rGO as an electrode material associated with lithium-sulfur batteries

Journal of Alloys and Compounds 785 (2019) 855e861

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Lithium storage performance and mechanism of VS4/rGO as an electrode material associated with lithium-sulfur batteries Liya Zhu 1, Chengsheng Yang 1, Yanan Chen, Jing Wang, Chenggang Wang, Xianjun Zhu* College of Chemistry, Central China Normal University, 152 Luoyu Rd, Wuhan, Hubei 430079, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 November 2018 Received in revised form 16 January 2019 Accepted 19 January 2019 Available online 23 January 2019

Nanorod vanadium sulfide anchored on reduced graphene oxide nanosheets composite is prepared by a simple solvothermal method. As an electrode material for lithium-ion batteries, the composite with 13 wt % reduced graphene oxide delivers 1886.3 and 1322.3 mAh g
1 specific capacities for the initial discharge and charge, respectively, resulting in 70.1% of Coulombic efficiency, and has a good capacity retention with 1181.9 mAh g
1 after 50 cycles, much higher than that of pure vanadium sulfide submicrospheres. It exhibits a good cycling stability and an enhanced high-rate capability. Lithium storage mechanism is investigated by cyclic voltammetry, electrochemical impedance spectrum and discharge-charge measurements. The results show that the vanadium sulfide with reduced graphene oxide composite can convert to sulfur þ vanadium with reduced graphene oxide composite after the first cycle, and the reduced graphene oxide can effectively inhibit the dissolution of polysulfide while metal vanadium increases electrode electronic conductivity, leading to an enhanced electrochemical performance. The composite is proved to be a promising functional material associated with lithium-sulfur batteries. © 2019 Elsevier B.V. All rights reserved.

Keywords: Vanadium sulfide Reduced graphene oxide Cathode Lithium-sulfur batteries

1. Introduction Lithium-ion batteries (LIBs) have been the predominant power sources in a variety of applications, from portable electronics to electric vehicles [1]. However, graphite as common commercial anode material for LIBs, hinders the development of high energy LIBs due to its low theoretical capacity (372 mAh g
1). Among all rechargeable batteries, lithium-sulfur (Li-S) batteries have a tremendous potential for high-energy storage devices, whose theoretical energy density is five times than that of LIBs [2,3]. But, its applications are still plagued by rapid capacity fade mainly resulting from the polysulfide shuttle. The dissolution of heteropolar polysulfide usually gives rise to losing active materials in the cathode, resulting in low Coulombic efficiency and poor cycling stability in Li-S batteries [4]. The poor electrochemical performance of Li-S batteries is because of the insulating nature of both sulfur and its discharging products (Li2S) as well as the dissolution of intermediate polysulfides. The common strategy to solve these issues is the confinement of sulfur in conductive carbon materials [5], including porous carbon [6,7] carbon nanotubes [8], graphene

* Corresponding author. E-mail address: [email protected] (X. Zhu). 1 L. Zhu and C. Yang equally contributed to this work. https://doi.org/10.1016/j.jallcom.2019.01.253 0925-8388/© 2019 Elsevier B.V. All rights reserved.

[9,10], heteroatom-doped carbon materials [11] and so on. For example, Wang et al. reported porous carbon supported sulfur cathode (with sulfur content of 72 wt%), delivering a high discharge capacity of 760 mA h g
1 after 150 cycles at 0.1 C and 455 mA h g
1 after 400 cycles at 1 C [6]. Zhang et al. used hollow graphene sphere through spray drying method as a sulfur host, achieving a reversible capacity of ~1340 mAh g
1 in the first discharge process and remains at ~780 mAh g
1 after 200 cycles at 0.2 C [10]. But, these nonpolar carbon materials fail to inhibit the polysulfide dissolution because of the only weak physical van der Waals adsorption. On the other hand, various polar materials with strong interactions with polysulfides, such as TiO2 [12], Co3O4 [13], TiN [14], Ni(OH)2 [15], Co9S8 [16] etc, have been regarded as alternative host materials for polysulfide adsorption and physical confinement. For instance, the Co3O4-S composite electrode exhibited high initial specific capacity of 1110 mAh g
1 at 0.1C. Even when the current density improved to 1 C, the capacity of Co3O4-S composite electrode still remained 796 mAh g
1 after 300 cycles [13]. Zhou et al. reported that the Co9S8@CNTs/S exhibited a high reversible specific capacity of 1425 mA h g
1 at 0.2 C, which is very close to the theoretical value of sulfur cathode [16]. Although the capacity can be improved, the long term cyclic performance is still unsatisfactory due to the high volume ratio of sulfur to the metal oxides/sulfides. Interestingly, some sulfides as electrode materials for LIBs

856

L. Zhu et al. / Journal of Alloys and Compounds 785 (2019) 855e861

exhibit the electrochemical characteristics of Li-S batteries [17,18]. It was found that MoS2 can be converted to Mo and Li2S after the first discharge. Then, the next electrochemical reactions happened between Li2S and S [19]. But, the dissolution of heteropolar polysulfide also gives rise to losing active reactions in Li-S batteries [20,21]. That means the electrochemical behaviour of metal sulfides is complicated because of the active materials involving in metal sulfides at the first discharge, and then sulfur after that instead. In addition, metal sulfides and sulfur have poor electronic conductivity as well as volume expansion during discharge and charge for Li-S batteries. Thus, it is an urgency to find novel hybrid materials to increase the capacity and conductivity of metal sulfides used for LiS batteries. Recently, vanadium sulfide (VS4) comes into sight [22,23]. It is a linear-chain compound, composing of S2
dimer connecting with neighbouring two V atoms (Fig. S1, see supporting information) [24]. Its crystallographic structure has weak interchain van der Waals forces, leading to a loosely stacked framework, which is favorable to Liþ insertion/desertion. VS4 is attracting considerable attention also because of its high content of sulfur and unique electrochemical behaviour [25,26]. For example, urchin-like VS4 as an anode material for LIBs delivers the first discharge capacity of 1305 mAh g
1 and the first charge capacity of 847 mAh g
1, giving rise to an initial coulombic efficiency of 65% at a current density of 100 mA g
1 [27]. VS4/MWCNTs nanoparticles have a reversible capacity of 922 mAh g
1 at 500 mA g
1 used as an anode for LIBs and also show excellent performance in Na-ion batteries [28]. Recently, Yao et al. employed VS4 as an alternative material for sulfur cathode for all-solid-state Li-S batteries, and the cell can deliver high reversible capacity of 611 mAh g
1 at 100 mA g
1 after 100 cycles, corresponding to 853 mAh g
1 based on the mass of sulfur [29]. But the poor electronic conductivity of VS4 hinders its application in the electrode. Graphene can be a promising candidate to solve the problem resulting from its high electronic conductivity and large surface area [30e32]. It is a key to make a composite in which VS4 particles can uniformly distribute on the surface of graphene nanosheets. However, there are rare reports about VS4 and reduced graphene oxide (rGO) composite (VS4/rGO, indicated as VSG) as a cathode material for Li-S batteries. It is not completely clear for the fundamental understanding of the insertion/desertion mechanism in the VSG composite for improving Liþ storage performance [31,33]. Therefore, it is necessary to investigate the role of the hetero-interface between VS4 and rGO for Liþ storage in VSG composite, and understand the synergistic effect of VS4 and rGO. In this study, VSG is synthesized by a simple hydrothermal method using NH4VO3, CH3CNH2S (TAA) and graphite oxide as precursors. In order to study the correlations between the heterointerface and Liþ storage performance, VSG composites with different contents of rGO are prepared. Compared with bare VS4 submicrospheres, the VSG composite exhibits enhanced electrochemical performance resulting from improving electron conductivity, suppressing polysulfide dissolution, and modifying interface between VS4 and rGO. The synergistic effect of VS4 and rGO is closely related to the hetero-interfacial area. After the first cycle, VS4 converts to S, and the electrochemical reaction occurs between Li and S, which is associated with Li-S batteries. The outcome will be favorable to the rational design of other transitional metal sulfides and graphene assembling as high-performance electrode materials for Li-S batteries. 2. Experimental 2.1. VSG preparation Graphite oxide was prepared from natural graphite using a

modified Hummers' method, and rGO was made as reported before [34]. VSG composite was synthesized by a hydrothermal method. Typically, NH4VO3 was dissolved in graphite oxide suspension (2.4 mg ml
1), and then TAA dissolved in ethylene glycol was introduced to obtain a mixture under continuous stirring. After ultrasonication for 1 h, the as-obtained solution was transferred into a Teflon-lined stainless steel autoclave with a volume of 100 ml. The autoclave was sealed and maintained at 160  C for 24 h. After being washed by deionized water and absolute alcohol several times, the product was dried in a vacuum oven at 80  C for 24 h. The total content of rGO in the composite was determined by chemical method. Typically, 200 mg VSG composite was dissolved completely in 100 ml of sodium hydroxide solution (20 wt%) under stirring, then the remaining suspension was filtered and washed by DI water several times. After being completely dried, the residue was weighed for calculating the rGO content in the VSG composite. In this study, three VSG composites are prepared, whose rGO contents are 7, 13 and 21 wt% (indicated as VSG-7, 13 and 21, respectively). Pure VS4 was also prepared using the same procedure without rGO. 2.2. Material characterizations Powder X-ray diffraction (XRD) for the samples was collected by using a D8 Advance X-ray diffractometer with Cu Ka X-ray source at room temperature. Raman spectroscopy (RENISHAW, England) was carried out with a 514.5 nm wavelength incident laser. Scanning electron microscope (SEM) images were acquired from Fieldemission scanning electron microscope. 2.3. Electrochemical measurements Galvanostatic charge-discharge measurements were performed in a potential range of 0.005e3 V vs Liþ/Li using a Land battery tester (China) by assembling 2032 coin cells in an argon-filled glove box. Lithium metal was used as the counter and reference electrode. The working electrode was obtained by mixing the assynthesized materials, acetylene black and polytetrafluoroethylene (PTFE) binder in a weight ratio of 8:1:1. The mass loading of VS4 active materials in the electrode was ~3 mg cm
2. The electrolyte was composed of 1 M LiPF6 dissolved in ethylene carbonate and diethyl carbonate with a volume ratio of 1:1, and Celgard 2300 as the separator. CV was conducted in the potential range of 0.005e3.0 V vs. Liþ/Li at a scan rate of 0.1 mV s
1, and electrochemical impedance spectroscopy (EIS) experiments was done using Autolab 302N potentio-galvanostat (Metrohm Autolab, Netherland) controlled by Nova 2.1 software. The excitation potential applied to the cells was 10 mV using the frequency ranged from 0.01 to 100 kHz. The specific capacity is based on the mass of VS4 in the composite. All of the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of pure VS4 and the as-prepared VSG composites. For pure VS4, all the diffraction peaks could be indexed as monoclinic phase VS4 in a space group of I2/a (JCPDS No. 211434). No peaks of any other impurities are detected in this pattern, indicating the high phase purity of VS4. For VSG composites, the intensities of the VS4 characteristic peaks decreased with the increase of rGO ratio, suggesting that the percentage of rGO in the VSG composites is crucial for preventing the aggregation of VS4 as well as rGO itself. It can be seen that monoclinic VS4 phase has formed in all of these composites as evidenced by its characteristic peaks. The decrease of VS4 characteristic peaks indicates the

L. Zhu et al. / Journal of Alloys and Compounds 785 (2019) 855e861

857

Fig. 1. (a) XRD pattern of VS4 and VSG composites. (b) Raman spectra of pure VS4 and VSG composites.

evolution of VS4-rGO hetero-interface with the increase of rGO ratio. Raman was performed to prove the nature of rGO, as shown in Fig. 1b. The bands located at 190 and 280 cm
1 are ascribed to the stretching A1 and bending B1 bands of VS4 [35]. The peaks located at 1340 and 1580 cm
1 are attributed to the D and G bands of graphene. Raman characterization confirms that the VSG composite consists of VS4 and rGO. The morphology and microstructure of the as-prepared VSG composites was analyzed by scanning electron microscopy (SEM).

Fig. 2 shows the SEM images of pure VS4, VSG-7, 13 and 21. Pure VS4 tends to aggregate due to the high surface energy and interlayer van der Waals attractions, leading to the formation of submicrospheres assembled by VS4 nanorods (Fig. 2aec). With the presence of rGO in the VSG composites, there is a crumpled flaky morphology, which is composed of VS4 nanorods grown on the surface of rGO nanosheets. Notably, for VSG-7 (Fig. 2def) with a low content of rGO (7 wt %), there appear VS4 submicrospheres besides flaky VS4-rGO hybrids,

Fig. 2. SEM images of VS4 (a, b, c), VSG-7 (d, e, f), VSG-13 (g, h, i) and VSG-21 (j, k, l).

858

L. Zhu et al. / Journal of Alloys and Compounds 785 (2019) 855e861

indicating that rGO is too low to prevent VS4 particles aggregation. For VSG-13 (Fig. 2gei) and VSG-21 (Fig. 2j-l), no VS4 submicrospheres were found in the composites. It indicates that VS4 particles were completely hampered to aggregate by rGO nanosheets, leading to forming VS4 nanorods with a size distribution of 50e100 nm in length and ~20 nm in width. The formation of separated VS4 nanorods has an advantage of accessibility of the reaction sites for Liþ. Moreover, VS4 nanorods have an advantage of the intimate contact with rGO nanosheets, resulting in a relatively strong interface interaction between VS4 and rGO. The content of rGO has a great effect on the hetero-interface of VS4 and rGO. As for VSG-13 with a medium content of rGO, it seems to have the morphology of thin flaky pieces, in which nanorod VS4 is wrapped completely by rGO and uniformly distributed on the surface of rGO nanosheets. Further increasing the rGO content of the VSG composite, although nanorod VS4 particles can be well covered by rGO nanosheets, more rGO sheets tend to aggregate to form multilayer rGO nanosheets. It can be seen from Fig. 2j-l that VSG-21 should have thicker flaky sheets than VSG-13. That is to say, a high content of rGO also increases the opportunity of rGO sheet aggregation in the VSG composites, which is unfavourable to the Liþ insertion/ desertion. In our study, VSG-13 should have an optimal amount of rGO among these composites, in which it would make a good hetero-interface between VS4 and rGO, leading to having an enhanced electrochemical performance. The electrochemical properties of VS4 and VSG samples are summarized in Fig. 3. Fig. 3a shows the first discharge and charge curves of VS4 and VSG samples in a potential window of 0.005e3.0 V at 100 mA g
1. All of the VSG samples in the first discharging curve have one plateau at ~1.9 V, corresponding to the Liþ intercalation into VS4 layers to become LixVS4. Following, there

appears a slope, indicating that Liþ further intercalation into LixVS4 to form Li2S. Meanwhile, Liþ can also insert or adsorb into/on the rGO nanosheets. Afterwards in the first charging curve, there is a long plateau at ~2.3 V, which is attributed to the conversion of Li2S into a high-order soluble polysulfide. Most notable is the lack of overpotential for the VSG composites in the initial charging process, indicative of a good VS4 interface with rGO nanosheets. After the first cycle, the discharge curve of the VSG composites is different, while the charge curve has the same tendency, meaning that the intercalation mechanism of the subsequent discharge is different from that of the first discharge (Fig. S3a, see support information). With the increase of rGO ratio in VSG samples, the discharge and charge capacities increase, resulting from the increment of rGO contribution from VSG samples. The reversible capacities of all VSG samples can exceed 147.1 mAh g
1 of pure VS4 (Fig. 3a). In addition, Fig. S3b (see supporting information) shows that rGO has a reversible capacity of 503.2 mAh g
1 under the same conditions. Based on the reversible capacities of VS4 (147.1 mAh g
1) and rGO (503.2 mAh g
1), in combination with their weight ratio of rGO and VS4 in VSG composites, the nominal capacity of VSG-7, 13 and 21 samples can be calculated to 172.0, 193.4 and 221.9 mAh g
1, respectively. For example, the nominal capacity of VSG-13 can be calculated as 503.2  0.13 þ 147.1  0.87 ¼ 193.4 mAh g
1. In this study, VSG-13 achieves an actual reversible capacity of 1322.3 mAh g
1 (1288.5 mAh g
1 Li2 S , based on the mass of Li2S in the electrode after the first discharge), beyond the simple principle of superposition between rGO and VS4. This clearly indicates a synergistic effect on the enhancement of Liþ storage capacity. As a comparison, the first reversible capacity of 1288.5 mAh g
1 Li2 S is close to the value of 1342.1 mAh g
1 of the initial reversible capacity for the MoS2Li2 S

Fig. 3. Electrochemical performance of VSG-7, 13 and 21 composites compared with that of pure VS4. (a) The first discharge-charge curves, (b) Cycling performance, (c) Rate performances in a potential range of 0.005e3.0 V at 100 mA g
1. (d) The CV curves of VSG-13 at a scanning rate of 0.1 mV s
1.

L. Zhu et al. / Journal of Alloys and Compounds 785 (2019) 855e861

derived Li2S@rGO cathode for Li-S batteries between 0.01 and 3.0 V in carbonate-based electrolytes reported in the literature [36]. On the other hand, with the increasing of the rGO ratio in the VSG composites, it is found from Table S1 (see supporting information) that the first reversible capacities of VS4, VSG-7, 13 and 21 increases from 147.1, 1240.8, 1322.3 to 1413.7 mAh g
1, respectively. But, their irreversible capacities decrease from 898.4, 602.2, to 564.0 mAh g
1, and then increases to 657.2 mAh g
1. This means that a high content of rGO in VSG samples can also lead to the increase of irreversible capacity due to the large surface specific area of the sample and the formation of solid electrolyte interphase (SEI). An optimal percentage of rGO in the VSG composites will have a maximum hetero-interfacial area for the high Liþ storage capacity. The favorable interfacial interaction also leads to significantly improved cycling performance of VSG samples compared with that of VS4. As shown in Fig. 3b, pure VS4 suffers from quick capacity decay from the 1st to the 2nd cycle. After 50 cycles, VS4 only delivers a capacity of ~17.2 mAh g
1. This could be due to poor conductivity, significant volume change and mechanical stress during Liþ insertion/extraction processes. Among VSG samples, VSG-13 exhibited the best cycling stability, with 92.3% capacity retention after 50 cycles at 100 mA g
1 compared to the capacity obtained in the second cycle, while the capacity retention of VSG-7 and VSG-21 are 77.0% and 74.2%, respectively. This demonstrates that the VSG composite can effectively increase the conductivity and buffer the volume change of VS4 during Liþ insertion/extraction processes. Fig. 3c shows the rate performance of VS4 and VSG samples in a potential window of 0.005e3.0 V. As the current density increases from 100 to 200, 500, 1000 and 2000 mA g
1, the discharge capacities of VSG-13 decrease from 1322.3 to 1236.8, 1085.3, 903.0 and 719.3 mAh g
1 correspondingly. After high rate measurement, the current density is back to 100 mA g
1, the discharge capacity recovers to 1208.5 mAh g
1. For VSG-7 and 21 samples, the discharge capacities are 566.5 and 796.6 mAh g
1 at 2000 mA g
1, respectively, then return to 1054.1 and 1213.2 mAh g
1 at 100 mA g
1, respectively. As a comparison, VS4 exhibits a reversible capacity of 13.3 mAh g
1 at 2000 mA g
1. When the current density returns to 100 mA g
1, the reversible capacity is 44.0 mAh g
1. These results indicate that the synergistic effect of VS4 and rGO on their hetero-interfaces can enhance the rate performances of VSG sample. Fig. 3d shows the CV curves of VSG-13 for the initial three cycles in a potential range of 0.005e3.0 V at a scan rate of 0.1 mV s
1. In the first negative sweep, there are two main reduction peaks. The shoulder peak located at ~1.80 V could be attributed to the lithium insertion into VS4 to form LixVS4; the main sharp peak at 1.63 V is likely to be a conversion reaction from LixVS4 to Li2S þ V; the other peak located at 0.49 V is assigned to the Liþ inserted into rGO. In the following negative sweep, the peak at 1.63 V disappears, suggesting that there is an irreversible reaction during the lithiation process. In the first positive direction, there is one oxidation peak at ~2.44 V, which represents the electrode reaction from Li2S to S. After the first cycle, VSG-13 shows the reduction peaks around 1.90 V, corresponding to the reaction from S to Li2S, and the broad oxidation peaks at ~2.49 V, which might be attributed to the reactions from Li2S to S [37,38]. That means the VSG composite after the first cycle converts to the S þ V/rGO composite, which can be associated with Li-S batteries. The typical peaks in the CV curves correspond well to the plateau positions in the discharge-charge curves. As a control experiment, the initial CV contrast diagram of VS4 and VSG composites are shown in Figs. S4 and S5 (see supporting information). It can be seen that the peak located at 0.49 V in the negative direction should be Liþ insertion into rGO. Obviously, all the reduction and oxidation peaks of VSG-13 are larger than those of VS4, VSG-7 or 21 samples, indicating that VSG-13 should have a higher specific

859

capacity. To further understand the lithiation/delithiation mechanism of VSG sample during discharge-charge process, visual beaker cell tests were carried out to observe the soluble polysulfides associated with Li-S batteries, as shown in Fig. 4. The colour changes of the electrolyte in the sealed beaker cells using VSG-13 as cathode and lithium foil as an anode were recorded at different time during the first discharging and charging process. As a contrast experiment, VS4 is also used as a cathode for a comparison. The VS4 mass loading in the electrode was ~3 mg cm
2. This is equal to ~2.1 mg cm
2 of S mass loading in the electrode because VS4 has 71.6% S in its molecular. For VS4, it can be seen from Fig. 4a, when the discharged time is 8.5 h (at d point), the colour of the electrolyte changes from colorless to light yellow, indicating that there is a little of polysulfide in the electrolyte. At this moment, the potential of the electrode is about 0.4 V. Further discharging to the end (at f point, ~10.5 h, 0.005 V), the colour of the electrolyte becomes bright yellows. Actually, the initial discharge process is a complex electrochemical reaction involving in the generation of polysulfide and a part of polysulfide dissolution into the electrolyte, resulting in the electrolyte yellow. The colour change from colorless to yellow is because polysulfide produced by the redox of VS4 gradually dissolves into the electrolyte. In the charging process (f/g/h/i/j), the colour of the electrolyte changes a little. For VSG-13 composite, Fig. 4b shows that the colour of the electrolyte almost has no colour change at the discharge time of ~15 h (at d' point, 0.4 V). After discharging over (at f' point, 0.005 V) and subsequently charging (f'/g'/h'/i'/j'), the colour of the electrolyte becomes light yellow, which is much lighter than that of VS4. It means that the concentration of polysulfide in the electrolyte is lower. According to the visual experimental results, it is believed that the presence of rGO in VSG-13 can suppress the polysulfide dissolution into the electrolyte [39,40]. That is to say, rGO has a great contribution to entrapping polysulfide in the electrode of VSG composites, resulting in high specific capacity as well as relatively high Coulombic efficiency and good cycle performance. The electrochemical impedance spectroscopy (EIS) measurements on the assembled cells for VSG sample were also used to investigate the lithiation/delithiation mechanism. The Nyquist plots at different cycles are shown in Fig. 5. The plots at the high frequency display a semicircle that stands for the charge transfer resistance (Rct), and an oblique line at the low frequency that is assigned to the semi-infinite Warburg diffusion process (Zw). The internal resistance (Rs) was shown in the junction point of the semi-circle and the real axis in high frequency. Obviously, the smaller the semicircle diameter is, the smaller the Rct is. At the beginning of the discharge-charge measurement for the VSG-13 electrode (before cycling), the Rct is high (curve 1). At the first cycle, the Rct (the radius of semi-circle) has a big change from curve 1 to curve 2. Following, the electrode resistance Rct decreases gradually from the first to 50th cycle (curve 2/curve 5). In this process, VS4 first transfers to LixVS4, then LixVS4 further inserted Liþ ions to become Li2S and metallic V at the first discharge. Meanwhile the product metallic V has a good conductivity, resulting in the decrease of Rct resistance. Following the first charging process, the Li2S change to S. Moreover, the product Li2S and S can uniformly distribute on the surface of rGO nanosheets during the chargingdischarging process, resulting in no increase of Rct resistance. After the first cycle, the electrode reaction becomes a conversion reaction between Li2S and S. The activation of the electrode during cycling can also lead to a little decrease of the resistance Rct. Combined with the results of CV and visual beaker cell test above, it suggests that lithiation/delithiation reaction occurs as follows [31]. The first cycle

860

L. Zhu et al. / Journal of Alloys and Compounds 785 (2019) 855e861

Fig. 4. Visual confirmation of polysulfide entrapment at different discharge-charge depths at 100 mA g
1. (a) VS4 and (b) VSG-13.

charge

þ Li2 S ƒƒƒƒƒ! ƒƒƒƒƒ S þ 2Li þ 2e
discharge

Fig. 5. Nyquist plots of VSG-13 electrode before cycling and after 1st, 10th, 20th, 50th cycle at a current density of 100 mA g
1.

discharge

VS4 þ x Liþ þ x e
ƒƒƒƒ! Lix VS4

(1) discharge

Lix VS4 þ ð8
xÞ Liþ þ ð8
xÞÞ e
ƒƒƒƒ! 4 Li2 S þ V charge

Li2 S ƒƒ! S þ 2 Liþ þ 2e
After the first cycle

(2)

(3)

(4)

At the first discharging, the lithiation process is indicated as equations (1) and (2). After the first discharge, the electrode resistance decreases, indicating that VS4 has transformed to Li2S þ V. The formation of metallic V greatly increases the conductivity of the electrode. Then in the first charging, Li2S converts to S. Ex-situ XRD indicates that there exist Li2S and V after the first full discharge, and there are S and V after the first cycle (Fig. S6, see supporting information). In the following discharging-charging process, there is no significant change in electrode resistance. The reaction can become the conversion reaction between Li2S and S as equation (4) shown. It is noted that an amorphous S phase is formed during the charging process. The product S is an insulating material which is unfavourable to the electrode conductivity. However, the amorphous S can be distributed on the surface of the good conductive rGO nanosheets during the discharge conversion reaction, which can keep the electrode having a good conductivity. Therefore, the electrode resistance has little change after the first cycle. Afterwards, the battery reaction occurs between Li and S associated with Li-S batteries. The promising electrochemical performance of VSG composites could be attributed to the good hetero-interfaces between nanorod VS4 and rGO. Particularly, the van der Waals interaction between rGO and VS4 leads to an intimate interfacial interaction, which can prevent the restacking and aggregating of VS4 nanorods as well as

L. Zhu et al. / Journal of Alloys and Compounds 785 (2019) 855e861

rGO nanosheets. Consequently, there are more active sites available for the electrode/electrolyte interaction. The intimate interfacial interaction also improves the interfacial electron transfer during lithiation/delithiation, which can maximize the conductivity of rGO. Furthermore, metallic V can increase electron transfer efficiency and be beneficial to the diffusion behaviors of Liþ, resulting in an enhanced Liþ storage ability of VSG composite for LIBs. 4. Conclusions In summary, VSG composites have been successfully synthesized by a solvothermal method. As an electrode material for LIBs, it has a high specific capacity and good cycling stability as well as high Columbic efficiency due to the synergistic effect of VS4 and rGO. In the first cycle, VSG has a conversion battery reaction from VS4/rGO to S þ V/rGO composite. From the second cycle on, the battery reaction can be associated with Li-S batteries. With the presence of rGO, the hetero-interfaces between VS4 and rGO are favorable to battery reactions, leading to having an enhanced electrochemical performance. Acknowledgements

[16]

[17] [18] [19]

[20]

[21]

[22]

[23]

[24]

This work was supported by the Program of Introducing Talents of Discipline to Universities of China (111 Program, B17019).

[25]

Appendix A. Supplementary data

[26]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.01.253.

[27]

References [1] Z. Liu, Q. Yu, Y. Zhao, R. He, M. Xu, S. Feng, S. Li, L. Zhou, L. Mai, Silicon oxides: a promising family of anode materials for lithium-ion batteries, Chem. Soc. Rev. 48 (2019) 285e309. [2] A. Eftekhari, D.-W. Kim, Cathode materials for lithiumesulfur batteries: a practical perspective, J. Mater. Chem. A 5 (2017) 17734e17776. [3] H.-J. Peng, J.-Q. Huang, X.-B. Cheng, Q. Zhang, Review on high-loading and high-energy lithiumesulfur batteries, Adv. Energy Mater. 7 (2017) 1700260e1700313. [4] X. Liang, C. Hart, Q. Pang, A. Garsuch, T. Weiss, L.F. Nazar, A highly efficient polysulfide mediator for lithium-sulfur batteries, Nat. Commun. 6 (2015) 5682e5689. [5] L. Zhang, Y. Wang, Z. Niu, J. Chen, Advanced nanostructured carbon-based materials for rechargeable lithium-sulfur batteries, Carbon 141 (2019) 400e416. [6] H. Gu, R. Zhang, P. Wang, S. Xie, C. Niu, H. Wang, Construction of threedimensional ordered porous carbon bulk networks for high performance lithium-sulfur batteries, J. Colloid Interface Sci. 533 (2019) 445e451. [7] F. Yu, H. Zhou, Q. Shen, Modification of cobalt-containing MOF-derived mesoporous carbon as an effective sulfur-loading host for rechargeable lithiumsulfur batteries, J. Alloys Compd. 772 (2019) 843e851.
mez-Urbano, J.L. Go
mez-Ca
mer, C. Botas, T. Rojo, D. Carriazo, Graphene [8] J.L. Go oxide-carbon nanotubes aerogels with high sulfur loadings suitable as binderfree cathodes for high performance lithiumesulfur batteries, J. Power Sources 412 (2019) 408e415. [9] X. Hong, J. Liang, X. Tang, H. Yang, F. Li, Hybrid graphene album with polysulfides adsorption layer for Li-S batteries, Chem. Eng. Sci. (2019) 148e155. [10] H. Li, L. Sun, Y. Zhao, T. Tan, Y. Zhang, Blackberry-like hollow graphene spheres synthesized by spray drying for high-performance lithium-sulfur batteries, Electrochim. Acta 295 (2019) 822e828. [11] Q. Zhu, H. Deng, Q. Su, G. Du, Y. Yu, S. Ma, B. Xu, A free-standing nitrogendoped porous carbon foam electrode derived from melaleuca bark for lithium-sulfur batteries, Electrochim. Acta 293 (2019) 19e24. [12] N. Li, Z. Chen, F. Chen, G. Hu, S. Wang, Z. Sun, X. Sun, F. Li, From interlayer to lightweight capping layer: rational design of mesoporous TiO2 threaded with CNTs for advanced LieS batteries, Carbon 143 (2019) 523e530. [13] Y. Chen, X. Ji, Bamboo-like Co3O4 nanofiber as host materials for enhanced lithium-sulfur battery performance, J. Alloys Compd. (2019) 688e692. [14] Y. Wang, R. Zhang, Y.C. Pang, X. Chen, J. Lang, J. Xu, C. Xiao, H. Li, K. Xi, S. Ding, Carbon@titanium nitride dual shell nanospheres as multi-functional hosts for lithium sulfur batteries, Energy Storage Mater. 16 (2019) 228e235. [15] H. Wu, Y. Li, J. Ren, D. Rao, Q. Zheng, L. Zhou, D. Lin, CNT-assembled

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

861

dodecahedra core@nickel hydroxide nanosheet shell enabled sulfur cathode for high-performance lithium-sulfur batteries, Nano Energy 55 (2019) 82e92. J. Zhou, X. Liu, J. Zhou, H. Zhao, N. Lin, L. Zhu, Y. Zhu, G. Wang, Y. Qian, Fully integrated hierarchical double-shelled Co9S8@CNT nanostructures with unprecedented performance for Li-S batteries, Nanoscale Horiz. 4 (2019) 182e189. S. Wu, Y. Du, S. Sun, Transition metal dichalcogenide based nanomaterials for rechargeable batteries, Chem. Eng. J. 307 (2017) 189e207. T. Wang, S. Chen, H. Pang, H. Xue, Y. Yu, MoS2-based nanocomposites for electrochemical energy storage, Adv. Sci. 4 (2017) 1600289e1600304. J. Xiao, X. Wang, X.Q. Yang, S. Xun, G. Liu, P.K. Koech, J. Liu, J.P. Lemmon, Electrochemically induced high capacity displacement reaction of PEO/MoS2/ graphene nanocomposites with lithium, Adv. Funct. Mater. 21 (2011) 2840e2846. H.B. Yao, G.Y. Zheng, P.C. Hsu, D.S. Kong, J.J. Cha, W.Y. Li, Z.W. Seh, M.T. McDowell, K. Yan, Z. Liang, V.K. Narasimhan, Y. Cui, Improving lithiumsulphur batteries through spatial control of sulphur species deposition on a hybrid electrode surface, Nat. Commun. 5 (2014) 1e9. H.J. Peng, Q. Zhang, Designing host materials for sulfur cathodes: from physical confinement to surface chemistry, Angew. Chem. Int. Ed. 54 (2015) 11018e11020. L. Zhang, Q. Wei, D. Sun, N. Li, H. Ju, J. Feng, J. Zhu, L. Mai, E.J. Cairns, J. Guo, Conversion reaction of vanadium sulfide electrode in the lithium-ion cell: reversible or not reversible? Nano Energy 51 (2018) 391e399. Y. Wang, Z. Liu, C. Wang, X. Yi, R. Chen, L. Ma, Y. Hu, G. Zhu, T. Chen, Z. Tie, J. Ma, J. Liu, Z. Jin, Highly branched VS4 nanodendrites with 1D atomic-chain structure as a promising cathode material for long-cycling magnesium batteries, Adv. Mater. 30 (2018) 1802563e1802572. €sch, E. Hellner, Die kristallstruktur R. Allmann, I. Baumann, A. Kutoglu, H. Ro des patronits V(S2)2, Naturwissenschaften 51 (1964) 263e264. G. Yang, B.W. Zhang, J.Y. Feng, H.H. Wang, M.B. Ma, K. Huang, J.L. Liu, S. Madhavi, Z.X. Shen, Y.Z. Huang, High-crystallinity urchin-like VS4 anode for high-performance lithium-ion storage, ACS Appl. Mater. Interfaces 10 (2018) 14727e14734. W.B. Li, J.F. Huang, L.L. Feng, L.Y. Cao, S.W. He, 3D self-assembled VS4 microspheres with high pseudocapacitance as highly efficient anodes for Na-ion batteries, Nanoscale 10 (2018) 21671e21680. G. Yang, B. Zhang, J. Feng, H. Wang, M. Ma, K. Huang, J. Liu, S. Madhavi, Z. Shen, Y. Huang, High-crystallinity urchin-like VS4 anode for high-performance lithium-ion storage, ACS Appl. Mater. Interfaces 10 (2018) 14727e14734. Y. Zhou, J. Tian, H. Xu, J. Yang, Y. Qian, VS4 nanoparticles rooted by a-C coated MWCNTs as an advanced anode material in lithium ion batteries, Energy Storage Mater. 6 (2017) 149e156. Q. Zhang, H. Wan, G. Liu, Z. Ding, J.P. Mwizerwa, X. Yao, Rational design of multi-channel continuous electronic/ionic conductive networks for room temperature vanadium tetrasulfide-based all-solid-state lithium-sulfur batteries, Nano Energy (2019). https://doi.org/10.1016/j.nanoen.2019.1001.1004. X. Xie, Z. Ao, D. Su, J. Zhang, G. Wang, MoS2/graphene composite anodes with enhanced performance for sodium-ion batteries: the role of the twodimensional heterointerface, Adv. Funct. Mater. 25 (2015) 1393e1403. X. Xu, S. Jeong, C.S. Rout, P. Oh, M. Ko, H. Kim, M.G. Kim, R. Cao, H.S. Shin, J. Cho, Lithium reaction mechanism and high rate capability of VS4-graphene nanocomposite as an anode material for lithium batteries, J. Mater. Chem. A 2 (2014) 10847e10853. Y.G. Zhang, L.C. Sun, H.P. Li, T.Z. Tan, J.D. Li, Porous three-dimensional reduced graphene oxide for high-performance lithium-sulfur batteries, J. Alloys Compd. 739 (2018) 290e297. S. Britto, M. Leskes, X. Hua, C.A. Hebert, H.S. Shin, S. Clarke, O. Borkiewicz, K.W. Chapman, R. Seshadri, J. Cho, C.P. Grey, Multiple redox modes in the reversible lithiation of high-capacity, Peierls-distorted vanadium sulfide, J. Am. Chem. Soc. 137 (2015) 8499e8508. X. Zhu, J. Hu, W. Wu, W. Zeng, H. Dai, Y. Du, Z. Liu, L. Li, H. Ji, Y. Zhu, LiFePO4/ reduced graphene oxide hybrid cathode for lithium ion battery with outstanding rate performance, J. Mater. Chem. A 2 (2014) 7812e7818. R. Sun, Q. Wei, Q. Li, W. Luo, Q. An, J. Sheng, D. Wang, W. Chen, L. Mai, Vanadium sulfide on reduced graphene oxide layer as a promising anode for sodium ion battery, ACS Appl. Mater. Interfaces 7 (2015) 20902e20908. M. Zensich, T. Jaumann, G.M. Morales, L. Giebeler, C.A. Barbero, J. Balach, A top-down approach to build Li2S@rGO cathode composites for high-loading lithiumesulfur batteries in carbonate-based electrolyte, Electrochim. Acta 296 (2019) 243e250. Y. Zhou, Y. Li, J. Yang, J. Tian, H. Xu, J. Yang, W. Fan, Conductive polymercoated VS4 submicrospheres as advanced electrode materials in lithium-ion batteries, ACS Appl. Mater. Interfaces 8 (2016) 18797e18805. R. Sun, Q. Wei, Q. Li, W. Luo, Q. An, J. Sheng, D. Wang, W. Chen, L. Mai, Vanadium sulfide on reduced graphene oxide layer as a promising anode for sodium ion battery, ACS Appl. Mater. Interfaces 7 (2015) 20902e20908. K. Xie, Y. You, K. Yuan, W. Lu, K. Zhang, F. Xu, M. Ye, S. Ke, C. Shen, X. Zeng, X. Fan, B. Wei, Ferroelectric-enhanced polysulfide trapping for lithium-sulfur battery improvement, Adv. Mater. 29 (2017) 1604724e1604729. T. Chen, B. Cheng, G. Zhu, R. Chen, Y. Hu, L. Ma, H. Lv, Y. Wang, J. Liang, Z. Tie, Z. Jin, J. Liu, Highly efficient retention of polysulfides in “sea urchin”-like carbon nanotube/nanopolyhedra superstructures as cathode material for ultralong-life lithium-sulfur batteries, Nano Lett. 17 (2017) 437e444.