Mechanism Investigation of High-Performance Li-Polysulfide Batteries

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Mechanism Investigation of High-Performance Li-Polysulfide Batteries Enabled by Tungsten Disulfide Nanopetals Shaozhuan Huang, Ye Wang, Junping Hu, Yew Von Lim, dezhi kong, Yun Zheng, meng ding, Mei Er Pam, and Hui Ying Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04857 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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Mechanism Investigation of High-Performance LiPolysulfide

Batteries

Enabled

by

Tungsten

Disulfide Nanopetals Shaozhuan Huang,† Ye Wang,†, Junping Hu,† , Yew Von Lim,† Dezhi Kong,† Yun Zheng,‡ Meng Ding,† Mei Er Pam,† Hui Ying Yang*,†

†

Pillar of Engineering Product Development, Singapore University of Technology and

Design, 8 Somapah Road, 487372, Singapore, *E-mail: [email protected] ‡

Institute of Materials Research and Engineering, Agency for Science, Technology and

Research (A*STAR), 2 Fusionopolis Way, 138634, Singapore

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ABSTRACT: Understanding the reaction kinetics and mechanism of Li-polysulfide batteries is critical in designing advanced host materials for improved performance. However, up to now, the reaction mechanism within the Li-polysulfide batteries is still unclear. Herein, we study the reaction mechanism of a high-performance Li-polysulfide battery by in situ X-ray diffraction (XRD) and density functional theory (DFT) calculations based on a multifunctional host material composed of WS2 nanopetals embedded in rGO-CNT (WS2-rGOCNT) aerogel. The WS2 nanopetal serves as “catalytic center” to chemically bond the polysulfides and accelerate the polysulfide redox reactions, and the 3D porous rGO-CNT scaffold provides fast and efficient e-/Li+ transportation. Thus, the resulting WS2-rGO-CNT aerogel accommodating the polysulfide catholyte enables stable cycling performance, excellent rate capability (614 mAh g-1 at 2 C) and high areal capacity (6.6 mAh cm-2 at 0.5 C). In situ XRD results reveal that the Li2S starts to form at an early stage of discharge (at a depth of 25% of the lower voltage plateau) during the discharge process, and β-S8 nucleation begins before the upper voltage plateau during the recharge process, which are different from the conventional Li-S battery. Moreover, the WS2 itself could be lithiated/delithiated during the cycling, making the lithiated WS2 (LixWS2, 0≤x≤0.3) a real host material for Lipolysulfide batteries. DFT calculations suggest that LixWS2 (0≤x≤0.3) exhibits moderate binding/anchoring interactions towards polysulfides with adsorption energies of 0.51-1.4 eV. Our work reveals the reaction mechanism of the Li-polysulfide batteries and indicates that the lithiated host plays an important role in trapping the polysulfides.

KEYWORDS: Tungsten disulfide nanopetals, chemical bonding, in situ XRD, lithium sulfur batteries, reaction mechanism


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Lithium sulfur (Li-S) battery, which has low cost and high theoretical energy density of 2600 Wh kg-1, is one of the most promising lithium-ion technologies to meet the requirements of these rapidly expanding markets for electric vehicles.1-4 However, the practical application of Li-S battery is still impeded by the poor cycling performance owing to the following challenges:5, 6 (i) the low electrical conductivity of S and its discharge product (Li2S), leading to large polarization and low active material utilization, (ii) the dissolution of polysulfide intermediates, which causes the polysulfides “shuttle effect” and leads to irreversible capacity loss and corrosion of Li-metal anode, and (iii) the large volume variation during the cycling (~80%), resulting in the cathode electrode pulverization. Extensive efforts have been devoted to solving the above-mentioned problems. The most common strategy is to immobilize the sulfur in various host materials including porous carbon,7-10 functionalized graphene,11, 12 polymers,13, 14 metal oxides15, 16 and metal sulfides,17, 18

etc. Recently, liquid polysulfides have been adopted as the cathode active material for Li-S

battery, since the lithium polysulfide systems show uniform active materials distribution, improved sulfur utilization and enhanced redox reaction kinetics.5, 19-24 For example, Zhou et al.5 reported the Li-polysulfide battery by injecting the Li2S6 to the nitrogen/sulfur co-doped graphene sponge, which delivered high specific capacity and long cycle life. Chuang et al.19 presented a lithium-polysulfide cell with a coaxial-graphene-coated cotton-carbon as the polysulfide host material, which exhibited high areal capacity and areal energy density. However, up to now, the phases transformation hidden in the Li-polysulfide battery has not been explored. Additionally, the underlying roles of lithiated host material in Li-polysulfide batteries are rarely uncovered. Therefore, it is desirable to explore the reaction mechanism within the Li-polysulfide batteries. In this work, we investigate the reaction mechanism of the Li-polysulfide batteries based on a rational designed cathode host material composed of tungsten disulfide (WS2)


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nanopetals uniformly embedded in 3D reduced graphene oxide/carbon nanotube (WS2-rGOCNT) aerogel. The polar WS2 nanopetals introduced in our system could provide abundant chemical anchoring sites for polysulfide adsorption and accelerate the polysulfide conversion reaction.25 When injecting the Li2S6 catholyte, the obtained WS2-rGO-CNT composite enables a high reversible capacity of 1227 mAh g-1 at 0.1 C after 100 cycles and excellent rate performance of 614 mAh g-1 at 2 C. Then the reaction mechanism of the Li-polysulfide batteries was investigated by ultilizing the in situ X-ray diffraction (XRD) and density functional theory (DFT) calculations. In situ XRD results reveal that the Li-polysulfide batteries show enhanced Li2S and β-S8 nucleation kinetics during the discharge and charge processes, respectively, which are different from the typical Li-S batteries. After the initial cycle, the Li-polysulfide batteries exhibit similar reaction processes to the typical Li-S batteries. Moreover, it is found that lithiated WS2 (LixWS2, 0≤x≤0.3) plays the real role in anchoring polysulfides during the cycling. DFT calculations suggest that LixWS2 (0≤x≤0.3) can provide moderate adsorption energies of 0.51-1.4 V eV to the polysulfides.

RESULTS AND DISCUSSION The WS2-rGO-CNT host material was prepared by a solvothermal method using the rGO, CNT and (NH4)2WS4 as precursors followed by freeze-drying. Figure S1 schematically illustrates the preparation process of the host material. During the solvothermal reaction, WS42- ions were anchored on the functionalized CNTs, and grew into WS2 nanopetals.26 Meanwhile, the GO and WS2-CNT formed into a 3D porous interconnected network due to the π–π stacking between the graphene oxide nanosheets.27 After freeze drying, the cylindric WS2-rGO-CNT aerogel was obtained (Figure S2). The obtained aerogel was laminated into plates for direct use as a host material for Li-polysulfides battery without any metallic current collector, binder or conductive additives. Owing to the strong chemical anchoring effect of


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WS2 nanopetals and physical adsorption of rGO-CNT framework for polysulfides, the WS2rGO-CNT absorbing Li2S6 polysulfide catholyte can serve as a stable cathode for Lipolysulfides battery.

Figure 1. Morphology and structural characterizations of WS2-rGO-CNT: (a) XRD patterns, (b) Raman spectra, inset in (b) is the enlarged view of the dashed area, (c) nitrogen adsorption-desorption isotherms. (d-f) SEM images of WS2-rGO-CNT aerogel, insets in (d) is the corresponding schematic diagram of 3D WS2-rGO-CNT. TEM images (g-h) and HRTEM image (i) of WS2-rGO-CNT, inset in (g) is the corresponding SAED pattern. The crystal structure of the 3D porous WS2-rGO-CNT host material was examined by X-ray diffraction (XRD), as shown in Figure 1a. The XRD pattern of the WS2-rGO-CNT shows four main peaks. The sharp peak at ~25.6° belongs to the carbonaceous CNTs and rGO.27 The other three peaks at 13.5°, 33.7°and 59.7° can be assigned to the (110), (006) and 5 ACS Paragon Plus Environment

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(110) planes of the hexagonal 2H-WS2 phase.28 The broad diffraction peaks indicates the nanoscale dimension of WS2 nanopetals. Further insight into the structure of WS2-rGO-CNT host was obtained from the Raman spectrum (Figure 1b). Obviously, the strong peaks at 1351 and 1589 cm-1 correspond to the disorder carbon atoms in the hexagonal graphite network (D band) and the in-plane vibrational mode of sp2-bonded carbon atoms (G band), respectively.29, 30

Beyond these peaks, there are two small peaks centered at 350 and 417 cm-1, which are

assignable to the longitudinal acoustic (2LA) E2g mode and the A1g mode in hexagonal WS2, respectively (inset in Figure 1b).31 The nitrogen adsorption-desorption isotherms of WS2rGO-CNT reveals that the WS2-rGO-CNT aerogel exhibits a typical IV type isotherm with remarkable H3 hysteresis (Figure 1c),32, 33 manifesting a dominant mesoporous framework. The WS2-rGO-CNT shows a high BET surface area of 315 m2 g-1, which is close to that of bare rGO-CNT (Figure S3a, 310 m2 g-1). The high surface area of the WS2-rGO-CNT aerogel enables sufficient contact between WS2 nanopetals and polysulfides, enhancing the polysulfide entrapment by WS2. The thermogravimetric analysis (TGA) reveals a weight loss around 56.1% from 100 to 700 °C, corresponding to the combustion of rGO and CNT in the oxygen, and the oxidation of WS2 to WO3 (Figure S3b).27, 34 Through calculation, the weight loss from the oxidation of WS2 to WO3 is around 6.4%. Therefore, the WS2 content in the WS2-rGO-CNT composite is around 46.9%. The morphology of the WS2-rGO-CNT host was performed by the scanning electron microscopy (SEM), as shown in Figure 1d-f. The WS2-rGO-CNT aerogel shows a 3D interconnected framework with macroporous quasi-periodic channels throughout the whole aerogel (Figure 1d). The highly porous rGO-CNT scaffold exhibits ordered channel with approximately 10 µm in diameter (Figure 1e). The ordered macroporous channels may result from the ice columns template that restructures and fortifies the framework during the freeze drying process.35 The 3D interconnected carbonaceous structure not only enables fast Li+


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diffusion and electron transportation, but also accommodates large volume change during the discharge/charge processes. The SEM image (Figure 1e-f) also reveals that plenty of WS2 nanopetals are homogeneously embedded in the rGO-CNT network with an average size of 400 nm. Interestingly, the nanopetals are tangled with CNTs instead of rGO. A possible explanation is that the WS42- ions are selectively adsorbed on the positively charged CNTs instead of negatively charged GO.36 EDX spectrum and elemental mapping images (Figure S4) indicates that the WS2-rGO-CNT composite mainly consists of C, O, W and S elements. The microstructure of the WS2-rGO-CNT composite was characterized by transmission electron microscopy (TEM), as depicted in Figure 1g-i. The low magnification TEM image (Figure 1g) shows that the WS2 nanopetals with an average size of 400 nm are uniformly distributed in the rGO-CNT network. The inset selected area electron diffraction (SAED) pattern exhibits four diffraction rings: the inner bright one corresponds to the (002) plane of rGO&CNTs and the other three weak ones correspond to the (100), (006) and (110) planes of hexagonal WS2, respectively. A higher magnification TEM image (Figure 1h) reveals that the carbon nanotubes are entangled with WS2 nanopetals and very thin rGO nanosheets cover on the surface of WS2 nanopetal. This hybrid structure ensures very fast and efficient electron transfer from WS2 to the current collector. High-resolution TEM (HRTEM) image shows that the WS2 nanosheet is composed of 8-16 layers of hexagonal WS2 in the (002) direction (thickness: 5-10 nm) with an inter-planar spacing of around 0.63 nm (Figure 1i).37


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Figure 2. (a) Ultraviolet-visible (UV) absorption spectra of Li2S6 solution before and after the addition of WS2-rGO-CNT and rGO-CNT, inset is the digital picture of the adsorption of Li2S6 by WS2-rGO-CNT and rGO-CNT, (b) XPS spectra of W4f in WS2 before and after mixing with Li2S6, (c) XPS spectrum of S2p in WS2-rGO-CNT/Li2S6 composites, (d) CV plots of the symmetric cells with identical electrodes at a scan rate of 2 mV s-1. To verify the chemical adsorption ability of WS2 for polysulfides, we performed the polysulfide adsorption test of the bare rGO-CNT and WS2-rGO-CNT composites by adding 10 mg of these two samples (with close surface area) separately into 3 mL of 3 mmol L-1 Li2S6 solution (Figure 2a). After aging for 2 h, the WS2-rGO-CNT composite shows more obvious color fading than the rGO-CNT, indicating the WS2 nanopetals can offer strong chemical anchoring for polysulfide. The corresponding ultraviolet-visible (UV) spectra show that the adsorption peak after adding WS2-rGO-CNT is weaker than that after adding rGOCNT, implying the improved polysulfide adsorption enabled by WS2.38 The better chemical 8 ACS Paragon Plus Environment

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adsorption ability of WS2-rGO-CNT is due to the formation of ionic bonding of W-S, as evidenced by X-ray photoelectron spectroscopy (XPS) in Figure 2b. In the W 4f spectrum, the pristine WS2 exhibits two peaks at 32.4 and 34.5 eV, corresponding to the W 4f7/2 and W 4f5/2 spin orbit doublet peaks of W (IV).39 Upon contact with Li2S6, the doublet peaks of W (IV) shifts towards lower binding energy significantly (Figure 2c), indicating that the chemical interaction between polysulfide and WS2 leads to electron transfer from Li2S6 molecules to W (IV) atoms.40 To further investigate the Li2S6-WS2 intercalation, the S 2p spectrum of the WS2-rGO-CNT/Li2S6 was collected, which shows several sulfur environments (Figure 2c). The S 2p3/2 peak at 162 eV comes from the WS2, which is confirmed by the S 2p spectrum of the pristine WS2 (Figure S5). The S 2p3/2 contributions at 162.8 and 164.2 eV are ascribed to the terminal (ST-1) and bridging sulfur (SB0) atoms of Li2S6, respectively.41 Besides, another two S 2p3/2 peaks at 167.5 eV and 169 eV appear. The former should arise from a surface redox reaction between Li2S6 and WS2, which may be ascribed to the binding energy of thiosulfate, and the latter is assigned to a polythionate complex, which comes from the further reaction between long-chain polysulfide and thiosulfate.41 The oxidation of Li2S6 to thiosulfate and polythionate complex is accompanied by a valence state decrease of W. The polythionate complex in the electrode is proven to be reversible and can serve as an anchor and transfer mediator to inhibit active materials (polysulfides) dissolution into the electrolyte.41 In order to further elucidate the roles of WS2 nanopetals in Li-polysulfide batteries, we performed the cyclic voltammetry (CV) in symmetric cells with identical working and counter electrodes in Li2S6 electrolyte, as shown in Figure 2d. The WS2-rGO-CNT electrode exhibits two obvious cathodic peaks around -0.09 and -0.45 eV, and two distinct anodic peaks at 0.09 and 0.45 V. The two cathodic peaks correspond to the reduction of S to Li2S6 and Li2S6 to Li2S, respectively. On the contrary, the two anodic peaks represent the oxidation


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of Li2S to Li2S6 and Li2S6 to S, respectively (see Figure S6 for more details).18 In contrast, without WS2 nanopetals, the rGO-CNT aerogel exhibits only one pair of peaks: a reduction peak at -0.75 V and a oxidation peak at 0.75 V. The broadened peaks and inferior peak separation reveal the reduced electrochemical reversibility and slower reaction kinetics of rGO-CNT, suggesting the WS2-rGO-CNT has better catalytic effect towards polysulfide conversion.

Figure 3. Cycling performances of the WS2-rGO-CNT and rGO-CNT cathodes with Li2S6 catholyte: (a) cycling performance and Coulombic efficiency at 0.1 C, (b) rate performance at various rates, (c) specific capacity of WS2-rGO-CNT with different sulfur loadings, (d) areal capacity of WS2-rGO-CNT with different sulfur loadings. To prove the advantages by incorporating WS2 nanopetals into the rGO-CNT framework, Li-polysulfide batteries were assembled by using the WS2-rGO-CNT and bare rGO-CNT as cathodes, Li2S6 solution as catholyte and metallic lithium foil as 10 ACS Paragon Plus Environment

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counter/reference electrode. The corresponding electrochemical performances are shown in Figure 3. The sulfur loading of the electrode is around 3.84 mg cm-2. Figure 3a compares the cycling performance of WS2-rGO-CNT and rGO-CNT at a rate of 0.1 C. The bare rGO-CNT electrode delivers a reversible capacity of 830 mAh g-1 after 100 cycles and shows a capacity retention of 80.6%. After embedding the WS2 nanopetals, the WS2-rGO-CNT electrode exhibits a higher capacity of 1227 mAh g-1 after 100 cycles, corresponding to an improved capacity retention of 90.7%. In addition, over the entire cycling process, the WS2-rGO-CNT electrode shows higher Coulombic efficiency than the rGO-CNT (98-99% versus 96-97%), indicating that the incorporation of WS2 nanopetals effectively mitigates the polysulfide shuttling to the anode . The rate capabilities of WS2-rGO-CNT and rGO-CNT were then evaluated at various rates from 0.1 C to 2 C (Figure 3b and Figure S7). The WS2-rGO-CNT electrode shows average capacities of 1270, 1140, 1030, 845 and 614 mAh g-1 at 0.1, 0.2, 0.5, 1 and 2 C, respectively. When the rate returns to 0.1 C, the capacity recovers to a high value of 1220 mAh g-1. In contrast, the rGO-CNT electrode exhibits lower capacities and poor capacity retention under the same conditions, indicating the worse reaction kinetics of the rGO-CNT/polysulfide system. EIS spectra of the two electrodes further confirm that the WS2-rGO-CNT electrode has smaller charge transfer resistance than the rGO-CNT (Figure S8). To further improve the energy density of the Li-S battery system and inspect the reaction kinetics, the areal sulfur loadings in the WS2-rGO-CNT electrode increase from 3.84 to 5.12 and 7.68 mg cm-2, respectively, by increasing the amount of catholyte. Figure 3c shows the cycling performances of the WS2-rGO-CNT electrode with areal sulfur loading of 3.84, 5.12 and 7.68 mg cm-2 at a rate of 0.5 C. It can be noted that with a high sulfur loading up to 7.68 mg cm-2, the WS2-rGO-CNT still approaches a high reversible capacity of 860 mAh g-1, which is close to the electrode with a sulfur loading of 3.84 mg cm-2. The large difference in sulfur loading only induces a minor capacity decrease, indicating the super-efficient mass


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transportation of the 3D interconnected framework. Accordingly, the WS2-rGO-CNT electrode delivers a high areal capacity up to 6.6 mAh cm-2 (Figure 3d), which is higher than most of the aerogel-based electrodes (Table S1). The strong redox reaction kinetics and excellent cycling performance indicate that the WS2-rGO-CNT aerogel is an attractive cathode host material for Li-polysulfide batteries. In order to prove the advantageous architecture of the WS2-rGO-CNT composite, the ex situ SEM images of the electrode after 20 cycles were obtained. As shown in Figure S9, the WS2-rGO-CNT electrode maintains its 3D porous structure after cycling, and WS2 still keeps its original nanopetal morphology with an average size of 400 nm. More importantly, the WS2 nanopetals do not agglomerate or detach from the rGO-CNT network, confirming the robust anchoring of WS2 nanopetals on the carbonaceous scaffold. The homogeneous distribution and strong anchoring of WS2 nanopetals in rGO-CNT network demonstrate the robust structure of WS2-rGO-CNT, assuring stable battery performance.


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Figure 4. (a) The first and second discharge-charge profiles of Li-polysulfide battery with WS2-rGO-CNT host at 0.1 C, (b) the first and second discharge-charge curves and the corresponding in situ XRD patterns in a contour plot, (c) Li2S formation during the first discharge process at 0.1 C based on the integrated area of (111) reflection, (d) Li2S consumption and β-S8 formation during the first recharge process at 0.1 C based on the integrated areas of (111) and (311) reflections, respectively. In order to reveal the reaction mechanism of the Li-polysulfide system, in situ XRD was performed during the discharge/charge processes (Figure 4). A prototype Li-polysulfide cell for in situ XRD measurement is shown in Figure S10, where the WS2-rGO-CNT aerogel was used as a freestanding cathode host material and Li2S6 catholyte was injected into the aerogel. The in situ cell was measured at a rate of 0.1 C, during which a series of XRD patterns at different discharge/charge potentials were collected. To better illustrate the peak intensities evolution during the cycling process, a contour plot with XRD patterns is displayed in Figure 4b. It should be noted that during the cycling, two notable diffraction peaks around 25.6° and 33.7° (labeled with vertical dashed lines) can be observed all along, corresponding to the carbonaceous rGO-CNT and WS2 (100), respectively. In a typical Li-S battery, the discharge profile consists of two potential plateaus, one at around 2.35 V and the other at around 2.1 V (Figure S11), corresponding to the reduction of α-S8 to polysulfide intermediates and subsequent reduction of these polysulfides to insoluble Li2S2/Li2S.2 However, in our Li-polysulfide system, the starting active material is liquid Li2S6 instead of solid S8. Thus, in the first discharge profile (Figure 4a), the upper plateau at around 2.35 V does not exist owing to the absence of transformation of S8 to Li2S6. This is also reflected by the CV curves (Figure S12), where the typical first cathodic peak in the first cycle cannot be observed. Accordingly, no Bragg peaks related to S8 can be observed at the first diffraction scan (at open circut voltage, ~2.3 V). The slope from open circuit voltage to


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2.1 V represents the liquid-liquid single-phase reduction of Li2S6 to Li2S4, consistent with the typical Li-S battery. In this region, no Bragg peaks appear (the so-called “soluble species” region). Further lithiation upon the long plateau at ~2.1 V, the Li2S4 starts to convert to Li2S2/Li2S. In this region, it is observed that the cubic Li2S (JCPDS No. 23-0369) nucleates quickly (~27.1° with triangle symbol) (Figure 4b and c), at around 25% depth of this plateau (Figure 4c), which is earlier than the previous reports.42,

43

The nucleation also happens

earlier than that in the rGO-CNT electrode (Figure S13b, 25% depth vs. 62.5% depth), indicating that the Li-polysulfide system with WS2 host enables enhanced kinetics for Lipolysulfide redox reaction. The peak intensity of the Li2S becomes stronger as the lithiation proceeds and reaches the strongest at the end of discharge (Figure 4c). During the recharge process, the peak of Li2S becomes weak gradually and completely disappears at the capacity of 1041 mAh g-1, corresponding to the transformation of Li2S to middle-chain Li2S4. After the Li2S decomposes completely, there is a “soluble species” region between 1041 and 1150 mAh g-1, corresponding to the conversion of Li2S4 to longchain polysulfides. After that, a new set of peaks around 23.3, 23.6, 24.1, 24.5, 26.0, 26.8 and 34.2° appears (with rhombus symbol) and becomes stronger, which belong to the β-S8 (JCPDS 071-0137).43, 44 Interestingly, the β-S8 starts to form (at 1150 mAh g-1) before the, which is different from the typical Li-S battery.2-4 This phenomenon means that the S8 deposition appears before the end of lower plateau, where several sulfur species including Li2S4, Li2S6, Li2S8 and β-S8 coexist. In contrast, the β-S8 nucleation in rGO-CNT electrode starts right at the beginning of the upper plateau around 2.4 V (Figure S13c). This should be ascribed to the fact that WS2 catalyst can facilitate the polysulfide conversion reaction and accelerate the β-S8 formation, as also confirmed by the symmetric battery in Figure 2d. As a whole, the initial charge capacity of WS2-rGO-CNT reaches 1356 mAh g-1, higher than the initial discharge capacity (~1280.6 mAh g-1).


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The in situ XRD patterns of the second cycle (Figure 4b) confirm the reappearance of Li2S and β-S8 during the second discharge and charge, respectively, indicating the Li-polysulfide batteries operate similarly to the typical Li-S battery after the initial cycle.

Figure 5. (a) Discharge-charge profiles of WS2-rGO-CNT at 167.3 mA g-1 without adding the Li2S6 catholyte (deduct the contribution from rGO-CNT), inset is the schematic illustration of Li intercalation/deintercalation in the WS2. (b) Adsorption binding energies for polysulfides on LixWS2 (0≤x≤0.3) and carbon. Recently, Liu et al.45 demonstrated that TiS2 host itself could be lithiated/delithiated during the cycling process in Li-S battery, which suggested that lithiated TiS2 was the real host material to trap the polysulfide. To further evaluate this phenomenon in our Li-polysulfide system, we study the lithiation/delithiation process of WS2, as shown in Figure 5a and Figure 15 ACS Paragon Plus Environment

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S14. The results reveal that WS2 delivers a low specific capacity of around 30 mAh g-1 within the voltage window of 1.7-2.8 V at 167.3 mA g-1. There are mainly two plateaus in the profiles: the discharge plateau around 1.95 V and charge plateau around 2.42 V. According to previous reports,37, 46-48 the discharge plateau is ascribed to the Li+ insertion to WS2 to form LixWS2 and the charge plateau corresponds to the Li+ extraction from LixWS2. Through calculation, the theoretical Li+ intercalation amount is around 0.3. The low Li+ intercalation content does not change the WS2 crystal structure significantly, as confirmed by the in situ XRD in Figure 4b. Compared with the Li-polysulfide battery profiles (Figure 4a), the discharge plateau of WS2 is slightly lower (1.95 vs. 2.1 V), indicating the Li+ will first react with sulfur species and then with WS2. This means the non-lithiated WS2 provides the chemical adsorption/anchoring to the polysulfides during the discharge process. While in the recharge process, the charge plateau of WS2 is higher than the long plateau of Li-polysulfide battery (2.42 vs. 2.3 V), indicating the Li+ will first extract from polysulfide and then from LixWS2. This means the LixWS2 traps polysulfides in this region. To further understand the chemical interactions between LixWS2 and polysulfides, DFT calculations were performed, as shown in Figure 5b. From the calculation results, it can be observed that the polysulfide can adsorb onto the LixWS2 surface with an adsorption energy of 0.51-1.4 eV, which is significantly larger than that of a graphitic carbon (0.1-0.51 eV). The anchoring effect becomes weaker when the Li+ intercalation concentration increases. However, owing to the low Li+ intercalation concentration (maximum 0.3 per unit), there is only a small difference of adsorption energy between WS2 and Li0.3WS2 (0 - 0.2 V). Therefore, the LixWS2 (0≤x≤0.3) can provide moderate chemical anchoring to the polysulfides over the entire cycling.

CONCLUSIONS


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In summary, we investigate the reaction mechanism of the Li-polysulfide battery based on a special designed 3D porous WS2-rGO-CNT host material. The interconnected rGO-CNT framework not only provides continuous electron transportation pathways and shortened Li+ diffusion channels, but also enables fast electrolyte uptake and accommodation of volume change. Moreover, the WS2 nanopetals show strong anchoring and catalytic effect towards the polysulfides, which greatly mitigate the polysulfide shuttling and enhance the Lipolysulfide redox reaction kinetics. Benefitting from these advantages, the WS2-rGO-CNT cathode exhibits stable cycling performance (90.7% capacity retention after 100 cycles), high Coulombic efficiency (>98% at 0.1 C), excellent rate capability (614 mAh g-1 at 2 C) and high areal capacity (6.6 mg cm-2 at 0.5 C). In situ XRD results reveal that the Li2S/S8 nucleate earlier than the traditional Li-S battery during the discharge/charge processes owing to the presence of WS2. After the initial cycle, the Li-polysulfide battery shows similar reaction mechanism to the typical Li-S battery. Additionally, during the cycling, it is the LixWS2 (0≤x≤0.3) that plays the real role in trapping and catalyzing the polysulfides. These encouraging results provide insightful view of the Li-polysulfide battery and offer important guidance to design better host material for high-performance Li-S batteries.

METHODS Synthesis of WS2-rGO-CNT aerogel. The WS2-rGO-CNT aerogel was prepared by a solvothermal method, according to our previous work with minor modifications.27 Typically, 15 mL of graphene oxide (GO) aqueous suspension with concentration of 4 mg mL-1 was dispersed in dimethyl formamide (DMF) and then centrifuged at 10000 rpm for 20 min. The product was washed by DMF and re-collected by centrifugation at 10000 rpm for 20 min. The obtained GO slurry was dispersed in 20 mL DMF with sonication for 30 min followed by adding 8 mg of CNT (positive charge). Then above suspension was added to 100 mg of


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(NH4)2WS4 powder with continuous stirring for 30 min. Next, the mixture was transferred to a 50 mL Teflon-lined stainless-steel autoclave and kept at 205 °C for 12 h. Then the asprepared cylindrical hydrogel was soaked in deionized water for two days followed by freeze-drying for 48 h. After that, the WS2-rGO-CNT aerogel was obtained after annealed at 500 °C for 2 h under N2/H2 (95%/5%). For comparison, the rGO-CNT aerogel was prepared following the same procedures without adding (NH4)2WS4. Adsorption test and symmetrical cell assembly. To prepare the Li2S6 solution (3 mmol L-1), 1:5 molar ratio of Li2S and sulfur was added into the DOL/DME solution (1:1, v/v) and stirred at 60 °C for 12 h. Then 10 mg of rGO-CNT or WS2-rGO-CNT aerogel was added to the above Li2S6 solution (3 mL, 3 mmol L-1). The suspension was stirred and aging for 2 h for adsorption measurements. For the symmetrical battery, the WS2-rGO-CNT (or rGO-CNT) aerogel was cut into square plate (5 × 5 mm2) with an average mass of 0.8 mg. Then the square plates were employed as identical counter and working electrodes, and 30 µL organic liquid (15 µL anolyte and 15 µL catholyte) containing 1/3 M Li2S6 and 1 M bis(trifluoroethanesulfony)imide lithium (LiTFSI, in DME/DOL, v/v = 1:1) was used as electrolyte. Cyclic voltammetry (CV) tests were performed on an VMP3 electrochemical workstation (France) at 2 mV s-1. Materials characterizations. XRD patterns of the as-prepared materials were collected by a D8 advance X-ray diffractometer (Bruker, Germany) with Cu Ka (λ = 0.154 nm) radiation. In situ XRD measurements were performed in a stainless-steel Swagelok-type cell (Figure S10) which was connected to a multichannel battery testing system (Neware, China). Raman spectra were recorded by a confocal Raman system with a 532 nm excitation laser (WITec Instruments Corp., Germany). SEM (JSM-7600F, JEOL) and TEM (JEM-2100F, JEOL) were used to reveal the morphology and structure of the aerogels. XPS (PHI 5600) spectra were recorded by using Al Kα X-ray (350 W) with a pass energy of 29 eV. TGA (DTG-60, 18 ACS Paragon Plus Environment

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Shimadzu) was used to determine the content of WS2 in composite at a temperature ramping rate of 5 °C min-1 under oxygen atmosphere. Electrochemical measurements. The aerogels were cut into square plate (5 × 5 mm2) with an average mass of 0.8 mg. Then 15 µL catholyte (in DOL/DME, v/v = 1:1) containing 1/3 M Li2S6 and 1 M LiTFSI was added into the square aerogel to form the cathode. The host weight/sulfur weight ratio is calculated to be 1:1.2. The sulfur loading of in both WS2-rGOCNT and rGO-CNT electrode is about 3.84 mg cm-2. For comparison, thicker WS2-rGO-CNT electrodes with higher sulfur loading of 5.12, 7.68 and 11.62 mg cm-2 were also prepared. Celgard 2400 was employed as the separator and lithium foil was used as the anode. 30 µL anolyte with 1 M LiTFSI in DME/DOL (v/v = 1:1) containing 0.3 M LiNO3 was added in the anode side. CV curves of the cells were collected using an VMP3 electrochemical workstation at 0.1 mV s-1. Galvanostatic charge/discharge was measured in a multichannel battery testing system (Neware, China) within potential range of 1.7-2.8 V. The specific capacities are calculated based on the mass of sulfur in Li2S6. Density functional theory calculations. All the calculations are based on density functional theory (DFT) using the plane-wave pseudopotentials with exchange-correlation of Perdew-Burke- Ernzerhof (PBE) formation as implemented in the Vienna Ab initio Simulation Package (VASP).49, 50 A cutoff energy of 450 eV is employed for the plane wave expansion of the wave functions. The Brillouin zone is sampled with 3 × 3 × 1 Monkhorst-Pack k-point mesh for the structural optimization. The convergence criteria for the total energy and ionic forces were set to 10-4 eV and 0.05 eV/Å, respectively. The construction with a 20 Å vacuum zone in the z direction to minimize the interactions between adjacent images. Meanwhile, considering the importance of van der Walls force in S-anchoring materials, we use a DFT-D2 method in all the calculations.


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The adsorption energy (Ead) of LixS8 on the WS2 monolayer with and without Li atoms is defined as Ead = ELiaW20 S40 Lim Sn - ELiaW20 S40 - ELim Sn , where ELiaW20 S40 Lim Sn and ELiaW20 S40 are the total energyies of W20S40 monolayer with (a=6) and without (a=0) 6 Li atoms adsorption. The adsorption sites of these 6 Li atoms are carefully tested, and the lowest energy adsorption model is chosen for the adsorption energy calculations. In addition, the W20S40 monolayer here is the 4 × 5 supercell of WS2 unit cell. ELim Sn is the total energy of single LimSn molecule, when m=0, n=8, and m=2, n=8, 6, 4, 2, 1, representing S8, Li2S8 Li2S6, Li2S4 Li2S2, and Li2S molecules, respectively.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: XX/acsnano.5b07977. The schematic illustration of the fabrication process, TGA curve, SEM-EDX elemental mapping, XPS spectrum, discharge-charge profiles, EIS spectra, Ex situ SEM images, and CV curves of WS2-rGO-CNT (Figures S1-S13).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS 20 ACS Paragon Plus Environment

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This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its NRF-ANR Joint Grant Call (NRF-ANR Award No. NRF2015-NRFANR000-CEENEMA).


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Table of Content A high-performance Li-polysulfide battery with stable cycling performance, excellent rate capability and high areal capacity has been demonstrated by using a WS2-rGO-CNT aerogel as a host material. In situ XRD reveals that Li-polysulfide battery shows different Li2S/S8 nucleation behaviors with the typical Li-S battery when loaded with WS2 host material. Moreover, the WS2 itself could be lithiated/delithiated during the cycling, making the lithiated WS2 (LixWS2, 0≤x≤0.3) a real host material for Li-polysulfide batteries.

ToC figure:


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Mechanism Investigation of High-Performance Li-Polysulfide Batteries

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