Rational Design of Highly Packed, Crack-Free Sulfur Electrodes by

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Rational design of highly packed, crack-free sulfur electrodes by scaffoldsupported drying for ultrahigh-sulfur-loaded lithium sulfur batteries Hobeom Kwack, Jinhong Lee, Wonhee Jo, Yun-Jung Kim, Hyungjun Noh, Hyunwon Chu, and Hee-Tak Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08006 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Materials & Interfaces

Rational Design of Highly Packed, Crack-Free Sulfur Electrodes by Scaffold-Supported Drying for Ultrahigh-Sulfur-Loaded Lithium Sulfur Batteries Hobeom Kwack†a, Jinhong Lee†a, Wonhee Joa, Yun-Jung Kima, Hyungjun Noha, Hyunwon Chua, and Hee-Tak Kim*a,b

aDepartment

of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science

and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea bAdvanced

Battery Center, KAIST Institute for the NanoCentury, Korea Advanced Institute of

Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon 34141, Republic of Korea

†

These authors contributed equally to this work.

* Corresponding author

E-mail: [email protected] (H.-T. Kim)

Keywords: lithium-sulfur batteries, high sulfur-loaded cathodes, electrode fabrication, crackfree electrodes, Li2S deposition

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ABSTRACT Despite the notable progress in the development of rechargeable lithium sulfur batteries over the past decade, achieving high performance with high sulfur-loaded sulfur cathodes remains a key challenge on the path to the commercialization of practical lithium sulfur batteries. This paper presents a novel method by which to fabricate a crack-free sulfur electrode with ultrahigh sulfur loading (16 mg cm-2) and a high sulfur content (64%). By introducing a porous scaffold on the top of a cast of sulfur cathode slurry, the formation of cracks during the drying of the cast can be prevented due to the lower volume shrinkage of the skin. The scaffold-supported sulfur cathode delivers notably high capacity of 10.3 mAh cm-2 and 473 mAh cm-3 after a prolonged cycle, demonstrating that the crack-free structure renders more uniform redox reactions at such high sulfur loading. The highly packed, crack-free feature of the sulfur cathode is advantageous given that it reduces the electrolyte uptake to as low as an E/S ratio of 4 μL mg-1, which additionally contributes to the high energy density. Therefore, the scaffoldsupported drying fabrication method as presented here provides an effective route by which to design practically viable, energy-dense lithium sulfur batteries.


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INTRODUCTION Rechargeable batteries with high energy density have been studied intensively over the past decade to meet the high energy requirements of electric vehicles and mobile electronic devices.1 Recently, rechargeable lithium-sulfur (Li-S) batteries have received much attention as a promising candidate to succeed lithium-ion batteries (LIBs) due to their high theoretical specific capacity (1,675 mAh g-1) and natural abundance of sulfur.2-5 However, several problematic features of Li-S, such as their poor redox kinetics due to the insulating nature of sulfur6-7 and the shuttle phenomenon8 caused by the dissolution of polysulfides (Li2Sn, 4  n  8) intermediates9-10 have impeded the commercialization of Li-S batteries. To resolve the hurdles inherent in the cell chemistry, many advances have been realized in a myriad of excellent works.11-26 One of the important factors when designing an energy-dense battery is the high areal or the volumetric capacity of the electrode. As the area-specific loading increases, the portion of active materials for the cell becomes larger, resulting in a higher energy density of the corresponding cell. Commercial LIBs, with an operating voltage of around 3.7V, conventionally have an areaspecific capacity of around 4 mAh cm-2. To be comparable with LIBs in terms of the energy density, Li-S batteries should have area-specific capacities which exceed 7 mAh cm-2, corresponding to area-specific sulfur loading rates higher than 6 mg cm-2 at 70% sulfur utilization.27 However, when a sulfur cathode with the aforementioned areal loading is fabricated with the conventional solvent drying method, the resulting sulfur cathode is often damaged with crack formation and delamination from the current collector due to the uncontrolled shrinkage of the cast on the skin during solvent evaporation and the consequent local stresses.28 The deterioration of the thick sulfur cathode disconnects the electrical network, aggravating the limited kinetics of the redox reactions.29 Furturemore, Li-metal anode, which is coupled with a high sulfur loaded electrode, should be also considered for preventing an unfavorable reaction between solouble polysulfides and Li metal.30-34 3 ACS Paragon Plus Environment


For this reason, to achieve a high-sulfur-loaded cathode (HSLC) with high sulfur utilization, many researchers have devised methods to achieve crack-free electrodes.27-29,35 For example, Yang et al. fabricated an integrated HSLC (7 mg cm-2) via the phase inversion method.29 With their fabrication method, they were able to induce zero-strain drying and achieved an initial discharge capacity of 951 mAh g-1. Although this approach addressed the limited ion and electron transport of high-sulfur-loaded cathodes, the large electrolyte-to-sulfur ratios and large pore volumes due to the high porosity of the sulfur cathode offset their energy density enhancement effect, as indicated by a low active mass density. Pang et al. suggested an in-situ cross-linked binder to fabricate densely packed and crack-free sulfur/N-graphene/g-C3N4 hybrid electrodes.35-38 Although their sulfur cathode with an areal loading of 15 mg cm-2 showed an initial capacity of 14.7 mAh cm-2, it did not operate for more than 10 cycles. The highly porous structures and insufficient cycling stability of previous HSLCs are not favored from the perspective of consumer applications. Very recently, Manthiram et al. noted that the three critical parameters of high sulfur loading (mg cm-2), a high sulfur content (wt%), and a low electrolyte/sulfur ratio (L mg-1) should be simultaneously addressed for practical Li-S batteries.39 The loading of sulfur electrodes should be increased simultaneously without overusing the electrolyte and conductive additives.40-42 However, many previous works on high-sulfur-loaded Li-S batteries have focused on a single parameter.43-45 Therefore, of great importance is a scalable fabrication route to realize high-quality crack-free HSLCs while also addressing the abovementioned critical parameters with acceptable cycling stability. Here, we present a novel fabrication route by which to realize highly packed and crack-free HSLCs, referred to as scaffold-supported drying (SSD). It features the covering of the cast of the cathode slurry with a mesh-type scaffold, which prevents volume shrinkage of the cast skin and consequently suppresses crack formation during the drying process. As described in Scheme 1, the cathode slurry is cast on a polyethylene terephthalate (PET) film and a porous scaffold is placed on the top of the wet cast. After drying, the dried electrode is removed from 4 ACS Paragon Plus Environment

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the PET substrate, and an HSLC having the scaffold on its one side is obtained. As a porous scaffold, a metal mesh (Figure S1) and carbon paper (Figure S2) were used in this work, considering their high porosity, high electric conductivity, and good mechanical integrity. Even without the use of an excessive carbon framework for the HSLC (64% sulfur content), high sulfur loading (up to 16 mg cm-2) and a high packing density (active mass density of 0.727 gSulfur

cm-3 and porosity of 53%) could be obtained with the proposed fabrication strategy. The

crack-free and highly packed structure guarantees a low electrolyte uptake (E/S ratio as low as 4) and close contact between the sulfur/carbon composite particles, resulting in effective electron transport and high sulfur utilization.

Scheme 1. Schematic illustration of the fabrication of HSLCs using the scaffold-supported drying method

EXPERIMENTAL SECTION Synthesis of the sulfur/carbon composite slurry. To synthesize the sulfur/carbon composite slurry, 75 wt% sulfur (>99.5%, Sigma-Aldrich) and 25 wt% Ketjenblack (EC 600JD, Infochems Co.) were placed in a 30 ml vial. Sulfur and Ketjenblack composites were prepared via a melt-diffusion approach at an ambient temperature at 155 C for 12 h. Next, the sulfur/carbon composite slurry (64% sulfur content) was prepared by mixing a 79.6 wt% sulfur/carbon composite, 10.4 wt% binder (polyethylene oxide (PEO, Sigma-Aldrich, 5wt%), 5 ACS Paragon Plus Environment


polyvinyl pyrrolidone (PVP, Sigma-Aldrich, 5wt%), carboxymethyl cellulose (CMC, LG Chem.), 0.4wt%), and 10 wt% vapor-grown carbon fiber (VGCF, Infochems) in DI water. Three types of binders (PEO, PVP, and CMC) are used simultaneously because they have shown the best results with Ketjenblack/Sulfur composite in our laboratoy. The ratio was empirically chosen. The slurry was then stirred for 24 h and used in the subsequent casting process. Fabrication of sulfur electrodes. For a conventional aluminum-based electrode, the stirred slurry was cast onto aluminum foil (LG Chem.), dried at 60C overnight, and cut into disks with diameters of 12mm. For a scaffold-dried electrode, the stirred slurry was cast onto a thoroughly washed polyethylene terephthalate (PET, 100 m thickness, SKC Co.) substrate (66 cm). Used as the scaffold for the solvent-drying procedure, a mesh-type current collector (66 cm) was dropped gently on top of the wet slurry. The intermediate product was then dried at 60 C overnight. Finally, the PET substrate was removed gently from the intermediate product and the crack-free electrode on a mesh-type current collector was obtained. Cell assembly. Here, 2032 coin cells were assembled in an Ar-filled glove box. A spacer (100 m), a sulfur cathode, a Celgard 2400 separator (26 m), and Li metal foil (450 m thickness, Honjo Metal) were stacked in the 2032 coin cells. An electrolyte consisting of 1 M bis(trifluoromethanesulfonyl)imide lithium (LiTFSI, Sigma-Aldrich) and 0.5 M of lithium nitrate (LiNO3, Sigma-Aldrich) in a mixture of 1,3-dioxolane (DOL, Sigma-Aldrich) and 1,2dimethoxyethane (DME, Sigma-Aldrich) (1:1, v/v) was added to the cells prior to sealing. 0.5 M LiNO3, which is the amount normally adopted in many papers that use high-sulfur-loaded cathodes, was added to protect lithium anodes from large amount of polysulfides in electrolytes from high-sulfur-loaded cathodes. The electrolyte/sulfur ratios for the cells were usually 12 L mg-1 to prevent early electrolyte depletion by Li metal corrosion during long cycling. For


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demonstrating the operation at low electrolyte/sulfur ratio, the ratio was varied as 7 and 4 L mg-1. Electrochemistry

measurements

and

materials

characterization.

Galvanostatic

charge/discharge tests were performed using a TOSCAT-3000U device (Toyo System) at 25 C. For an electrochemical impedance spectroscopy (EIS) analysis, a three-electrode pouch cell was utilized. A sulfur electrode was used as a working electrode and lithium metal foil was laminated onto SUS foil as both a counter and a reference electrode. Two lithium electrodes (22 cm), two sheets of separators (33 cm), and a sulfur electrode (12mm in diameter) were stacked in an Al-laminated pouch with the abovementioned electrolyte. Lastly, the pouch cell was thoroughly sealed. EIS measurements were conducted using a Solartron 1255; the AC amplitude was 10 mV and the frequency range was from 1 MHz to 5 mHz. To study the changes in the morphologies of the sulfur cathodes, pre-cycle sulfur cathodes and sulfur cathodes cycled at 0.1 C were prepared by an aluminum-based conventional approach and the scaffold-supported solvent drying approach, respectively. The cycled sulfur cathode was charged at the end of the 80th cycle and disassembled. Next, the disassembled sulfur cathodes were rinsed gently with 1,2-dimethoxyethane (DME, Sigma-Aldrich) and subsequently subjected to an analysis of the surface and the cross-section with a field-emission scanning electron microscope (FE-SEM, Sirion, FEI). Surface characterization of sulfur cathodes was conducted with by means of X-ray diffraction spectroscopy (XRD) to detect the irreversible amount of Li2S. XRD was conducted on a highresolution powder X-Ray diffractometer (Smartlab, RIGAKU) with a Cu Kα radiation source at a scan rate of 5 °/min. Aluminum-based conventional electrodes and crack-free electrodes were cycled at 0.1 C for 10 and 80 cycles, respectively. Fully charged at the end of the 10th and the 80th cycles, these sulfur electrodes were disassembled and rinsed thoroughly with DME to remove any electrolyte residue. To avoid contact with air or moisture, the cycled electrodes 7 ACS Paragon Plus Environment


were thoroughly sealed with Kapton tape. All the analyte preparation steps were performed in an Ar-filled glove box.

RESULTS AND DISCUSSION Conventionally, sulfur cathodes are fabricated by casting a sulfur cathode slurry onto a current collector, with a subsequent solvent evaporation step. During the conventional drying process, electrode deterioration often occurs due to local tension stress, and it intensifies with an increase in the thickness of the cast.28-29 As shown in Figure 1, an increase in the areal sulfur loading (6 mg cm-2 for Figures 1a, b and 10 mg cm-2 for Figures 1c, d) typically accompanies crack formation and delamination of the sulfur, which emphasizes the challenge during the fabrication of crack-free HSLCs. To demonstrate the crack-free fabrication of HSLCs with the SSD method, two sulfur cathodes with areal sulfur loadings of 6 and 16 mg cm-2 were fabricated using a metal mesh. These are denoted as s-HSLC/MM-6 and s-HSLC/MM-16, respectively. As shown in the SEM images (Figure 1e-l), HSLC/MMs were highly integrated without any cracks, in sharp contrast to those after the conventional drying method (Figure 1a-d). The electrode thicknesses were 110 m for s-HSLC/MM-6 and 220 m for s-HSLC/MM-16. The sulfur cathode and mesh were interconnected, ensuring tight interfacial bonding. Upon electrode punching, detachment of the sulfur cathode at its edge did not occur, as typically shown in the optical image of s-HSLC/MM16 (Figure S3). The effects of the porous scaffold are twofold. The scaffold can prevent aggressive volume shrinkage of the cast skin and lessen the stress induced by the shrinkage rate difference, suppressing crack formation. It also retards solvent evaporation on the skin, inducing a compact packing of the cathode materials. Therefore, SSD enables close contact between the sulfur/carbon composite particles as well as between the cathode and the current collector, preventing the deterioration of HSLCs (Scheme 2). 8 ACS Paragon Plus Environment

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Figure 1. SEM images of c-HSLC-6: a) top surface, b) cross-section and of c-HSLC-10: c) top surface, d) cross-section). SEM images of s-HSLC/MM-6: e) top surface, f) cross-section, and g), h) backside and of s-HSLC/MM-16: i) top surface, j) cross-section, and k), l) backside

It is highly meaningful that without any changes of the electrode material or composition, densely packed and crack-free HSLCs can be fabricated by simply placing a porous scaffold onto one surface. Due to their crack-free structure, the s-HSLC/MMs showed a porosity rate of 53%, which was 5% lower than that of the control sample fabricated by the conventional drying method (c-HSLC) (Table S1). The pore size distributions of c-HSLC-6 and s-HSLC/MM-6 as measured using mercury porosimetry are compared in Figure S4. The pore volume below 15 m was somewhat smaller for s-HSLC/MM due to a slower solvent evaporation rate and consequent dense packing of the solid components. The pore volume from large pores (> 15 m) originating from cracks was notably smaller for s-HSLC/MM. The compact cathode 9 ACS Paragon Plus Environment


structure is practically important as it reduces the amount of electrolyte needed to fill the cathode pores. The porosity of s-HSLC/MM is even lower than those of the previously reported HSLCs (> 70%)27-29 because it does not include any 3D carbon frames inside the electrode. Therefore, our highly packed structure provides the additional benefit of an increasing specific energy density level.

Scheme 2. Schematic illustration of different solvent drying mechanisms: a) conventional drying and b) scaffold-supported drying

To assess the advantages of the SSD method in enhancing the electrochemical performances of Li-S batteries, a galvanostatic cycling test was conducted for s-HSLC/MM-6 and c-HSLC6. At a sulfur loading rate of 6 mg cm-2, the conventional HSLC with an aluminum current collector (c-HSLC-6) could be fabricated without severe delamination. However, at 16 mg cm-2, the corresponding control (c-HSLC-16) underwent electrode deterioration and delamination from the current collector. Therefore, s-HSLC/MM-6 and c-HSLC-6 were selected for comparison. After three formation cycles, s-HSLC/MM-6 showed an initial capacity of 818 mAh g-1 and stable cycling at 0.92 mA cm-2 with a high capacity retention rate of 78% (680 mAh g-1) at the 150th cycle (Figure 2a). The coulombic efficiencies for the 1st cycle and the 100th cycle are maintained as high as 99.47% and 95.71%, respectively (Figure S5). For c-HSLC-6, the discharge capacities were increased with fluctuations during the early and late cycles, as 10 ACS Paragon Plus Environment

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observed in previously reported HSLCs.38 This behavior can be attributed to the redistribution of sulfur species over the carbon matrix.22 At the 80th cycle, c-HSLC-6 delivered merely 500 mAh g-1, which is lower by 234 mAh g-1 than s-HSLC/MM-6. The discharge capacities of sHSLC/MM and c-HSLC were also compared at difference rates ranging from 0.8 to 8 mA cm2

(Figure 2b) in order to assess their rate capabilities. The c-HSLC exhibited limited capacities

of 811, 153, and 45 mAh g-1 at 0.8, 1.6, and 8.0 mA cm-2, respectively. The capacities of cHSLC dropped by 94% when the current densities increased from 0.8 to 8.0 mA cm-2. In contrast, s-HSLC/MM exhibited superior rate performance at various C-rates, showing capacities of 812, 639, and 372 mAh g-1 at 0.8, 1.6, and 8.0 mA cm-2, respectively. The sHSLC/MM showed not only a smaller capacity drop of 46% when the current densities increased from 0.8 to 8.0 mA cm-2 but also 417% and 827% greater capacities than the c-HSLC at 1.6 and 8.0 mA cm-2, respectively. The typical charge and discharge voltage profiles of s-HSLC/MM-6 and c-HSLC-6 were compared at the 3rd and 80th cycle, as shown in Figure 2c, d. Two conventional plateaus of Li-S are clearly indicated in Figures 2c, d. The discharge curve of s-HSLC/MM-6 exhibited a less polarized lower plateau than that of c-HSLC-6. Furthermore, after 80 cycles, the lower plateau of s-HSLC/MM-6 (Figure 2c) remained nearly invariant, while that of c-HLSC-6 (Figure 2d) was significantly decreased by 41%. The higher stability of the lower plateau of s-HSLC/MM-6 suggests enhanced dimensional stability of the carbon matrix. The discharge capacity ratio of the lower (2.1V) and higher (2.3V) plateaus, which is theoretically 3, indicates the conversion of the formed polysulfides to solid discharge products (Li2S or Li2S2). At the 80th cycle, the capacity ratio was remained as high as 2.72 for the s-HSLC/MM-6 sample, 35% higher than that for c-HSLC-6 (2.01). This indicates that s-HSLC/MM-6 achieved an enhanced conversion of polysulfides to solid discharge products. Upon cycling, s-HSLC/MM-6 successfully converted most of the dissolved polysulfides to solid discharge products, while c-HSLC-6



experienced early passivation of the carbon matrix by Li2S due to the deterioration of the carbon matrix. To examine the superior performance outcomes of the s-HSLC/MM electrode in more depth, three-electrode pouch cells were prepared with s-HSLC/MM-6 and c-HSLC-6, as illustrated in Figure S6, and an electrochemical impedance spectroscopy (EIS) analysis was conducted with the pouch cells. Nyquist plots of the impedances before and after 50 cycles are compared for sHSLC/MM-6 and c-HSLC-6 in Figures 2e, f, respectively. Nyquist plots that compare the impednaces before and after 50 cycles only for s-HSLC/MM-6 are shown in Figure S7. Four different impedance values could be observed in the Nyquist plots of s-HSLC/MM-6 and cHSLC-6. First, the intercept on x-axis corresponds to the ohmic resistance that is associated with the lithium ion migration resistance through electrolyte phase. Second, the high-frequency semicircles at 3200 Hz often referred to as Rpore represent the Li ion migration through the porous sulfur cathode. Third, the semicircles observed at 2800 Hz are related to the charge transfer resistance (Rct) of soluble polysulfide. Finally, the semicircles detected at low frequency (10 Hz) reflect the charge transfer resistance (Rct) through the solid-state Li2S.46 Nyquist plots measured before and after cycle showed significantly smaller semicircles for sHSLC/MM-6 (Figure 2e) over the entire regions. The contact between C/S composite active materials and between the cathode and the current collector brought about such differences. More interestingly, there is a big difference in the tendency of the changing semicircles after cycle. While the Rpore (3200 Hz) and Rct (2800 Hz) impedance factors for s-HSLC/MM-6 did not change significantly after cycle, the impedance factors for c-HSLC-6 decreased notably over 50 cycles due to the dissolution of polysulfides into vacant pore volume between C/S composites.22 Furthermore, in contrast to s-HSLC/MM-6, c-HSLC-6 showed a significant increase from 12 to 21  cm2 in the charge transfer resistance at 10 Hz, which is related to the irreversible Li2S charge transfer resistance (Figure 2f). This can be understood in terms of the electrical network of carbon matrix; electrode cracking disconnects the electrical network in the 12 ACS Paragon Plus Environment

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electrode, increasing the electrically disconnected inactive portion. Therefore, the welldeveloped electrical network enabled by the SSD method preserves the uniform active sites for the formation of solid discharge products, easing the charge transfer reaction. The accumulation of irreversible Li2S, which remains undecomposed upon charging up to 2.8 V, is regarded as a cause of capacity fading. At higher sulfur loadings, the formation of irreversible Li2S can become more severe due to the inhomogeneous electrochemical reaction and deterioration of the electrode structure. The formation of irreversible Li2S was traced via the X-ray diffraction (XRD) of a fully charged s-HSLC/MM-6 (Figure 2g) and c-HSLC-6 (Figure 2h) samples after 10 and 80 cycles.47



Figure 2. a) Cycling performances (charge/discharge current density: 0.92 mA cm-2) and b) rate capability test results when varying the current density from 0.8 to 8 mA cm-2 for the sHSLC/MM-6 and c-HSLC-6 samples. The discharge/charge voltage profiles at the 3rd and 80th cycles for c) s-HSLC/MM-6 and d) c-HSLC-6. Nyquist plots of s-HSLC/MM-6 and c-HSLC6 e) before and f) after 50 cycles. X-ray diffraction patterns of g) s-HSLC/MM-6 and h) cHSLC-6 at the end of the 10th and 80th charge. (★ and ▼ indicate the peaks from the metal mesh and Li2S, respectively.)

The assignment of the peaks was made based on the XRD patterns of each ingredient (Figure S8). For c-HSLC-6, the Li2S peaks clearly increased with an increase in the number of cycles, indicating the accumulation of irreversible Li2S.48,49 In contrast, s-HSLC/MM-6 did not show any appreciable increase in the characteristic Li2S peaks, which is a feature of enhanced reversibility. The cycling stability, EIS and XRD analysis outcomes collectively suggest that the SSD method improves the mechanical integrity of sulfur cathodes, providing significantly higher reversibility. In order to confirm the high mechanical integrity, a post-mortem analysis was conducted for s- and c-HSLC-6 after 80 cycles. As shown in Figure S9, c-HSLC-6 features an increased number of cracks which were also larger upon cycling, whereas s-HSLC/MM-6 preserved its crack-free morphology even after cycling. Stable contact with the current collector, which is the mesh metal, could also be shown in Figure S10. Furthermore, s-HSLC/MM-6 maintained its close contact with the mesh current collector (Figure S11).


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Figure 3. a) Cycling performance of s-HSLC/MM-16 (charge/discharge current density: 1.34 mA cm-2). b) Comparison of the areal sulfur loading and volumetric capacity for s-HSLC/MM16 and sulfur electrodes reported in recent publications with sulfur loading rates in the range of 10-25 mg cm-2.

The advantage of the crack-free structure is highlighted with a higher sulfur loading of 16 mg cm-2. As described earlier, the conventional preparation method did not allow the fabrication of a mechanically integrated cathode upon sulfur loading, which prevented any electrochemical investigation of c-HSLC-16. For s-HSLC/MM-16, a galvanostatic cycling test was conducted at a current density of 1.34 mA cm-2. This current density was selected to prevent the deterioration of the lithium metal anode and to avoid large electrode polarization.35-38 The N/P ratio of 4 was applied to accommodate severe Li-metal degradation. Figure 3a shows the galvanostatic cycling result for s-HSLC/MM-16. It is interesting that the s-HSLC/MM-16 cell exhibited an initial areal-specific capacity of 11.5 mAh cm-2, an initial specific capacity of 726 mAh g-1, and a high initial volumetric capacity of 523 mAh cm-3. Within the first 10 cycles, the discharge capacity decreased and then re-increased; however, as explained in a previous 15 ACS Paragon Plus Environment


study,29 this behavior results from the gradual electrolyte-wetting of the thick electrode and the activation of the active mass. More importantly, the cell showed a high capacity retention rate of 90% up to 78 cycles, indicating the preservation of its mechanical integrity during the cycling process. However, after 78 cycles, s-HSLC/MM cell exhibited sudden capacity fading. To delineate the cause of the cell failure after 78 cycles, a post mortem analysis was conducted, as described in the work of Qie et al.50 Preservation of mechanical integrity of sHSLC/MM-16 with significant deterioration of the lithium metal anode was observed in the cycled cell (Figure S12). After reassembling the cycled s-HSLC/MM-16 with a fresh lithium metal anode and electrolyte, regaining of the coulombic efficiency was observed (Figure S13). From these results, we concluded that the sudden death of the Li-S cell can be attributed to the harsh stripping and plating of the lithium metal and the decomposition of the electrolyte. To gauge the efficacy of the SSD method, the high-sulfur-loaded sulfur electrodes from recent publications were compared with those fabricated by SSD in terms of the areal sulfur loading and volumetric capacity. In consideration of both capacity and cycle stability, the volumetric capacities at the final cycle were compared. Figure 3b compares the volumetric capacity as a function of the areal sulfur loading rate for this work and previous works, which utilized a sulfur loading rate in the range of 10-25 mg cm-2.51-55 The high volumetric capacity highlights the benefit of the dense electrode structure derived by the SSD method.


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Figure 4. a) Schematic illustration of the s-HSLC/CP and c-HSLC/CP samples. SEM and optical images of the s-HSLC/CP-12: b) surface, c) cross-section) and of the c-HSLC/CP-12: d) surface, e) cross-section. f) Cycling performances of s-HSLC/CP-12 and c-HSLC/CP-12

Although a SUS mesh (350 mesh) of 19.29 mg cm-2 was employed for proof of concept, a lighter material is preferred as a scaffold to increase the energy density. To generalize the SSD 17 ACS Paragon Plus Environment


method, an HSLC was fabricated with a light carbon paper (CP) (3.27 mg cm-2), the specifications of which are provided in Table S2. CP, which is widely used as a gas diffusion layer in fuel cells, has high porosity and good mechanical integrity. It should be noted that the CP here consists of 10-m-thick carbon microfibers, which differs in structure from a nanotubebased or a nanofiber-based 3D carbon frame.36 One sulfur cathode was made using the conventional drying method (c-HSLC/CP) and the other was created using the SSD method (sHSLC/CP) while employing the CP as a current collector, as illustrated in Figure 4a. To form the c-HSLC/CP sample, the cathode slurry was cast onto the CP and dried with a free surface, and to fabricate the s-HSLC/CP sample, it was cast onto PET film and dried with a CP cover. With the CP, a crack-free HSLC with a sulfur loading rate of 12 mg cm-2 (s-HSLC/CP-12) was successfully fabricated, as shown in Figures 4b, c. In contrast, a control electrode with a sulfur loading rate of 12 mg cm-2 (c-HSLC/CP-12) showed large cracks on its surface (Figures 4d, e). The comparison of the cycling stability outcomes between the two HSLC/CPs again demonstrates the advantage of the SSD method. The HSLC from the conventional drying showed cell failure at only the 10th cycle, whereas that from SSD exhibited an initial specific capacity of 829 mAh g-1 at 1 mA cm-2 and capacity retention of 94.6% (784 mAh g-1) at the 42nd cycle (Figure 4f). The charge capacities and voltage profiles of the 5th and the 8th cycles are shown in Figure S14, S15, S16, respectively. Both discharge curves of s-HSLC/CP-12 at 5th and 8th cycle exhibited a less polarized plateau than that of c-HSLC/CP-12. This result suggests that various types of porous scaffolds can be used for designing HSLCs with the SSD method.


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Figure 5. Cycling performances of s-HSLC/CP samples with E/S ratios of 4 and 7 (charge/discharge current density: 0.8 mA cm-2). The corresponding charge capacities and voltage profiles of the 5th cycle are provided in Figure S17 and Figure S18, respectively.

The compact and crack-free sample here enables stable cycling even at low electrolyte/sulfur ratios considering that the electrolyte phase in the crack does not effectively contribute to the electrochemical reaction. The cycling stability test for the s-HSLC/CPs with low E/S ratios of 4 and 7 (Figure 5) showed that the initial discharge capacities did not vary significantly with the E/S ratio; however, the cycling stability was enhanced after lowering the E/S ratio, contrary to previous reports. This indicates that an E/S ratio of 4 is just enough to allow a facile redox reaction in the compact, crack-free electrode owing to the absence of electrolyte loss by cracking. With an increase in the E/S ratios, polysulfide dissolution into the excess electrolyte phase outside the s-HSLC/CP may increase, causing more severe capacity fading. We observed that below an E/S ratio of 4, the discharge capacity was decreased due to large polarization. However, the minimum E/S ratio for stable cycling is clearly lower for s-HSLC compared to the other types of crack-free HSLCs.29 These results also suggest that the need for a large amount of electrolyte for the stable operation of HSLCs is partly attributed to cracking in sulfur cathodes. By achieving a highly uniform and compact structure and not wasting electrolyte by filling cracks, the amount of electrolyte for the facile redox reactions of sulfur species can be



reduced. Therefore, we conclude that the s-HSLC would also be effective in a lean electrolyte system, making it feasible to use in the design of energy-dense Li-S batteries.

CONCLUSION In summary, we proposed a scaffold-supported drying method which enables the fabrication of crack-free, compact sulfur cathodes at ultra-high sulfur loadings (up to 16 mg cm-2). With two types of porous scaffolds (metal mesh and carbon paper), the efficacy of the SSD method to achieve high-quality sulfur cathodes at high sulfur loadings was demonstrated. Due to the prevention of volume shrinkage on the top surface, the resulting sulfur cathodes did not show cracks or delamination from the current collector for both a metal mesh and a carbon paper scaffold. A sulfur cathode of 16 mg cm-2 fabricated by SSD exhibited high initial capacities of 11.5 mAh cm-2 and 523 mAh cm-3 with 90% capacity retention after more than 70 cycles. Due to the compact, crack-free electrode structure, a volumetric capacity of 523 mAh cm-3 was achieved with a sulfur content of 64%. Given that the compact, crack-free electrode structure effectively confines the electrolyte and dissolves the active mass in the electrochemically active area of the electrode without waste to fill cracks, the Li-S cell with a low E/S ratio of 4 showed a high initial capacity of 938 mAh g-1 and capacity retention of 79% over 60 cycles. The impedance, XRD, and the results of a postmortem analysis demonstrated the high mechanical durability of the sulfur cathodes. Therefore, the scaffold-supported drying method and resulting crack-free, compact electrode structure offer a practically viable route to high-energy-density Li-S batteries.


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ASSOCIATED CONTENT Supporting Information SEM images and phogo images of electrolyte, pore size distribution of electrode, shematic illustration of 3-electrode pouch cell, XRD of SUS electrode, physical property of carbon paper. This material is available free of charge on the ACS

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel.: +82-42-350-3916; Fax: +82-42-350-3910 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Technology Develpoment Program to Solve Climate Changes through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (2018M1A2A2063807) and by the KAIST Institute for Nano-Century (KINC).



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Rational Design of Highly Packed, Crack-Free Sulfur Electrodes by

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