Co-reduction self-assembly of reduced graphene oxide nanosheets coated Cu2O sub-microspheres core-shell composites as lithium ion battery anode materials
Introduction
Lithium ion batteries (LIBs) have been widely applied due to their advantages such as high energy density and capacity [1], [2], [3]. Compared to the traditional graphite anode material, metal oxides demonstrate much higher capacity for lithium storage [4]. However, the metal oxides always suffer from low conductivity, side reactions occurring at the electrode-electrolyte interface and a large specific volume change [5]. The above disadvantages in turn lead to a growing interest in surface coating [5], [6], [7], [8], [9]. Carbonaceous materials as coating layers for metal oxides could be an effective way to solve the above problems [10], [11], [12]. In terms of select suitable carbonaceous materials, graphene, especially the chemical reduced graphene oxide (rGO) might be one of the most appealing matrices due to its low cost, favorable electrical conductivity, high surface area, excellent mechanical intensity and flexibility [13], [14], [15], [16], [17], [18].
For the attractive advantages such as the morphological diversity, environmentally friendly, low cost, Cu2O has received much attention as a potential substitute for the LIBs anode materials [19], [20], [21], [22], [23], [24], [25], [26]. Recently, Cu2O-graphene nanohybrids were fabricated and exhibited an enhanced reversible specific capacity [27]. However, some problems have still to be existed like unsatisfactory and large irreversible capacity. In order to obtain a better synergistic effect between Cu2O and rGO, structural design might be essential. A suitable morphology and size of active electrode materials have important effect on the currently large scale production and performance of commercial LIBs. Practical cathode material of LiCoO2 and anode material of graphite are micro or sub-micro spherical powders due to the high tap density with high volume capacity density, facile electrode fabrication, and good morphology stability, etc [28], [29]. For these reasons, Cu2O is designed as the sub-microscale core. Moreover, rGO is chose for the encapsulation of Cu2O. As some good preliminary evidence has been demonstrated [30], [31], [32], the graphene-encapsulated structure could provide the active materials with nano feature as well as prevent the aggregation of particles during the continuous discharge-charge process.
Herein, Cu2O sub-microspheres encapsulated in rGO nanosheets are fabricated via a facile co-reduction process. Through the formation of encapsulated structure, Cu2O sub-microshere is then as a skeleton to transform the two-dimensional rGO nanosheets to three-dimensional architecture. The controlled deformation of rGO could offer 3D conductive paths and increased active sites. Moreover, the protective graphene layer on the surface of Cu2O might be capable of meeting the requirement of minimizing the direct contact of the active material with the electrolyte as well as accommodating the large volume expansion of Cu2O.
Section snippets
Materials
Cupric citrate powders (C6H4Cu2O7∙2.5H2O, AR) were purchased from Aladdin reagent Co., Ltd. Graphite powder (particle size ≤ 30 μm, C% ≥ 99.85%, C.P.) and hydrazine hydrate (N2H4∙H2O, 85%, AR) were bought from Sinopharm Chemical Reagent Co., Ltd. Ultra-pure water (18MΩ∙cm) was used in the experiments.
Synthesis of graphene oxide (GO)
The synthesis of GO nano-sheets was based on a modified Hummers method from graphite powders [33], [34]. Typically, concentrated H2SO4 (46 ml) was added into a mixture of graphite powders (1 g) and NaNO3
Results and discussion
As schematically shown in Fig. 1, the overall preparation of Cu2O@rGO is through the co-reduction of cupric citrate powders and GO nanosheets by N2H4 in solution at room temperature. After the reduction with N2H4, Cu2O nucleuses with spherical form appear and GO nanosheets are reduced to rGO nanosheets. At the same time, the reduced GO nanosheets self-assembly on the Cu2O spherical powders to form core-shell structured Cu2O@rGO with 3D architecture in the co-reduction process.
SEM images in Fig.
Conclusions
Cu2O sub-microspheres encapsulated in rGO nanosheets are obtained by a facile co-reduction approach from cupric citrate and GO nanosheets precursors at room temperature. Compared with the simple physical mixture of Cu2O and rGO, the Cu2O@rGO composites deliver the better lithium storage ability and capacity retention. In this kind of Cu2O@rGO structure, the rGO nanosheets would deform into a 3D structure and serve as a buffer to accommodate the volume variation of Cu2O during the continuous
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 21203236), Guangdong and Shenzhen Innovative Research Team Program (No. 2011D052, KYPT20121228160843692), Shenzhen Electronic Packaging Materials Engineering Laboratory (2012-372), Shenzhen Electronic Packaging and Device Assembly Key Laboratory (ZDSYS20140509174237196).
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