graphene interlayer as an efficient polysulfide barrier for advanced lithium-sulfur batteries

graphene interlayer as an efficient polysulfide barrier for advanced lithium-sulfur batteries

Electrochimica Acta 256 (2017) 28–36 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electac...

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Electrochimica Acta 256 (2017) 28–36

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Research Paper

Dual functional MoS2/graphene interlayer as an efficient polysulfide barrier for advanced lithium-sulfur batteries Pengqian Guo, Dequan Liu, Zhengjiao Liu, Xiaonan Shang, Qiming Liu, Deyan He* School of Physical Science and Technology, and Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China

A R T I C L E I N F O

Article history: Received 17 August 2017 Received in revised form 20 September 2017 Accepted 1 October 2017 Available online 7 October 2017 Keywords: Lithium-sulfur batteries Shuttle effect MoS2 Graphene Interlayer

A B S T R A C T

A dual functional interlayer consisted of composited two-dimensional MoS2 and graphene has been developed as an efficient polysulfide barrier for lithium-sulfur batteries (LSBs). With such a configuration, LSBs show a superior rate capacity and improved cycling capacity. The excellent electrochemical performance can be attributed to the strong bonding interactions between the MoS2/ graphene interlayer and the formed lithium polysulfides (LiPSs) as well as the good electrical conductivity of the MoS2/graphene composite. The MoS2/graphene interlayer can physically block LiPSs by the graphene nanosheets and chemically suppress the dissolution of LiPSs by the polar MoS2 nanoflowers. Such a dual functional interlayer further provides a good contact with the surface of the sulfur cathode, acts as an upper current collector and greatly improves the sulfur utilization and the rate capability of LSBs. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Advanced energy storage technologies based on rechargeable batteries are becoming increasingly important in today’s world [1,2]. Lithium-sulfur batteries (LSBs) with a high theoretical capacity of 1675 mAh g 1 and a high energy density of 2600 Wh kg 1, almost one order of magnitude higher than those of the current lithium-ion batteries, are regarded as a promising candidate for the next-generation batteries. Sulfur also has advantages of natural abundance, low cost, nontoxicity, and environmental friendliness [3,4]. However, practical application of LSBs is impeded by several factors in which the intrinsically poor electrical conductivity of sulfur and the fast capacity degradation caused by the dissolution of long-chain lithium polysulfides (LiPSs) are two main obstacles [3–6]. Much effort has been devoted to address the above issues, including to design various host materials for immobilizing sulfur and LiPSs [7–12], develop new electrolyte additives [13,14] and other electrode protection strategies [15–17], however, shuttle effect is still inevitable. Besides, the fabrication processes in the mentioned approaches cannot avoid high cost, elaborate

* Corresponding author. E-mail address: [email protected] (D. He). https://doi.org/10.1016/j.electacta.2017.10.003 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

procedures and harsh conditions, which are serious drawbacks for the commercialization of LSBs. Changing the cell configuration by modifying separator or adding an interlayer between the sulfur cathode and separator have been proven to significantly suppress the shuttling of LiPSs [18–22]. Many carbonaceous materials, such as porous carbons [18], conductive polymers [23], carbon nanotubes (CNTs) [24], and graphene [25–27] have been used as bifunctional interlayers in LSBs, since the carbon interlayers could not only reduce the shuttle effect but also enhance the electrical conductivity of the sulfur cathodes. However, weak interaction between the non-polar carbon materials and the polar LiPSs is not sufficient to suppress the diffusion of LiPSs in organic electrolyte [12,19,20]. A number of metal oxides and metal sulfides which can chemically bond with LiPSs have been introduced to enhance the trapping of LiPSs and improve the electrochemical performance of LSBs. Xiao et al. reported a TiO2/graphene interlayer which effectively immobilized LiPSs [28]. Liu et al. explored a class of regenerative polysulfide-scavenging layers based on CNTs and oxides, LSBs with these functional interlayers as a shuttle inhibitor exhibited high areal capacity and extended cycling life [29]. Very recently, two-dimensional (2D) layered transition metal disulfides were shown to have strong chemical interactions with LiPSs and thus efficiently depressed the shuttle effect. Lei et al. designed a freestanding [email protected] composite as a host material for sulfur

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Fig. 1. Schematic of the electrode configuration for the designed LSB with an MoS2/graphene interlayer.

cathode, resulting in an enhanced electrochemical performance [30]. Ghazi et al. reported a MoS2/Celgard composite separator as a new barrier for LiPSs, LSBs with such a separator showed high coulombic efficiency and long-cycle stability [31]. MoS2-x/rGO composite was also used as a catalyst for LiPS conversion in sulfur cathode to improve the battery performance [32]. Considering the excellent electrical conductivity of graphene nanosheets and strong chemical interactions between MoS2 and LiPSs, here we design a dual functional MoS2/graphene interlayer with better conductivity to control the shuttling of LiPSs in LSBs, on the basis of a combination of physical and chemical adsorption for LiPSs. Graphene nanosheets in the MoS2/graphene interlayer are well connected to form a three-dimensional (3D) conductive network, which effectively improves conductivity of the sulfur cathode. It can be found that the prepared MoS2/graphene

interlayer is able to efficiently mitigate the shuttle effect and decrease the polarization of the sulfur cathode, and the resultant LSBs exhibit an improved cycling capacity and an excellent rate performance. 2. Experimental 2.1. Synthesis of MoS2/graphene composite Graphene oxide (GO) was first prepared using natural graphite powder as a starting material by a modified Hummers method [33]. MoS2/graphene composite was synthesized via a one-step hydrothermal reaction followed by a thermal treatment. Briefly, 60 mg GO was dispersed in 45 ml of deionized water. After ultrasonication for 1 h, sodium molybdate dehydrate (400 mg) and

Fig. 2. (a, b) SEM and (c, d) TEM images of the prepared MoS2/graphene composite.

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Fig. 3. (a) SEM image of the used pristine separator. (b, c) Top-surface and cross-section SEM images of the MoS2/graphene composite-coated separator. (d) Photograph of the pristine separator (left) and the MoS2/graphene composite-coated separator (right).

thiourea (400 mg) were added into it. The formed solution was then transferred into a 60 ml of Teflon-lined stainless steel autoclave and kept at 200  C for 24 h. Subsequently, the obtained black hydrogel was washed with deionized water and freeze-dried for 12 h. The foam-like product was finally annealed at 800  C for 6 h under an argon atmosphere.

2.2. Preparation of MoS2/graphene interlayer The interlayer was prepared using a vacuum-filtration method. Typically, the prepared MoS2/graphene composite was dispersed in N-methyl-2-pyrrolidone (NMP) at a concentration of 0.1 mg ml 1 and sonicated for 6 h. The resultant suspension was mixed

Fig. 4. (a) Open-circuit voltage and (b) EIS spectra of the fresh LSBs with/without the MoS2/graphene interlayer. (c) CV curves of the LSBs with/without the MoS2/graphene interlayer in the 2nd cycle. (d) Discharge-charge voltage profiles of the LSBs with/without the MoS2/graphene interlayer in the 2nd and the 50th cycle at a current density of 0.2 A g 1.

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Fig. 5. (a) Cycle capacity of the LSBs with/without the MoS2/graphene interlayer at a current density of 0.2 A g 1. (b) The corresponding discharge-charge voltage profiles of the LSB with the MoS2/graphene interlayer. (c) Rate performance of the LSBs with/without the MoS2/graphene interlayer. (d) Cycling performance of the LSBs with/without the MoS2/graphene interlayer and a contrastive LSB with a graphene interlayer at a current density of 0.5 A g 1.

with 10 wt% polyvinylidene fluoride (PVDF) binder and then vacuum filtered through a polypropylene membrane (Celgard 2400). The obtained MoS2/graphene composite-coated membrane was dried at 60  C overnight and punched into disks with a diameter of 19 mm. The mass density of the MoS2/graphene coating is around 0.5 mg cm 2. The pure graphene coated membrane was also fabricated using a similar method. 2.3. Fabrication of sulfur cathode To fabricate the sulfur cathode for LSBs, 60 wt% sublimed sulfur, 30 wt% Ketjen black (EC600JD) and 10 wt% PVDF in NMP were mixed to form a slurry. The slurry was then casted onto an

aluminium foil. After the slurry coated aluminum foil was vacuum dried at 50  C for 12 h, it was punched into circular electrodes. The sulfur mass loading was determined to be 0.8–1.2 mg cm 2. 2.4. Characterization method The structures and morphologies of the samples were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800), high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2 F30), micro-Raman spectrometer (Jobin–Yvon Horiba HR800 with an excitation wavelength of 532 nm), and X-ray powder diffraction (XRD, Rigaku RINT2400 with Cu Ka radiation). X-ray photoelectron spectroscopy (XPS)

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analysis was carried out using a Kratos Axis Ultra DLD instrument with Al Ka probe beam. 2.5. Electrochemical measurement Electrochemical tests were carried out using CR-2032 coin cells assembled in a high-purity argon-filled glovebox (H2O < 0.5 ppm, O2 < 0.5 ppm, MBraun, Unilab) with lithium foil as the reference/ counter electrode. The electrolyte used was 1 M lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixture of 1,2-dimethoxyethane (DMC) and 1,3-dioxolane (DOL) (in a volume ratio of 1:1) containing 2.0 wt% of LiNO3. The prepared MoS2/ graphene composite-coated membrane was used as the separator. Celgard 2400 only was also used for comparison. Galvanostatic charge-discharge tests were performed in a voltage window of 1.72.6 V using a multichannel battery tester (Neware BTS-610). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tests were carried out at room temperature by using an electrochemical workstation (CHI 660C). 3. Result and discussion A schematic illustration of the designed electrode configuration is shown in Fig. 1. Different from the conventional LSBs, an MoS2/ graphene interlayer is introduced between the cathode and the separator. The functional MoS2/graphene interlayer acts as a selective filter enabling the flow of lithium ions while inhibiting the shuttle of LiPSs. The interconnected graphene sheets can physically trap LiPSs and server as a conductive network. The polar MoS2 nanoflowers chemically suppress the dissolution of LiPSs. The morphology of the prepared MoS2/graphene composite was observed by SEM and TEM, as shown in Fig. 2. It can be seen that the MoS2 nanoflowers are uniformly wrapped into the

graphene nanosheets without aggregation. The size of the MoS2 flowers is around 400 nm. These fully encapsulated MoS2 nanoflowers in the conductive graphene nanosheets can serve as an effective barrier for LiPSs. The high-resolution TEM image in Fig. 2d displays clear fringes with a d-spacing of about 0.68 nm, which is in good agreement with the interplanar spacing of the (002) plane of MoS2 [34]. As shown in Fig. 3a, the used commercial polymer separator displays abundant pores of around 100 nm in size. Fig. 3b shows the top-surface SEM image of the MoS2/graphene compositecoated separator, in which MoS2 nanoflowers wrapped in highly crimped graphene nanosheets can be observed clearly. The MoS2 nanoflowers still anchor into the graphene nanosheets tightly even after an ultrasonic treatment for 6 h, suggesting a strong interaction between the graphene sheets and the MoS2 nanoflowers. The cross-section SEM images (Fig. 3c and Fig. S1) further display that the interlayer is composed of the MoS2/graphene composite with a thickness of around 60 mm. Fig. 3d shows a typical photograph of the commercial separator and the MoS2/ graphene composite-coated separator. To evaluate the efficiency of the MoS2/graphene interlayer for improving the electrochemical performance of LSBs, coin cells were assembled using bare sulfur electrode as the cathode and lithium foil as the anode. Celgard 2400 microporous polypropylene film with/without the MoS2/graphene interlayer was employed as the separator. Open-circuit voltages (OCVs) of the fresh LSBs with/ without the MoS2/graphene interlayer were first monitored to evaluate the self-discharge phenomenon. As shown in Fig. 4a, OCV decreases rapidly with time in case of the LSB with a pristine separator, while it presents a much better stability with time up to 300 h for the LSB with a separator coated by the MoS2/graphene interlayer. Fig. 4b compares the EIS spectra of the fresh LSBs with/ without the MoS2/graphene interlayer. According to the fitting

Fig. 6. (a) Cycling performance and (b, c) the representative discharge-charge voltage profiles of the LSBs with/without the MoS2/graphene interlayer at a current density of 1 A g 1.

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Fig. 7. SEM images of the cycled Li anodes in the LSBs with the (a) MoS2/graphene interlayer and (b) pristine separator. (c) SEM image of the MoS2/graphene interlayer after 200 cycles. (d) Raman spectra of the MoS2/graphene interlayer before and after 200 cycles.

result (Fig. S2), the charge transfer resistance of the battery with the MoS2/graphene interlayer (41 V) is much smaller than that of the battery with a bare separator (179 V), this is because the MoS2/ graphene interlayer has abundant reaction area and excellent conductivity. However, the lines in low frequency region show almost the same slope, which means that the addition of the MoS2/ graphene interlayer has little effect on the diffusion rate of lithium ions into the active material [35]. The improved conductivity contributes greatly to the enhancement of the rate performance and capacity retention of LSBs. CV curves of the LSBs with/without the MoS2/graphene interlayer in the 2nd cycle were tested at a scan rate of 0.05 mV s 1 and shown in Fig. 4c. Both LSBs present two cathodic peaks, which correspond to the conversion of sulfur (S8) to high-order LiPSs (Li2Sx, 4 < x  8) and the reduction of highorder LiPSs to lithium sulfides, respectively. For the LSB with a pristine separator, only one anodic peak can be observed at 2.4 V, which represents the oxidation of lithium sulfides to high-order LiPSs and S8. It can be found that there are two anodic peaks at 2.31 and 2.4 V for the LSB with the MoS2/graphene interlayer, indicating the oxidation of lithium sulfides to high-order LiPSs and the further oxidation of high-order LiPSs to S8, respectively [28,29]. Thus, by inserting the MoS2/graphene interlayer, the electrode exhibits sharp and symmetric redox peaks with a relatively low potential polarization. Moreover, the LSB with the MoS2/graphene interlayer shows much lower voltage hysteresis (DE) compared with the LSB without the MoS2/graphene interlayer (Fig. 4d), indicating a low charge transfer resistance and facilitated electrochemical reactions [28]. Cycling stability of the LSBs with/without the MoS2/graphene interlayer was investigated at a current density of 0.2 A g 1. As shown in Fig. 5a, the LSB with the MoS2/graphene interlayer delivers an initial discharge capacity of 1642 mAh g 1, and the reversible capacity remains at 720 mAh g 1 after 100 cycles. In contrast, the LSB with a pristine separator shows serious capacity fading, reaching a capacity of only 440 mAh g 1 after 100 cycles. The correspongding discharge-charge voltage profiles of the LSB

with the MoS2/graphene interlayer (Fig. 5b) display two typical discharge plateaus at 2.3 and 2.1 V. The higher discharge plateau comes from sulfur being reduced to high-order LiPSs, and the lower discharge plateau corresponds to the conversion of highorder LiPSs to lithium sulfides. The latter shows much slower reaction kinetics due to formation of the solid-state lithium sulfides. The observed two plateaus in the charge curves respectively represent the reactions from lithium sulfides to LiPSs and S8 [4,5]. The obtained galvanostatic discharge-charge voltage profiles are in good agreement with the cathodic and anodic peaks observed by CV test. The flat and stable plateaus with a low polarization of 160 mV suggest a kinetically efficient conversion process of the high-order LiPSs to lithium sulfides. It should be point out that the first coulombic efficiency of the LSB with the MoS2/graphene interlayer is only 73% due to the irreversible side reactions of the MoS2/graphene interlayer during the first discharge [22]. Rate capability of the LSBs with/without the MoS2/graphene interlayer was evaluated at current densities from 0.5 to 3.0 A g 1 as shown in Fig. 5c. For the LSB with the MoS2/graphene interlayer, its specific capacities are around 850, 770, 701, and 600 mAh g 1 at current densities of 0.5, 1.0, 2.0, and 3.0 A g 1, respectively, whereas the LSB with a pristine separator accordingly delivers specific capacities of only 680, 510, 217, and 158 mAh g 1. Moreover, the LSB with the MoS2/graphene interlayer exhibits outstanding cycle stability after the rate test, retaining a reversible capacity of 718 mAh g 1 in the 200th cycle. The corresponding voltage profiles at different current densities are shown in Fig. S3. Voltage profiles of the LSB with the MoS2/graphene interlayer present much lower polarization and higher capacity compared to the LSB with a pristine separator, especially at the higher rates of 2.0 and 3.0 A g 1. The excellent rate performance and high capacity retention can be attributed to the substantial decrease in the charge-transfer resistance and strong chemical interactions between the MoS2/ graphene interlayer and LiPSs. Fig. 5d displays the long-term cycling performances of the LSBs with/without the MoS2/graphene

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Fig. 8. (a) Schematic of the reactions for the LSB with the MoS2/graphene interlayer during the first 2 cycles. (b, c) Ex-situ XPS spectra of the MoS2/graphene interlayer at the specific states.

interlayer and with a graphene interlayer at a current density of 0.5 A g 1. The LSB with the MoS2/graphene interlayer still retains a discharge capacity of 689 mAh g 1 after 200 cycles, indicating an excellent electrochemical reversibility. For the LSB with a graphene interlayer, a reversible discharge capacity of 516 mAh g 1 is remained after 200 cycles. Whereas, the discharge capacity of the LSB with a pristine separator is severely faded to 442 mAh g 1 after 200 cycles. The enhanced electrochemical performance of the LSB with the MoS2/graphene interlayer suggests that the migration of LiPSs has been greatly suppressed by the introduced interlayer as a result of physical and chemical interactions between the dual functional MoS2/graphene composite and the dissolved LiPSs. Galvanostatic discharge-charge tests were carried out at a current density of 1 A g 1 after the CV test. As shown in Fig. 6, in the initial 5 cycles, the LSB with the MoS2/graphene interlayer delivers a high reversible capacity around 840 mAh g 1. Even after 200 cycles, the capacity is still as high as 620 mAh g 1. By contrast, the LSB with a pristine separator shows an initial capacity of 573 mAh g 1 and only maintains a capacity of 360 mAh g 1 after 200 cycles. The improved electrochemical performance of the LSB

with the MoS2/graphene interlayer could be related to the unique structure and outstanding chemical properties of the functional MoS2/graphene interlayer. The polar MoS2 nanoflowers serve as a chemical adsorption for the dissolved LiPSs, and the ultrathin and interconnected graphene nanosheets can physically block the shuttling of LiPSs. Furthermore, the MoS2/graphene composite forms a 3D network which can act as a fast transport path for electron and ion during the discharge-charge processes. The surface morphologies of the metallic Li anodes in the cycled LSBs were observed to further demonstrate the suppressed shuttle effect by the MoS2/graphene interlayer. As shown in Fig. 7a, the surface of the Li anode in the LSB with the MoS2/graphene interlayer is uniform and lower surface roughness can be observed, indicating that the introduction of the MoS2/graphene interlayer markedly inhibits the shuttling of LiPSs in the electrolyte. However, the surface of the Li anode in the LSB with a pristine separator is uneven and loosely packed after 200 cycles, suggesting serious corrosion of the Li anode during cycling, and thus the loss of the active material and then severely capacity fade [17,22,36]. An SEM image of the cycled MoS2/graphene interlayer (Fig. 7c) displays

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that the structure of the MoS2/graphene composite is still preserved after 200 cycles. Raman spectra of the MoS2/graphene interlayer before and after cycles show no obvious change for the characteristic peaks [37], indicating the robust chemical stability of the MoS2/graphene interlayer during cycles. XPS analysis was performed to probe the above-mentioned chemical interactions between the MoS2/graphene interlayer and LiPSs. Fig. 8a illustrates the transition from sulfur to lithium sulfide during cycles. The high-resolution XPS spectra of the MoS2/ graphene interlayer were recorded at the five voltage states (A to E) during the first two discharge-charge cycles. As shown in Fig. 8b, the spectra of S 2p core level in the interlayer before cycle (A state) show two peaks at 162.4 eV and 163.7 eV which correspond to SMo bonds [32,37]. The sulfur signal at 165.1 eV was also observed due to a close contact between the barrier layer and the sulfur cathode [32]. The presence of the S-Li bonds at voltage states of B and D demonstrates the formation of LiPSs and lithium sulfide as reported previously [19]. Elemental sulfur was detected at voltage states of C and E, which is consistent with the transition from lithium sulfide to sulfur during charge processes. The highresolution XPS spectra of Mo 3d in the MoS2/graphene interlayer show two peaks at binding energies of 232.9 and 229.7 eV, which are corresponding to the levels of Mo4+ 3d3/2 and Mo4+ 3d5/2, [31,32,34] respectively. A slight peak shift can be observed for Mo 3d spectra in the interlayer during the discharge-charge processes, which further confirms the interactions between the MoS2/ graphene interlayer and LiPSs [31]. The XPS analysis suggests that the chemical interactions between the MoS2/graphene interlayer and LiPSs are reversible. 4. Conclusions In summary, we have developed a dual functional MoS2/ graphene interlayer for LSBs. The unique structural and chemical properties of the MoS2/graphene interlayer are beneficial to improve the electrochemical performance of LSBs. Functional MoS2/graphene interlayer can physically trap LiPSs by graphene nanosheets and chemically suppress the dissolution of LiPSs by MoS2 nanoflowers. Furthermore, the MoS2/graphene interlayer form a 3D network, acting as an upper current collector which facilitates electron and ion transfer during cycles. As a result, the obtained LSB with the MoS2/graphene interlayer shows a superior rate capacity and improved cycling capacity. This design of compositing 2D materials as a selective interlayer may have diverse potential applications in the fields of catalysis, energy storage, and separating techniques. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 11674138 and 11504147) and the Fundamental Research Funds for the Central Universities (Grant No. lzujbky-2017-184). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.electacta.2017.10.003. References [1] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652– 657. [2] R. Mukherjee, R. Krishnan, T.-M. Lu, N. Koratkar, Nanostructured electrodes for high-power lithium ion batteries, Nano Energy 1 (2012) 518–533.

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