S Cathode for Lithium

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Three-Dimensionally Hierarchical Ni/ Ni3S2/S Cathode for Lithium–Sulfur Battery Zhe Li, Shiguo Zhang, Miao Xu, Ryoichi Tatara, Kaoru Dokko, Masayoshi Watanabe, and Jiaheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11065 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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ACS Applied Materials & Interfaces

Three-Dimensionally

Hierarchical

Ni/Ni3S2/S

Cathode for Lithium–Sulfur Battery Zhe Li, Shiguo Zhang, Jiaheng Zhang, Miao Xu, Ryoichi Tatara, Kaoru Dokko, and Masayoshi Watanabe* Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Keywords: Lithium–sulfur battery, metal sulfide, sulfur, polysulfide, 3D cathode.

1 ACS Paragon Plus Environment


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Abstract

Lithium–sulfur (Li–S) batteries have attracted interest as a promising energy-storage technology due to their overwhelming advantages such as high energy density and low cost. However, their commercial success is impeded by deterioration of sulfur utilization, significant capacity fade, and poor cycle life, which are principally originated from the severe shuttle effect in relation to the dissolution and migration of lithium polysulfides. Herein, we proposed an effective and facile strategy to anchor the polysulfides and improve sulfur loading by constructing a threedimensionally hierarchical Ni/Ni3S2/S cathode. This self-supported hybrid architecture is sequentially fabricated by the partial sulfurization of Ni foam by a mild hydrothermal process, followed by physical loading of elemental sulfur. The incorporation of Ni3S2, with high electronic conductivity and strong polysulfide adsorption capability, can not only empower the cathode to alleviate the shuttle effect, but also afford a favorable electrochemical environment with lower interfacial resistance, which could facilitate the redox kinetics of the anchored polysulfides. Consequently, the obtained Ni/Ni3S2/S cathode with a sulfur loading of ~4.0 mg/cm2 demonstrated excellent electrochemical characteristics. For example, at high current density of 4 mA/cm2, this thick cathode demonstrated a discharge capacity of 441 mAh/g at the 150th cycle.

Introduction In recent times, the ubiquitous Li-ion batteries (LIBs) have significantly altered the manner in which we interact with people and information. Although LIBs have played a major role in revolutionizing personal electronics devices, their energy densities (no more than 387 Wh/kg) are not sufficient for powering transportation (such as electric vehicles) and large-scale grid-based energy storage systems.1-2 As one of the most promising next-generation rechargeable systems,


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lithium–sulfur (Li–S) batteries, which conventionally consist of a metallic lithium anode, an organic electrolyte, and a sulfur cathode, have attracted ever-increasing attention.3-8 In addition to its low cost, natural abundance, and environmental benignancy, sulfur is also electrochemically active. It can provide a theoretical specific capacity of 1672 mAh/g and a theoretical energy density of 2600 Wh/kg when fully converted to Li2S (16Li + S8→ 8Li2S); these values are considerably higher than those of the currently available LIBs.9-11 Despite remarkable advances in Li-S systems, there remain several challenges that hinder their commercial success. For example, lithium anodes with high reactivity suffer from drawbacks like non-uniform solid electrolyte interphase (SEI) and dendritic growth, resulting in poor cycling stability and potential safety issues.12-13 Recently, our group proposed strategies to tackle this challenging issue by adopting the Li2S cathode to combine with Li-free anode (graphite or silicon).14-16 Additionally, sulfur and its discharge product, Li2S, are electronically and ionically insulating. To achieve the necessary electronic conductivity at the cathode and to accommodate the volumetric expansion (~80%) from S8 to Li2S, suitable carbons with high conductivity and porosity have been explored and incorporated into the sulfur cathode.13, 17-18

However, those low-density carbon additives, such as porous carbon, carbon nanotubes,

carbon hollow spheres, and graphene, could increase the electrolyte uptake and limit the energy density of the cell.10, 18-19 The most critical issue facing Li–S systems is that polysulfide intermediates are prone to dissolve into the ether solvent-based electrolyte, leading to a redox shuttle phenomenon.20-21 For instance, the dissolved lithium polysulfides (Li2Sn) diffuse to the Li anode and can be chemically reduced to low-order polysulfides. This undesired shuttle effect could give rise to a pronounced loss of active material, rapid capacity decay, anode corrosion, low Coulombic efficiency, and poor cycling performance.20 Unfortunately, traditional cathodic carbon supports are not very effective at



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alleviating the shuttle effect by restricting the polysulfides within the carbon barrier.19, 22 This is because the affinity between the nonpolar carbon surfaces and polar polysulfide intermediates is weak, and the physically confined polysulfides will be eventually transported to the electrolyte forced by the chemical potential gradient.23-25 In order to circumvent the shuttle effect, functional carbon interlayers, modified separators, novel electrolytes, and electrolyte additives have been developed, with the aim of suppressing the diffusion and dissolution of lithium polysulfides.20-21, 26-35

Another effective approach, quite different from the aforementioned strategies, is to rationally design the matrix materials with “sulfiphilic” surfaces as alternative cathode ingredients.19, 22, 36-39 In this case, the generated polysulfide molecules can be chemically adsorbed and anchored at the sulfiphilic surfaces in situ, making them available for the electrochemical redox reactions.22-23 For example, the cathodic carbon matrix can be functionalized by doping nitrogen, oxygen, sulfur, and boron atoms, which could facilitate the polysulfide adsorption due to the strong interaction between the heteroatoms and Li2Sn.40-41 In addition, studies have also found that metal oxides, metal sulfides, and metal-organic frameworks could work as efficient polysulfide adsorbents to inhibit the shuttle effect and enhance the capacity retention during cycling.23-25, 37, 41-48 Nevertheless, some of these materials are electronic insulators, which will hamper the redox characteristics of the directly trapped polysulfide species, and of course, deteriorate the ability to obtain a highloading sulfur cathode.24-25 It is notable that the sulfur loadings for most of the reported metal sulfide (or oxide)-based cathodes are usually low (0.3–1.5 mg/cm2),42 which will reduce the energy density of the Li-S batteries. Hence, matrix materials that exhibit both strong polysulfide adsorption capability and good conductivity are of significance to accelerate the redox kinetics of the adsorbed polysulfides and improve the cell performance.


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Nickel sulfides have been extensively studied as cathode materials during the early-stage development of rechargeable lithium batteries due to their chemical stability, high electronic conductivity, attractive electrochemical properties, and good compatibility with organic solvents.49-50 Motivated by this knowledge, for the first time, we chemically incorporated nickel sulfide (Ni3S2) into the sulfur cathode design in this study. In order to sequestrate the polysulfide and achieve high sulfur loading, a 3D hierarchical and porous Ni/Ni3S2/S cathode is constructed by partially sulfurizing the commercial Ni foam through a facile hydrothermal method, followed by physical loading of the elemental sulfur. The resultant Ni/Ni3S2/S hybrid cathode functions in multiple aspects. First, the innermost Ni metal provides the electron transport pathways and effectively increases the cathode conductivity. Second, density functional theory (DFT) calculations demonstrate that the introduced Ni3S2 has stronger binding interactions with polysulfides than traditional carbon, and can prevent the dissolution of polysulfides from the electrode surface. Third, the Ni3S2 which is formed in situ on the Ni metal surface has a fairly low resistivity (1.8 × 10−5 Ω▪cm at room temperature),51-52 which could benefit the redox reactions of the adsorbed polysulfides. Fourth, compared with traditional carbon matrices, Ni3S2 with a higher density (5.8 g/cm3) could improve the entire density of the cathode materials and hence decrease the electrolyte uptake. Lastly, the high porosity of this self-supported cathode architecture could permit high sulfur loading and accommodate the volume change of the active material during cycling. As a result, even without any binder, this hierarchical Ni/Ni3S2/S cathode with a sulfur loading of 4.0 mg/cm2 still exhibits excellent electrochemical performance with higher sulfur utilization, faster reaction kinetics, and better cycling performance than the cathode without Ni3S2. Results and discussion



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The schematic two-step procedure for the synthesis of 3D Ni/Ni3S2/S hybrid cathode is shown in Fig. 1. The sulfurization of the commercial Ni foam is realized via a facile hydrothermal process. More specifically, a piece of cleaned Ni foam (NF), elemental sulfur, and hydrazine monohydrate solution were taken in a Teflon-lined autoclave. The sealed autoclave was maintained at 160 °C for 18 h, followed by natural cooling to room temperature. By this procedure, with the assistance of hydrazine, the reaction between Ni metal and sulfur can generate Ni3S2, in situ, on the surface of the Ni foam. After washing by ethanol and water and drying in vacuum, the Ni/Ni3S2 foam (denoted as NSF) was obtained. As shown in Fig. 2, the color of the foam changed from gray to brown, due to the Ni3S2 coating. Here, the areal loading of Ni3S2 is around 1.5 mg/cm2. To ensure the necessary electronic conductivity of the outermost sulfur layer of this hybrid cathode, elemental sulfur was mixed with a small amount of carbon black (weight ratio of 71:29), and heated in a sealed vial at 155 °C for 6 h (melt–diffusion method). The as-prepared sulfur composite was dispersed in N-methyl-2-pyrrolidinone (NMP) to form a slurry, which was then loaded onto the surface of the Ni/Ni3S2 foam by an absorption method. Eventually, the Ni/Ni3S2/S cathode (denoted as NSF-S) was obtained. X-ray photoelectron spectroscopy (XPS) was carried out to elucidate the surface composition of NSF (Fig. 3). The Ni 2p spectra reveal two signals at 856.1 and 873.7 eV, which can be attributed to the Ni 2p3/2 and Ni 2p1/2 in Ni3S2, respectively, while the satellite peaks associated with Ni 2p3/2 and Ni 2p1/2 appeared at 861.7 and 879.7 eV, respectively.53 In the case of the S 2p spectra, the peaks at 162.7 and 163.9 eV, characteristic of S2-, could be assigned to the S 2p3/2 and S 2p1/2 in Ni3S2, respectively.53-54 X-ray diffraction (XRD) was also used to investigate the chemical composition and crystal structure of the NF, NSF, and NSF-S samples (Fig. 4). For NF, the diffraction peaks at 44.4, 51.7, and 76.3 can be indexed as the (111), (200), and (220) reflections


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of cubic Ni (JCPDS 65-2865), respectively. After sulfurization, the NSF sample exhibited the same three diffraction peaks of NF, while the other five observed peaks can be assigned to the (101), (110), (003), (211), and (122) reflections of rhombohedral Ni3S2 (JCPDS 44-1418), clearly revealing the conversion of Ni into Ni3S2. In the case of NSF-S, in addition to the diffraction peaks originating from NSF, all the other peaks correspond to the diffraction pattern of orthorhombic sulfur (JCPDS 08-0247). Thus, the XRD results strongly confirm the successful construction of the hierarchical Ni/Ni3S2/S hybrid cathode.

Fig. 1. Schematic illustration of the synthesis of the Ni/Ni3S2/S hybrid cathode.



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Fig. 2. Optical photographs of the Ni foam and Ni/Ni3S2 foam.

Fig. 3. High-resolution XPS spectra of Ni 2p (a) and S 2p (b) of NSF.


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Fig. 4. XRD patterns of NF, NSF, and NSF-S.

Fig. 5 exhibits the field emission scanning electron microscopy (FESEM) images of NF and NSF. The NF samples possess a three-dimensional ductile network structure with abundant porous spaces (Figs. 5a and b), which are well retained after sulfurization (Figs. 5d and e). At high magnification, it is observed that the surface of NF (Fig. 5c) is smooth, whereas a rough/bumpy surface morphology is observed for NSF (Fig. 5f), resulting from the formation of Ni3S2. Energydispersive X-ray spectroscopy (EDX) elemental mappings of NSF clearly demonstrate a homogeneous distribution of nickel (Fig. 5h) and sulfur (Fig. 5i) throughout the NSF (Fig. 5g), suggesting that the Ni3S2 is grown uniformly and densely on the surface of the Ni foam. Also, Ni/S atom ratio is close to 3:2 according to EDX quantification, implying the stoichiometric composition of Ni3S2. After applying the slurry-absorption approach, the sulfur active material can be tightly anchored to the 3D skeleton of the NSF (Fig. 6). As seen in the EDX elemental mappings



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of NSF-S, besides the nickel (Fig. 6e) and sulfur (Fig. 6f) signals, carbon mapping signals (Fig. 6g) from the sulfur-carbon black composite are also well distributed over the selected region (Fig. 6d), verifying the uniform coating of the sulfur layer. It is noteworthy that the porosity of the architecture was maintained in NSF-S, and these porous spaces can channel the electrolyte to wet the cathode material, improving the electrolyte penetration during cycling and cell assembly.

Fig. 5. FESEM images of NF (a, b, and c) and NSF (d, e, f, and g) at different magnifications. EDX elemental mappings of nickel (h) and sulfur (i) over image (g).


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Fig. 6. FESEM images of NSF-S (a, b, c, and d) at different magnifications. EDX elemental mappings of nickel (e), sulfur (f), and carbon (g) over the highlighted region in image (d).

In order to evaluate the electrochemical performance of the obtained NSF-S cathode, 2032-type coin cells were assembled using Li metal as anode and 1.0 M LiTFSA in a binary mixture of 1,3dioxolane (DOL) and 1,2-dimethoxyethane (DME), containing 0.2 M LiNO3 as electrolyte. For comparison, a Ni foam-sulfur (NF-S) cathode was fabricated by directly loading the sulfur slurry onto the unmodified Ni foam under the same conditions. Figs. 7a and b present the charge– discharge profiles of the NSF-S and NF-S cathodes within the potential window of 1.7–2.8 V in the 1st, 2nd, 10th, and 20th cycles at a high current density of 1 mA/cm2. During the discharge process, the typical electrochemical characteristics of Li–S batteries were observed for both NSF-S and NF-S cathodes, with two well-defined plateaus. The upper plateau, at 2.3 V, represents the transitions of sulfur to long-chain lithium polysulfide (Li2Sn, 4 < n < 8), whereas the lower plateau, at 2.1 V, corresponds to the formation of short-chain lithium sulfide and lithium disulfide.13, 18 When the current density was increased from 1 to 2 mA/cm2, the NSF-S cathode displays a relatively lower polarization in comparison with NF-S, as indicated in Figs. 7c and d. The voltage hysteresis between the discharge and charge curves reduced from 0.5 to 0.4 V, which is related to



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the incorporation of Ni3S2 into the sulfur cathode. Moreover, the initial discharge curve of the NSF-S at high current density of 2 mA/cm2 is different from the one at other cycles (Fig. 7c), implying that the electrolyte wetting is improved after the initial cycle and thereafter the batteries operate steadily. The cycling performance and Coulombic efficiency of NSF-S and NF-S cathodes are also compared in Fig. 7e. The NSF-S cathode with a sulfur loading of 4.0 mg/cm2 could deliver an initial discharge capacity of 655 mAh/g at 1 mA/cm2, which is higher than that of the NF-S cathode (518 mAh/g), reflecting an increased utilization of the active material. After 80 cycles, a discharge capacity of 654 mAh/g was preserved for NSF-S, implying virtually no capacity fade, while the discharge capacity of NF-S declines to 480 mAh/g. It should be mentioned that Ni3S2 itself cannot contribute to the capacity of the NSF-S cathode, as shown in Fig. S1 A significant increase in the Coulombic efficiency is also achieved on account of the incorporation of the Ni3S2 layer. At a current density of 1 mA/cm2, the average Coulombic efficiencies of the NSF-S and NFS cathodes are ~95% and ~91%, respectively, over 80 cycles. The enhanced capacity, reduced polarization, and increased energy efficiency of the NSF-S cathode were ascribed to its unique Ni3S2 layer, which can serve as a bifunctional matrix material that combines inherently high conductivity with the good polysulfide adsorption capability to facilitate the redox kinetics of polysulfides, as will be further demonstrated. The electrochemical impedance spectroscopy (EIS) profiles of NSF, NSF-S, and NF-S cathodes before cycling are presented in Fig. 7f, where the diameter of the depressed semicircle in the high frequency reflects the charge transfer resistance (Rct). Obviously, the NSF-S cathode exhibits a lower Rct than NF-S, providing the significantly better electrochemical environment with a higher interfacial conductivity. Hence, by introducing conductive Ni3S2 into the sulfur cathode, charge


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transfer at the electrode–electrolyte interfaces become much faster, and the redox kinetics of the active materials is enhanced.



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Fig. 7. Galvanostatic charge–discharge profiles of the (a) NSF-S and (b) NF-S cathodes in the 1st, 2nd, 10th, and 20th cycles at a current density of 1 mA/cm2. Charge–discharge profiles of the (c) NSF-S and (d) NF-S cathodes at 2 mA/cm2. (e) Cycling performance and Coulombic efficiency of NSF-S and NF-S cathodes over 80 cycles at 1 mA/cm2. (f) Nyquist plots of NSF, NSF-S, and NF-S cathodes before cycling. For both NSF-S and NF-S cathodes, the sulfur (S8) loading was around 4.0 mg/cm2.

In order to validate the interactions between the different matrix materials and the polysulfide intermediates, visualized ex situ adsorption measurements were performed. Li2S4 dissolved in tetrahydrofuran (THF) was employed as a representative lithium polysulfide for static adsorption. The NF and NSF with the same geometric size were immersed in the light yellow Li2S4 solutions as adsorbents. As shown in Fig. 8a, the NSF could capture the polysulfide yielding an almost colorless solution. On the other hand, the NF surface did not have any observable interactions with the polysulfide, because the color of the Li2S4 solution remained the same. This merit of NSF is attributed to the Ni3S2 layer, which affords a relatively strong affinity to the polar polysulfide intermediates at an interfacial scale.


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Fig. 8. (a) Visualized ex-situ adsorption measurement of Li2S4 (in THF) using NF and NSF with the same size. (b) Binding geometry configurations of different lithium polysulfides on the surface of Ni3S2 (yellow=S, red=Li, blue=Ni). (c) Binding energies of the different lithium polysulfides on the Ni3S2 surface ((110) plane) and traditional carbon surface (hexatomic carbon ring-based network).

Density functional theory (DFT) calculations were also conducted using the Vienna ab initio simulation package (VASP) to further elucidate the inherent binding effect of Ni3S2 with polysulfides at the molecular level. A semiempirical DFT-D2 force-field approach was adopted to include the influence of van der Waals interactions in these calculations. When compared with the freestanding lithium polysulfide molecules (Fig. S2), the geometries of the polysulfides trapped



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by the Ni3S2 surface are somewhat distorted (Fig. 8b) and indicative of a strong binding effect. Previously, Zhang et al. pointed out that the chemical binding originates from interactions between the Li atoms in the Li2Sn species and the S (or O) atoms in the two-dimensional layered materials.36 Consistent with this point of view, Li (in Li2Sn) –S (in Ni3S2) binding was clearly observed in the case of Ni3S2. However, for example, in the system of Ni3S2 with Li2S4 (Fig. 8b), we find that the Ni atoms in Ni3S2 prefer to bond with the neighboring S atoms or Li atoms in Li2S4, and the very weak S-S binding between sulfurs in Li2S4 and Ni3S2 also exists. Here, the existence of S-S interaction could also be indicated by other literatures.41, 55 Those induced interactions could possibly increase the binding strength and contribute to the strong capability of Ni3S2 to adsorb polysulfides. For contrast, the binding effect between the polysulfides and the most frequently utilized carbon host material is also investigated, and the corresponding binding geometry configurations are shown in Fig. S3. Since the carbon host with nonpolar C-C bonds does not have a polarized charge distribution, its interaction with polar polysulfides is weak and no obvious distortion of the polysulfide configurations was observed. Fig. 8c compares the binding energies (Eb) on different surfaces. Impressively, the binding energies for S8, Li2S8, Li2S6, Li2S4, Li2S2, and Li2S on the (110) plane of Ni3S2 are computed to be 1.09, 1.92, 2.15, 2.29, 3.90, and 4.89 eV, respectively, which are remarkably higher than those on the carbon surface (hexatomic carbon ring-based network). This sharp contrast suggests that Ni3S2 is more effective in anchoring the polysulfide intermediates than traditional carbonaceous materials. In addition, it is evident that the binding energy on the Ni3S2 surface increases as the lithiation of sulfur progresses (S8→Li2S8→ Li2S6→Li2S4→ Li2S2→Li2S). This is because more and more charge will transfer onto the Ni3S2 with the increasing lithiation of sulfur.36 It should be noted that although Ni3S2 can bind strongly to polysulfides in theory, it is still not sufficient to prevent all the polysulfide loss to the electrolyte,


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due to the high sulfur loading (~4.0 mg/cm2). Hence, the Coulombic efficiency of the NSF-S cathode is not so high (Fig. 7e), and further studies on its enhancement are currently underway by optimizing the components of the hybrid cathode, changing the electrolyte type, and so on.

Fig. 9. Rate capability of NSF-S and NF-S cathodes at various current densities from 1 to 5 mA/cm2. The cycle durability measurement was conducted at 4 mA/cm2. For both NSF-S and NF-S cathodes, the sulfur loading was ~4.0 mg/cm2.

The rate capability of the NSF-S and NF-S cathodes in the range 1−5 mA/cm2 is also investigated to confirm the role of Ni3S2 on enhancing the redox kinetics of the polysulfides. As shown in Fig. 9, the discharge capacity of the NSF-S cathode reduced as the current density increased; however, this hybrid cathode with a sulfur loading of ~4.0 mg/cm2 could still deliver a capacity of 441 mAh/g after 150 cycles, at a high current density of 4 mA/cm2. On the contrary, in the absence of Ni3S2, a lower 150th cycle discharge capacity of 309 mAh/g is obtained for the NF-S cathode.



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Moreover, cycling of both cathodes at elevated current densities exhibits a gradual increase in discharge capacity, which is probably due to the slow electrolyte wetting within the high-massloading cathode structures.56 Fig. S4 presents the charge–discharge profiles of different cathodes at the 150th cycle. It is apparent that the NSF-S cathode with a relatively lower polarization could still maintain the two discharge plateaus around 2.2 and 1.9 V, indicating the good activity of elemental sulfur at a high current density. The decreased polarization, improved sulfur utilization and accelerated redox kinetics of the polysulfides could be attributed to the participation of Ni3S2 in the cathode reactions. Thus, the sulfiphilic Ni3S2 can not only introduce highly favorable interactions with the polysulfides and serve as the adsorbent, but also provide the anchored polysulfides with activation sites, where the lower interfacial resistance could promote further redox reactions of the polysulfides. Notably, the outermost sulfur layer in this NSF-S hybrid cathode exploits only the carbon black and bulk sulfur, but this electrode engineering already enables an excellent cell performance, and we expect a drastic improvement when other advanced carbon (such as graphene, porous carbon, and carbon nanotubes) or sulfur nanomaterials are involved. Conclusions In summary, we developed a facile, cost-effective, and controllable approach to build a threedimensionally hierarchical Ni/Ni3S2/S cathode, with the aims of capturing the polysulfides and achieving high sulfur loading. Herein, Ni3S2 was first synthesized in situ and coated homogenously onto the surface of the Ni foam by a hydrothermal process at 160 °C, followed by physically coating elemental sulfur onto the Ni/Ni3S2 foam to obtain the Ni/Ni3S2/S hybrid cathode. This was confirmed by XPS, XRD, FESEM, and EDX elemental mappings. For this cathode architecture, the innermost Ni metal network could serve as electron transport pathways to facilitate the charge-


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discharge reactions, while the sufficient pore spaces can enable a high sulfur loading and endure the volume change coming from the active material. Additionally, a combination of density functional theory (DFT) calculations and experimental studies revealed that the Ni3S2 layer in this cathode possesses both good polysulfide adsorption capability and high electronic conductivity. When applied in Li–S batteries, this Ni/Ni3S2/S cathode with a high sulfur loading of ~4.0 mg/cm2 demonstrated enhanced capacities, reduced polarization, fast reaction kinetics, and superior rate capability, as compared to the sulfur cathode without the Ni3S2 layer. For example, at a high current density of 4 mA/cm2, this Ni/Ni3S2/S cathode delivered a discharge capacity of 441 mAh/g after 150 cycles, which is significantly higher than the cathode without Ni3S2. The enhancement in cell performance was ascribed to the following factors: (i) the incorporation of the Ni3S2 layer could introduce a strong interaction with the polysulfides and suppress the shuttle effect, and (ii) the electrochemical environment due to the highly conductive Ni3S2 could enhance the redox kinetics of the polysulfides. All the above results show that this Ni/Ni3S2/S architecture is a promising cathode design for practical high-performance Li-S batteries. In addition, this facile fabrication strategy is generally applicable, and we believe that the present study could pave the way for other novel metal/metal sulfide/sulfur hybrid cathodes. Experimental Preparation of 3D Ni/Ni3S2 foam (NSF). The 3D Ni/Ni3S2 foam was fabricated by sulfurizing the Ni foam (NF) through a simple hydrothermal method. In detail, a piece of Ni foam (Sumitomo Electric Industries, size: 3.4 cm × 3.4 cm) was washed with acetone, methanol, and ultrapure water in succession. The Ni foam was then statically soaked in 2 M HCl aqueous solution for 15 min at room temperature (RT). The Ni-



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foam was taken out, washed in ultrapure water, and dried under vacuum for 12 h at 60 °C. Prior to use, the cleaned Ni foam was compressed to reduce the thickness. This Ni foam piece, 6.1 mg sulfur (Wako Pure Chemical Industries Ltd.) and 5 % (v/v) hydrazine monohydrate solution (60 mL) were loaded into a Teflon-lined autoclave for the hydrothermal reaction. The autoclave was sealed and maintained at 160 °C for 18 h, followed by natural cooling to RT. Finally, the product (Ni/Ni3S2 foam, denoted as NSF) was collected, washed with ultrapure water and methanol in turn, and dried under vacuum for 12 h. The mass of Ni3S2 in NSF can be determined from the equation, mNi3S2= (240.2 × Δm)/64.1, where Δm is the difference between the mass of the Ni foam before and after the hydrothermal treatment. Preparation of the 3D Ni/Ni3S2/S hybrid cathode. The sulfur powder (Wako Pure Chemical) was ground with carbon black (Super C65, TIMCAL) for 30 min, followed by heating in a sealed vial at 155 °C for 6 h. The sulfur content in this sulfur/carbon composite was ~71 wt%, which was confirmed by thermogravimetric analysis (TGA). Subsequently, the sulfur/carbon composite was directly dispersed into N-methyl-2pyrrolidinone (NMP), without the addition of a binder. After stirring for 48 h, the resulting lowviscosity slurry was applied to the Ni/Ni3S2 foam via an absorption method, followed by drying at 60 °C for 18 h to remove NMP. Finally, the Ni/Ni3S2/S cathode (denoted as NSF-S) was collected, where the areal loading of sulfur was around 4.0 mg/cm2. For comparison, the slurry was directly coated onto the Ni foam (NF), and the Ni Foam-S (NF-S) cathode with a sulfur loading of 4.0 mg/cm2, was prepared under similar conditions. The mass ratio of Ni, Ni3S2 and S in the final electrode is about 80:3:8. Characterization


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The surface composition was studied by an X-ray photoelectron spectroscope (ULVAC-PHI Quantera SXM). X-ray diffraction (XRD) analysis was carried out on a Rigaku Ultima IV X-ray diffractometer using a Cu Kα radiation source. Field emission scanning electron microscopy (FESEM, JEOL JSM-7001F) equipped with an energy-dispersive X-ray spectroscopy (EDX) was used to evaluate the structure, surface morphology, and elemental distribution of the samples. In order to measure the polysulfide adsorption, the Li2S4 solution was prepared by dissolving 2.84 mg of Li2S4 powder in 20 mL of tetrahydrofuran (THF), followed by stirring at 60 °C for more than 24 h. Here, Li2S4 was compositionally obtained by grinding Li2S with S in a molar ratio of 1:3 for 20 min. The NF and NSF with the same geometric size were then added into the Li2S4 solutions for static adsorption. Electrochemical measurements 1.0 M lithium bis(triﬂuoromethanesulfonyl)amide (LiTFSA, Solvey) and 0.2 M LiNO3 in a binary mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1, v/v) was prepared as the electrolyte. 2032-type coin cells consisting of the as-prepared NSF-S or NF-S cathode, a Celgard 2400 separator, a lithium foil anode, and electrolyte were assembled in an argon-filled glovebox, where the H2O level was below 1 ppm. Here, the electrolyte-to-sulfur ratio is around 16 uL/mg. Prior to the electrochemical measurements, these coin cells were equilibrated for 12 h. A Nagano BTS-2004 battery testing system was used to characterize the galvanostatic charge/discharge behaviors of the coin cells at 30 °C. The cells were operated in the voltage range 1.7–2.8 V (vs. Li/Li+) at different current densities, and the charge/discharge process was defined as the following: 1st discharge → 2nd charge → 2nd discharge →3rd charge →3rd discharge, and so on. The specific capacity of the cell was calculated in terms of the sulfur mass. Electrochemical impedance spectroscopy measurements of the fresh cells (before cycling) were performed at the



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open circuit voltage (OCV) by a Bio-Logic SAS VMP3 electrochemical workstation with a sinusoidal excitation amplitude of 5 mV in the frequency range 200 kHz–10 mHz. Theoretical calculation Density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package (VASP) to investigate the binding effect between Ni3S2 and the polysulfides.5760

The Blöchl’s all-electron-like projector augmented wave (PAW) method was used to describe

the interactions between valence electrons and ion cores,61-62 while the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was employed to describe the exchange-correlation interactions.63 A semiempirical DFT-D2 force-field approach was used to include the physical van der Waals (vdW) interaction in our calculations.64-65 The electron occupancies were determined based on the Fermi scheme with an energy smearing of 0.1 eV. The expansion of the electron wave functions in plane waves was conducted using a cut-off energy of 400 eV. In our geometry optimization process, the energy and force were converged to 1.0 × 10−5 eV/atom and 0.01 eV/Å, respectively. Monkhorst-Pack k-point sampling with a 3 × 3 × 1 mesh was used for the Brillouin zone integrations.66 To simulate the Ni3S2, a three-layer slab was chosen, and a 15 Å vacuum layer between repeating slabs was employed to avert the periodic interactions. For comparison, traditional carbon (hexatomic carbon ring-based network) was also studied by choosing a single-layer slab. In this work, we investigated the full set of sulfur-related species (S8, Li2S8, Li2S6, Li2S4, Li2S2, and Li2S), which reflect the lithiation stages during the battery reactions. The binding energy, Eb, between the matrix (Ni3S2 or carbon) and sulfur-related species (denoted as SS) can be calculated as follows: Eb = Ematrix + ESS – Ematrix+ss


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where Ematrix, ESS, and Ematrix+SS represent the ground-state energy of the matrix, the sulfur-related specie, and the combined system, respectively.

ASSOCIATED CONTENT Supporting Information. Details for electrochemical characterization.

AUTHOR INFORMATION Corresponding Author [email protected]

ACKNOWLEDGMENT This work was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). We would like to acknowledge Zhijian Wu, Feng He and Kai Li (State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, China) for the density functional theory calculations.



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Table of Contents


S Cathode for Lithium

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