CHEMSUSCHEM MINIREVIEWS DOI: 10.1002/cssc.201300782

Functionalized Graphene-Based Cathode for Highly Reversible Lithium–Sulfur Batteries Jin Won Kim,[a] Joey D. Ocon,[a] Dong-Won Park,[c] and Jaeyoung Lee*[a, b, c] In this article, we highlight the salient issues in the development of lithium–sulfur battery (LSB) cathodes, present different points of view in solving them, and argue, why in the future, functionalized graphene or graphene oxide might be the ultimate solution towards LSB commercialization. As shown by previous studies and also in our recent work, functionalized graphene and graphene oxide enhance the reversibility of the

charge–discharge process by trapping polysulfides in the oxygen functional groups on the graphene surface, thus minimizing polysulfide dissolution. This will be helpful for the rational design of new cathode structures based on graphene for LSBs with minimal capacity fading, low extra cost, and without the unnecessary weight increase caused by metal/metal oxide additives.

1. Introduction Ever increasing demand for mobile electronic devices and the continued search for high capacity batteries for the transportation sector undoubtedly drive the scientific and technological innovations in rechargeable Li batteries. Moreover, advanced Li batteries are vital in a diverse range of applications in the future, from electric cars to energy storage systems in smart grids.[1] Among secondary Li batteries, Li ion batteries are the state-of-the-art technology and are still the best energy storage solution for a wide array of applications.[2–4] Despite the impressive growth of the Li ion battery industry worldwide in the past decades, the chemistry behind the current technology has been criticized for its slow advancement: achievements in battery technology are dwarfed when compared to the rate of progress in the electronic industry, for which the memory capacity doubling approximately every two years, as predicted by Moore’s Law.[5] The challenge has always been to move beyond the present Li ion chemistry to produce batteries that

[a] J. W. Kim,+ J. D. Ocon,+ Prof. J. Lee Electrochemical Reaction and Technology Laboratory School of Environmental Science and Engineering Gwangju Institute of Science and Technology (GIST) Gwangju, 500-712 (South Korea) E-mail: [email protected] [b] Prof. J. Lee Ertl Center for Electrochemistry and Catalysis Research Institute for Solar and Sustainable Energies Gwangju Institute of Science and Technology (GIST) Gwangju, 500-712 (South Korea) [c] Dr. D.-W. Park, Prof. J. Lee Laboratory of Energy Storage Systems Research Institute for Solar and Sustainable Energies Gwangju Institute of Science and Technology (GIST) Gwangju, 500-712 (South Korea) [+] These authors contributed equally to this work. Part of a Special Issue on “The Chemistry of Energy Conversion and Storage“. To view the complete issue, visit: http://onlinelibrary.wiley.com/doi/10.1002/cssc.v7.5/issuetoc.

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are safer, less expensive, operate for longer, and have a higher energy density. In this Minireview, we begin with a brief discussion on the working principles of the Li-S battery (LSB) and present the challenges facing its development as a commercial battery, specifically with regard to S cathodes. We then focus our attention on the importance of the electrochemical behavior of various graphene–S composites in improving the S utilization and reversibility of the positive S electrodes. Finally, perspectives and approaches are introduced for solving the key issues from the perspective of materials science and materials chemistry. In recent years, several reviews covering nanostructured carbon and related materials and their application in LSBs have been published.[6, 7] However, a review on LSBs with particular emphasis on the potential of graphene–S cathodes has, to the best of our knowledge, not been done before. Nowadays, LSBs are becoming attractive and promising next-generation Li batteries when compared with other batteries. First introduced by Herbet and Ulam in 1962[8] and later developed by Argonne National Laboratory in 1967, it can theoretically store energy at 2500 Wh kg1 or 2800 Wh L1 in terms of weight and volume, respectively.[9] When considered separately, Li and S have specific capacities of 1673 and 3861 mAh g1, respectively, with the S cathode having the highest theoretical capacity among solid elements. Highly abundant S exhibits vastly distinct reactions with Li in comparison to the commercially successful intercalation compounds. Furthermore, S is a particularly noteworthy cathode material as it has a capacity that is significantly higher than state-of-theart LiMnO2–graphite batteries and advanced Li ion batteries utilizing nanostructured Si anode and is environmentally friendly compared to existing cathodes based on transition metals.[10] With huge improvements in LSB technology in the coming years, it is posed to meet the requirement of viable electric vehicle of 500 km driving range between charges.[11–13]

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Similar to how conventional batteries function, LSBs store energy within the electrode structure through charge transfer reactions. Conventional Li–S cells are composed of a Li anode, an organic ion-conducting liquid electrolyte, and a Sbased cathode; an anode based on high capacity Si and Sn has also been reported.[14, 15] When a load is attached to the battery, Li ions from the anode spontaneously transfer to the S cathode, while the electrons flow through the external load, as il- Figure 1. Diagram of a typical Li–S battery (left), illustrating the movement of Li ions from the anode to the cathode during discharge. Sulfur electrochemistry in an ideal charge–discharge process in the LSB (right). lustrated in Figure 1. In the cathode, S rings (S8 electrons from the S cathode are forced to move towards the being the most stable allotrope at room temperature) are cleaved to produce short S chains, eventually forming Li2S if anode. A charge and discharge cycle of a Li–S cell can be dethe battery is fully discharged. During charging, Li ions and scribed by the formation of Li2S in the discharge process and

Jin Won Kim received his Master’s degree from the Department of Environmental Science and Engineering at the Gwangju Institute of Science and Technology (GIST) in 2011 and is currently a PhD candidate at GIST, South Korea, specializing in electrochemistry under Prof. Dr. Jaeyoung Lee. His research interests include the development of electrocatalysts for both oxygen reduction and evolution reactions in aqueous/organic electrolytes and electrodes for rechargeable energy storage systems such as Li–S and metal (especially, semiconductor and Li anode)–air batteries.

Dong-Won Park received his BSs, MSs, and PhD degrees from the Department of Chemistry in Chonnam National University (South Korea). After postdoctoral work at the Korea Atomic Energy Research Institute (KAERI), he worked as a senior researcher at the Research Institute for Solar and Sustainable Energies (RISE), GIST (2009– 2013). Currently, he is working as a senior scientist at the Institut fr Technische Thermodynamik (ITT), Deutsches Zentrum fr Luft- und Raumfahrt (DLR) in Germany. His current research interests are focused on synthesis, characterization, and modeling of new electrode architectures/materials for rechargeable batteries.

Joey D. Ocon is a PhD candidate at Prof. Dr. Jaeyoung Lee’s Electrochemical Reaction & Technology Laboratory (ERTL) group at GIST (South Korea). In 2008 and 2011, he obtained his BSc and MSc degrees, respectively, from the Department of Chemical Engineering of the University of the Philippines, Diliman (the Philipines), where he is presently a faculty member on studyleave. His current scientific interests include the development of new semiconductor–air alkaline batteries (germanium–air and silicon–air), electrode synthesis and testing in Li batteries (Li–ion and Li–S), and catalyst development for oxygen electrocatalysis in fuel cells and metal–air batteries.

Jaeyoung Lee received his doctoral degree in 2001 from the Fritz Haber Institute and FU Berlin, Germany. He was a senior scientist at the RIST (2002–2004) and at the Fuel Cell Research Center, KIST (2004–2007). Currently, he is Vice Director of the Ertl Center for Electrochemistry and Catalysis and an Associate Professor of the School of Environmental Science and Engineering at GIST, South Korea. His current research interests include the optimization of the oxygen reduction and evolution reaction, fuel production from CO2, liquid-fuel oxidation in electrochemical power production, and chemical energy storage systems.

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the reverse during the charge process, according to the reversible redox couple shown in Equation (1). E  ¼ 2:15 V vs: Li=Liþ

S8 þ 16 Li $ 8 Li2 S

ð1Þ

As seen in the magnitude of the equilibrium potential above, the Li–S reaction occurs at a considerably lower potential than that of conventional transition metal oxide cathode materials; however, this fact is offset by the higher gravimetric capacity of S and improved safety associated with a lower operating potential. Therefore, there has been increased interest in LSB research in recent years, as shown in the inset of Figure 2. Although the reaction appears simple at first, the actual reaction occurring at the cathode during discharge is far more complex than the intercalation reactions at transition metal oxide cathodes, proceeding through a series of reactions as described below [Eqs. (2)–(6)].[16, 17]

Figure 2. Comparison between different rechargeable battery chemistries in terms of gravimetric and volumetric energy densities, showing the potential of LSBs to meet tomorrow’s energy storage requirements. The figure has been redrawn to include the theoretical values for the Li batteries.[5] The inset displays the number of publications about Li S battery in Elsevier’s Scopus Database.

S8 þ 2 e ! S8 2

ð2Þ



2

ð3Þ

2 S6 2 þ 2 e ! 3 S4 2

ð4Þ

S4 2 þ 4 Liþ þ 2 e ! 2 Li2 S2

ð5Þ

3 S8

2

þ 2 e ! 4 S6

þ



Li2 S2 þ 2 Li þ 2 e ! 2 Li2 S

ð6Þ

The voltage profile and the reactions that occur during the charge–discharge cycles are illustrated in Figure 1. The high voltage plateau between 2.15 and 2.4 V in the discharge profile corresponds to the breakage of the eight-atom S rings to form short polysulfide chains. The flatter and longer discharge plateau at 2.1 V marks the formation of the insoluble Li2S2 and  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Li2S compounds at the cathode’s surface. The final reaction involving the formation of Li2S, governed by slow solid-state diffusion, limits the overall rate of the discharge reactions. Consequently, the cell voltage drops quickly at the onset of full conversion to Li2S.

2. How Essential is Graphene to the Future of LSBs? 2.1. Solving key issues in LSB cathodes Despite the outstanding theoretical characteristics of LSB, it faces considerable challenges before it can truly replace Li ion batteries in our everyday electronics. These issues, which require new solutions to achieve LSB commercialization, can be grouped into two main impediments: maximizing the utilization of S and enhancing the reversibility. In comparison with commercial Li ion batteries, LSB exhibited poor cycle performance, degrading to a considerably lower capacity than the initial capacity within a few cycles.[18, 19] It is well established that the dramatic capacity fading in LSBs is caused by dissolution of intermediate discharge products into the organic electrolyte, a phenomenon called “shuttle mechanism”. As described earlier, solid elementary S (S8) is reduced to Li polysulfides (Li2Sx, 2 < x < 8) during cell discharge. As the Li polysulfides are liquid, it can easily dissolve into the electrolytes and migrate across the electrolytes to the Li anode, where it then forms a passivation layer on the anode’s surface, which results in self-discharge. In addition, the high S volume change (  80 %) decreases the mechanical integrity of the electrode, limiting the charge–discharge cycle stability. Another very important constraint is the low S utilization caused by the poor electrical conductivity of S (5  1030 S cm1), making the use of pure S inefficient and requiring the use of additional conducting materials in the cathode. Many innovative approaches in the development of S cathodes have been subsequently implemented to remedy these problems (Figure 3). A few well-known examples illustrate the impact of controlling the carbon and S network to enhance the electronic transport in S cathodes. In particular, mesoporous carbon was generally synthesized for use in LSBs because of their excellent electrical conductivity and high surface area. Towards this end, for example, KOH-activated hierarchical bimodal carbon (both meso- and microporous) and CMK-3 (a versatile mesoporous carbon) have been used.[20, 21] The S particles that were well distributed in the mesoporous regions serve as adsorption sites for Li ions and Li polysulfides.[22] Increasing the electrical conductivity and trapping Li polysulfides at the contact area between the conductive carbon and insulating S significantly enhanced the S utilization rate and the overall cycle performance. Another important consideration in the rational design of LSB cathodes is the confinement of low-numbered S molecules to improve the reversible capacity. Recalling that there are two regions in a typical discharge curve for LSBs, the first one involves the formation of long-chain polysulfides (  2.3 V), whereas the formation of short-chain polysulfides (  2.1 V) ChemSusChem 2014, 7, 1265 – 1273

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Figure 3. Representative schematics for various approaches (meso- and microporous carbon design, development of porous absorbent and polymer coating of S cathode) in LSB cathodes to overcome dramatic capacity fading.[19, 21, 32, 49]

occurs at the other one. Because the reversibility of LSBs is determined by the kinetics of the lower plateau, retaining the short-chain polysulfides within the electrode structure helps to enhance cycle durability. In this respect, a study reported a stable capacity of around 800 mAh g1 during 100 cycles at 400 mA gS1 when using microporous carbon spheres to capture the polysulfides.[23] For the purpose of blocking the dissolution of Li polysulfides, several studies also utilized the deposition of S into the channels, mostly involving S diffusion into the pores when melted over 110 8C.[21, 23–27] Using this concept, Cui et al. reported a high specific capacity of 730 mAh g1 at 0.2C after 150 cycles using carbon-coated anodic aluminum oxide (AAO) templates,[28] whereas another group obtained a stable cycle performance with polyethylene glycol (PEG)-coated CMK as carbon framework.[21] Several reports have already suggested the use of porous materials composed of metal or metal oxides as polysulfide absorbents; however, the mechanism of the interaction of polysulfides with the absorbents is not yet well understood.[29–31] Examples of additive absorbents that have been introduced are porous carbon, silica, alumina, and transition metal oxides and their derivatives.[32–36] Choi’s group reported a stable capacity of 660 mAh g1 using nanosized Al2O3 as additives, thus improving the specific capacity by 165 %.[31] A representative study on the use of metal oxide was performed by Ji et al.,  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

who used SBA-15 mesoporous silica as a reservoir for holding the polysulfides.[33] The mesoporous silica matrix contributed to the reversible adsorption–desorption of Li polysulfides during the charge–discharge process, leading to a capacity of 650 mAh g1 after 40 cycles at 0.2C, and significantly enhancing the Coulombic efficiency. Moreover, another study by DemirCakan compared the performance of SBA-15 silica matrix and a metal-organic-framework (MOF) as cathode additives, with both structures showing better reversible capacity.[34] To investigate Li polysulfide adsorption–absorption, a comparison between SBA-15 and TiO2-containing SBA-15 was also performed.[29] The authors first thought that TiO2-containing SBA15 would show a more stable performance because TiO2— being the more electropositive—electrostatically attracts the negatively charged polysulfides. However, no exact proof whether the polysulfides were bound to the electropositive metal or to the considerable more negative oxides could be found when performing FTIR analysis. Furthermore, these additional materials used to trap the Li polysulfides are difficult to fabricate and can increase the total weight of the cell, thereby lowering the gravimetric capacity and increasing the total cost. Experiments have also shown that surrounding S with polymers, such as polyacrylonitrile (PAN) and other polyanilinebased polymers, enhances S utilization by compensating for the low electrical conductivity of S, blocks Li polysulfide dissolution, and minimizes S volume expansion.[37–40] However, there ChemSusChem 2014, 7, 1265 – 1273

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CHEMSUSCHEM MINIREVIEWS are still many problems associated with this approach, such as the material’s low electrical conductivity, difficulty in scaling up the synthesis method, and low performance under high current densities. 2.2. Graphene as an essential S cathode component Carbon materials are widely used as electrode materials because of their many available forms, robust chemistry, good conductivity and stability, and abundance.[7] Among various nanostructured carbon materials considered as cathode components in LSBs, derivatives of carbon nanofibers (CNFs), carbon nanotubes (CNTs), and graphene are extensively studied.[41–43] For example, CNFs, derived from the carbonization of polymer precursors such as PAN, was utilized to form a CNF/S composite and was shown to a have good initial capacity of 850 mAh g1 and a capacity of 600 mAh g1 after 50 cycles.[41] Addition of CNFs prevented capacity loss because of a decrease in S and Li polysulfide agglomeration.[42] CNF cycling performance, however, is limited during high-current charge–discharge and long-term operation because of the poor electrical conductivity of the carbon fibers. To overcome drawbacks of CNFs, CNTs were used in several reports[43] because of its excellent electrical conductivity. Another key advantage of using CNTs as carbon material in LSB cathodes is the possibility of infiltrating S within the CNT structures. Successful incorporation of S inside CNTs improved S utilization because the S particles are well connected electrically, resulting in higher specific capacity and lower capacity fading.[43] Unfortunately, use of CNT-based cathode materials is still hindered by their poor solution processability, difficult Li accessibility across the carbon plain, and low S loading within the CNTs. Despite its short history, the importance of graphene as an essential carbon-based component in S cathodes is increasing fast. Graphene, the parent of all graphitic forms of carbon, is well known of having excellent electron mobility (200 000 cm2 V1 s1), high surface area (  2600 m2 g1), and high Young’s modulus (  1.0 TPa), properties that are ideal in solving the two main issues in LSBs at once.[45–47] In addition, graphene has a flexible two-dimensional sheet morphology, which can block the dissolution of Li polysulfides when wrapped around S and suppress S volume expansion. In terms of its superior material properties, graphene has the potential to improve the performance of LSB cathodes, thus enabling potential disruptive technologies in almost all facets of life.[48] As a robust yet flexible material, graphene provides almost infinite possibilities for modification or functionalization of its carbon backbone. Using graphene produced using various synthesis and reduction methods, diverse S cathode compositions can be obtained. For example, detailed studies have been performed using graphene-enveloped S (Figure 4 a), S particledecorated graphene (Figure 4 b), and graphene layers infiltrated with a S layer (Figure 4 c).[49–61] These structures showed excellent performance as shown by the higher specific capacity and improved cycle reversibility. Dai et al. first reported the possibility of using graphene for LSBs, obtaining a stable spe 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org cific capacity of 600 mAh g1 over 100 cycles when using graphene-enveloped S structures.[50] They successfully loaded S particles on graphene through disproportionation of thiosulfate followed by PEG chains coating to synthesize the graphene–S composite. Subsequently, various modifications of graphene–S cathodes were developed, including a S particle-decorated graphene and a sandwich-type structure, where S particles are rationally inserted between graphene sheets. Sulfur decoration on graphene was achieved through S precipitation using an acid. During graphene oxide reduction, surface sites previously occupied by oxygen functional groups played a major role in the nucleation and growth of S particles. A S layer between graphene sheets can be prepared by restacking of Nafion and Scoated graphene layers or allowing S diffusion into an expanded graphite during melting.[52–54] These studies enabled sustained interest in the use of graphene for minimization of Li polysulfide dissolution because of it closed structure and its strong interaction with polysulfides. As seen in the examples above, the graphene–S composite compensated for the low electrical conductivity of S, which in turn enhanced S utilization.[50, 54] Recently, graphene hybrid materials for LSB cathodes were also developed, such as graphene–CNF-S- cathodes[47] and graphene–CNT–S cathodes,[48] to offset the drawbacks of each material. Their results improved the cycle performance; however, the inherent problems resulting from the original material could not be solved. Interestingly, according to a recent report, graphene materials are also shown to be superior materials for use in LSBs because of the strong interaction between S and the graphene surface.[44] The contact angle of liquid S measured on various substrates showed that the graphene–S interface has the lowest contact angle, as displayed in Figure 5 a (bottom). It is believed that because the four lone pairs of S bind with the anti-bonding conjugated p* states on the graphene plane, S and the polysulfides are strongly coupled to the surface (Figure 5 a, top).[44] Despite the chemical compatibility between elemental S and graphene, however, graphene–S composite cathodes still showed unsatisfactory performance during fast charge and discharge cycles. The commonly developed S cathode structures are similar in nature to the illustration in Figure 5 b (top). These structures—although effective at low current densities—are inefficient in preventing Li polysulfide dissolution at high current densities. Even though close-type cathode structures are efficient in blocking Li polysulfides from the electrolyte, Li ions cannot penetrate across graphene in the lateral direction. Thus, the most basic question to overcome this challenge is how to disperse the short-chain polysulfides or nanosized S particles within graphene sheets while avoiding S agglomeration, as illustrated in Figure 5 b (bottom). Similarly, for the purpose of achieving high charge–discharge rates, the number of defect sites on graphene, which decrease electron mobility, should be reduced and the contact area between S particles and graphene should be considerably increased.

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Figure 4. Representative schematics, morphologies, and corresponding cycle performance of different types of S cathodes: a) graphene-wrapped S, b) S particle-decorated graphene, and c) infiltrated S layer between graphene layers.[40, 53, 59]

2.3. Lithium polysulfide absorption by oxygen-rich functionalized graphene cathodes

Figure 5. a) Wetting properties of S on different substrates and illustration of the electron orbit of graphene and S, and b) graphene-wrapped S particles and S molecules dispersed on graphene sheets.[44]

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Again, the key issue in the development of LSB cathodes is stopping the shuttling of short-chain polysulfides near the cathode surface. For all approaches that minimize the dissolution of Li polysulfides, understanding the physicochemical behavior of polysulfides remains a prerequisite. In the development of S cathodes, the surface functionality and mechanical structure are extremely important because specific functional groups, especially oxygen-rich sites in the cathode material, play a critical

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CHEMSUSCHEM MINIREVIEWS role in retaining the soluble Li polysulfides within the cathode. The importance of polysulfide absorbents, which was described in detail in Section 2.1, can be easily recognized by the numerous reports that used hydrophilic surfactants, additional absorbents, and binders. In graphene-enveloped S particles, Dai’s group argued that PEG, the main chain of Triton X-100 surfactant used for coating the S particles, trapped the polysulfides and limited the size of the S particles.[50] Their result is consistent with other studies using aqueous binders such as Nafion, polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone–polyethyleneimine (PVP–PEI), and hydrophobic polyvinylidene difluoride (PVDF) binders, providing enough evidence for the role of coupling of the Li polysulfides with the hydrophilic surface or oxygenated sites of the graphene-based electrodes.[62, 63] Despite the good interaction between Li polysulfides and the hydrophilic binders, PVDF is still commonly used because of its better adhesion characteristics and thermal stability. Clearly, these results emphasize that the strategy of using the inherent interaction between polysulfides and oxygenated graphene in the cathode is as important as developing porous materials to capture Li polysulfides. Proponents of the use of graphene in LSBs avoided using heavy metals and metal oxides as additives. Furthermore, absorbents or additives in cathodes have adverse effects on the cost and weight of the cell. Thus, the best method to use graphene materials is to optimize the oxygen functionalization on the surface of graphene oxide. In this respect, the electrochemical behavior of graphene oxide materials becomes more attractive than that of pure graphene or highly reduced graphene oxide. The interest increasingly shifts from GO as a precursor of graphene to the properties of GO itself and its applications in different fields.[64] Zhang and co-workers obtained a stable capacity of 954 mAh g1 at 0.1C over 50 cycles with 96.7 % Coulombic efficiency, and 370 mAh g1 at a higher current density (2C).[65] They elucidated the excellent performance as arising from the graphene oxide component in the GO–S composite, which was partially reduced during the chemical synthesis, as observed through Xray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) analysis. In addition, the incorporated S slightly influenced the valence band states of graphene oxide.[66] These results have two important implications: i) S incorporation involves the enhancement of electrical conductivity of insulating graphene oxides by partial reduction, and ii) interaction between S and graphene oxides, albeit weak, prevents Li polysulfide dissolution. To further illustrate the extent of the value-adding properties of graphene oxide in S cathodes, we studied the electrochemical behavior of S and graphene oxide during the charge–discharge process.[67] As seen in the CV curves in Figure 6 a, the upper potential plateau (UPP) region at around 2.4 V, which corresponds to the production of Li polysulfides, decreased, while the lower potential plateau (LPP) at approximately 2.1 V, which generally involves the reversible formation of Li2S, continuously increased with cycle number. Concurrently, the oxidation peak shifted to a more negative potential, while the two  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 6. a) Cyclic voltammograms (CVs) during cycling and b) Raman spectra of a GO-containing S cathode before and after discharge.[67]

reduction peaks moved to a more positive potential. As the potential difference between the redox peaks is getting smaller, this provides conclusive evidence on improved cycleability of the LSBs with increasing number of cycles. This implies that the simple mixture of graphene oxide and S enhanced the reversibility during the charge and discharge cycles. Furthermore, the D/G ratio of the GO-containing S cathode dramatically increased after cell discharge, as observed from the Raman spectra in Figure 6 b. It is believed that during cell operation the charge–discharge cycles partially reduce graphene oxide in addition to polysulfides binding with the oxygen functional groups in graphene oxide during discharge up to 2.0 V, as demonstrated by the Raman peak at around 1000 cm1. According to a recent report by Zu and Manthiam, graphene functionalized with oxygen by hydroxylation showed superior performance at high current densities.[68] The specific capacity reached 1277 mAh g1 at 0.5C, 1021 mAh g1 at 1C, and 647 mAh g1 at 2C. In addition, the cycle durability remained very stable during 100 cycles. The excellent performance resulted from the strong interaction between hydroxylated functional groups on graphene and S, as shown by XPS and UV/VIS absorption spectroscopy. This report is meaningful in elucidatChemSusChem 2014, 7, 1265 – 1273

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CHEMSUSCHEM MINIREVIEWS ing the effect of oxygen-rich functional groups on the battery performance at high current densities. Overall, the above results strongly suggest that optimizing the surface functional groups in graphene to obtain hydrophilic (or oxygenated) surfaces or using graphene oxide can achieve huge improvements in S cathodes. Although referred to as disordered materials, it is the inherent disorder caused by the presence of the functional groups that provides opportunities for tailoring the chemical properties of functionalized graphene and graphene oxide to overcome the two main issues in LSBs, low S utilization and poor stability. The fact that it is possible to improve the reversibility of LSBs, as proven by previous results, gives some hope that this is one step closer to realizing the goal of commercializing LSBs. There are still a number of challenges ahead, such as elucidating the origin of the interaction between polysulfides and the functionalized graphene/graphene oxide surface through further mechanistic studies.

3. Summary and Outlook Today, it seems that demand for power-hungry mobile electronics has finally caught up with the advances in Li battery technology. We can no longer rely on just Li ion batteries to power gadgets or store energy for electric cars in the future. Although tremendous progress in advanced Li ion anodes (Si, Ge, and Sn) has been made in recent years, new battery chemistries must be explored and developed simultaneously. LSBs have shown great promise because of their high theoretical energy density; however, its commercialization is still in question. This Minireview summarizes the key issues surrounding the development of next-generation LSBs, primarily based on the rational design of nanostructured S cathodes that can increase S utilization and charge–discharge reversibility. Significant attention has been focused on the shuttling phenomenon during the electrochemical charge and discharge process, which was induced by the dissolution of Li polysulfides into the electrolyte. As illustrated throughout this Minireview, many research groups have laid out several approaches, although each with varying degrees of success, in solving the problems in S cathodes. Specifically, we emphasized here the use of meso- and microporous carbons, polysulfide absorbents, and polymer coatings. Although the advantages of these materials have been well documented, choosing a material that has all the benefits of high porosity, excellent electrical conductivity, and immense flexibility in its morphology is more beneficial. Indeed, graphene-based materials provide all these advantages in addition to other unique characteristics such as high solution processability and easy complexation with many organic and inorganic systems. More importantly, graphene can immobilize S and polysulfides through electrostatic interactions. However, it has also been shown that graphene-wrapped S, S-coated graphene, and S layer–graphene sandwich structures are only effective at low current densities. These structures suffer from S agglomeration and inadequate graphene–S interfacial sites, leading to poor performance at high current densities. Thus,  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org the ideal structure would be, for example, a highly dispersed nanosized S and short-chain polysulfides on graphene layers. We believe that functionalized graphene and graphene oxide are key aspects to unlocking the many benefits of graphene without its negative consequences. As shown in several cases, Li polysulfides strongly interact with the oxygen functional groups in graphene, thus suppressing the shuttle effect. To fully solve the challenges in the development of highly reversible S cathodes, however, we should ask more fundamental questions. Understanding the complex interaction between the polysulfides and graphene surface is a first step to knowingly design materials with the right chemical functionality and properties. To move forward, chemists and material scientists from various backgrounds will be required to further tune the properties of functionalized graphene or graphene oxide for specific use in LSBs. Theoretical methods, such as density functional theory (DFT) and molecular modeling can shed light on the surface reactions in detail, in the same way that these techniques have been widely applied in the computational design of improved heterogeneous catalysts.[69, 70] Another strategy that would complement the molecular and quantum mechanical models is the use of existing and upcoming in situ and in operando characterization techniques to elucidate the nanoscale phenomena during LSB charge and discharge at operationally relevant conditions. With these developments, only then, perhaps, LSBs can become the “next best thing” among energy storage devices of the future.

Acknowledgements This work was supported by the Core Technology Development Program for Next-generation Energy Storage of the Research Institute for Solar and Sustainable Energies (RISE), GIST. J.D.O. gratefully acknowledges the ERDT Faculty Development Program of the University of the Philippines Diliman and the Department of Science and Technology (DOST) of the Philippines. Keywords: batteries · graphene oxide · lithium · reversibility · sulfur [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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Received: July 31, 2013 Revised: October 1, 2013 Published online on January 24, 2014

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