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Two-Dimensional Tin Disulfide Nanosheets for Enhanced Sodium Storage Wenping Sun,†,‡ Xianhong Rui,† Dan Yang,† Ziqi Sun,^ Bing Li,# Wenyu Zhang,† Yun Zong,# Srinivasan Madhavi,*,†,§ Shixue Dou,*,‡ and Qingyu Yan*,†,§ †
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore, ‡Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia, §Energy Research Institute@NTU, Nanyang Technological University, Research Techno Plaza, Singapore 637553, Singapore, ^School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Gardens Point, Brisbane, QLD 4000, Australia, and #Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Singapore 117602, Singapore
ABSTRACT Sodium-ion batteries (SIBs) are considered as
complementary alternatives to lithium-ion batteries for grid energy storage due to the abundance of sodium. However, low capacity, poor rate capability, and cycling stability of existing anodes significantly hinder the practical applications of SIBs. Herein, ultrathin two-dimensional SnS2 nanosheets (34 nm in thickness) are synthesized via a facile refluxing process toward enhanced sodium storage. The SnS2 nanosheets exhibit a high apparent diffusion coefficient of Naþ and fast sodiation/desodiation reaction kinetics. In half-cells, the nanosheets deliver a high reversible capacity of 733 mAh g1 at 0.1 A g1, which still remains up to 435 mAh g1 at 2 A g1. The cell has a high capacity retention of 647 mA h g1 during the 50th cycle at 0.1 A g1, which is by far the best for SnS2, suggesting that nanosheet morphology is beneficial to improve cycling stability in addition to rate capability. The SnS2 nanosheets also show encouraging performance in a full cell with a Na3V2(PO4)3 cathode. In addition, the sodium storage mechanism is investigated by ex situ XRD coupled with high-resolution TEM. The high specific capacity, good rate capability, and cycling durability suggest that SnS2 nanosheets have great potential working as anodes for high-performance SIBs. KEYWORDS: tin disulfide . nanosheets . anode . sodium-ion batteries
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odium-ion batteries (SIBs) are considered as complementary alternatives to lithium-ion batteries (LIBs) for largescale grid energy storage systems due to the abundance of sodium.1,2 To date, the major obstacle in realizing practical SIBs is that the current developed electrode materials cannot meet the demands for high capacity, fast, and durable sodium storage. Thus, ever-growing research attention has been devoted to exploring high-performance electrode materials recently.24 On the basis of the database of cathode materials for LIBs, the research on cathode materials for SIBs is very progressive. Various layered oxides and phosphate-based materials have been investigated as cathode materials for SIBs and showed promising electrochemical performances, such as P2-Na2/3MnO2-based oxides57 and Na3V2(PO4)3.812 Some organic compounds also SUN ET AL.
attracted considerable attention for SIB cathodes due to resource renewability, lower cost, and better safety features.1315 In contrast, progress on anode materials is relatively limited. Graphite, which is the state-of-the-art anode for commercial LIBs, is nearly electrochemically inactive for sodium storage.2,3 Although some lower crystalline carbons (e.g., hard carbons) and carbon-based nanostructures show improved sodium storage performance,1620 the sodium storage ability in terms of specific capacity and rate capability is still unsatisfactory toward SIBs with high energy density. Similar to lithium storage by the conversion reaction, many metal oxides possess high theoretical capacities for sodium storage, such as NiCo2O4,21 NiO,22,23 Fe3O4,24 Fe2O3,23,25,26 Co3O4,23,27,28 and CuO.29,30 However, nearly all the reported metal oxides deliver reversible capacities of VOL. 9
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* Address correspondence to
[email protected] (S. Madhavi);
[email protected] (S. Dou);
[email protected] (Q. Yan). Received for review August 21, 2015 and accepted October 20, 2015. Published online October 20, 2015 10.1021/acsnano.5b05229 C 2015 American Chemical Society
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supercapacitors.22,44,46,49 In this work, 2D SnS2 nanosheets (34 nm in thickness) were synthesized via a facile refluxing process by controlling the reaction temperature. The formation mechanism of the SnS2 nanosheets was proposed based on the time-dependent morphology evolution of the products. The SnS2 nanosheets exhibited excellent sodium storage capability with high specific capacity, good cycling stability, and promising rate capability. The corresponding sodium storage mechanism of SnS2 was also investigated by ex situ X-ray diffractometry (XRD) coupled with highresolution transmission electron microscopy (HRTEM) technology.
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less than 400 mAh g1, which is far lower than those for lithium storage. The low reversible capacity is very possibly induced by the sluggish reaction kinetics between sodium and metal oxides. Recently, tin-based compounds have attracted considerable attention and have been investigated as anode materials for SIBs due to their high theoretical capacities.3143 Tin has a theoretical capacity of 847 mAh g1 by the alloying reaction forming Na15Sn4. Considering the additional capacity contribution from the conversion reaction, tin oxides and sulfides possess higher theoretical capacities compared to metallic tin. Moreover, tin sulfides appear to be more promising due to their superior delivered performance. On the basis of the combined conversion and alloying reaction, SnS2 and SnS can deliver a theoretical capacity of 1136 and 1022 mAh g1, respectively. The hybrid nanostructured composite composed of reduced graphene oxide (RGO) and SnS delivered a high specific capacity of 492 mAh g1 after 250 cycles at a current density of 810 mA g1.38 Choi et al. reported a SnS-C composite prepared through spray pyrolysis, and the composite showed 433 mAh g1 after cycling at 500 mA g1 for 50 cycles with capacity retention of 89%.42 The SnS2/RGO composite exhibited 500 mAh g1 after 400 cycles at 1 A g1, and 84% capacity retention was retained.39 Considering the higher theoretical capacity, SnS2, which possesses a CdI2-type layered hexagonal structure, is a more competitive anode material toward high-performance sodium storage. Worthy of noting, bare SnS or SnS2 exhibits poor rate and cycling stability,39,41,42 which is mainly caused by the sluggish reaction kinetics and severe volume change during the sodiation/desodiation process. Although the performance was improved after incorporating a considerable amount of RGO or carbon to accommodate volume change, this would be detrimental to the energy density of the electrode because of the low capacity of RGO and carbon.18,38,39,41 Therefore, it is urgent to develop other efficient strategies to enhance the sodium storage capability of bare SnS2. In addition to high theoretical capacity, the delivered reversible capacity of the electrode materials is also closely related with the reaction kinetics during the charging/discharging process. In this case, engineering nanostructures and reducing dimensionality would play a critical role. It has been strikingly highlighted that two-dimenstional (2D) nanostructures can significantly shorten the diffusion pathway and accelerate the diffusion efficiency of metal ions, consequently enhancing the electrochemical performance for lithium and sodium storage.4449 Moreover, 2D nanostructures are more capable of buffering volume change during the charging/discharging process compared to bulk materials, thereby leading to improved cycling stability, which has been widely demonstrated in electrode materials for batteries and
RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of the as-prepared SnS2. All the peaks in the patterns can be indexed to layered hexagonal SnS2 (JCPDS card No.79-5206). Clearly, the peak intensity increases with increasing reaction temperature, suggesting better crystallinity at a higher reaction temperature. Notably, for the samples synthesized at 140 C (140SS) and 160 C (160SS), the relative intensity of the (001) peak is much weaker compared to the standard pattern and the sample synthesized at 180 C (180SS), indicating that the growth of the (001) crystal plane is suppressed at low temperatures. Figure 1bd show the scanning electron microscopy (SEM) images of SnS2 synthesized at different temperatures. As can be seen, the synthesis temperature has a significant influence on the morphology of SnS2. SnS2 synthesized at 140 C shows a hierarchical sphere morphology (around 1 μm in size), on the surface of which nanosheets appear. The product shows crumpled sheet morphology when the synthesis temperature is increased to 160 C. According to the AFM result (Figure S1, Supporting Information), the nanosheets are 34 nm in thickness. On further increasing the temperature to 180 C, the morphology evolves to nanoplates with a thickness of about 1015 nm. The nanoplates tend to aggregate to assemble hierarchical spheres (around 2 μm in size). In such a growth process, the mass diffusion and precipitationsolubility rates are higher at 180 C, and this would induce a relatively faster lateral growth, eventually resulting in the formation of thicker SnS2 nanoplates. The detailed morphology and crystal structure of SnS2 were analyzed by TEM and HRTEM. Figure 2a shows a typical TEM image of 140SS. It confirms that nanosheets are grown on the surface of the solid SnS2 sphere. In the HRTEM image (Figure 2b), a few crystallized nanograins can be observed, which is consistent with the broad diffraction peak in the XRD result (Figure 1a). The lattice spacing of 0.32 nm can be assigned to the (100) crystal plane. The TEM image of 160SS (Figure 2c) confirms the nanosheet morphology. As can be seen, the lateral size of the nanosheet is a few VOL. 9
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ARTICLE Figure 1. XRD patterns (a) and SEM images of SnS2 synthesized at 140 (b), 160 (c), and 180 C (d). The insets in panels c and d are the corresponding high-magnification SEM images.
micrometers and appears crumpled. In the HRTEM image of SnS2 nanosheets (Figure 2d), the lattice spacings of 0.28 and 0.32 nm correspond to the (011) and (100) crystal planes, respectively. The corresponding fast Fourier transform (FFT) pattern shows a spot pattern along the [011] zone axis. This indicates that these nanocrystals show major exposure of the (011) facets. Figure 2e shows the TEM image of 180SS, which is also in good agreement with the SEM result that the nanoplates are assembled into hierarchical flower-like structures. The (111) and (100) crystal planes with lattice spacings of 0.28 and 0.32 nm can be resolved from the HRTEM image (Figure 2f). The formation mechanism of 160SS nanosheets was investigated based on time-dependent morphology evolution of the product (Figure S2, Supporting Information). As is shown, the morphology of the products is time-dependent. Nanoparticles are formed during the initial nucleation stage when the reaction temperature just reaches 160 C (Figure S2a,b, Supporting Information). The nanosheet formation can possibly be interpreted by the mechanism of Ostwald ripening, in which process low-energy facets are exposed to reduce the surface energy. Due to the high intrinsic anisotropic property of SnS2, the initial formed spheres prefer to grow into 2D nanosheets with exposure of (011) facets as the primary surfaces. With prolonging the reaction time, more and more nanosheets are formed because of mass diffusion and Ostwald SUN ET AL.
ripening (Figure S2ce, Supporting Information). Eventually, the spheres are exhausted and are transformed into crumpled nanosheets (Figure S2f, Supporting Information). The dependence of chemical composition on the synthesis temperature was also studied. X-ray photoelectron spectroscopy (XPS) analysis was used to confirm the oxidation states of Sn in the three samples. Figure 3 presents the high-resolution XPS spectra of Sn 3d for SnS2 synthesized at different temperatures. As can be seen, the Sn 3d5/2 and 3d3/2 peaks clearly split into two peaks for 140SS (Figure 3a), which is due to two different oxidation states of Sn.50 The peaks of 496.0 and 487.6 eV are ascribed to Sn 3d3/2 and 3d5/2 of Sn4þ, respectively, and the peaks at 495.1 and 486.7 eV are assigned to Sn 3d3/2 and 3d5/2 of Sn2þ, respectively. Furthermore, the Sn2þ/Sn4þ ratio measured with XPS analysis is estimated to be about 1.4 for 140SS. For 160SS (Figure 3b) and 180SS (Figure 3c), both Sn 3d5/2 and 3d3/2 peaks are located at around 496.0 and 487.6 eV, and Sn 3d peaks corresponding to Sn2þ account for only a very small fraction based on the fitting results, indicating that Sn exists mainly in the oxidation state of Sn4þ in 160SS and 180SS. As indicated in Figure 1a, 140SS is not well crystallized and contains a considerable amount of amorphous phase. Sn2þ might mainly come from the amorphous phase. On increasing the reaction temperature, most Sn2þ would be oxidized to Sn4þ, correspondingly generating VOL. 9
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ARTICLE Figure 2. TEM and HRTEM images of SnS2 synthesized at 140 C (a, b), 160 C (c, d), and 180 C (e, f). The insets in panels d and f are the corresponding fast Fourier transform (FFT) patterns.
well-crystallized SnS2. Besides, as observed from the energy-dispersive X-ray spectra (EDS) of SnS2 (Figure 3d), the S peak of 160SS and 180SS shows higher intensity and larger area compared to that of 140SS, further confirming that Sn4þ accounts for a higher proportion in 160SS and 180SS, which is well in agreement with the XPS results. The S/Sn ratio is calculated to be 1.58, 1.88, and 1.89 for 140SS, 160SS, and 180SS, respectively. Sodium storage via the conversion reaction contributes significantly to the total capacity of SnS2. 160SS and 180SS predominated by Sn4þ would contribute more electrons for the conversion reaction, and this is vital to deliver high sodium storage capacity. To evaluate the sodium storage ability, coin-type half-cells were assembled and the electrochemical SUN ET AL.
measurements were performed at room temperature. The cyclic voltammetric (CV) curves of the cell with a 160SS anode tested at a scan rate of 0.1 mV s1 are shown in Figure 4a. During the first cathodic scan, there are two cathodic peaks at around 1.66 and 0.59 V, respectively. The cathodic peak located at 1.66 V can be assigned to the intercalation of Naþ into SnS2, forming NaxSnS2.39,41 The broad peak located at 0.59 V corresponds to the conversion reaction accompanied by the formation of metallic Sn and Na2S and the alloying reaction between Sn and Na. Meanwhile, the formation of a solid electrolyte interphase (SEI) might also occur at this stage. This cathodic peak was split into two peaks located at 0.64 and 0.86 V in the subsequent cycles. In the anodic processes, the broad oxidation peak located at 1.26 V corresponds to the reversible VOL. 9
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ARTICLE Figure 3. XPS spectra of SnS2 synthesized at 140 C (a), 160 C (b), and 180 C (c) and EDS spectra of the three SnS2 samples (d).
Figure 4. (a) CV curves of a fresh SIB with a 160SS electrode at a scan rate of 0.1 mV s1 within a potential range of 0.001 to 3.0 V (vs Na/Naþ). (b) CV curves of the 160SS electrode at various scan rates. (c) Linear relationship of the cathodic peak current (ip) and the square root of the scan rate (ν1/2) for 140SS, 160SS, and 180SS. (d) Linear relationship of the anodic peak current (ip) and the square root of the scan rate (ν1/2) for 140SS, 160SS, and 180SS.
dealloying reaction and the restitution of tin sulfide.39 The CV curves for the following four cycles overlap well SUN ET AL.
with each other, indicating the excellent reversible sodium storage of SnS2 after the initial activation cycle. VOL. 9
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ARTICLE Figure 5. (a) First chargedischarge profiles of the cells with 140SS, 160SS, and 180SS electrodes at a current density of 0.1 A g1. The inset is the enlarged profiles of the discharge curves between 1.3 and 2.2 V. (b) Chargedischarge profiles of the cell with a 160SS electrode at 0.1 A g1. The inset is the enlarged profiles of the discharge curves between 1.3 and 2.2 V. (c) Cycling performances of the cells with 140SS, 160SS, and 180SS electrodes at 0.1 A g1. The inset is the Coulombic efficiency (CE) of the three cells. (d) Rate capabilities of the three cells with 140SS, 160SS, and 180SS electrodes.
Figure 4b shows the CV curves of the 160SS anode at different scan rates ranging from 0.1 to 2 mV s1. The intensity of the cathodic and anodic peaks increases with increasing the scan rate. Theoretically, the peak area divided by the scan rate yields the capacity of the electrode, which is considered to be constant. Thus, the peak intensity of the CV curve varies with the scan rate. When the scan rate increases from 0.1 mV s1 to 2 mV s1, the CV profile of 140SS (Figure S3a, Supporting Information) is seriously distorted, while the basic CV profiles of 160SS and 180SS (Figure S3b, Supporting Information) are well preserved. Obviously, 160SS and 180SS are capable of sustaining a quick CV response to a fast potential sweep as compared with 140SS. Such quick CV response to scan rate indicates that 160SS and 180SS might possess a better rate capability for sodium storage. The dependence of the cathodic and anodic peak currents on the square root of the scan rate (ν1/2) is presented in Figure 4c,d. It can be clearly seen that both cathodic and anodic peaks show a linear relationship with the square root of the scan rate, which suggests that the sodiation/desodiation reaction rate is diffusion-controlled. Herein, the classical Randles Sevchik equation (eq 1) for a semi-infinite diffusion of Naþ into the SnS2 anode can be applied to further interpret this phenomenon.51,52 ip ¼ (2:69 105 )n3=2 SD1=2 Cν1=2 SUN ET AL.
(1)
where ip is the peak current (A), n is the chargetransfer number, S is the electrode area, D is the diffusion coefficient of Naþ (cm2 s1), C is the concentration of sodium ions, and ν is the potential scan rate (V s1). Given the three electrodes were prepared and tested by the same procedure, the Randles Sevchik equation can be simplified as ip ¼ AD1=2 ν1=2
(2)
where A is supposed to be constant for the three cells and AD1/2 is defined as the apparent diffusion coefficient of Naþ in the cells, which can be calculated by fitting the linear curves shown in Figure 4c,d. In this case, the peak current variation of CV curves for the three cells is determined by the Naþ diffusion and scan rates. On the basis of the fitting results, it can be found that 160SS shows the highest apparent diffusion coefficient of Naþ and 140SS exhibits the lowest diffusion coefficient. As stated in the introduction, the ultrathin 2D nanostructure of 160SS significantly shortens the Naþ transport pathway and is the most possible reason for the efficient diffusion of Naþ.44,47,49 The accelerated Naþ diffusion behavior would improve the sodiation/desodiation reaction kinetics and consequently enhance the sodium storage ability of 160SS. Figure 5a shows the first galvanostatic charge discharge profiles of the cells with SnS2 anodes VOL. 9
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at 160 mA g1.27 However, it has to be mentioned that capacity degradation induced by the volume change still exists, and hence special attention is needed to further mitigate this problem. Figure 5d presents the rate performance of the three cells. As can be seen, the cell with a 160SS electrode exhibits the best rate capability. The second charge capacities of the cell are 685, 660, 647, 567, 496, and 435 mAh g1 at current densities of 0.2, 0.3, 0.5, 1, 1.5, and 2 A g1, respectively. Moreover, the cell still has a charge capacity retention of 710 mAh g1 when the current is switched to 0.1 A g1, further confirming the good cycling durability of 160SS. In contrast, the second charge capacity is only 211 and 258 mAh g1 at 2 A g1 for 140SS and 180SS, respectively. As can be seen from the charge discharge curves at 1 A g1 (Figure S5, Supporting Information), the profile of 160SS is well preserved. Also, 160SS shows a much lower polarization compared to the other two samples. Lower polarization suggests faster charge transfer and mass diffusion kinetics, which is critical for the superior rate performance of 160SS. The rate performance of 160SS, which shows observable superiority to other reported SnS2 and SnO2 electrodes,33,36,38,39,41,43 is one of the best for SIB anodes. The promising rate performance of 160SS well validates the aforementioned results that the unique ultrathin morphology is beneficial to achieve fast sodium diffusion kinetics. The possible mechanism for the sodiation/desodiation reaction was further studied by ex situ XRD and HRTEM analysis of the SnS2 electrodes at various discharge/ charge potentials in the first cycle, as shown in Figure 6. During the discharge process at a plateau of around 1.6 V (stage B), the electrode keeps the same crystal structure as the pristine one, indicating that sodium inserts into SnS2 forming NaxSnS2.39,41 The electrode becomes amorphous after this discharging stage (stage C), and this suggests that the crystal structure of SnS2 breaks down upon intercalating sodium ions because the layered structure cannot accommodate too many sodium ions. The conversion reaction occurs following the sodium intercalation reaction. SnS can be resolved when the cell is discharged to around 0.8 V (stage E), which is confirmed by XRD and ex situ HRTEM results (Figure 6b). Additionally, as can be seen from Figure 6b, lattice fringes corresponding to β-Sn can also be observed, indicating the conversion reactions of SnS2 to SnS and SnS to β-Sn occur simultaneously during discharging. Then, SnS gradually transforms to the β-Sn phase in the following discharge process via conversion reaction, and this process might also be accompanied by the alloying reaction, forming NaySn.38,55 No obvious peaks corresponding to Na2S and NaySn can be observed from the XRD patterns, indicating that Na2S and NaySn are not well crystallized during the sodiation reaction.38,55 The β-Sn phase still exists when the cell is discharged to 0.005 V (stage G), VOL. 9
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between 0.005 and 3.0 V at a current density of 0.1 A g1. The discharge plateau located at around 1.65 V that corresponds to the intercalation of Naþ into SnS2 is most significant for 180SS, and this is well consistent with the CV results (Figure S4, Supporting Information). The first discharge capacity is 1171, 1232, and 1128 mAh g1 for 140SS, 160SS, and 180SS, respectively; correspondingly, the first charge capacity is 500, 733, and 644 mAh g1. The first discharge capacities are close to the theoretical capacity of SnS2 (1136 mAh g1). Considering that SEI formation and electrolyte decomposition also contribute to the first discharge capacity for SIBs,3,41 it may be deduced that the electrode materials are not fully involved in the sodiation reaction. The significant charge capacity difference can be well interpreted by the chemical composition and morphology variation of the three electrodes. Compared to 160SS, the relatively slow sodium diffusion kinetics is a response to the lower capacity of 180SS. For 140SS, not only the particle morphology (corresponding to slower sodium diffusion) but also the chemical composition (corresponding to lower theoretical capacity) contribute to its lower capacity. Figure 5b shows the galvanostatic charge discharge profile of the cell with a 160SS anode during different cycles at 0.1 A g1. From the second cycle, the cell shows similar chargedischarge profiles, suggesting that sodium storage is well reversible, which is in agreement with the CV result (Figure 4a), and the discharge plateau at around 1.65 V disappears, which confirms that the sodium intercalation process in the first cycle is irreversible. Besides, the Coulombic efficiency (CE) increases accordingly from 59% to around 94% over the cycling, and this can be ascribed to the passivation of the electrode.3 Figure 5c shows the cycling performance of the cells at 0.1 A g1. The cell with a 160SS electrode shows not only the highest reversible sodium storage capacity but also the best cycling durability and still delivers a specific charge capacity of 647 mAh g1 during the 50th cycle at 0.1 A g1. In contrast, the capacity decays to 335 and 417 mAh g1 for 140SS and 180SS, respectively. The improved cycling stability of 160SS is possibly attributed to the crumpled ultrathin 2D structure, which can effectively accommodate the volume change during the charging/discharging process and hence helps to mitigate electrode pulverization.22,44,46,49 The cycling performance of 160SS is also dramatically enhanced compared to other reported bare SnS2 electrode and is even comparable with the SnS2/RGO composite electrodes.38,39,41 The specific capacity of SnS2 nanosheets is much superior to some other materials that store sodium by means of conversion reaction.23,25,2730,53,54 Fe2O3@RGO exhibited a reversible specific capacity of 400 mAh g1 at 100 mA g1,25 and the Co3O4@CNTs composite showed a reversible specific capacity of 425 mAh g1
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ARTICLE Figure 6. (a) Ex situ XRD patterns of the SnS2 electrode collected at various points, as indicated in the corresponding chargedischarge profiles: (A) fresh electrode, (B) after first discharge to 1.65 V, (C) after first discharge to 1.6 V, (D) after first discharge to 1.2 V, (E) after first discharge to 0.8 V, (F) after first discharge to 0.5 V, (G) after first discharge to 0.005 V, (H) after first charge to 1.2 V, and (I) after first charge to 3.0 V; ex situ HRTEM images of the electrode after first discharge to 0.8 V (b), after first discharge to 0.005 V (c), and after first charge to 3.0 V (d).
as presented in Figure 6a and c. The presence of β-Sn suggests that the electrode material is not fully involved in the sodium storage reaction. Given the first discharge capacity of SnS2 is very close to the theoretical value (Figure 5a), it can be inferred that SEI formation and electrolyte decomposition contribute significantly to the capacity. During the charge process, the peaks corresponding to β-Sn vanish gradually (Figure 6a). The dealloying reaction of NaySn occurs followed by the reaction between β-Sn and Na2S, generating tin sulfide. In addition to SnS2, the lattice fringes of SnS, Sn, and Na2S can still be found in the ex situ HRTEM image of the electrode when the cell is charged to 3.0 V (Figure 6d). The result reveals that sodium storage via the conversion reaction in SnS2 is not fully reversible, possibly due to the sluggish reaction kinetics. It should also be mentioned that the charged products are not well crystallized after the first cycle, as indicated by the XRD result. To further investigate 160SS as an anode material for practical SIB applications, a full cell was demonstrated SUN ET AL.
as a proof of concept constructed with a 160SS anode and a Na3V2(PO4)3 cathode.11 The first six charge discharge curves of the full cell at 0.1 A g1 are shown in Figure 7a. The first specific charge and discharge capacity is 559 and 375 mAh g1, showing a Coulombic efficiency of 67%, and the charge capacity remains at 251 mAh g1 during the 12th cycle (Figure 7b). The preliminary result demonstrates that SnS2 nanosheets have potential to be considered as an anode material for SIBs, although further optimization is urgent to improve the cycling stability to meet the demand of commercial applications. CONCLUSIONS 2D SnS2 nanosheets (34 nm in thickness) were synthesized via a facile refluxing process and evaluated as an anode material for SIBs. The SnS2 nanosheets delivered a high reversible specific capacity of 733 and 435 mA h g1 at 0.1 and 2 A g1, respectively, and still exhibited a high capacity retention of 647 mA h g1 during the 50th cycle at 0.1 A g1. The excellent performance can be attributed to the unique ultrathin VOL. 9
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ARTICLE Figure 7. (a) Chargedischarge profiles of a full cell consisting of a 160SS anode and Na3V2(PO4)3 cathode between 0.8 and 3.5 V at a current density of 0.1 A g1. (b) Cycling performance of the full cell at 0.1 A g1.
morphology of SnS2 nanosheets, which is beneficial to achieve fast sodiation/desodiation reaction kinetics and to accommodate electrode volume change. A full cell combined with a Na3V2(PO4)3 cathode was also successfully demonstrated as a proof of concept and
showed encouraging performance. All the results indicate that ultrathin SnS2 nanosheets are a promising candidate for achieving high-performance sodium storage with high specific capacity, good rate capability, and cycling stability.
METHODS
galvanostatic charge and discharge in the voltage range 0.0053.0 V. For coin-type full-cell testing, the cathode was a mixture of Na3V2(PO4)3, CNTs, and PVDF at a weight ratio of 8:1:1. The 160SS anode was operated for one discharge/charge cycle (0.0053.0 V) in a half-cell before assembling the full cell. The weight ratio between cathode and anode material was around 5:1. The full cell was tested in the voltage range 0.83.5 V, and the specific capacity was calculated based on the mass of the 160SS anode. Conflict of Interest: The authors declare no competing financial interest.
Synthesis. The synthesis process was carried out in a threeneck flask (100 mL) by a refluxing process. In a typical synthesis, 1.6 mmol of SnCl2 (anhydrous, Alfa Aesar, 98%) and 3.2 mmol of 1,3,4-thiadiazole-2,5-dithiol (DMCT) (Sigma-Aldrich, 98%) were added to the flask containing 30 mL of ethylene glycol (EG) (Alfa Aesar, 99%) and 10 mL of 1-octadecene (ODE) (Sigma-Aldrich, 90%) under vigorous stirring. Then, argon gas was introduced to purge the suspension for about 20 min. The suspension was quickly heated to 140, 160, or 180 C. During heating, SnCl2 and DMCT were completely dissolved, forming a yellow transparent solution. The solution was maintained at 140, 160, or 180 C for 2 h under stirring, and a brown product was gradually formed with time. After the reaction, the reaction mixture was naturally cooled to room temperature. The brown product was collected by centrifugation followed by repeated washing with hexane and ethanol for several times and then dried at 60 C overnight in a vacuum oven. The product synthesized at 140, 160, and 180 C is abbreviated as 140SS, 160SS, and 180SS, respectively. Characterization. Phase structure of the samples was investigated by a Bruker AXS D8 Advance X-ray diffractometer using Cu Ka radiation. The morphology was determined by a fieldemission scanning electron microscopy (FESEM) (JEOL, model JSM-7600F), and the nanostructure was characterized by a transmission electron microscope (TEM) (JEOL, model JEM2100) operating at 200 kV. Atomic force microscopy (AFM) (Digital Instruments) was used to determine the thickness of the nanosheets. X-ray photoelectron spectroscopy measurements were performed using a VG ESCALAB 220i-XL instrument (base pressure
