Tungsten Carbide as a Highly Efficient Catalyst for Polysulfide

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C: Energy Conversion and Storage; Energy and Charge Transport

Tungsten Carbide as a Highly Efficient Catalyst for Polysulfide Fragmentations in Li-S Batteries Jihwan Choi, Tae-Gyung Jeong, Byung Won Cho, Yousung Jung, Si Hyoung Oh, and Yong-Tae Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02096 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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The Journal of Physical Chemistry

Tungsten Carbide as a Highly Efficient Catalyst for Polysulfide Fragmentations in Li-S Batteries Jihwan Choia,b, Tae-Gyung Jeonga,b, Byung Won Cho,b Yousung Jung,c Si Hyoung Ohb* and Yong-Tae Kima*

a

Department of Energy Systems, Pusan National University, Busan 46241, Republic of

Korea. b

Centre for Energy Convergence Research, Korea Institute of Science and Technology, Seoul

02792, Republic of Korea c

Graduate School of EEWS, Korea Advanced institute of Science and Technology, Daejeon

34141, Republic of Korea

Corresponding Author *E-mail: [email protected] (Y.-T. Kim), [email protected] (S. H. Oh)

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ABSTRACT The sluggish disproportionation of short-chain lithium polysulfides, Li2Sx, is known as one of the major causes to limit the rate capability of lithium sulfur batteries. Herein, we report that a tungsten carbide not only affords strong sulfiphilic surface moieties, but also provides an efficient catalysis to enhance the polysulfide fragmentation, leading to a drastic improvement in the electrode kinetics. We show that a tungsten carbide acts as a superb anchoring material for the long-chain polysulfide, and also promotes the dissociation of short-chain polysulfide during the electro-reduction process. This leads to a high rate performance of the composite cathode loaded with a tungsten carbide, delivering a markedly enhanced discharge capacity of 780 mA h g-1 at a high current rate of 5 C, when it is applied with a combination with a carbon-coated separator for the polysulfide confinement. Hence, this work presents a new strategic approach for a high-power lithium sulfur battery.


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In the emerging markets of electric vehicles and commercial energy storage systems, there is a growing demand for new medium- and large-scale rechargeable batteries possessing a high energy density and cost-competitiveness.1 To cope with this, world-wide intensive researches have continued to search for the feasible next-generation lithium-ion batteries, which is superior in energy and power density as well as cycling stability.2-4 Lithium sulfur batteries are attractive candidates to meet these requirements because of their high theoretical energy density (~2600 Wh kg-1) compared to conventional lithium-ion batteries (~400 Wh kg-1),5 not to mention that sulfur is inexpensive and environmentally-benign.6-7 However, there are still many technical challenges to overcome in order to be commercialized such as intrinsically low electrical conductivity of sulfur, the redox shuttle caused by soluble reaction intermediates, and large volume changes in the electrodes during the discharge-charge processes. These cause a poor rate capability and unsatisfactory long-term cycle-life in lithium sulfur batteries.8-14 A lot of intensive attempts have been made so far to devise an effectual method which can control a redox shuttle by soluble polysulfides. For instance, various materials including porous carbon,15-18 carbon nanotubes19-23 and carbon interlayers24-29 or coating methods30-34 were introduced to the cathode structure as a physical barrier to entrap the polysulfides. In general, these methods were effective to some extent in improving the characteristics associated with capacity and cycle-life. Recently, a new strategy has been proposed, which utilizes transition metal compounds like metals, oxides, sulfides, and nitrides to provide favorable chemical moieties to adsorb or reactivate polysulfides.31, 35-47 As one of those approaches, we recently reported the improved discharge capacity and rate capability with TiN as a heterogeneous catalyst for lithium sulfur batteries.42 It is well-known that the slow disproportionation reaction at the second plateau causes a limitation of the 3 ACS Paragon Plus Environment


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lithium sulfur battery.48 As a promising solution for this problem, we interestingly found that the TiN efficiently promoted the chemical disproportionation reaction of polysulfide intermeditates extending the length of the second discharge plateau. In this study, we report a further enhanced catalyst exceeding the previously reported TiN to enhance the rate capability and the cycle stability of Li-S batteries. It was interestingly revealed that tungsten carbide (WC) catalyzes the chemical disproportionation of polysulfide, while providing a strong sulfiphilic surface moieties to capture the soluble polysulfides by simulating ‘tungsten disulfide (WS2)’-like surface moieties. It is widely recognized that transition metal dichalcogenides have strong adsorption properties toward chalcogenide species because of dangling bonds at edge sites.39-40, 44, 49 Particularly, WS2 is a well-known catalyst for hydrodesulfurization form these properties, as reported that it can be a good additive for lithium sulfur batteries due to this anchoring effect.40, 50 Unfortunately, since WS2 possesses an intrinsic semiconducting characteristics, a high dose can lead to a critical increase of ohmic resistance in the cathode. WC examined in this work has however a metallic conductivity, while being covered with fragmented sulfurs providing similar surface properties to WS2. For this reason, WC demonstrated unique properties for the facilitation of chemical disproportionation and for the adsorption of dissolved polysulfide with a negligible ohmic loss. The material and methodology developed in this work will offer a new strategy to overcome the technical challenges of Li-S batteries.


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Figure 1. (a) EDS elemental maps of WC loaded cathode, (b) X-ray diffraction pattern of Sulfur, WC, S/WC cathode.

The morphology and the homogeneity of the composite electrode consisting of sulfur, carbon black and tungsten carbide additives was investigated through scanning electron microscopic (SEM) images with the elemental mapping by energy dispersive X-ray spectroscopy (EDS). The EDS mapping and XRD patterns in Figure 1 show that all components including the tungsten carbide are uniformly dispersed on the electrode. The tungsten carbide used in this work was round-shaped with a diameter about 0.5 to 1.0 µm (Figure S1) and the BET surface area was measured around 2.24 m2 g-1 (Table S1).



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Figure 2. (a) Charge-discharge profiles at 0.2 C. (b) Cyclability of the bare, WC, WO3 electrode. (c) Rate capability profiles from 0.2 C to 5 C. (d) Discharge capacity ratio of WC electrode to bare electrode. (e) CV curves of lithium sulfur cells with and without WC. (f) CV curves of lithium sulfur cells with and without WO3.

To investigate the effect of tungsten carbide addition on the electrochemical properties, three composite electrodes with (i) no additive, (ii) WC, and (iii) WO3 were prepared for comparison. Here, WO3 is included primarily to assess the effect of the difference in the electrical conductivity, which is summarized in Table S2. Figure 2a shows the galvanostic discharge-charge profiles (on the 3rd cycle) of the composite electrodes at a 6 ACS Paragon Plus Environment

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slow current rate of 0.2 C (= 335 mA g-1), which can be conveniently divided into two plateau regions (region I and II, respectively). The discharge process for Li-S batteries is known to consist of three major reaction steps particulary in those electrolytes made of lowdielectric solvents like 1,2-dimethoxyethane and 1,3-dioxolane.48, 51-53 The first step is known to mainly involve the reduction of elemental S8 to highly-soluble long-chain polysulfides, (R1: S8 + 2e- → S82-) and primarily associated with the reactions occuring in the region I in the discharge profiles.48 This is followed by the second step, where long-chain polysulfides are electrochemically reduced to the short-chain ones (e.g., R2: S82- + 2e- → 2S42-).48 Finally, disproportionation of short-chain polysulfides (R3: 2S42- → (6/7)S82- + (8/7)S12-) occurs to produce Li2S and long-chain S82-.48 It is notable that S82- from the third step is recycled into the second step repeatedly to attain the full theoretical capacity of sulfur, 1675 mA h g-1. The second and the third steps account for the reactions for the region II. Among these reaction steps, the electro-reductions of polysulfides in the first and the second steps proceed quickly. However, the chemical dissociation of polysulfides in the third step occurs slowly, severely limiting the overall electrode kinetics during the discharge process. Hence, the introduction of proper catalysts that accelerate this ‘bottleneck’ reaction (R3) is critical to improve the poor rate capability of Li-S batteries, since higher dissociation rate of polysulfides results in larger discharge capacity for region II due to the contribution from the regenerated S82- from S42-. In Figure 2a, all three electrodes show a similar discharge capacity for region I (~270 mA h g-1), indicating that a similar amount of long-chain polysulfides was produced from the first step reaction regardless of additives. Slightly higher capacity than the value theoretically expected from S8 + 2e- → S82- (~210 mA h g-1) is probably contributed by regenerated S8 from the disproportionation of some S82- (e.g., S82- → (1/4)S8 + S62-) shortly after the first step. Interestingly, the composite electrode with a WC additive exhibited much higher discharge capacity for region II compared to other electrodes (464 mA h g-1 for bare, 497 mA h g-1 for 7 ACS Paragon Plus Environment


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WO3 and 572 mA h g-1 for WC). This indicates that the addition of tungsten carbide promotes the disproportionation of polysulfides, i.e., S42- for the third step effectively. Moreover, the electrode with a WC additive shows much improved cyclic performance compared to other electrodes as shown in Figure 2b. After 100 cycles at 0.2 C, the electrode with WC additive still maintains 81 % of its initial capacity (excluding the first three cycles with 0.05 C), while bare and WO3-loaded electrodes show only 60 % and 44 %, respectively. This reflects that the addition of tungsten carbide enables more efficient utilization of the polysulfide in the electrolyte. Furthermore, this approach can deliver much higher reversible capacity compared to typical cathodes for Li-ion batteries (Figure S2). In order to demonstrate the catalytic dissocaiton of polysulfide by tungsten carbide more clearly, the performance of the composite electrode on various C-rates from 0.5 C to 5 C was evaluated, showing that the WC-loaded electrode shows much superior rate capability (Figure 2c) and lower overpotentials during the high-rate discharge (Figure S3). At a low current rate of 0.2 C, the discharge capacities for bare, WO3- and WC-loaded electrodes were measured to be 738, 769 and 843 mA h g-1, respectively. However, the difference in the capacity becomes more distinct at higher current rates: i.e., 134, 27 and 438 mA h g-1 at 5 C. This tendency is well reflected in Figure 2d, where the ratio of the discharge capacities of WC-loaded to the bare electrodes soars from 1.13 at the scan rate of 0.2 C to 3.22 to 5 C. This is a clear evidence that tungsten carbide enhances the electrode kinetics involved with the dissocation of polysulfide. Moreover, the high electrical conductivity (~0.5 × 105 S cm-1) of tungsen carbide offers the electrons with an easy access to the reaction sites, facilitizing redox rections of adsorbed polysulfide on the spot.54 We also chekced the electrochemical behavior of tungsten carbide to understand if tungsten carbide itself produces a discharge capacity without active sulfurs (Figure S4). As expected, the tungsten carbide did not exhibit 8 ACS Paragon Plus Environment

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any tangible electrochemical activity with lithium ions under the experimental condition, indicating that the higher discharge capacity of WC-loaded cathode obviously originates from the catalytic activity of tungsten carbide on the dissociation of polysulfide. A cyclic voltammograms of the composite electrodes are shown in Figure 2e, f. During the cathodic sweep are observed two apparent peaks around 2.3 V (Epc1) and 2.1 V (Epc2), which is associated with the region I and region II in the galvanostatic discharge curves, respectively (Figure 2a). Interesting features were observed in the capacity and the position related to the cathodic peaks. Firstly, a considerable increase in the capacity (20%) associated with cathodic peak, Epc2, for the WC-loaded composite electrode was observed compared to other electrodes, consistent with the observation that a WC-loaded composite electrode showed a higher discharge capacity (23%) for region II during the galvanostatic discharge. Furthermore, the cathodic peak, Epc2, for tungsten carbide is located at slightly higher potential by ~13 mV compared to other electrodes. This indicates that tungsten carbide lowers the activation energy and thus increases the kinetics for the cathodic reaction at Epc2, which involves a disproportionation of short-chain polysulfides.51-53



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Figure 3. UV-vis. spectra measured with (a) carbon and (b) WC in the electrolyte containing 50 mM Li2S4, photographs of electrolyte with (c) carbon and (d) WC.

To demonstrate the effectiveness of tungsten carbide on promoting the dissociation of short-chain polysulfides more clearly, we designed a chronoamperometric experiment (held at 2.0 V vs. Li/Li+ for a controlled period of time) for the solution containing Li2S4 polysulfides using the well-polished glassy carbon and polycrystalline tungsten-carbide electrodes (see the experimental for detail). The UV-vis. spectroscopy was employed to visualize the concentration variation of the S42- species by the chemical disproportionation. The resultant spectra indicate that tungsten carbide has a remarkable effect on promoting the disproportionation of Li2S4 (Figure 3a, b), as the absorption peak centered at 410 nm (which is associated with Li2S4) for tungsten carbide electrode decreases much faster along with reaction time than glass carbon electrode.55-56 This tendency is also visually observed in Figure 3c, d, where much thinner color is observed for the solution treated with WC electrode. This result supports the analysis for the discharge profiles carried out in Figure 2a and the CV curves in Figure 2e that WC acts as a superb catalyst for the dissociation of polysulfide. 10 ACS Paragon Plus Environment

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The sulfiphilic character of the tungsten carbide towards long-chain polysulfides was investigated by stirring the tungsten carbide powders in the solution containing Li2S8. In Li-S batteries, the sulfiphilic moieties of the cathode additives are important since they can provide effective anchoring sites for the soluble polysulfide.40,

57

This can suppress the

problematic redox shuttle reactions, and also facilitate the rapid dissociation of adsorbed polysulfide, as well as help the transformation to sulfur during charge. The UV-vis. spectra and mapping images in Figure S5 clearly shows that a tungsten carbide captures polysulfide efficiently, proving that tungsten carbide possesses a strong sulfiphilic property. The greatly improved cyclic performance of WC-loaded cathode is attributed to the higher utilization of the polysulfide by tungsten carbide acting as a good anchoring material. These explanations accord with Y. Cui et al.’s results that anchoring materials with strong (but not too strong) adsorption energies with polysulfide and low diffusion barrier will work efficiently for Li-S batteries.57-58 The surface structure of tungsten carbide upon the interaction with elemental sulfur was characterized by X-ray photoelectron spectroscopy (XPS). Figure S6 shows that the binding energy of the S 2p electron from elemental sulfur and the S-decorated tungsten carbide, which was prepared by removing elemental sulfur from the ball-milled S/WC composite with a plenty of toluene. A shift of S 2p3/2 peak was observed from 163.98 eV for elemental sulfur to 163.26 eV for S-decorated WC. This red shift of the peak results from the increase in the electron density around the sulfur atom as is the case with metal sulfide (e.g., binding energy of 161.9 eV for WS2).59-60 We believe that this originates from nanoscale layers of sulfur atoms specifically adsorbed on tungsten carbide, creating ‘tungsten disulfide’like surfaces. Recently, WS2 was reported to retain a good sulfiphilic property to acts as an excellent anchoring material and also catalyze the disproportionation of polysulfides in Li-S 11 ACS Paragon Plus Environment


batteries.40,

61-62

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We believe that a tungsten carbide with a sulfur-decorated surface can

function in a similar way to WS2 due to their similar surface chemistry. This mechanism is consistent with our previous work that S-decorated TiN exhibits properties similar to TiS2 which is known as a good anchoring material and an efficient catalyst for the dissociation of polysulfide.42,

57

In addition, XPS for Li2S4-adsorbed WC was utilized to investigate the

interaction between WC and polysulfide (Figure S7). From W 4f7/2 spectra, a relatively small shift of -0.31 eV to lower binding energy was observed, indicative of electron donation from polysulfide to WC and formation of chemical bonding.63 This suggests that WC may act as an efficient anchoring material. Moreover, this may contribute to the destabilization of chemical bonding in adsorbed Li2S4 molecules and their disproportionation.

Figure 4. Rate capability of the bare electrode, WC electrode, WC electrode with C coated separator and WC electrode with C/WC coated separator cells.

To develop a comprehensive methodology to cope with various critical problems of Li-S battery, the current method was combined with a known polysulfide confinement technique. We employed WC-loaded cathode to enhance the lethargic disproportionation of polysulfide and also utilized carbon-coated separator as a physical barrier to suppress 12 ACS Paragon Plus Environment

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polysulfide redox shuttle.31-34 The separator was coated with either carbon black alone or a 3:1 mixture of carbon black and tungsten carbide. Figure 4 demonstrates the performance of the various electrochemical cells equipped with either WC catalysts or C- or C/WC-coated separators. The use of WC catalyst or coated separator turned out to be highly effective in improving the performance at high current rates. Particularly, the electrochemical cell with both a WC-loaded cathode and a C/WC-coated separator shows the best performance, delivering 780 mA h g-1 at 5 C rate. This indicates that the combination of a catalyst for polysulfide disproportionation and physical barrier for polysulfide confinement produces synergic effect to significantly improve the electrochemical performance. A comparison with the results found in other literatures is shown in Table S3. In this work, tungsten carbide was investigated as a promising cathode additive to achieve a high-power Li-S battery. From the electrochemical studies, the addition of tungsten carbide on the composite cathode resulted in a remarkable increase in the reversible capacity and the rate capability. This is explained by the catalytic disproportionation of short-chain polysulfide by tungsten carbide and the repeated use of recycled long-chain polysulfide in the electro-reduction process. Tungsten carbide also works as a good anchoring material due to the highly sulfiphilic surface moieties, which form S-decorated surface chemistry to function similarly to WS2. The current method works synergically in a combination with polysulfide confinement technique, delivering a discharge capacity of 780 mA h g-1 even at a high current rate of 5 C. We expect that the material and methodology developed in this work will help greatly overcome the technical challenges and the commercialization of Li-S batteries.

Supporting Information. 13 ACS Paragon Plus Environment


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The following files are available free of charge. Experimental details, physical properties of additives, additional electrochemical data, SEM images, spectra for UV-vis., and XPS. (PDF)

Conflicts of interest The authors declare no competing financial interests.

Acknowledgement This

work

was

supported

by

(2015R1A2A1A10056156,

the

grant

of

National

2017R1A4A1015533,

Research

Foundation

2014M1A8A1049348,

2015M1A2A2056556), and Korea Institute of Energy Technology Evaluation and Planning (20153030031510, 20153010041750) funded by the Korea government.

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Tungsten Carbide as a Highly Efficient Catalyst for Polysulfide

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