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Keywords:

  • cobalt;
  • composites;
  • electrochemistry;
  • graphene;
  • nanoparticles

Recently, electrochemically active metals and metal oxides, such as Sn,[1] Si,2 Co3O4,3, 4 and Fe2O3,5 have attracted much attention as anode materials for lithium ion batteries due to their high theoretical capacities and promising potential. However, a large specific volume change commonly occurs in the host matrix of these metals and metal oxides during the cycling process. The resulting partial pulverization of the electrodes leads to a decrease in electrical conductivity and reversible capacity.1, 6 To circumvent these problems, graphitic carbons with high electrical conductivity have been widely used as matrices for metals and metal oxides to improve their cycle performances.7, 8 Although remarkable progress has been made, the metals and metal oxides unavoidably aggregate after long cycles since they are mainly located on the surface of the graphitic carbon.7

Graphene, an integral part of graphite, is a 2D aromatic monolayer of carbon atoms not only exhibiting superior electrical conductivity and high surface area,3, 9 but also possessing structural flexibility, chemical tolerance, and reassembly properties.1013 Such merits suggest that graphene sheets hold promise as matrices for metals and metal oxides to improve their electrochemical performance. Graphene can generally be produced by the chemical reduction of readily available exfoliated graphite oxide with reducing agents, such as hydrazine or dimethylhydrazine.1417 Recently, SnO2/graphene,11 carbon nanotube/graphene, and fullerene/graphene10 composites were fabricated by the assembly of graphene sheets in the presence of inorganic precursors. Highly reversible capacities and good cycle performances were achieved when using these as anode materials for lithium ion batteries. Nevertheless, these inorganic precursors are still prone to strong aggregation and thus preclude a homogenous dispersion in graphene-based composites.

In this Communication, we describe a new strategy by choosing planar metallo-organic molecules, such as cobalt phthalocyanine (CoPc), for the purpose of fabricating organic metal/graphene composites. Due to their pronounced π-interactions with graphene sheets,18 they enable a homogenous dispersion of cobalt and cobalt oxide into/onto the graphene sheets by a simple pyrolysis and oxidation process. As a consequence, the graphene sheets (GS) in composites can not only efficiently buffer the volume change of cobalt oxide during charging and discharging processes but also preserve the high electrical conductivity of the overall electrode. One thus expects highly reversible capacity, good cycle performance, and good rate capability of the cobalt oxide/graphene composite as anode materials for lithium ion batteries.

As illustrated in Scheme 1, graphite oxide was suspended in a solution of ultrapure water and ammonia (25 wt % in water) to create a brown dispersion with a concentration of 0.05 wt %. The dispersion was then homogeneously mixed with CoPc molecules by ultrasonication, in which the weight ratio between graphite oxide and CoPc was fixed to 1:2. After sonication for 3 h, hydrazine solution (35 wt % in water) was added and the mixture was stirred at 40 °C for 12 h, giving rise to a homogenous dispersion of CoPc molecules in the reduced graphene sheets. The resulting precipitates were collected by filtration and subsequently pyrolyzed at 800 °C under Ar atmosphere, or upon further thermal treatment at 400 °C in air, yielding the Co/graphene and Co3O4/graphene composites, respectively.

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Scheme 1. The fabrication of Co/GC and Co3O4/GC: 1) dispersion of graphite oxide and CoPc in water by ultrasonication and subsequently chemical reduction of graphite oxide; 2) pyrolysis at 800 °C; and 3) oxidation at 400 °C.

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To elucidate the morphology and structure of graphene composites during fabrication, transmission electron microscopy (TEM) measurements were firstly carried out. As shown in Figure 1, for the CoPc/graphene composite pyrolyzed at 800 °C, a large quantity of inorganic particles with a size of about 15 nm (Figure 1 b) are uniformly dispersed onto/into the graphene layers to form compact composites as large as several micrometers. The selected area electron diffraction (SAED) pattern of these nanoparticles in the composite presents obvious spot rings (see the Supporting Information, Figure S1 b), which correspond to the lattice spacings of metallic cobalt. Cross-sectional atomic force microscopy (AFM, Figure 2) analyses were further conducted to investigate the structural features of Co/graphene composites (denoted as Co/GC in context). The thickness of the composites is about 7 nm, which suggests that they consist of several graphene layers. It is remarkable that some cobalt nanoparticles are decorated onto the graphene sheets and some confined into the slot between graphene layers (Figure 2 b), which is consistent with the TEM results.

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Figure 1. a) Low- and b) high-magnification TEM images of Co/GC showing the homogenous dispersion of cobalt nanoparticles in the graphene sheets.

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Figure 2. a), b) AFM images of Co/GC at two different magnifications. Note that the resultant composite consists of several graphene layers and many cobalt nanoparticles are confined in the graphene sheets.

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The as-synthesized Co/GC was treated in air at relatively low temperatures. Therefore, the graphene sheets could be partially burned off and at the same time the cobalt was oxidized to cobalt oxide, resulting in the formation of Co3O4/graphene composites (denoted as Co3O4/GC). Interestingly, many hollow nanoparticles were formed within the graphene sheets (Figure 3 a), which is in strong contrast to the solid nanoparticles before oxidation treatment. The X-ray diffraction (XRD) pattern (Figure 3 b) further disclosed that the diffraction peaks of the sample are perfectly indexed to the cubic spinel Co3O4 with a lattice parameter of a=8.01 Å, consistent with the values in the standard card of Co3O4 (JCPDS card No. 42-1467).19 The additional peak at 2θ=26.5° can be ascribed to the (002) plane of graphene sheets. Based on these results, the hollow particles can be undoubtedly identified as Co3O4 hollow nanocrystals, the formation of which should be similar to that synthesized from the reaction of cobalt nanocrystals with oxygen (Kirkendall effect).20 Thermogravimetric analysis (TGA) of the Co3O4/GC (see the Supporting Information, Figure S2) revealed that the weight content of Co3O4 in the composites was around 66 %. Such a high content of Co3O4 nanoparticles should result in high capacity when the Co3O4/GC is applied as an anode material for lithium ion batteries.

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Figure 3. a) TEM image and b) XRD pattern of the Co3O4/GC.

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Galvanostatic discharge–charge (Li insertion–extraction) experiments were carried out to evaluate the electrochemical performance of Co3O4/GC. For comparison, commercially available Co3O4 nanoparticles (with a size of 20–30 nm) and GS were also tested under the same electrochemical conditions. The first two discharge–charge curves of Co3O4/GC, Co3O4, and GS electrodes at a current density of 74 mA g−1 are presented in Figure 4 a, b, and c, respectively. It is remarkable to note a very high reversible capacity (754 mA h g−1) of Co3O4/GC in the voltage range from 0.01 to 3.00 V, which is two times higher than that of graphite (372 mA h g−1) and close to those of pure Co3O4 (about 900 mA h g−1)4 and commercial Co3O4 (791 mA h g−1). The charging curves of the three electrodes are distinctly different, which can be supported by their differential capacity versus cell voltage plots (see the Supporting Information, Figure S3). In the case of the Co3O4/GC electrodes, there are three peaks at 0.1, 1.2, and 2.0 V, respectively. The first two peaks can be ascribed to the Li extraction from graphene layers and related cavities, as they appear in the differential capacity versus cell voltage plot of the GS electrode, but disappear in that of the Co3O4 electrode. The third peak appearing in the case of both Co3O4/GC and Co3O4 electrodes should correspond to the Li extraction process from Li2O to form Co3O4 and Li ions according to literature reports,3,4,21 indicating that both graphene and Co3O4 are electrochemically active components for Li ion storage. The excellent cycle performance and rate capability of Co3O4/GC are demonstrated in Figure 4 d. The reversible capacity of the Co3O4/GC electrode is as high as 760 mA h g−1 after 20 cycles at the current density of 74 mA g−1, whereas that of the Co3O4 electrode rapidly decays to 388 mA h g−1. At the higher current density, the capacity retention of Co3O4/GC electrode is more evident. For example, at the current density of 1860 mA g−1, the reversible capacity of Co3O4/GC electrode is about 500 mA h g−1. This value is in sharp contrast to that of pure Co3O4 which decays to nearly 0 mA h g−1. Therefore, such pronounced electrochemical performance can be ascribed to the unique structure of Co3O4/GC with a variety of favorable properties: firstly, the Co3O4 nanoparticles are homogeneously dispersed into/onto the graphene sheets, which act as a structural buffer for the large volume change of Co3O4 during cycling processes; secondly, the compact graphene sheets significantly decrease the contact resistance of active particles in the composites and provide a high electrical conductivity for the electrode; thirdly, both graphene and Co3O4 are electroactive components for Li storage and contribute to the overall capacity of the electrode.

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Figure 4. First two charge–discharge curves of a) Co3O4/GC, b) Co3O4, and c) GS electrodes cycled at a current density of 74 mA g;−1 cycle performances and rate capabilities of d) Co3O4/GC, e) Co3O4, and f) GS electrodes at a variety of current densities.

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In summary, we have established a new approach to fabricate organic metal/graphene composites through π-interactions of disc-shaped phthalocyanine molecules with graphene sheets during the chemical reduction of graphite oxide. This process enables a homogenous dispersion of Co and Co3O4 nanoparticles into/onto the graphene sheets after simple pyrolysis and oxidation. It can be envisioned that Fe2O3, CuO, and other metal or metal oxide-based graphene composites can be easily synthesized in the same manner by using the corresponding metallo-organic precursors. The Co3O4/graphene composites exhibit remarkable lithium storage performance including highly reversible capacity, good cycle performance, and good rate capability. Thus, they shed light on the utility of graphene sheets to improve the electrochemical performance of a variety of metal and metal oxide nanoparticles.

Experimental Section

  1. Top of page
  2. Experimental Section
  3. Acknowledgements
  4. Supporting Information

The overall fabrication procedures of nanosized Co and Co3O4/graphene composites are shown in Scheme 1. Graphite oxide (GO) was synthesized from natural graphite flake by a modified Hummers method,22 which has been described previously.23 Graphene was produced by thermal reduction of graphite oxide at 800 °C for 2 h in argon.

The morphology, microstructure, and composition of the samples were investigated by TEM (Philips EM 420), AFM, XRD, and TGA (Mettler TG 50) measurements. Electrochemical experiments were carried out using two-electrode Swagelok-type cells. The working electrodes were prepared by mixing the Co3O4/graphene composite, carbon black, and poly(vinyl difluoride) (PVDF) at a weight ratio of 80:10:10 and pasting the mixture on pure Cu foil (99.6 %, Goodfellow). A glass fiber from Whatman was used as a separator. Pure lithium foil (Aldrich) was used as the counter electrode. The electrolyte consisted of a solution of LiPF6 (1 M) in ethylene carbonate/dimethyl carbonate (1:1 v/v) obtained from Ube Industries Ltd. The cells were assembled in an argon-filled glove box. The electrochemical performance was tested at different current densities in the voltage range of 0.01–3.00 V on an Arbin MSTAT battery test system.

Acknowledgements

  1. Top of page
  2. Experimental Section
  3. Acknowledgements
  4. Supporting Information

This work was financially supported by the Max Planck Society through the program ENERCHEM, the German Science Foundation (Korean–German IRTG) and DFG Priority Program SPP 1355.

Supporting Information

  1. Top of page
  2. Experimental Section
  3. Acknowledgements
  4. Supporting Information

Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors.

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