Graphene and graphene-based materials for energy storage applications.

Graphene

Graphene and Graphene-Based Materials for Energy Storage Applications Jixin Zhu, Dan Yang, Zongyou Yin, Qingyu Yan,* and Hua Zhang*

From the Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Graphene-Based Materials for Li-ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Graphene-Based Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . 7 4. Graphene and Graphene-Based Composites for Lithium Sulfur Batteries . . . . . . . . . . . . . 12 5. Graphene and Graphene-Based Composites for Lithium Air/Oxygen Batteries . . . . . . . . . 14 6. Conclusions and Outlook. . . . . . . . . . . . . . . 16

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With

the increased demand in energy resources, great efforts have been devoted to developing advanced energy storage and conversion systems. Graphene and graphenebased materials have attracted great attention owing to their unique properties of high mechanical flexibility, large surface area, chemical stability, superior electric and thermal conductivities that render them great choices as alternative electrode materials for electrochemical energy storage systems. This Review summarizes the recent progress in graphene and graphene-based materials for four energy storage systems, i.e., lithium-ion batteries, supercapacitors, lithium-sulfur batteries and lithium-air batteries.

© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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1. Introduction Hydrocarbon fossil fuels such as petroleum oil, coal and natural gas are accounted for the most significant portion of the global electricity generation. However, the increasing consumption and the rapid depletion of fossil fuels has driven the major research focus to exploitation and utilization of renewable energy such as wind energy, tidal energy and solar energy for the past few decades. To provide widespread usage of renewable energies, efficient energy storage and conversion technologies are required. Among these applications, portable electric vehicles and hybrid electric vehicles have been significantly developed to the need of our informationrich, mobile society. Inspired by this, the electrochemical systems available for energy storage and conversion including lithium ion batteries, electrochemical capacitors and metalair batteries are designed and developed for advanced energy conversion and storage devices.[1–5] To enhance the energy density and power density of energy storage devices, great efforts have been invested to the syntheses of advanced electrode materials with tailored structure, composition and morphology. Significantly, engineering the nanostructured active materials into highly conductive matrix offers desirable functionality and great potential to achieve excellent energy storage, high rate capabilities and long lifespan for electrode materials.[6–10] Graphene, a two-dimensional (2D) sheet composed of sp2bonded single-layer carbon atoms with the honeycomb lattice structure, has attracted great research interest and wide application potential in physics, chemistry, and materials science.[11] With regard to its unique structural features of high surface area, flexibility, chemical stability, superior electric and thermal conductivity,[12,13] graphene has been used as ideal building blocks for graphene-based materials with desirable functionality as alternative electrode materials. In this review, we mainly focus on the latest advances on the application of graphene and graphene-based materials for four frontier electrochemical energy storage devices named as lithium-ion batteries, supercapacitors, lithium-sulfur batteries and lithium-oxygen batteries.

2. Graphene-Based Materials for Li-ion Batteries The lithium-ion battery (LIB), as an effective electrochemical energy storage device, has attracted great interests due to their high energy density (120–170 Wh kg−1), longer life and better safety compared to the tranditional batteries that have been developed. However, although their energy density is high, LIBs still suffer from a much lower power density compared to electrochemical capacitors (ECs), another important energy storage device, e.g., a ∼6 h charging time is normally necessary for the mobiles phones or laptops that are equipped with a lithium ion battery in the present market. To simultaneously achieve high power and large energy density at fast charge and discharge rates from several minutes to seconds still remains a great science and engineering challenge for the development of LIB. In principle, the charging

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and discharging process of a LIB is realized through the insertion/deintercalation of Li ions transporting between the anode and cathode material.[4,10] The power capability of a LIB depends critically on the speed at which Li ions and electrons migrate through the electrolyte and electrode. Therefore, the key to create high-power LIBs is developing new materials with high electrical conductivity for fast electron transport and a large surface area and well-developed nanostructures with shortened diffusion length for Li ions. In this aspect, graphene, due to its superior electrical conductivity, excellent mechanical flexibility, good chemical stability, and high surface area (2630 m2 g−1), is expected to be a good candidate.[14–16] However, it is reported that LIBs with pristine graphene anodes cannot provide stable potential outputs, which sets obstacle for its practical applications.[17] To circumvent such problem and further improve the performance of graphene electrodes, variable strategies have been developed, of which the advantages and challenges will be discussed in the following section.

2.1. Graphene-Based Materials for Anodes 2.1.1. Graphene and its Derivatives for Anodes The theoretical capacity of commercially used graphite LIB anode is 372 mAh g−1 based on the formation of LiC6.[4] Increasing this value is one of the key issues to develop LIB with higher energy density and power density. The pioneering work of exploring the possibility of increasing lithium storage capacity by graphene nanosheet (GNS) material is achieved by Yoo et al. in 2008.[18] They controlled the layered structure of graphene nanosheet through an exfoliation and reassemble process. The specific capacity of GNS was found to be 540 mAh g−1, and this was further increased up to 730 mAh g−1 and 784 mAh g−1, respectively, by the incorporation of CNT and C60 to the GNS. This improved performance was attributed to the different electronic structure between graphite and graphene nanosheet and the additional sites for accommodation of lithium ions due to the expansion in the d-spacing of the graphene layers.[18–22] Besides of GNS, the graphene papers have shown great potential in the usage as electrodes in flexible energy storage devices.[23,24] Via reduction of prefabricated graphene oxide paper with hydrazine hydrate, non-annealed graphene paper was employed as a binder-free lithium ion battery anode, exclusive the usage of polymer binders and other additives required for the

Dr. J. Zhu, D. Yang, Dr. Z. Yin, Prof. Q. Yan, Prof. H. Zhang School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore, 639798, Singapore E-mail: [email protected]; [email protected] Dr. J. Zhu, D. Yang, Prof. Q. Yan TUM CREATE, 1 CREATE Way, #10–02 CREATE Tower, Singapore, 138602, Singapore DOI: 10.1002/smll.201303202


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Graphene and Graphene-Based Materials for Energy Storage Applications

Figure 1. a) Schematic structure of the binding conditions of Nitrogen (N) and Boron (B) in a graphene lattice, indicated by magenta dotted rings. b) Ragone plots for the pristine graphene, N-doped graphene, B-doped graphene, graphene oxide (GO), and thermally reduced graphene oxide at 500 °C (GO500) based cells with lithium metal as the counter/reference electrode. The calculation of gravimetric energy and power density was based on the active material mass of a single electrode. Reproduced with permission.[32] Copyright © 2011, American Chemical Society.

conventional electrodes. On the other hand, chemically prepared graphene exhibits distinguishable electrochemical properties compared to graphite, which was attributed to the presence of some residual oxygen containing groups.[20,25] However, even chemically derived graphene could exhibit a high reversible capacity up to 1264 mAh g−1 (∼2 times higher than that of conventional graphite anode that has been used in commercial LIBs) at a low charge-discharge rate (such as 50–100 mA g−1); at a high charge-discharge rate (500 mA g−1 or higher), it would undergo large capacity fluctuation. This is closely related to the surface side reactions, in particular, the formation of solid electrolyte interphases (SEI) films. Furthermore, oxygen will be released from the delithiated state because of the decomposition of the oxygen-containing functional groups and will partly oxidize the electrolyte and consequently induce electrochemical instability of the electrode. Although the oxygen content of graphene films can be significantly reduced by thermal annealing, the limited rate capability of chemically derived graphene still remains a big deficiency. Among the numerous strategies that have been applied to improve the rate capabilities of graphene electrodes, nitrogen (N)-and boron (B)-doped graphene structure has attracted great interests.[26–32] In particular, Cheng’s group has reported an electrode with extremely high charge-discharge rate and large capacity made by heteroatom (N, B)-doped chemically derived graphene as shown in Figure 1.[32] At a low charge/discharge rate of 50 mA g−1, the doped graphene electrodes exhibited a high capacity of 1043 mAh g−1 for N-doped graphene and 1540 mAh g−1 for B-doped graphene. More importantly, the doped graphene could be quickly charged and discharged for a very short time from 1 h to several tens of seconds, achieving high rate capability and long-term cyclability at the same time (Figure 1b). For example, at an ultrafast charge/discharge rate of 25 A g−1 (∼30 s to full charge), the electrodes could still retain a significant capacity of ∼199 mAh g−1 for the N-doped graphene and ∼235 mAh g−1 for the B-doped graphene. It was proposed by the author that such superior performance of this doped graphene electrode is due to disordered surface morphology, heteroatomic defects, better small 2014, DOI: 10.1002/smll.201303202

electrode/electrolyte wettability, increased intersheet distance, improved electrical conductivity, and thermal stability which are beneficial to rapid surface Li ion absorption and ultrafast Li+ diffusion and even electron transport. Moreover, the successful syntheses of large area monolayer and multilayer graphene and their feasibility to be transferred onto any substrate provide an opportunity to explore numerous fundamental science issues. For example, due to the coexistence of both edge plane and basal plane in graphite, the diffusion mechanism of lithium ions in graphite is ambiguous.[33] With the aid of a chemical vapor deposition (CVD) method, two types of graphene samples, with well-defined basal plane enriched and edge plane enriched have been fabricated on a Cu and Ni substrate, respectively (Figure 2a and b).[34] It was found that Li ion diffusion perpendicular to the basal plane of graphene is facilitated by defects, whereas diffusion parallel to the plane is limited by the steric hindrance that originates from aggregated Li ions adsorbed on the abundant defect sites as illustrated in Figure 2c. In addition, self-heating arised from the short-circuiting or fast charging/discharging processes in a LIB would sometimes lead to cell rupture or even severe safety concerns, such as fire and explosion. The attempt to achieve higher power density made efficient heat removal an even more crucial and challenging issue for practical applications of LIBs.[35–37] With its extremely high intrinsic thermal conductivity, i.e., the thermal conductivity of a single-layer graphene sheet is measured to be as high as ∼5000 W mK−1 at room temperature, graphene has been introduced in practical LIB packs to achieve effective heat transfer. As demonstrated by the Balandin‘s group, hybridizing graphene with conventional phase-change materials (PCMs) could increase the thermal conductivity by two orders of magnitude compared to the conventional PCMs, which inhibited the temperature rise inside a LIB pack efficiently. 2.1.2. Graphene-Based Composites as Anodes Metal oxides (MO, e.g., M = Fe, Co, Ni, Sn, or Cu), Sn, Ge and Si are important members of anode family, which


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Figure 2. Optical micrographs of a) Cu-grown single layer graphene (SLG). b) Ni-grown multilayered graphene (MLG) on SiO2/Si substrate. White dashed lines indicate wrinkles. Some portion of thicker graphene is indicated by arrows. c) Schematic of I) single layer graphene (SLG) with a well-defined basal plane and II) edge plane enriched multilayered graphene (MLG). d) Micro-Raman spectra of single layer graphene (SLG) and MLG. Confocal Raman mapping of D/G intensity ratio of e) single layer graphene (SLG). f) multilayered graphene (MLG) from squared positions of (a) and (b). The contrast is normalized to 0.4 to visualize the defect distribution for both images. g) Wavelength-dependent transmittance (values are provided at a wavelength of 550 nm) and h) optical photographs of different number of graphene layers on Polyethylene terephthalate (PET) substrate. Reproduced with permission.[34] Copyright © 2012, American Chemical Society.

have been widely investigated due to their high theoretical Li-ion storage capacities (>600 mAh g−1) compared to the conventional graphite anode (372 mAh g−1).[38–43] However, these materials often suffer from the low electrical conductivity and poor capacity retention due to the pulverization process. Graphene can host the nanostructured electrode materials by providing a support for anchoring nanoparticles and work as a highly conductive matrix for good contact between electrode and current collector.[44–48] More importantly, graphene layers can prevent the volume expansion/contraction and the aggregation of nanoparticles effectively during charge and discharge process. Meanwhile, the integration of inorganic nanostructures with the graphene layers may reduce the restacking of graphene sheets and consequently maintain the high surface area. In this regard, both the lithium storage capacity and the cycling performance of graphene-based composites can be improved. Consequently, great efforts have been devoted to develop new synthetic methods of metal oxide/graphene sheets composites.[46,47,49,50] For example, reduced

graphene oxide-wrapped Fe3O4 composites (rGO/Fe3O4) have been prepared using a hydrothermal method followed by a post-annealing process.[51] It was found that the rGO/ Fe3O4 composite demonstrated much better rate capability as well as stable cyclability compared to the commercial Fe3O4 and bare Fe2O3 particles. In particular, when the current density reached 1750 mA g−1, the specific capacity of the rGO/Fe3O4 composite still remained to be 520 mAh g−1 that ca. 53% of the initial capacity. As a proof of concept, Cui and co-workers has reported a two-step hydrothermal method to prepare Mn3O4 nanoparticles attached onto the rGO sheets, referred to as Mn3O4/rGO (Figure 3a).[44] The Mn3O4/rGO composite showed superior rate capability as compared to the bare Mn3O4 nanoparticles (Figure 3b) when tested as LIB anodes. The specific capacity of Mn3O4/ rGO was ∼390 mAh g−1 even at a high current density of 1600 mA g−1, which is higher than the theoretical specific capacity of graphite (∼372 mAh g−1). For comparison, a low specific capacity of ∼100 mAh g−1 was delivered at a current density of 40 mA g−1 for bare Mn3O4.

Figure 3. a) SEM image of the cross-section of graphene nanosheet (GNS)/Fe3O4 composite. b) Rate capability of the commercial Fe3O4 particles, graphene nanosheet (GNS)/Fe3O4 composite, and bare Fe2O3 particles at different current densities. Here, actually graphene nanosheet (GNS) is reduced graphene oxide (rGO). Reproduced with permission.[51] Copyright © 2010, American Chemical Society.



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Figure 4. FESEM images of a) reduced graphene oxide (rGO)-encapsulated SiO2. b) rGO@Co3O4. c) Cycling performance of rGO@Co3O4, mixed Co3O4/rGO composite, and pure Co3O4 electrodes over 30 cycles. d) Cycling performance and Coulombic efficiency of the rGO@Co3O4 electrode during 130 cycles at current density of 74 mA g−1. Reproduced with permission.[48] Copyright © 2010, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In order to further improve the performance of graphenebased materials, control over the microstructures has proved to be of great importance.[52–54] For example, Müllen and coworkers prepared different types of rGO-encapsulated oxide nanoparticles (e.g., SiO2, Co3O4) with the addition of organic molecules as “binder” (Figure 4a and b).[48] This unique architecture can effectively suppress the aggregation of oxide nanoparticles and accommodate the volume expansion of the active material to enhance its electrochemical performance. The rGO-encapsulated Co3O4, referred to as rGO@ Co3O4, showed higher capacity than that of the mixed Co3O4/ rGO composite or pure Co3O4 (Figure 4c), and also exhibited excellent cycling stability (Figure 4d). It can be observed that a highly reversible capacity of about 1100 mAh g−1 was delivered in the initial 10 cycles for the rGO@Co3O4 electrode at a current density of 74 mA g−1. And a discharge capacity of 1000 mAh g−1 can still be retained at the 130th cycle. For comparison, the specific capacities of the mixed Co3O4/rGO and pure Co3O4 electrodes dropped quickly to 67% (∼680 mAh g−1) and 52% (∼400 mAh g−1) of their initial capacities during the 30th cycle, respectively. This concept has been extended to fabrication of other rGO-encapsulated metal oxide composites, such as rGO@Fe2O3, rGO@CuO and rGO@CoO.[55] In such preparation process, the surface of metal oxides was firstly modified by poly (allylamine hydrochloride) and then wrapped with GO. Besides inorganic/ rGO composites, preparation of organic molecular/rGO composites as anode materials has also been reported.[56] It small 2014, DOI: 10.1002/smll.201303202

has been found that grafting redox-active organic molecules onto rGO can effectively reduce the dissolution of these molecules into the electrolyte during the charge/discharge process and largely improve their cycling stability.[56] Finally, the electrochemical performance of typical graphene and graphene-based materials as anode electrodes for LIBs has been listed and compared in Table 1. Importantly, the hybrid structures which combine graphene with other functional materials such as metal oxides or organic molecules have shown better performance compare to the pristine graphene. Doping of heteroatoms that introduces more surface defects and improves electrical conductivity of pure graphene also gives the superior enhancement in performance. Further breakthrough in increasing energy density of graphene-based anode for LIB may lie in the development of more hybrid systems or more delicate structure manipulation, such as well-defined porous, 2D or 3D structures.

2.2. Graphene-Based Materials for Cathodes To this end, limited capacity of current cathode materials remains a bottleneck for further breakthroughs in LIB technology. Over the past decades, extensive efforts have been devoted into increasing the capacity and energy density of existing cathode materials as well as exploring their possible alternatives to satisfy the high power demand in the future electronics market. In particular, graphene and its derivatives



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Table 1. Comparison of electrochemical performance of graphene and graphene-based materials as anode electrodes for lithium-ion batteries. Materials

Specific capacities Current densities References (A g−1) (mAh g−1)

Graphene nanosheet (GNS)

540

0.05

[18]

GNS/CNT

730

0.05

[18]

GNS/C60

784

0.05

[18]

N-doped graphene

1043

0.05

[32]

199

25

[32]

1540

0.05

[32]

235

25

[32]

520

1.75

[51]

B-doped graphene

rGO/Fe3O4 Mn3O4/rGO

390

1.6

[44]

rGO/Co3O4

1000

0.074

[55]

Organic molecule/rGO

415

3

[56]

315

5

236

10

have been extensively introduced into the cathode system to compensate for some deficiencies suffered by common cathode materials, such as the poor electrical conductivity, sluggish kinetics of electron and Li ion transportation, low specific capacities and particle agglomeration generated from their nanostructures.

For example, olivine-structured LiMPO4 (M = Fe, Mn, Co, or Ni) has been intensively investigated as promising cathode materials for rechargeable LIBs owing to their high capacity, excellent cycle life, thermal stability, environmental benignity, and low cost.[57–60] However, the inherently low ionic and electrical conductivities of such cathode materials have hindered their practical application. Several reports have demonstrated certain enhancement in rate capacity using the reduced graphene oxide (rGO)/LiFePO4 composite material as the cathode, such as the three dimentional (3D) porous LiFePO4/graphene hybrid and graphene wrapped LiFePO4/C composites.[61,62] In particular, the LiMn1–xFexPO4/rGO composed of small LiMn1–xFexPO4 nanorods and rGO sheets has been reported by Cui and co-workers.[63] The excellent rate capabilities were demonstrated in such sample with specific capacities of 107 mAh g−1 at 50 C, and 65 mAh g−1 at 100 C. In addition, it also showed high cycling stability with a specific capacity of 155 mAh g−1 till to the 100th cycle at 0.5 C. Such outstanding Li-storage performance of the LiMn0.75Fe0.25PO4/rGO cathode was ascribed to rapid ion and electron transportation offered by the intimate interactions between the nanorods and the underlying rGO sheets. Recently, a high reversible capacity and ultrafast charging and discharging capability is achieved in the VO2-graphene nanoribbons as shown in Figure 5. The results clearlydemonstrate that well-defined crystalline VO2-graphene nanoribbons can provide fastdischarging and charging capability with

Figure 5. Electrochemical performance of VO2-graphene architectures under various temperatures. (a) Cycle performance of VO2-graphene architectures under various temperatures from 25 to 75 °C at a current rate of 5C (1.9 A g−1) over the potential range of 1.5–3.5 V vs Li+/Li. After the cycle performance test at 75 °C, the environmental temperature is recovered to 25 °C for another 30 cycles. (b) Nyquist plots of VO2-graphene architecture (78%) after 30 cycles under various selected temperatures (25, 45, 60, and 75 °C), obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range of 100 kHz to −0.01 Hz. (c) Capacity retentions of VO2-graphene architectures under the highest temperature of 75 °C at a current rate of 28C. 1′ and 2′ are denoted as VO2-graphene architectures with the VO2 contents of 78% and 68%, respectively.[64] Copyright © 2013 American Chemical Society.



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high capacity andlong cycle performance even at high temperatures. This would lead to a breakthrough in graphenebased cathode materials for high energy density and power densitylithium ion batteries. Vanadium pentoxide (V2O5) is another promising cathode candidate for LIBs because its theoretical capacity (440 mAh g−1) is much higher than those of the current commercial cathodes (200 000), high power capability, and compatibility with flexible substrates were well preserved. The prominent enhancement was believed to be attributed to the N-configuration at the basal planes which enlargers the binding energy to accommodate larger amount of ions in the electrolyte on the electrode surface. In addition, a series of graphene-polymer composites have been prepared.[85–87] For example, graphite oxide has shown efficient heterogeneous catalytic acitivity for the polymerization of various olefin monomers, which after thermal treatment, displayed a high


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Figure 7. a) Schematic illustration of the assembled supercapacitor structure alongside an SEM image showing a top view of the device. b) Charging and discharging curves of N-doped rGO and pristine rGO measured by galvanostatic characterization. c) Gravimetric capacitances of supercapacitors based on various N-doped rGO and pristine rGO measured at a series of current densities. The numbers in the legend indicate the plasma durations in minutes. d) Gravimetric capacitances of N-doped rGO on different substrates measured at different current densities. Inset: A photograph showing that a wearable UC wrapped around a human arm can store the electrical energy to light up a LED. e) The cycling tests for the supercapacitors based on Ni and paper substrates up to 10000 cycles. f) The specific capacitances measured in aqueous and organic electrolytes. Reproduced with permission.[28] Copyright © 2011, American Chemical Society.

specific capacitance of 25–120 F g−1 and low equivalent series resistance (14–27 Ω).[88] Poly(ionic liquid)-modified graphene yielded a maximum energy density of 6.5 Wh kg−1 with a power density of 2.4 kW kg−1 when it was used as electrodes for supercapacitors.[89] As inspired by the aforementioned work, other functional polymers, such as polydopamine, were also considered for the design of future electrodes in energy storage devices, such as ECs and LIBs.[90,91] Besides of effective prevention on the restacking of graphene, a high-quality interfacial contact between the current collector and the active materials is highly desirable

to reduce the contact resistance and lead to supercapcitors with high power and rate capabilities. Recently, vertically oriented graphene (VG) nanosheets have been grown on various substrates (e.g., planar or cylindrical metals, and carbon nanotubes) through plasma-enhanced chemical vapor deposition (PECVD) as illustrated in Figure 8.[92] As graphene shows a higher in-plane than out-of-plane electrical conductivity, these perpendicular graphene can serve as ideal electrical “bridges” linking the current collector and active materials. The as-proposed VG bridged supercapacitor, showed an ultrahigh power density of 112.6 kW kg−1

Figure 8. a) High-magnification SEM image of the graphene bridges standing vertically on the nickel-foam surface. b) Ragone plots for different working electrodes. c) Schematic illustration of electron transport between active materials and Foil-current collector (Foil-CC), Foam-current collector (Foam-CC), and VG/Foam-current collector, respectively. Reproduced with permission.[92] Copyright © 2013, WILEY-VCH Verlag GmbH & Co. KGaA. small 2014, DOI: 10.1002/smll.201303202



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Figure 9. a-c) Schematic diagram showing the fabrication process for laser-scribed graphene microsupercapacitors (LSG-MSC). a). Copper tape is applied along the edges to improve the electrical contacts, and the interdigitated area is defined by polyimide (Kapton) tape b). An electrolyte overcoat is then added to create a planar micro-supercapacitor c). d,e) This technique has the potential for the direct writing of micro-devices with high areal density. More than 100 micro-devices can be produced on a single run. The micro-devices are completely flexible and can be produced on virtually any substrate. f) Volumetric stack capacitance of LSG-MSC in the sandwich and interdigitated structures as calculated from the charge/ discharge curves at different current densities. Data for a commercial AC-SC are shown for comparison. Reproduced with permission.[96] Copyright © 2013, Nature Publishing Group.

(specific capacitance of 130 F g−1) at a current density of 600 A g−1. On the other hand, in recent years, the continuous miniaturization trend in the current portable electronics has spurred development of the micro-supercapacitors with enhanced functionality and reliability, which holds great potential in future commercialization. However, due to the insufficient energy density, downsizing of the necessary components remains a big challenge and supercapacitors with conventional sandwich structure cannot be well incompatible with integrated circuits.[93] Progress in micro-fabrication technology has enabled on-chip micro-supercapacitors in an interdigitated planar form. Most important among these studies, a new type of all-carbon, monolithic supercapacitor devices have been demonstrated by Ajayan and co-workers using a laser-induced reduction followed by the patterning of graphite oxide film.[94] The substantial amounts of trapped water in the graphite oxide made it simultaneously a good ionic conductor and an electrical insulator, allowing it to serve as both an electrolyte and an electrode separator with ion transport characteristics. Device from this simple and promising technique exhibited outstanding cycling stability and energy storage capacities comparable to that of the conventional thin film supercapacitor. However, the major shortage for this technique is the poor frequency response and the large internal resistance (6.5 kΩ). In order to solve these issues, graphene/CNT composite micro-supercapacitors were fabricated by micro-fabrication techniques and electrostatic spray deposition.[95] Even at a very high scan rate of 50 V s−1, a specific capacitance of 2.8 mF cm−2 (stack capacitance of 3.1 F cm−3) was recorded. The addition of CNT, electrolyte-accessible and binder-free microelectrodes, as well as an interdigitated in-plane design resulted in a highfrequency response with resistive-capacitive time constant as


low as 4.8 ms. Despite of the above advances, developing a simple, cost-effective technique that does not require masks, or sophisticated processing while producing high-performance micro-devices is still challenging. Most recently, the UCLA researchers have developed a groundbreaking technique that used a DVD burner to fabricate micro-scale graphene-based supercapacitors (see Figure 9).[96,97] More than 100 micro-supercapacitors can be easily manufactured and readily integrated into small devices on a single disc in less than 30 minutes. These supercapacitors exhibited an ultrahigh power of ∼200 W cm−3 and excellent frequency response with an resistor-capacitor (RC) time constant of only 19 ms. More prominently, they were also highly bendable and twistable, which could be potentially useful as energy-storage devices in flexible electronics like roll-up displays and TVs, e-paper, and even wearable electronics. So far, mass production of the above micro-supercapacitors is ready to be realized through the commercialization of this cost effective, large scalable and high power technique, as expected by its inventors. In terms of progress made on the energy density improvement for current supercapacitor electrodes, another crucial work worth mentioning is the porous yet densely packed carbon electrodes with high ion-accessible surface area and low ion transport resistance reported by Li’s group refered to Figure 10.[98] Taking advantage of chemically converted graphene’s intrinsic microcorrugated two-dimensional configuration and self-assembly behavior, porous yet densely packed graphene was readily formed by capillary compression of adaptive graphene gel films in the presence of a nonvolatile liquid electrolyte. This simple soft approach enables subnanometer scale integration of graphene sheets with electrolytes to form highly compact carbon electrodes with a continuous ion transport network. As a result, supercapacitors


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Figure 10. Characterization of liquid electrolyte-mediated chemically converted graphene (EM-CCG) films. A) A photograph showing the flexibility of the film. B and C) SEM images of cross sections of the obtained EM-CCG films containing, B) 78.9 volume percent (vol.%) and C) 27.2 vol.% of H2SO4, respectively, corresponding to r = 0.42 g cm−3 and r = 1.33 g cm−3. Reproduced with permission.[98] Copyright © 2013, American Association for the Advancement of Science.

fabricated from the as-resulted films could achieve volumetric energy densities approaching 60 Wh L−1. Despite some exciting and promising breakthroughs have been realized in improving the specific capacitances and energy densities of EDLCs, the capacitance derived from the EDLCs is still not sufficient for most energy storage devices. To address this problem, another class of capacitors, so-called pseudocapacitors, has been developed. Pseudocapacitors usually take advantage of the redox reaction on the surface of active metal oxides to store energy. Through the synergistic effects of the active material and the highly conductive carbonaceous materials, the specific pseudocapacitance could largely exceed that of the conventional electrical double layer capacitor.[99–102] MnO2 has attracted great research interests as a promising supercapacitor electrode due to its high theoretical specific capacitance, low cost and easy synthesis.[100] However, the poor electrical conductivity of MnO2 would lead to the low specific capacitance at relative high current density. Graphene and its derivatives were often introduced to overcome this obstacle.[99,103–106] For example, MnO2 attached to the rGO-covered textile has been reported by Bao and coworkers.[107] The rGO/MnO2-textile electrode was fabricated by electrodeposition to form a network structure, where the MnO2 particles were decorated onto the surface of textile covered with rGO. The rGO layer largely increased the electrical conductivity of the electrode. The specific capacitance of rGO/MnO2-based nanocomposite, i.e., ∼315 F g−1 at a scan rate of 2 mV s−1, is substantially higher than that of the purerGO textile owing to the additional redox responses of MnO2 with electrolyte ions. Based on similar mechanism, the high specific charge capacitance was aslo achieved in other metal oxides/rGO composites.[54] Moreover, graphene grafted with polymers, such as graphene/polyaniline,[108,109] has also been widely studied as flexible supercapacitors. As an example, in situ electrodeposition of polyaniline (PANI) nanorods on the surface of small 2014, DOI: 10.1002/smll.201303202

reduced graphene oxide (rGO) patterns has been reported (see Figure 11). The resulting micro-supercapacitor possessed electrochemical capacitance as high as 970 F g−1 at a discharge current density of 2.5 A g−1.[110] Recently, novel three-dimensional graphene networks (3DGNs) on Ni foam prepared by CVD with post-treatment have been reported.[111,112] The pure porous 3DGNs were obtained after the removal of Ni foam by acid and the porous structure was preserved. Furthermore, the NiO nanoparticles can be further deposited on the surface of 3DGNs, which largely prevented the aggregation or stacking of active materials during the electrochemical charge-discharge cycling. In addition, the porous 3DGNs offers free channels to allow effective interaction between the electrolyte and the active materials. Due to the aforementioned advantages, the formed NiO/3DGN composite demonstrated good rate capability and superior stability. Its specific capacitances evaluated from the CV curves were as high as 816 and 573 F g−1 at scan rates of 5 and 40 mV s−1, respectively.[113] Further evaluation of rate capability demonstrated that a high specific capacitance of 745 F g−1 was released at a discharge current density of 1.4 A g−1. Here, the reliable electrical contact between the graphene sheets and the current collectors allows the rapid charge transfer from active materials to current collector, while the highly porous 3DGNs with large surface area facilitate the easy accessibility of electrolyte ions to NiO active materials. Besides of the 3D NiO/GN composite, cobalt oxide (Co3O4) nanowires were in situ synthesized on threedimensional (3D) graphene foam grown by chemical vapor deposition, which delivered a high specific capacitance of ∼1100 F g−1 at a current density of 10 A g−1 with excellent cycling stability.[114] To sum up, the electrochemical performance of graphene and functional graphene-based composites, that were used as electrodes and tested under different conditions for supercapacitors, has been compared and summarized in Table 3. Although still far below the theoretical



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Figure 11. a) Fabrication of reduced graphene oxide (rGO)/polyaniline (PANI) microelectrodes. b) Specific capacitance of gold/PANI (triangle), PANI in the rGO/PANI composites (square) and rGO/PANI (circle) based all-solid-state micro-supercapacitors at different charge/discharge current densities. c) Cycle stability of the all-solid-state devices at 2.5 A g−1 of the gold/PANI (triangle) and rGO/PANI (circle) based all-solid-state microsupercapacitors. The high voltage of the test is 1 V. Reproduced with permission.[110] Copyright © 2012, WILEY-VCH Verlag GmbH & Co. KGaA.

capacitance expected from graphene sheets, current efforts of treating graphene with different means, such as doping, chemical modification, microwave activation, polymer modification, etc. have undoubtedly achieved higher practical capacitance of graphene. Hybridizing graphene with other active materials, such as metal oxides, CNTs, and PANI, gave much higher pseudocapacitance. The stability of asobtained supercapacitor relies on the reversibility of the redox reaction of the active material. Better anchoring of active material and keeping the integration of their structures would be the main challenge for the future development of supercapacitors.

4. Graphene and Graphene-Based Composites for Lithium Sulfur Batteries The study of lithium-sulfur (Li-S) battery started in the 1940s. From then on, numerous efforts have been invested to its commercialization. Theoretically, a high capacity of 1672 mAh g−1 and a specific energy of 2600 Wh kg−1 can be yielded by elemental sulfur (S) upon the complete redox reaction with lithium (Li) (S8+16 Li = 8 Li2S, which lies ∼2.2 V with respect to Li+/Li).[5,115,116] Other advantages of Li-S batteries include its natural abundance, low cost and low toxicity of sulfur. However, significant challenges remain

Table 3. Comparison of electrochemical performance of graphene and graphene-based composites electrodes for supercapacitor devices under different test conditions. Material

Capacitance

Condition

Reference

130 F g−1

Aqueous KOH electrolyte, 1.3 A g−1

[76]

99 F g−1

Organic electrolyte, 1.3 A g−1

Hydrazine reduced GO

264 F g−1

Aqueous KOH electrolyte, 0.1 A g−1

[77]

Exfoliated GO

120 F g−1


[78]

Chemically modified graphene

Exfoliated GO

−1

75 F g

Ionic liquid, 100 mV

s−1

[79]

Solvated graphene films

156.5 F g−1

Aqueous H2SO4 electrolyte, 1080 A g−1

[81]

Microwave activated GO

165 F g−1


[84]

−1

Aqueous KOH electrolyte, 1 A

g−1

[28]

N-doped rGO

282 F g

Poly modified graphene

185 F g−1

Ionic liquid, 8 A g−1

[89]

Vertically oriented graphene

130 F g−1

Aqueous KOH electrolyte, 600 A g−1

[92]

2.8 mF cm−2

KCl electrolyte, 50 V s−1

[95]

MnO2/rGO

315 F g−1

Aqueous Na2SO4 electrolyte, 2 mV s−1

[107]

PANI/graphene

970 F g−1

All-solid-state, 2.5 A g−1

[110]

Aqeous KOH electrolyte, 10 A g−1

[114]

Graphene/CNTs

3D NiO/graphene foam


−1

1100 F g


small 2014, DOI: 10.1002/smll.201303202


that hinders the commercialization of this technology. One major problem is the inherent low electrical conductivity of S (5 × 10−30 S·cm−1), which results in limited active material utilization efficiency and rate capability.[117] Another issue is the polysulfide anions formed as reaction intermediates in the charge-discharge process are highly soluble.[118–120] During cycling, they can migrate through the separator to the Li anode and be reduced to solid precipitates (Li2S2 and/or Li2S), causing the loss of active mass. Additionally, the severe volumetric change (∼80%) of S during charge and discharge process will gradually decrease the mechanical integrity and stability of the electrode over cycling. So far, the possible solution to the above issues is to encapsulate S in a carbon matrix, which is supposed to enhance the electrical conductivity of S cathode, trap soluble Li2Sn intermediates, and accommodate volume variation of electrode during cycling. Therefore, nanostructures based on different carbon materials, such as ordered mesoporous carbon, hollow carbon spheres, multi-walled carbon nanotubes and so forth, have been widely investigated.[115,121–124] Among these carbonaceous materials, application of graphene in Li-S battery is very promising due to its unique 2D structure, high conductivity and superior mechanical flexibility. Besides, the surface functional groups of graphene can be tuned flexibly to immobilize S/Li2Sx on the graphene surface during the cycling process. For the graphene-based electrode materials, rational design of the microstructure is of critical importance to realize its full advantage for Li-S battery. One way to deposit sulfur onto graphene is taking advantage of the strong interaction between sulfur and carbon and directly impregnating aggregated graphene sheets with melted sulfur. This method was proved to be only effective in increasing the electrical conductivity but not in constraining the polysulfides intermediates as they can still diffuse out from the irregular pores formed by the aggregated graphene and undermine the stability of the cell accordingly.[120] In contrast to this direct melting method, micro-sized sulfur nanoparticles enveloped by the rGO was synthesized by a scalable solution-based oxidation process,[125] which achieved a high loading amount of sulfur (87%) embedded into the rGO matrix. Polysulfides were proved to be trapped effectively by the highly conductive network through strong hydrophilic-hydrophilic interactions. However, insulating domains of Li2S may be formed on the microsized particles due to the incomplete oxidation during charge process, which resulted in poor cycling stability and drastic capacity fading within 50 cycles. Chemical modification over the surface structure is another important strategy that has been developed to enhance interaction between graphene and sulfur/polysulfide species and achieve optimized performance for Li-S battery. As reported by Zhang's group, nano-S was deposited onto graphene oxide (GO) sheets by chemical reaction in a microemulsion system.[126] Epoxy and hydroxyl groups on the modified GO surface were proved to be efficient in immobilizing the S to the C-C bonds and preventing polysulfides species from dissolving. As a result, the cell delivered a reversible capacity of 954 mAh g−1 (with a high coulombic efficiency of about 96.7%) for 50 cycles with negligible capacity fading, small 2014, DOI: 10.1002/smll.201303202

indicating very stable reversibility of the electrochemical reactions and excellent capacity retention. In a recent work, dense nanopores (mean pore size around 3.8 nm) were created on the surface of the graphene sheets as “microreactor” to constrain the sulfur and polysulfide species through modified chemical activation of hydrothermally reduced graphene oxide (rGO), which also resulted in improved cycling stability.[127] Besides, it has been demonstrated by Nazar’s group that polymer modification of the carbon surface can provide a chemical gradient that retards diffusion of these large anions out of the electrode, thus facilitating more complete reaction.[115] Inspired from this work, a series of hybrid graphene-polymer-sulfur composites have been fabricated.[128] For example, as reported by Dai’s group, in which sulfur nanoparticles were coated with poly (ethylene glycol) (PEG) first and then wrapped by the carbon black decorated graphene oxide sheets. In such architecture, the graphene coating layers and the PEG “cushions” acted cooperatively in minimizing the dissolution and diffusion of polysulfides, accommodating the volume expansion and finally leading to a stable capacity up to ∼600 mAh g−1 at a rate of 0.2 C (1 C equals to 1672 mA g−1) over 100 cycles.[129] Furthermore, two stable interfaces have been designed based on rGO/S composites in a recent work and the tailored cathode system exhibited an outstandingly long cycle life.[130] As illustrated in Figure 12, polydopamine (PD) molecules were first coated on the rGO/S composites as a soft buffer to accommodate the volume expansion and avoid leakage of polysulfide during cycling, and then a cross-link reaction was built between PD buffer and poly (acrylic acid) (PAA) binder to integrate individual rGO/S composites into a whole system. In this way, two stable interfaces between rGO/S and buffer layer and between buffer layer and binder were built. Benefiting from this unique interface feature, a specific capacity of 530 mAh g−1 was well retained after 800 cycles at a high current density of 1 A g−1 when testing this system as cathode material for Li-S battery. Besides, in the respect of long cycle stability, another work worth mentioning is a multilayer and coaxial graphenesulfur-carbon nanofibers (G-S-CNFs) composite, which survived with a discharge capacity of ∼273 mAh g−1 after 1500 charge-discharge cycling at 1 C (1 C equals to 1675 mA g−1) with an extremely low decay rate (0.043% per cycle after 1500 cycles).[131] However, the loading amount of sulfur was also compromised (∼33%) in order to achieve the multilayered hybrid structure, which may not be favorable to achieve an overall high energy density for the nanocomposite. Recently, translating 2D graphene sheets into welldefined 3D macroscopic architecture has become popular since it can provide large surface area, high mechanical strength, and fast mass and electron transportation through the combination of the porous structure and intrinsic advantages of graphene.[132] For example, a 3D graphene/singlewalled carbon nanotube (SWCNT) hybrid was synthesized by the catalytic growth on layered double hydroxide at a high temperature.[133] A high electrical conductive pathway was constructed and the internal spaces between the stacked graphene layers and SWCNTs offered room for sulfur storage, which resulted in a capacity as high as 650 mAh g−1 after 100 cycles even at a high current rate of 5 C (1 C equals to



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Figure 12. a) Schematic of the Formation of polydopamine (PD)-coated reduced graphene/sulfur (rGO/S) Composite; b) Discharge capacity retention of cross-linked PD-coated rGO/S composite electrode cycled at 0.5 A g–1 and the first two cycles and the third cycle discharged at the current density of 0.1 A g–1 and 0.2 A g–1, respectively.[130] Copyright © 2013 American Chemical Society.

1675 mA g−1). Similarly, multiwalled carbon nanotube/sulfur (MWCNT @ S) composte with core-shell structure was successfully embedded into the interlay galleries of graphene sheets (GS) through a facile two-step assembly process.[134] Furthermore, the 3D hierarchical sandwich-type architecture showed a high initial capacity of 1396 mAh g−1 at 0.2 C (1 C equals to 1675 mA g−1) with efficient usage of sulfur active material (83%). Recently, graphene-sulfur(G-S) hybrid materials with sulfur nanocrystals anchored on interconnected fibrous graphene were reported by Cheng’s group.[135] With the aid of a freeze-drying process, the assembled structure could be further cut and pressed into pellets to be directly used as lithium-sulfur battery cathodes without using a metal current collector, binder, and additional conductive additive as shown in Figure 13. The hybrid showed a high capacity and an excellent high-rate performance, which may be attributed to the porous network and sulfur nanocrystals that enable rapid ion and Li+ transport, the highly conductive electron pathway provided by interconnected fibrous graphene, and strong binding between polysulfides and oxygen-containing groups.


5. Graphene and Graphene-Based Composites for Lithium Air/Oxygen Batteries As potential next-generation energy storage devices, the lithium air/oxygen (Li-O2) batteries abandon the intercalation electrodes in traditional Li-ion batteries. The Li ions react directly with the oxygen in a porous electrode.[5,136,137] Such unique battery chemistry and electrode architecture provide a greatly increased theoretical specific energy (∼3500 Wh kg−1), holding significant potential to meet the targets set for batteries in automotive applications (∼1700 Wh kg−1). The fundamental electrochemical reactions for aqueous and nonaqueous Li-air battery are illustrated as Equation (1)–(4).[137]Anode: Li ↔ Li+ + e −

(1)

Cathode: Alkaline O2 + 2 H2 O + 4e− ↔ 4OH− × (E0 = 3.43V vs Li /Li+ )


(2)

small 2014, DOI: 10.1002/smll.201303202


Figure 13. a) Illustration of the formation process of the graphene-Sulfur hybrid and schematic of fabrication of a self-supporting electrode. b) Capacity at different current densities of the graphene-with 63% sulfur (G-S63) cathode. c) Cyclic performance and Coulombic efficiency of the G-S63 cathode at 0.75 A g−1 for 100 cycles after the high current density test. Reproduced with permission.[135] Copyright © 2013 American Chemical Society.

Acid O2 + 4e− + 4H+ ↔ 2H2 O(E0 = 4.26 V vs Li/Li+ )

(3)

Nonaqueous 2 Li+ + 2 e− + O2 ↔ Li2 O2 × (E0 = 2.96V vs Li /Li+ )

(4)

Compared to the aqueous Li-air batteries, nonaqueous Li-air batteries exhibit higher theoretical energy density and alleviated parasitic corrosion of Li metal at the anode and therefore have attracted much more interests than aqueous Li-air batteries. As illustrated in Equation (4), the desired electrochemical reaction product for a rechargeable Li-air battery should be Li2O2, which is the only desired product to enable the rechargeability of the Li-oxygen battery. However, the intermediates during oxygen reduction, e.g., O2−, O22−, LiO2/LiO2− are rather reactive and can easily decompose most organic electrolytes.[138–140] Therefore, normally the discharge products of a Li-air battery are mainly composed of various side products instead of Li2O2. For example, Li2CO3, lithium alkylcarbonates, and LiOH are the main products when using carbonates as electrolytes.[141,142] Besides, the sluggish kinetics of oxygen reduction reaction (ORR) (during discharge) and oxygen evolution reaction (OER) (during charging) in Li+-containing aprotic electrolytes is another critical challenge that limits the practical use of this technology.[143,144] Development of effective electrocatalysts is vital for the development of Li-air battery. This cannot only prohibit the side reactions but also promote the slow kinetics related to the oxygen reactions. Porous carbon materials (i.e., carbon black, active carbon, diamond-like carbon) and carbon supported precious/non-precious metals (i.e., Pt, Au, CuFe),[145–147] metal oxides (i.e., MnO2, Co3O4, small 2014, DOI: 10.1002/smll.201303202

RuO2)[143,148–151] have been widely investigated as electrocatalysts. In particular, two-dimensional graphene has proved to be quite effective in enhancing the cathode performance because of its high chemical stability, superior electrical conductivity and large surface area. Therefore, graphene and its derivatives were extensively investigated as electrodes for Li-O2 battery. In most cases, graphene was employed as a support to achieve a good dispersion of metal/metal oxides and improve charge transfer during the charge/discharge process. For example, ruthenium-based nanomaterials supported on rGO have been tested as catalyst for the oxygen reduction (ORR, Li2O2 formation) and oxygen oxidation (OER, Li2O2 reconversion), in the Li/O2 cells.[143] The authors claimed that, although rGO itself could not boost the Li2O2 formation and oxidation reaction, the hybrids, especially the hydrated ruthenium supported on the rGO (RuO2·0.64 H2O-rGO), had shown a superior catalytic activity. It remarkably reduced the charge potential to ∼3.7 V and delivered a high capacity of 5000 mAh g−1 at a high current density of 500 mA g−1. On the other hand, as graphene prepared by the chemical method has many edge and defect sites, some researchers tried to explore the catalytic effect of the graphene without addition of other active materials (metal/metal oxide nanoparticles), named as metal-free graphene. For example, metal-free graphene nanosheets (GNSs) were examined as air electrodes in a Li-air battery with a hybrid electrolyte at 0.5 mA cm−1. The GNSs showed a high discharge voltage close to that of the 20 wt% Pt/carbon black.[152] In particular, the heattreated GNSs not only provided attractive catalytic activity in reducing oxygen, but also showed a much better cycling stability compared to the non-annealed GNSs. The authors



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Figure 14. Morphologies of the graphene-based air electrode. a, b) SEM images of as-prepared functionalized graphene sheets (FGS) (carbon/ oxygen (C/O) = 14) air electrodes at different magnifications. c, d) Discharged air electrode using FGS with C/O = 14 and C/O = 100, respectively. e, f) Electrochemical performances of Li-air batteries using FGS as the air electrodes. e) The discharge curve of a Li-O2 cell using FGS(C/O = 14) as the air electrode (Pressure O2 = 2 atm). f) The discharge curve of a Li-O2 cell using FGS (C/O = 100) as the air electrode (PressureO2 = 2 atm). Reproduced with permission.[156] Copyright © 2011 American Chemical Society.

attributed the improved performance to the presence of sp3 bonding associated with the edge and defect sites and the removal of some adsorbed functional groups during the heattreatment process. Interestingly, as shown in another work reported recently,[153] remarkably improved electrochemical performance for Li-O2 battery was achieved by using hightemperature annealed 3D graphene foams as electrode. The improvement was claimed to be due to efficient removal of structural defects by the high-temperature treatment. However, such heat-treated 3D graphene foams didn't show any catalytic effect towards either ORR or OER in the system. Thus, more studies are suggested to be carried out to verify many hypothesis related to graphene-based oxygen cathodes. Furthermore, it has been reported that graphene doped with hetero atoms such as nitrogen (N) and sulfur (S) exhibited superior catalytic activity for ORR.[154,155] For example, N-doped graphene-rich composites were synthesized via the graphitization of a heteroatom polymer, polyaniline (PANI).[155] Compared to the metal-free graphene catalysts, a higher level of quaternary and pyridinic N was detected in the nanocomposites, which was in concomitant with their much optimized catalytic activity for ORR. Besides of its function as electrocatalyst, graphene has been considered for tailored design of the microstructure for a Li-O2 battery. At first, porous carbon, which can provide large surface area and enough active sites, was considered as an ideal component to construct well-performed Li-O2 battery. However, further study revealed that the discharge capacity for a Li-oxygen battery is closely related to the volume of mesopores with pore size in the range of 2–50 nm.[157] Small sized pore could lead to blockage of the pore channels by extensive deposition of Li2O2.[7,158] Large sized pores might be flooded with the electrolytes, which formed two-phase region instead of three-phase ones. This is not favorable for the formation of Li2O2. By employing hierarchically porous graphene as electrode, Xiao et al. has


built a dual pore system, in which numerous large tunnels facilitate continuous oxygen flow into the air electrode while other small “pores” provide ideal triphase regions for the oxygen reduction (Figure 14). Such 3D air electrode achieved an extremely high capacity of around 15000 mAh g−1.[156] As demonstrated by the above works, employment of graphene and its derivatives (i.e., annealed graphene, doped graphene) was somehow efficient in improving the cathode performance of Li-O2 battery. However, whether the improvement was resulted from catalytic effect of graphene itself or other features of graphene (i.e., high surface area, high electrical conductivity, well-defined porosity) which help to maximize the effect of catalyst particles is still open to question. For the current Li-O2 battery system, the presence of active sites, the pore size/struture of support, and the electrical conductivity of the reaction products may all exert certain effects on the battery performance. However, it is still difficult to determine the most limiting factor. Sometimes differences in the test conditions or the air electrode structures from different research groups could even lead to different conclusions in their Li-air batteries.[122] After all, Li-O2 battery is still in the early stage of development, many fundamental issues remain to be classified.

6. Conclusions and Outlook The past decades have witnessed the rapid development of numerous technologies in the field of energy conversion and storage. In particular, incorporation of graphene and its derivatives into the traditional active materials have brought about many remarkable advances in the frontier energy storage systems, that is, Li-ion battery, supercapacitor, Li-S battery and Li-O2 battery. For the future design and optimization of supercapacitors, 3D structures based on the self-assembly behavior of 2D graphene sheets and their further hybridization with active materials (e.g., metal oxide,


small 2014, DOI: 10.1002/smll.201303202


metal sulfides) may be very promising. As such structure is potential in achiving tunable pore size and structures, large surface areas, fast mass and electron transportation. However, for the design of the future 3D structure, the restacking behavior of the graphene sheets still need to be avoided. And what's more, the formation mechanism of these 3D structures is also necessary to be elucidated in the future investigation. In practical application, large scale, cost-effective, and simple fabrication techniques for the production of graphene with little compromise in its intrinsic advantages is always favored. The micro-fabrication techniques which keep up with the miniaturization trend in current portable electronics market offered more opportunities for graphenebased materials, and as stated above, micro-scale graphenebased supercapacitors are ready to be commercialized in flexible electronics like roll-up displays and TVs, e-paper, and even wearable electronics. For the battery development, conventional energy density of LIB can reach 200–250 Wh kg−1. Signicant increase of this value may still be challenging in the near future. Emerging of Li-O2 and Li-S battery brought about remarkable leap in the energy density and impressive reduction in the cost of batteries. For the Li-O2 battery, the theoretical specific energy density is reported to be 5200 Wh kg−1, while when it comes to reality, only a value of 500–900 Wh kg−1 can be expected so far. For the Li-S battery, a value of ∼600 Wh kg−1 is expected from its theoretical specific energy density 2567 Wh kg−1. Further breakthrough in practical energy density of these two types of battery may lie in the clarification of certain fundamental issues, such as the charge storage mechanism, the effectiveness of catalyst, the structure-property relationship and etc. With regard to this, the well-defined 2D structure of graphene may help to provide some clues. For example, an efficient interface design between graphene and sulfur or Li2S species seems to be very vital to achieve better performance. Previous results have demonstrated that size of sulfur materials on the graphene surface played an important role in determining the electrochemical performances. It would be appealing if we can further tune the morphology of the sulfur materials on the surface of graphene. Construction of different interfaces based on different crystal planes between the nano-sized sulfur material and graphene sheets may bring about more attractive properties, and provide some fundamental understanding in Li-S battery. At the meantime, theoretical calculations togetehr with the experimental data would also be very helpful for better understanding of the mechanism of the complicated and confusing Li-S and Li-O2 systems.

AcRF Tier 1 (RG 61/12, RGT18/13) and Start-Up Grant (M4080865.070.706022) in Singapore. This research is also funded by the Singapore National Research Foundation and the publication is supported under the Campus for Research Excellence And Technological Enterprise (CREATE) programme (Nanomaterials for Energy and Water Management).

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

[27]

Acknowledgements

[28]

Dr. Jixin Zhu, Dan Yang contributed equally to this work. This work was financially supported Singapore A*STAR SERC grant 1021700144 and Singapore MPA 23/04.15.03 grant, and Singapore National Research Foundation under CREATE program: EMobility in Megacities. This work was also supported by MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2–1–034), small 2014, DOI: 10.1002/smll.201303202

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Received: October 8, 2013 Revised: November 10, 2013 Published online:


19

Nanoarchitectured graphene-based supercapacitors for next-generation energy-storage applications.

Three dimensional graphene based materials: Synthesis and applications from energy storage and conversion to electrochemical sensor and environmental remediation.

2D materials. Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage.

Graphene-Based Materials for Stem Cell Applications.

Copper ferrites@reduced graphene oxide anode materials for advanced lithium storage applications.

Green preparation of reduced graphene oxide for sensing and energy storage applications.

Materials and structures for stretchable energy storage and conversion devices.

Materials for sustainable energy production, storage, and conversion.

Multidimensional materials and device architectures for future hybrid energy storage.

Ionic liquid-induced three-dimensional macroassembly of graphene and its applications in electrochemical energy storage.

Graphene Hybrid Materials in Gas Sensing Applications.

Bioinspired nanoscale materials for biomedical and energy applications.

Integrated smart electrochromic windows for energy saving and storage applications.

Energy storage materials synthesized from ionic liquids.

Materials science: Energy storage wrapped up.

3D Printed Graphene Based Energy Storage Devices.

Composite materials for thermal energy storage: enhancing performance through microstructures.

Computational chemistry for graphene-based energy applications: progress and challenges.

The role of graphene for electrochemical energy storage.

Nanostructured Mo-based electrode materials for electrochemical energy storage.

Development of hybrid materials based on sponge supported reduced graphene oxide and transition metal hydroxides for hybrid energy storage devices.

Multifunctional Carbon Nanostructures for Advanced Energy Storage Applications.

Solution synthesis of metal oxides for electrochemical energy storage applications.

oxide hexagonal ring-graphene hybrids through chemical etching of metal hydroxide platelets by graphene oxide: energy storage applications.