Elsevier

Nano Energy

Volume 1, Issue 1, January 2012, Pages 107-131
Nano Energy

Review
Graphene/metal oxide composite electrode materials for energy storage

https://doi.org/10.1016/j.nanoen.2011.11.001Get rights and content

Abstract

Recent progress on graphene/metal oxide composites as advanced electrode materials in lithium ion batteries (LIBs) and electrochemical capacitors (ECs) is described, highlighting the importance of synergistic effects between graphene and metal oxides and the beneficial role of graphene in composites for LIBs and ECs. It is demonstrated that, when the composites are used as electrode materials for LIBs and ECs, compared to their individual constituents, graphene/metal oxide composites with unique structural variables such as anchored, wrapped, encapsulated, sandwich, layered and mixed models have a significant improvement in their electrochemical properties such as high capacity, high rate capability and excellent cycling stability. First, an introduction on the properties, synthesis strategies and use of graphene is briefly given, followed by a state-of-the-art review on the preparation of graphene/metal oxide composites and their electrochemical properties in LIBs and ECs. Finally, the prospects and future challenges of graphene/metal oxide composites for energy storage are discussed.

Highlights

► Advance in graphene/metal oxide composites for energy storage is reviewed. ► The importance of synergistic effects of graphene and metal oxides is highlighted. ► The beneficial roles of graphene in the composites is highlighted. ► Six basic structures of graphene/metal oxide composites are summarized. ► The future of graphene/metal oxide composites for energy storage is predicted.

Introduction

Graphene is a one-atom-thick sheet of sp2-bonded carbon atoms in a honeycomb crystal lattice, which is at the cutting-edge of materials science and condensed matter physics research [1], [2], [3], [4]. It is the thinnest known material in the world and conceptually a basic build block for constructing many other carbon materials. It can be rolled into one-dimensional carbon nanotubes (CNTs), and stacked into three-dimensional (3D) graphite. With the addition of pentagons it can be wrapped into a spherical fullerene. In one sense, it is the mother of all graphitic materials [3].

In 2004, Geim and Novoselov reported their experimental investigation of the exfoliation, characterization and electronic properties of this two-dimensional (2D) carbon by repeatedly cleaving graphite with an adhesive tape [1]. Theoretical work on this structure has being carried out for decades [5], but isolated graphene and other 2D atomic layers are considered to be thermodynamically unstable. By using the same top-down approach and starting with other bulk 3D crystals with a layered structure, several stable 2D crystal nanosheets, such as boron nitride, dichalcogenide and Bi–Sr–Ca–Cu–O superconductor, were also produced by an exfoliation process. This finding shows that free-standing 2D crystals do exist and are stable at ambient temperature [6]. Graphene has been already drawn a wealth of research activities in its production, versatile unique properties and many high-tech applications.

From the viewpoint of its electronic properties, graphene is a zero-gap semiconductor with unique electronic properties originating from the fact that charge carriers in graphene are described by a Dirac-like equation, rather than the usual Schrödinger equation [2]. As a consequence of its perfect crystal structure, low-energy quasiparticles in it obey a linear dispersion relation, similar to massless relativistic particles. This essential characteristic of a gapless semiconductor has led to many observations of peculiar electronic properties [3], [7], [8], [9] including ballistic transport, pseudospin chirality based on the “Berry phase”, a room-temperature half-integer “chiral” quantum Hall effect, and conductivity without charge carriers, that make it a promising choice for future electronic materials, both as a device and as an interconnect. For example, graphene has the fastest electron mobility of ∼15,000 cm2 V−1 cm−1 or 106 Ω cm (lower than Ag), a superhigh mobility of temperature-independent charge carriers of 200,000 cm2 V−1 s−1 (200 times higher than Si), and an effective Fermi velocity of 106 m s−1 at room temperature, similar to the speed of light.

More importantly, graphene possesses not only unique electronic properties, but also excellent mechanical, optical, thermal and electrochemical properties that are superior to other carbon allotropes such as graphite, diamond, fullerene and CNTs, as shown in Table 1 [3], [4], [10], [11], [12], [13]. Single-layer graphene has excellent mechanical properties with a Young’s modulus of 1.0 TPa and a stiffness of 130 GPa, optical transmittance of ∼97.7% (absorbing 2.3% of white light), and superior thermal conductivity of 5000 W m−1 K−1 (about 100 times that of Cu). It also has a high theoretical specific surface area of 2620 m2 g−1, extreme electrical conductivity and good flexibility. Due to its unique properties, it is speculated that in many applications graphene will out-perform CNTs, graphite, metals and semiconductors where it is used as an individual material or as a component in a hybrid or composite material.

Since 2004, much work has been related to the synthesis of graphene, because its availability is an important pre-condition for its use in research and development into possible applications. So far, there are various intriguing strategies for producing single-layer and few-layer graphene that can be broadly categorized into the following six groups:

  • (i)

    Micromechanical cleavage of highly oriented pyrolytic graphite or natural graphite flakes using a Scotch tape. It is the first method to be used to produce graphene and is suitable for fundamental research due to the high structural and electronic quality of the graphene produced. This technique allows reliable and easy preparation but suffers from a low yield [1].

  • (ii)

    Epitaxial growth of graphene on SiC [14], [15] and metal (Ru, Pt) [16] single-crystal substrates at high temperature and in ultrahigh vacuum. It can grow large-size and high-quality graphene, but requires high-vacuum conditions, high-cost fabrication systems and suffers from the difficulty in transferring the graphene from the substrates as well as low yield.

  • (iii)

    Thermal- or plasma-enhanced chemical vapor deposition (CVD) of graphene from the decomposition of hydrocarbons at high temperatures on metal substrates (such as Ni, Cu, Pt) or metal oxide (Al2O3, MgO) particles. It allows for fast, uniform, large-area, high-quality graphene production, but its disadvantages are high-cost and relatively low yield. However, this strategy has great potential for further improvement [17], [18], [19].

  • (iv)

    Chemical exfoliation of graphitic materials. It involves oxidation, intercalation, exfoliation and/or reduction of graphene derivatives [20], [21], [22], [23], such as graphite, graphite oxide, expandable graphite, CNTs, graphite fluoride and graphite intercalation compounds. It can potentially afford a bulk quantity of graphene, especially, from graphite oxide. The exfoliation and reduction of graphite oxide has now been demonstrated to be a primary low-cost strategy that can yield a large quantity of reduced graphene oxide (GO) with high processability.

  • (v)

    A bottom-up synthesis strategy from organic compounds. It is used to synthesize nano/micrographene and graphene-based materials from structurally defined precursors, such as polycyclic aromatic hydrocarbons. This approach can precisely control the formation of molecular graphene (<5 nm), nanographene (5–500 nm) and integrated macrographene (>500 nm) with well-defined structures and high processability. However, it suffers from the drawback of low productivity [24], [25].

  • (vi)

    Other methods such as electrochemical exfoliation of graphite [26], graphene growth from solid state carbon [27], direct arc discharge of graphite [28], reduction of ethanol by sodium metal [29], and the thermal splitting of SiC granules [30]. Each method has its merits and shortcomings in terms of both scalability of the method and the quality of the graphene produced. Detailed discussion can be found in several excellent reviews [13], [31], [32].

It should be emphasized that, currently, only the chemical exfoliation method is considered as a common route toward the production of graphene at low cost and in a large quantity [20]. It first involves the oxidation of well-stacked graphite to graphite oxide [33], and is then followed by chemical reduction of GO to obtain reduced GO (rGO) [34] or thermal exfoliation of graphite oxide [35], [36], [37], [38] to produce graphene. Generally, oxidation results in an increase of the d-spacing and intercalation between adjacent graphene layers, and thus weakens the interaction between adjacent sheets, and finally leads to the delamination of GO in an aqueous solution. Reduction using chemical compounds such as NH2NH2 [34], KOH [39], NaBH4 [40], HI [41] or thermal exfoliation of graphite oxide [35], [36], [37], [38] is commonly performed to obtain graphene from GO. For example, Schniepp et al. proposed a thermal exfoliation method to produce graphene nanosheets (GNS), where a rapid heating process is involved to exfoliate graphite oxide by quickly moving it into a furnace preheated to a high temperature [35]. Wu et al. proposed a controllable oxidation and rapid heating exfoliation strategy to tune the number of graphene layers produced by selecting a suitable starting graphite [37]. It is found that the higher the heating rate, the greater both the exfoliation and de-oxygenation degrees of graphite oxide. High annealing temperature is essential to remove structural defects. Therefore, Wu et al. developed a hydrogen arc discharge exfoliation method (>2000 °C) for the synthesis of high-quality GNS from graphite oxide with excellent electrical conductivity (∼2×103 S cm−1) and good thermal stability (∼601 °C). Complete exfoliation and considerable de-oxygenation of graphite oxide and defect elimination can be simultaneously achieved during the hydrogen arc discharge exfoliation process [38]. In addition to the easy bulk synthesis, a major advantage of both GO and rGO is the controlled attachment of oxygen species on the edges and surfaces of the graphene sheets. This enables the formation of stable GO or rGO dispersions and easy functionalization in aqueous and organic solvents [42], thus offering a variety of opportunities for the simple processing of structure-dependant functionalized graphene-based materials [43]. Future methods for the production of graphene will be focused on innovation in its low-cost, large-area, large-scale production for applications.

The following main applications of graphene that take advantage of its electronic properties are expected to be major breakthroughs: (i) graphene-based electronics and optoelectronics, partially replacing conventional silicon-based electronics, because graphene has ultrafast terahertz electron mobility that gives it a very bright future for building smaller, faster, cheaper electronic devices such as ballistic transistors [44], spintronics [45], field effect transistors [46], and optoelectronics [47]. (ii) Graphene-filled polymer composites with high electrical and thermal conductivity, good mechanical strength, and low percolation threshold, which, in combination with low-cost and large-scale production, allow a variety of performance-enhanced multifunctional use in electrically conductive composites, thermal interface materials, etc. [48], [49]. (iii) Large-area CVD-grown graphene that is suitable to replace indium tin oxide (ITO) as cheaper, transparent conducting electrodes in various display applications such as touch screens, which is considered to be one of the immediate applications in a few years [50], [51]. Another advantage over ITO is that ITO suffers from being brittle and is incapable of bending, which does not allow it to meet the requirements for flexible devices, while graphene is a more competitive solution for flexible, transparent and processable electrodes. (iv) Graphene-based electrochemical storage energy devices such as high-performance LIBs and ECs because of their greatly improved electrochemical performance of capacity, cyclability and rate capability due to its unique 2D structure and excellent physiochemical properties [52]. (v) Recent research indicates many other potential applications in gas- [11], bio-, electrochemical, and chemical sensors [53], dye-sensitized solar cells [54], organic solar cells [55], field emission devices [56], catalysts [57] and photocatalysts [58], nanogenerators [59], hydrogen storage [60], etc. Graphene may offer other advantageous properties that outperform those of CNT and graphite, resulting in the development of new and unexpected applications.

Detailed descriptions of the properties, synthesis and applications of graphene can be found in some recent published papers [13], [31], [53], [61], [62], [63]. Here, we only try to provide an overview of the recent progress in graphene/metal oxide composites as advanced electrodes for high-performance LIBs and ECs, highlighting the importance of synergistic effects between graphene and metal oxides in the composites and the improvement of their electrochemical properties including high capacity/capacitance, increased rate capability, excellent cyclic stability, and high energy density and power density.

Section snippets

Performance of graphene for LIBs and ECs

Graphene has attracted intense interest in electrochemical energy storage due to its large surface area, good flexibility, good chemical and thermal stability, wide potential windows, rich surface chemistry, and extraordinary electrical, thermal and mechanical properties [61], all of which are advantageous for energy storage and conversion systems. Therefore, graphene has been explored as an electrode material in electrical energy storage devices such as LIBs and ECs just after its large-scale

Structural models of graphene/metal oxide composites

As described above, one of the intractable issues for the use of graphene in LIBs and ECs is that chemically derived graphene generally suffers from serious agglomeration and re-stacking after removal of dispersed solutions and drying due to the van der Waals interactions between GNS, consequently lowering the electrochemical performance of GNS in LIBs and ECs.

To fully use all the potential advantages of graphene in LIBs and ECs, fabrication of graphene/metal oxide composites is expected to be

Perspectives and challenges

We have reviewed the recent advance in electrochemical applications of graphene/metal oxide composite materials, highlighting them as a new and promising class of advanced electrode materials for LIBs and ECs. In these graphene-based materials, emphasis is given to synergistic effects between graphene and metal oxides. The beneficial role of graphene in the composites is due to its unique structures and properties such as high surface area, ultra-thin thickness, excellent electrical and thermal

Acknowledgment

This work was supported by the Key Research Program of Ministry of Science and Technology, China (no. 2011CB932604), the National Natural Science Foundation of China (no. 50921004), Chinese Academy Sciences (KGCX2-YW-231) and the Jinlu Limited.

Zhong-Shuai Wu received his Bachelor’s degree in Chemistry and Master degree in Inorganic Chemistry from Liaoning University in 2004 and 2007, and Ph.D. in Materials Science from Institute of Metal Research, Chinese Academy of Sciences in 2011 supervised by Prof. Hui-Ming Cheng. He is currently a postdoctoral fellow working with Prof. Dr. Klaus Müllen at Max-Plank Institute for Polymer Research. His research mainly focuses on synthesis and application explorations of graphene and nanomaterials

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    Zhong-Shuai Wu received his Bachelor’s degree in Chemistry and Master degree in Inorganic Chemistry from Liaoning University in 2004 and 2007, and Ph.D. in Materials Science from Institute of Metal Research, Chinese Academy of Sciences in 2011 supervised by Prof. Hui-Ming Cheng. He is currently a postdoctoral fellow working with Prof. Dr. Klaus Müllen at Max-Plank Institute for Polymer Research. His research mainly focuses on synthesis and application explorations of graphene and nanomaterials for energy storage and conversion systems in batteries, supercapacitors and fuel cells.

    Guangmin Zhou received his Bachelor’s degree from the Department of Materials Science and Engineering at Nanjing University of Science and Technology in 2008. He is currently pursuing his Ph.D. under the supervision of Prof. Hui-Ming Cheng at Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences. His research interests mainly focus on synthesis and application explorations of carbon-based materials for lithium–sulfur batteries and lithium ion batteries.

    Li-Chang Yin received his Ph.D. in Particle and Nuclear Physics from Jilin University, Changchun, the People’s Republic of China, in 2002. He started his postdoctoral work at the Advanced Carbons Research Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Science from 2003, and then joined its faculty from 2006. He worked at the Tohoku University of Japan in 2009. His research interests center on the theory and computational simulation of low dimensional nano-materials.

    Wencai Ren received his Ph.D. degree in materials science from Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS) in 2005. He has been an assistant and associate professor of materials science at IMR, CAS since 2005, and was promoted to be a professor in 2011. From 2009 to 2010, he worked with Prof. Andre K. Geim at the University of Manchester as a visiting researcher. His current research interests include the synthesis, properties and applications of graphene and carbon nanotubes.

    Feng Li is a professor of the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Ph.D. in materials science at IMR, CAS in 2001 supervised by Prof. Hui-Ming Cheng. He mainly works on the nanomaterials for clean energy such as electrode materials for lithium ion battery and supercapacitor.

    Hui-Ming Cheng received his Ph.D. in Materials Science from the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He worked at AIST and Nagasaki University in Japan, and MIT in USA. Currently he is Professor and Director of the Advanced Carbons Research Division at Shenyang National Laboratory for Materials Science, IMR, CAS. His research interests focus mainly on carbon nanotubes, graphene, high-performance bulk carbons, materials for batteries and electrochemical capacitors, and photocatalytic materials for hydrogen production and CO2 conversion.

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    These authors have equally contributed.

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