Elsevier

Journal of Alloys and Compounds

Volume 702, 25 April 2017, Pages 636-643
Journal of Alloys and Compounds

Microwave synthesis of SnO2 nanocrystals decorated on the layer-by-layer reduced graphene oxide for an application into lithium ion battery anode

https://doi.org/10.1016/j.jallcom.2017.01.245Get rights and content

Highlights

  • Graphene-SnO2 is synthesized within 10 min by microwave irradiation.

  • SnO2 nanocrystals are uniformly decorated on the graphene sheets.

  • The restacking of layer-by-layer structured graphene is inhibited.

  • Graphene-SnO2 shows the high capacity, improving rate and cyclic capabilities.

Abstract

We demonstrate the microwave synthesis of tin oxide (SnO2) nanoparticles and direct deposition on the surface of restacking inhibited reduced graphene oxide (rGO) nanosheets for an application into lithium ion battery anodes. The mesoporous rGO-SnO2 nano-composite (G-SnO2), where the SnO2 nanoparticles are intercalated in the layer-by-layer structure of the restacking rGO nanosheets, can be synthesized within 10 min by microwave irradiation, simultaneously promoting the reduction of graphene oxides (GO). The size of SnO2 nanoparticles ranges from 5 to 10 nm and they are highly crystalline structure along with the change in the oxidation states from Sn2+ to Sn4+ in the process of the microwave synthesis. The G-SnO2 anodes show 1200 mAh g−1 at 50 mA g−1 and their specific capacity is preserved up to 1000 mAh g−1 during the 100 cycles. The coulombic efficiency keeps 97% after the 1st cycle and the high specific capacity of 747 mAh g−1 is maintained with 66.3% of capacity retention even when the current density increases from 50 mA g−1 to 300 mA g−1. These results indicate that the improvement of specific capacity, rate capability and cycle stability is attributed to the mesoporous layer-by-layer structure of G-SnO2, where the well-defined SnO2 nanoparticles are deposited on the restacking inhibited rGO nanosheets.

Introduction

Rechargeable lithium ion batteries (LIBs) are a promising energy source for electrical vehicles, electrical devices and energy storage system (ESS) because of the high energy density, high operation voltage and long life cycle [1]. Although graphite has been mostly used as a commercial anode in LIBs, it has not been satisfied in the demand of high energy density because of the its intrinsically limited specific capacity of 372 mAh g−1 and sluggish ion and electron transport kinetics through the van der Waals gaps of interlayers [2], [3]. In order to resolve the afore-mentioned bottlenecks, various kinds of anode materials have been developed for the design of the high performance LIBs.

Graphene-derived materials are also considered as the promising energy storage materials because there are several advantages such as high theoretical specific capacity (744 mAh g−1), superior electrical conductivities, large specific surface area (∼2600 m2 g−1) and good mechanical properties [4], [5], [6], [7]. To apply those advantages for LIBs, researchers have tried to make layer-by-layer structure by using chemical vapor deposition [8], [9], vacuum filtration [10], [11] and so on [12], [13], but the restacking of graphene sheets such as face-to-face aggregation could hamper fast electrolyte diffusion into the storage sites. Moreover, it results in diminishing available surface area. In order to prevent the restacking problem, researchers have investigated to use carbon nanotubes [14], [15], [16], [17], [18], carbon nanofibers [19], conducting polymer [20], [21], [22] and metal oxides [23], [24] as a role of pillars to hold spaces between the graphene sheets that allows to maximize the intrinsically good properties of graphene for the improved electrochemical performances [25]. Among the various kinds of pillars, metal oxides could be a better solution for LIBs because the rGO could be in a form of composites with metal oxide on a nanoscale and the synergizing effects of integrating high capacity metal oxide with high conductivity and large specific surface area of rGO on the battery performances. The representative metal oxides to incorporate with graphene are tungsten oxide [26], iron oxides [27], [28], [29], copper oxides [30], [31], cobalt oxides [32], [33], [34], manganese oxides [35], [36], nickel oxides [37], [38] and tin oxides [39], [40]. Especially, SnO2 is an attractive anode material because of high theoretical reversible capacity (781 mAh g−1 for SnO2), less toxicity and excellent processability for LIBs [41], [42], [43], [44], [45], [46], [47], [48]. If SnO2 nanoparticles are randomly decorated on the rGO sheets, it is possible to solve the restacking problem and to enhance the rate and cyclic capabilities along with the high specific capacity.

The SnO2 nanoparticles decorated onto the graphene sheets have been investigated by various synthesis methods such as microwave, hydrothermal, gas-liquid interfacial reaction, laser irradiation, co-precipitation and ultrasonication synthesis [39], [49], [50], [51], [52], [53], [54], [55]. Particularly, the microwave synthesis is a representative method to fabricate the nanocomposites efficiently because of ultra-fast reaction time, high energy efficiency and simple process. In addition, it is eco-friendly, economical and has better reproducibility, compared to the conventional methods [56], [57], [58].

Herein, we demonstrate the method to make layer-by-layer yet restacking inhibited structured G-SnO2 with the well-defined uniform SnO2 nanoparticles by utilizing a vacuum filtration and microwave method. The precursors of SnO2 are inserted into the interlayers of rGO sheets and directly transformed to the metal oxide nanoparticles through the nucleation and growth induced by the microwave irradiation. Thus, the SnO2 nanoparticles are randomly decorated onto the surfaces of rGO interlayer, while acting as pillars to provide both mechanical exfoliation and high capacity.

Section snippets

Synthesis of rGO

The GO was prepared by Hummers method [59]. GO (40 mg) were uniformly dispersed into the deionized water (7.6 mL) by sonication. Hypophorous acid (H3PO2, SAMCHUN Chemical, 50 wt%) and iodine (I2, DAEJUNG, 99.0%) was blended with the dispersed mixture into a vial. Then, the mixture was placed into the 80 °C oven for 12 h. After the gel was formed, the gel was washed by deionized water. When it became pH 7, the gel was put into the freeze-dryer for 72 h, and the rGO was obtained.

Microwave synthesis of SnO2 on rGO sheets

The rGO (5 wt%)

Results and discussion

As shown in Fig. 2(a), the SnO2 nanoparticles were randomly decorated on the rGO sheets because the microwave was uniformly irradiated to the reactor bottles and nucleation was very fast. Such randomly deposited SnO2 nanoparticles were well distributed onto the mesoporous surface of rGO without significant local aggregation. Moreover, the surface of G-SnO2 became rough and crumpled after the microwave irradiation. From Fig. 2(b), the layer-by-layer structure which was formed by vacuum

Conclusion

In summary, G-SnO2 was simply synthesized by vacuum filtration and microwave irradiation. G-SnO2 created a layer-by-layer structure in the course of the vacuum filtration, while the exfoliation of rGO sheets was preserved with the synthesis and decoration of SnO2 nanoparticles during the microwave irradiation. Through those processes, SnO2 nanoparticles were randomly inserted into the rGO sheets, playing important roles of pillars to prevent the restacking problem and of active material to

Acknowledgment

This research was supported by a grant from R&D Program of the Korea Railroad Research Institute, Republic of Korea.

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