reduced graphene oxide nanocomposites as functional coating layer for polysulfide shuttle suppression and lithium anode protection of Li-S battery

Journal of Colloid and Interface Science 566 (2020) 11–20

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Colloidal dispersion of Nb2O5/reduced graphene oxide nanocomposites as functional coating layer for polysulfide shuttle suppression and lithium anode protection of Li-S battery Qingyan Ma a, Mengfei Hu a, Yuan Yuan a, Yankai Pan a, Mingqi Chen a, Yayun Zhang a,⇑, Donghui Long a,b,⇑ a b

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Key Laboratory of Specially Functional Polymeric Materials and Related Technology (ECUST), Ministry of Education, Shanghai 200237, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Nb2O5-rGO composite was consisted

Colloidal dispersion of Nb2O5/reduced graphene oxide nanocomposites as functional coating layer for polysulfide shuttle suppression and lithium anode protection of Li-S battery.

of homogeneously Nb2O5 nanoparticles.  Colloidal dispersion of Nb2O5-rGO was coated onto the separator for Li-S battery.  Nb2O5-rGO coating layer showed ultrathin thickness and ultralight weight.  Nb2O5-rGO coating layer could suppress lithium polysulfides shuttle.  Nb2O5-rGO coating layer could retard lithium surface corrosion and dendrite growth.

a r t i c l e

i n f o

Article history: Received 28 November 2019 Revised 18 January 2020 Accepted 18 January 2020 Available online 20 January 2020 Keywords: Separator Functional coating Polysulfide shuttle suppression Lithium anode protection Lithium-sulfur battery

a b s t r a c t Functional separator, which bridges anode, electrolyte and cathode together, has the potential to offer a good solution for efficient polysulfide diffusion inhibition and anode protection of Li-S battery. Herein, a novel ultra-thin multifunctional separator is prepared by a facile coating of colloidal dispersion of Nb2O5/reduced graphene oxide nanocomposites (rGO) onto porous polypropylene (PP) matrix. Benefiting from the physical blocking effect of rGO layer and chemisorption of Nb2O5, the shuttle of polysulfides has been greatly suppressed. Meanwhile, the rGO layer functioning as a conductive upper current collector can improve the sulfur utilization, while the Nb2O5 with high activity promotes the transformation of sulfur-containing species. With the assistant of Nb2O5-rGO function layer, the sulfur cathode shows significantly improved electrochemical performance with a high specific capacity of 1328 mAh g 1 at 0.2C and 754 mAh g 1 retained after 200 cycles. The sulfur cathode also exhibits excellent rate capability and stable Coulombic efficiency of 91% without the addition of LiNO3 in the electrolyte. Moreover, the presence of thin Nb2O5-rGO layer also prevents the lithium surface corrosion and the dendrite growth in the lithium anode. Ó 2020 Published by Elsevier Inc.

⇑ Corresponding authors at: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China (D. Long). E-mail addresses: [email protected] (Y. Zhang), [email protected] (D. Long). https://doi.org/10.1016/j.jcis.2020.01.066 0021-9797/Ó 2020 Published by Elsevier Inc.

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1. Introduction With the increasing demands of portable electrical devices and vehicle electrification, it is crucial to develop advanced electrochemical energy storage systems with high energy density [1,2]. Among various rechargeable battery systems such as sodium sulfur batteries [3,4], zinc-air batteries [5,6], and lithium-air batteries [7,8], lithium-sulfur batteries have been promising candidates due to their ultrahigh theoretical energy density of 2600 Wh kg 1 (three to five times of commercial lithium-ion batteries) [9,10]. However, the practical application of lithium-sulfur (Li-S) batteries has been hindered by severe shuttle effect, originating from multistep conversion of element sulfur. During the lithiation of sulfur, the solid cyclo-S8 transforms into high-order soluble polysulfides and ultimately to solid low-order polysulfides. Driven by the concentration gradient, the soluble intermediate high order polysulfides migrate from cathode side to anode side and react with lithium metal [11–13]. The diffusion of polysulfides are thought to be the main reason for low utilization of active sulfur and corrosion of lithium metal, leading to the severe capacity degradation and short cyclic life of the Li-S batteries. To suppress the undesired shuttle effect of polysulfides, great endeavors have been devoted. The initial strategy was achieved by the encapsulation of sulfur in the nanopores of conductive carbons, such as CMK-3 mesoporous carbons [14], microporous carbon [15,16], carbon nanotubes [17,18] and graphene [19–21]. These porous structures can offer abundant pore volume capable of achieving adequate amount of sulfur loading, while highsurface-area frameworks can effectively hinder the polysulfides shuttling through physical adsorption [22]. However, such nonpolar carbon materials have weak adsorption for polar polysulfides, which alone cannot serve as the perfect host. Various types of polar functional groups on carbon-based materials have been demonstrated to increase the interaction between Li2Sn species and the electrode; these materials can generally be categorized into three types: heteroatom-doping (N [23–25], O [26], S [27]), metal oxides (TiO2 [28], MnO2 [29], La2O3 [30]), and transition-metal disulfides [31] (TiS2, ZrS2, VS2). Except the enhanced chemical adsorption, some metal oxides or sulfides such as Nb2O5 [32], MoS2 [33], CoS2 [34] have the ability of electrocatalyst, which could accelerate the polysulfide redox kinetics and thus to alleviate the polysulfide shuttling. Nevertheless, it is still a big challenge to rationally design a hybrid cathode system to reach a satisfactory polysulfides trapping, outstanding electronic conductivity and high sulfur loading. Moreover, these cathode materials intend to confine the soluble polysulfides within the electrodes, which cannot cope with already dissolved polysulfides.

Separator, as an essential component of electrochemical system, possesses interconnected channels with sub-micro diameters which plays the primary function in separating cathode and anode for preventing short circuit. This porous polymer membrane guarantees ion diffusion and electrolyte permeation but also provides pathway for polysulfides migration. Recently, the modification of separator with functional coating has proven to be effective in suppressing the migration of polysulfides across the separator [35– 37]. This is typically realized by coating the separator with a electronegative polymeric layers, such as Nafion [35], lithium perfluorinated sulfonyl dicyanomethide [36], reduced graphene oxide/sodium lignosulfonate [37] and poly(acrylic acid) [38] to reject polysulfide anions via electrostatic repulsion. Conductive layer such as super P [39], activated carbon [40], carbon nanotube [41] or graphene films [42] were also used to block the migration of polysulfides as well as to serve as ‘‘upper current collector”. Being similar to the cathode design, polar metal oxide nanoparticles have been introduced into coating layer to enhance the chemisorption ability towards polysulfides via Lewis acid-base interactions [43]. Although these separators could effectively block the migration of polysulfides, it should be noted that the functional coating is mostly achieved by a slurry casting method. The thick coating layer in the system increases the weight of inactive component and the electrolyte uptake, which inevitably compromise the cell performance. Thus, the development of effective but lightweight coating layers is very important to the practical applications of the functional separator. In this work, we demonstrate a multifunctional separator modified with an ultrathin conductive Nb2O5/reduced graphene oxide (rGO) nanocomposite (Nb2O5-rGO) layer for high effective blocking of the polysulfides shuttle and inhibiting the dendrite growth in the lithium metal anode (illustrated in Scheme 1). Nb2O5 is a unique electronic semi-conductor which has a very fast Li+ intercalation behavior in its bulk structure [44]. Our work revealed that Nb2O5 nanoparticles could deliver an electrocatalytic effect of sulfur redox, which dynamically promoted the kinetics of the polysulfides redox reaction, especially for the reduction of soluble Li2S6/ Li2S4 to insoluble Li2S2/Li2S [32]. Herein, the ultra-thin Nb2O5 nanoparticles could be sufficiently and uniformly anchored in the rGO layer through a one-pot polyol synthesis. The present synthesis allows the obtained Nb2O5-rGO to form a highly dispersed colloidal solution, based on which the ultrathin coating layer is obtained through a simple vacuum-filtration without the addition of polymeric binder. The cooperative combination of high active Nb2O5 nanoparticles and conductive rGO layer should afford a good construction strategy to chemically and physically trap polysulfides within a functional coating layer. In addition, the presence

Scheme 1. Schematic diagram of batteries applying conventional PP separator and novel Nb2O5-rGO/PP separator.

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of the Nb2O5 nanoparticles between condense coating oriented from the stack of rGO layer could create adequate transport channels for electrolyte and Li+ diffusion. Therefore, the modified separator enables high-sulfur-content cathode to achieve high specific capacity of 1328 mAh g 1 at 0.2C and 754 mAh g 1 retained after 200 cycles, without compromising the rate capability of Li-S batteries. Moreover, the presence of thin Nb2O5-rGO layer could also induce homogeneous Li+ deposition and prevent the lithium surface corrosion induced by polysulfide shuttle effect. These results indicate the modified separator is promising to optimize the electrochemical performance for Li-S batteries. 2. Materials and methods

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(1:1 by volume) without the addition of LiNO3 is used as electrolyte. The matrix of Nb2O5-rGO/PP separator is Celgard 2500. The working and counter electrodes in Li2S6 symmetric batteries is Al foil coated with active material (Nb2O5-rGO/rGO powder) and PVDF with the mass ratio of 8:2. 40 ll electrolyte of 0.1 M Li2S6 is added into the battery. The galvanostatic discharge–charge performance of as-assembled coin cell is tested using LAND CT2001A in the voltage range of 1.5–2.8 V (vs. Li+/Li). The cyclic voltammetry (CV) tests is performed with an Arbin potentiostat (BT2000, USA) with the scan rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) is carried out on an electrochemical working station Bio-Logic VSP, FR in the frequency range of 100 kHz to 0.01 Hz. The symmetric Li-Li battery is assembled with Li foil as working electrode and counter electrode.

2.1. Preparation of Nb2O5-rGO colloidal solution Graphene oxide (GO) is prepared from natural graphite flakes by a modified Hummers method. Typically, 0.3 g Niobium chloride (NbCl5) is dispersed in 30 mL ethylene glycol (EG) to get a homogeneous solution with magnetic stirring for 30 min. Then, NbCl5/EG solution and GO/EG colloidal solution (5 mg mL 1) are mixed under continuous magnetic stirring for another 2 h. Afterward, the obtained NbCl5-GO/EG suspension are added into a Teflon-lined stainless steel autoclave and heats at 180 °C for 24 h. After cooling down to room temperature naturally, the Nb2O5-rGO/EG colloidal solution is collected. 2.2. Preparation of Nb2O5-rGO/PP multifunctional separator Nb2O5-rGO is diluted by isopropanol and ultrasonicate for 15 min. The Nb2O5-rGO layer is deposited on PP separator (Celgard 2500) via facile vacuum filtration. The resulting Nb2O5-rGO/PP multifunctional separator is vacuum dried at 50 °C for 12 h. Finally, the obtained membrane is cut into discs with diameter for 19 mm. 2.3. Preparation of Li2S6 solution Sulfur and lithium sulfide (Li2S) with a molar ratio of 5:1 are mixed in 1,3-dioxolane (DOL) and dimethoxymethane (DME) (1:1 by volume) under magnetic stirring in glovebox to prepare Li2S6 solution (3 mM and 0.1 M). 2.4. Characterization The morphology and microstructure of samples is observed by using scanning electron microscopy (SEM, JOEL 7100F) and transmission electron microscopy (TEM, JEOL 2100F). Elemental mapping analysis is carried out using a scanning electronic microcopy (SEM, FEI Q-300). The thermogravimetric analysis (TGA) is carried out through a TA Instrument Q600 Analyzer with a rate of 10 °C min 1 from room temperature to 800 °C under an N2 or air flow. The X-ray diffraction (XRD) pattern is performed using Cu Ka radiation (k = 1.5406 Å) on a Rigaku D/Max 2550 diffractometer with the 2h = 10–80°. The Raman spectra is recorded on a Spex 1403 Raman spectrometer. The surface chemical state is performed using the X-ray photoelectron spectroscopy (XPS). 2.5. Electrochemical tests The performance of LIR2016-type coin cell with Nb2O5-rGO/PP separator is assembled in an argon-filled glovebox with lithium foil as the anode. The cathode in this report is mesoporous carbon (MC) with ~75 wt% sulfur content and sulfur loading is around 1 mg cm 2. 1.0 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME)

3. Results and discussion 3.1. Material characterization The key to the modified separator is using highly dispersed Nb2O5 nanoparticle/reduced graphene oxide (Nb2O5-rGO) colloidal solution, based on which the flexible separators are obtained through a simple vacuum-filtration without the addition of any conductive agents or polymeric binder. The Nb2O5-rGO colloid solution is prepared by a facile solvothermal reaction of NbCl5 and GO in ethylene glycol (EG) solution, as illustrated in Fig. 1a. During the solvent thermal reaction, GO is reduced to rGO while Nb2O5 nanoparticles are homogenously anchored on the surface of rGO nanosheets. In this case, EG is served both as reducing agent for GO and stabilizing agent for Nb2O5 nanoparticles, which restricts the nanoparticle growth and suppress the nanoparticle agglomeration and aggregation. Furthermore, the existence of Nb2O5 nanoparticles could prevent the formation of 3D reduced graphene oxide gel, which plays an important role in attaining highly dispersed colloidal solution. The total synthesis is easy to perform and does not require multistage steps. Notably, the asprepared Nb2O5-rGO nanocomposites could be easily dispersed in the EG, DMF or ethanol to form homogeneous and stable dispersion, without any precipitate after several days (Fig. 1b). TEM images demonstrate that the ultrathin Nb2O5 nanoparticles have the average particle size of ~ 3 nm, which are uniformly anchored in rGO nanosheet (Fig. 1c and d). The Nb2O5 content in the nanocomposites is approximately 32 wt% determined by TG as shown in Fig. 1e. To further understand the properties of Nb2O5-rGO coating, the X-ray diffraction (XRD) spectra of Nb2O5-rGO and GO are shown in Fig. 1f. There are no sharp peaks at 10.8° which is a characteristic peak of GO, suggesting the reduction of GO via solvothermal reaction. And the crystal structure of Nb2O5 crystal is hard to determine due to its ultra-thin particle size. There is a broad diffraction peak located at ~25°, which should be attributed to the amorphous graphitic structure. Form the Raman spectra of the Nb2O5-rGO (Fig. S1), two sharp peaks at 1345 cm 1 (D-band) and 1590 cm 1 (G-band) with intensity ratio of 1.15 exhibits, which corresponding to structural defects and graphitic structure of rGO, further confirming the reduction of GO. For further chemical structure analysis, XPS analysis is carried out (Fig. 1g) to identify chemical composition. In the high-resolution of Nb 3d spectra, there exhibits one major doublet at 210.0 eV and 207.3 eV, corresponding to Nb 3d3/2 and Nb 3d5/2 respectively. These binding energies are consistent with the values reported [45], affirming the existence of Nb2O5 nanoparticle in the ultrathin coating. Through a vacuum-filtration, the Nb2O5-rGO layer could be easily coated on one side of commercial PP separator. The resulting modified PP separator shows excellent flexibility, strong adhesion,

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Fig. 1. (a) Schematic of the fabrication process of Nb2O5-rGO/PP separator, (b) digital photograph of Nb2O5-rGO colloidal dispersion in EG, (c, d) TEM image of Nb2O5-rGO composite, (e) TGA curve of Nb2O5-rGO nanocomposite, (f) XRD patterns of GO and Nb2O5-rGO composite, g) High-resolution Nb 3d spectra of Nb2O5-rGO.

and macroscopically smooth surface with a shiny black color (shown in Fig. S2). No cracks and dusting could be observed after repeatedly folding and scraping, further indicating the close contact between Nb2O5-rGO layer and PP matrix (Fig. 2a). The overall areal loading amount of Nb2O5-rGO coating could be controlled from 0.01 to 0.1 mg cm 2 by changing the volume of colloidal solution used. The initial PP separators are full of long-narrow pores with the size of 400–500 nm (Fig. 2b). Followed by the coating of Nb2O5rGO layer, there are no obvious pore structures could be observed from top view as shown in Fig. 2c and Fig. S3a. Elemental mapping images of Nb2O5-rGO/PP in Fig. 2d manifest the homogenous distribution of C, Nb and O. From the cross-section image (Fig. 2e, Fig. S3b), the layer-by-layer stacking of Nb2O5-rGO nanosheet could be observed. With a low coating amount of 0.05 mg cm 2, the thickness of the coating layer is ca. 200 nm, much lower than those of previous reported coating layers. No obvious differences could be discerned between the N2 adsorption isotherms of PP and Nb2O5-rGO/PP separators (Fig. S4) due to the low mass ratio of coating layer. Furthermore, the presence of Nb2O5 nanoparticles should effectively inhibit the Van de Walls assemble of rGO nanosheets, ensuring that the liquid electrolyte penetrates easily into the whole separator structure, and so that the electrochemical reactions proceed. It is pointed out that the polar surface of Nb2O5-rGO layer has great significance in improving modified separator surface wettability, which facilitates electrolyte maintaining and diffusion over the battery. As depicted in Fig. 2f, after the Nb2O5-rGO coating is applied, the contact angle decrease from 40° to 15° which indicates

the improved interfacial compatibility between separator and liquid electrolyte and therefore benefits lithium ion welldistribution. Thermal dimensional stability is also crucial for batteries safety performance. As shown in Fig. S5, after heat treatment at 150 °C for 2 h, the PP separator shows significant dimensional change. As for Nb2O5-rGO/PP separator, there is no obvious thermal shrinkage due to the shield and drafting of Nb2O5-rGO layer. The improved thermal stability is beneficial for cell safety which is of great significance in boosting the practical application of LiS batteries. 3.2. Electrochemical testing To investigate the effectiveness of Nb2O5-rGO/PP separators, electrochemical evaluations have been carried out using 1 M LiTFSI in DOL/DME (1:1) electrolyte without the addition of LiNO3. And the sulfur cathode is mixture of elemental sulfur and mesoporous carbon with a sulfur loading of 75% (Fig. S6a). The cyclic voltammetry (CV) curves at the scan rate of 0.1 mV s 1 (Fig. S6b), both cells with conventional PP separators and Nb2O5rGO/PP separators exhibit two major sharp peaks at voltage of 2.3 V and 2.0 V, corresponding to the element sulfur’s lithiation to high-order polysulfides (Li2Sx, x = 4–8) and further reduction to terminal discharge products Li2S2/Li2S, respectively. In the subsequent anodic scan, the broad peak at 2.4 V could be attributed to the oxidation of lithium polysulfide and lithium sulfide. With repeated 5 scans, the CV curves (Fig. 3a) of battery applied with Nb2O5-rGO/PP separator show well-overlapped cathodic and anodic peaks, indicating highly reversible electrochemical process

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Fig. 2. (a) Fold and recovery state of Nb2O5-rGO/PP separator, SEM image of the top surface of (b) PP and (c) Nb2O5-rGO/PP separator, (d) Mapping of Nb2O5-rGO/PP separator surface, (e) Cross-sectional SEM image of Nb2O5-rGO/PP separator, (f) contact angle of electrolyte on PP and Nb2O5-rGO/PP separator.

Fig. 3. (a) CV curves of battery with Nb2O5-rGO/PP separator at a scan rate of 0.1 mV s 1, (b) galvanostatic discharge-charge profiles of battery with Nb2O5-rGO/PP separator at 0.2C, (c) Cycling performance of PP and Nb2O5-rGO/PP separator at 0.2C, (d) EIS of the Li-S batteries with different separators, (e) rate capability of PP and Nb2O5-rGO/PP separator at various current rate of 0.1C, 0.2C, 0.5C, 1C and 2C, (f) Long-cycling performance of Nb2O5-rGO/PP separator at 3C.

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and favorable stability. Also, the galvanostatic discharge-charge profiles of battery with Nb2O5-rGO/PP (Fig. 3b) and PP (Fig. S7a) separator in different cycles at 0.2C display two typical discharge plateaus and one charge plateau, in consistent with CV curves discussed above. There is no representative peak that can be attributed to the lithiation of Nb2O5, indicating that Nb2O5 nanoparticle doesn’t take part in discharge-charge process, as inactive materials with affinity for polysulfides. The upper plateau corresponds to the formation of long-chain soluble polysulfides with theoretical capacity of 419 mAh g 1. In Fig. S7b, the battery with Nb2O5-rGO/PP displays much higher upper plateau capacity

than that with PP separator, retaining 83% of its initial value after 50 cycles. The improved upper plateau capacity indicates the excellent trapping and reutilization of Nb2O5-rGO layer. The cycling performances of the cells with different separators are performed at a current rate of 0.2C (1C = 1675 mA g 1), as displayed in Fig. 3c. Obviously, the Nb2O5-rGO/PP separator exhibits a higher specific capacity of 1328 mAh g 1 than that of PP separator of 1071 mAh g 1, increasing the sulfur utilization from 64% to 79%. The macro-pores of PP separator can accommodate a large number of migrated polysulfides which intend to precipitate on the closest conducting surface [46]. Therefore, such higher sulfur utilization

Fig. 4. CV curves of symmetric batteries with electrodes of (a) Nb2O5-rGO and (b) rGO.

Fig. 5. (a, b) SEM images of Nb2O5-rGO coating and (c–g) corresponding elemental mapping, (h) SEM images of Nb2O5-rGO/PP separator’s anode side, (i, j) PP cathode side and anode side, the inserts are high resolution images at a charged state of 2.8 V.

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should be due to the introduction of conductive Nb2O5-rGO layer, which increase conductive surface area for the deposition and reutilization of sulfur-related species. As the cycle number increasing, batteries with Nb2O5-rGO/PP separator performs much more stable specific capacity with reversible capacity of 754 mAh g 1 remained after 200 cycles, confirming the effective retardation of polysulfides shuttling by Nb2O5-rGO layer. The Coulombic efficiency of the cathode with Nb2O5-rGO/PP separator maintains at ~91%, which displays significantly higher than that of routine PP separator (~85%), especially in the later cycling. The electrochemical impedance spectroscopy (EIS) is performed to prove the benefits of Nb2O5-rGO layer in Li-S batteries. As shown in Fig. 3d, the Nyquist plots of the cell impedance are composed of a typical semicircle at middle-frequency and inclined line in the low-frequency, representing the charge-transfer resistance (Rct) and Warburg diffusion resistance. Batteries with Nb2O5-rGO layer show similar charge-transfer resistance compared with that with PP separator, indicating that the introduction of Nb2O5-rGO layer, serving as ‘‘upper current collector”, doesn’t have significant influence on the charge transfer. The increased contact area with conductive surface could reuse the sulfur-related species for higher active material utilization and effectively capture the migrating polysulfides. To explore the rate performance of Nb2O5-rGO/PP separator, batteries are evaluated at different current densities of 0.1C, 0.2C, 0.5C, 1C and 2C (Fig. 3e). As is known, the reversible capacities decrease with the increase of current rates. When the Nb2O5-rGO/PP separator is applied, the additional upper current collector leads to higher utilization of the active materials than that with PP separator as shown in Fig. S8a, which achieves

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1240, 1097, 1008, 843, 775 mAh g 1 respectively. The discharge capacity of 1116 mAh g 1 is acquired when the current density is switched back to 0.1C, investigating a slight improvement in rate capabilities with the introduction of Nb2O5-rGO layer. The cathode with Nb2O5-rGO/PP separator could deliver a high reversible specific capacity of 990 mAh g 1 at 0.5C and 714 mAh g 1 at 1C after 250 cycles as shown in Fig. S8b. To investigate more long-term cycling performance, we carry out a 500-cycle discharge/charge test at a high current density of 3C, as presented in Fig. 3f. The battery with the Nb2O5-rGO/PP separator could perform relatively stable performance with a reversible specific capacity of 378.4 mAh g 1 and Coulombic efficiency of 93% (without the addition of LiNO3 in the electrolyte) at 3C after 500 th cycling. The good capacity retention at different current densities could be attributed to the uniformly dispersion of Nb2O5 nanocrystals on the conductive rGO layer, which combine the physical blockage and chemical adsorption to effectively inhibit the polysulfides shuttling. It should be noted that the thickness of Nb2O5-rGO layer do not distinctly affect the cycling performance of sulfur cathode. As shown in the Fig. S9, the separator with different loadings of Nb2O5-rGO have very similar cycling performance. Considering the applicable capacity retention ratio and the whole mass of separator, we chose the separator with middle thickness for the otherwise testing. Furthermore, the electrochemical performance of battery applied with Nb2O5-rGO/PP separator and those reported based on functional modified separators are summarized in Table S1. The results demonstrate that Nb2O5-rGO coating exhibits superior properties, with ultralight mass loading and ultrathin thickness, achieving competitive electrochemical performance.

Fig. 6. (a, b) cycled lithium surface of Li anode with PP separator, (c, d) cycled lithium surface of Li anode with Nb2O5-rGO/PP separator.

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To further investigate the electrocatalytic effect of Nb2O5, symmetric batteries with Li2S6 (0.1 M) electrolyte are carried out cyclic voltammetry (CV) test within a voltage of 1.0 to +1.0 V with a scan rate of 1 mV s 1 for 3 cycles. As shown in Fig. 4, the battery with Nb2O5-rGO composite shows two pairs of redox peaks at 0.1 V/0.48 V and 0.1 V/ 0.48 V, while the battery with rGO electrode only displays a broad peak at 0.2 and 0.2 V. Compared with rGO electrode, the Nb2O5-rGO shows much broader peaks with wide separation, indicating the promoted sulfur-related species redox kinetics and electrocatalytic activity of Nb2O5 nanoparticles. What’s more the existence of Nb2O5 increase the peak current, proving the increased sulfur-related species utilization. After 100 cycles (at a charged state of 2.8 V), batteries applied with Nb2O5-rGO/PP separator and PP separator are disassembled to further investigate the state of separator and lithium surface. SEM images and corresponding mapping images of the Nb2O5-rGO/PP separator are shown in Fig. 5a-g. The Nb2O5-rGO layer still strongly adheres to the separator as an integrated electrode without significant structure change after cycling (Fig. 5a). According to Fig. 5b, the intercepted active materials that are immobilized by Nb2O5-rGO coating could be observed clearly. The corresponding elemental mappings in Fig. 5c–g and Fig. S10 reveal strong sulfur signals that are uniformly distributed on the coated layer which manifests soluble polysulfides remain in the

cathode side. Meanwhile, the other side of separator shows a relatively clean surface (Fig. 5h), confirming that the polysulfides cannot migrate across the Nb2O5-rGO layer. Therefore, the restrained sulfur-related species could be reused on the surface of upper current collector successively to obtain a higher active material utilization. On the contrast to the cycled PP separator, there are many particles deposited both on the cathode side and anode side (Fig. 5i and j), responding to the formation of isolated and insoluble sulfur or Li2S particles. Furthermore, the morphology and structure of Nb2O5-rGO/PP separator after 500 cycles at 3C also be characterized as shown in Fig. S11. The surface SEM images still present smooth morphology without obvious changes. These results strongly confirm that the Nb2O5-rGO/PP separator could perform stable performance during long-term discharge/charge cycling. The polysulfides blocking behavior of Nb2O5-rGO/PP separator is further verified through a visualized diffusion test as shown in Fig. S12. The little sample bottle with Li2S6 solution, inserting inside a centrifuge tube with fresh DOL/DME (volume ratio of 1:1) solvent, shows dark brown. The two bottles are separated by PP separator and as-prepared Nb2O5-rGO/PP separator respectively. In the case of PP separator, the solvent phase shows significant yellow brown after resting for 1.0 h. The color turns deeper with the time increasing because of the rapid polysulfide diffusion under the concentration gradient. In contrast, the fresh

Fig. 7. (a) Galvanostatic discharge/charge voltage profiles of PP and Nb2O5-rGO/PP separator in symmetric batteries, SEM images of Li surface morphology fabricated with PP (b, c) and Nb2O5-rGO/PP separator (d, e).

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solvent employed Nb2O5-rGO/PP separator shows much slower color change, indicating the polysulfide diffusion behavior is significantly suppressed. This phenomenon is a direct proof which manifests the ultrathin Nb2O5-rGO layer can stop the polysulfide migration. Furthermore, we put our sight on the anode side to study the protection of lithium anode by the Nb2O5-rGO layer. After 100 cycles, the batteries are disassembled, and lithium anodes are characterized. The Li metal foil assembled with pure PP separator have uneven, rough surface with cracks and loose structure, indicating inhomogeneous Li+ deposition and corrosion induced by polysulfide shuttle effect (Fig. 6a and b). Contrastively, Li foil cycled with Nb2O5-rGO/PP separator delivers much smoother and more compact surface, manifesting the effective blocking of migrated polysulfides (Fig. 6c and d). The improved lithium deposition state comes from the regulation of Li-ion deposition and reduction of LixSy corrosion to lithium anode. The symmetric Li-Li batteries are fabricated with PP and Nb2O5-rGO/PP separator to further investigate the contribution of Nb2O5-rGO coating to uniform lithium distribution and lithium dendrite growth suppression. The batteries are cycled from 0 to 1 V at current density of 50 lA to stabilize the lithium surface firstly and then the cycling capacity is 0.5 mAh cm 2. As shown in Fig. 7a, the battery with Nb2O5-rGO/PP separator exhibits excellent cycling stability and low voltage hysteresis of 8 mV, which indicates the introduction of Nb2O5-rGO layer will not increase the resistance of lithium ion transfer and deposition. The cycled batteries are disassembled to observe the surface state of Li metal as shown in Fig. 7. The lithium metal applied with PP separator exhibits mossy surface morphology with wire-like lithium dendrites (Fig. 7b), and there is irregular and loose lithium deposition from the high-resolution SEM images (Fig. 7c). In sharp contrast, the Li metal fabricated with Nb2O5-rGO/PP separator shows relatively dense and uniform deposition morphology (Fig. 7d and e), which verifies the promotion of Nb2O5-rGO coating towards lithium distribution. Therefore, the Nb2O5-rGO/PP separator not only reduce the corrosion of lithium metal caused by polysulfide shuttle, but also attribute to homogeneous Li+ distribution, and thus inhibit the growth of Li dendrite. 4. Conclusions In summary, Nb2O5-rGO colloidal dispersion have been prepared by solvothermal reaction and coated on commercial PP separator via facile vacuum filtration to apply as modified separators for Li-S batteries. With the assistant of Nb2O5-rGO functional layer, the resulting sulfur cathode exhibits excellent cycling stability and high Coulombic efficiency as well as good rate capability. The sulfur cathode shows higher specific capacity of 1328 mAh g 1 at 0.2C and 754 mAh g 1 retained after 200 cycles. The enhanced electrochemical performance can be attributed to the shuttle effect suppression of physical barrier rGO and chemical adsorption of Nb2O5. Furthermore, the conductivity of rGO, which functioning as upper current collector, and electrochemical catalytic effect of Nb2O5 have improved the reutilization rate of sulfur-related species. Except for the promotion on cathode side, the Nb2O5-rGO coating plays an important role in the lithium metal protection via reducing the corrosion of lithium metal and regulating Li+ distribution. Therefore, this work promotes the application of Li-S batteries to become next generation energy storage system. CRediT authorship contribution statement Qingyan Ma: Conceptualization, Writing - original draft. Mengfei Hu: Formal analysis, Methodology. Yuan Yuan: Software.

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Yankai Pan: Visualization, Validation. Mingqi Chen: Software, Validation. Yayun Zhang: Formal analysis, Supervision. Donghui Long: Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was partly supported by National Natural Science Foundation of China (No. 21576090), and Fundamental Research Funds for the Central Universities (222201718002). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2020.01.066. References [1] Y.Z. Song, W.L. Cai, L. Kong, J.S. Cai, Q. Zhang, J.Y. Sun, Rationalizing electrocatalysis of Li-S chemistry by mediator design: process and prospects, Adv. Energy Mater. (2019) 1901075. [2] M.Q. Chen, Z. Su, K. Jiang, Y.K. Pan, Y.Y. Zhang, D.H. Long, Promoting sulfur immobilization by a hierarchical morphology of hollow carbon nanosphere clusters for high-stability Li-S battery, J. Mater. Chem. A 7 (2019) 6250–6258. [3] T.T. Yang, B.S. Guo, W.Y. Du, M.K. Aslam, M.L. Tao, W. Zhong, Y.M. Chen, S.J. Bao, X. Zhang, M.W. Xu, Design and construction of sodium polysulfides defense system for room-temperature Na-S battery[J], Adv. Sci. 6 (23) (2019) 1901557. [4] T.H. Hwang, D.S. Jung, J.S. Kim, B.G. Kim, J.W. Choi, One-dimensional carbonsulfur composite fibers for na-s rechargeable batteries operating at room temperature[J], Nano Lett. 13 (9) (2013) 4532–4538. [5] D.X. Ji, L. Fan, L.L. Li, S.J. Peng, D. Yu, J.N. Song, S. Ramakrishna, S.J. Guo, Atomically transition metals on self-supported porous carbon flake arrays as binder-free air cathode for wearable zinc-air batteries, Adv. Mater. 31 (2019) 1808267. [6] C. Guan, A. Sumboja, W.J. Zang, Y.H. Qian, H. Zhang, X.M. Liu, Z.L. Liu, D. Zhao, S. J. Pennycook, J. Wang, Decorating Co/CoNx nanoparticles in nitrogen-doped carbon nanoarrays for flexible and rechargeable zinc-air batteries, Energy Storage Mater. 16 (2019) 243–250. [7] J.B. Wang, M.L. Fan, W.M. Tu, K. Chen, Y.F. Shen, H.N. Zhang, In situ growth of Co3O4 on nitrogen-doped hollow carbon nanospheres as air electrode for lithium-air batteries, J. Alloys Compd. 777 (2019) 944–953. [8] Y. Hang, C.F. Zhang, X.M. Luo, Y.S. Xie, S. Xin, Y.T. Li, D.W. Zhang, J.B. Goodenough, a-MnO2 nanorods supported on porous graphitic carbon nitride as efficient electrocatalysts for lithium-air batteries, J. Power Sources. 392 (2018) 15–22. [9] Y. Yang, G. Zheng, Y. Cui, Nanostructured sulfur cathodes, Chem. Soc. Rev. 42 (2013) 3018. [10] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Li-O2 and Li-S batteries with high energy storage, Nat. Mater. 11 (1) (2012) 19. [11] Z.A. Ghazi, X. He, A.M. Khattak, N.A. Khan, B. Liang, A. Iqbal, J.X. Wang, H. Sin, L. S. Li, Z.Y. Tang, MoS2/celgard separator as efficient polysulfide barrier for longlife lithium-sulfur batteries, Adv. Mater. 29 (2017) 1606817. [12] W.B. Kong, L.J. Yan, Y.F. Luo, D.T. Wang, K.L. Jiang, Q.Q. Li, S. Fan, J.P. Wang, Ultrathin MnO2/graphene oxide/carbon nanotube interlayer as efficient polysulfide-trapping shield for high-performance Li-S batteries, Adv. Funct. Mater. 27 (2017) 1606663. [13] K. Liao, P. Mao, N. Li, M. Han, J. Yi, P. He, Y. Sun, H.S. Zhou, Stabilization of polysulfides via lithium bonds for Li-S batteries, J. Mater. Chem. A 4 (2016) 5406. [14] X.L. Ji, K.T. Lee, L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries, Nat. Mater. 8 (2009) 500–506. [15] B. Zhang, X. Qin, G.R. Li, X.P. Gao, Enhancement of long stability of sulfur cathode by encapsulating sulfur into micropores of carbon spheres, Energy Environ. Sci. 3 (2010) 1531–1537. [16] S. Xin, L. Gu, N.H. Zhao, Y.X. Yin, L.J. Zhou, Y.G. Guo, L.J. Wan, Smaller sulfur molecules promise better lithium-sulfur batteries, J. Am. Chem. Soc. 134 (2012) 18510–18513. [17] G.Y. Zheng, Y. Yang, J.J. Cha, S.S. Hong, Y. Cui, Hollow carbon nanofiberencapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries, Nano Lett. 11 (2011) 4462–4467. [18] J.C. Guo, Y.H. Xu, C.S. Wang, Sulfur-impregnated disordered carbon nanotubes cathode for lithium-sulfur batteries, Nano Lett. 11 (2011) 4288–4294.

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