Enhanced Sulfur Redox and Polysulfide Regulation via Porous VN

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Letter

Enhanced Sulfur Redox and Polysulfide Regulation via Porous VN-Modified Separator for Li-S Batteries Yingze Song, Shuyang Zhao, Yiran Chen, Jingsheng Cai, Jia Li, Quan-Hong Yang, Jingyu Sun, and Zhongfan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22014 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Materials & Interfaces

Enhanced

Sulfur

Redox

and

Polysulfide

Regulation via Porous VN-Modified Separator for Li−S Batteries Yingze Songab†, Shuyang Zhaoc†, Yiran Chena†, Jingsheng Caia, Jia Lic*, Quanhong Yangcd*, Jingyu Suna* and Zhongfan Liuae aCollege

of Energy, Soochow Institute for Energy and Materials InnovationS (SIEMIS), Key

Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou, Jiangsu 215006, P. R. China bState

Key Laboratory for Environment-Friendly Energy Materials, Southwest University of

Science and Technology, Mianyang, Sichuan 621010, P. R. China cDivision

of Energy and Environment, Graduate School at Shenzhen, Tsinghua University,

Shenzhen 518055, P. R. China dNanoYang

Group, State Key Laboratory of Chemical Engineering, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China eCenter

for Nanochemistry (CNC), Beijing Science and Engineering Center for Nanocarbons,

Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China

ACS Paragon Plus Environment

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KEYWORDS: Li–S batteries, vanadium nitride, separator modification, polysulfide regulation, sulfur redox kinetics

ABSTRACT: Lithium–sulfur (Li–S) batteries have now emerged as the next-generation rechargeable energy storage system owing to the high energy density and theoretical capacity. However, the notorious “lithium polysulfide (LiPS) shuttle” and sluggish kinetics in sulfur redox have posted great threat to their practical applications. Herein, we develop a VN-modified separator as an effective promoter to regulate the LiPSs and accelerate the electrochemical kinetics of Li–S batteries. Benefiting from the dense packing structure and polar surface of porous VN, the VN-modified separator favorably synergizes bifunctionality of physical confinement and chemical entrapment toward LiPSs whilst affords smooth lithium ion migration. In addition, the superb electrical conductivity of VN also propels the LiPS conversion. With these advantages, thusintegrated batteries with VN-modified separator exhibit an average capacity decay of 0.077% per cycle at 1 C for 800 cycles. A reasonable areal capacity of 4.2 mAh cm–2 is achieved even with a high sulfur mass loading of 3.8 mg cm–2 at 0.2 C. The present work offers a rational strategy to regulate the LiPS behavior and guide the sulfur redox kinetics toward effective and long-life Li–S batteries.


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The ever-growing demand for high-performance energy storage systems has driven the rapid development of state-of-the-art lithium–sulfur (Li–S) battery, owing to its conspicuous merits pertaining to high energy density (2600 Wh kg–1)/theoretical capacity (1672 mAh g–1), nontoxicity, low cost, and natural abundance of sulfur.1 However, the practical applications of Li–S batteries have been greatly hindered by the insulating nature of sulfur, the shuttling of soluble lithium polysulfides (LiPSs), the volume variation of cathode materials upon discharge-charge, and the stability of lithium anodes.2-5 Of particular note, Li–S batteries present a complicated sulfur electrochemistry experiencing the solid-liquid-solid transformation between sulfur and lithium sulfide. During the charging process, the soluble long-chain LiPSs diffuse from sulfur cathode to lithium anode and are reduced to short-chain LiPSs. Subsequently, the short-chain LiPSs migrate back to cathode and are oxidized back to long-chain LiPSs. As a result, the LiPS migrating back and forth between cathode and anode is so-called “LiPS shuttle”, which gives rise to irreversible loss of active materials, low Coulombic efficiency, impeded sulfur utilization, and limited lifespan of systems. In response, implementing an efficient LiPS management is considered as a promising strategy to effectively restrain the LiPS shuttle, promote the redox kinetics and boost the electrochemical performances of Li–S batteries.6-8 To achieve a rational regulation of LiPSs, key efforts have been exerted on developing sulfur hosts and/or their additive materials such as carbons9 as well as metal oxides/sulfides/nitrides10-12. Furthermore, emerging candidates encompassing MXenes,13 metal organic frameworks (MOF),14


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covalent organic frameworks,15 and porphyrin organic framework16 have recently been explored. The employment of hosts and additives has undeniably realized the immobilization and conversion of LiPSs to a certain extent. Nevertheless, their incorporation within the cathode is at the cost of lowering the sulfur content and hence losing the energy density. Additionally, the soluble LiPSs generated upon cycling are highly likely to migrate through the pores of the traditional separator and reach the Li anode, causing a fast capacity decay. In light of this, designing a thin and functional interlayer between the cathode and separator is a noteworthy technique to maintain the active material loading whilst alleviate LiPS shuttle, thereby advancing the electrochemical performances. Along this line, commercial separators have been modified by simply depositing various materials such as carbons,17 polypyrrole,18 MOF/GO,14 and Co9S8,19 targeting efficient suppression of undesired LiPS migration with the presence of such a deposited layer. Despite steady progresses made in this direction, it is still very challenging to develop an ideal interlayerintegrated separator for practical Li–S batteries with high energy density and long cycle life.20 This is due to the fact that, on one hand, the carbon-based nanostructures normally exhibit weak affinity with polar LiPS stemming from their nonpolar surfaces; on the other hand, a plethora of polar materials possessing poor electrical conductivity can only impart a limited conversion of adsorbed LiPSs, and accordingly result in an inferior rate capability. Recently, vanadium-based materials such as VO2,21 V2O3,22 V2O5,23 VS2,24 VN,12,25 and VNmodified hierarchical porous carbon aerogel26 have been employed by others and us as sulfur hosts and additives in the realm of Li–S batteries. Despite advantageous trapping ability for LiPSs, VO2, V2O3, and V2O5 present a low-efficiency conversion of LiPSs due to their intrinsically poor conductivity. In contrast to vanadium oxides, VN displays a favorable electrical conductivity of ~106 S m–1 at room temperature, endowing it with a fast Li2S nucleation on the surface.12 Latest


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studies also revealed the catalytic effects of VN serving as hosts and additives on the enhancement of sulfur redox kinetics.12,25 However, applying VN for the functional interlayer in Li–S systems has rarely been demonstrated thus far. In this letter, we report the design of VN-membrane based separator to rationally guide LiPS management throughout simply depositing porous VN nanobelts onto conventional Celgard separator. Such a VN-modified separator in combination with sulfur cathode manifests several key advantages: (i) the thin yet dense membrane formed by porous and intertwined VN nanobelts serves as a physical barrier to block LiPSs whilst selectively sieve lithium ions; (ii) the polar VN surface acts as chemical reservoirs to trap LiPSs; and (iii) the superb electrical conductivity of VN enables the accelerated conversion of LiPSs. By synergizing these outstanding features, appropriate regulation of LiPSs could be gained by our VN-modified separator, which is ultimately beneficial to propelling the reaction kinetics and improving the electrochemical performances. The results demonstrated herein would enrich the strategies of LiPS regulation and inspire the design of versatile separators with multifunctionality and low cost toward advanced Li–S batteries. In view of the cell configuration of Li–S batteries, the normal PP separator suffers from weak blocking capability of LiPSs owing to the distribution of large-sized pores and limited electrical conductivity because of its insulating nature, giving rise to severe LiPS shuttle and inferior rate performance of the system. To address these challenges, modified separators have been proposed and demonstrated with the aid of the incorporation of selected functional materials. An ideal modified separator should meet the following requirements: (i) synergizing of structural blockage and chemical entrapment of LiPSs, (ii) possessing a conductive surface for Li2S nucleation, (iii) maintaining a favorable lithium ion conductivity, and (iv) realizing a low-mass-load of functional materials to ensure a reasonably high sulfur content. Our as-prepared VN nanobelts could be in-


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target deposited onto commercial PP separator by means of vacuum filtration to produce VNmodified functional separator. Such a material design and an interfacial engineering pertaining to polar and conductive VN would be beneficial to depressing the LiPS shuttle and improving the power performances targeting the effective LiPS regulation (Figure 1a). As shown in Figure 1a, polar conductive VN can chemically anchor LiPS and maintain efficient charge transport. Therefore, surface reaction and subsequent Li2S precipitation occur at the ample active sites of the VN. In our work, the porous VN nanobelts were accordingly synthesized by a simple wetchemistry process in combination with NH3 annealing procedure, as illustrated in Figure 1b.

Figure 1. Schematic showing the fabrication process of VN-modified separator. (a) Comparison of the LiPS migration and lithium ion transport between pristine and VN-modified separators. (b) Preparation procedure for VN-modified separator.


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Our obtained VN product was subject to detailed characterizations. The SEM image in Figure 2a discloses the intertwined nature of VN nanobelts with uniform size and shape distributions. Figure 2b displays a low-magnified TEM image of an individual VN nanobelt, showing its size information (several μm in length and ca. 50 nm in width). It is noted that ample micropores within the VN nanobelt can be clearly witnessed (Figure S1, Supporting information), which would be beneficial to facile ion transport. As such, our synthesized VN enables smooth transport channels for lithium ions due to its porous structure, as well as effective adsorption sites for LiPSs because of its polar surface. Close observation by employing high-resolution TEM (HRTEM) (Figure 2c) and corresponding selected-area electron diffraction (SAED) (Figure 2d) reveals that the highlycrystallized VN possesses the lattice fringe spacing of ~0.21 nm, implying the existence of (200) facet. The atomic force microscopy (AFM) image also corroborates the thin nature of VN nanobelt, exhibiting a thickness of 39.2 nm (Figure S2). In turn, VN-modified separators were produced by punching the vacuum-filtrated VN/PP film into standard circle disks with a diameter of 19 mm (Figure 2e, Figure S3). Such a separator manifests remarkable uniformity and flexibility, which could serve as a thin barrier to restrain the LiPS migration owing to the dense packing of intertwined VN. In addition, the as-formed VN layer also exhibits favorable electrical conductivity (with a sheet resistance of 121 Ω sq–1; Figure S4), thereby having the potential for the acceleration of LiPS conversion. Cross-sectional SEM/EDS characterizations (Figure 2f-h) demonstrate that the thickness of VN depositing layer is ca. 25 μm, featured by uniform elemental distributions of vanadium and nitrogen. The crystal structure of obtained product was further probed by XRD (Figure 2i), where all the detected signals can be completely assigned to cubic VN (JCPDS card No. 35-0768). Note that the (200) peak is predominant, suggesting that the (200) facet can be employed as the representative plane for theoretical simulations. As illustrated in Figure 2j, the


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surface area value of VN nanobelts derived by Brunauer-Emmett-Teller (BET) method reaches 30.0 m2 g–1 in combination with a total pore volume of 0.23 cm3 g–1. Such porous VN could not only possess high permeability for lithium ions but also offer abundant active sites for regulating LiPSs, accordingly propelling the sulfur redox kinetics and enhancing the electrochemical performances of the battery systems. XPS spectra reveal the chemical constitution of our VN (Figure S5, Figure 2k); The V 2p3/2 signal within the high-resolution V 2p spectrum deconvolutes into peaks with binding energies located at 516.3, 517.6 eV, and 515.1 eV (Figure 2k), which can be ascribed to V4+, V5+ and V3+ ions, respectively. V4+ and V5+ signals with minor contributions are attributed to the discontinuous VO2 and V2O5 passivation layers. Such layers are surface-bound due to the air-sensitive nature of VN. The N 1s spectrum displays a dominating V-N signal and a small contribution from V-N-O (Figure 2l), implying the successful fabrication of VN by our synthetic strategy.


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Figure 2. Characterization of as-synthesized VN nanobelts and VN-modified separator. (a) SEM image of VN. (b-d) TEM (b), HRTEM (c) images and corresponding SAED pattern (d) of VN. (e) Photographs of VN-modified separators. (f-h) Cross-sectional SEM images and corresponding elemental maps of VN-modified separator. (i) XRD pattern of VN. (j) N2 adsorption/desorption isotherm of VN. (k,l) XPS V 2p (k) and N 1s (l) spectra of VN.

To evaluate the blockage efficiency of VN-modified separator with respect to LiPS migration, Li2S6 permeation tests were systematically carried out by the employment of homemade H-shaped devices (Figure 3a). Typically, the VN-modified separator was sealed into the compartment of the two quartz tubes, where 4 mL of Li2S6 solution (4 mmol L–1) was injected into the left-handed tube and 4 mL of blank electrolyte including was filled within the other tube. The blank electrolyte was


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achieved by dissolving 1.0 mol L–1 LITFSI and 2 wt% LiNO3 in a mixed solution of DME/DOL. To clearly evaluate the anchoring of LiPSs, the LiPS migration was driven by the obvious concentration pressure generated by Li2S6 solution and blank electrolyte. In parallel, control test based on a pristine PP separator was also performed under identical conditions. As illustrated in Figure 3a, the migration of LiPSs was markedly suppressed in terms of the VN-modified separator within 12 h. In contrast, as for the pristine separator (Figure S6), the blank electrolyte within the right-handed tube turned completely yellow after 6 h, suggesting an easy pass of Li2S6 through the unmodified separator. Our VN nanobelts after such Li2S6 adsorption were inspected with the aid of Scanning TEM (STEM)-EDS characterization (Figure 3b, Figure S7). The STEM image and corresponding elemental maps over an area of interests with regards to an individual VN nanobelt exhibit uniform distributions of sulfur on the surface of VN, implying the strong affinity between VN and Li2S6 (Figure 3b). After 12 h of diffusion through VN-modified separator, the solution within the right-handed tube was also collected for ultraviolet-visible absorption test. As shown in Figure S8, in the visible light range, the absorption signals of Li2S6 solution vanish upon the employment of VN/PP separator, implying the efficient blockage of VN/PP separator toward LiPS migration. To further verify their interaction, theoretical simulations employing density functional theory (DFT) calculation were conducted to construct the optimal binding configurations between VN and sulfur species. As aforementioned, VN (200) facet was selected as the representative plane for modelling because of its dominant and stable state. Figure 3c depicts the side-view binding geometric configurations (with top-view configurations showing in Figure S9) between VN and various sulfur species, with the corresponding adsorption energy values indicated. Clearly, the adsorption energies between the (200) facet of VN and S8, Li2S8, Li2S6, Li2S4, Li2S2, and Li2S clusters are –2.02, –2.93, –2.72, –2.88, –3.24, and –3.36 eV, respectively, corroborating strong


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interaction between VN and LiPSs. Both these theoretical and experimental results substantiate that the dense packing and polar surface of porous VN nanobelts indeed pose a synergistic impact on the physical confinement and chemical immobilization of LiPSs, which is in favor of effective LiPS management toward the high-efficiency VN-modified separator.

Figure 3. LiPS permeation tests and theoretical simulation of the binding affinity between VN and sulfur species. (a) H-shaped permeation devices equipped with a VN-modified separator and pristine separator, respectively. (b) TEM, STEM images and corresponding V, N, and S maps of VN after Li2S6 adsorption. (c) Binding configurations and related adsorption energies of sulfur species with the (200) plane of VN.

In addition to the efficient anchoring of LiPSs, fast diffusion of lithium ions via VN at the separator would be beneficial to sulfur transformation chemistry, thereby ultimately boosting the electrochemical performance of Li–S batteries. To this end, CI-NEB simulation based on the dominant and stable VN (200) facet was further carried out to unravel the diffusion barrier for


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lithium ions on the surface of VN. As depicted in Figure 4a, the magnitude of the diffusion barrier on the energy profile (along the diffusion coordinate) lies in as low as 0.11 eV, with the corresponding diffusion pathway included. This result well suggests a facile diffusion of lithium ions on the surface of VN and better reaction kinetics of VN-separator incorporated batteries. As such, redox kinetics of LiPSs with the employment of VN was evaluated by redox current characterizations in combination with Li2S nucleation tests. The redox current curves were collected by the CV measurements of symmetric cells with CP-VN (or bare CP) simultaneously serving as the working and counter electrodes at a scan rate of 50 mV s–1 in a voltage range from -0.8 to 0.8 V. As shown in Figure 4b, VN-incorporated electrode presents an enhanced redox current in contrast to the bare CP one under a polarization of 0.8 V, implying outstanding catalytic ability of VN toward LiPS redox. This is probably due to the conductive nature of VN enabling an efficient charge transfer for LiPSs. In parallel, Li2S nucleation tests were performed by first galvanostatically discharging the cell to 2.06 V at 0.112 mA and then potentiostatically discharging under 2.05 V until the current was below 10–5 A. The Li2S nucleation was driven by using 0.1 V overpotential. Figure 4c and 4d record the Li2S precipitation curve of CP-VN and CP, respectively, with the capacity contributions in response to the reduction of Li2S8 and Li2S6 displayed in Figure S10. Calculated from the remaining current integration, the capacity of Li2S precipitation on CP-VN and CP reaches 128.4 and 71.1 mAh g–1, respectively. This suggests that the presence of VN leads to promoted Li2S nucleation. Note that the fast conversion of LiPSs to Li2S has been identified as an effective strategy to address the issues of LiPS shuttle and sluggish sulfur redox, which would be beneficial to improving sulfur utilization and cycling lifespan of Li–S batteries. Post-mortem SEM characterization of Li2S precipitation on CP-VN and CP were further performed by examining the electrodes from disassembled cells. In Figure 4e, the polar


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and conductive VN enables a high density of nucleation sites and large surface coverage of Li2S. Nevertheless, the bare CP surface is clean, with barely-discernible Li2S precipitants (Figure 4f), indicative of limited nucleation of Li2S. Additionally, digital photographs of separators from these disassembled cells are displayed in Figure S11, again showing an effective LiPS management by incorporating VN. In light of the decisive roles played by the surface activity and electrical conductivity in Li2S nucleation, polar and conductive VN could indeed accelerate the Li2S nucleation by promoting LiPS conversion. Our results as a whole imply that the introduction of VN-modified separator could realize the comprehensive regulation in terms of LiPS immobilization and conversion, which is expected to result in propelled sulfur redox kinetics and thereby enhanced electrochemical performances of Li–S batteries.

Figure 4. LiPS redox kinetics. (a) Energy profile for diffusion processes of lithium ions on VN. (b) CV of symmetric cells using CP-VN and CP as electrodes. (c,d) Potentiostatic discharge


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profiles of a Li2S8/tetraglyme solution on CP-VN and CP under 2.05 V. (e,f) SEM images of the Li2S precipitation on CP-VN and CP.

To reveal the conspicuous merits of VN-modified separators for boosting the electrochemical performances of Li–S batteries, standard coin cells with a sulfur content of 63.0 wt% were accordingly assembled. The sulfur mass loadings were 1.4-1.6 mg cm–2 for common sulfur cathodes and 2.4-3.8 mg cm–2 for high-load sulfur cathodes. As illustrated in Figure 5a, S@VN/PP by applying our VN separator manifests favorable rate capacities as compared to that of S@PP. As such, the discharge capacities of S@VN/PP at 0.2, 0.5, 1.0, and 2.0 C are 1280, 1043, 895, and 760 mAh g–1, respectively. When the current density is switched back to 0.2 C, the cathode manages to maintain a durable discharge capacity of 1065 mAh g-1. This result indicates that the employment of VN-modified separator enables the improvement of rate capability of sulfur cathodes, possibly throughout the creation of conductive environment in between the cathode and separator. Figure 5b displays the cycling performances of S@VN/PP and S@PP at a relatively low current density of 0.5 C. S@VN/PP delivers an initial capacity of 1050 mAh g-1 with a Coulombic efficiency of 99.3% and preserves a capacity of 860 mAh g-1 with a capacity decay of 0.18% per cycle after 100 cycles, manifesting a higher utilization of sulfur and superior cyclic stability in comparison with S@PP (capacity decay: 0.40%). In response, the discharge-charge profile of S@VN/PP at 0.5 C shows a prolonged low-voltage plateau and a smaller voltage hysteresis compared with those of S@PP, confirming the strong anchoring effect of LiPSs, propelled reaction kinetics, and remarkable electrical conductivity of the S@VN/PP cell (Figure 5c). In this respect, the porous VN network built upon the PP separator could offer abundant voids to accommodate electron and ion transport, thereby improving the utilization of sulfur. The superb


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conductivity of VN would endow the sulfur cathodes with promoted LiPS conversion, which is beneficial to enhancing the electrochemical performances of batteries. The S@VN/PP with higher sulfur loadings of 2.4-3.8 mg cm–2 were further constructed to meet the requirement for commercial applications. EIS measurements were conducted to probe the charge-transfer ability of the cells upon the separator modification with different sulfur loadings (Figure 5d, Figure S12). After charging to 2.8 V, the charge transfer resistances (Rct) of S@VN/PP were 14.0, 22.8, and 24.7 Ω with sulfur loadings of 1.6, 2.4, and 3.8 mg cm–2, respectively, indicating favorable conductivities of systems. As a control, the S@PP with sulfur loadings of 1.6 and 3.7 mg cm–2 display much higher Rct values. This result suggests that the introduction of dense VN layer could efficiently guide the Li-S chemistry and propel the redox kinetics. Figure 5e present the cycling performances of S@VN/PP with sulfur loadings of 3.1 and 3.8 mg cm–2 at 0.2 C. The S@VN/PP with a sulfur loading of 3.1 mg cm–2 delivers an initial areal capacity of 3.6 mAh cm–2 and restores 2.7 mAh cm–2 after 100 cycles. When the sulfur mass loading is further increased to 3.8 mg cm–2, the cell exhibits an initial areal capacity of 4.2 mAh cm–2 and remains an areal capacity of 2.7 mAh cm–2 after 100 cycles. In addition, the S@VN/PP with a sulfur loading of 1.6 mg cm–2 also exhibits a low capacity decay of 0.27% per cycle at 0.2 C after 80 cycles (Figure S13). These results imply outstanding cyclic stability in terms of relatively high sulfur loadings, which could be envisaged for practical usages. The enhanced electrical conductivity and accelerated sulfur redox kinetics with the aid of VNdecorated separator has also enabled remarkable cyclic stability under high rates. The long-term cycling performances of S@VN/PP were also evaluated at 1 C with sulfur loadings of 1.6 and 2.4 mg cm–2. As depicted in Figure 5f, S@VN/PP with a normal sulfur loading of 1.6 mg cm–2 displays an initial capacity of 960 mAh g–1 and a capacity decay of only 0.077% per cycle after


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800 cycles, demonstrating superb cycling performance as compared with S@PP. Note that the S@VN/PP manages to maintain a favorable cycling stability (capacity decay of 0.15%) at 1 C within 330 cycles even at a relatively high sulfur loading (2.4 mg cm–2). Post-mortem tests on the cathodes, separators, and anodes after 100 cycles at 1 C cycling were additionally carried out. First, SEM observations of cathodes after cycling reveal the structural stability of S@VN/PP in contrast to the scenario of S@PP, indicating efficient LiPS regulation upon applying the VN-modified separator (Figure S14). Second, the post-mortem SEM images and elemental maps of VN/PP separator also verify the structural stability of VN interlayer and its effective blockage toward LiPSs (Figure S15 and S16). Finally, the morphologies of lithium anodes after cycling were also inspected. Figure S17 exhibits the SEM images of lithium anodes pertaining to VN/PP and PP separators after cycling at 1 C. Obviously, the anode with VN/PP presents a smoother surface, less cracks and holes in contrast to that with pristine PP, implying that VN/PP can indeed lower the side reaction between lithium and LiPSs by suppressing the shuttle effect. This observation is in good accordance with those by applying other composite materials reported in the literatures.27-29 Furthermore, the pristine sulfur cathode was also electrochemically evaluated by employing the VN-modified separator. The pristine sulfur cathode was fabricated by vigorous mixing of 80 wt% sulfur, 10 wt% super P carbon black, and 10 wt% LA132 aqueous binder. As expected, the pristine sulfur cathodes show low initial discharge capacities of 852 and 720 mAh g-1 at 0.5 C and 1 C, respectively, manifesting an inferior sulfur utilization (Figure S18). These experimental and theoretical investigations corroborate that the employment of VN-based separator could realize an effective LiPS management and hence an advanced electrochemical performance of Li–S batteries. The role of our VN-separator could be summarized as follows: (i) synergizing the physical confinement and chemical immobilization of LiPSs via the dense packing structure and polar


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surface of VN; (ii) enabling smooth lithium ion diffusion through the porous structure of VN; (iii) attaining high-efficiency LiPS conversion owing to the conductive nature of VN. In this sense, a “three in one” strategy has been demonstrated for the VN-modified separator to ultimately regulate the behavior of LiPSs. Such a comprehensive regulation over the LiPS immobilization and conversion indeed leads to the accelerated sulfur redox kinetics and hence enhanced electrochemical performances of Li–S systems. In this sense, the battery performance enabled by our VN-modified separator compares favorably with those of state-of-the-art separator modified work (Table S1). In summary, we designed a multifunctional VN-modified separator for Li–S battery by simple vacuum-filtration of prepared VN dispersion through Celgard sheet. The dense packing structure and polar surface of VN endowed such functional separator with advanced abilities to enable dual structural/chemical LiPS immobilization and allow for facile lithium ion transportation. Benefiting from the superb electrical conductivity, the VN-modified separator also presented a fast conversion of LiPSs. Therefore, a comprehensive LiPS regulation in terms of the strong anchoring and favorable conversion with the aid of VN-modified separator was achieved, thereby promoting the sulfur redox kinetics and accordingly boosting the battery performance. Furthermore, the VNmodified separator demonstrated great implication for practical usage upon integration with highload sulfur cathodes. The LiPS regulation strategy in the present work might offer insights into rational management of sulfur redox and innovative design of high-energy and long-life Li–S batteries.


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Figure 5. Electrochemical performances of S@VN/PP and S@PP in Li−S batteries. (a) Rate capabilities of S@VN/PP and S@PP. (b) Cycling stabilities of S@VN/PP and S@PP at 0.5 C. (c) Galvanostatic discharge-charge profiles of S@VN/PP and S@PP at 0.5 C. (d) EIS curves of highload S@VN/PP and S@PP. (e) Cycling stabilities of S@VN/PP with sulfur mass loadings of 3.1 and 3.8 mg cm−2 at 0.2 C. (f) Long-term cycling performances of S@VN/PP with sulfur mass loadings of 1.6 and 2.4 mg cm−2 at 1 C.

ASSOCIATED CONTENT


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Supporting Information. Experimental procedures, supplementary methods, additional characterizations including SEM, AFM, TEM, XRD, XPS, BET, elemental analysis, and electrochemical measurements. The Supporting Information is available free of charge on the ACS Publications website at DOI: .

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Y. Sun) *E-mail: [email protected] (Q. H. Yang) *E-mail: [email protected] (J. Li) Author Contributions †These

authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work was supported by the National Natural Science Foundation of China (51702225), National Key Research and Development Program (2016YFA0200103) and Jiangsu Youth Science Foundation (BK20170336). Y.Z.S acknowledges the support from the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17-2023).

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Enhanced Sulfur Redox and Polysulfide Regulation via Porous VN

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