Conductive porous vanadium nitride/graphene composite as chemical anchor of polysulfides for lithium-sulfur batteries

Abstract

Although the rechargeable lithium-sulfur battery is an advanced energy storage system, its practical implementation has been impeded by many issues, in particular the shuttle effect causing rapid capacity fade and low Coulombic efficiency. Herein, we report a conductive porous vanadium nitride nanoribbon/graphene composite accommodating the catholyte as the cathode of a lithium-sulfur battery. The vanadium nitride/graphene composite provides strong anchoring for polysulfides and fast polysulfide conversion. The anchoring effect of vanadium nitride is confirmed by experimental and theoretical results. Owing to the high conductivity of vanadium nitride, the composite cathode exhibits lower polarization and faster redox reaction kinetics than a reduced graphene oxide cathode, showing good rate and cycling performances. The initial capacity reaches 1,471 mAh g^-1 and the capacity after 100 cycles is 1,252 mAh g^-1 at 0.2 C, a loss of only 15%, offering a potential for use in high energy lithium-sulfur batteries.

ARTICLE

Received 30 Jul 2016 | Accepted 17 Jan 2017 | Published 3 Mar 2017

Although the rechargeable lithiumsulfur battery is an advanced energy storage system, its practical implementation has been impeded by many issues, in particular the shuttle effect causing rapid capacity fade and low Coulombic efciency. Herein, we report a conductive porous vanadium nitride nanoribbon/graphene composite accommodating the catholyte as the cathode of a lithiumsulfur battery. The vanadium nitride/graphene composite provides strong anchoring for polysuldes and fast polysulde conversion. The anchoring effect of vanadium nitride is conrmed by experimental and theoretical results. Owing to the high conductivity of vanadium nitride, the composite cathode exhibits lower polarization and faster redox reaction kinetics than a reduced graphene oxide cathode, showing good rate and cycling performances. The initial capacity reaches 1,471 mAh g 1 and the capacity after 100 cycles is 1,252 mAh g 1 at 0.2 C, a loss of only 15%, offering a potential for use in high energy lithiumsulfur batteries.

DOI: 10.1038/ncomms14627 OPEN

Conductive porous vanadium nitride/graphene composite as chemical anchor of polysuldes for lithium-sulfur batteries

Zhenhua Sun1, Jingqi Zhang1, Lichang Yin1, Guangjian Hu1, Ruopian Fang1, Hui-Ming Cheng1,2 & Feng Li1

1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. 2 Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, Shenzhen 518055, China. Correspondence and requests for materials should be addressed to F.L. (email: mailto:[email protected]

Web End [email protected] ).

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Large-scale electrical energy storage involves transportation and stationary applications ranging from plug-in hybrid electric vehicles and full electric vehicles to the widespread

use of intermittent renewable energy in the modern electrical grid, all of which require advanced battery systems1. The high capacity and low cost of lithiumsulfur (LiS) batteries are essential for achieving practical applications2,3. These batteries possess high specic energy of 2,500 Wh kg 1 and 2,800 Wh l 1, and although their average working voltage is as low as 2.15 V, their high theoretical specic capacity of 1,672 mAh g 1 can compensate for this limitation4. The practical energy density for packaged LiS batteries may reach as high as 500600 Wh kg 1or 500600 Wh l 1, which is sufcient for driving an electric vehicle 500 km57.

Despite these attractive properties, one of the major issues with LiS batteries is their sluggish reaction kinetics stemming from the high electronic resistivity of sulfur and lithium suldes. As the resistivity of sulfur is as high as 1024 O cm, it is necessary to be combined with conductive materials8. In addition, the resistivity of Li2S is 41014 O cm and the Li ion diffusivity in Li2S is low9.

Once an insoluble insulation layer composed of Li2S2 and/or Li2S is plated on the electrode, it would increase the internal resistance, resulting in polarization that decreases energy efciency. Moreover, the 79% volume expansion of sulfur upon cycling induces the pulverization of active materials, which often results in poor contact with current collectors to further slow reaction kinetics10. The other major issue is polysuldes (Li2S4Li2S8) dissolving in the electrolyte and migrating between the anode and the cathode, which causes the so-called shuttle effect in a process in which polysuldes participate in reduction reactions with lithium and re-oxidation reactions at the cathode11,12. Despite the fact that the shuttle effect provides an

overcharge protection, it causes low discharge energy capacity, thermal effects, self-discharge and low Coulombic efciency13,14.

Porous carbon-based materials used as barriers and hosts have been demonstrated to be a simple approach to suppress the polysulde shuttle effect1518. Owing to the large specic surface area, macropores and mesopores can encapsulate a large amount of sulfur and facilitate fast ion transport19. A microporous sulfur/carbon composite has been produced that had an unusual capacity between 1.5 and 2 V, indicating a mixture of the two elements at the atomic level20. Nevertheless, because of the distinct non-polarity of carbon and the polarity of the Li2Sn species, the connement of polysuldes inside the pores is mainly a result of weak physical interactions21. Some advantages of porous carbon are conicting; for instance, a large surface area of Li2S2 and Li2S deposition is prone to cause an open structure and lead to ineffective trapping of polysuldes22, but a small pore volume limits the sulfur loading23,24. Functionalized graphene materials, such as graphene oxide obtained by the hydrothermal method, are decorated with hydroxyl and epoxide functional groups, and have chemical interactions with polysuldes25. Functional groups containing nitrogen and/or sulfur also show strong binding and are capable of anchoring polysuldes26,27. However, these functional groups are often unstable and it is difcult to control their contents28. Because of this, many groups have used polar oxides for chemically adsorbing polysuldes. For instance, MnO2 nanosheets were used to spatially locate and control the deposition of both Li2S/Li2S2 and sulfur by offering an active interface via the thiosulfate intermediate29. Silica has also been used as a polysulde adsorbent, because of its excellent stability and high specic surface area. In conjunction with a polyethylene oxide coating on a separator, self-discharge was increased due to

Hydrothermal Freeze-drying

NH3, 550 C

Graphene oxide NH4VO3

VOx/G composite VN/G composite

Cut and compress

Lithium foil

Separator

VN/G electrode

Electrolyte

L2S6 catholyte

Cell assembly

Free-standing VN/G composite electrode

Figure 1 | Schematic of fabrication process of VN/G composite and cell assembly. Schematic of the fabrication of a porous VN/G composite and the cell assembly with corresponding optical images of the material obtained. Scale bar, 500 nm.

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a b

c d

e f

0.20 nm (200)

FFT

Figure 2 | Morphology and structural characterization of the VN/G composite. (a) Low-magnication SEM image, (b) high-magnication SEM image, (c) high-angle annular dark-eld (HAADF) STEM image and (d,e) TEM images of the as-prepared porous VN/G composite. (f) High-resolution TEM (HRTEM) image, with inset showing the fast Fourier transform (FFT) pattern. Scale bars, (a) 100 mm; (b) 2 mm; (c) 500 nm; (d) 500 nm; (e) 50 nm;

(f) 5 nm.

the strong polysulde-silica interactions causing polysulde diffusion from the cathode30. Nonetheless, insulating oxides ultimately impede electron transport and interrupt paths for Li ion movement, thus leading to low sulfur utilization and rate capability. It is worth noting that introducing highly conductive polar materials into the sulfur electrode is an effective means of alleviating the above issues. For example, the surface of added metallic Ti4O7 triggers the reduction of sulfur and oxidation of

Li2S by forming an excellent interface with polysuldes31. Similarly, the addition of MXene phase Ti2C introduces exposed terminal metal sites that bond with sulfur as a result of an interface-mediated reduction32. Metal nitrides with a high electrical conductivity can be an ideal anchoring material. A generalized gradient approximation and local density approximation analysis of a series of transition metal

nitrides (TiN, VN, CrN, ZrN and NbN) indicate the metallic behaviour of these materials with no resolved band gap33. Among metal nitrides, vanadium nitride (VN) has a number of desirable properties for a potential host materials for sulfur including the following: (1) a strong chemical adsorption for polysuldes that can effectively inhibit the shuttle effect, (2) a high electrical conductivity (1.17 106 S m 1 at room temperature)

(Supplementary Table 1) that is conducive to the electrochemical conversion of adsorbed sulfur species on the surface and(3) catalytic properties similar to the precious metals that may facilitate redox reaction kinetics.

Here we report a highly conductive porous VN nanoribbon/ graphene (VN/G) composite accommodating a suitable amount of Li2S6 catholyte as the cathode of LiS batteries without using carbon black and binder. The free-standing three-dimensional

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a

(200)

Intensity (a.u.)

20 30 40 50 60 70 80 90

2[afii9835] ()

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20

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60

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200 400 600 800 1,000

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Figure 3 | Compositional information of the VN/G composite. (a) X-ray diffraction (XRD) pattern and (b) thermogravimetric-differential scanning calorimetry (TG-DSC) curve of the VN/G composite.

(3D) interconnected network of the graphene facilitates the transportation of electrons and lithium ions, and the VN not only shows strong chemical anchoring of the polysuldes, but also accelerates the redox reaction kinetics. The anchoring of polysuldes by VN is investigated in a dissolved polysulde system and further veried by theoretical calculations. The VN/G cathode delivers a high specic capacity of 1,461 mAh g 1 at 0.2 C, a Coulombic efciency approaching 100%, and a high-rate performance of 956 mAh g 1 at 2 C.

ResultsSynthesis and characterization of VN/G composite. As illustrated in Fig. 1, the synthesis of a porous VN/G composite involves two steps. We rst obtained a vanadium oxide/graphene (VOx/G) hydrogel by a hydrothermal method using graphene oxide and NH4VO3 as precursors. VOx was grown in situ on the surface of the graphene oxide and simultaneously assembled into a 3D foam. After immersion in deionized water, the product was subjected to freeze-drying and a VOx/G macrostructure was formed. After annealing in a NH3 atmosphere, the free-standing VN/G composite was obtained. The nal product can be cut and pressed into plates for direct use as LiS battery electrodes without a metal current collector, binder and conductive additive.

The morphology and microstructure of the VN/G composite were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as shown in Fig. 2. SEM images reveal that 3D interconnected network of VN nanoribbons and reduced graphene oxide (RGO) sheets. Numerous voids, several micrometres in size, are able to hold a large amount of sulfur and provide good penetration of electrolyte (Fig. 2a,b). This skeleton structure not only enhances the electron and lithium ion transportation but also accommodates the volume expansion of sulfur. The elemental mappings of vanadium, nitrogen, carbon and oxygen further reveal the hybrid

structure of the VN/G composite (Supplementary Fig. 1). To see this more clearly, we then characterized the structure using a high-angle annular dark-eld scanning TEM (STEM) and TEM in Fig. 2ce. The VN nanoribbons are typically 50100 nm wide and 12 mm long. Compared with the product before annealing in

NH3 (Supplementary Fig. 2), VN nanoribbons contains a large number of mesopores ranging from 10 to 30 nm in diameter, which are benecial for both the ion transportation and the adsorption of polysuldes in the electrochemical process. A representative high-resolution TEM image and the fast Fourier transform pattern are also shown in Fig. 2f, revealing lattice fringes with a spacing of 0.20 nm, which is in agreement with spacing of the (200) plane of VN. The graphene in the VN/G composite provides a supporting framework to prevent the aggregation of the VN nanoribbons.

The crystal structure of the 3D VN/G composite was further examined by X-ray diffraction (Fig. 3a). The major peaks are assigned to cubic VN (JCPDS card number 73-0528) with a wide peak around 26 corresponding to graphene stacking. Thermogravimetric-differential scanning calorimetry analysis suggested that the VN content was 30% (Fig. 3b). The specic surface area of the VN/G was 37 m2 g 1 with mesopores 18 nm in diameter (Supplementary Fig. 3), which is consistent with the

TEM observation. In contrast, the specic surface area of the RGO was as high as 296 m2 g 1 (Supplementary Fig. 4).

The electrochemical performance of VN/G cathodes. A series of electrochemical measurements were carried out to evaluate the performance of the VN/G cathode. In the cell assembly process, Li2S6 catholyte was directly added to VN/G (Fig. 1). The nal areal sulfur loading of the electrode was 3 mg cm 2. Typical cyclic voltammetry (CV) proles for the RGO and VN/G electrodes were obtained within a potential window of 1.72.8 V at a scan rate of 0.1 mV s 1 (Fig. 4a), both showing two cathodic peaks and two anodic peaks. The two representative cathodic peaks can be attributed to the reduction of sulfur to long-chain lithium polysuldes (Li2Sx, 3rxr8) at the higher potential and the formation of insoluble short-chain Li2S2/Li2S at the lower potential. When scanning back, the anodic peaks corresponded to the oxidation of Li2S/Li2S2 to polysuldes and then to sulfur. It is interesting to note that the reduction peaks with the VN/G cathode (2.0 and 2.35 V) appeared at higher potentials than those with the RGO cathode (1.88 and 2.24 V). The distinguishable positive shift in the reduction peaks and negative shift in the oxidation peaks of the VN/G cathode indicate the improved polysulde redox kinetics by VN. According to recent reports, Pt as an electrocatalyst can help to convert polysulde deposits back to soluble long-chain polysulde and hence enhance reaction kinetics and retain high Coulombic efciency, and the catalytic activities of VN resemble those of noble metal Pt34,35. These results suggest that VN has similar catalytic activity to that of precious metals, which can improve the redox reaction kinetics. Galvanostatic charge/discharge tests (Fig. 4b) were further performed at a constant current rate of 0.2 C (based on the mass of sulfur in the cell, 1 C 1,675 mA g 1). The charge

discharge proles of VN/G consist of two discharge plateaus at2.35 and 2.05 V, and two charge plateaus between 2.2 and 2.45 V, respectively, which are in agreement with the CV curves. The plateaus were longer and atter with a higher capacity and a lower polarization than those using the RGO electrode, suggesting a kinetically efcient reaction process. Figure 4c shows the cycling performance of the VN/G and RGO cathodes. The VN/G cathode delivered an excellent initial discharge capacity of 1,471 mAh g 1 and, more importantly, it was able to maintain a stable cycling performance with a Coulombic

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Figure 4 | Electrochemical performances of VN/G and RGO cathodes. (a) CV proles of the VN/G and RGO cathodes at a scan rate of 0.1 mVs 1 in a potential window from 1.7 to 2.8 V. (b) Galvanostatic chargedischarge proles of the VN/G and RGO cathodes at 0.2 C. (c) Cycling performance and

Coulombic efciency of the VN/G and RGO cathodes at 0.2 C for 100 cycles. (d) Rate performance of the VN/G and RGO cathodes at different current densities. (e) Cycling stability of the VN/G cathode at 1 C for 200 cycles.

efciency above 99.5% for 100 chargedischarge cycles at 0.2 C, indicating that dissolution of polysuldes into the organic electrolyte was effectively mitigated in the VN/G electrode. The LiNO3 additive in the electrolyte also has a positive effect on the

Coulomb efciency and cyclic performance of LiS batteries36. It was also conrmed that the VN/G host contributed almost nothing to the measured capacity (Supplementary Fig. 5). In contrast, the RGO cathode showed a lower discharge capacity of 1,070 mAh g 1 in the initial cycle and rapid capacity decay with a capacity retention of 47% after 100 cycles, implying low sulfur utilization with severe polysulde dissolution into the electrolyte. In the electrochemical impedance spectroscopy measurements (Supplementary Fig. 6), the Nyquist plots obtained consist of two parts, a semicircle in the high-frequency region representing the charge transfer resistance and a straight line in the low-frequency region associated with the mass transfer process. The VN/G cathode has a smaller resistance (28 O) than that of the RGO cathode (95 O), which can be explained by enhanced interfacial

afnity between VN and polysuldes, and the high electrical conductivity of metal nitrides comparable to their metal counterparts, as shown in Supplementary Table 1. In addition, the VN/G composite also exhibits an electrical conductivity of E1,150 S m 1 measured by the four-point probe method, which is over four times larger than that of RGO (about 240 S m 1), even though RGO contains doping nitrogen (about 4.6%) after

NH3 annealing (Supplementary Fig. 7). Although N-doped graphene can improve the performance of LiS batteries, but the electrochemical performance of VN/G composite electrode was much better than that of RGO electrode in the same condition. As shown in Fig. 4d, when the electrode was cycled at different rates of 0.2 C, 0.5 C, 1 C, 2 C and 3 C, the cell was able to deliver discharge capacities of 1,447, 1,241, 1,131, 953 and 701 mAh g 1, respectively. In contrast, the RGO electrode exhibited lower discharge capacity and poorer stability under the same conditions. Moreover, a stable discharge capacity of 1,148 mAh g 1 was recovered as soon as the current density was

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a

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S

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2.61

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Li

S

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Li

H

Li2S6-N doped graphene

Li2S6-VN (200)

Figure 5 | Demonstration of the strong interaction of VN/G composite with polysuldes. (a) Ultraviolet/visible absorption spectra of a Li2S6

solution before and after the addition of RGO and VN/G. Inset image shows a photograph of a Li2S6 solution before and an 2 h after the addition of graphene and VN/G. (b) Side view of a Li2S6 molecule on a nitrogen-doped graphene surface, the binding energy between Li2S6 and pyridinic N-doped graphene is calculated to be 1.07 eV. (c) Side view of a Li2S6 molecule on VN (200) surface, the binding energy between Li2S6 and VN is calculated to be 3.75 eV.

restored to 1 C. Figure 4e shows the long-term cyclability of VN/ G electrode at 1 C, indicating an excellent cycling stability. The initial capacity was as high as 1,128 mAh g 1 and retained 81%

of the initial capacity (917 mAh g 1) after 200 cycles. Although higher polarization occurred in the electrodes at higher rates due to slower dynamics of sulfur, the chargedischarge proles still consist of two plateaus even at a very high current density (Supplementary Fig. 8). In contrast, the VOx/G electrode displayed rapid capacity decay and low Coulombic efciency (about 93% after 100 cycles), which probably resulted from the low conversion efciency of polysuldes adsorbed on non-conductive VOx surfaces (Supplementary Fig. 9). The excellent electrochemical performance of the VN/G cathode can be attributed to the following factors. First, the porous VN host provides a polar surface and a strong chemical interaction with polysuldes, effectively inhibiting the shuttle effect. Second, the high electrical conductivity of VN enhances redox electron transfer and reduces interfacial impedance, and accelerates the polysulde conversion. Third, VN has similar catalytic activity to that of the precious metals, which improves the redox reaction kinetics.

DiscussionTo verify the strong anchoring of VN for polysuldes, as shown in Fig. 5a, we compared the polysulde adsorption ability of

RGO and the VN/G composite, after adding 20 mg of their powders to Li2S6 solution for 2 h. The VN/G completely decoloured the polysulde solution, whereas the solution containing RGO remained the same bright yellow colour. Ultraviolet/visible absorption measurements were also made to investigate the concentration changes of Li2S6 solutions after adding RGO or

VN/G. It can be clearly seen that the absorption peak of Li2S6 in the

visible light range apparently disappeared after adding VN/G, but remained after adding RGO (Fig. 5a). This difference suggests strong adsorption of Li2S6 molecules to polar VN, owing to ionic bonding of VS. The surface compositions of VN/G composite were measured by X-ray photoelectron spectroscopy survey spectra indicates that the surface of the VN also contains small amounts of VNO and VO bonds, which have a high afnity for polysuldes (Supplementary Fig. 10)37. The strong interaction between VN and lithium polysuldes was further veried by an evaluation of the binding energies between Li2S6 and

VN based on density functional theory calculations (Supplementary Note 1). As shown in the Supplementary Fig. 7, the pyridinic-N is the dominant dopant in N-doped graphene synthesized in this work. For comparison, the binding energy between Li2S6 and pyridinic N-doped graphene was considered, and it has been reported to be 1.07 eV38. In contrast, the binding energy between Li2S6 and VN was calculated to be much larger(3.75 eV). This is mainly due to the much stronger polarpolar interactions between Li2S6 and VN than those between Li2S6 and pyridinic N-doped graphene. In comparison with the case of Li2S6 on pyridinic N-doped graphene (Fig. 5b), the strong polar

polar interaction between Li2S6 and VN results in an obvious deformation of the Li2S6 molecule (Fig. 5c), forming three SV and one LiN bonds. The bond lengths of these SV (2.492.61 ) and LiN (2.08 ) bonds are very close to the corresponding bond lengths in bulk VS (2.42 ) and LiNH2 (2.06 ), respectively39,40.

These results clearly show the good afnity and strong chemical anchoring of polar VN for polysuldes. In addition, the non-polarity of graphene in the VN/G composite can also be benecial for the redeposition of the charging product sulfur. The hetero-polar VN/G electrodes provide both polar (VN) and non-polar (graphene) platforms to facilitate the binding of solid LixS and sulfur species to the electrodes. STEM elemental mapping was performed to track the sulfur distribution in the VN nanoribbons after cycling. The high-angle annular dark-eld STEM image and corresponding elemental maps of vanadium, nitrogen and sulfur show that the sulfur species were uniformly distributed and strongly adsorbed on the surface of the VN nanoribbons (Fig. 6). This result veries the experimental observations and corresponding theoretical calculations.

In summary, we have used a 3D highly conductive porous VN/G composite to solve the shuttle effect in LiS batteries. This composite combines the advantages of both graphene and VN. The 3D free-standing structure composed of a graphene network facilitates electron and ion transportation, but is also benecial to electrolyte absorption. In addition, VN showed a strong anchoring effect for polysuldes and its high conductivity also accelerated the polysulde conversion. The VN/G electrode exhibited excellent specic capacity with a Coulombic efciency reaching 499% compared with the RGO electrodes.

We believe that other highly conductive metal nitrides can also be used for high-energy LiS batteries and our design opens a new direction of the electrochemical use of transition metal nitrides for energy storage.

Methods

Preparation of a 3D porous VN/G composite. The VN/G composites were prepared using hydrothermal method, according to the previously reported

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a

b

STEM

V map

c

d

N map S map

Figure 6 | Sulfur distribution in the VN nanoribbons after cycling. (a) STEM image of a VN nanoribbon after cycling with the corresponding elemental maps of (b) vanadium, (c) nitrogen and (d) sulfur. Scale bars, 100 nm.

procedure41. Specically, 0.05 g NH4VO3 was dissolved in a mixture of 45 ml water and 5 ml ethanol, followed by slowly adding drops of HCl (2 M) to adjust the pH of the solution to 23. Next, 30 ml of a graphene oxide suspension (5 mg ml 1) was added to the solution under continuous stirring. The mixture was then transferred to a 100 ml Teon-lined autoclave, which was heated to 180 C where it was maintained for 24 h. The as-prepared sample was rinsed with deionized water several times followed by freeze-drying for 2 days. Finally, the obtained product was heated at 550 C for 3 h in an NH3 (30 s.c.c.m.) atmosphere. For comparison, a 3D RGO structure was prepared following the same procedure. The 3D porous VOx/G composite was also synthesized using a process similar to that for the synthesis of VN/G composite, except that the atmosphere of the heat treatment was changed from ammonia to argon.

Preparation of the Li2S6 solution. Sulfur and Li2S at a molar ratio of 5:1 were added to an appropriate amount of 1,2-dimethoxyethane and 1,3-dioxolane by vigorous magnetic stirring at 50 C until the sulfur was fully dissolved.

Polysulde adsorption test. A solution with a Li2S6 concentration of50 mmol l 1 (calculated based on sulfur content) was used. Twenty milligrams of

VN/G and RGO powder were separately added to 2.0 ml of Li2S6 solution and the mixtures were stirred to obtain thorough adsorption. A blank glass vial was also lled with the same Li2S6 solution as a comparison.

Preparation of sulfur electrodes. A VN/G composite was cut and compressed into 1.5 mg VN/G electrode. Next, inside an Argon-lled glovebox, 30 ml Li2S6

catholyte equal to 1.92 mg of sulfur and 60 ml of electrolyte was used to form the sulfur electrode. The nal areal sulfur loading of the electrode was determined about 3 mg cm 2.

Materials characterization. The morphology and structure of the materials were characterized using a SEM (FEI Nova NanoSEM 450, 15 kV). TEM imaging was performed on a FEI CM120 microscope. High-resolution TEM images, STEM images and energy dispersive X-ray spectroscopy (EDX) elemental maps were obtained on a FEI Tecnai F20 microscope equipped with an Oxford EDX analysis system with an acceleration voltage of 200 kV. X-ray diffraction patterns were obtained on a Rigaku diffractometer (Cu Ka, l 0.154056 nm). Thermogravi-

metric-differential scanning calorimetry analysis (TGA) was performed with a NETZSCH STA 449 C thermo balance in air with a heating rate of 10 C min 1 from room temperature to 1,000 C. The X-ray photoelectron spectroscopy measurements were carried out in an ultra-high vacuum ESCALAB 250 set-up equipped with a monochromatic Al Ka X-ray source (1486.6 eV; anode operating at 15 kV and 20 mA). Ultraviolet/visible absorption spectroscopy analysis (Cary 5000) was performed to evaluate the polysulde adsorption capability of RGO and VN/G. The electrical conductivities were measured by a standard four-point-probe resistivity measurement system (RTS-9, Guangzhou, China). N2 adsorption/

desorption isotherms were determined using a Micromeritics ASAP2020M instrument. Before the measurements, the samples were degassed at 200 C until a manifold pressure of 2 mm Hg was reached. The surface area and pore size distribution were determined based on the BarrettJoynerHalenda method.

Electrochemical measurements. Stainless steel coin cells (2,032-type)were assembled inside an Ar-lled glovebox. The electrolyte was lithium bis-triuoromethaesulphonylimide (99%, Acros Organics, 1 M) dissolved in 1,3-dioxolane (99.5%, Alfa Asea) and 1,2-dimethoxyethane (99.5%, Alfa Aesar) (1:1 ratio by volume) with 0.2 M lithium nitrate (LiNO3, 99.9%, Alfa Aesar) as the additive. Lithium metal foil was used as the anode and Celgard 2400 as the separator. A Landian multichannel battery tester was used to perform electro-chemical measurements. The charge-discharge voltage range was 1.72.8 V. The CV and the electrochemical impedance spectroscopy measurements were performed on a VSP-300 multichannel workstation.

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Data availability. The authors declare that the data supporting the ndings of this study are available within the article and its Supplementary Information les. All other relevant data supporting the ndings of this study are available from the corresponding author on request.

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Acknowledgements

We acknowledge nancial support from MOST (2016YFA0200100, 2014CB932402 and 2016YFB0100100) and the National Science Foundation of China (numbers 51525206, 51521091, 51472249, 51372253, 51272051 and U1401243), Youth Innovation Promotion Association of the Chinese Academy of Sciences (number 2015150), the Natural Science Foundation of Liaoning province (number 2015021012), the Institute of Metal Research (number 2015-PY03) and Strategic Priority Research Program of the Chinese Academy of Sciences (XDA09010104), the Key Research Program of the Chinese Academy of Sciences (grant number KGZD-EW-T06) and the CAS/SAFEA International Partnership Program for Creative Research Teams. The theoretical calculations were performed on TianHe-1(A) of National Suercomputer Center in Tianjin. We thank Dr Wei Lv and Professor Quanhong Yang for helping in experiments.

Author contributions

Z.S. and F.L. designed the research. Z.S. conducted the electrochemical experiments and characterization of materials, and J.Z. prepared the materials. L.Y. performed density functional theory calculations. G.H. and R.F. contributed to the discussion of the results. Z.S., J.Z., H.-M.C. and F.L. wrote the paper. All the authors commented on and revised the manuscript.

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How to cite this article: Sun, Z. et al. Conductive porous vanadium nitride/graphene composite as chemical anchor of polysuldes for lithium-sulfur batteries. Nat. Commun. 8, 14627 doi: 10.1038/ncomms14627 (2017).

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