) 4325-4329. [44] L. Liu, G. Zeng, J. Chen, L. Bi, L. Dai, Z. Wen, N-doped porous carbon nanosheets as pH-universal ORR electrocatalyst in various fuel cell devices, Nano Energy, 49 (2018) 393-402. [45] C. Zhang, L. Fu, N. Liu, M. Liu, Y. Wang, Z. Liu, Synthesis of Nitrogen-Doped Graphene Using Embedded Carbon and Nitrogen Sources, Advanced Materials, 23 (2011) 1020-1024. [46] N. Wang, Z. Xu, X. Xu, T. Liao, B. Tang, Z. Bai, S. Dou, Synergistically Enhanced Interfacial Interaction to Polysulfide via N,O Dual-Doped Highly Porous Carbon Microrods for Advanced Lithium–Sulfur Batteries, ACS Applied Materials & Interfaces, 10 (2018) 13573-13580. [47] Y. Pang, J. Wei, Y. Wang, Y. Xia, Synergetic Protective Effect of the Ultralight MWCNTs/NCQDs Modified Separator for Highly Stable Lithium–Sulfur Batteries, Advanced Energy Materials, 8 (2018) 1702288. [48] F. Pei, T. An, J. Zang, X. Zhao, X. Fang, M. Zheng, Q. Dong, N. Zheng, From Hollow Carbon Spheres to N-Doped Hollow Porous Carbon Bowls: Rational Design of Hollow Carbon Host for Li-S Batteries, Advanced Energy Materials, 6 (2016) 1502539. [49] W. Kong, L. Yan, Y. Luo, D. Wang, K. Jiang, Q. Li, S. Fan, J. Wang, Ultrathin MnO2/Graphene Oxide/Carbon Nanotube Interlayer as Efficient Polysulfide-Trapping Shield for High-Performance Li–S Batteries, Advanced Functional Materials, 27 (2017) 1606663. 28

[50] L. Shi, F. Zeng, X. Cheng, K.H. Lam, W. Wang, A. Wang, Z. Jin, F. Wu, Y. Yang, Enhanced performance of lithium-sulfur batteries with high sulfur loading utilizing ion selective MWCNT/SPANI modified separator, Chemical Engineering Journal, 334 (2018) 305-312. [51] G. Zhou, Y. Zhao, A. Manthiram, Dual-Confined Flexible Sulfur Cathodes Encapsulated in Nitrogen-Doped Double-Shelled Hollow Carbon Spheres and Wrapped with Graphene for Li–S Batteries, Advanced Energy Materials, 5 (2015) 1402263. [52] L. Wang, Y.-B. He, L. Shen, D. Lei, J. Ma, H. Ye, K. Shi, B. Li, F. Kang, Ultra-small self-discharge and stable lithium-sulfur batteries achieved by synergetic effects of multicomponent sandwich-type composite interlayer, Nano Energy, 50 (2018) 367-375.

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Highlights 1. The novel NMC has excellent adsorption capacity for polysulfides 2. A functional sandwich-structure CNT/NMC/CNT interlayer is prepared 3. LSB with the CNT/NMC/CNT interlayer shows good electrochemical performance 4. The CNT/NMC/CNT interlayer can well suppress the self-discharge effect of LSB