PROGRESS ARTICLE NATURE MATERIALS DOL:10.1038/NMAT4170 Table1 Grap e prop pared with othe us material Carbon nanotube Fullerene Graphite Dimensions Highest (for .non-elastic Experimental SSA (m2g) 1500 1300 80-90 10-20 Electrical conductivity (S cm) 2.000 Structure-dependent 10 Anisotropic:2-3x1046 Thermal conductivity(WmK)4,840-,300 3,500 0.4 Anisotropic:1,500-2.000',5-10 Table 2|Dimension-based graphene nomenclature. Thickness(n,number of layers) nsion D(nm) ati(nth:width) 10 D=100 100=D=102 >10 Single-ayer Nano Micro- -Ribbon monolaye Table3Graphene's structural defect typologies ntrinsic arbon atomsin graphene's chemical natomsin graphene'schemical composition Hybrid structures an active material when it take ew.No rfectly summ dthe charges on the electr er (as in electroc e w hene has the sam metal-arba Cs),or functioning as a catalyst in nbitious chall Lithium-ion batteries.In lithiur )The mo t of ions hosted per e material st few Similar to Fie an xpabtain graphene.Pristinened and subseque host. stoiegeo city of 744 mAhg e that o tion intre ngraphene-like c an be easily nge of solvents s peculia an theo re Li'ions th ough an adsorptio tive ials(such as ctive polymers the can be used as such,or ateratively can be further reduced to ch a takes place at low potentials (.5 V versu Graphen cbased mat s from the terials have been prop sed for use in all ides ele as an active materialor aninactive component lly on-qualese NATURE MATERIALS ADVANCE ONLINE PUBLICATION www.nature.com/naturemateriab 2014 Mac lan 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 cur￾rent 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 ‘out￾standing performance’ graphene is the most ambitious challenge to address before its widespread application5 . This aspect is particu￾larly 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 graphene￾based 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, alter￾natively, oxidized to obtain graphite oxide25,26. The latter, after liquid￾phase exfoliation, yields GO, which is then reduced to form RGO20. This approach is different from other types of application as it is par￾ticularly useful for energy-storage materials. In fact, although oxida￾tion introduces defects that cannot be entirely removed during the reduction process20, this synthetic pathway facilitates the prepara￾tion of composites. In contrast with graphene (including RGO), GO can be easily dispersed in a wide range of solvents10. This peculiar￾ity 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 compos￾ites 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 electro￾static 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 con￾tinuously shuttle between a lithium-releasing cathode (commonly a layered lithium metal oxide) and a lithium-accepting anode (com￾monly 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 stoichiom￾etry 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 (<0.5 V versus Li/Li+). However, it differs from the characteristic staging behaviour of graphite because graphene provides electronically and geometri￾cally non-equivalent sites32. As a result of this unique mechanism, Table 1 | Graphene properties compared with other carbonaceous materials. Graphene Carbon nanotube Fullerene Graphite Dimensions 2 1 0 3 Hybridization sp2 Mostly sp2 Mostly sp2 sp2 Hardness Highest (for single layer) High High High Tenacity Flexible, elastic Flexible, elastic Elastic Flexible, non-elastic Experimental SSA (m2  g–1) ~1,500 ~1,300 80–90 ~10–20 Electrical conductivity (S cm–1) ~2,000 Structure-dependent 10–10 Anisotropic: 2–3 × 104*, 6† Thermal conductivity (W m–1 K–1) 4,840–5,300 3,500 0.4 Anisotropic: 1,500–2,000*, 5–10† *a direction, † c direction. Table 2 | Dimension-based graphene nomenclature. Thickness (n, number of layers) Lateral dimension D (nm) Aspect ratio (length:width) 1 2 ≤ n ≤ 10 D ≤ 100 100 ≤ D ≤ 105 ≤10 >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