Elsevier

Electrochimica Acta

Volume 155, 10 February 2015, Pages 251-256
Electrochimica Acta

Self-assembly of SiO/Reduced Graphene Oxide composite as high-performance anode materials for Li-ion batteries

https://doi.org/10.1016/j.electacta.2014.12.124Get rights and content

Abstract

SiO/reduced graphene oxide (SiO/rGO) composite was prepared by a simple hydrothermal process involving the capsulation of SiO particles with graphene hydrogel followed by the HF treatment. The prepared composite electrode exhibited higher discharge capacity and better rate capability than the pristine SiO electrode. Such enchancement could be attributed to the efficient conducting/buffering rGO matrix and the successful corrosion of SiO2 shell on the surface of SiO.

Introduction

As promising anode materials for lithium-ion batteries, Si-based materials have attracted considerable attention due to their abundant sources, low cost, and high theoretical capacities (for example, ∼4200 mAh g−1 for Si and 2043 mAh g−1 for SiO) [1], [2], [3], [4]. As compared to Si, SiO suffers from less volume expansion during charging and discharging, which is atttributed to the formation of lithium oxide and lithium silicate acting as buffer layers [2]. Therefore, SiO anodes appear particularly promising for their long cycle life in practical applications. However, some intrinsic drawbacks of SiO including rapid capacity fading, poor initial columbic efficiency and poor electrical conductivity remain to be solved to improve the electrochemical performance. Moreover, solid SiO is thermodynamically unstable at all temperatures since SiO is air- and moisture-sensitive and it disproportionates to Si and SiO2 easily. Such limitations hinder the practical application of SiO anodes [1], [2], [5]. In order to overcome these problems and improve the electrochemical performance, many strategies have been employed in recent studies. Guo et al. successfully prepared SiO/graphene nanocomposite by an in situ chemical synthesis approach, which demonstrated excellent cyclic performance of 890 mAh g−1 after 100 cycles and good rate capability [6]. Si et al. reported that the ball-milled SiO/carbon-nanofibre composite exhibited a reversible capacity of ∼700 mAh g−1 after 200 cycles [7]. Unfortunately, despite significant progress for SiO anodes, SiO2 shell inevitably formed on SiO due to its thermodynamical unstability, which hindered further enhancements of the specific reversible capacity and the cycle life for SiO anodes. Very recently Yu et al. proposed a strategy that involved the surface corrosion of SiO2 shell through NaOH treatment [5]. However, there generally still remains SiO2 shell after the NaOH treatment, which prevents the SiO from reacting with lithium-ions on a certain degree [1]. In this context, they demonstrated that the SiO2 shell could be removed completely through the HF treatment [8].

On the other hand, recent study has focused on the 3D graphene hydrogel macrostructure for its high electrical conductivity, mechanical flexibility and thermal stablility, which can effectively contribute to excellent rate performance. In especial, the graphene can act as buffering matrix for the volume variation of SiO during lithiation/delithiation [9], [10]. For instance, Xu’s group reported 3D graphene hydrogel macrostructure self-assembled from 2D graphene nanosheets via a one-step hydrothermal process, and the formation of the graphene hydrogel is driven by π–π stacking interactions of graphene sheets [9].

Here, we employed a simple self-assembly process involving the capsulation of SiO particles with graphene hydrogel. After a sintering process, the SiO particles are well dispersed into the rGO matrix. The method applied here is simple and environmental friendly. In addition, as compared to SiO/graphene composites prepared via ultrasonic mixing, the SiO capsulated by graphene hydrogel repesents a more efficient conducting/buffering matrix. Finally, HF solution was used to remove the inevitable SiO2 shell on the surface of SiO. It is found that the SiO/rGO composite anode exhibits a more stable reversible capacity and a better rate capability compared to the pristine SiO anode.

Section snippets

Experimental

Graphite oxide was synthesized from natural graphite flake (Sinopharm, ≤30 μm) by a modified Hummers method [11]. After that, ultrasonication and centrifugal were applied to exfoliate the graphite oxide to generate graphene oxide sheets. Besides, the milled SiO powder (Aladdin, 99.99%, referred as m-SiO) was obtained using high energy mechanical milling (HEMM, planetary ball mill). Then 0.8 g of m-SiO powder and 0.2 g of graphene oxide (GO) were mixed in 40 ml de-ionized water (DI) via the

Results and discussion

As shown in Fig. 2, the rGO shows a broad peak at 20–26° which indicates the disordered agglomeration of graphene nanosheets [12]. As a whole the XRD patterns of the other three samples (Fig. 2a–c) have a broad peak located around 23°, which indicates the amorphous states of the SiO [2], [4]. There is no peak at about 11.24° which corresponds to GO for the F-SiO/H-rGO composite (Fig. 2a), confirming the successful conversion of GO to rGO after the thermal treatment in the Ar/H2 mixed atmosphere

Conclusions

In summary, a uniform dispersion of SiO particles in the rGO matrix is achieved at the expense of graphene hydrogel structure via a simple hydrothermal process. The F-SiO/H-rGO composite exhibited a discharge capacity of 744 mAh g−1 at 120 mA g−1 after 50 cycles and high-rate capability of 620 mAh g−1 at 3600 mA g−1. The excellent electrochemical performance of the F-SiO/H-rGO composite should be attributed to several aspects as follows. First, the successful etching of the insulation SiO2 layer

Acknowledgements

Financial support from the national natural science foundation of China (No.11174292, No.11374306, No.51202252, and No. 10904144) is gratefully acknowledged.

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