NATURE MATERIALS DOL:10.1038/NMAT4170 PROGRESS ARTICLE Wrapped Sandwich-lik Active materia ）Carbon black 0 Graphene Free Li'path Blocked upath Figure 4 S ck of tic of the diff ing th ial particles are pped by multiple graph well-repr pacitor electrodes.i d to the graphene surface.This structure is ver ant for metal-ioo Nesa an raphene-composite m although not widespread,is used for LIB cathodes.Layered lel Active-material nar and graphene-(right)based e es in the mixed structura sed toim mount of dat hemic activ cal properties of GO-b and RO-based comp pects of ou lives.This is pa that can meet the markes peo the vanadium permeation and preventing ionic cross-mix t thatr eal breakthroughs s are still to c ome.As was the case fo obtained with ban plica cessful commercial VRFB containing graphene is still far away Conclusions luction,and so a the rush to find new applications for this exciting material is more NATURE MATERIALSIADVANCE ONLINE PUBLICATION INATURE 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 electrochemi￾cal properties of GO-based91, RGO90,92 and RGO-based compos￾ites93–95 for application in VRFBs. All of these reports show promising electrochemical performance for graphene-supported carbon elec￾trodes, 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 addi￾tives 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 suc￾cessful 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 develop￾ment 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 applica￾tions. The first challenge lies in the production of graphene. Owing to its peculiar nature, the electrochemical properties of this mate￾rial 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 one￾pot 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