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SnS2 Nanowall Arrays toward High Performance Sodium Storage Peng Zhou, Xiao Wang, Wenhao Guan, Dan Zhang, Libin Fang, and Yinzhu Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13613 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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ACS Applied Materials & Interfaces

SnS2 Nanowall Arrays toward High Performance Sodium Storage Peng Zhou, Xiao Wang, Wenhao Guan, Dan Zhang, Libin Fang, Yinzhu Jiang* State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China *E-mail: Prof. Y. Jiang ([email protected])

Abstract Cost-effective sodium ion batteries (SIBs) are springing out as desirable alternative choice of lithium ion batteries (LIBs), in terms of application in large-scale energy storage devices. SnS2 is regarded as a potential anode material for SIBs due to its unique layered structure and high theoretical specific capacity. However, the development of SnS2 was hindered by the sluggish kinetics of diffusion process and the inevitable volume change during repeated sodiation/desodiation processes. In this work, SnS2 with a unique nanowall arrays (NWAs) structure is fabricated by one-step pulsed spray evaporation chemical vapor deposition (PSE-CVD), which could be used directly as binder-free and carbon-free anodes for SIBs. SnS2 NWAs electrode achieves a high reversible capacity of 576 mAh g-1 at 500 mA g-1 and enhanced cycling stability. Attractively, an excellent rate capability is demonstrated with ~370 mAh g-1 at 5 A g−1, corresponding to a capacity retention of 64.2% at 500 mA g-1. The superior sodium storage capability of SnS2 NWAs electrode could be attributed to 1 / 32

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outstanding electrode design and rational growth process, which favor fast electron and Na-ion transport, as well as provide steady structure for elongated cycling. Keywords: Sodium ion batteries; Anode; SnS2 nanowall arrays; PSE-CVD; Rate capability.

Introduction Recently, sustainable electrical energy storage (EES) technologies for stationary applications have attracted an increasing attention because of the intermittent nature of renewable energy sources.1, 2 Compared with lithium ion batteries (LIBs), which have been widely utilized in commercial portable electronics, sodium ion batteries (SIBs) are more promising for large-scale energy storage system owing to the lower cost and the much higher abundance of sodium.3 Fruitful achievements have been made in SIB cathodes by applying the knowledge learned from the exploration of LIBs.4-7 Unfortunately, graphite, the commercial anode for LIBs, turns out to be almost inactive in SIBs because the superior difficulty in forming staged Na-intercalation compounds.8,9 The exploitation of suitable anode materials is still a great challenge. Metal sulfides8-16 (SnS2, Sb2S3, Bi2S3, MoS2, and etc.), offering high theoretical capacity via conversion and/or alloying with Na+, have been under active investigation in recent years. Among these anode materials, SnS2 undergoes both conversion reaction between Na+ and SnS2 and alloying reaction of Sn with Na, resulting in highest theoretical sodiation/desodiation capacity (~1136 mAh g-1).10-12, 19-25

Additionally, SnS2 has a classical CdI2-type crystal structure, which consists two 2 / 32


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closed-packed layers of S atom layers with Sn atoms sandwiched between them. The adjacent sulfur layers of these triple layers with an interlayer distance of 0.59 nm are bound by weak van der Waals interactions, thus, SnS2 crystals could allow fast intercalation/deintercalation of Na+ in the first step of conversion reaction.19,

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Unfortunately, the poor electrical conductivity and the huge volume change during sodiation/desodiation lead to the pulverization of active materials and the sudden decrease in capacity, which extremely impede its practical applications.21,23 Building SnS2/carbon composites is the most common strategy to address the above challenge. Xiong et al. demonstrate the enhanced cycling stability through bonding SnS2 nanocrystals on amino-functionalized graphene.11 Li et al. prepared SnS2 cross-linked by carbon nanotubes (CNTs) which achieved a high reversible capacity of 758 mAh g-1 at 100 mA g-1 and a superior rate capability as well.27 However, these composite materials generally adopt high cost graphene or CNTs, and in the meantime, the lengthy and complicated processes will also inevitably limit the potential large-scale applications. On the other hand, nanoarchitectured arrays (NAAs) represents a promising architecture that has been successfully utilized as battery electrodes.28-34 Depending on the structure and integrated way of the active materials, various building units have been demonstrated for the construction of NAAs such as nanowires, nanotubes and nanowalls.28,29,34-37 In the NAAs electrodes, the oriented nanoarrays directly grown on the conductive substrates constructs directional pathways enabling fast electron transport. Moreover, the porous networks of NAAs ensure the good wetting of the electrolyte and therefore the fast ion access to 3 / 32



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electrolyte; in the meantime, the nanoscopic nature of the building nanounits could shorten the diffusion length and enable the rapid ion transport. In the consideration of the typical layered structure of SnS2, it is greatly desirable to construct NAAs through two dimensional (2D) nanowall units, namely nanowall arrays (NWAs). Besides the features of NAAs presented above, NWAs could provide steady interconnected networks to resist the volume fluctuation during sodiation/desodiation processes. Herein, for the first time, one-step pulsed-spray evaporation chemical vapor deposition (PSE-CVD) is selected to prepare SnS2 NWAs as anodes of SIBs. The nature of direct growth of SnS2 nanowalls assures the robust adhesion to the current collector without using any binder or carbon black. Compared to the counterpart of SnS2 nanoparticles (NPs) electrode, SnS2 NWAs electrode exhibits a high reversible capacity of 576 mAh g-1 at 500 mA g-1 with enhanced cycling stability. In the meantime, an superior rate capability is realized with ~370 mAh g-1 at 5 A g−1, which corresponds to a capacity retention of 64.2% at 500 mA g-1.

Experimental section Preparation of SnS2 NWAs and SnS2 NPs A home-built PSE-CVD reactor, presented in our previous report,38-40 was used for depositing the SnS2 NWAs. PSE-CVD has been verified as a powerful technique for the fabrication of metal, metal oxide and composite films.41-43 This is the first attempt to apply PSE-CVD for the deposition of metal sulfide. Beyond material composition, the particle size and nanostructure of target films can be also controlled by the direct pulsed liquid delivery (DPLD) units of PSE-CVD. 40,44-45 The ethanol 4 / 32


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solution of CN2H4S (30 mM) and that of SnCl4·5H2O (15 mM) were used as precursor solutions for all deposition. These two solutions were alternately introduced into the chamber as a fine spray with a pulse width of 15 ms and a spacing interval of 485 ms in between. Figure 1 demonstrate the sequences (time) for the growth of SnS2 NWAs. The total deposition time was 1.5 h. The evaporation zone and transport zone of this set-up were fixed at 160 °C and 180 °C, respectively. Stainless steels were used as substrates, which were kept at 280 °C during the deposition process. The mass flow rate of carrier gas (N2) was 800 sccm and the system pressure during deposition was kept at 1000 Pa. For comparison, SnS2 NPs was also prepared under a similar route except that the working pressure and temperature of substrates were kept at about 10000 Pa and 300 °C, respectively. Materials characterization X-ray diffraction (XRD) analysis of as-deposited films was performed on a Rigaku D/Max-2550pc X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å), ranging from 20 ° to 80 ° at a slow rate of 1 °/min. Scanning electron microscopy (SEM, Hitachi S-4800) was conducted to observe the surface morphology of films. Transmission electron microscopy (TEM) was conducted using a FEI Tecnai G2 F20 microscope worked at 200 kV. Electrochemical measurement A microbalance (Sartorius CPA26P, accuracy 0.002 mg, Germany) was used for the calculation of the mass loading of the active materials by weighing the mass of the stainless steel substrates before and after the deposition directly. The specific mass 5 / 32



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loading is ~0.3 mg cm-2. The as-deposited films were directly applied as the working electrodes, and sodium foils were used as the counter/reference electrode. The electrolyte was 1 M NaPF6 in solvent mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) (4:4:2 by volume) with 5% fluoroethylene carbonate (FEC) added. Celgard 2300 microporous polypropylene was used as the separator of the cells. The CR2025 coin type cells were assembled in an argon filled glove box with both moisture and oxygen levels less than 1 ppm. Galvanostatic measurements and cyclic voltammetry (CV) tests were carried out in the voltage range of 0.01-2.5 V (vs. Na/Na+) using a battery test system (Neware BTS-5) and a CHI660C electrochemistry work station, respectively. Electrochemical impedance spectroscopy (EIS) measurements were conducted under open circuit conditions in the frequency range of 100 kHz-0.01 Hz by an amplitude of 5 mV at room temperature.

Results and discussion As shown in Figure S1, SnS2 NWAs are successfully grown directly on stainless steel substrate in the whole temperature range of 260 to 300 °C with neglected difference when the chamber pressure is fixed at 1000 Pa, indicating that the substrate temperature seems not to be the key factor affecting the morphology in our deposition temperature range. As shown in Figure 2b-c, the SnS2 NWAs have been uniformly and vertically grown composed of interconnected 2D SnS2 nanowalls with a length of 500-1500 nm and thickness around 50 nm. Upon increasing the chamber pressure to 10000 Pa, the nucleation rate is greatly accelerated and the SnS2 film has transformed 6 / 32


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to be an accumulation morphology of densely agglomerated nanoparticles (NPs) with a wide range of sizes of 50-500 nm, which is included for electrochemical comparison (Figure 2d). The thickness of the SnS2 NWAs and NPs films are around 5.1 µm and 810 nm, respectively. The microstructures of as-prepared SnS2 NWAs and SnS2 NPs were determined by XRD as shown in Figure 2a. It can be seen the SnS2 NWAs exhibits high crystallinity with sharp diffraction peaks, which can be readily assigned to a hexagonal phase (PDF No. 23-0677). In the case of the SnS2 NPs, the only peak at around 2θ = 14.88 ° indicates that the preferred direction for the SnS2 NPs is along (001) plane, which is in agreement with the sample prepared by the spray pyrolysis method.46 TEM analysis was conducted to identify the refined morphology and the microstructure of SnS2 NWAs and SnS2 NPs (Figure 3), which is in agreement with the SEM images. The lattice fringes with d-spacings of 0.216 and 0.278 nm are clearly observed in high resolution TEM (HR-TEM) images of both SnS2 NWAs and SnS2 NPs (Figure 3b,e), corresponding to the (102) and (101) planes of hexagonal SnS2 crystals. Meantime, the bright spots in orienting ordered arrangement in selected area electron diffraction (SAED) pattern reveal that the individual SnS2 nanowall is monocrystalline with high crystallinity (Figure 3c), while the SAED pattern of SnS2 nanoparticle indicates its poor polycrystalline nature (Figure 3f). In a further step, the detailed formation mechanisms of NWAs and NPs have been summarized based on the time-dependent deposition of film (Figure 4): (1) when the chamber pressure is fixed at 1000 Pa, initially after 10 min deposition, a golden film was obtained and a small quantity of nanosheets observed clearly on the stainless steel 7 / 32



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surface (Figure 4a). While the deposition time was 30 min, there are more and bigger nanosheets spreading vertically on the surface of substrate (Figure 4b). Upon further increasing the deposition time to 60 min, the nanosheets grew continuously and completely covered the substrate surface to form a nanowall architecture (Figure 4c), which is similar to that obtained after 90 min deposition. (2) when the chamber pressure is increased to 10000 Pa, at the early reaction stage, there are numerous nanoparticles which are densely and uniformly distributed on the substrate (Figure 4d). After 30 min deposition, the nanoparticles tend to aggregate together with bigger size observed (Figure 4e). When the deposition was further extended to 60 min, the microsphere-like particles was formed densely on the substrate, as displayed in Figure 4f. According to the time-dependent results, the growth processes of SnS2 NWAs and SnS2 NPs are schematically summarized in Figure 4g. The morphologies of the SnS2 NWAs and SnS2 NPs are determined by mutual effects of growth kinetics and thermodynamics.47 At a lower chamber pressure (1000 Pa), the partial pressure of the reactants is relatively low so the SnS2 crystal nucleus can be only formed on the defects of the substrate first, and then the primary crystal nucleus would grow up. As reported in literature, the (001) plane of SnS2 possesses the lowest surface energy so the crystal growth rate perpendicular to the (001) plane is much lower than that parallel to the (001) plane,48 which results in the formation of an interconnected nanowall architecture. Correspondingly, the spacing between the nanowalls might be tuned through choosing another kind of substrate with different defect density, which has been verified by the enlarged spacing of nanowalls on the glass substrate (Fig. S2). 8 / 32


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In the case of higher chamber pressure of 10000 Pa, there will be a dramatic increase of nucleation rate owning to the much larger partial pressure of the reactants. The nucleation tends to occur on the whole surface followed by the growth of nanoparticles. In a further stage, the stronger interaction between particle and particle over particle-substrate will facilitate the ordered growth of dense film for the minimization of the surface energy, leading to the c-textured films (as verified by XRD analysis) due to the lowest surface energy of (001) SnS2 plane.49,50 The as-deposited SnS2 NWAs and SnS2 NPs, as working electrodes directly without using any conductive carbon or binder, were subsequently assembled into 2025-type coin cells to evaluate their sodium storage performance. The sodiation/desodiation processes of the SnS2 NWAs were first evaluated by cyclic voltammetry (CV, Figure 5a). During the first discharge process, three cathodic peaks are presented at 1.67, 1.06, and 0.49 V (vs. Na/Na+). A small shoulder at 1.67 V could be attributed to the intercalation of Na+ ions into SnS2 layers (xNa+ + SnS2 + xe- → NaxSnS2), while the peak at 1.06 is ascribed to the conversion of NaxSnS2 into metallic Sn and the formation of Na2S (NaxSnS2 + (4-x)Na+ → Sn + 2Na2S).51 The other cathodic peak at 0.49 V can be assigned to the Na-Sn alloying process (Sn + 3.75 Na+ + 3.75 e- → Na3.75Sn), and the formation of solid electrolyte interface (SEI) layer.24,27 Correspondingly, in the following negative scan, there are also three anodic peaks at 0.24, 0.77 and 1.25 V. The oxidation peak at 0.24 V is due to the de-alloying of Na3.75Sn,41 and the other two peaks centered at 0.77 and 1.25 V can be attributed to the formation of NaxSnS2 and the extraction of sodium ions from the layered structure, 9 / 32



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respectively.23 The subsequent CV scans overlap together indicating the excellent reversibility of the reactions.22 Typical galvanostatic discharge-charge voltage profiles of SnS2 NWAs were measured between 0.01 and 2.5 V at a current density of 50 mA g−1 (Figure 5b). In consistence with the CV curves, three plateaus and a sloping curve can be observed in the initial discharge curve, corresponding to the formation of NaxSnS2 (~1.75 V and 1.25 V), the conversion reaction (~0.70 V) and alloying reaction (0.01-0.50 V), respectively.51 The initial discharge and charge capacity are 765 and 576 mAh g−1, suggesting a comparatively high initial Coulombic efficiency (~75%) among the previous reports on anode materials for SIBs.1, 2 Figure 5c presents the cycling performance of the SnS2 NWAs electrode at 500 mA g−1 in the range of 0.01-2.5 V vs. Na/Na+. The SnS2 NPs electro de was also included for comparison. The SnS2 NWAs electrode exhibits a stable cycling with a high reversible capacity of 510 mAh g-1 after 100 cycles, suggesting a high capacity retention of 85%. The Coulombic efficiency since the 2nd cycle has increased to be ~99% and maintained in the subsequent cycles, implying the high reversibility of the electrochemical reactions. In contrast, the SnS2 NPs electrode delivers a much lower capacity of 380-430 mAh g-1 during the whole cycling test. The results clearly demonstrate the superiority of SnS2 NWAs structure allowing better play to sodiation/desodiation, while the NPs counterpart is poor in electrochemical access of active sites. Rate capability of SnS2 NWAs was further evaluated through galvanostatic discharging/charging at different current densities from 100 mA g-1 to 5 A g-1, as shown in Figure 5d. At the initial lower current density of 100 mA g-1, it 10 / 32


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exhibits a high reversible capacity of 730 mAh g-1. Upon increasing the current density to 200, 500, 1000 and 2000 mA g-1, the specific capacity decrease slightly to 636, 580, 533, 474 mAh g-1, respectively. Even cycled at an ultrahigh current density of 5000 mA g-1, the SnS2 NWAs electrode still delivers a reversible capacity of 370 mAh g-1, suggesting ~51% retention of capacity at 100 mA g-1, although the current density is 50 times higher. When the current density turns back to 100 mA g-1, the specific capacity could recover to over 710 mAh g-1. Contrarily, for the SnS2 NPs electrode, the specific capacity is much lower than that of NWAs electrode under all tested current densities, and only ~100 mAh g-1 capacity is retained at 5000 mA g-1, indicating the poor rate capability as well. Such an excellent rate capability is far beyond the previous results of bare SnS2 electrode and also competitive to those SnS2/graphene and SnS2/CNTs electrodes (Table S1).19,21-24,51,52 Electrochemical impedance spectroscopy (EIS) has been verified as a powerful technique to investigate the electrochemical kinetics, which was employed to understand the difference in electrochemical behaviors between the SnS2 NWAs and SnS2 NPs. Figure 6a-b presents the EIS results of the pristine state, the 1st discharge state and 1st charge state for both electrodes. All plots are composed of a semicircle in the high-medium frequency region and an inclined line in the lower frequency range. Both electrodes undergo a decrease in charge transfer resistance (Rct) after discharge, due to the improved electronic conduction of the discharge products. The Rct values for all plots are listed in Figure. 5c based on the equivalent circuit simulation. As compared to the counterpart of SnS2 NPs, the SnS2 NWAs electrode demonstrates 11 / 32



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much lower charge transfer resistance (Rct) in all corresponding states, indicating superior electrochemical kinetics of SnS2 NWAs electrode. To obtain further insight into the electrochemistry of sodiation/desodiation processes, a series of CV scans at different scan rates ranging from 0.1 to 2 mV s-1 were recorded to calculate the diffusion coefficient for both electrodes, as shown in Figure 6d-e. It can be seen that the intensity of both the anodic and cathodic peaks increase with the increased scan rate, since the areas of close CV curves divided by the scan rate yield the capacity of the electrode, which is theoretically a constant. As shown in Figure 6f, the linear relationship between the peak current (Ip) and the square root of the scan rate (v1/2) indicates that the electrochemical kinetics is controlled by the process of Na+ ion diffusion. Thus, the Na+ coefficient can be calculated by the classical Randles-Sevcik equation:53,54 Ip = 2.69 ×105 n3/2AD1/2 C0 v1/2

(1)

Where n is the number of electrons transferred in the reaction, D is the Na+ diffusion coefficient, A is the electrode area and C0 is the concentration of Na+ in the solution. According to Equation (1), the Na+ diffusion coefficients of SnS2 NWAs and SnS2 NPs electrodes are calculated to be 3.17×10-15 and 1.28×10-15 cm2 s-1, respectively. It is obvious that the Na+ diffusion coefficient of SnS2 NWAs is almost 2.5 times the value of SnS2 NPs. The much higher diffusion coefficient further verifies the superior sodium storage performance of SnS2 NWAs. The morphology of cycled electrode was further characterized to investigate the difference in sodiation/desodiation processes for both SnS2 NWAs and SnS2 NPs, as 12 / 32


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shown in Figure 7. In the case of SnS2 NWAs, the open space between nanowalls with high surface areas provides effective electrolyte accessible channels for fast ion transportation so that excellent rate capability could be acquired. Meanwhile, these interconnected nanowalls leaves enough space and also provides a good support for inevitable volume change, maintaining the integrity of the geometry during the discharge and charge process. Oppositely, for the SnS2 NPs, the spontaneous atom migration and huge volume change during sodiation/desodiation processes induce the restack of active materials; moreover, the dense structure of SnS2 NPs and non-existed Na+ diffusion channels prevents the electrochemical reactions occurring in the inner layer, resulting in huge stress between surface layer and inner one, and therefore large cracks formed as shown in Figure 7. All the characterization presented above clearly elucidates the advantages of SnS2 NWAs in sodium storage capability, which could be ascribed to its unique morphology and growth process: (1) the present one step CVD-grown NWAs grown vertically on current collector constructs fast pathway for electron transport, in the meantime, the nature of growth ensures the firm contact between active materials and current collectors, stabilizing the array structure during sodiation/desodiation; (2) the 2D nanoscopic SnS2 and its intrinsically layered structure are beneficial for shortened diffusion length and rapid ion transport; (3) the interconnected NWAs networks preserve enough void space both for electrolyte wetting and volume expansion during sodiation, leading to fast kinetics and enhanced cycling stability. Furthermore, there is no need for any binder or conductive carbon since bare SnS2 could be directly grown 13 / 32



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on current collectors, which can be also extended to the growth of arrays of other active materials for superior sodium storage.

Conclusions In conclusion, a rational design of SnS2 NWAs is successfully demonstrated for high performance sodium storage through direct growth on stainless steel substrates via a novel PSE-CVD technique. The as-prepared SnS2 NWAs display a network architecture with thin SnS2 nanowalls growing vertically on the surface of substrates. This unique structure combines the vertically grown arrays for fast electron transport and the nanoscopic nature of nanowalls for rapid ion transport, as well as the steady structure upon sodiation/desodication, leading to fast and cycle-stable sodium storage. Such NWAs electrode presents a high reversible capacity of 576 mAh g-1 at 500 mA g-1 with enhanced cycling stability, and a superior rate capability with 370 mAh g-1 at 5 A g−1. The rational growth technique and the unique structure provide great potentials in extending to other electrode materials for SIBs in the future.

Supporting Information SEM images of SnS2 NWAs deposited at 260, 280 and 300 °C, SEM images of the as-deposited SnS2 NWAs on glass, Comparison of rate capability of the present SnS2 NWAs in this work with those of others reported in literature.

Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 14 / 32


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21373184), Public Projects of Zhejiang Province (2015C31039), Fundamental Research Funds for the Central Universities (2016QNA4007 & 2016XZZX005-07), and Opening Project of CAS Key Laboratory of Materials for Energy Conversion (KF2016002).

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Vijayakumar,

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Sanjeeviraja,

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M.;

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Yan,

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Figure Captions Figure 1 the deposition sequence (time per pulse) used for the growth of SnS2 NWAs and SnS2 NPs. Figure 2 (a) X-ray diffraction pattern of as-deposited SnS2 NWAs and SnS2 NPs. (b) SEM image and (c) HRSEM image of the as-deposited SnS2 NWAs. (d) HRSEM image of the as-prepared SnS2 NPs. Figure 3 (a) TEM images and (b) HRTEM image of SnS2 NWAs. (c) Corresponding SAED pattern of SnS2 NWAs. (d) TEM images and (e) HRTEM image of SnS2 NPs. (f) corresponding SAED pattern of SnS2 NPs. Figure 4 SEM images of SnS2 NWAs obtained at 1000 Pa for the time of (a) 10 min, (b) 30 min and (c) 60 min; SnS2 NPs obtained at 10000 Pa for the time of (c) 10 min, (d) 30 min and (e) 60 min; (g) schematic mechanism for the fabrication of SnS2 NWAs and SnS2 NPs on stainless steel. Figure 5 (a) CV curves up to three cycles for SnS2 NWAs at a scan rate of 0.1 mV s−1 23 / 32



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in the voltage range of 0.01 to 2.5 V. (b) Charge/discharge curves of cycling performance of SnS2 NWAs. (c) Cycling performance of SnS2 NWAs and SnS2 NPs anodes at a current density of 500 mA g-1 between 0.01 and 2.5 V. (d) Rate performance of SnS2 NWAs and SnS2 NPs. Figure 6 The electrochemical impedance spectroscopy of (a) SnS2 NWAs and (b) SnS2 NPs before discharge, after 1st discharge, and after 1st charge. (c) The corresponding equivalent circuit used to simulate EIS curves and electrochemical impedance spectra of SnS2 NWAs and SnS2 NPs electrodes, experimental (dot) and simulated (line). The CV curves in the potential range of 0.01-2.5 V at scanning rates of 0.1, 0.2, 0.5, 1.0, and 2.0 mV s−1 for (d) SnS2 NWAs and (e) SnS2 NPs. (f) The relationship between Ip and v1/2 for SnS2 NWAs andSnS2 NPs. Figure 7 Schematic illustration of morphological changes of SnS2 NWAs and SnS2 NPs during the sodiation/desodiation processes.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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