Microwave solvothermal synthesis of flower-like SnS2 and SnO2 nanostructures as high-rate anodes for lithium ion batteries
Graphical abstract
Introduction
Rechargeable lithium ion batteries have attracted intensive research attention due to their high energy density, long cycle life, and no memory effect. To meet the requirement of new-generation high-power lithium-ion batteries, electrode materials with high capacity, long cycle life and especially high-rate performances are highly desired [1], [2], [3], [4], [5]. Sn-based anodes have been suggested as promising anodes owing to their higher theoretical capacity than commercial graphite. SnS2 has a layered hexagonal CdI2-type crystal structure, which consists of two layers of close-packed sulfur anions with tin cations sandwiched between them [6], [7], [8]. The electrochemical Li-ion storage mechanism of SnS2 has been proposed as follows [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]:
In the first discharge (lithium insertion) cycle, lithiation leads to the decomposition of SnS2 into metallic tin and Li2S. The reversible tin alloying and de-alloying process results in a high theoretical capacity (645 mA h g−1) based on the formation of Li4.4Sn [6], [7], [8]. Following the similar above Li-ion storage mechanism by substituting S with O, SnO2 has a higher theoretical capacity (781 mA h g−1) [4], [5]. However, both SnS2 and SnO2 suffer from large volume change associated with lithium insertion and extraction, which would lead to a fast capacity fading [1], [2].
In recent years, nanoscale electrode materials have been widely used in lithium-ion batteries. Various SnS2 nanomaterials including nanoparticle [6], [7], [8], microsphere [9], nanosheet [10], [11], [12], nanobelt [13], nanoflake [14], [15], [16], nanowall [17], nanoflower [18], [19], [20], [21], have been reported. In particular, flower-like SnS2 nanostructures have been prepared by solvothermal/hydrothermal method [18], [19], [20] or direct thermal evaporation technique [21]. Flower-like SnO2 was also prepared by hydrothermal [22] or surfactant-assisted calcination method [23] for their applications in Li-ion batteries. The porous follower-like nanostructure may exhibit shortened pathway for lithium ions/electrons and improved lithium storage, however it is noted that these Sn-based nanoflowers have not been explored with respect to their high-rate Li-ion storage properties. It is suggested that hydrothermal or solvothermal method is the most commonly used technique for the synthesis of SnS2 nanostructures [6], [7], [8], [9], [10], [11], [12], [13], [14], [17], [18], [19], [20], in which long reaction time and high energy consumption are usually required. In comparison, microwave irradiation method has been increasingly used in the fabrication of various oxides and metals because it is fast, simple, energy efficient [24], [25], [26]. The rotational motion of polar molecules with the rapidly changing electric field and the ionic motion in microwave-susceptive materials lead to the product with good uniformity control in terms of size and morphology. Microwave technique has been used to prepare Sn-based nanostructrues such as SnS2 nanoflakes [15], [16] and SnS2–SnO2 composite nanosheets [27] in a household microwave oven. It is believed that reaction environment needs to be improved because reaction temperature cannot be adjusted and controlled and inhomogenous microwaves are present in the multi-mode household microwave oven.
In this work, flower-like SnS2 structures were prepared, for the first time, by a single-mode microwave solvothermal approach, which integrates the advantages of both solvothermal and microwave techniques. The uniform SnS2 nanoflowers were obtained from SnCl4 and thiourea in a sealed glass tube at 160–200 °C after 5–20 min microwave irradiation in a professional single-mode microwave reactor. The growth process and mechanism at various experimental conditions were explored in details. Flower-like SnO2 was subsequently obtained by calcinating SnS2 in air. The SnO2 product showed a very good high-rate performance as an anode material for applications in rechargeable Li-ion batteries.
Section snippets
Preparation of flower-like SnS2 and SnO2
All chemical reagents were analytical grade and used as received without further purification. In a typical experiment, 30 mL of 0.1 M thiourea (CH4N2S, Sinopharm Chemical) ethylene glycol solution was mixed with 30 mL of 0.025–0.1 M SnCl4.5H2O (Sinopharm Chemical) ethylene glycol solution. After stirring for a few minutes, the mixture was transferred into a specialized glass tube and followed by microwave-irradiation with continuous magnetic stirring in a single mode microwave reactor (Nova, EU
Crystal structure
The XRD patterns of the as-prepared SnS2 products prepared from SnCl4 and thiourea by single-mode microwave heating and the corresponding SnO2 products after calcination are shown in Fig. 1a. The characteristic peaks of the as-prepared product after microwave irradiation were assigned to hexagonal SnS2 (PDF 23-0677) in Fig. 1a. After calcination in air, the appeared new peaks could be perfectly indexed to tetragonal SnO2 (PDF 41-1445). There was no any peak attributed to SnS2 in the sample,
Conclusions
In summary, flower-like SnS2 nanostructures were successfully fabricated by a modified microwave solvothermal technique. The microwave-irradiation temperature, reaction time and precursor concentrations have critical influences on their morphologies. The obtained flowerlike SnS2 could be converted to SnO2 nanoflowers without substantial morphology change. The obtained flower-like SnS2 and SnO2 were explored as anode materials for rechargeable lithium ion batteries. The porous flower-like SnO2
Acknowledgements
The authors gratefully acknowledge the financial support from the Program for Professor of Special Appointment (Eastern Scholar) in Shanghai, National Natural Science Foundation of China (51271105) and Shanghai Municipal Government (11JC1403900, 11SG38, S30109).
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