Suppressing Polysulfide Dissolution via Cohesive Forces by

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Suppressing polysulfide dissolution via cohesive forces by interwoven carbon nanofibers for high-areal-capacity lithium-sulfur batteries Jong Hyuk Yun, Joo-Hyung Kim, Do Kyung Kim, and Hyun-Wook Lee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04425 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Suppressing polysulfide dissolution via cohesive forces by interwoven carbon nanofibers for highareal-capacity lithium-sulfur batteries Jong Hyuk Yun, Joo-Hyung Kim, Do Kyung Kim*,†, and Hyun-Wook Lee*,‡ †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea. ‡

School of energy and Chemical Engineering, Ulsan National Institute of Science and

Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea. *Corresponding author E-mail: [email protected] (D. K. Kim), [email protected] (H.W. Lee)

ABSTRACT

Nanostructural design renders several breakthroughs for the construction of highperformance materials and devices, including energy-storage systems. Although attempts made toward electrode engineering have improved the existing drawbacks, nanoengineering is still hindered by some issues. To achieve practical applications of lithium-sulfur (Li-S) batteries, it is difficult to attain a high areal capacity with stable cycling. Physical encapsulation via

ACS Paragon Plus Environment

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nanostructural design not only can resolve the issue of lithium polysulfide dissolution during the electrochemical cycling but also can offer significant contact resistance, which in turn can decrease the kinetics, particularly at a high sulfur loading. Thus, we demonstrate an electrospun carbon nanofiber (CNF) matrix for a sulfur cathode. This simple design enables a high mass loading of 10.5 mg cm-2 with a high specific capacity and stable cycling. The CNF-sulfur complex can deliver a high areal capacity of greater than 7 mAh cm-2, which is related to the excellent electrical conductivity of one-dimensional species. Moreover, we have observed that the reacted sulfur species have adhered well to the junction of the CNF network with specific wetting angles, which is induced by the cohesive force between the narrow gaps in the matrix that trapped the viscous polysulfides during cycling. The results of this study open new avenues for the design of high-areal-capacity Li-S batteries.

KEYWORDS Lithium-sulfur batteries; high mass loading; electrospun carbon nanofibers; polysulfide dissolution; cohesive force

TEXT To satisfy the ever-demanding energy requirements for large-scale energy-storage systems and electric transportation with high energy density and cost-effectiveness, it is desirable to develop rechargeable batteries that can outperform the state-of-the-art Li-ion batteries.1-4 In this regard, lithium-sulfur (Li-S) batteries have been widely reported to be one of the promising energystorage devices because of their high theoretical specific capacity (1,675 mAh g-1) and energy


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density (2,500 Wh kg-1 or 2,800 Wh l-1).5-7 Despite these considerable merits, the commercialization of Li-S batteries is limited because of the low electrical conductivity of covalent-bonded sulfur (5 × 10-30 S cm-1 at 25 °C); high solubility and diffusivity of polysulfide intermediates;8-11 large volume expansion during lithiation; and side reactions caused by the shuttle effect. Hence, poor material utilization and loss of active materials from the electrode are observed for Li-S batteries. To resolve these drawbacks, the physical encapsulation of sulfur by various metal oxides12-16 or carbonaceous materials, including porous carbon,17-23 carbon nanotubes,24-29 and graphene,30-32 has been reported to enhance the electrical conductivity of sulfur and suppress the dissolution of polysulfides into the electrolyte. Although these attempts in terms of electrode engineering have led to the improvement of several drawbacks of Li-S batteries, complicated synthetic processing and limited mass loading of sulfur materials have been reported. Previously, the physical encapsulation of sulfur has been predominantly reported using zerodimensional (0D) carbon support materials, including meso/microporous carbon spheres,18-20 carbon black,33 and hollow carbon spheres.17, 21 Although physical encapsulation has improved the electrochemical properties of Li-S batteries, the 0D structures induce considerable contact between supported carbon materials,34, 35 leading to substantial contact resistance (Figure 1(a)). Such high contact resistance in an electrode leads to the temperature increase of the electrode during cycling, corresponding to Joule heating. Joule heating significantly limits the high-areal density of sulfur electrodes. In contrast, one-dimensional (1D) carbon materials render a large surface area and a long-range conduction path for electrons and lithium ions,36, 37 indicative of fast kinetics, high-rate capability, and excellent mechanical properties as a good electron path and high surface-to-volume ratio enhance the contact between active materials. Crossed


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junctions via interconnected networks are created by the 1D interwoven structures as shown in Figure 1(a), which also exhibit advantages of the deposition of some active materials because the junction area is a thermodynamically more stable region compared to the surfaces along the 1D direction. Compared with a simple spherical 0D structure, such a network structure benefits to decreased contact resistance between particles and nanowires. As 1D carbon materials exhibit less contact resistance and a junction area for the deposition of sulfur, the interwoven matrix is suitable to achieve high-performance Li-S batteries with an areal capacity of greater than 7 mAh cm-2; this goal is a considerable challenge for Li-S batteries because of their poor intrinsic electrical properties. In addition, the 1D carbon matrix has a benefit on the volumetric changes during cycling, whereas the contact between 0D structures is easily altered or diminished, leading to the electrical isolation of particles. As the polysulfide species that dissolve in the electrolyte during the electrochemical reaction are viscous, the conformal encapsulation of sulfur species is known to be necessary to suppress polysulfide dissolution. However, considering the dissolution issue originated by the interaction between polar polysulfide and polar electrolytes, capillary force can also become another corresponding energy to decrease the energy associated with the dissolution of polysulfides. With all of the issues related to the surface tension, the specific binding energy increases the junction between fibers (Figure 1(b)), enabling new regions for the infusion of sulfur. From this viewpoint, randomly distributed carbon nanofibers (CNFs) are thought to be suitable for high-areal-capacity Li-S batteries by taking advantage of the high electrical conductivity with the suppressed dissolution of polysulfides. In this regard, we have demonstrated a sulfur cathode using an electrospun CNF matrix for trapping dissolved polysulfide and enabling the high mass loading of active materials. Using a


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facile immersing method, the CNF-S electrode comprising CNF and sublimed sulfur particles is obtained. As the CNF networks exhibit sustainable binding between fibers, this CNF-S electrode also serves as a current collector; therefore, the use of polymeric binders such as polyvinylidene fluoride or polyvinylpyrrolidone is not required. This CNF-S electrode exhibited an enhanced reversible capacity of ~752 mAh g-1 with a high sulfur loading of 10.5 mg cm-2, corresponding to a high areal capacity of 7.90 mAh cm-2. An areal capacity of 7.14 mAh cm-2 is retained even after 100 cycles. The results obtained from ex situ scanning electron microscopy (SEM) indicated that after the charge and discharge processes, sulfur and Li2S are concavely rearranged through the junctions between the CNF networks, with the retention of the wetting angles. As the polysulfide intermediates are viscous, they are thought to be adsorbed into the fibrous entanglement via cohesive forces during the electrochemical reaction. Figure 1(c) and 1(d) show a schematic for the fabrication of the CNF-S electrode and the corresponding SEM images, respectively. The average diameter of the as-electrospun PAN nanofibers is 344 nm, which decreases to 280 nm after stabilization at 280 °C in air and then further decreases to 144 nm after carbonization at 1500 °C under nitrogen. The structural and mechanical property of this carbon nanofiber are demonstrated in Figure S1. The nitrogen adsorption/desorption isotherm obtained by Brunauer-Emmett-Teller (BET) measurement is demonstrated in Figure S1(a). The CNF substrate exhibits a typical BET surface area for carbon nanofibers as 22.88 m2 g-1. The average pore size distribution calculated by Barret-JoynerHalenda (BJH) is shown in Figure S1(b). The adsorption/desorption average pore diameter by BET analysis is 5.51 nm and 3.94 nm respectively, and BJH adsorption/desorption is 23.51 nm and 17.43 nm respectively. In accordance with those results, it is evident that the porous CNF substrate provides electrochemically active sites for the cathode materials and improves the


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surface contact between the cathode and the liquid electrolyte. The mechanical property of the CNF substrate was measured by tensile stress test, and the stress-strain curve was obtained as illustrated in Figure S1(c). The CNF substrate endures over 30 MPa of tensile stress; hence, it exhibits a good mechanical strength. The inset in Figure S1(c) is photograph of a bent CNF sheet, which presents a flexibility. After the immersion of this heat-treated CNF sheet in a mixture of the sublimed sulfur powder and 1-methyl-2-pyrrolidone (NMP), the sulfur particles were successfully intertwined between the nanofibers as shown in Figure 1(d). The fabrication process is relatively simple compared with the conventional lengthy melt-infiltration or precipitation method.38-42 Ex situ SEM images were conducted to analyze the surface morphology of the initial discharged and charged electrodes. Figure 2 shows the SEM images of the representative junctions after the electrochemical reaction. The discharged and charged CNF-S electrodes exhibit a good distribution of the active materials of sulfur and Li2S throughout the CNF matrix. The solid Li2S and sulfur particles form a concavely wet shape at the junction between nanofibers, indicating that the solid Li2S phases in Figure 2(a) are well adhered between the intertwined networks even after passing through liquid polar intermediates. During the electrochemical reaction of sulfur, the weak interaction between the nonpolar carbon matrix and polar polysulfide intermediate phase deteriorates the adhesion of Li2S2 and Li2S to the carbon substrate after discharge, leading to the dissolution of the polysulfide in the electrolyte.29-31 However, the CNF-S matrix adsorbs the liquid polysulfide intermediates into the micropores via cohesive forces, similar to the adsorption of liquid by a fibrous fabric via capillary pressure, thereby suppressing the dissolution of polysulfides. This absorption ability of the CNF matrix was confirmed by a droplet test of Li2S6 catholyte, as described in Figure S2. 60 µl of 0.25M, 0.5M, and 1M Li2S6 catholyte were


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dropped on the CNF substrate, and these correspond to 2.5, 5.1, and 10.2 mg cm-2 of sulfur loading, respectively. The liquid polysulfides are rapidly absorbed within the substrate in all cases. There is no significant change before and after dropping polysulfide catholyte, as displayed in Figure S2(a) and (b). As shown in Figure S2(c), 10.2 mg cm-2 of sulfur as a form of the liquid phase is easily diffused into the CNF network. The adsorption test for 1M Li2S6 is supporting media. To further understand the wettability of the utilized sulfur, a geometric model is developed by considering the wetting angles of sulfur and Li2S in the CNF-S electrodes (Figure 2(f) and (g)). By this simplified model, the mass of the utilized sulfur throughout the CNF matrix can be assumed. As sulfur participating in the electrochemical reaction contributes to the specific capacity, the amount of concave sulfur associated with the total sulfur loading (4.4 mg cm-2) contributes to a cell’s specific capacity. This calculated specific capacity is comparable to the actual specific capacity as a function of acquired the battery data. The calculation is explained below, and Figure S3 summarizes the computational details. The concave wetting of sulfur species in the CNF junctions is considered to be a two-dimensional polygon with a thickness similar to the CNF diameter by taking into account the height because of the layer-by-layer stacking of the electrospun nanofibers. Hence, by the solidification of polysulfide intermediates, sulfur also undergoes wetting almost two-dimensionally on the layer (Figure 2(a)-(e)). Most of the polygons comprise concave quadrangles (Figure 2(e) and (f)). As the average width and wetting angles are 1.38 µm and 21.8o, respectively, the area of the quadrangle can be calculated as 1.82 µm2; hence, it is simplified to be a square with a length of 1.35 µm. As the average distance among the polygons is 2.89 µm, the areal ratio of the sulfur-occupied sites is determined as the occupied area by sulfur per total area of a unit cell.


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occupied area by sulfur = The ratio of sulfur-occupied sites = total area

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+ 2 ∗ 2

= +

Once the occupied area of the sublimed sulfur per total area is determined, the ratio of the sulfur-occupied sites throughout the CNF substrate is obtained. By the substitution of x and y by 2.89 µm and 1.35 µm, respectively, the ratio of the sulfur-occupied sites is calculated to be 0.1014, indicating that the sublimed sulfur covers 10% of the total area. Based on the ratio of the sulfur-occupied sites to the total volume of the CNF-S cathode, the volume occupied by sulfur can be obtained. As shown in Figure 2(i), the sulfur is distributed throughout the cathode. As the area and thickness of the cathode are determined to be 1.13 cm2 and 103.7 µm (Figure 2(h)), respectively, the total volume is calculated to be 1.17 × 10-2 cm3. Hence, the volume occupied by sulfur from the total volume is 1.19 × 10-3 cm3. Considering the density of sulfur as 2.07 g cm-3, the mass of the loaded sulfur is approximately 2.46 mg, and the mass corresponds to an areal loading of 2.17 mg cm-2. Compared to the total sulfur loading of 4.40 mg cm-2 estimated empirically, 49.32% of sulfur contributes to a specific capacity of 826.1 mAh g-1. This value is in agreement with a theoretical capacity of 1,675 mAh g-1. According to this geometric consideration, the cohesively wet sulfur adheres well to the junctions between the CNF matrixes during electrochemical cycling; hence, almost all of the loaded sulfur contributes to the cell capacity. The CNF-S networks exhibit remarkable battery performance as shown in Figure 3. Due to cohesive interactions during cycling, the impedance after cycling is expected to be less than that of the initial state. In Figure 3(a), the Nyquist plots of fresh and cycled cells indicated that the charge-transfer impedance of the sample after 10 cycles considerably decreases, indicative of good adhesion between the sulfur active material and a CNF current collector. Based on the EIS


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characterization, the liquid phase of active materials is well wetted throughout the CNF substrate; hence, the electrical conductivity of the cell is enhanced, thereby effectively expediting the utilization of sulfur.43-45 The CV data of the CNF-S cell exhibits good reversibility during repeated scans (Figure 3(b)). The CV curve shows typical cathodic peaks at approximately 2.3 V and 2.0 V for the Li-S cell, corresponding to the reduction of sulfur to high-order polysulfides (Li2S8 to Li2S4) and low-order polysulfides (Li2S2 to Li2S). In addition, anodic peaks are observed at typical potentials, without any peak shifting or fading. In addition, the cell exhibits a narrow peak separation, indicative of low cell polarization. Hence, the CNF matrix is thought to facilitate the redox chemistry. Based on the good reversibility and electrical conductivity, the cell rate capability was investigated at sulfur loadings of 2, 4, and 6 mg cm-2 (Figure 3(c)). Although the specific capacity slightly decreases with the increase in the loading amount of sulfur, a cell with a sulfur loading of 4 mg cm-2 still maintains reversible capacities even at a high rate of 2 C, indicating that the CNF matrix can satisfy the high-areal sulfur loading. Based on the decent battery performance, we therefore examined thick CNF-S electrodes with mass loadings of up to 4.4, 6.0, and 10.5 mg cm-2, and the weight ratio between carbon and sulfur was determined by thermogravimetric analysis in Figure S4. The mass of the circular CNF substrate used for electrode is 2.80 mg consistently for the sulfur loading of 4.4, 6.0, and 10.5 mg cm-2 cathode. The individual cathodes show high sulfur content of 61.18 wt%, 67.85 wt%, and 78.99 wt%, respectively. As demonstrated in Figure 3(e), each of the corresponding cells reach specific discharge capacities of 1,139, 1,036, and 753 mAh g-1, with the maintenance of the specific discharge capacities of 847, 812, and 680 mAh g-1 after 100 cycles at a rate of 0.1 C. Even with a high sulfur loading of 10.5 mg cm-2, 90.4% of the initial discharge capacity is retained, with a decay of only 0.092% with each cycle. Such good cycle stability is related to the


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interwoven networks of CNFs, which exhibit high electrical conductivity and stability with sulfur via cohesive forces in the porous cage. With the increase in the loading amount of sulfur, the initial charge/discharge profiles of the three cells show a decreasing trend (Figure 3(d)). The sharp peaks at the initial charge state imply that the cell polarization is caused by the insulating property of high sulfur loading. After the activation of insulating sulfur, the high sulfur loading can also contribute to a high-areal capacity as illustrated in Figure 3(f). The cells with sulfur loadings of 4.4, 6.0, and 10.5 mg cm-2 achieve high-areal capacities of 5.07, 6.22, and 7.91 mAh cm-2, with high capacity retention values of 3.77, 4.87, and 7.14 mAh cm-2 after 100 cycles, respectively. The performance of the CNF-S electrode at high mass loading is superior to the areal capacities reported recently for lithium-ion batteries,46, 47 indicative of the successful CNFS design. Since the sulfur species adhere well to the CNF matrix, the chemical bonding of CNF-S electrodes should also be considered. To verify the chemical interaction, we have performed Xray photoelectron spectroscopy profiles of the CNF matrix, pristine CNF-S electrode, discharged CNF-S electrode, and charged CNF-S electrode as shown in Figure 4 and Figure S5. Several studies have reported that lithium polysulfides are strongly chemisorbed onto nitrogen-doped carbon materials.48-51 Accordingly, the nitrogen groups possibly attract the sulfur species because the N groups in PAN, which is the precursor for CNFs, possibly remain as pyridinic, pyrrolic, or quaternary N after carbonization. Moreover, a marked nitrogen-related peak is not observed for the CNF matrix and pristine CNF-S electrodes (Figure 4(a)); however, C-N peaks are observed (Figure 4(b) and (c), and Figure S5(a) and (b)). Hence, this remaining C-N bond also facilitates the cycling stability of the CNF-S cathode, similar to the chemisorption effect as a result of nitrogen doping. Strong N, O, and F peaks corresponding to the discharged and charged CNF-S


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are observed, corresponding to the lithium bistrifluoromethanesulfonimidate (LiTFSI) salt. Figure 4(f) and 4(h) show the S 2p peaks for the discharged and charged CNF-S electrodes. For the discharged CNF-S, only peaks typical of polysulfides, corresponding to bridging sulfur (162.4 eV) and terminal sulfur (163.3 eV), are detected (Figure 4(f)).52 The charged CNF-S electrode also shows typical S 2p3/2 (163.9 eV) and S 2p1/2 (165.0 eV) peaks corresponding to elemental sulfur; hence, there is less interaction of the chemical encapsulation of sulfur and polysulfide species (Figure 4(h)). As notable peaks corresponding to the C-S bonds or N groups are not observed for all samples, the chemisorption of CNF matrix is less effective to capture polysulfides. Accordingly, sulfur and polysulfide are more physically adsorbed into the porous space of the CNF network. To obtain further evidence for the physical confinement of sulfur, CNF-S electrodes were compared to a flat, non-porous carbon electrode as a control sample (referred to as Flat CarbonS, Figure 5). Since the precursor and annealing process are exactly the same as that utilized for the preparation of the CNF matrix, the only difference between CNF-S and the Flat Carbon-S is the structural property of the non-porous and porous species. After heat treatment, a freestanding polymer is obtained after drying the as-casted PAN sheet (Figure 5(a)). The yellow polymer changes into a dark sheet after carbonization at 1500 oC under nitrogen atmosphere (Figure 5(b)). SEM images confirmed that Flat Carbon-S exhibits a flat microstructure (Figure 5(c) and (e)), whereas CNF-S comprises a network structure (Figure 5(d)). Figure 5(g) shows the cycle performance of CNF-S and Flat Carbon-S wherein the sulfur loading for both electrodes is identical to 4 mg cm-2. Although the initial discharge capacity of the Flat Carbon-S cell reaches 867.6 mAh g-1, it decays to 180.4 mAh g-1 after 100 cycles. After cycling, Flat Carbon-S and CNF-S cells were disassembled to verify the dissolution of polysulfide. As shown in Figure 5(f),


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the electrolyte color using the Flat Carbon-S cathode completely changed to yellow; this result provides clear evidence for the dissolution of polysulfide. In contrast, the CNF-S cell still maintains the original transparent color of the electrolyte. From all of the data, the physical interactions between a current collector and the polysulfide species are thought to play a significant role as a stable host for suppressing the dissolution without physical encapsulation. As several physical encapsulation methods using nanotechnology are expensive for processing, this facile fabrication is envisioned to fulfill the requirements for the practical approach of Li-S batteries. In summary, we have demonstrated electrospun CNFs to realize high-areal-capacity Li-S batteries. The free-standing, binder-free CNF electrode has accommodated a high sulfur loading of 10.5 mg cm-2 with high energy density and stable cycling. The intertwined CNF matrix has physically adsorbed polysulfide via cohesive forces, enabling high mass loading with dramatic suppression of the polysulfide dissolution. Consequentially, the CNF-S cell exhibits a reversible capacity of 90.3% after 100 cycles, with a high areal capacity of greater than 7 mAh cm-2, which surpasses that of the current lithium-ion batteries. In addition, CNFs have also provided the excellent electrical conductivity, leading to both fast kinetics and high capacity. To realize commercial Li-S batteries, we believe that the physical interaction should also be considered to satisfy both scientific and practical intrigue.


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FIGURES

Figure 1. Schematic of (a) the electron paths in 0D and 1D electrodes and their cell resistance. (b) Schematic of the deposition of sulfur species during the electrochemical reaction via the cohesive force of viscous polysulfides. (c) Fabrication of the CNF-S electrode using electrospun PAN, and (d) the corresponding SEM images of each product.


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Figure 2. SEM images of (a)-(b) the first discharged electrode containing Li2S at the junction between the nanofibers, and (c)-(e) the first charged electrode. (f) The geometrical model of the concave sulfur polygon with a specific wetting angle and (g) its repeated unit cell. (h) SEM image of the cross-sectional CNF-S electrode after cycling and the corresponding energydispersive spectroscopy maps of (i) sulfur and (j) carbon.


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Figure 3. Electrochemical measurements of the CNF-S electrode. (a) Electrochemical impedance spectroscopy, (b) cyclic voltammetry, (c) rate capability of the CNF-S electrodes with sulfur loadings of 2, 4, and 6 mg cm-2, (d) voltage profiles as a function of the specific capacity, (e) cycle performance, and (f) the areal capacity plot of the electrodes with sulfur loadings of 4.4, 6.0, and 10.5 mg cm-2.


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Figure 4. X-ray photoelectron spectra of the CNF-S electrode. (a) Entire element scan; (b), (c), (e), and (g) The C 1s scan; and (d), (f), and (h) The S 2p scan spectra of the CNF matrix, pristine, discharged, and charged CNF-S electrode.


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Figure 5. Comparison of the non-porous Flat Carbon-S and CNF-S electrodes. Images of (a) the as-casted flat PAN sheet and (b) the carbonized flat PAN sheet. SEM images of (c), (e) the Flat Carbon sheet, and (d) the CNF. (f) Photograph of disassembled coin cells of the cycled Flat Carbon-S and CNF-S cells. (g) Comparison of the cycle performance of the Flat Carbon-S and CNF-S cells at a rate of 0.1 C.


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ASSOCIATED CONTENT The following files are available free of charge. Experimental methods, and Figure S1-5 (PDF) Video of adsorption test (MP4)

AUTHOR INFORMATION Corresponding Authors *E-mail: (D. K. Kim) [email protected] *E-mail: (H.-W. Lee) [email protected] ORCID Jong Hyuk Yun: 0000-0002-0798-4669 Do Kyung Kim: 0000-0001-9092-9427 Hyun-Wook Lee: 0000-0001-9074-1619 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Climate Change Research Hub of EEWS from KAIST (Grant No. N11170059) and the National Research Foundation of Korea (NRF) grant funded by the


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Korea government (MSIT) (No.2017R1A2B2010148). H.-W.L. acknowledges support from International Cooperation in S&T Program (NRF-2016K2A9A1A06934767) and Basic Research Lab Program (NRF-2017R1A4A1015533) through the National Research Foundation of Korea funded by the MSIT.

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Suppressing Polysulfide Dissolution via Cohesive Forces by

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