Designable fabrication of flower-like SnS₂ aggregates with excellent performance in lithium-ion batteries

Abstract

Flower-like SnS₂ aggregates have been prepared and show better electrochemical performances than nanoplates, which could be attributed to their structural matrix with the functions of facilitating the Li⁺ diffusion and electron transfer as well as reducing the crumbling and cracking of the electrode.

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As one of the most important tin-based materials, tin disulfide (SnS₂) has attracted more and more attention in lithium-ion batteries. SnS₂ has a layered CdI₂-type structure, which can facilitate the intercalation of lithium ions and also tolerate the volume change during the lithiation–delithiation process. Thus, much effort has been made to synthesize SnS₂ with an improved electrochemical performance for their use as alternative anode materials.^1–5 In particular, nanostructured SnS₂ with controlled morphologies has become favorable due to their unique properties such as larger surface area, greater accessibility to electrolyte, faster transportation of Li⁺, and accelerated phase transitions.^6,7 However, compared to those well-reported simple SnS₂ nanostructures including nanoplates, nanowires, nanorods, nanobelts and nanotubes,^4,5,9–13 there are only limited reports on the fabrication of SnS₂ electrodes with hierarchical structures, which are known to have the advantage of not only facilitating the Li⁺ diffusion and electron transfer, but also effectively reducing the crumbling and cracking of electrodes^2,3,6–8 Thus, it is necessary to explore novel protocols for the synthesis of SnS₂ with hierarchical structures.

In this work, we have successfully developed a method to synthesize hierarchical SnS₂ structures (i.e., flower-like aggregates) under hydrothermal conditions without any additives. By controlling the reaction conditions, we can also fabricate SnS₂ nanomaterials from pure nanoplates to flower-like aggregates. Our method has two excellent merits: i) it is designable for this synthetic method, based on our knowledge of the principle of the lowest energy in our synthetic system; ii) it is a simple synthetical protocol with no need for any organic additive, compared with some other reported technologies.^2,3,14,15 Our design of flower-like SnS₂ aggregates is also promising for the synthesis of other inorganic nanomaterials with hierarchical structures. Moreover, electrochemical experiments demonstrate that the flower-like SnS₂ aggregates have an excellent electrochemical performance in lithium storage and can act as an alternative anode material in lithium-ion batteries.

Fig. 1a and b display typical SEM images of the as-synthesized products. It's clear that such as-prepared SnS₂ samples (Fig. 1a) were mainly nanoplates with a regular shape. A magnified SEM image in Fig. 1b reveals that most of the SnS₂ nanoplates are hexagons with an angle of 120° between adjacent sides. Also, the thickness of each nanosheet is around 10 to 15 nm, as revealed from the cross section of those nanoplates standing up on their edges. The SnS₂ nanoplates are further studied by HRTEM for further structural information. As shown in Fig. 1c, the lattice spacing of the surface of a representative nanoplate is measured to be 0. 31 nm, which reveals that the surfaces are composed of (100) planes.¹⁴ The XRD pattern in Fig. 1d is in good agreement with the standard SnS₂ one (JSPDF: 23-0677), implying a high purity of the final product.

Fig. 1 (a and b) low and high magnification SEM images; (c) HRTEM image and (d) XRD pattern of spherical SnS₂ microstructures.

SnS₂ is built on a layered CdI₂-type structure,¹⁶ which in turn governs its growth and facilitates the formation of nanoplates. Therefore, it is easy to grow SnS₂ into plate-like shapes under proper conditions, as shown in the above-mentioned synthesis. Our effort also shows the possibility to prepare a nanoplate-based structural matrix, which has been favored for improved electrochemical performance.^2,3,6–8 Thus, one important task for our synthetical effort is to focus on the controlled formation of SnS₂ into a well-organized hierarchical structure. Herein, we employed the strategy of concentration control to facilitate the assembly of preformed building blocks of nanoplates. This strategy is based on our knowledge of the principle of the lowest energy in a synthetic system: the preformed nanoplates will tend to aggregate together through heterogeneous nucleation. Fig. 2 shows the SEM images of the as-prepared flower-like aggregates. It's clear that an increase of concentration will lead to the formation of a large amount of flower-like SnS₂ aggregates assembled by nanoplates. In an enlarged SEM image, as shown in Fig. 2c, the thickness of the nanoplates from these SnS₂ aggregates is in the range from 10 to 15 nm. The XRD pattern in Fig. 2d identifies such SnS₂ aggregates as pure SnS₂ (JSPDF: 23-0677).

Fig. 2 (a) low magnification SEM image; (b and c) high magnification SEM images and (d) XRD pattern of flower-like SnS₂ aggregates.

To further understand the growth mechanism of the flower-like SnS₂ aggregates, we carried out the time-dependent synthesis and investigated the products at different stages by SEM and XRD techniques. From the XRD patterns in Fig. S1,† the SnS₂ phase will form pretty fast, after reacting for just 2 h, which is especially different from the reaction system with thiourea as the sulfur source.¹⁶ This can be attributed to the difference in the hydrolysis speed of the different sulfur sources.¹⁷ Fig. S2a–c shows the morphological evolution from the nanoparticles formed at the beginning, and to the final flower-like aggregates.† Usually two different processes are involved during this morphological transformation: nucleation and Ostwald ripening. The nanoparticles (Fig. S2a for 2 h) firstly grew into aggregates with irregular shapes (Fig. S2b for 5 h) through anisotropic nucleation following its crystal growth habit,⁴ and then evolved into flower-like SnS₂ aggregates (Fig. S2c for 20 h) via Ostwald ripening.† In addition, two possible reasons can be explained why the higher precursor concentration favored the formation of flower-like SnS₂ aggregates. Firstly, when the precursor concentration is increased, the nucleation rate increases and it may result in different particle sizes as shown by Fig. 1a and b and 2a–c. It seems that the flower-like sample exhibits a wider particle size distribution and varied surface energy for individual nanoplate particles. For particles with a high aspect ratio, the atoms on the surface and edges are not fully coordinated. In larger particles, the lack of coordination has a greater effect at the edges, thus results in edge-to-surface interactions, which produces the as-obtained flower-like structure.¹⁸ Secondly, at higher precursor concentration, the presence of more electrolytes could promote coagulation and three-dimensional aggregation effects,¹⁹ thus facilitating the formation of flower-like SnS₂ aggregates. In summary, the result is in accordance with our designed SnS₂. The whole process is illustrated in Scheme 1 for a better understanding.

Scheme 1 Schematic formation process of stacked SnS₂ nanoplates.

As reported by many groups,^2,3,6–8 assembled structures have better electrochemical performance than simple nanostructures. After the preparation of different SnS₂ structures, we also test the performances of the as-prepared samples with different morphologies to understand the structure-performance relationship of our SnS₂ nanomaterials. Fig. 3a shows the charge–discharge curves of the two electrodes made from the SnS₂ nanoplates and flower-like SnS₂ aggregates. All the data are collected between 0.05 V and 1.2 V vs. Li⁺/Li at a current density of 100 mA g⁻¹. In the first discharge process, the initial charge–discharge curves display a similar pattern, which implies that they have the same electrochemical processes,^1,17 expressed by the following:

SnS₂ + 4Li⁺ + 4e⁻ →Sn + 2Li₂S (1)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (2)

Fig. 3 (a) initial charge–discharge curves and (b) cycling performances for the two SnS₂ samples tested at a current density of 100 mA g⁻¹ in the potential range of 0.05–1.2 V.

It's interesting that both SnS₂ electrodes show high initial discharging capacities (1228 mAh g⁻¹ for nanoplates and 1095 mAh g⁻¹ for flower-like aggregates). This might be due to the high surface area of the SnS₂ nanoplates, which could promote side-reactions with the electrolytes and the lithium intercalation of the SnS₂ layers without phase decomposition.²⁰

The effects of different morphologies on the electrochemical performance can be further manifested through the further cycling of the prepared electrodes.^21,22 Herein, we investigated the charge–discharge cycling performance for the two electrodes. In Fig. 3b, it is clear that the aggregated SnS₂ has a higher capacity. The initial capacity of the flower-like SnS₂ aggregates is 1095 mAh g⁻¹, and decreases gradually to 387 mAh g⁻¹ after the 50th cycle, while for the pure SnS₂ nanoplates, their capacity decreases from 1288.5 to 331.7 mAh g⁻¹. Moreover, it can be seen that the efficiency was kept at a value of about 95% from the 2nd cycle. In conclusion, our flower-like SnS₂ aggregates could be potentially used as an anode material of lithium-ion batteries which is obviously superior to other reported chalcogenides (Sb₂Se₃ and Bi₂S₃).^23,24

In summary, we have successfully controlled the formation of flower-like SnS₂ aggregates based on the reaction system for preparing simple nanoplates. The mechanism of such a shape transformation has been discussed in detail. The electrochemical results shows that the electrochemical properties of SnS₂ electrodes are morphologically dependent and flower-like SnS₂ aggregates show a high discharge capacity and improved cyclability, which endows them with great potential in lithium-ion batteries.

This work was financially supported by the National Natural Science Foundation of China (No. 20971070, 21073095 and 21003041) and MOE (No. RT0927).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra00965j

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