Enabling rapid polysulfide conversion kinetics by using functionalized carbon nanosheets as metal-free electrocatalysts in durable lithium-sulfur batteries

Journal Pre-proofs Enabling Rapid Polysulfide Conversion Kinetics by Using Functionalized Carbon Nanosheets as Metal-free Electrocatalysts in Durable Lithium-Sulfur Batteries Songju Ruan, Zhichao Huang, Wendi Cai, Cheng Ma, Xiaojun Liu, Jitong Wang, Wenming Qiao, Licheng Ling PII: DOI: Reference:

S1385-8947(19)33255-3 https://doi.org/10.1016/j.cej.2019.123840 CEJ 123840

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

31 October 2019 10 December 2019 14 December 2019

Please cite this article as: S. Ruan, Z. Huang, W. Cai, C. Ma, X. Liu, J. Wang, W. Qiao, L. Ling, Enabling Rapid Polysulfide Conversion Kinetics by Using Functionalized Carbon Nanosheets as Metal-free Electrocatalysts in Durable Lithium-Sulfur Batteries, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.123840

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Enabling

Rapid

Polysulfide

Conversion

Kinetics

by

Using

Functionalized Carbon Nanosheets as Metal-free Electrocatalysts in Durable Lithium-Sulfur Batteries Songju Ruana, Zhichao Huanga, Wendi Caia, Cheng Maa, Xiaojun Liua, Jitong Wanga,b,*, Wenming Qiaoa,b and Licheng Linga,b a

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Shanghai 200237, China. b

Key Laboratory of Specially Functional Polymeric Materials and Related Technology, East

China University of Science and Technology, Shanghai 200237, China *

Corresponding author e-mail: [email protected]

Abstract Propelling polysulfide conversion and regulating the precipitation of lithium sulfides by introducing electrocatalysts has been proven as an effective strategy to enhance the durability of the lithium-sulfur (Li-S) batteries. Herein, the two-dimensional N-doped carbon nanosheets with appropriate distribution of micro/mesopores are proposed as a novel barrier to modify the commercial polypropylene separator, which significantly reduce the shuttle effect of soluble polysulfides via physical confinement and chemical bonding and immobilize them as active materials within the cathode side. In addition, the efficient electrocatalysis of the functionalized carbon nanosheets on improving sulfur redox electrochemistry is revealed by systematic characterization, leading to rapid polysulfide conversion kinetics and uniform deposition of

lithium sulfides. Due to the smooth trapping-adsorption-conversion process of polysulfides by applying the modified separator, the Li-S batteries with the simple carbon black/S cathodes exhibit outstanding lithium storage capacity (1338 mAh g-1 at 0.1 C) and satisfactory cycling stability (0.029% capacity decreasing per cycle over 700 cycles at 3 C). More importantly, a high reversible areal capacity of 4.3 mAh cm-2 could be maintained at high sulfur loading (5.8 mg cm-2), showing the striking commercial potential. These findings not only provide a feasible method to enhance electrochemical performance of Li-S batteries, but also open up a new sight on developing carbon nanomaterials as metal-free electrocatalysts for multistep reactions in battery electrochemistry. Key words Lithium-sulfur battery; Separator; Carbon nanosheet; Metal-free electrocatalyst

1. Introduction The urgent demand for efficient utilization of renewable energy, as well as the rapid popularization of hand-held electronics and electric vehicles, have greatly motivated the research enthusiasms on the exploit of various advanced battery technologies [1-4]. Owing to great energy density (2600 Wh kg-1), good eco-friendliness and abundant natural resource of sulfur, lithium-sulfur (Li-S) batteries hold the most promise to succeed the current commercial lithium ion batteries (LIBs), whose performance has almost approached the theoretical limitation [5-7]. The superior lithium storage capacity (1675 mAh g-1) of Li-S batteries mainly originates from the multi-step redox reactions of sulfur cathodes, involving a series of phase transformation and migration, nevertheless, bringing several stubborn obstructions on the pace

towards commercialization [8, 9]. First and foremost, the well-known shuttle effects of soluble long-chain polysulfides (LiPSs) would gradually induce the loss of active materials on the cathode side, resulting in poor cycling stability and low coulombic efficiency [10, 11]. Secondly, the conversion from the liquid-phase polysulfides to the solid-phase Li2S as an endothermic reaction, presents relatively sluggish kinetics, reducing the practical power output of Li-S batteries [12, 13]. Furthermore, the uneven deposition of insulating Li2S on the surface of conductive framework would partially inactivate the subsequent redox conversion and lead to low sulfur utilization [14, 15]. Therefore, inhibiting the overall shuttling behavior and improving the polysulfide conversion process hold the key to promote the practicability of LiS batteries. Tremendous strategies have been proposed to suppressing the shuttling effects of LiPSs, such as cathode structure designing, sulfur composition regulating, electrolyte optimizing, lithium metal anode protection and so on [16-20]. Recently, inserting a barrier layer between the separator and cathode, blocking the polysulfide diffusion pathway directly, has been proved as a feasible method to address this problem [21-24]. Carbon materials (graphene, carbon black, carbon nanotube, etc.) with outperforming electrical conductivity, exceptional mechanical properties and cost-effective large-scale production, are the most employed barrier materials coating on cathode side of the conventional polypropylene (PP) separators [25-27]. Porosity is one of the key characteristics of carbon materials and an important issue to be considered here. As the continuous dissolution of LiPSs from the cathodes and the following diffusion to the interlayers, the coated carbon requests much more accommodation to serve as the “second current collector”, reutilizing the trapped active materials. Construction of abundant mesopores

would satisfy the demands for enough space to localize dissolved LiPSs as well as promote the lithium diffusion kinetics across the coated separators [28, 29]. However, it is noteworthy that carbons only show weak physical confinement towards LiPSs because the nonpolar surface exhibits poor affinity to the polar LiPSs and unsatisfactory electrolyte wettability [11, 30]. Some of other polar materials, such as metal oxides, metal sulfides, metal nitrides and so on, are introduced into the carbon materials to provide chemical bonds with LiPSs or further act as electrocatalysts to propel redox kinetics of sulfur species [12, 14, 24, 31-33]. But the relative low conductivity of metal compounds and the limited number of exposed active sites would not allow the cathode with high sulfur loading and content, being against the concept of commercialization. From the perspective of carbon skeleton modification, the modest heteroatom doping (N-, S-, O-, B-, etc.) could improve the conductivity of the carbon matrixes, optimize the interface properties and offer strong chemical adsorption to anchor LiPSs [28, 30, 34, 35]. What’s more, the electrocatalysis effect of heteroatom-doped carbon materials on polysulfide conversion, which has been neglected for a long time, is gradually revealed by recent studies, pointing out a new evolution strategy for carbon materials [11, 13, 36, 37]. It is also believed that the large specific surface area decorated with numerous catalytic heteroatoms could ensure the uniform growth of Li2S on the conductive interface. Thus, it is highly hopeful to achieve smooth trapping-adsorption-conversion process of polysulfides via oriented functionalization of carbon materials. Inspired by the above consideration, herein, we report a functionalized separator by coating two-dimensional (2D) nitrogen-doped carbon nanosheets on commercial polypropylene (PP) membranes to improve the overall electrochemical performance of Li-S batteries. The 2D

nitrogen-doped carbon nanosheets were fabricated by using graphene as the structure formwork, resorcinol and formaldehyde as the carbon source and melamine as the nitrogen source. The in situ generated silica nanoparticles from tetraethoxysilane (TEOS) hydrolysis serve as the hard template to tailor the pore structure. The conductive 2D carbon layer with abundant micropores and mesopores coated on the commercial PP separator reduces the shuttle effects via physical adsorption towards polysulfides and offers physical places to accommodate them, retaining them as active materials within the cathode side. Furthermore, the doping of nitrogen in the carbon framework and the surface residual oxygen-containing groups originating from the silica removal, not only provide strong chemical adsorption sites to anchor polysulfides, but also exhibit synergistic electrocatalytic effects on promoting the conversion kinetics of sulfur specifies. The solid lithium sulfides are controlled to uniformly nucleate on the conductive carbon surface, ensuring the high sulfur utilization in a working Li-S battery. In virtue of the above favorable characteristics, the Li-S battery using the modified separator and the carbon black/sulfur cathode demonstrates a high sulfur utilization superior rate capability, ultra-long lifespan and high reversible areal capacity. These results suggest the superb protentional of the developed carbon nanosheets towards commercialization of Li-S batteries, from another point of view, broaden the application of carbon materials as metal-free electrocatalyst in electrochemical energy conversion.

2. Experimental section 2.1 Material preparation 2.1.1 Synthesis of functional carbon nanosheets

All of the starting materials (unless other-wise noted) were purchased from Shanghai Titan Technology Co., Ltd. and used as received. Graphene oxides (GO) were freshly fabricated from natural flake graphite via the modified hummer’s method and the ultimate GO suspension was calibrated to 12 mg ml-1 for convenience [38, 39]. In a typical synthesis procedure of the mesoporous carbon nanosheets (Fig. 1a), 10 ml GO suspension was firstly dispersed in the mixed solution of 20 ml deionized water (DIW) and 240 ml ethanol with ultrasonic treatment. Subsequently, 0.2 g cetyltrimethylammonium bromide (CTAB) was dissolved into 20 ml DIW and dropped into the GO dispersion, followed by the addition of 4 ml ammonia aqueous solution (28%) with vigorous stirring. Next, 6 ml tetraethoxysilane (TEOS) was dissolved in 40 ml ethanol and injected into the above solution at an approximate rate of 5 ml min-1. The mixed solution was stirred at 40 °C for 24 h after the dropwise of the resorcinol-melamine-formaldehyde (RMF) polymer solution. The GO@RMF/SiO2 was collected via filtration and air drying at 50 °C. As for the preparation of RMF polymer solution, 1.6 g melamine and 3.2 g formaldehyde (37%) were dissolved in 5 ml DIW at 80 °C while 1.4 g resorcinol and 2.1 g formaldehyde (37%) were dissolved in 5 ml DIW at 40 °C. Then, they were mixed up to form the RMF polymer solution [40]. The GO@RMF/SiO2 was further carbonized at 400 °C for 1 h and 800 °C for 3 h under inert atmosphere. Finally, the mesoporous carbon nanosheets were obtained after silica removal by 1 M NaOH solution and freeze drying, named as GMC. For comparison, the microporous carbon nanosheets were prepared by the same procedure, except the addition of TEOS solution, marked as GPC. 2.1.2 Fabrication of carbon-coated separators

The carbon-coated separators were fabricated via casting the carbon slurry on commercial porous polypropylene separators (Celgard 2500). To prepare the carbon slurry, the active materials, carbon black (Super C) and LA133 (7:1:2 in mass ration) were dispersed in a mixed solvent of n-propanol and DIW (3:1 by mass) via virous stirring. The coated separators were vacuum dried at 50 °C for 24 h and cut into 19 mm disks for cell assembly. The total mass loading of the coated separators was controlled at 0.28 mg cm-1. The composite separators with the coating layer of GMC, GPC and carbon black were marked as GMC/PP, GPC/PP and SPC/PP, respectively. 2.2 Characterization and test 2.2.1 Material characterization The micro morphologies of samples were investigated by a field-emission scanning electron microscopy (FE-SEM, Nova Nano SEM 450) and a transmission electron microscopy (TEM, JEOL JSM-7000F). The element mapping images were collected on SEM and TEM equipped with energy dispersive X-ray spectroscopy (EDS). The X-ray diffraction patterns were obtained at 40 kV/40 mA on a polycrystalline diffractometer (Bruker D8 advance), using Cu Kα radiation (l = 1.5406 Å). The Raman spectra was recorded at an excitation wavelength of 514.5 nm (Renishaw inVia Reflex). The nitrogen adsorption experiments were conducted on a Quadrasorb SI analyzer at 77 K and all the samples were degassed at 393 K over 12 h before measurements. The surface chemistry properties were characterized by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD), operating at 15 kV/10 mA with the working pressure lower than 2 × 10-8 Torr. The electrolyte infiltration on the separators was observed on a contact

angle meter (DataPhysics OCA-30). The functional group information was analyzed by Fourier transform infrared spectrum (FTIR, Thermo Fisher Nicolet 6700). 2.2.2 Polysulfides diffusion and adsorption experiments The Li2S4 solution was prepared by dissolving sulfur and Li2S powders (3:1 by molar) in the mixed solution of DME and DOL (1:1, v/v). As for polysulfides diffusion experiments, a glass bottle, which was filled of 1 ml Li2S4 solution (25 mmol) and sealed with the separators, was invert onto 4 ml DME/DOL mixture. The color changes as the time going were recorded by photos. To conduct polysulfides adsorption experiments, 10 mg samples were soaked in 5 ml polysulfide solution (3 mmol) to observe the process of color fading. 2.2.3 Electrochemical measurement The slurry of S/C cathodes was prepared by dispersing pure S, Super C and LA133 (8:1:1 by weight) in n-propanol/DIW mixture (3:1 by mass), which was then casted on the aluminum foil. After vacuum drying at 50 °C overnight, the electrodes were cut into circular pieces. The areal sulfur loading was controlled from 1.0 to 6.6 mg cm-2. The electrochemical performances were investigated by assembling CR2016-type coin cells with S/C cathodes, carbon-coated separators and lithium metal anodes. The sulfur content calculated based on total mass including the coating layer and cathode is around 50 wt.% (with 1 mg cm-2 cathode) and 70 wt.% (with 3.8, 5.3, 5.8, 6.6 mg cm-2 cathode). The electrolyte was 1 M bis sulfonamide lithium salt (LiTFSI) dissolved in DOL and DME solution (1:1 v/v) with 1 wt.% LiNO3 and the addition was about 40 μL per cell. The galvanostatic charge/discharge tests were conducted on a LAND CT2001A battery test system in the voltage range of 1.7 V - 2.8 V. The cyclic voltammogram

(CV) curves were collected on an Arbin BT2000 electrochemistry workstation. Electrochemical impedance spectroscopy (EIS) measurements were performed on a Gamry instrument (Reference 600+) from 100 kHz to 0.01 Hz with a sinusoidal excitation voltage of 5 mV. The detailed experiment procedures of lithium diffusion behavior tests, electrocatalysis effect evaluation and Li2S nucleation measurements could be found in the Supporting information. The ambient temperature of the above measurements was kept at 25 °C.

3. Results and discussion

Fig. 1. (a) Schematic illustration of the synthesis procedure of GMC; FE-SEM images of (b-c) GMC and (d) GPC; (e-g) TEM images of GMC.

The functional carbon nanosheets were synthesized via a facile hard template method, as schemed in Fig. 1a. The preparation procedure started with co-assembly of the in situ generated SiO2 primary particles and pre-polymerized resorcinol-melamine-formaldehyde resin onto the surface of graphene oxides under the guidance of CTAB and the catalysis of ammonium hydroxide, forming two-dimensional sandwich-liked G@RMF/SiO2 nanosheets. After carbonization and silica removal, the two-dimensional architecture was well preserved with high dispersibility, as presented in the FE-SEM images (Fig. 1b-c). In respect of material designing, graphene serves as a 2D framework to host the co-assembly of SiO2 and RMF, meanwhile, remaining in the final nanosheets as highly conductive skeleton. The SiO2 nanoparticles generating from the hydrolysis of TEOS were employed as the hard template to improve the porosity of carbon nanosheets, which could attach with -OH of resorcinol and achieve the uniform dispersion in the RMF polymer [41]. In stark contrast to the carbon nanosheets without the addition of TEOS (Fig. S1a and Fig. 1d), the obtained GMC displays numerous pores spreading on the surface of nanosheets (Fig. 1c), which is further confirmed by TEM images of two samples (Fig 1e-g and Fig. S1b-d). In TEM images of GMC (Fig. 1e), the micromorphology of graphene sandwiched by porous carbon skin is demonstrated clearly and the total thickness of nanosheets is around 25 nm. The amorphous carbon structure could be determined in high-regulation TEM images (Fig. 1g and Fig. S1d) as well, corresponding to the broad peaks around 26° in XRD patterns (Fig. S2). The 3D interpenetrated pores of several nanometers (Fig. 1f-g) across the carbon skin would endow the carbon nanosheets with much more space to immobilize the dissolved LiPSs and provide a significant improvement of ion supply for electrochemical reaction.

Fig. 2. (a) Nitrogen adsorption-desorption isotherm curves and (b) DFT pore-size distribution of GMC and GPC; (c) Full survey XPS spectrum of GMC and GPC; (d) High resolution XPS spectra of O 1s. The nitrogen adsorption-desorption tests are conducted for further evaluation of the pore structure. As shown in Fig. 2a, the reversible type IV isotherm of GMC with H3-type hysteresis loop, which is entirely different from the type I isotherm of GPC, corresponds to the mesopores templated from silica nanoparticles. The calculated Brunauer-Emmett-Teller (BET) surface area and the total pore volume of GMC are 861.9 m2 g-1 and 1.173 cm3 g-1 while the values for GPC with predominant micropores are only 681.8 m2 g-1 and 0.4 cm3 g-1. The enlarged surface area contributed from the mesopores could expose more active sites for efficacious anchoring

of the polysulfide species and facilitate their phase transition reaction on the interface between electrolyte and the carbon framework. Notably, the pore-size distribution analyzed by Density Functional Theory (DFT) reveals the hierarchical pore structure of the prepared GMC, which is equipped with massive micro/mesopores (Fig. 2b). The micropores holding stronger physical adsorption protentional could improve the sulfur-immobilization ability of carbon surface, providing synergic effects combined with the mesopores on trapping LiPSs [42-44]. The above results drive a conclusion that the elaborate 2D carbon nanosheets with favorable porous texture are well constructed in this work. From another perspective of carbon functionalization, heteroatom-doped modification is adopted to regulate the surface chemistry of carbons. The elemental composition of the carbon nanosheets is illustrated in EDS mapping images (Fig. S3), indicating that the carbon framework is decorated with a certain amount of nitrogen atoms due to the pyrolysis of nitrogen-enriched RMF resin. Equipped with moderate heteroatom doping, the surface polarity of the carbons is enhanced, which is confirmed by the conspicuous increasing of ID/IG ration from 0.86 of the pristine GO to 1.00 of GMC and 0.99 of GPC in Raman spectrum (Fig. S4) [11]. XPS analysis is performed to quantify the surface chemical composition of carbon nanosheets, as given in Fig. 2c-d and Fig. S5a-b. In addition to the dominant carbon atoms, the peaks on behalf of O and N atoms could be detected in the full survey of GMC and GPC (Fig. 2c). However, it is worth mentioning that the oxygen content of GMC (8.36%) is much higher than GPC (3.66%) because the oxygen-rich SiO2 template would tend to replace the nitrogen atom. Furthermore, the breaking out of the O-Si bonds between the pyrolytic carbon and SiO2 during the alkali etching process reserves considerable hydroxyl on the carbon surface [45].

This could be proved by the enhancement of the corresponding peaks in high resolution O 1s spectrum (Fig. 2d) and FT-IR spectrum (Fig. S5c). It is well documented that hydroxyl groups could afford strong adsorption sites towards LiPSs via large polarization caused by asymmetrical charge distribution [46]. The doped nitrogen is investigated by N 1s spectrum (Fig. S5b), which has previously been demonstrated to have a more favorable interaction with LiPSs by calculation [11, 45]. Beyond strong chemical adsorption towards LiPSs, the N dopants also play other roles in Li-S batteries, which is gradually revealed in these years, including reducing the energy barrier for polysulfide conversion reaction and motivating the reactivity between oxygen functional groups and sulfur [13, 47].

Fig. 3. (a) Visualized adsorption of Li2S4 by GMC and GPC; (b) Li 1s and (c) S 2p spectra of

polysulfides before and after adsorption; (d) CV curves of symmetric cells with GMC and GPC; Potentiostatic discharge profiles at 2.05 V on surfaces of (e) GMC and (f) GPC; (g) Nyquist plots of GPC and GMC after potentiostatic discharge; SEM images of (h) GMC and (i) GPC electrodes after potentiostatic discharge. Visualized adsorption experiments are performed to figure out the trapping ability of the functional carbon nanosheets towards polysulfides. After immersion of the carbon nanosheets for 12 h, the color fading comparison of Li2S4 solution indicates the better affinity of GMC on LiPSs, which is attributed to the cooperation of physical and chemical confinement (Fig. 3a). The chemical interaction between the carbon surface of GMC and polysulfides is further determined by XPS analysis, as given in Fig. 3b-c. After adsorption, the Li-S bond of Li2S4 is shifted to the higher binding energy while the two weak new peaks located at 56.3 and 59.2 eV are observed, standing for Li-O and Li-N respectively (Fig. 3b) [15, 48]. It provides the convincing evidence for the formation of strong interaction between polysulfides and nitrogen dopants and oxygen-containing groups. The electron cloud density in sulfur atoms are reduced so that the peaks representing terminal (ST-1) and bridging sulfur (SB0) move to the higher bonding energy range in S 2p spectrum (Fig. 3c). What’s more, the appearance of thiosulfates ([O3S-S]2-, 167.2 eV) and polythionates ([O3S2-(S)x-2-S2O3]2-, 168.4 eV) species suggests the sulfur-chain catenation effects on improving sulfur conversion electrochemistry. A reasonable inference is given that the carboxyl and carbonyl groups of carbon nanosheets, activated by the contiguous nitrogen dopants, tend to react with sulfur species and transfer as thiosulfates grafted on the carbon surface. The adsorbed polysulfides (Sx2-, x ≥ 4) would catenate the nearby thiosulfates, then, create intermediate surface-bound polythionates and induce the nucleation of

lithium sulfides on the carbon surface [23, 49, 50]. As a result, the functional carbon nanosheets exhibit superb electrocatalytic effects on propelling polysulfide redox kinetics, which is further confirmed by CV curves of symmetric cells employing two identical carbon-coated electrodes and Li2S6 electrolyte (Fig. 3d). In contrast to symmetric cells without Li2S6 (Fig. S6a-b), the curves of GMC and GPC exhibit pairs of redox peaks on behalf of reversible polysulfide conversion reactions. The stronger response currents and sharp peaks of the GMC curves are recognized as a distinct signal of the ultrafast catalytic redox reactions of soluble LiPSs. The peak A during the cathodic scan is related to the reduction of Li2S6 on the working electrode and simultaneous oxidation of Li2S6 on the counter electrode. The peak B is on behalf of the reformulation of Li2S6 due to the oxidation of Li2S or Li2S2 on the working electrode. The peak C is ascribed to the further oxidation of Li2S6 to generate element sulfur on the working electrode, followed by the reduction of sulfur to Li2S6 on position of the peak D [14, 51]. The reduction and oxidation peaks of GMC still remain clearly even at a high rate of 200 mV s-1 (Fig. S6c-d). However, the GPC only show a pair of broad current response peaks, where the typical peaks regarding multistep conversion of polysulfides could not be clearly recognized. And the voltage hysteresis of GPC between the cathodic and anodic peaks is much higher than GMC due to the limited active sites and sluggish lithium diffusion. Besides, the ununiform deposition of solid Li2S on carbon surface, which greatly prevents the subsequent reactions from smooth proceeding, is another explanation to the large polarization of GPC. Therefore, the Li2S precipitation experiments on the solid-liquid interface are designed [8, 12], as potentiostatic discharge profiles shown in Fig. 3e-f. It is rather obvious that the

responsivity of Li2S nucleation on GMC is earlier than that on GPC and the corresponding peak of GMC displays spiculate shape, demonstrating the significant reduction of overpotential for the initial Li2S nucleation. The evaluated capacity of Li2S precipitation according to the Faraday’s law are 223.6 and 183.9 mAh g-1 for GMC and GPC, respectively [52]. The higher precipitation capacity of GMC is attributed to the larger reactive interface and superior electrocatalytic activity facilitating the polysulfide conversion as proved above. EIS measurements are conducted to investigate the charge transfer ability across the carbon nanosheets after fully deposition of Li2S. The equivalent circuit is given in Fig. S7, corresponding to the impedance parameters in Table S1. The lower electron transfer resistance of GMC-Li2S indicates the better distribution of Li2S on the conductive surface, which can be observed visually in SEM images of the disassembled electrodes (Fig. 3h-i). Despite the higher capacity of Li2S precipitation, the porous channels are well reserved on the surface of GMC due to the great accommodating ability of mesopores (Fig. 3h). In contrast, the relatively low specific surface area of GPC results in the limited exposed functional groups and poor reaction interface. The dissolved polysulfides are merely adsorbed on the outside surface of carbon nanosheet due to the small size of the micropores. The dense stacking of insulating Li2S nanoparticles covered the surface of GPC could be detected (Fig. 3i). There is no doubt that the electron transfer would be severely hampered during the subsequent reactions, resulting in large overpotential for redox conversion.

Fig. 4. (a-b) Cross-sectional and (c-d) Top-down SEM images of GMC/PP; (e) Electrolyte wettability measurements of PP and GMC/PP; (f) Lithium ions transference number of GMC/PP, GPC/PP and PP separators; (g) Impedance plots estimating lithium conductivity; (h) The calculated lithium diffusion coefficients of Li-S cells with the GMC/PP and GPC/PP separators. Based on the above findings, the functional carbon nanosheets are employed as a polysulfide barrier to coat on the commercial PP separators via a facile slurry-casting process, which is quite mature and makes it much more possible for the composite separators to achieve large-scale application in battery industry (Fig. S8). As displayed in the cross-sectional images, the carbon nanosheets are evenly coated on the cathode side of PP separator with the thickness of 20 μm and cut into round disks for cell assembling (Fig. 4a-b). A certain macropores

reserved on the surface of the composite separator are benefit for electrolyte infiltration (Fig. 4c-d), which is confirmed by the smaller contact angle for electrolyte wetting the surface of GMC/PP (Fig. 4e). The electrolyte is quickly assimilated by GMC/PP within 3 s while the droplet is clear enough to be recognized on the surface of pristine PP separator. The better electrolyte infiltration ability of GMC manifests the lithium diffusion pathway would not been blocked by the functional barrier so that the rapid electrochemical reaction kinetics could be well maintained. To drive a more believable conclusion, a series of tests are performed to evaluate the lithium diffusion behavior across the composite separators. As results shown in Fig. 4f, the lithium ion transference number of GMC/PP is estimated to be 0.863, approaching the value of the pristine PP separator (0.897) but much higher than GPC/PP (0.612). The similar tendency could be observed in the ionic conductivity, which are calculated to be 7.65×10-4, 7.09×10-4 and 7.70×10-4 S cm-1 for GMC/PP, GPC/PP and PP respectively (Fig. 4g). These phenomena could be ascribed to the abundant mesopores of GMC that could provide highway for lithium ion transfer while the microporous GPC presents considerable resistance for lithium ion conduction. For further demonstrating the effect of the mesopores on enhancing lithium transfer during electrochemical reactions, the lithium ion diffusion coefficients of the sulfur redox conversion (I: S8→Li2S4/Li2S6, II: Li2S4/Li2S6→Li2S2/Li2S, III: Li2S2/Li2S→Li2S8/S8) are calculated by adopting Randles-Sevick equation (Fig. S9a-d) [32, 53]. Apparently, the lithium ion diffusion coefficients of GMC/PP are all higher than GPC/PP, especially during the process II and III, which involves the slowly multi-phase transformation and migration (Fig. 4h). It is probably ascribed to the enrichment of lithium ions in these mesopores accelerating lithiation and delithiation process, resulting in facile lithium ion diffusion.

Fig. 5. Mechanism illustration of functional carbon nanosheets reducing shuttle effects and propelling LiPSs conversion kinetics. After the carbon coating, the composite separator possesses the ability to trap the dissolved polysulfides via physical blocking, as illustrated in Fig. 5. The visible polysulfide diffusion experiments are designed to prove the sulfur-immobilizing ability of carbon-coating layer (Fig. S10). Driving by the concentration gradient, polysulfides have a tendency to permeate across the separator from one side to the other, accompanied by the apparent color changing. It is obvious that the diffusion rate of LiPSs becomes much slower when the separator is modified

by carbon materials, indicating the shuttle effects in Li-S cells would be considerably reduced by applying the modified separator. Subsequently, the trapped polysulfide species are anchored on the polar surface of the mesopores via strong chemical bonding and physical adsorption and reserved as active materials for reutilization. The massive mesopores not only provide large specific area to expose enough active sites for seizing LiPSs but also contribute to improved lithium ion diffusion behavior. With the help of highly conductive graphene framework, the functional carbon nanosheets act as a role of “second current collector” for further conversion of polysulfides. It is demonstrated that the emerging thiosulfates could catenate with polysulfides to create surface-bond polythionates and lithium sulfides, as schemed in Fig. 5. The new reaction process enhances the surface interaction of carbon nanosheets and LiPSs, moreover, ensures the uniform nucleation of solid Li2S in mesopores. The homogeneous deposition of Li2S on the surface of carbon nanosheets enables the significant reduction of the overpotentials for following-up redox reactions. Therefore, the dissolved polysulfides are effectively reutilized via the proposed smooth trapping-adsorption-conversion process, resulting in the superior electrochemical performance of Li-S batteries.

Fig. 6. (a) Cycling and (b) rate performance comparisons between Li-S cells employing PP, SPC/PP, GPC/PP and GMC/PP separators; (c) Capacity of the upper (QH) and lower (QL) discharge plateaus at various rates; (d) Overpotentials for conversion between LiPSs and Li2S2/Li2S in the discharge/charge process at 0.1 C. The electrochemical performances of Li-S batteries with the modified separators are systematically investigated to prove the superiority of proposed strategy. The galvanostatic charge/discharge tests are firstly conducted by applying the modified separators and simple S/C cathodes (ca.1 mgS cm-2), as showing in Fig. 6a. It is obvious that the lithium storage capacity and cycling performance get remarkable improvement by using carbon-coated separators and

the cell with the GMC/PP separator reserves the highest capacity of 848 mAh g-1 after 200 cycles at 0.5 C, indicating the significant inhibition of shuttle effects. During the evaluation of rate capability, the cell with the GMC/PP separator delivers an initial capacity of 1338 mAh g1

1,

at 0.1 C, which is higher than that with GPC/PP, SPC/PP and PP (1022, 980 and 248 mAh grespectively). The enhanced sulfur utilization is mainly ascribed to the excellent sulfur-

immobilizing ability and unique electrocatalysis on polysulfide conversion kinetics of functional carbon nanosheets. Correspondingly, the favorable lithium storage capacity of 1029, 936, 827, 736, 666 mAh g-1 at 0.2, 0.5, 1, 2 and 3 C could be achieved by the S/C cathode coupled with GMC/PP. The discharge-charge profiles are illustrated in Fig. S11a-c to analyze the intrinsic electrochemical reaction behaviors. There are two pairs of distinct charge/discharge voltage plateaus occurring on behalf of multistep sulfur redox reactions. On discharge curves, the upper plateau is related to the formation of soluble LiPSs while the lower one is associated with the precipitation of solid lithium sulfides (Li2S2/Li2S), which contributes three quarters of the theoretical capacity but presents relative sluggish kinetics. The capacities of upper and lower plateaus are extracted from the total discharge capacity, marked as QH and QL (Fig. 6c). As a fast electrochemical reaction process from solid to liquid phase, the values of QH with the carbon-coated separators are fairly close and do not show dramatical decline under high rate. However, there are apparent differences between the values of QL. Due to the accelerated polysulfide conversion kinetics and the uniform Li2S precipitation, the cell with GMC/PP delivers higher capacity on the lower voltage plateaus. The disparity becomes much more significant as current density increasing. Likewise, the polarization of cells with GMC/PP are

always kept at a comparatively low level at various current densities (Fig. S11d). The overpotentials for the conversion between soluble LiPSs and insoluble Li2S/Li2S are estimated by enlarged discharge/charge profiles in Fig. 6d. The overpotentials of the cell with GMC/PP could be easily determined as the lowest one. These analysis drives us a reasonable conclusion that the originally slow polysulfides conversion kinetics get conspicuous improved indeed by applying the elaborated carbon nanosheets, as predicted above.

Fig. 7. (a) Top-down element mapping images and EDS spectrum of GMC/PP after 200 cycles (full charge); (b-c) High-resolution SEM images of GMC/PP after 200 cycles; Element mapping images of lithium metal anodes with (d) GMC/PP and (e) PP separators.

For further confirming the role of carbon coating layer during the cycling, the cells after 200 cycles are disassembled and characterized. The element composition on the cathode side of GMC/PP are revealed by mapping images in Fig. 7a, indicating the homogeneous distribution of C, N, O and S on the surface of the coating layer. The pronounced sulfur signal on EDS spectrum suggests that the dissolved sulfur species are well anchored on the functional carbon nanosheets, which could be demonstrated by the high-resolution SEM images as well (Fig. 7b-c). In a state of charge-up, the surface of carbon nanosheets is fully covered with the elemental sulfur. That means the electrochemical reactions of dissolved sulfur species has transferred from the electrode to the coating layer so that this part of active material could be efficiently reutilized. The surface chemistry status of cycled lithium anodes is investigated as results given in Fig. 7d-e. It is noted that the sulfur signal on the lithium surface of the cell with GMC/PP is much weaker than that with the pristine PP separator, demonstrating fewer migration of polysulfides across the separator. The polysulfides migrating towards the anode side would react with lithium metal and generate insulating Li2S2/Li2S depositions on the lithium surface, resulting in the corrosion and degradation of the lithium anodes and low coulombic efficiency [8, 28]. Owing to the significant reduction of shuttling effects, the surface of lithium anode with GMC/PP is smoother without any apparent damage (Fig. S12). Hence, by applying the modified separator, the degradation rate of the lithium anode would become slower and the lifespan of Li-S cells could be prolonged.

Fig. 8. (a) Long cycling performance of Li-S batteries with GMC/PP separators at high rates; (b) Cycling performance of the Li-S battery with the GMC/PP separator and S/C cathode of 3.8 mgS cm-2; (c) Cycling performance of Li-S batteries with the GMC/PP separators and highloading S/C cathodes; (d) Comparison of areal capacity with different sulfur-loading cathodes after 120 cycles. The long cycling stability of Li-S cells with GMC/PP is tested under high rate of 1, 2 and 3 C, as results presented in Fig. 8a. After 700 cycles, the high capacity retention of 70.5%, 72.8% and 79.5% could be achieved with the capacity fading of only 0.042%, 0.039% and 0.029% per cycle at 1, 2 and 3 C, respectively. That is to say the high-power output and long lifespan could

be simultaneously acquired in the Li-S battery by employing the functional carbon nanosheets as the efficient barrier. For validating the superior performance of GMC in more practical condition, the S/C cathodes with sulfur loading from 3.8 to 6.6 mgS cm-2 are evaluated using the GMC/PP separators. The Li-S cells with high-loading cathodes exhibit satisfactory cycling stability as shown in Fig. 8b-c. For instance, the cell with the cathode of 3.8 mgS cm-2 delivers a favorable capacity of 724 mAh g-1 after 300 cycles at 0.2 C. Even based on total mass including the coating layer and cathode, the reversible capacity could be calculated as high as 506 mAh gtotal-1. The capacity retention is calculated to be 88.0%, which is ascribed to the admirable sulfur-immobilizing ability of GMC. By applying the 5.8 mgS cm-2 cathode, a high areal capacity of 4.3 mAh cm-2 after 120 cycles suggests its considerable practical potential (Fig. 8d). Its superiority is further demonstrated by comparison with previous works, as listed in Table S2. The electrochemistry performance obtained in this work is better or at least comparable to the recent literatures (Fig. S13). In addition, the Li-S cell using the high-loading cathodes and the GMC/PP separator lights up the led lamps with the pattern of “ECUST”, suggesting the stable and strong output of the cell.

4. Conclusions In summary, the two-dimensional carbon nanosheets were elaborately functionalized and applied as the modified layer attached on the cathode side of a conventional separator, aiming to realize the smooth trapping-adsorption-conversion process of polysulfides in durable Li-S batteries. In view of our systematic characterization, the functional carbon nanosheets significantly reduce the shuttle effects and improve the utilization of sulfur in Li-S batteries, which is mainly ascribed to the subsequent reasons: (i) the abundant mesopores provide

sufficient room to settle the dissolved LiPSs and facilitate lithium diffusion behavior for electrochemical reaction as well; (ii) the polar surface decorated with nitrogen dopants and oxygen-containing groups has strong affinity to LiPSs via chemical bonding; (iii) the emerging thiosulfates grafted on the regulated surface could catenate with polysulfides and accelerate redox conversion of polysulfides. The synergy of the above benefits renders the carbon nanosheets serving as advanced electrocatalysts in the working Li-S batteries, enabling rapid polysulfide conversion kinetics and unobstructed phase transformation. Therefore, the ultralong lifespan, excellent rate capability and high reversible areal capacity are well achieved by applying the modified separators and simple carbon black/S cathodes, indicating the carbon nanosheets possess the enormous potential on promoting the commercialization of Li-S batteries. These findings point out a feasible strategy to develop carbon materials as metal-free electrocatalysts in Li-S batteries and motivate the related researches in the future.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work is partly supported by the National Natural Science Foundation of China (No. U1710252, 21978097), the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the Fundamental Research Funds for the Central Universities (222201817001, 50321041918013) and the Shanghai Rising Star Program (17QB1401700).

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Graphic abstract

Highlights 

The novel carbon nanosheets are prepared and coated on the separators.



The functional coating layer suppresses the shuttle effects in the working LSBs.



The GMC exhibits significant electrocatalysis on polysulfide conversion kinetics.



The LSBs with GMC/PP separators show excellent electrochemical performance.