nature materials PROGRESS ARTICLE PUBLISHED ONLINE:22 DECEMBER:10.1038/NMAT4170 The role of graphene for electrochemical energy storage Rinaldo Raccichini23,Alberto Varzi23,Stefano Passerini23*and Bruno Scrosati24* in the feld.h h as an active mat rial and as an inactive co ent from lithiu on batteries and elec hen Icapacitors to emerg address the penefitsand ofgraphene-based as outline the most promising resultsand. GD e to fun tions,su as to hig el-cstuablishe ed technique in s ger rally unsuitabl oduct purity and rathe king density (EESDs) o wil next-generation batteries y,the abl d graphite Furtherm mance met nake it sui efor producing phe e fo s-hond is produced by a de free flat rbon monolay xide must be redu rder to t reviewed ina Carbon Editoria this graphene family include ed in thi of bo he pre nce is the facto ects (edges and defo nsic detects (C que ed for bulk quantiti for producing commercially available aphene for EESDS other Uni rsity of Muenster.D-4814 M any."Helmholtz Institute Ulm ViMor-16163 Genova. NATURE MATERIALS IADVANCE ONLINE PUBLCATION IWr 2014 MaNATURE 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 build￾ing block for carbonaceous materials of other dimensionali￾ties (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 ‘gra￾phene 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 differ￾ent 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 advan￾tages of graphene, they often lack an objective analysis of its limita￾tions 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 met￾rics 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, lat￾eral 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, conse￾quently, 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 emerg￾ing 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 bottom￾up synthesis from structurally defined organic precursors9,10 neces￾sarily 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 stor￾age 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 aforemen￾tioned 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 graph￾ite’), 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 high￾quality graphene sheets5,13. Unfortunately, the low yield of this pro￾cess 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-to￾medium quality of the obtained material due to the presence of both intrinsic defects (edges and deformations) and extrinsic defects (O￾and H-containing groups), these methods allow the production of bulk quantities with high yield and contained costs. Although liq￾uid-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