Effective Trapping of Lithium Polysulfides Using a Functionalized

Oct 16, 2017 - (46-49) Another effective strategy would be to enhance the cycling ...... Son , B. K.; Coskun , A.; Choi , J. W. Rational Sulfur Cathod...
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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38445-38454

Effective Trapping of Lithium Polysulfides Using a Functionalized Carbon Nanotube-Coated Separator for Lithium−Sulfur Cells with Enhanced Cycling Stability Rubha Ponraj,† Aravindaraj G. Kannan,† Jun Hwan Ahn,† Jae Hee Lee,† Joonhee Kang,‡ Byungchan Han,‡ and Dong-Won Kim*,† †

Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Korea

‡

ACS Appl. Mater. Interfaces 2017.9:38445-38454. Downloaded from pubs.acs.org by KAROLINSKA INST on 01/24/19. For personal use only.

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ABSTRACT: The critical issues that hinder the practical applications of lithium−sulfur batteries, such as dissolution and migration of lithium polysulfides, poor electronic conductivity of sulfur and its discharge products, and low loading of sulfur, have been addressed by designing a functional separator modified using hydroxyl-functionalized carbon nanotubes (CNTOH). Density functional theory calculations and experimental results demonstrate that the hydroxyl groups in the CNTOH provoked strong interaction with lithium polysulfides and resulted in effective trapping of lithium polysulfides within the sulfur cathode side. The reduction in migration of lithium polysulfides to the lithium anode resulted in enhanced stability of the lithium electrode. The conductive nature of CNTOH also aided to efficiently reutilize the adsorbed reaction intermediates for subsequent cycling. As a result, the lithium−sulfur cell assembled with a functional separator exhibited a high initial discharge capacity of 1056 mAh g−1 (corresponding to an areal capacity of 3.2 mAh cm−2) with a capacity fading rate of 0.11% per cycle over 400 cycles at 0.5 C rate. KEYWORDS: lithium−sulfur cell, carbon nanotube, lithium polysulfide, functional separator, cycling stability

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polymers,26−28 using core−shell structured materials,29 optimizing the electrolyte composition,30−32 utilizing amino polar binder,33 and using lithium sulfide as an active material.34 Recently, the functional materials such as carbon nanotube and graphene oxide have been used as an interlayer between the cathode and the separator to trap the polysulfides within the cathode side, resulting in enhanced cycling performance.35−42 For instance, Su and Manthiram37 demonstrated that a carbon nanotube interlayer reduced the charge transfer resistance of the sulfur cathode and trapped lithium polysulfides within the cathode side, resulting in enhanced utilization of the active material and improvement of cycling performance. Another promising approach is the modification of the separator using conductive materials, which integrate the functionalities of the interlayer while the separator acts as an electrically insulating membrane. Since the conductive coating layer is supported by the polymeric membrane, its weight and coating thickness can be reduced in comparison to the freestanding interlayer to achieve higher specific energy density. In addition, integration of the functional layer on to the separator avoids the complications related to modifying the cell configuration to accommodate the interlayer. Various types of carbons, such as Super P carbon, multiwalled carbon nanotubes (MWCNTs),

INTRODUCTION Development of advanced energy storage devices with high energy density and long cycle life is crucial for powering electric vehicles and utilizing renewable energy sources such as wind and solar energy. In this regard, lithium−sulfur (Li−S) batteries are highly promising due to their high theoretical energy density.1−3 In addition, sulfur is abundantly available, environmentally benign, and inexpensive. In Li−S batteries, cyclic sulfur is converted to Li2S through the formation of various lithium polysulfide intermediates during the discharge reaction.4 Unfortunately, higher order lithium polysulfide intermediates (Li2Sx, 4 ≤ x ≤ 8) dissolve in the electrolyte solution and migrate to the lithium anode side, resulting in the loss of active sulfur and damage of the lithium electrode surface.5 Also, sulfur and its discharge products are highly insulating in nature, which further limit their electrochemical performance. These characteristics of the sulfur cathode result in rapid capacity fading and low Coulombic efficiency, which hinder their practical applications. To address these drawbacks, sulfur has been embedded into various forms of carbons, such as hierarchically porous carbon,6,7 hollow carbon spheres,8−10 carbon nanotubes,11 carbon nanofibers,12 and graphene.13−16 Also, metal oxides including TiO2,17,18 MnO2,19−21 SiO2,22 and MgO23 have been utilized as functional additives in the sulfur cathode to trap the soluble lithium polysulfides within the cathode. Other strategies involve protecting the lithium anode,24,25 coating the active material with conductive © 2017 American Chemical Society

Received: July 20, 2017 Accepted: October 16, 2017 Published: October 16, 2017 38445

DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

Research Article

ACS Applied Materials & Interfaces

2 g of MWCNT was dispersed in 80 mL of 2 M sodium hydroxide solution and ultrasonicated for 30 min to achieve a homogeneous dispersion. The solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and placed in a preheated oven at 180 °C for 4 h. The reaction mixture was allowed to cool down naturally, and the product was washed with methanol, ethanol, and water sequentially until the pH became neutral. Finally, the obtained product was dried overnight in a vacuum oven at 60 °C and used for separator coating. For separator coating, 2.5 mg of CNTOH was dispersed in 300 mL of isopropanol using ultrasonication for 2 h, and the solution was vacuum filtered through a commercial polyethylene (PE) separator (Asahi ND420) to achieve uniform coating on one side of the separator. The coated separator was dried in a vacuum oven at 80 °C for 12 h and punched into circular disks for performance evaluation. The weight of the CNTOH-modified PE separator was about 1.34 mg cm−2. A CNTcoated separator was also prepared as a control sample through the same procedure, where CNTOH was replaced with CNT. Electrode Preparation and Cell Assembly. The sulfur cathode was prepared by mixing sulfur powder, Teflonized acetylene black, and Ketjen black carbon in a weight ratio of 70:15:15 in ethanol. The obtained slurry was pressed onto a stainless steel mesh and dried at 60 °C under vacuum for 15 h. The loading of sulfur was controlled to be 3 mg cm−2. A 200 μm thick lithium foil (Honjo Metal Co. Ltd.) pressed onto copper foil was used as the anode. A coin-type (CR2032) Li−S cell was assembled by sandwiching the surface-modified separator between the lithium anode and the sulfur cathode. The cell was then injected with 30 μL of electrolyte solution consisting of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2, LiTFSI) dissolved in a mixed solvent of 1,3-dioxalane (DOL) and 1,2dimethoxyethane (DME) (50:50 by volume) containing 0.4 M LiNO3 as an additive. All cells were assembled in an argon-filled glovebox. Characterization and Measurements. The morphological analysis was carried out using a scanning electron microscope (SEM, JEOL JSM 6701F) equipped with energy dispersive spectroscopy (EDS). A high-resolution transmission electron microscope (HRTEM, JEOL, JEM 2100F) was used to observe the morphology of the CNT and CNTOH samples. The quantitative analysis of surface functional groups in CNTOH was performed using X-ray photoelectron spectroscopy (XPS, VG Multilab ESCA System, 220i). Raman spectroscopy was carried out using a Dongwoo Optron, MonoRa 780i spectrometer. Fourier transform infrared (FTIR) spectra of CNT and CNTOH were recorded using a JASCO 760 IR spectrometer. AC impedance measurements were conducted in order to measure electrolyte resistance and interfacial resistances using a Zahner Electrik IM6 impedance analyzer in the frequency range of 1 mHz to 100 kHz with an amplitude of 10 mV. Charge and discharge cycling tests of the Li−S cells were carried out using battery test equipment (WBCS 3000, WonA Tech Co., Ltd.) at 25 °C. Lithium Polysulfide (Li2S4) Adsorption Test. Lithium polysulfide was synthesized according to the procedure previously reported.57 In a typical synthesis, 2.75 mol of sulfur was dissolved in 1 mol of superhydride solution, and the resulting solution was dried under vacuum to obtain a yellow powder. Then, the product was washed with toluene and dried to obtain a fine yellow powder of Li2S4. For the lithium polysulfide adsorption test, 5 mg of Li2S4 was dissolved in 5 mL of tetrahydrofuran and 20 mg of CNT or CNTOH was added into the solution and stirred for 1 h. Digital photographs of the solution were taken before and after stirring to observe its color change. The precipitated product was dried under vacuum for XPS analysis. All procedures were carried out in an argon-filled glovebox. Computational Studies. Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) program with the Perdew−Burke−Ernzerhof (PBE) exchange-correlation functionals and the projector-augmented wave (PAW) as implemented.58,59 Total energy and force were converged within 10−5 eV and 0.05 eV Å−1, respectively. The Brillouin zone in the reciprocal space was integrated by k-mesh grids 1 × 1 × 3 for CNT and CNTOH with a cutoff energy of 500 eV. van der Waals interaction was described by the DFT-D3 correction method.

acetylene black carbon, and Ketjen black carbon, have been successfully explored as conductive coating layers on the separator.43−45 However, these carbon-based materials exhibited weak interaction with lithium polysulfides, which can trap lithium polysulfides only to a certain extent.18 To enhance the trapping ability of the carbon-based coating layers through chemical interaction, various strategies such as doping carbon with heteroatoms and adding additional functional materials have been adopted.46−49 Another effective strategy would be to enhance the cycling performance of lithium−sulfur cells by modifying the surface of carbon to achieve strong interactions with lithium polysulfides. Most of the reports utilizing a surfacemodified separator in Li−S cells have paid little attention to areal capacity, which is a critical parameter to elevate these systems to the practical level.50 Although functionalized carbons have been explored in sulfur cathodes51−53 and as interlayers54,55 in Li−S cells, to the best of our knowledge, the modification of separators using hydroxyl functionalized carbon materials in high areal density Li−S cells has been rarely explored so far. In this work, we demonstrate the effectiveness of using a hydroxyl functionalized carbon nanotube (CNTOH)-coated polyethylene (PE) separator to trap lithium polysulfides and enhance sulfur utilization in Li−S cells, as schematically demonstrated in Figure 1. The hydroxyl groups in CNTOH

Figure 1. Schematic illustration of trapping lithium polysulfides by the CNTOH-coated separator. (a) The soluble lithium polysulfides migrate through the PE separator to the anode side, leading to low sulfur utilization. (b) For the CNTOH-coated separator, the soluble lithium polysulfides are trapped by the hydroxyl functional groups and reutilized to form short-chain polysulfides.

exhibited a strong interaction with lithium polysulfides, resulting in effective trapping of lithium polysulfides within the cathode side. The reduction in migration of lithium polysulfides to the anode side also resulted in enhanced stability of the lithium electrode. As a result, the Li−S cell assembled with the CNTOH-coated separator exhibited higher initial discharge capacity and more stable cycling performance compared to the cells assembled with a pristine PE separator and a CNT-coated separator.

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EXPERIMENTAL SECTION

Surface Coating of Separator with CNTOH. MWCNT (Hanwha Chemical, Korea) was surface-functionalized using a hydrothermal method, as previously reported.56 In a typical synthesis, 38446

DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

Research Article

ACS Applied Materials & Interfaces

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RESULTS AND DISCUSSION MWCNTs were surface functionalized with hydroxyl groups using a hydrothermal process, as illustrated in Figure 2, which is a simple and easy procedure to scale up for large-scale applications.

impurities are present in the sample. It is noticeable that the intensity of the oxygen peak significantly increased in CNTOH, which suggests that additional oxygen functional groups are incorporated onto the surface of CNTOH. The amounts of oxygen in the CNT and CNTOH samples were measured to be 2.1 and 9.4 at. %, respectively, indicating that additional oxygencontaining functional groups are attached to the CNT surface upon hydrothermal reaction. To elucidate the types of functional groups incorporated onto the CNTOH surface, the high-resolution C 1s XPS spectra of CNT and CNTOH were resolved, as shown in Figure 4c,d, respectively. The C 1s spectrum of CNT exhibited three peaks centered at 284.6, 287.2, and 288.5 eV, which can be ascribed to sp2-hybridized carbon, carbonyl, and carboxyl groups, respectively. In contrast, the high-resolution C 1s spectrum of CNTOH showed a peak at 285.9 eV in addition to three peaks detected in the CNT sample. This additional peak can be attributed to the hydroxyl functional groups attached to carbon, thereby confirming that CNT was successfully functionalized with hydroxyl groups. To further validate these results, FTIR spectra of CNT and CNTOH were recorded and are given in Figure S1. When compared to the CNT spectrum, CNTOH showed additional peaks at 1634 and 3422 cm−1, where the band at 3422 cm−1 corresponds to the stretching vibrational mode of hydroxyl groups, and the band at 1634 cm−1 can be ascribed to the stretching mode of hydroxyl in an enol group.56 This result further confirms the successful hydroxylation of CNT using hydrothermal method. CNTOH was coated on one side of a separator using the vacuum infiltration method. The SEM image of the pristine separator (Figure S2a) shows a uniform porous morphology. In contrast, the CNTOH-coated separator (Figure S2b) exhibits a homogeneous morphology with uniform distribution of CNTOH without agglomeration. The pore structure of the CNTOH-coated separator is important for polysulfide confinement in the Li−S cell. Since the functionalized CNT was coated on the commercial separator using vacuum filtration of the dispersed solution, it resulted in a highly intertwined, random network structure of CNTOH on the separator surface. The thickness of the coating layer was found to be 8.4 μm from the cross-sectional SEM image of the CNTOH-coated separator presented in Figure 5a, and the mass loading of CNTOH on the PE separator was about 0.14 mg cm−2. Although the surface SEM image of the CNTOH-coated separator (Figure 5b) shows a porous morphology, the pores are not continuous through the coating layer and are random in nature. It increases the tortuosity for polysulfide migration to the anode side, thereby allowing ample interaction with the hydroxyl groups on the coating layer. As shown in Figure 5b−d, the CNTOH-coated separator showed uniform distribution of carbon and oxygen elements in the SEM EDS elemental mapping of the surface. These results confirm that the PE separator is uniformly coated with CNTOH. It can be also seen that the integrity of the coated separator remains intact with no visual damage on the edges even after punching (Figure S2c,d), which suggests that CNTOH is firmly adhered to the PE separator. After the separator was coated with CNTOH, its ionic conductivity when soaked with liquid electrolyte remained almost unchanged (6.2 × 10−4 S cm−1) compared to that of a pristine separator (6.4 × 10−4 S cm−1). The cycling performance of the Li−S cells assembled with different separators was evaluated at a constant current rate of

Figure 2. Surface modification of CNT using the hydrothermal method to form CNTOH.

The morphologies of pristine CNT and CNTOH were characterized using SEM and TEM, and the resulting images are given in Figure 3. Both the SEM and TEM images of CNT

Figure 3. SEM images of (a) CNT and (b) CNTOH. TEM images of (c) CNT and (d) CNTOH.

and CNTOH samples did not show any difference in the morphology, indicating that the hydrothermal method used to functionalize MWCNTs gave negligible influence on its tubular morphology. To investigate the influence of functionalization on the structural features of CNT, Raman spectra were obtained, and the results are shown in Figure 4a. Both CNT and CNTOH samples showed characteristic bands at 1572, 1356, and 2681 cm−1, corresponding to the G-, D-, and G′-bands, respectively. The G-band arises from the sp2-hybridized carbon structures, and the D-band is from the scattering from a defect which breaks the basic symmetry of carbon material. The G′-band is attributed to the overtone of the D-band. As given in Figure 4a, the I D /I G ratio increased from 0.695 to 0.859 after functionalization, indicating that functionalization of CNT with hydroxyl groups induced structural distortions. The amount and nature of functional groups on the CNTOH surface was further characterized using XPS. The survey spectra of CNT and CNTOH in Figure 4b show the presence of carbon and oxygen in both samples. The detection of oxygen in CNT indicates that a trace amount of oxygen-containing 38447

DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

Research Article

ACS Applied Materials & Interfaces

Figure 4. (a) Raman and (b) XPS survey spectra of CNT and CNTOH. High-resolution C 1s XPS spectra of (c) CNT and (d) CNTOH.

lithium polysulfides that are soluble in the electrolyte solution. The lower discharge plateau arises from subsequent reactions to form insoluble Li2S. During charging, Li2S is reverted to form sulfur through a two-step reaction. All cells showed similar capacity emanating from the upper discharge plateau, indicating that a similar amount of sulfur was utilized in all cells to form lithium polysulfides. However, the capacity corresponding to the lower discharge plateau was the largest for the Li−S cell with the CNTOH-coated separator and the least for the cell with the pristine separator. This result shows that the formed soluble lithium polysulfides were not effectively utilized during subsequent reactions in the cell with the pristine separator. This implies that the soluble polysulfides migrated to the anode side, thereby resulting in reduced capacity from the lower discharge plateau. In contrast, the soluble lithium polysulfides were trapped by CNTOH on the CNTOH-coated separator and reutilized during the subsequent reaction. This postulate was validated using a lithium polysulfide migration test (Figure S3). In this test, a vial containing a solution of lithium polysulfide dissolved in a DOL/DME solvent mixture was allowed to migrate through the separator to another vial containing a solvent mixture without the dissolved polysulfides, and the color change was monitored with time. In the case of the pristine separator, the color of the solvent mixture turned from colorless to yellow, indicating the migration of dissolved lithium polysulfides through the separator. In contrast, for the CNTOH-coated separator, the solvent mixture remained transparent after 2 h, which demonstrates the effective trapping of soluble lithium polysulfides by the CNTOH-coated separator.

Figure 5. (a) Cross-sectional SEM image of the CNTOH-coated separator. (b) Surface SEM image of the CNTOH-coated separator and its EDS elemental mappings corresponding to (c) carbon and (d) oxygen.

0.5 C, where 1.0 C corresponded to a current density of 1675 mA g−1. Prior to the cycling performance evaluation, the Li−S cells were preconditioned at the 0.2 C rate for one cycle in the voltage range of 1.9−2.6 V, and the resulting charge−discharge voltage profiles are given in Figure 6a. All cells showed an upper discharge plateau and a lower discharge plateau around 2.35 and 2.10 V, respectively. The upper discharge plateau corresponds to the conversion of cyclic sulfur to higher order 38448

DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

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ACS Applied Materials & Interfaces

Figure 6. (a) Preconditioning charge−discharge voltage profiles of Li−S cells with different separators, (b) charge−discharge voltage profiles at various cycles of the Li−S cell assembled with CNTOH-coated separator at 0.5 C rate, (c) discharge capacities of the Li−S cells with different separators at 0.5 C rate as a function of cycle number, and (d) discharge capacities of the Li−S cells with the C rates increasing from 0.1 to 2.0 C every five cycles.

Table 1. Comparison of Initial Capacity and Cycling Stability of the Li−S Cell in This Work with Those Reported in the Cells Using Various Surface-Modified Separators initial specific capacity (mAh g−1)

initial areal capacity (mAh cm−2)

final areal capacity (mAh cm−2)

no. of cycles

fading rate (% per cycle)

C rate

ref

1056 1307 1389 1350 1073 1216 1090 946 1216 1362 1028 1225 1259 953 1012

3.20 2.61 1.57 2.36 2.15 1.88 1.74 1.67 1.95 1.91 1.64 4.90 5.50 6.00 1.42

1.71 1.19 0.97 1.29 1.24 1.12 1.24 1.44 1.28 1.42 0.91 3.87 4.00 3.15 0.99

400 500 200 500 300 500 100 100 500 100 1200 30 100 100 100

0.11 0.11 0.19 0.10 0.14 0.08 0.28 0.13 0.07 0.25 0.037 0.70 0.27 0.47 0.30

0.5 C 0.2 C 0.5 C 0.2 C 1.0 C 0.5 C 0.5C 1.0 C 1.0 C 0.2 C 0.5 C 0.2 C 0.2 C 0.2 C 0.2 C

this work 43 44 45 60 61 62 63 64 65 46 49 66 63 67

the cell with the CNTOH-coated separator exhibited the best cycling performance in terms of discharge capacity and cycling stability up to 400 cycles at 0.5 C rate in the voltage range of 1.8−2.6 V. Although, the cell with the CNT-coated separator exhibited better cycling stability than the cell with pristine separator, its cycling performance was inferior to the cell with the CNTOH-coated separator. The discharge capacity delivered by the Li−S cell with the CNTOH-coated separator after 400 cycles at the 0.5 C rate is similar to the discharge

The long-term stability of the Li−S cells with different separators was evaluated at 0.5 C, and the charge−discharge voltage profiles of the cell employing the CNTOH-coated separator are given in Figure 6b. The initial discharge capacity was 1057.6 mAh g−1 based on the mass of sulfur in the cathode with an initial Coulombic efficiency of 98.4%. The cell delivered a discharge capacity of 570.0 mAh g−1 after 400 cycles. The cycling performances of the Li−S cells assembled with different separators are compared in Figure 6c. As shown in this figure, 38449

DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

Research Article

ACS Applied Materials & Interfaces capacity exhibited by the cell with the CNT-coated separator after 100 cycles. The Li−S cell with CNTOH-coated separator maintained a high and stable Coulombic efficiency over 99.5% throughout cycling after initial cycles. In contrast, the cells with the CNT-coated separator and pristine separator exhibited a slightly low initial Coulombic efficiency and the Coulombic efficiencies fluctuated in the later cycles, which arise from the partial dissolution of lithium polysulfides and their migration to the anode side. These results suggest that the hydroxyl groups in CNTOH provided stronger interaction with lithium polysulfides, thereby trapping them more effectively, which resulted in increased utilization of sulfur in the cathode and enhanced cycling stability. To the best of our knowledge, the cycling data of the Li−S cell with the CNTOH-coated separator are better than the results reported so far for Li−S cells using other surface-modified separators considering the relatively high areal capacity and low fading rate, as given in Table 1. Note that the areal capacity of the Li−S cell with the CNTOH-coated separator in the present work is 3.2 mAh cm−2, which is higher than the other reported works. The effect of CNTOH on the rate capability was evaluated at various current rates with 5 cycles at each current rate. In the rate capability test, the cells were fully charged at the same low current rate (0.1 C) and discharged at different current rates with increasing current rate from 0.1 to 2.0 C every five cycles. As shown in Figure 6d, the cell with the CNTOH-coated separator delivered higher discharge capacities at all current rates than the cell with the pristine separator and CNT-coated separator. The main reason for the good rate performance is the conductive nature of CNTOH, which aided in reducing the charge transfer resistance.43 The enhanced rate performance can be also attributed to the increased stability of the lithium anode in the cell with the CNTOH-coated separator due to the trapping of dissolved lithium polysulfides within the cathode side. Note that the discharge capacities of the cells measured at 0.5 C rate (Figure 6d) are higher than those presented in Figure 6c. It can be ascribed to different current rates during charging cycles. In the cycling test, the cells were charged at 0.5 C rate and discharged at the same current rate (0.5 C) to the 400th cycles after one preconditioning cycle at 0.2 C. In contrast, the cells for the rate capability test were charged at a lower current rate (0.1 C) for all the cycles and discharged at different current rates from 0.1 to 2.0 C every 5 cycles, which enabled higher utilization of sulfur during the charging cycle, resulting in higher discharge capacity. SEM analysis of the lithium electrode was performed before and after cycling, and the resulting images are shown in Figure 7. The pristine lithium electrode showed uniform morphology before cycling (Figure 7a,b). When the Li−S cell with the pristine separator was cycled, the lithium electrode exhibited the extensive surface growth that was protruded from the electrode surface (Figure 7c,d). In contrast, the lithium electrode cycled with the CNTOH-coated separator gave a smoother surface morphology (Figure 7e,f). To further investigate the chemical composition on the lithium surface, SEM EDS mapping was carried out and the results are given in Figure S4. The lithium electrode cycled with the pristine separator exhibited uniform and dense distribution of sulfur throughout the surface, indicating that the migrated lithium polysulfides got deposited on the surface of lithium metal. Although some sulfur could be detected on the lithium electrode cycled with the CNTOH-coated separator, its intensity was very weak. The smoother morphology and

Figure 7. (a, b) SEM images of pristine lithium metal, (c, d) the lithium anode cycled with pristine separator, and (e, f) the lithium anode cycled with CNTOH-coated separator. Li−S cells were cycled in the voltage range of 1.8−2.6 V at 0.5 C for 100 cycles.

relatively less intense sulfur distribution on the lithium electrode cycled with the CNTOH-coated separator demonstrate that the trapping of lithium polysulfides within the cathode side enhances the stability of the lithium electrode. To further understand the influence of the CNTOH-coated separator on the electrochemical performance of Li−S cells, AC impedance of the cell was measured before and after cycling, and the results are presented in Figure S5. All the impedance spectra show the presence of two semicircles. The semicircle observed at the high to medium frequency region is related to the resistance of lithium ions (Rf) in the surface film formed on the electrode, and the semicircle in the low frequency range is attributed to the charge transfer resistance (Rct) at the interface between electrolyte and electrode. The X-axis intercept at the high frequency is corresponding to the electrolyte solution resistance (RS). Before cycling (after one preconditioning cycle), all the cells showed the almost same resistance. After 400 cycles, both electrolyte resistance (RS) and interfacial resistances (Rf and Rct) increased for all cells. Interestingly, the cell with the CNTOH-coated separator exhibited the lowest resistances among the cells, and the cell with the pristine separator showed the highest resistance. These results demonstrate that the hydroxyl groups in CNTOH effectively trapped and reutilized soluble lithium polysulfides within the cathode side, resulting in reduced Rf and Rct. The effect of hydroxyl groups in CNTOH on the interaction of soluble lithium polysulfides was investigated using XPS analysis after the Li2S4 adsorption test, and the results are presented in Figure 8. The Li2S4 adsorption test was carried out to examine the behavior of CNT and CNTOH in a simulated environment within the Li−S cell during cycling. The initial color of the Li2S4 solution with CNTOH (Figure 8a) or CNT (Figure 8b) was yellow, and then the yellow solution turned 38450

DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

Research Article

ACS Applied Materials & Interfaces

Figure 8. Photographs of the Li2S4 solution in THF containing (a) CNTOH and (b) CNT, and the Li2S4 solution containing (c) CNTOH and (d) CNT after stirring for 1 h. High-resolution S 2p XPS spectra of (e) Li2S4 and (f) CNTOH-Li2S4 mixture after stirring with CNTOH for 1 h.

transparent after stirring with CNTOH for 1 h (Figure 8c). In contrast, the color of the solution with CNT added remained dark yellow (Figure 8d) even after stirring for 1 h. These results demonstrate that the hydroxyl groups in CNTOH effectively can trap lithium polysulfides. To further elucidate the type of interaction between lithium polysulfide and CNTOH, XPS analysis was performed on the Li2S4-adsorbed CNTOH. As shown in Figure 8e, the resolved high-resolution S 2p XPS spectrum of Li2S4 showed peaks corresponding to bridging (SB0) and terminal sulfur atoms (ST−1), which is consistent with spectra of lithium polysulfides reported in previous studies.21,23,57 Upon interaction with CNTOH, the S 2p XPS spectrum of CNTOH-Li2S4 provided the additional peaks at higher binding energy, as depicted in Figure 8f. The deconvoluted peak at higher binding energy indicates the presence of thiosulfate and the polythionate complex, which denotes a strong interaction between CNTOH and Li2S4.21,68 The polythionate complex was formed by the catenation reaction, and it acted as a transfer mediator for the insoluble short-chain lithium polysulfides. The formation of thiosulfate was attributed to the interaction of Li2S4 with the hydroxyl groups in CNTOH. From these results, a strong interaction between lithium polysulfide and hydroxyl groups in CNTOH could be validated. In order to understand the atomistic mechanism of the performance enhancement, binding energies between the discharge products of Li2Sn intermediates and CNTOH were obtained by DFT calculations. The model systems of the CNT and CNTOH consist of 140 carbon atoms with a diameter of 9.58 Å. Binding energies of Li2Sn on CNT or CNTOH were defined as eq 1

ΔE B = E(CNT) + E(Li 2Sn) − E(CNT + Li 2Sn)

(1)

where CNT can be CNT or CNTOH. As shown in Figure 9, the binding energies of Li2Sn clusters on CNT and CNTOH

Figure 9. Theoretical calculations of binding energies for Li2Sn intermediates at six different lithiation stages (Li2S, Li2S2, Li2S3, Li2S4, Li2S6, and Li2S8) on CNT and CNTOH. The green and yellow balls represent lithium and sulfur, respectively.

revealed that the CNTOH shows stronger binding strength than CNT due to the additional interaction between Li and O in the hydroxyl group. This result suggests that the OH in CNTOH enhances interaction with lithium polysulfides and effectively confines Li2Sn in the cathode. Also, the possible binding configurations of Li2Sn clusters on CNT and CNTOH are demonstrated in Figure S6, denoting various possible 38451

DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

Research Article

ACS Applied Materials & Interfaces

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orientations. Moreover, the binding energies between Li2Sn species and CNTOH are much higher than those between Li2Sn clusters and the recently reported amino groups,33 which indicates strong adsorption of lithium polysulfides by the OH in CNTOH to trap them within the cathode.

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CONCLUSIONS We demonstrated that a functional separator coated with CNTOH effectively trapped lithium polysulfides within the cathode side, resulting in improvement of the cycling performance of Li−S cells. The enhanced cycling performance was attributed to (a) the strong adsorption of lithium polysulfides by the hydroxyl groups in CNTOH to trap them within the cathode side, (b) the conductive nature of CNTOH aiding in the effective reutilization of the adsorbed reaction intermediates, and (c) the reduction in the migration of soluble lithium polysulfides to the anode side, resulting in a stable lithium electrode. These results highlight the importance of using functional groups in separators to strongly bind the soluble lithium polysulfides, and it suggests that using such functional separators is an effective strategy to realize the potential of Li−S batteries on a practical level.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10641. FTIR spectra of CNT and CNTOH, SEM images of pristine PE separator and CNTOH-coated separator, lithium polysulfide trapping test using pristine and CNTOH-coated separators, SEM and EDS mapping images of lithium electrodes after cycling, AC impedance spectra of the Li−S cells, and illustration of the binding configurations of Li2Sn clusters on CNT and CNTOH (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Byungchan Han: 0000-0002-2325-6733 Dong-Won Kim: 0000-0002-1735-0272 Author Contributions

R. Ponraj and A. G. Kannan contributed equally in this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT, and Future Planning (2014R1A2A2A01002154 and 2016R1A4A1012224).

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DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454

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DOI: 10.1021/acsami.7b10641 ACS Appl. Mater. Interfaces 2017, 9, 38445−38454