SnS2 with Expanded Interlayer

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Sandwich-Like SnS2/Graphene/SnS2 with Expanded Interlayer Distance as High-Rate Lithium/Sodium-Ion Battery Anode Materials Yong Jiang, Daiyun Song, Juan Wu, Zhixuan Wang, Shoushuang Huang, Yi Xu, Zhiwen Chen, Bing Zhao, and Jiujun Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03330 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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Table of contents graphic 271x107mm (300 x 300 DPI)

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Sandwich-Like SnS2/Graphene/SnS2 with Expanded Interlayer Distance as High-Rate Lithium/Sodium-Ion Battery Anode Materials Yong Jiang a,c, Daiyun Song b, Juan Wu b, ZhixuanWang a,c, Shoushuang Huang b, Yi Xu a,*, Zhiwen Chen b, Bing Zhao a,c*, and Jiujun Zhang c,* a

School of Environmental and Chemical Engineering, Shanghai University, Shanghai

200444, China. b

Shanghai Applied Radiation Institute, Shanghai University, Shanghai 201800,

China. c

Institute for Sustainable Energy, Shanghai University, Shanghai 200444, China.

Drs. Y. Jiang and D. Song contributed equally to this work. * Address correspondence to [email protected] (B. Zhao); [email protected] (Y. Xu); [email protected] (J. Zhang).


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ABSTRACT: SnS2 materials have attracted broad attention in the field of electrochemical energy storage due to its layered structure with high specific capacity. However, the easy restacking property during charge/discharge cycling would lead to electrode structure instability and serve capacity decrease. In this paper, we report a simple one-step hydrothermal synthesis of SnS2/graphene/SnS2 (SnS2/rGO/SnS2) composite with ultrathin SnS2 nanosheets covalently decorated on both sides of reduced graphene oxide sheets via C‒S bonds. Owing to the graphene sandwiched between two SnS2 sheets, the composite presents an enlarged interlayer spacing of ~8.03 Å for SnS2, which could facilitate the insertion/extraction of Li+/Na+ ion with rapid transport kinetics, as well as inhibiting the re-stacking of SnS2 nanosheets during the charge/discharge cycling. The density functional theory calculation reveals the most stable state of the moderate interlayer spacing for the sandwich-like composite. The diffusion coefficients of Li/Na-ions from both molecular simulation and experimental observation also demonstrate that that this state is the most suitable for fast ions transport. In addition, numerous ultra-tiny SnS2 nanoparticles anchored on the graphene sheets can generate dominant pseudocapacitive contribution to the composite especially at large current density, guaranteeing its excellent high-rate performance with 844 and 765 mAh g-1 for Li/Na-ion batteries even at 10 A g‒1. No distinct morphology changes occur after 200 cycles, and the SnS2 nanoparticles still recover to pristine phase without distinct agglomeration, demonstrate that this composite with high rate capabilities and excellent cycle stability are the promising candidate for lithium/sodium storage. KEYWORDS: tin disulfide/graphene; expanded interlayer distance; pseudocapacitive contribution; density functional theory; lithium/sodium storage


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Rechargeable lithium/sodium ion batteries (LIBs/SIBs) have been considered the important technologies in energy storage and conversion applications including portable electronics devices, electric vehicles and grids.1,2 Further enhance the performance through increasing energy/power densities and cycle-life is the major effort in research and development of such batteries3-6 One of the approaches to achieve high performance is to realize the fast and reliable Li/Na-ions insertion/extraction at during the charge/discharge at the anodes, in which anode materials of such Li/Na-ion batteries play on of the most important roles. Regarding anode materials, various carbonaceous materials, metal/alloy and metal oxide/sulfide anodes, alloy-based materials have stood out due to their high capacities for lithium/sodium ion batteries.7-9 Recently layered transition-metal sulfides, such as MoS2 and SnS2,10-15 have received increasing attention due to their high Li/Na storage capacities. As a typical two-dimensional material, SnS2 is a layered semiconductor with a CdI2-type structure composed of a three-layer stacked atomic layer (S‒Sn‒S) connected by van der Waals force.16-18 A large layer spacing (0.59 nm) makes SnS2 suitable for insertion of Li/Na-ions without apparent volumetric expansion.19,20 However, the Li/Na storage performance of the pure SnS2 electrode is practically unsatisfactory due to its poor conductivity and severe pulverization. Previous studies have reported that the hybridization of SnS2 to conductive substrates can effectively improve the electrochemical properties of SnS2-based electrodes.21 Actually, the layered SnS2 materials can offer several advantages for Li/Na storage

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compared to other materials. First, the two-dimensional (2D) layered materials have excellent structure compatibility with graphene nanosheets; Second, the layered structure with larger interlayer distance is conducive to the insertion/extraction of Li+/Na+ and is more resistant to volume expansion/shrinkage during the repeated cycling; and third, the stacking of the layered material in the c direction can be suppressed by the interaction with second component. For instance, the introduction of graphene oxide (GO) shows favorable effects in reducing the sheet thickness due to the van der Waals interaction between adjacent layers,22,23 thereby reducing the diffusion distance of Li/Na ions. Therefore, the accessibilities of Li+/Na+ ions are expected to be enhanced by reducing the thickness and lateral dimension of the layered materials. Regarding Li+/Na+-ion battery anode materials, the migration rates of Na+ or Li+ ions in the anodes can be a limiting factor in achieving a high performance of the batteries. To speed up the ions migration rates, the reduction in particle sizes of the bulk materials and increase of the contact area between the material and the electrolyte, could promote the pseudocapacitive behavior of the anodes, making contribution to the energy storage capacity.24 In addition, suitable adjusting and controlling the interlayer spacing of the layered anode materials may bring many promising electrochemical performances, especially for high-rate anode materials for lithium/sodium-ion batteries. Since the Van der Waals interactions between adjacent metal dichalcogenide monolayers is weak, interlayer expansion can be achieved by intercalating or trapping foreign species (e.g., ions, molecules, quantum dots,


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graphene sheets, etc.) into the interlayer gaps during “top-down” and ‘‘bottom-up’’ approaches.

Among

them,

the

‘‘bottom-up’’

hydrothermal/solvothermal

reaction

provides

interlayer-expanded

sulfide

nanosheets

metal

a

process

facile by

way

trapping

based to

on

synthesize

foreign

species

simultaneously. Recently, interlayer-expanded MoS2 has been widely studied, the larger interlayer spacing promotes more Li/Na ions accommodation and faster ions transport. Density functional theory (DFT) simulations also indicate that interlayer expansion in 2H-MoS2 is benefit to lower both the ion intercalation energy and the ion diffusion energy barrier with an optimal interlayer spacing value.25 In this paper, the ultrathin SnS2 nanosheets are covalently decorated on both sides of reduced graphene oxide sheets (rGO) to form an anode composite material of SnS2/rGO/SnS2. This sheet-like sandwich structure endows an enlarged interlayer spacing of ~8.03 Å for SnS2, which can facilitate the insertion/extraction of Li+/Na+ with rapid transport kinetics, as well as inhibiting the re-stacking of SnS2 nanosheets during the cycling. The density functional theory calculation reveals the most stable state of the moderate interlayer spacing of the sandwich-like composite. The diffusion coefficients of Li+/Na+ ions from both molecular simulation and experimental measurements also demonstrate that this stable state is the most suitable for the fast transport of ions. As expected, the as-prepared SnS2/rGO/SnS2 composite performs ultra-high current rate capability with 844 and 765 mAh g-1 at 10 A g‒1 for lithium-ion battery and sodium-ion battery, respectively. Besides the enhancement of Li+/Na+ transport kinetics, the dominant pseudocapacitive behavior can also be observed when


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increasing the discharge current, which makes a significant contribution to high-rate performance. There is no distinct morphology change observed after 200 cycles, and the SnS2 nanoparticles can still recover to the pristine phase without distinct agglomeration after 200 cycles, demonstrating that this sandwich-like SnS2/rGO/SnS2 composite can be a promising candidate for lithium/sodium storage.

RESULTS AND DISCUSSION

Figure 1. (a) XRD pattern and (b) TEM image of SnS2/rGO/SnS2 composite; (c) Schematic illustration of the sandwich-like structure; High-resolution (d) Sn 3d, (e) S 2p, and (f) C 1s XPS spectra.

Figure 1a shows the XRD pattern of as-prepared SnS2/rGO/SnS2 composite, in which the majority of the diffraction peaks can be indexed well to SnS2 (JCPDS No.: 23–0677). It can be seen that the (001) diffraction peak at 2θ = 15.0° disappears, and Page 6 of 40 ACS Paragon Plus Environment

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two diffraction peaks at around 10.99° and 16.79° can be found. The corresponding layer spaces of these two peaks are 8.03 and 5.27 Å, respectively. Figure 1b shows that the thin layers of SnS2 nanosheets are uniformly decorated on both sides of the rGO surface to form a sheet-like sandwich structure (bright-field and dark-field images are presented in Figure 3b and c), supporting the disappearance of (001) diffraction peak and two peaks with the enlarged intercalating space. Combining the XRD and TEM results, we can speculate that the even deposition of SnS2 nanosheets on both sides of rGO with van der Waals attractive force might lead to the expanded interlayer between upper and lower SnS2 layers (as illustrated in Figure 1c and Figure S1), and these two peaks may be attributed to the diffraction between adjacent SnS2 sheets and carbon layers.28,29 The Raman spectrum (Figure S2a) of SnS2/rGO/SnS2 shows a visible peak located at 307 cm‒1, corresponding to the A1g mode of SnS2. 30,31 And two more peaks at about 1329 and 1586 cm‒1 can be indexed to the D and G bands of graphene, respectively, which confirms the coexistence of SnS2 and graphene in the nanocomposite. 32-36 The high intensity ratio (ID/IG = 1.33) compared those of pristine GO (1.07) and rGO (1.15) with the similar hydrothermal treatment indicates more defects incorporated into the prepared sample. 37,38 The content of SnS2 in SnS2/rGO/SnS2 nanocomposite is calculated by TGA performed in air (Figure S2b). It can be seen that the mass loss from 300 to 700 °C should be associated with the chemical oxidations of SnS2 to SnO2 and C to CO2.39,40 Therefore, weight ratio of SnS2 in the composite is about 89.9%. The nitrogen adsorption-desorption isotherm curve (Figure S3) shows that the specific surface area is 38.7 m2 g-1 with a plenty of


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mesopores, which can facilitate easy entry of electrolyte in the SnS2/rGO/SnS2 composite. To clarify the chemical composition and valence state of the sample surface, XPS analysis was performed. The full spectrum of SnS2/rGO/SnS2 can indicate the elements such as Sn, S and C (Figure S4). The high-resolution Sn 3d spectrum (Figure 1d) shows two peaks corresponding to Sn 3d5/2 and Sn 3d3/2 binding energy, respectively, indicating the presence of Sn4+ without apparent Sn2+ peak.41,42 In S 2p spectrum, the binding energies of 161.95 and 163.10 eV correspond to S 2p3/2 and S 2p1/2 of S2‒, respectively, indicating the existence of SnS2 in SnS2/rGO/SnS2. In addition, an apparent shoulder peak is presented, and the divided two peaks at 163.56 and 164.47 eV can be assigned to S 2p3/2 and S 2p1/2 of the C‒S bond, respectively.43 The result indicates that the SnS2 layer and the rGO sheet are linked by a C‒S covalent bond, which is further confirmed by the C 1s spectrum (Figure 1f). The main peak centered at 284.52 eV is attributed to the C‒C/C=C bond, while the peak centered at 285.19 eV is indexed to C in C‒O/C‒S with a binding energy with a slightly up-shifting, agreeing with the sulfur-doping graphene-based composites in the previous reports44,45. XPS results indicate that SnS2 may be chemically bonded to rGO via C‒S bond to form SnS2/rGO/SnS2 sandwich structure, which explains rationally that the interlayer distance is smaller than the sum of interlayer spacing between SnS2 (0.59 nm) and graphite (0.334 nm).


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Figure 2. (a) Molecular model of the sandwich structured SnS2/rGO/SnS2; (b) Dependence of system energy on the interlayer space (d) in the SnS2/rGO/SnS2 composite crystals; (c) The 1×2×1 supercell model of SnS2/rGO/SnS2 composite containing Li+/Na+, (d) Dependence of diffusion coefficient (D) of Li+ and Na+ on the interlayer space.

According to the TEM and XPS analysis results, and for a fundamental understanding, a SnS2/rGO/SnS2 sandwich structure is constructed by using the Forcite module of Materials Studio 8.0, and the SnS2 layer and the graphene sheet are connected by C‒S bond as shown in Figure 2a (the detailed modeling and theoretical


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calculation process is provided in the supporting information, Figure S5 and S6). Thus, the dependence of system energy on the interlayer space (d) of the SnS2/rGO/SnS2 composite crystals can be calculated (in the d range of 7.7-8.7 Ǻ). As shown in Figure 2b, the system energy (E) is closely related to the interlayer space (d) in the SnS2/rGO/SnS2 composite crystal. The SnS2/rGO/SnS2 with a d value of ~8.03 Ǻ has the minimum system energy, which may be the most stable state for the sandwich composite. To evaluate the electrochemical reaction kinetics of the sandwich composites, Li+ (Na+) ions are added to the 1×2×1 supercell of SnS2/rGO/SnS2, acting as diffusible substances, as displayed in Figure 2c. On this basis, the diffusion coefficient (D) in this composite crystal can be calculated according to the Einstein's relation, which is then employed to evaluate the kinetics superiority. As demonstrated in Figure 2d, both of DLi+ (2.134 × 10‒5 cm2 s‒1) and DNa+ (5.076 × 10‒5 cm2 s‒1) reach maximum when d ≈ 8.03 Å indicating that this state is the most suitable for fast transport of lithium/sodium ions during the charge/discharge processes, which is in accordance with the electrochemical experimental results as described below.


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Figure 3. Morphology and structural characterization of SnS2/rGO/SnS2 composite. (a) TEM image; (b) Bright-field and (c) dark-field images; (d,e) HRTEM image, insets of (d) are the size distribution of ultra-small SnS2 nanoparticles in SnS2/rGO/SnS2 composite. (f) SAED pattern; (g) HAADF image and the corresponding Sn, S and C elemental mapping; (h) AFM image and corresponding height profile.

The detailed morphology and microstructure of the SnS2/rGO/SnS2 composite are further investigated using both HRTEM and STEM, as shown in Figure 3. The Page 11 of 40 ACS Paragon Plus Environment

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enlarged TEM image shows that the small-sized SnS2 nanosheets are evenly distributed on the surface of graphene sheets (Figure 3a); the transparent characteristic of graphene indicates that the coating is very thin, probably few-layers. Figure 3b and c are bright and dark field images of SnS2/rGO/SnS2 nanocomposites. In the dark field image, it can be clearly seen that the nanosheets and ultra-small-sized SnS2 nanoparticles are supported on the graphene sheet, which are not displayed in the bright field. This further proves that the SnS2 nanosheets are distributed on both sides of the graphene sheet, forming a sandwich-like structure. The (d) region drawn in the yellow frame in Figure 3b is further enlarged, as shown in Figure 3d. Many ultrafine SnS2 particles are found, which are distributed homogeneously and densely on the surface of the graphene nanosheets and the size distribution in the inset picture shows that the diameter of SnS2 nanoparticles primarily ranges from 3-5 nm. It is well known that the diffusion times of lithium/sodium ions diffusion time have linear relationships with the square of the diffusion length (t = L2/D). Therefore, the migration efficiency of Li+/Na+ ions can be significantly improved by a thin two-dimensional nanosheet with short diffusion distance so as to improve the high current discharge performance. There are also many nanovoids between the SnS2 particles, which are not only advantageous for accommodating the volume expansion of SnS2, but also for the active material sufficiently to contact with the electrolyte in a cell.46,47 The (e) region drawn in the yellow frame in (d) is further enlarged, which shows that the lattice spacings are 0.28 nm and 0.32 nm, corresponding to the (101) and (100) plane of hexagonal SnS2, respectively, which are consistent with the XRD


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results. The selected area electron diffraction (SAED) pattern shows three distinct diffraction rings corresponding to (100), (110), and (200) planes of SnS2, respectively, indicating its polycrystalline features. The corresponding element mapping results shown in Figure 3g confirm that the Sn and S signals are distributed on the entire graphene sheets except of the visible one-hundred-nanometer SnS2 sheets, confirming the complete coverage and uniform distribution of SnS2 ultrafine nanoparticles on the surface of graphene sheets. Figure 3h shows the 2D AFM morphology and height distribution of SnS2 nanosheets. The height between two red lines corresponds to the thickness of graphene (ca. 2.31 nm), showing the few-layer characteristic of graphene. The height between two green lines corresponds to the thickness of SnS2, which is about 4.43 nm, presenting the small size and few layer characteristics of SnS2 decorated on the thin graphene sheets.


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Figure 4. Electrochemical performances of the SnS2/rGO/SnS2 for lithium-ion battery. (a) Cyclic voltammograms at a scan rate of 0.1 mV s−1; (b) Typical discharge/charge curves at a current density of 0.1 A g−1; (c) Cycling performance at 0.1 A g−1; (d) Differential charge-capacity plots at typical cycles; (e) Reversible charge capacities versus cycles occurring at potential ranges of 0.01–1.0 and 1.0–3.0 V; (f) Rate performance at various current densities; (g) Cycling performance at 1 A g−1.

In order to reveal the lithium storage electrochemical process in the as-prepared samples, CV and galvanostatic charge/discharge tests are conducted in a voltage range of 0.01-3.00 V. The cyclic voltammetry (CV) curves shown in Figure 4a present three reduction peaks at 1.5-1.8 V in the 1st cathodic scan, which can be assigned to multi-step lithium ions insertion into LixSnS2.48 The peaks at around 1.2 and 0.7 V can be ascribed to the decomposition of LixSnS2 into metallic Sn and Li2S, and formation of a solid electrolyte interface (SEI) film. The broad peak at 0.1 V represents the reversible formation of the LixSn alloy. In the reversed scanning, a visible peak of 0.57 V is attributed to the LixSn de-alloying process,49 another anodic peak at 1.9 V can be assigned to reversible transformation of Sn and Li2S to SnS2.50 The peak at high voltage of 2.1 V is related to Li+ extraction from SnS2 without phase decomposition.51 After the first activation, the CV curves almost overlap at the following cycles; and distinct pair of reduction and oxidation peaks are continuously exhibited with stable intensities, suggesting the highly reversible conversion and


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alloying reactions and excellent cycle performance of the electrode. Figure 4b shows the typical discharge and charge profiles during the cycling. The inconsistent platforms (redox potentials) for charge and discharge may be due to low electronic conductivity of the metal sulfide materials, resulting in polarization, which behaves as potential deviation during discharge and charge processes. Generally, the platforms are consistent with the cathode and anode peaks in the CV curves and literature recorded SnS2–based composites.

10,15,23,35

The first discharge (lithiation)

and charge (delithiation) capacities of SnS2/rGO/SnS2 electrode are 1733.6 and 1404.1 mAh g‒1, respectively, and the corresponding initial coulombic efficiency (ICE) is about 81.0%. The capacity loss at the first cycle is mainly caused by the inevitable formation of SEI layer on the electrode surface and the partial irreversibility of the reversed conversion reaction (Sn + 2Li2S → SnS2 + 4Li+ + 4e‒). After that, the curves overlap even after 200 cycles, indicating the stable capacity. This phenomenon can be confirmed by the cycling performance of the electrode as shown in Figure 4c. It shows that the SnS2/rGO/SnS2 electrode can maintain a reversed capacity of 1357 mAh g-1 at the 200th cycle, corresponding to a reversible capacity retention rate of 96.6%. The differential charge capacity plots (DCPs) and the reversible charge capacities at potential ranges can clearly reveal the capacity contribution during the repeated cycles. Figure. 4d exhibits two distinct peaks, including a sharp peak for LixSn dealloying process at about 0.5 V, and multi-peaks for the reverse conversion from Sn/Li2S to SnS2 in the voltage range of 1.0–3.0 V. Even after 200 cycles, the intensity of the two peaks remains quite stable, suggesting


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that the de-alloying process and reverse transformation reaction of Li2S and Sn are still reversible. The reversible charge capacity variation in these two potential ranges also exhibit stable capacity retention with increasing cycle number (Figure 4e). These results suggest a good Sn agglomeration confinement for alloy/de-alloy and conversion reaction due to the stable sandwich-like sheet-on-sheet composite structure. Figure 4f shows the rate performance tests. It can be seen that the SnS2/rGO/SnS2 can deliver specific capacities of 1342, 1244, 1067 and 844 mAh g‒1 at current densities of 1, 2, 5 and 10 A g‒1, respectively. It is worth noting that the high rate capability is superior to almost all of the reported SnS2-based composites, including ultrafine SnS2 nanoparticles/graphene,52 SnS2/S-doped graphene composites,53 hierarchal SnS2@rGO,55

nanostructure

SnS2/MoS2/3D

SnS2/graphene

graphene,54

nanoscroll/nanosheet

Sandwich-structured

aerogels,56

SnS2/graphene

nanosheets,57 and conducting polypyrrole@SnS2@carbon nanofiber,58 etc. (The detailed capacities and retention rates are summarized in Table S1). The electrode can also recover the pristine capacity (1406 mAh g-1) when the current density is turned back to 0.1 A g‒1, indicating the stable structure even after high current density cycles. The EIS measurements (Figure S7) also show the decrease of the semicircle diameter in the high-middle frequency region and increase of the slope for the straight line in the low frequency region after cycling, suggesting the enhancement of electron and Li-ion transport. Because the graphene sheet is sandwiched between the SnS2 layers, it can improve the conductivity of the material, and more importantly, effectively


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prevent the restacking of the SnS2 layer and keep the structural stability with ample buffer space. The sandwich-like structure with an enlarged interlayer distance can promote facile ion transport. At the same time, the SnS2 size is small, which facilitates the diffusion of electrolyte and Li+/Na+ ions into the electrode, shortens the electron and ion transport paths, thus maintain the high-speed performance of the composite. To further clarify the significant large current for Li storage, the long-term cycling of the battery was evaluated at 1 A g‒1. As shown in Figure 4g, even after 200 cycles, the capacity remains at 909 mA h g‒1. In addition, the coulombic efficiency increases rapidly during the initial cycles and then stabilizes at almost 100% during the following cycles. Therefore, the excellent high-rate cyclic performance, once again, indicates the stable structure of electrode materials of SnS2/rGO/SnS2 with an expanded interlayer distance.


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Figure 5. Electrochemical performances of SnS2/rGO/SnS2 electrode for sodium-ion battery. (a) CV profiles at a scan rate of 0.1 mV s-1. (b) Galvanostatic discharge/charge profiles at a current density of 0.1 A g‒1. (c) Cycle performance at 0.1 A g‒1. (d) Rate performance at various current densities.

The sodium storage performance of the SnS2/rGO/SnS2 nanocomposite was also investigated. The CV curves (Figure 5a) present a broad cathodic peak at high voltage (1.69 V), which corresponds to the sodium intercalation into SnS2 layer59-61 and a large peak at 1.32 V from the conversion reaction. The peak at 0.53 V can be indexed to the Na-Sn alloying process,62 meanwhile the irreversible SEI might also be formed at this stage.63, 64 In anodic sweeps, the predominant oxidative peak at 1.25 V can be assigned to the desodiation reaction of NaxSn, and the subsequent peak at 1.58 V may Page 18 of 40 ACS Paragon Plus Environment

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be related to the restitution of SnS2.65 Apparently, from the second cycle, the oxidation/reduction peaks at 1.25/0.64 V and 1.58/0.92 V are clearly emerged and remained

steady,

representing

the

excellent

reversibility

of

both

the

alloying/dealloying and conversion reactions. The peaks in the CV curves overlap in the following cycles, suggesting that the SnS2/rGO/SnS2 electrode has a good stability and reversibility during the sodium storage. The galvanostatic voltage curves (Figure 5b) shows the initial discharge/charge capacities of the SnS2/rGO/SnS2 composite are 1860 and 1243 mA h g‒1, respectively. The charge/discharge curves and potential platforms overlap well from the second until 100th cycle. An ultra-stable sodium storage capacity is maintained with a reversible capacity of 1133 mAh g‒1 and 91.2% capacity retention at 100th cycle, and the coulombic efficiency is stably high above 99.5% (Figure 5c), demonstrating the excellent Na+ intercalation/deintercalation reversibility. Benefited from the improved conductivity and expanded layer spacing through the SnS2/rGO/SnS2 sandwich-like structure, this composite can give an excellent rate performance (Figure 5d). A reversible capacity of 1295 mAh g‒1 is delivered at 0.1 A g‒1. When the current density is increased to 0.2 and 0.5 A g‒1, the capacity remains essentially constant and stable. Even at high current densities of 5 and 10 A g‒1, the specific capacities still maintain at 950 and 765 mA h g‒1, respectively. Also, the specific capacity can facilely recover 1250 mAh g‒1 when the current density returns to 0.1 A g‒1, indicating the excellent stability even at high current density due to its strong SnS2/rGO/SnS2 sandwich structure with covalently bonding enlarged layer spacing.


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The lithium/sodium storage kinetics were studied by evaluating the lithium/sodium diffusion coefficients of the composite material. By changing CV sweep rates from 0.1 to 1 mV s‒1 and linear fitting according to Randles Sevcik equation,66 the slopes of SnS2/rGO/SnS2 when used as the anodes of lithium-/sodium-ion batteries were obtained to be 1.42 and 2.64, respectively (Figure S8). According to the charge number of the redox reaction and the specific area of the electrode, the lithium and sodium diffusion coefficients of SnS2/rGO are1.386 × 10−5 and 4.785 × 10−5 cm2 s‒1, respectively. These measured values coincide well with the results of the molecular dynamics simulation presented above as shown in Figure 2c and d, which are also superior to majority of SnS2-based composites. The higher diffusion coefficient values of SnS2/rGO/SnS2 further verify the speculation that the enlarged interlayer space with C‒S covalent bonding can dramatically shorten Li+/Na+ diffusion distance and improve the electrical conductivity, which also means the enhancement of ionic and electronic transport kinetics, thus synergistical improvement of Li- and Na-storage performances.


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Figure 6. Quantitative capacitive analysis of sodium storage behavior. (a) CV curves at different scan rates. (b) Relationship between logarithm cathodic/anodic peak current and logarithm scan rates. (c) Normalized contribution ratio of capacitive capacities at different scan rates. (d) Capacitive contribution (red) and diffusion contribution (blue) at 0.8 mV s−1.

To further explain the excellent rate capability of SnS2/rGO/SnS2, especially for the higher sodium diffusion coefficient and superior large-current sodium storage property, the kinetic analysis of the electrode material was also carried out. Figure 6a shows that the shape of CV curves is well preserved when the scan rate is increased from 0.1 to 1.0 mV s‒1, suggesting the excellent electronic/ionic conductivity and low Page 21 of 40 ACS Paragon Plus Environment

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polarization of the composite. Based on the relationship between the measured peak current (i) and the scanning rate (v) of the CV curves (i = avb), the capacitance and diffusion-controlled contribution of the entire capacity can be quantify analyzed.67,68 The value of b can be calculated from the slope of the log i vs. log v plots (Figure 6b). It shows that the slopes for cathodic and anodic peaks are all near to 1.0, which means much favorable capacitive kinetics of the SnS2/rGO/SnS2 electrode. According to Dunn et al.,70 a quantitative analysis could be performed to determine the region of capacitance contribution in the CV curve by separating the current response i at the fixed potential V into a capacitive contribution (k1v) and diffusion-controlled process (k2v1/2), then the contribution of the Na-ion capacitive effects can be quantified. Figure 6c shows the calculated capacitive contribution at various scan rates; and the voltage curve for the capacitive contribution (red) compared to the entire area (blue) at 0.8 mV s−1 is shown in Figure 6d. It can be seen that with increasing the scan rate, the diffusion contribution is suppressed, and the ratio of capacitive contribution increases, reaching a maximum of ~86% at 1.0 mV s‒1. This means that the pseudocapacitive Na-storage contribution can account for most of the entire capacity. This is not surprising, because the sandwich structure of SnS2/graphene/SnS2 expands the interlayer space, increases the connection area of the electrolyte with the active substrate, which are beneficial to pseudocapacitive behavior71-74. At the same time, the widespread ultra-tiny SnS2 nanoparticles would facilitate the charge transfer near the surface and further benefit the pseudocapacitive property. The quantitative capacitive contribution analysis during the lithium storage


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process (shown in Figure S9) also reveal that the dominant capacitive and less stored charge coming from diffusion-controlled process at the scanning rate range, could give a typical capacitive contribution of ~78% at 1.0 mV s‒1. This result explains very well the excellent rate capabilities of the SnS2/rGO/SnS2 electrode with specific capacities of 844 and 765 mA h g‒1 for lithium-/sodium-ion batteries, respectively, even at a high discharge current density of 10 A g‒1.

Figure 7. Morphology and structural characterizations of SnS2/rGO/SnS2 electrode after 200 charge/discharge cycles: (a) TEM image and (b,c) HRTEM images. (d) HAADF image and the corresponding C, Sn and S elemental mapping.

The SnS2/rGO/SnS2 electrode after 200 lithium insertion/extraction cycles at 1 A g‒1 is performed to study the structural evolution. As shown in Figure 7a and b, no significant particle agglomeration can be found, and the ultrafine nanoparticles are still uniformly anchored on the surface of graphene nanosheets, similar to the original electrode, indicating its excellent structural integrity. The size distribution of the


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nanoparticles is also shown in the inset of Figure 7b; the average size (~9 nm) is slightly increased after a long-term cycling, suggesting that the sandwich-like structure could inhibit the agglomeration and migration of the generated Sn nanocrystals and LixSn particles. The enlarged area marked by the yellow frame in Figure 7b is shown in Figure 7c, and lattice fringes of 0.32 and 0.28 nm are presented in the magnified HRTEM image, which are corresponded to the (100) and (101) planes of hexagonal SnS2, respectively. These results indicate that the generated Sn and Li2S can recover to the original SnS2 phase. Moreover, the element mappings of Sn, S and C completely overlap each other, suggesting the intact structure of the electrode over long-term cycling. These results represent the superior structural integrity as well as tight electronic connection between SnS2 and the graphene sheets in the sandwich-structured SnS2/rGO/SnS2 composite.

CONCLUSIONS In this work, a hybrid SnS2/rGO/SnS2 composite, with few-layered SnS2 nanosheets covalently decorating on both sides of reduced graphene oxide (rGO) sheets, is synthesized by a simple one-step hydrothermal method. Different from previously reported SnS2-based composites, the sheet-like composite can present an enlarged interlayer spacing of ~8.03 Ǻ for SnS2. The SnS2 nanosheets may be chemically bonded to graphene via C‒S bond to form SnS2/rGO//SnS2 sandwich-like structure. The density functional theory calculation reveals that such a sand-witched structure is the most stable state with a moderate interlayer spacing. The diffusion coefficients of


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Li+/Na+ ions from both theoretical simulation and experimental observation also demonstrate that this state is the most suitable for fast ions transport. As expected, the as-prepared SnS2/rGO/SnS2 composite exhibited excellent large-current rate capability and long-term cycle stability for both lithium-/sodium-ion batteries. The reversible capacity can maintain at 1357 mAh g‒1 (for lithium-ion battery after 200 cycles) and 1133 mAh g‒1 (for sodium-ion battery after 100 cycles). Even at a high current density of 10 A g‒1, the ultra-high specific capacity of 844 and 765 mAh g-1 can be retained, respectively. Because the graphene sheet is sandwiched between the SnS2 layers, it can improve the conductivity of the material, and more importantly, effectively prevent the restacking of the SnS2 layer and keep the structural stability with ample buffer space. Particularly, the excellent rate properties could be assigned to the dominant pseudocapacitive contribution during charge/discharge process. the enlarged interlayer spacing and the numerous ultrafine SnS2 nanoparticles evenly distributed on the surface of graphene sheets, which can facilitate the insertion/extraction of Li+/Na+ ions with rapid transport kinetics and suppress the re-stacking of SnS2 nanosheets during continuous cycling. No distinct morphology changes occur after 200 cycles, and the SnS2 nanoparticles still recover to pristine phase without distinct agglomeration, demonstrating that the sandwich-like SnS2/rGO/SnS2 composite are the promising candidate for lithium/sodium storage.


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EXPERIMENTAL SECTION Synthesis of the SnS2/rGO composite. Graphene oxide (GO) was synthesized by the modified Hummers method according to the previous reports.26,27 The SnS2/rGO composite was synthesized by a simple hydrothermal method. First, 30 mL of GO solution (concentration: 3 mg mL‒1) and 20 mL of ethanol were placed in a 100 mL beaker and magnetic stirring for 2 hours to form a brown GO suspension. Then 0.7 g of SnCl4·5H2O was added dropwise to this GO suspension to form a Sn4+ and GO containing suspension. Thereafter, 0.45 g of CH3CSNH2 and 2 mL of glacial acetic acid (HAc) were added to this suspension to obtain a brown suspension, which was then transferred to a Teflon-lined autoclave and maintained at 180 °C for 12 hours. Finally, the resulting composite was washed separately with water and ethanol twice and freeze-dried for 24 hours, and the collected sample was marked as SnS2/rGO/SnS2. The possible fabrication procedure of the composite is illustrated at Figure S1. Materials characterizations. X-ray diffraction (XRD) of the composite product was conducted by using a Rigaku D/max-2500 with Cu Kα radiation (λ = 1.54178 Å) over a 2θ range of 5‒80°. The microstructures were investigated by a JEOL 200CX transmission electron microscope (TEM), and JEOL JSM-2010F high-resolution transmission electron microscope (HRTEM). X–ray photoelectron spectroscopy (XPS, ESCA LAB 250Xi, Al Kα radiation) and Raman spectra (Renishaw Raman Microscopy, 514.5 nm Ar+ ion laser) were conducted to reveal the surface electronic states and chemical compositions. BET specific surface area was determined by


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Quantachrome SI analyzer. Thermogravimetry (TG) was performed in air atmosphere and heated at a rate of 10 ºC min‒1 (NETZSCH STA 409 PC Instrument). The atomic force microscope (AFM, SPM-9600, Shimadzu) was used to assess the sheet height of the composite. Electrochemical measurements. The coin-cells (2032) were assembled for electrochemical measurements in an Ar-filled glovebox with H2O and O2 concentrations below 0.5 ppm. The working electrode slurry was prepared by mixing 80 wt.% active material, 10 wt.% acetylene carbon black (conductive agent) and 10 wt.% poly(vinylidene fluoride) (PVDF, binder) in N-methyl-2-pyrrolidone, which was blade-coated on copper foil with an active material mass loading of about 1.0 mg cm-2, and then dried at 100 ºC in vacuum overnight. 1.0 M LiPF6 (or NaPF6) dissolved in 1:1 volume ratio of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte. Lithium metal and sodium metal were used as the cathode materials for lithium-ion battery and sodium-ion battery, respectively. A LAND CT2001A battery tester system was used to measure the Li/Na storage performances at a potential window between 0.01 and 3.00 V. Electrochemical workstation (CHI 660C) was used to study the cyclic voltammetry (CV) performance at a scan rate of 0.1 mV s‒1. Electrochemical impedance spectroscopy (EIS) tests were measured in a frequency between 0.01 and 100 kHz at an amplitude of 5 mV.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (B. Zhao) *E-mail:[email protected] (Y. Xu) *E-mail: [email protected] (J. Zhang) ORCID Yong Jiang: 0000-0002-1414-7404 Shoushuang Huang: 0000-0002-8895-0443 Zhiwen Chen: 0000-0001-9625-8556 Bing Zhao: 0000-0001-6388-4999 Jiujun Zhang: 0000-0002-6858-4060 Author Contributions Drs. Y. Jiang and D. Song contributed equally to this work.

Acknowledgement. This work was supported by the National Natural Science Foundation of China (11575105, 21501119 and 21671130). We also acknowledge the High-Performance Computing Center for providing the Materials Studio software.

ASSOCIATED CONTENT Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx Schematic illustration of the synthesis of the sandwich structured SnS2/rGO/SnS2.


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Raman spectrum, TG curves, N2 adsorption–desorption isotherm and pore size distribution, full XPS spectrum, theoretical calculation, different sweep speeds’ CV curves, quantitative capacitive analysis of lithiuim storage behavior of SnS2/rGO/SnS2 composite. The molecular model of SnS2 and GO. Nyquist plots of the initial electrode and the electrode after 10th cycle. Comparison of rate performance of sandwich SnS2/rGO/SnS2 in this work with those of others reported in the literatures.


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Y.;

Wang, Z.; Huang, S.;

In-situ

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43. Zhu, Y.; Chu, Y.; Liang, J.; Li, Y.; Yuan, Z.; Li, W.; Zhang, Y.; Pan, X.; Chou, S.-L.;

Zhao,

L.,

Tucked

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Composite

for

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SnS2 with Expanded Interlayer

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