Graphene nanosheets for enhanced lithium storage in lithium ion batteries
Introduction
Lithium-ion batteries currently are ubiquitous power sources for portable electronics, using the chemistry of lithium cobalt oxide (LiCoO2) cathode and graphite anode [1], [2], [3]. The energy density and performance of lithium-ion batteries largely depend on the physical and chemical properties of the cathode and anode materials. The possibilities for the improvement of cathode materials are quite limited due to the stringent requirements such as high potential, structural stability, and inclusion of lithium in the structure [4], [5]. Nevertheless, there is considerable room for exploring new anode materials, owing to many materials having reversible lithium storage capability.
Recently, graphene, a single layer of carbon (carbon atoms in a two-dimensional (2D) honeycomb lattice), was found to exist as a free-standing form and exhibits many unusual and intriguing physical, chemical and mechanical properties [6], [7]. Due to the high quality of the sp2 carbon lattice, electrons were found to move ballistically in graphene layer even at ambient temperature [8], [9]. Graphene powders have been successfully applied in polymers to produce highly conductive plastics [10]. Despite the optimistic expectation on graphene-based electronics, it is unlikely that this will appear in next two decades. Current research mainly focuses on fundamental research. In the meantime, one of exciting possibility is the use of bulk graphene powders as anode materials for reversible lithium storage in lithium-ion batteries [11].
The maximum specific lithium insertion capacity for graphite (3D network of graphene) is 372 mAh/g, corresponding to the formation of LiC6 – a first stage graphite intercalation compounds (GIC). During the intercalation process, lithium transfers its 2s electrons to the carbon host and is situated between the carbon sheets. High capacity carbon materials have also been reported. This could be mainly ascribed to (i) lithium insertion within the “cavities” in the material [12], (ii) lithium absorbed on each side of the carbon sheet [13], (iii) lithium binding on the so called “covalent” site [14], and (iv) lithium binding on hydrogen terminated edges of graphene fragments in carbon materials [15]. Owing to its large surface-to-volume ratio and highly conductive nature, graphene may have properties that make it suitable for reversible lithium storage in lithium-ion batteries. This is because lithium ions could be bound not only on both sides of graphene sheets, but also on the edges and covalent sites of the graphene nanoplatelets. Therefore, it is expected that graphene could overtake its 3D counterpart (graphite) for enhanced lithium storage in lithium-ion batteries. Herein, we report the chemical synthesis of graphene nanosheets and their electrochemical performance as anodes in lithium-ion cells.
Section snippets
Chemical synthesis of graphene nanosheets
In a typical synthesis process, natural graphite powders (SP-1, Bay Carbon, MI, USA) were oxidized to graphite oxide using a modified Hummers method [16]. One gram graphite powder and 0.5 g sodium nitrate were poured into 70 ml concentrated H2SO4 (under ice bath). Then 3 g KMnO4 was gradually added. The mixture was stirred for 2 h and then diluted with de-ionised (DI) water. After that, 5% H2O2 was added into the solution until the colour of the mixture changed to brilliant yellow. The as-obtained
Results and discussion
Fig. 1a shows a FEG-SEM image of bulk graphene nanosheets at low magnification. The loose graphene nanosheets tend to stick together to form fluffy agglomerates with a flower-like appearance. A magnified view of one such agglomerate is shown in Fig. 1b, from which we can clearly see the nanosheets forming flower petal-like shape. Multilayer graphene nanosheets stick together if there is no perturbation by an external force. Graphene nanosheet petals are naturally crumpled and curved, which is
Conclusions
In summary, we have prepared graphene nanosheets in large quantity by a soft-chemistry approach. FEG-SEM observation revealed that loose graphene nanosheets agglomerated and crumpled naturally into shapes resembling flower-petals. HRTEM analysis, Raman spectroscopy and UV–Vis spectroscopy confirmed the graphitic crystalline structure of graphene nanosheets. We applied graphene nanosheets as anodes for lithium storage in lithium-ion cells. Graphene nanosheet anodes exhibited an enhanced
Acknowledgment
Financial support from the Australian Research Council (ARC) through ARC Discovery Project DO0772999 is gratefully acknowledged.
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