Layered SnS₂ cross-linked by carbon nanotubes as a high performance anode for sodium ion batteries

Abstract

A SnS₂@CNT nanocomposite as a high performance sodium-ion battery anode material is prepared via a facile sintering route. Benefiting from the high conductivity and strong mechanical properties of the CNTs, the SnS₂@CNT with a unique nanostructure of 2D SnS₂ sheets cross-linked by 1D CNTs, delivers a high reversible capacity of 758 mA h g⁻¹ at 100 mA g⁻¹ and an excellent cyclability with a capacity retention of 87% over 300 cycles. More impressively, over 445 mA h g⁻¹ can be obtained at a high current density of 5 A g⁻¹, demonstrating a superior rate capability.

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Introduction

Over the past several decades, lithium ion batteries (LIBs) have dominated the energy market of portable devices and electric vehicles owing to their high energy and power densities. However, for large-scale applications, there is growing concern regarding the increasing cost and uneven geological distribution of lithium sources.¹ Sodium ion batteries (SIBs) have been considered as promising alternatives to LIBs for large-scale energy storage due to the natural abundance and widespread terrestrial reserves of sodium resources.² However, since the Na⁺ ions are 55% larger than Li⁺ ions (Na⁺ 1.02 Å vs. Li⁺ 0.76 Å), finding suitable host materials with sufficiently large interstitial space to accommodate Na⁺ ions and allow reversible and rapid ion insertion and extraction is more difficult than that for Li⁺ ions. In recent years, significant breakthroughs have been realized in cathode materials for SIBs, such as phosphates/fluorophosphates,^3–6 layered sodium transition-metal oxides^7,8 and Prussian blue.^9,10 In contrast, the choice of Na-storage anode is relatively limited. Most studies focused on non-graphitizing carbon materials, including hard-carbon, carbon fiber, reduced graphene oxide and some pyrolytic carbon due to their large interlayer distance and disordered structure. Unfortunately, the application of these materials are greatly limited by the relatively low capacity (100–300 mA h g⁻¹) and poor cycling stability.^11–14 Except that insertion/extraction mechanism,^15–21 another effective approaching to achieve high specific capacity is to develop Na-alloying/de-alloying metallic anodes, for instance, 450–733 mA h g⁻¹ for Sb-based^22–26 and 420–860 mA h g⁻¹ for Sn-based materials.^27–33 Among these materials, Sn and Sn-based alloys/oxides have shown great potential as high-capacity SIB anodes due to the superior theoretical capacity based on the stoichiometry of Na₁₅Sn₄ (847 mA h g⁻¹).²⁷ Liu et al. reported Sn composite anode, consisting of an array of nanorods with carbon outer shell, delivers an initial capacity of 722 mA h g⁻¹ at 50 mA g⁻¹ and 405 mA h g⁻¹ after 150 cycles.³⁴ Anode based on Al₂O₃/SnO₂/carbon cloth (375 mA h g⁻¹ after 100 deep cycles with 80% capacity retention)²⁸ and SnO₂@graphene nanocomposite (638 mA h g⁻¹ after 100 cycles at 20 mA g⁻¹)³⁵ have also been developed. Kim and co-workers examined Sn₄P₃ composite as SIB anode with the reversible capacity of 718 mA h g⁻¹ and good cycling performance.³⁶ SnS@graphene hybrid nanocomposite, prepared by hydrothermal method, delivers reversible capacity of 492 mA h g⁻¹ at 810 mA g⁻¹ and 308 mA h g⁻¹ at 7.29 A g⁻¹ after 250 cycles, respectively.³⁷ However, the large volume expansion in Na–Sn alloy (420% volume expansion after Na insertion) seriously hinders their cycling stability.

Recent researches have shown that layered transition metal disulfide (MoS₂, WS₂ and SnS₂ et al.) could exhibit excellent Li or Na storage performances.^38,39 SnS₂ has a typical layered structure composed of three stacked atom layers (S–Sn–S) held together by van der Waals forces. The large interlayer spacing (d = 0.5899 nm) enables the convenient reversible insertion/extraction of the large Na⁺ ions, but also provide a large buffer matrix for the volume changes in Na–Sn reactions. The electrochemical performance of the layered SnS₂ have been significantly improved by the integration of SnS₂ with graphene. The 2D graphene sheets can easily self-assemble into conductive networks, offering a beneficial conductivity environment for charge transport. Furthermore, the structural compatibility between the two layered compounds also lead to a robust structure with high reversible capacity and good rate performances.^40–43 Besides the RGO, carbon nanotubes, due to its unique properties of high conductivity and flexible 1D structure, have been considered as powerful carbon materials to functionalize other materials aiming at improving their electrochemical performances and mechanical properties. Herein, we proposed to introduce the 1D carbon nanotubes (CNTs) to the 2D SnS₂, constructing a hybrid nanostructure with layered SnS₂ cross-linked by the CNTs. The high conductivity and strong mechanical properties of the CNTs can not only enhance the conductivity of the composite but also enable a robust structure to alleviate the volume expansion during cycling, realizing long-term cycling stability. The SnS₂@CNT nanocomposite, prepared by a one-pot sintering method, demonstrates an ultrahigh capacity of 758 mA h g⁻¹ at 100 mA g⁻¹ and superior rate performances. The simple synthesis method and the excellent Na-storage performances of the SnS₂@CNT nanocomposite show great potential for the energy storage applications.

Experimental section

Materials synthesis and characterization

Multi-walled carbon nanotubes were purchased from XIAN FENG nano-materials (China) and pre-treated with nitric acid at 80 °C. DBTA (dibutyltin diacetate) and sulfur powder from Aladdin were used as tin and sulfur source, respectively. The synthesis process of SnS₂@CNT composite is shown in Scheme 1 with typical process as follows: 0.45 g carbon nanotubes were dispersed in 30 ml carbon disulfide solution with sulfur (1.6 g) under stirring, then, 0.01 mol DBTA were added to the above mixture and sonicated for 30 min at 50 °C to evaporate the carbon disulfide and got a pasty mixture. The pasty product was then heated in Ar atmosphere at the rate of 2 °C min⁻¹ to 400 °C and kept at this temperature for 2 h. Pure SnS₂ was synthesized under the same conditions without adding the multi-walled carbon nanotubes. All products prepared above were characterized by field emission scanning electron microscopy (FESEM) on a FEI Nova NanoSEM 450 operating at 5 kV. Transmission electron microscopy (TEM) and High Resolution TEM (HRTEM) were carried on a FEI Tecnai G2 F30 operating at 200 kV. Powder X-ray diffractions (XRD) were recorded on a Shimadu XRD-6000 using Cu Kα radiation. Thermogravimetric (TG) analysis was conducted on a DSCqQ1000 from room temperature to 800 °C in air with the heating rate of 10 °C min⁻¹.

Scheme 1 Schematic diagram illustrating the preparation of the SnS₂@CNT nanocomposite, and SEM image of the typical structure of SnS₂@CNT.

Electrochemical test

For electrochemical measurements, SnS₂@CNT composite materials were mixed with super-p carbon and polyacrylic acid/carboxyl methyl cellulose (PAA/CMC = 1 : 1) in an 80 : 10 : 10 weight ratios, distilled water was used as solvent in the mixture to form dense slurry. The slurry was then casted on a copper foil and dried at 100 °C in vacuum overnight to form the working electrodes. Each electrode typically contained 1.2–1.6 mg active materials. In this work, the specific capacity was calculated based on the mass of SnS₂ and the specific current density was set based on the total mass of SnS₂@CNT composite. The 2016 coin cells were assembled in an Ar filled glove box (O₂/H₂O < 1 ppm). Sodium disk as the counter electrode, Celgard® 2400 (Celgard, LLC Corp., USA) as the separator, and 1 mol L⁻¹ sodium hexafluorophosphate (NaPF₆) in ethylene carbonate (EC)/dimethyl carbonate (DEC) (volume ratio = 1 : 1) with the addition of 10% fluoroethylene carbonate (FEC) as the electrolyte. Cyclic voltammetry were performed on PGSTAT302N potentiostat/galvanostat (AUTOLAB, Netherlands) and galvanostatic cycling tests were conducted on LAND2001A battery test system in the range of 0.01–2.5 V.

Results and discussion

Fig. 1 presents the structural characterization of the SnS₂@CNT composite. X-ray diffraction (XRD) patterns of the SnS₂, SnS₂@CNT composite and carbon nanotubes are presented in Fig. 1a. As can be seen, the bare CNTs display a single diffraction peak at 26.5°, corresponding well to the (002) planes. All the diffraction peaks of SnS₂ and the main peaks of SnS₂@CNT can be well assigned to the 2T-type layered structure (JCPDS Card no. 23-0677). The weaken peak of the CNTs at around 26.5° in the XRD pattern of SnS₂@CNT confirms that the CNTs are well wrapped in the layered SnS₂ sheets. As shown in Fig. 1b, Sn, S, C and O elements in SnS₂@CNT and the valent state of Sn were further characterized by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 1c, the binding energy at 487.5 and 495.9 eV are assigned to the Sn 3d_5/2 and Sn 3d_3/2 of Sn⁴⁺, indicating the presence of SnS₂.⁴¹ The products of different ratios of sulfur and DBTA were also studied in this work and the XRD patterns are shown in Fig. S1.† The constituent of the products are controllable through adjusting the molar ratio of the raw materials. The products of SnS₂ mixed with SnS were obtained when the ratio of S and DBTA below 5 : 1. While, the pure SnS₂ can be obtained when the molar ratio of S and DBTA is 5 : 1. Fig. 1d presents the thermogravimetry analyses (TGA) curves of the as prepared SnS₂@CNT, in which two obvious platforms are corresponded to the oxidizing reactions of SnS₂ and carbon, respectively. The carbon contents in SnS₂@CNT can be calculated to be 28 wt% by a combined mass loss of SnS₂@CNT and mass addition from SnS₂ to SnO₂.⁴⁴

Fig. 1 (a) XRD patterns of SnS₂, SnS₂@CNT and CNT; (b) XPS survey of SnS₂@CNT and (c) Sn 3d XPS of SnS₂@CNT; (d) thermogravimetric (TG) result of SnS₂@CNT in air with heating rate of 10 °C min⁻¹.

The morphology of the as-prepared SnS₂@CNT composite were further characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). As shown in Fig. 2, layered SnS₂ sheets are wrapped with CNTs and stack to a porous structure in which all SnS₂ sheets are linked through the criss-cross CNTs (Fig. 2a), which are unambiguous in high resolution SEM image (Fig. 2b) and TEM image (Fig. 2c). As for comparison, SEM image of bare SnS₂ was shown in Fig. S3.† HRTEM images (Fig. 2d) suggest that the prepared SnS₂ have a few layers (less than 10) of thin sheets stacking together and tightly anchored onto the carbon nanotubes. Meanwhile, the regular interlayer spacing of 2.784 Å exhibited in the HRTEM image is ascribed to the (101) plane of hexagonal SnS₂. The corresponding SAED pattern (Fig. 2d inset) of hybrid composite can be obviously indexed to the (101) and (110) planes, which are consistent with the XRD patterns. Furthermore, the STEM-EDS mapping in Fig. 2e reveals the homogeneous distribution of sulfur, carbon and tin, indicating that the layered SnS₂ is uniformly wrapped with CNTs, thus providing a better electronic conducting network with 3D channels.

Fig. 2 (a and b) Typical SEM images of SnS₂@CNT architecture, which reveal the porous structure and CNTs wrapped with layered SnS₂ nano-sheets. (c) Representative TEM image of SnS₂@CNT and (d) the HRTEM image of SnS₂@CNT with the corresponding SAED pattern (inset). (e) Element mapping images of SnS₂@CNT with corresponding TEM image.

The electrochemical reactivity of the SnS₂@CNT composite was investigated by cyclic voltammetry (CV). As shown in Fig. 3a, during the first cathodic process, two broad peaks at 1.3 V and 0.3 V are presented. The ∼1.3 V peak is commonly attributed to sodium intercalation of SnS₂ layers without phase decomposition (eqn (1))^40,43 and the ∼0.3 V peak could be attributed to the conversion and alloying reactions (eqn (2) and (3)),⁴⁶ as well as the formation of irreversible solid electrolyte interphase (SEI) in the initial cycle.⁴⁰ These peaks shift to 0.55 V and 1.5–1.7 V in the subsequent scans, respectively, and the 0.2 V peak appear, corresponding to the alloying reaction of Na–Sn.⁴³ In the anodic scan, the broad oxidation bands at 0.55 V and 0.75 V in all cycles can be attributed to the desodiation reaction of Na_xSn. The oxidation peak at ∼1.3 V can be assigned to the restitution of the SnS₂@CNT.^40,45 It could be noticed that all the peaks overlapped during the subsequent cycles, suggesting a good reversibility of the SnS₂@CNT composite for sodium storage.

xNa⁺ + SnS₂ + xe⁻ → Na_xSnS₂ (1)

4Na⁺ + SnS₂ + 4e⁻ → 2Na₂S + Sn (2)

Sn + 3.75Na⁺ + 3.75e⁻ → Na_3.75Sn (3)

Fig. 3 Electrochemical performance of SnS₂@CNT as anode for SIBs. (a) First five cycles cycle voltammograms (CV) curves of the SnS₂@CNT electrode at a scan rate of 0.2 mV s⁻¹ with a voltage range of 0–3.0 V. (b) Charge–discharge curves of first three cycles under 0.01–2.5 V voltage range. (c) Cycling performance of SnS₂@CNT, SnS₂ and CNTs electrodes at 100 mA g⁻¹, respectively. (d) Rate performance of SnS₂@CNT as anode for SIB. (e) Long-term cycling performance of the SnS₂@CNT electrode, first five cycles at 100 mA g⁻¹ and 1 A g⁻¹ for the next 300 cycles.

The electrochemical performances of SnS₂@CNT composite were further evaluated by galvanostatic cycling in the voltage range of 0.01–2.5 V. As given in Fig. 3b, at the current density of 100 mA g⁻¹, the initial discharge and charge capacities of SnS₂@CNT electrodes are 1076 mA h g⁻¹ and 787 mA h g⁻¹, respectively, with an initial coulombic efficiency of ca. 73%. The capacity loss in the first cycle could be mainly attributed to the SEI film formation.⁴⁰ The SnS₂@CNT electrodes demonstrate excellent reversibility and cycling stability after the first cycle, with the coulombic efficiency up to 99%. For comparison, the charge–discharge profiles of SnS₂@CNT, SnS₂ and CNTs under the same test conditions are presented in Fig. S5.† The carbon addictive (CNTs) delivers limited reversible capacity of less than 100 mA h g⁻¹. While the SnS₂ electrode exhibits a reversible Na storage capacity of 300 mA h g⁻¹, with severe fading in the first few cycles which may be resulted from the low electronic conductivity of the unsupported SnS₂ and unrestrained aggregations of Sn or Na_xSn.⁴¹ Fig. 3c shows the cycling performances of the SnS₂@CNT, SnS₂ and CNTs. Compared to the bare SnS₂, the SnS₂@CNT electrode exhibits much better cycling stability with a capacity retention of 91% over 100 cycles. The noteworthy Na-storage performances of the SnS₂@CNT composite could be ascribed to the improved conductivity resulted from the uniformly dispersing of criss-cross CNTs on the layered SnS₂, providing abundant electrochemically active areas for Na storage. Electrochemical impedance spectroscopy (EIS) results further prove the enhancement of electrical conductivity by the introduction of the CNTs. As shown in Fig. 4, SnS₂@CNT composite exhibits lower surface film and charge transfer resistance (R(sf + ct) 23.1 Ω) than the bare SnS₂ electrode (85.7 Ω) based on the equivalent circuit simulation, respectively, which indicates higher conductivity and faster charge transfer kinetics of SnS₂@CNT nanocomposite than the bare SnS₂.^46–48

Fig. 4 Nyquist plots of the SnS₂ and SnS₂@CNT hybrid electrodes at fully charge state after 5 cycles, obtained by applying a sine wave with amplitude of 10 mV in the frequency from 10 mHz to 100 kHz. Insert is the equivalent circuit.

The rate performance of batteries is an important factor for both grid-scale electricity storage and electric vehicle applications, especially under complex working conditions. Fig. 3d shows the rate capability of SnS₂@CNT nanocomposite from 0.1 to 5 A g⁻¹. The reversible Na storage capacities at current densities of 0.1, 0.4, 1.6 and 3.2 A g⁻¹ are 758, 692, 600 and 546 mA h g⁻¹, respectively. Even at a 50-fold increase in current density (5 A g⁻¹), a high reversible capacity of 445 mA h g⁻¹ can be obtained (corresponding to ∼59% of the capacity at 0.1 A g⁻¹), which could be attributed to high electronic and ionic conductivity of the hybrid nanostructure of the SnS₂@CNT composite.

In order to evaluate the long-term cycling stability of SnS₂@CNT electrode, cells with the SnS₂@CNT electrodes were galvanostatically charged and discharged at 1 A g⁻¹ for 300 cycles (0.1 A g⁻¹ for the first five cycles, Fig. 3e). The composite delivers capacities of 626 mA h g⁻¹, 625 mA h g⁻¹, 602 mA h g⁻¹ and 556 mA h g⁻¹ after of 50, 100, 200 and 300 cycles, respectively, with a capacity retention of 87% after 300 cycles. The as-prepared SnS₂@CNT composite in this work exhibits much better Na-storage performances compared to other Sn-based anodes reported previously,^41–44 in terms of capacity and cyclability, which are listed in Table S1 in ESI.†

Furthermore, the architectural and morphological changes in the typical electrodes after sodiation/desodiation cycling were investigated by SEM. Fig. 5 shows the SEM images of the electrode films of fresh, 50 and 100 cycles, respectively. It is obviously that the SnS₂@CNT composite still maintain the porous structure, while the thickness of SnS₂ sheets has a noticeable increment. The adhered spheres in the Fig. 5b and c were proved to be the SEI film or reaction products of the sample with air after taken out from the glove box (Fig. S7†).^41,42

Fig. 5 SEM images of SnS₂@CNT electrode after different cycles at fully charged state, (a) fresh electrode, (b) 50 cycles and (c) 100 cycles.

The superior rate capability and cycling performances of the SnS₂@CNT composite may be originated from the porous and conductive layered structure. The criss-cross CNTs well disperse with the SnS₂ sheets and provide electronic conducting internet. The expanded spaces by crossing CNTs with layered SnS₂ could provide effective 3D rapid charge transport channels. Moreover, the network structure of the CNTs serves to effectively protect the electrode structure from collapsing caused by the volume expansion/contraction during cycling, as well as to suppress the aggregation of the Sn particles, which ensures the cycling stability during sodiation/desodiation processes.

Conclusions

In summary, the SnS₂@CNT hybrid nanocomposite was successfully prepared via a facile one-pot sintering. The SnS₂@CNT electrode demonstrates a high reversible capacity of 758 mA h g⁻¹ at 100 mA g⁻¹, excellent rate performance (445 mA h g⁻¹ at 5 A g⁻¹) and long cycle life (87% retention after 300 cycles at 1 A g⁻¹). The superior electrochemical performances could categorically be attributed to the unique nanostructure consisting of the 2D SnS₂ sheets and 1D criss-cross CNTs, which provides multiple dimension pathways for the electron and ion transport, thus resulting in the enhancement of the conductivity. On the other hand, the strong mechanical properties of the CNTs enable a robust structure to alleviate the volume expansion during cycling, which is benefit for the stabilization of the solid electrolyte interphase (SEI), leading to better cycling performances. The results suggest that the SnS₂@CNT composite with hybrid nanostructure would be a promising candidate for the next generation of anode material for sodium ion batteries with high capacity and low cost.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant 51307069), 973 Program (2015CB258400) and the National Thousand Talents Program of China. The authors thank Analytical and Testing Center of HUST for XRD, SEM and FETEM measurements.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04941a

‡ These authors contributed equally to this work.

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