《材料科学》课程教学资源（参考文献）The role of graphene for electrochemical energy storage.pdf_大学文库

NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 1 Graphene, a carbon monolayer packed into a 2D honeycomb lattice, was for a long time considered to be merely a building block for carbonaceous materials of other dimensionalities (that is, graphite, fullerenes and carbon nanotubes)1 . Initially labelled as an ‘academic material’, graphene was thought not to exist in a free state until 2004, when Novoselov and co-workers isolated a single-atom-thick layer of carbon2 . Since then, interest in graphene has grown continuously, giving rise to what might be called the ‘graphene gold rush’1 . Recently, intense research efforts — motivated by graphene’s many appealing properties — have been boosted by multimillion-dollar funding from both the European Union and China3 . Despite its wide range of potential applications4 and very promising array of features5 with respect to other structurally different forms of carbon (Table 1)5,6, it is not yet clear whether graphene has the potential to revolutionize many aspects of our lives. In recent years, a large number of publications have discussed the application of graphene in electrochemical energy-storage devices (EESDs). However, although such discussions always highlight the advantages of graphene, they often lack an objective analysis of its limitations and drawbacks. This leaves us with a number of key questions. Will the employment of graphene be limited to niche applications, or will next-generation batteries and capacitors be graphene-based? Graphene’s properties vary strongly as a function of its production method. Hence, which typologies of graphene can be produced with today’s available technologies? Could these significantly outperform state-of-the-art materials? Furthermore, which performance metrics are more relevant for predicting the potential use of graphene in EESDs? This Progress Article aims to address these open questions. Properties and production methods Graphene — a defect-free flat carbon monolayer — is the only basic member of a much larger family of 2D carbon forms. As carefully reviewed in a Carbon Editorial7 , this ‘graphene family’ includes materials with very different properties in terms of morphology, lateral dimensions, number of layers and defects (Tables 2 and 3)1,7,8. Among these characteristics, the presence of defects is the factor that primarily affects the quality of the end material8 and, consequently, its electrochemical features. The methods adopted for graphene production5,6,9, the most common shown in Fig. 1, play a crucial role in determining the properties of the final product. The role of graphene for electrochemical energy storage Rinaldo Raccichini1,2,3, Alberto Varzi2,3, Stefano Passerini2,3* and Bruno Scrosati2,4* Since its first isolation in 2004, graphene has become one of the hottest topics in the field of materials science, and its highly appealing properties have led to a plethora of scientific papers. Among the many affected areas of materials science, this ‘graphene fever’ has influenced particularly the world of electrochemical energy-storage devices. Despite widespread enthusiasm, it is not yet clear whether graphene could really lead to progress in the field. Here we discuss the most recent applications of graphene — both as an active material and as an inactive component — from lithium-ion batteries and electrochemical capacitors to emerging technologies such as metal–air and magnesium-ion batteries. By critically analysing state-of-the-art technologies, we aim to address the benefits and issues of graphene-based materials, as well as outline the most promising results and applications so far. Owing to limited scalability and high production costs, methods such as mechanical exfoliation2,10, synthesis on SiC5,10 and bottomup synthesis from structurally defined organic precursors9,10 necessarily restrict the use of graphene to fundamental research and niche applications, such as touch screens and high-frequency transistors. Similarly, chemical vapour deposition of hydrocarbons5 , although a well-established technique in industry, seems generally unsuitable for mass-production of graphene for electrochemical energy storage because of its high cost, moderate product purity and rather low yield10. Nevertheless, chemical vapour deposition has been reported as an efficient method for producing vertically oriented graphene nanosheet electrodes11, although the packing density of the as-obtained graphene is very low12. Beyond the aforementioned techniques, two methods are widely employed for the bulk production of graphene: liquid-phase exfoliation, and reduction of graphene oxide. In liquid-phase exfoliation, pristine or expanded graphite particles, obtained by thermal expansion of graphite intercalation compounds (usually known as ‘expandable graphite’), are first dispersed in a solvent to reduce the strength of the van der Waals attraction between the graphene layers. Afterwards, an external driving force such as ultrasonication13, electric field14 or shearing15 is used to induce the exfoliation of graphite into highquality graphene sheets5,13. Unfortunately, the low yield of this process leaves a considerable amount of unexfoliated graphite, which must be removed15. Nevertheless, the high scalability and low cost of liquid-phase exfoliation13 make it suitable for producing graphene in bulk quantities16. In the second method, graphene oxide (GO), a highly defective form of graphene with a disrupted sp2 -bonding network, is produced by strong oxidation of pristine graphite17,18 followed by stirring or ultrasonication in liquid media19. Graphene oxide must be reduced in order to restore the π network, which is the characteristic of conductive graphene20. Chemical, thermal and electrochemical processes are commonly employed in this order to produce reduced graphene oxide (RGO)10,20,21. Despite the low-tomedium quality of the obtained material due to the presence of both intrinsic defects (edges and deformations) and extrinsic defects (Oand H-containing groups), these methods allow the production of bulk quantities with high yield and contained costs. Although liquid-phase exfoliation and reduction of GO are the primary methods for producing commercially available graphene for EESDs, other 1 Institute of Physical Chemistry, University of Muenster, Corrensstrasse 28/30, D‑48149 Muenster, Germany. 2 Helmholtz Institute Ulm, Helmholtzstrasse 11, D‑89081 Ulm, Germany. 3 Karlsruhe Institute of Technology, PO Box 3640, D‑76021 Karlsruhe, Germany. 4Istituto Italiano di Tecnologia, Graphene Labs and Nanochemistry Department, Via Morego 30, I‑16163 Genova, Italy. *e-mail: stefano.passerini@kit.edu; bruno.scrosati@gmail.com PROGRESS ARTICLE PUBLISHED ONLINE: 22 DECEMBER 2014 |DOI: 10.1038/NMAT4170 © 2014 Macmillan Publishers Limited. All rights reserved

2 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials techniques are available (such as carbon nanotube unzipping22 or direct arc-discharge23). However, owing to their higher costs, these techniques remain relatively marginal and thus unsuitable for bulk production. In their review, Novoselov et. al.5 perfectly summarized the current state of affairs: “Graphene will be of even greater interest for industrial applications when mass-produced graphene has the same outstanding performance as the best samples obtained in research laboratories.” As a matter of fact, the large-scale production of ‘outstanding performance’ graphene is the most ambitious challenge to address before its widespread application5 . This aspect is particularly relevant with regard to the introduction of graphene in EESDs for powering millions of electric cars in the near future. Over the past few years, many studies have explored graphenebased materials for electrochemical energy storage24. In most of these, graphene was produced from graphite. As shown in Fig. 2, expandable graphite can be thermally expanded and subsequently exfoliated to obtain graphene. Pristine graphite can also be directly exfoliated to give graphene through liquid-phase methods or, alternatively, oxidized to obtain graphite oxide25,26. The latter, after liquidphase exfoliation, yields GO, which is then reduced to form RGO20. This approach is different from other types of application as it is particularly useful for energy-storage materials. In fact, although oxidation introduces defects that cannot be entirely removed during the reduction process20, this synthetic pathway facilitates the preparation of composites. In contrast with graphene (including RGO), GO can be easily dispersed in a wide range of solvents10. This peculiarity enables, through different chemical routes, the functionalization of GO with electroactive materials (such as conductive polymers and metal oxides) to form GO-based composites27. These composites can be used as such, or alternatively can be further reduced to obtain RGO-composites28. Graphene-based materials have been proposed for use in all kinds of EESD, either as an active material or an inactive component. Graphene as an active material Graphene can be considered to be an active material when it takes part in an energy-storage mechanism. This can range from hosting ions (such as Li+ or Na+ in metal-ion batteries) to storing electrostatic charges on the electrode double-layer (as in electrochemical double-layer capacitors, EDLCs), or functioning as a catalyst in metal–air batteries. Lithium-ion batteries. In lithium-ion batteries (LIBs), Li+ ions continuously shuttle between a lithium-releasing cathode (commonly a layered lithium metal oxide) and a lithium-accepting anode (commonly graphite)29. The amount of ions hosted per gram of material determines the capacity — and thus the energy — of the battery. Similar to graphite, graphene can be used as an anode for hosting Li+, both as such and as a carbonaceous matrix in composites with other materials also capable of storing lithium. Graphene as an Li+ host. As originally suggested by Dahn et al. in 1995, an anode comprising single layers of graphene can host two times as many Li+ ions as conventional graphite30,31. The storage of one lithium ion on each side of graphene results in a Li2C6 stoichiometry that provides a specific capacity of 744 mAh g–1 — twice that of graphite (372 mAh g–1)30. This primeval concept of lithium hosting in graphene-like carbons was retrieved following the first isolation of graphene in 20042 . Differently from graphite, in which lithium is intercalated between the stacked layers32, single-layer graphene can theoretically store Li+ ions through an adsorption mechanism, both on its internal surfaces and in the empty nanopores that exist between the randomly arranged single layers (accordingly to the ‘house of cards’ model)30,31. Similarly to other disordered carbons, such a process mainly takes place at low potentials (10 Single-layer monolayer Few-layer multilayer Nano- Micro- -Sheet -Flake -Plate -Platelet -Ribbon Table 3 | Graphene’s structural defect typologies. Intrinsic (removal or introduction of carbon atoms in graphene’s chemical composition) Extrinsic (introduction of non-carbon atoms in graphene’s chemical composition) Vacancies Edges Deformations Hybrid structures O, H and other foreign atoms PROGRESS ARTICLE NATURE MATERIALS DOI: 10.1038/NMAT4170 © 2014 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 3 the amount of lithium stored by graphene-based anodes is more strongly dependent on the production method of both the material and the electrode. In most reported studies, RGO is the material of choice for lithium-ion storage33. During the first Li+ insertion, RGO exhibited incredibly high-capacity values of >2,000 mAh g–1 (ref. 33), which is higher than the theoretical capacity of single-layer graphene. However, this amazing capacity is not fully released after de-insertion due to the massive irreversibility of the first lithiation step33. This phenomenon, also observed for other Li-ion anode materials32,34, can be attributed mainly to the irreversible reduction of the electrolyte to form a surface passivation layer on the active particles; namely, the ‘solid electrolyte interphase’32. As shown in Fig. 3a, the solid electrolyte interphase strongly depends on the specific surface area (SSA) of the active material. Accordingly, the extremely high SSA of graphene, when compared with common graphite (Table 1), results in a very high initial irreversible capacity6 (Fig. 3b). In the following de-insertion cycle, graphene displays a high reversible capacity, although delivered mostly at potentials of 1–3 V versus Li/Li+, which is rather higher than typical graphite values (0–0.4 V versus Li/Li+). This leads to the occurrence of a large voltage hysteresis upon insertion and de-insertion of Li+ (Fig. 3b), which results in poor energy efficiency for cells employing such electrodes. Such a drawback, together with the large cathode quantities needed to supply the initial charge for the irreversible capacity, makes graphene-based cells unfeasible. The voltage hysteresis, also observed in several nanotube-shaped materials35 and high-specific-charge carbons32, is caused, among other reasons, by Li storage on defects such as edges and/or oxygen- and hydrogen-containing surface groups32,36. It is thus advisable to limit the number of such defects in graphene-based anodes, particularly because they are also responsible36 for the low Coulombic efficiency in the first cycle. In addition, the progressive reduction of oxygen-containing groups (for example, in RGO) leads to graphene layer re-stacking, which lowers the storage capacity over repeated cycling33. All of these aspects affect the value of the reversible capacity, which, after a few tens of cycles, is rarely comparable to that of commercially available graphites33. Graphene quality is therefore a crucial issue that must be addressed before the graphite in LIBs can be replaced. Even when graphene is finally available in large quantities at reasonable cost, graphite will probably still be the active material of choice for widespread hard-case batteries, unless we develop effective strategies to prevent initial lithium ion consumption and avoid graphene layer re-stacking. In this regard, pre-lithiation37,38, controlled surface functionalization6 and the use of composites39 might be promising strategies. At the same time, the development of flexible LIBs, which require lightweight and ultrathin active materials, could benefit from the use of graphene. However, even if different studies demonstrate graphene as a promising anode in flexible LIBs, the aforementioned drawbacks still represent major obstacles for practical applications40. Graphene-based composite anodes. Several composites have recently been developed in an effort to overcome the energy-storage limitations and poor cycling behaviour of bare graphene negative electrodes4 . The addition of electroactive materials, such as metal (or metal oxide) nanoparticles, provides reversible alloying (with SnO2 or Si nanoparticles), insertion (with TiO2) or conversion (with Fe2O3 or Co3O4) reactions with lithium, thus allowing considerably higher storage capacities than those of bare graphene or graphite6,41. During the composite preparation, graphene can act as a support for the growth of electroactive nanostructures that, in turn, hinder re-stacking by lowering the van der Waals forces among the layers. As a result, graphene-based composites are less affected by agglomeration during electrode preparation, as well as by capacity fading during cycling6 . Moreover, the extensive and highly conductive carbon matrix established by graphene layers improves the electrical conductivity of the composite and buffers eventual volume changes taking place in electrodes based on alloying or conversion materials during cycling42. Despite these promising properties, however, graphene-based composites suffer, similarly to bare graphene, from the huge irreversible charge consumption of 30–50% during the first charge/discharge cycle6,43. Results achieved so far with graphene composite anodes are very encouraging towards not only the development of high-energy LIBs, but also future applications such as wearable EESDs40. Among the proposed composite graphenebased materials, some of the most promising in terms of reversible capacity are Co3O4/RGO (1,500 mAh g–1)44, silicon nanoparticles/ RGO (1,150 mAh g–1)44, N- and S- co-doped RGO (900 mAh g–1)45 and SnO2/RGO (700 mAh g–1)46. Nevertheless, the optimization of structural arrangement and weight ratio distribution between the composite components are still key issues that must be addressed to achieve good electrochemical performance and extended cycle life6 . Sodium-ion batteries. The development of sodium-ion batteries (SIBs), seen as a cheaper alternative to LIBs, is promoting extensive research to identify a suitable anode active material because, owing to their large ionic radius, Na+ ions do not intercalate into graphite47 (Fig. 3c). In this regard, graphene seems to be a good candidate as an active anode in SIBs. Graphene as an Na+ host. The use of RGO as an anode material in SIBs was first reported in 201348, where it showed promising electrochemical behaviour, good cycle life and excellent rate capability. Such remarkable performance is related to the presence of defects (for example, residual oxygen-containing groups), which increase the graphene interlayer distance (0.37 nm, compared with 0.34 nm in graphite). However, as observed in LIBs, the presence of defects represents a serious drawback in term of Coulombic efficiency for SIBs48. Recently, Ding et al. reported the synthesis of different kinds of few-layer graphene (produced from biomass precursors) and their performance as anode materials in SIBs49. Interestingly, the obtained Mechanical exfoliation Chemical vapour deposition Reduction of graphene oxide Synthesis on SiC 0 = None or N/A 1 = Low 2 = Average 3 = High Liquid-phase exfoliation Bottom-up synthesis P S C G Y P S C G Y P S C G Y P S C G Y P S C G Y P S C G Y 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 Figure 1 | Schematic of the most common graphene production methods. Each method has been evaluated in terms of graphene quality (G), cost aspect (C; a low value corresponds to high cost of production), scalability (S), purity (P) and yield (Y) of the overall production process. NATURE MATERIALS DOI: 10.1038/NMAT4170 PROGRESS ARTICLE © 2014 Macmillan Publishers Limited. All rights reserved

4 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials graphenes exhibited different Na+ ion host mechanisms depending on the synthesis temperature (600–1,400 °C). Lower temperatures yielded average-quality graphenes with an Na+ storage capacity similar to that of RGO. In contrast, higher temperatures enabled the formation of better quality graphene, with an interlayer spacing of 0.38 nm and promising insertion performance. Indeed, this report discloses one of the best-performing graphene-like materials for SIB anodes49, showing up to 300 mAh g–1 specific capacity and good retention over 200 cycles, even though the Coulombic efficiency for the first cycle remains poor. Such results give hope for the successful employment of graphene in SIBs, insofar as it can compete with other recently developed anode materials50. Additionally, the lower insertion potential of graphene-based anodes makes it more advantageous in terms of specific energy49,50. Similarly to LIBs, graphene-based composites enable SIBs with higher specific capacity, better rate capability and longer cycle life than bare graphene51–55. Electrochemical capacitors. Electrochemical capacitors (also called supercapacitors) exploit fast charge-storage mechanisms to enable considerably higher power densities than those available in LIBs or SIBs. Electrochemical capacitors can be subdivided into two classes: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, the energy is physically stored through the adsorption of ions on the surface of the electrodes, whereas in pseudocapacitors, electrochemical energy storage is enabled by fast redox reactions occurring between the electrode active material and the electrolyte56. Electrochemical double-layer capacitors. In EDLCs, the electrode’s active materials are electrochemically stable, do not undergo any Faradaic processes and, above all, possess large SSAs56. The amount of charge stored per unit mass (F g–1), volume (F cm–3) or area (F cm–2) is indeed directly proportional to the surface available for the formation of the double layer (that is, the area in contact with the electrolyte)57. In principle, graphene, with its theoretical SSA of 2,675 m2 g–1 (ref. 8) and capacitance of 550 F g–1 (ref. 58), would be a perfect candidate for boosting the energy density of such devices59. However, this does not seem to be the case in practice, as the difficulty of even approaching the theoretical SSA of graphene (for instance, average values for RGO are in the range of 300–1,000 m2 g–1)43 results in a lower practical gravimetric capacitance (100–270 F g–1 and 70–120 F g–1 with aqueous and organic electrolytes, respectively)43,58. Additionally, spontaneous graphene layer re-stacking, which occurs during both electrode manufacturing and cycling, strongly reduces the practical surface available for charge storage (Fig. 3d,e). Different approaches have been introduced to mitigate these detrimental effects. As reported by Ruoff et al., RGO can be chemically activated to create an extended 3D meso- and microporous network (with an SSA of up to 3,100 m2 g–1) of highly curved graphene walls that prevent re-stacking during cycling. Such ‘activated graphene’ enables high gravimetric capacitances with both organic (166 F g–1) and ionic liquid electrolytes (200 F g–1)60 and, moreover, operates across a wide temperature range of –50 °C to 80 °C61. Alternatively, graphene layer re-stacking can be minimized by optimizing the electrode-manufacturing process. In this regard, RGO sheets could be vertically aligned with respect to the current collector plane, thus granting better ion accessibility and enabling higher packing density. Moreover, high and reversible volumetric (171 F cm–3) and areal (1.83 F cm–2) capacitances in aqueous electrolyte could be obtained12. In summary, although activated graphene and vertically aligned RGO show promising performance, the large majority of graphenelike materials cannot yet compete with the cheaper and well-established activated carbons62. The majority of the results reported for graphene-based supercapacitors were obtained with very low density electrode materials (for example, aerogels and foams), which possess a large number of void spaces (macropores). These pores are filled by the electrolyte, thus increasing both the weight and volume of the final device to a point where they are unsuitable for use in EDLCs58,63. In contrast, graphene would probably fit in the approaching era of small-scale supercapacitors required to power the next generation of wearable- and micro-electronic devices64. Pseudocapacitors. In pseudocapacitors, the presence in the active material of electroactive species such as oxygen-containing functional groups, conducting polymers or transition metal oxides enables higher energy densities with respect to EDLCs58,59. Nevertheless, pseudocapacitors are inferior to EDLCs in terms of power density (limited by the poor electrical conductivity of the active materials) Graphite Expandable Direct exfoliation Liquid phase (ultrasonication, electrochemical, shear mixing) Liquid phase (stirring, ultrasonication) Gr Exfoliation aphite oxide GO GO composite O Exfoliation Liquid phase (ultrasonication) Expansion Thermal Standard or modified Hummers method Oxidation Graphene-based materials for electrochemical energy-storage devices Graphene or RGO RGO composite Functionalization with electroactive materials Pristine Chemical, thermal, electrochemical Reduction OH O OH Figure 2 | The most common synthetic pathways for producing graphene-based materials (GO, RGO, GO- and RGO-based compounds) for use as electrode active materials in EESDs. PROGRESS ARTICLE NATURE MATERIALS DOI: 10.1038/NMAT4170 © 2014 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 5 and cycle life62. In this regard, graphene-based electrodes could be viable candidates for improving the performance of pseudocapacitors62. Despite its lower electrical conductivity, GO, owing to its large number of oxygen-containing groups, has a higher pseudocapacitance than RGO65. However, as previously discussed, these groups may negatively affect the electrochemical behaviour of the electrode by reducing the cycling stability and reversibility62,65 (Fig. 3e). Various graphene-conducting polymer and graphene– metal-oxide composites have also been developed and investigated for use as pseudocapacitors6,62. In these composites, graphene provides a support matrix for the growth of the electroactive species at the nanoscale, which results in a larger SSA and thus enhances the electrochemical performance by increasing electrical conductivity and mechanical stability62,66. It seems the key to exploiting the full potential of graphene in pseudocapacitors relies on the development of composite materials that offer the synergistic effect of the graphene substrate and the electroactive component, along with an optimized spatial orientation of the graphene sheets12,39,66. Lithium–air batteries. The growing demand for energy has led to the development of new EESDs with higher energy densities than metal-ion batteries. In this regard, the lithium–air battery (LAB), which offers a theoretical energy density of 5,200 Wh kg–1 (ref. 67), represents one of the best candidates. Although lithium–air chemistry was introduced in 1976, the rechargeability of this system was brought to the attention of the scientific community only in 2006 by Bruce and colleagues68. Although different LABs may employ different typologies of electrolyte, they are generally composed of metallic lithium and oxygen (or air) as, respectively, the anode and cathode. The rechargeability of the system relies on the conversion of reduction products (LiO2 and, mainly, Li2O2) formed during discharge (oxygen-reduction reaction), back to the original reagents during charge (oxygen-evolution reaction)39. Unfortunately, the entire system suffers from a low energy efficiency, short lifetime and low rate capability (discharge capacity of 400 mAh g–1 after 50 cycles at a specific current of 100 mA g–1)68–70. Reports indicate a maximum of only 100 capacity-limited (1,000 mAh g–1) cycles71. Among the various factors that influence the performance of LABs, the morphology of the air electrode (cathode) is particularly important for obtaining high discharge capacity. In fact, the SSA and porosity of the air electrode determine the morphology and amount of discharge products. It was demonstrated that RGO, with its large SSA, could deliver higher capacities than other carbon substrates (for example, 8,700 mAh g–1 with respect to 1,000–2,000 mAh g–1 in the first cycle). Defects and functional groups can also play a catalytic role for the formation of discharge products69 (Fig. 3f). So far, the use of RGO as a bare material or substrate for other catalyst72 in LAB cathodes has improved performance, although achieving the theoretical energy density is still far away. Different aspects are still unclear and further studies are needed to demonstrate an effective role of graphene in LABs. Further investigations of graphene with stable electrolytes are needed before we can assess its effective role in such batteries69. Sodium–air batteries. Over the past five years, sodium–air batteries (SABs), despite having an energy density half that of LABs, Figure 3 | Features and limitations of graphene as an active material in different EESDs. a, Graphite and graphene in LIB anodes. Correlation of characteristics in terms of defect amount, SSA and ratio between reversible (Crev) and irreversible (Cirr) capacity during the first charge/discharge cycle. b, Typical voltage profiles of graphite and graphene (RGO) during constant current Li+ insertion/de-insertion. c, Li+ and Na+ insertion mechanisms in graphene and graphite. d, Layers re-stacking in graphene during electrode manufacturing and electrochemical cycling. Re-stacking is a serious issue that affects the performances of all graphene-based EESDs. e, Generic voltammetric behaviour of graphene-based electrochemical capacitors over prolonged cycling. Top: Effect of graphene layers re-stacking (such as in RGO) on the double-layer capacitance. Bottom: Effect of surface group degradation (such as in GO) on pseudocapacitance. f, Catalytic effect of graphene defects (vacancies, deformations and presence of surface groups) in metal–air batteries. Capacitance Voltage Graphene Graphite Ionic radius Electrode manufacturing Electrochemical cycling Surface group degradation (for example in GO) Decrease of pseudocapacitance Hysteresis Cirr 0 1,000 1 2 3 2,000 a d f b c e Graphite Graphene Graphite Graphene Layers re-stacking (for example in RGO) Decrease of double-layer capacitance Specific gravimetric capacity (mAh g–1) Defects M = Li, Na Interlayer spacing Potential versus Li/Li+ (V) SSA Crev /Cirr 2M+ + O2 + 2e– M2O2 Li+ Na+ Na+ Li+ NATURE MATERIALS DOI: 10.1038/NMAT4170 PROGRESS ARTICLE © 2014 Macmillan Publishers Limited. All rights reserved

6 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials have been increasingly proposed owing to their low production cost and the availability of the required raw materials73. In contrast with lithium, sodium is capable of reversibly forming during discharge a stable superoxide (NaO2) with low overpotentials73. This enables SABs to cycle with a charge efficiency of 80–90% after the first cycle. The formation of peroxide (Na2O2), however, is kinetically hindered as it requires a suitable catalyst. RGO has demonstrated, under dry air conditions, remarkable catalytic properties towards the formation of Na2O2 (ref. 74), which are not exhibited by conventional carbon. As reported by Liu et al.74, the micro- and nanostructures of the graphene air electrode enable one of the highest specific discharge capacities for SABs. These results suggest that RGO can efficiently function as a catalyst for both oxygen-reduction and oxygen-evolution reactions. Nitrogen-doped RGO nanosheets have also been investigated in this respect. The defective sites introduced by nitrogen doping enable a more uniform and smaller size distribution of the discharge products and, therefore, a higher specific discharge capacity with respect to the undoped graphene75. Even though this technology is in the very early stages of development, the few reports available in this field depict a quite promising scenario for graphene in SABs. Graphene as an inactive component Graphene could play an important role in EESDs, even without being actively involved in the electrochemical reaction. Owing to its impressive electrical conductivity (Table 1), graphene was proposed as a conductive agent in metal-ion battery electrodes as well as an encapsulating carbon matrix in, for example, lithium–sulphur batteries. Besides enabling efficient electron transport, its superior thermal conductivity (Table 1) may be advantageous for dissipating the heat generated in the case of high current loads or/and abuse conditions5 . This would result in devices with improved intrinsic safety. The variety of structures reported can be classified into six different models (Fig. 4a)6 . Lithium-ion batteries. LiCoO2, LiMn2O4 and LiFePO4 (hereafter referred to as LMO) are some of the most commonly used cathode materials in LIBs. The cycle life and rate capability of these materials are generally limited by their poor electrical conductivity. Introducing low-cost conductive additives (for example, carbon black) into the composite electrodes commonly solves this issue. Nevertheless, owing to their amorphous structure, carbon blacks possess a rather low electrical conductivity, with respect to more crystalline carbons such as graphene5,76. Recently, Kucinskis et al.76 reviewed state-of-the-art graphenebased composite cathode materials. Among the vast number of reports, most employ GO as a source for the formation of graphene conductive networks. Additionally, in a large part of these works, GO is reduced to RGO simultaneously with the LMO precursors (one-pot synthesis) to produce graphene-based composites. This approach, which is different from simply mixing the carbon additive with the LMO active material during electrode preparation, promotes the formation of small-size LMO particles (leading to improved Li+ diffusion) directly onto the RGO matrix76. Moreover, the RGO 3D network is reportedly capable of preventing the dissolution of some LMOs39, thus extending the cycle life of the batteries. However, it was also suggested that when RGO is mixed in a manner similar to conventional carbon additives during electrode preparation, it could negatively affect the Li+ mobility, thus worsening the electrochemical performance of the composite cathode76 (Fig. 4b). Regardless of this fact, RGO is generally reported to enhance the rate capability of the cathode with respect to conventional carbon additives. Depending on the active material, improvements of up to 160% of the discharge capacity (at the same current rate) have been observed76. Nevertheless, it is not yet clear whether RGO may replace carbon blacks, which are much cheaper and easier to handle76. Sodium-ion batteries. As explained previously for the negative electrode, the larger size of Na+ with respect to Li+ restricts the choice of available cathode material for SIBs47. Several layered oxides with promising electrochemical performance have recently been developed50. However, like their LIB analogues, they usually possess poor electrical conductivity and thus limited rate capability. So far, only a few reports are available on graphene-containing composite cathodes for SIBs77–80. However, in all cases, the RGO matrix seems to enhance the electrical conductivity of the composite, thus improving the rate capability compared with bare cathode materials77–80. Lithium–sulphur batteries. Lithium–sulphur batteries (LSBs), through the redox reaction of metallic lithium (anode) and elemental sulphur (cathode), could provide a remarkably high theoretical specific energy of up to 2,600 Wh kg−1 (ref. 39). Despite the intrinsic advantages of sulphur in terms of low cost, abundance, low toxicity and high theoretical specific capacity (1,672 mAh g–1), LSBs are affected by several drawbacks: (1) slow kinetics owing to the low electrical conductivity of the redox reaction products; (2) low energy efficiency; (3) poor cycle life as a direct result of the dissolution of the intermediate reaction products (polysulphides) in the electrolyte; and (4) large volume changes during the electrochemical reaction69. Graphene has been proposed as a good candidate to address these issues because of its high electrical conductivity and capability of trapping the charge/discharge products39. Several studies have reported that RGO and GO are suitable substrates for the deposition of sulphur micro- and nanoparticles39,69. Good encapsulation seems to be achieved in both cases; however, the presence of epoxy and hydroxyl groups in GO promotes the immobilization of sulphur and thus prevents its dissolution39. Promising performance — with specific capacities >1,000 mAh g–1 — has also been obtained with hybrid graphene–polymer–sulphur composites, although an acceptable capacity retention during cycling has not yet been achieved41. Recently, different graphene–sulphur composites have been synthesized and tested in LSBs81–83. They exhibit a good performance in terms of capacity, Coulombic efficiency and stability during cycling81–84, even if the values reported are not higher than those obtained with other carbon-encapsulated sulphur cathodes, such as ordered mesoporous carbon84. From these results, graphene might be a possible candidate for encapsulating sulphur on LSB cathodes. However, real advances in the field require improvements to the sulphur/graphene interface in order to achieve stable electrochemical performance39. Developing applications of graphene The recent outbreak of graphene in the field of electrochemical energy storage has spurred research into its applications in novel systems such as magnesium-ion batteries (MIBs), which is one of the newest members of the metal-ion battery family. MIBs have been proposed as a high-energy-density and environmentally friendly replacement for LIBs85. Although research in this field is still at an early stage, a few graphene-based composites have recently been proposed as MIB cathode materials86,87. Although the results obtained so far show very poor electrochemical behaviour, they could represent the first attempts to use graphene in MIBs. Interestingly, Wang et al. have already patented a rechargeable magnesium-ion cell based on graphene active materials88. The employment of graphene has also been considered for the improvement of vanadium redox-flow batteries (VRFBs). Patented in 198689, VRFBs enable energy storage using V3+/V2+ and [VO2+]/[VO2 +] redox couples as negative and positive acid electrolytic solutions, respectively90. Carbon-based materials, such as cloths or graphite felts, are used as electrodes because of their electrochemical stability and wide operating potential91. Unfortunately, despite their high SSAs, these electrodes do not exhibit satisfactory electrochemical properties92. Accordingly, the use of graphene-based materials has PROGRESS ARTICLE NATURE MATERIALS DOI: 10.1038/NMAT4170 © 2014 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 7 been proposed to improve electrical conductivity, kinetic reversibility and electrochemical activity of these electrodes91,92. Over the past few years, a small number of studies have investigated the electrochemical properties of GO-based91, RGO90,92 and RGO-based composites93–95 for application in VRFBs. All of these reports show promising electrochemical performance for graphene-supported carbon electrodes, specifically in terms of high peak current density, reduced overpotential and decreased charge-transfer resistance. Additionally, GO96 and commercial graphene97 have recently been tested as additives in VRFB ion-exchange membranes, with the aim of reducing the vanadium permeation and preventing ionic cross-mixing. The results achieved so far seem promising when compared with those obtained with bare membranes. However, the development of a successful commercial VRFB containing graphene is still far away. Conclusions It has been ten years since the beginning of the graphene era, and the rush to find new applications for this exciting material is more vibrant than ever. However, despite the enormous amount of data produced throughout research laboratories across the globe, it is still not clear whether graphene has the potential to revolutionize many aspects of our lives. This is particularly appropriate for the field of electrochemical energy storage, in which ‘graphene fever’ has reached rather high levels due to the continuous need for new materials that can meet the market’s performance requirements. Graphene promises to increase substantially the energy- and power-density of practical systems, as well as enable the development of next-generation devices. However, the results so far tend to suggest that real breakthroughs are still to come. As was the case for many other innovative materials in the past, the main task is to close the gap between laboratory-scale research and practical applications. The first challenge lies in the production of graphene. Owing to its peculiar nature, the electrochemical properties of this material are strictly dependent on its method of production, and so are its chances of finding an application in EESDs. Nowadays, the vast majority of graphene-based materials are produced by the reduction Figure 4 | Structural models and a possible drawback of graphene composites. a, Schematic of the different structures of graphene composite electrode materials. All models (except where specifically indicated) refer to composites in which graphene and the active material are synthesized through onepot processes. Encapsulated: Single active-material particles are encapsulated by graphene, which acts as either an active (for example, LIB anodes) or an inactive (for example, LIB cathodes) component. Mixed: Graphene and active materials are synthesized separately and mixed mechanically during the electrode preparation. In this structure, graphene may serve as an inactive conductive matrix (for example, LIB cathodes) or an active material (for example, LIB anodes). Wrapped: The active-material particles are wrapped by multiple graphene sheets. This structure well-represents pseudocapacitor electrodes, in which graphene is the active material, as well as metal-ion battery cathodes, where graphene is an inactive component. Anchored: This is the most common structure for graphene composites, in which electroactive nanoparticles are anchored to the graphene surface. This structure is very relevant for metal-ion battery anodes and pseudocapacitors, where graphene serves as an active material, as well as for metal-ion battery cathodes and in LSBs, where graphene acts as an inactive component. Sandwich-like model: Graphene is used as a template to generate active material/graphene sandwich structures. This graphene-composite model, although not widespread, is used for LIB cathodes. Layered model: Active-material nanoparticles are alternated with graphene sheets to form a composite layered structure, which has been proposed for use in metal-ion battery anodes and cathodes. b, Li+ paths in carbon black- (left) and graphene- (right) based electrodes in the mixed structural model. The figure highlights a possible drawback of graphene in terms of Li+ mobility. Anchored Encapsulated Sandwich-like Mixed Wrapped Layered Active material Carbon black Graphene Free Li+ path Blocked Li+ path a b NATURE MATERIALS DOI: 10.1038/NMAT4170 PROGRESS ARTICLE © 2014 Macmillan Publishers Limited. All rights reserved

8 NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials of GO to RGO. This method is relatively cheap and has the potential for scalability — mandatory properties for widespread adoption — but introduces both intrinsic and extrinsic defects, which strongly affect electrochemical properties. Although for some applications the defects function as catalyst sites to improve cell kinetics (for example, in lithium–air and sodium–air batteries), in other cases they strongly reduce the performance. For example, RGO cannot compete with the widespread carbonaceous materials commonly employed in commercial LIBs. In fact, despite their very promising initial performance, RGO electrodes show a limited cycle life98 compared with well-established graphite electrodes. With respect to EDLCs, the limited cycle life and low density of RGO-based electrodes prevents their transition to the commercial stage. However, some strategies for improving the packing density of graphenebased materials have been proposed. Nevertheless, the macroporous nature of graphene, in general, seriously affects its volumetric energy density. In this case, the common practice of evaluating EDLC performance through gravimetric data might lead to misleading conclusions. Because low-density and few-micrometre-thick electrodes are often reported, volumetric data are surely more appropriate63. In view of the funding and human resources devoted worldwide to this unique material, we may expect to see a turnover in the not-too-distant future. Some important results support this vision. In fact, a growing body of research into graphene-based full LIBs37,38,99,100 is continuing to prove the benefits of graphene for this important application. In addition, it has been demonstrated that graphene (or RGO) may find its true role when employed in composite electrodes. Here, graphene layers and electroactive particles work symbiotically, with the former providing a stiff and conductive matrix, which can buffer eventual volume changes, and the latter helping to avoid layer re-stacking. Graphene-based composites have, in fact, shown outstanding performance in LIBs44–46, SIBs51–55, pseudocapacitors58,59,62,66 and LSB81–83. Moreover, a few preliminary studies into full SIBs51,55 have confirmed graphene exploitation in the ‘beyond lithium’ battery generation. Nevertheless, the most crucial point is the nano-architecture of the composite. Indeed, if proper nanoscale engineering is achieved, such compounds will surely play a crucial role in the progress of the field. Winning the graphene ‘gold rush’ requires consistent investment and commitment from industry and research-funding institutions. 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Electrochem. Commun. 12, 378–381 (2010). 100. Chae, C., Noh, H.-J., Lee, J. K., Scrosati, B. & Sun, Y.-K. A high-energy Li-ion battery using a silicon-based anode and a nano-structured layered composite cathode. Adv. Func. Mater. 24, 3036–3042 (2014). Acknowledgements R.R., A.V. and S.P. acknowledge the financial support of Bundesministerium für Bildung und Forschung (BMBF) within the project ‘IES, Innovative Elektrochemische Superkondensatoren’ (contract number 03EK3010). B.S. is grateful to the Helmholtz Institute Ulm for a six-month visiting professorship position. Author contributions R.R. and A.V. designed the outline of the Progress Article, wrote the manuscript and conceived the figures and tables. S.P. and B.S. supervised and revised the writing. Additional information Reprints and permissions information is available online at www.nature.com/reprints. Correspondence should be addressed to S.P. or B.S. Competing financial interests The authors declare no competing financial interests. NATURE MATERIALS DOI: 10.1038/NMAT4170 PROGRESS ARTICLE © 2014 Macmillan Publishers Limited. All rights reserved