Elsevier

Electrochimica Acta

Volume 136, 1 August 2014, Pages 483-492
Electrochimica Acta

A magnesiothermic route to multicomponent nanocomposites of FeSi2@Si@graphene and FeSi2@Si with promising anode performance

https://doi.org/10.1016/j.electacta.2014.05.121Get rights and content

Abstract

The multicomponent nanocomposites of FeSi2@Si@graphene and FeSi2@Si are synthesized via the magnesiothermic reduction of core-shell Fe3O4@SiO2 nanoparticles with/without graphene oxide shell. In the course of the magnesiothermic reaction, the SiO2 and Fe3O4 components in the Fe3O4@SiO2 core-shell particles are transformed into elemental Si and FeSi2, respectively. The formation of intimately-coupled composite structure consisting of Si and FeSi2 domains as well as the coating of graphene layer is verified by high resolution-transmission electron microscopy. Both the nanocomposites of FeSi2@Si@graphene and FeSi2@Si show promising anode performance for lithium ion batteries, indicating a beneficial role of the electrochemically inactive FeSi2 domains in alleviating the drastic expansion/contraction of elemental Si during the electrochemical cycling. The better cyclability and rate characteristic are obtained for the FeSi2@Si@graphene nanocomposite than for the graphene-free FeSi2@Si one, which is attributable to the depression of pulverization and the enhancement of electrical conductivity upon the coating of graphene layer. The present work highlights that the magnesiothermic reaction provides a powerful synthetic route to multicomponent Si-based nanocomposites with tailored composition and complex geometry.

Introduction

A great deal of research effort is devoted for the exploration of new efficient anode materials for lithium secondary batteries to replace graphite material currently used in commercial Li+ ion cell [1]. Among many promising anode materials for lithium ion batteries, elemental silicon boasts the largest theoretical capacity of ∼4000 mAh g−1 [2]. However, the commercialization of this material is frustrated by its poor cyclability and inferior rate characteristic, which originate from severe volume change during electrochemical cycling and low electrical conductivity, respectively [3]. To alleviate the breakdown of electrical contact between silicon and current collectors caused by drastic volume change, various attempts are made such as the formation of porous nanostructures, the incorporation of electrochemically inactive secondary phase, and the composite formation with other nanostructured species [4], [5], [6], [7]. In particular, the hybridization with highly conductive material helps to improve the poor rate performance of pure silicon via the enhancement of electrical conductivity [8], [9], [10]. There are many reports about the synthesis of carbon–silicon nanocomposites including silicon–graphene nanocomposites [11], [12], [13], [14], [15], [16]. In fact, the theoretical investigation about the effect of carbon species on the electrode performance of Si- and Sn-based nanocomposites predicts that the graphene nanosheet shows much better performance as a matrix to improve the electrode performance of composite material compared with other forms of carbon species [17]. In the most chemical syntheses of the silicon-based nanocomposites, elemental silicon is used as a precursor [18], [19], [20], [21], [22]. In nature, however, most of silicon elements exist in the form of tetravalent Si4+ compounds. Since a strong binding of Si4+ ion with ligand anions like oxide ions makes very difficult the chemical synthesis of elemental silicon from Si4+-containing precursors [23], a lot of electrical energy is currently consumed to produce elemental silicon via an electrochemical method. Additionally there are considerable limitations in the manipulation of elemental silicon precursor in terms of the control of morphology and size. In comparison with the elemental Si precursor, Si4+-containing precursors like SiO2 and silane compounds can provide much greater flexibility in tailoring the structure, morphology, composition, and chemical bonding nature of elemental silicon [24]. Thus, these Si4+-containing precursors are much more convenient not only in preparing the size- and morphology-controlled elemental silicon but also in synthesizing Si-based composite materials with complex morphology and tailored composition. Recently the reaction of SiO2 with Mg is reported to provide an effective and facile synthetic method to elemental Si using Si4+-containing precursor (i.e. magnesiothermic reaction) [25]. To dates, there are several reports about the magnesiothermic synthesis of morphology-controlled elemental silicon [26], [27], [28], [29]. But at the time of the publication of the present study, we are aware of no study about the application of magnesiothermic method for the synthesis of Si-based nanocomposites with tailored morphology and composite structure.

In the present study, the intimately-coupled multicomponent nanocomposites of FeSi2@Si and FeSi2@Si@graphene are synthesized by the magnesiothermic reaction of core-shell Fe3O4@SiO2 nanoparticles without/with graphene coating layers. The crystal structure and composite structure of the resulting nanocomposites are systematically investigated along with their chemical bonding nature. These multicomponent nanocomposites are tested as anode materials for lithium ion batteries to examine the effect of composite formation on the electrode performance of elemental silicon.

Section snippets

Synthesis of positively-charged reduced graphene oxide (rG-O+) nanosheets

An aqueous suspension of exfoliated graphene oxide (G-O) nanosheet was prepared by modified Hummer's method [30]. The obtained G-O suspension was reacted with hydrazine hydrate and polydiallyldimethylammonium (PDDA) chloride at 90 °C for 1 h to induce the reduction and surface modification of the G-O precursor, respectively [31]. After the reaction, the aqueous suspension of positively-charged reduced graphene oxide (rG-O+) nanosheets was obtained.

Synthesis of core-shell Fe3O4@SiO2 and Fe3O4@SiO2@graphene precursors

The core-shell Fe3O4@SiO2 precursor was

Powder XRD analysis

The left panel of Fig. 2 represents the powder XRD patterns of the precursor Fe3O4@SiO2 core-shell particle and its magnesiothermically reduced products without/with chemical etching. The pristine Fe3O4@SiO2 material shows a very broad hump peak of amorphous SiO2 phase at 2θ = ∼24° as well as weak and diffuse Bragg reflections of Fe3O4 phases. After the magnesiothermic reduction at elevated temperature, intense Bragg reflections of Si and MgO phases appear whereas the XRD feature of SiO2 phase

Conclusions

The intimately-coupled multicomponent FS and FS-G nanocomposites are synthesized by the magnesiothermic reduction method using core-shell Fe3O4@SiO2 nanoparticles without/with coated G-O layer as precursors. Both of the nanocomposites commonly display the intimately-coupled composite structure of FeSi2 nanoparticles surrounded by Si domains without/with outer graphene coating layers. These two materials show promising anode performance for lithium ion batteries, indicating the beneficial role

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MEST)" (NRF-2010-C1AAA001-2010-0029065), by the Core Technology of Materials Research and Development Program of the Korea Ministry of Intelligence and Economy (grant No. 10041232), by the National Research Foundation of Korea(NRF) grant funded by the Korea government (MEST) (NRF-20011-001419), and by the National Research Foundation of Korea (NRF) Grant funded by the Korean

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