The Regulating Role of Carbon Nanotubes and Graphene in Lithium Ion and Lithium Sulfur Batteries

Transcription

1 REVIEW Lithium Batteries The Regulating Role of Carbon Nanotubes and Graphene in Lithium Ion and Lithium Sulfur Batteries Ruopian Fang, Ke Chen, Lichang Yin, Zhenhua Sun, Feng Li,* and Hui-Ming Cheng* The ever-increasing demands for batteries with high energy densities to power the portable electronics with increased power consumption and to advance vehicle electrification and grid energy storage have propelled lithium battery technology to a position of tremendous importance. Carbon nanotubes (CNTs) and graphene, known with many appealing properties, are investigated intensely for improving the performance of lithium ion (Li ion) and lithium sulfur (Li S) batteries. However, a general and objective understanding of their actual role in Li ion and Li S batteries is lacking. It is recognized that CNTs and graphene are not appropriate active lithium storage materials, but are more like a regulator: they do not electrochemically react with lithium ions and electrons, but serve to regulate the lithium storage behavior of a specific electroactive material and increase the range of applications of a lithium battery. First, metrics for the evaluation of lithium batteries are discussed, based on which the regulating role of CNTs and graphene in Li ion and Li S batteries is comprehensively considered from fundamental electrochemical reactions to electrode structure and integral cell design. Finally, perspectives on how CNTs and graphene can further contribute to the development of lithium batteries are presented. 1. Introduction The development of advanced battery systems is being driven by a drastically increasing demand for more powerful, efficient, R. P. Fang, K. Chen, Dr. L. C. Yin, Dr. Z. H. Sun, Prof. F. Li, Prof. H.-M. Cheng Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences Shenyang, Liaoning, China fli@imr.ac.cn; cheng@imr.ac.cn R. P. Fang Graduate School University of Chinese Academy of Sciences Beijing, , China K. Chen School of Physical Science and Technology Shanghai Tech University Shanghai , China Prof. H.-M. Cheng Center of Excellence in Environmental Studies King Abdulaziz University Jeddah 21589, Saudi Arabia Prof. H.-M. Cheng Tsinghua Berkeley Shenzhen Institute Tsinghua University Shenzhen , China DOI: /adma and economic energy storage, which can be used from portable electronics, electric vehicles, to smart grids. [1 3] Lithium ion (Li ion) batteries, since their successful launch in 1991, have revolutionized the present information-rich and mobile society. [4] They have dominated the consumer market for portable electronic devices, including mobile phones, digital cameras, and laptops because of their high energy density and long lifespan compared with their lead acid, nickel cadmium, or nickel metal hydride battery counterparts. [3] Moreover, it has been reported that Li ion battery technology dominates the present global battery market, with an equivalent electricity storage of 38 GWh being commercialized in [5] In recent years, improvements in energy density have been the primary driving force for battery technologies. [6] The lack of reliable high energy batteries has held back the electrification of vehicles rather than use an internal combustion engine. Indeed, the intrinsic intercalation mechanism of Li ion batteries places an upper limit on their energy density due to the limited weight and volume of the cathode and the anode host which lithium ions intercalate. [4] Therefore, in order to satisfy next-generation electrochemical energy storage requirements from personal devices to automobiles, effort needs to be made in two aspects: optimization of state-of-the-art Li ion technology and exploration of high-energy alternatives beyond the Li ion intercalation electrochemistry. Lithium sulfur (Li S) batteries, using elemental sulfur and metallic lithium as the cathode and anode materials, respectively, have been regarded as one of the most promising alternatives to meet future needs due to their overwhelming advantage of a high energy density. [6 10] Different from the intercalation mechanism of Li ion batteries, Li S batteries involve the conversion of sulfur to lithium sulfides by accepting two electrons per sulfur atom during the electrochemical process (S + 2Li + + 2e Li 2 S), which delivers a theoretical energy density of 2600 Wh kg 1, much higher than that of today s Li ion batteries (387 Wh kg 1 for LiCoO 2 /graphite batteries). [6] However, the two-electron-transfer mechanism between sulfur and lithium that fundamentally enables the high energy density also causes the Li S battery system to have inherent problematic issues, including the high solubility of polysulfide intermediates and the use of a metallic lithium anode. These lead to a low specific capacity, poor cycling stability, and safety concerns, which have (1 of 22)

2 prevented the Li S battery technology advancing from lab-scale demonstration to large-scale production. Continuous efforts have been made to build better lithium batteries with a higher energy density and wider applicability, including both current state-of-the-art Li ion batteries and near-term Li S batteries. Because the behavior of a rechargeable battery is mainly based on the performance of its anode and cathode, designing advanced electrode materials as well as electrodes with tailored compositions and structures has been the main research focus in recent years. [11,12] Carbon nanotubes (CNTs) and graphene, with highly appealing advantages of high electrical conductivity, good structural stability, tunable surface functionality, and excellent mechanical properties, have aroused intense research effort in the lithium battery field, and it is widely accepted that they can give a lithium battery a better performance. [13 18] In most reports, the advantages of CNTs and graphene are always highlighted for giving for the observed performance improvement, but a general and objective understanding of their effects on the operation of lithium battery systems is often lacking. It is not yet clear what the actual role of CNTs and graphene is in the internal electrochemical processes of an electrode, and how they contribute to the improved performance of a cell, and whether they could really lead to progress in the lithium battery field. In order to obtain insight into and further advance the applications of CNTs and graphene, it is of great importance to discuss the above questions for the use of CNTs and graphene in building better lithium batteries. Generally, the development of lithium batteries can be divided into three parts based on considerations from materials, electrodes, and cell levels. First is the identification of electroactive materials, which determines the fundamental electrochemistry, that are required to show intrinsically desirable properties, such as a high capacity, suitable potential, and good stability. Second is the construction of electrode structures in which the active materials can be appropriately assembled with ion and electron-conducting components. Third is the realization of functional electrodes and cells, aimed at satisfying modern flexible applications, including touch screens, wearable electronics, and diagnostic implements. In this review, the role of CNTs and graphene in lithium batteries is discussed based on these three perspectives. To begin with, it is worth noting that CNTs and graphene are not appropriate for use as electroactive materials that participate in the lithiation/delithiation process for three reasons. First, when CNTs and graphene are used as an anode, they often exhibit high specific capacities during the first lithiation step, but this cannot be fully released during the subsequent delithiation process. [18,19] This means that a large fraction of lithium ions are irreversibly consumed instead of reversibly stored, leading to a low Coulombic efficiency of the cell. Second, on the one hand, a graphene-based anode delivers discharge capacities mostly at a potential of 1 3 V (vs Li/Li + ) rather higher than V (vs Li/Li + ) for a conventional graphite anode, leading to a large voltage hysteresis in the charge/discharge curves. [20] On the other hand, a CNT-based anode has been reported to lack a voltage plateau with large change in voltage during discharge. [21] These result in poor energy efficiencies of their cells. Third, despite their high initial capacities, graphene and CNTbased anodes often suffer from fast capacity decay after a few Ruopian Fang is currently a Ph.D. student in the Institute of Metal Research, Chinese Academy of Sciences. She obtained her B.S. degree in 2012 in materials science and engineering from Wuhan University of Technology in China. Her research interests focus on the design and fabrication of novel carbon-based materials for highenergy-density battery systems. Feng Li is a professor of the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Ph.D. degree in materials science at IMR, CAS, in His main research interests focus on the nanomaterials for electrochemical energy storage and conversion. Hui-Ming Cheng is Professor and Director of both Advanced Carbon Research Division of Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, and the Low-Dimensional Material and Device Laboratory of the Tsinghua-Berkeley Shenzhen Institute, Tsinghua University. His research activities focus on carbon nanotubes, graphene, other 2D materials, energy storage materials, photocatalytic semiconducting materials, and bulk carbon materials. He is recognized as a Highly Cited Researcher in both materials science and chemistry fields by Thomson Reuters. tens of cycles, resulting in a low capacity that cannot compete with commercial graphite anodes. Therefore, it is not feasible to use CNTs and graphene as active lithium storage materials in lithium batteries. However, CNTs and graphene are widely expected to be used in a lithium battery with specific electrochemical reactions, and their roles are more like a regulator: they do not react with lithium ions and electrons during the electrochemical process, but serve to modify the lithium storage behavior of a specific electroactive material and increase the range of applications of a lithium battery. In addition, lithium (2 of 22)

3 metal anode has recently received tremendous research attention due to its significance in increasing the battery energy density, but safety concerns induced from uncontrollable lithium dendrite growth have hindered its pace of application. In this respect, CNTs and graphene have also shown effectiveness in regulating the behavior of lithium dendrite growth. Indeed, these regulating roles are rarely discussed systematically and are of equal importance to the active materials themselves because they have a significant influence on the electrochemical processes and are decisive in the realization of flexible electrodes and devices. Various review articles have summarized recent progress on the uses of CNTs and graphene in both Li ion and Li S batteries, [13 15,18,22 24] and we do not intend to give another research update. Instead, we aim to provide a systematic understanding of how the regulating role of CNTs and graphene helps build better lithium batteries. We begin with a brief introduction of the principles of lithium batteries and methods for evaluating them. Then the regulating role of CNTs and graphene in Li ion and Li S batteries is comprehensively discussed from the viewpoints of fundamental electrochemical reactions to electrode structure and integral cell design. Finally, perspectives on how they can further contribute to the development of lithium batteries are presented, and important research directions for the use of CNTs and graphene in the lithium battery field are considered. 2. Principles and Evaluation Methods of Lithium Batteries 2.1. Principles Typically, a lithium battery consists of a cathode and an anode where the electrochemical processes occur, in an electrolyte solution for lithium ion transfer separated by a polymer separator (Figure 1a). [5] Once the two electrodes are connected externally, redox reactions involving lithium ions and electrons proceed spontaneously at both electrodes, which liberate electrons and produce an output current. The chemical energy stored in the battery is calculated from capacity and voltage, which are primarily determined by the properties of the electrodes. Figure 1b shows the energy levels involved in a lithium battery. The electrochemically stable window (E g ) in a lithium cell is determined by the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the electrolyte. The open-circuit voltage of the cell (V oc ) corresponds to the difference between the chemical potentials of Li in the cathode and the anode. During charge/discharge, the voltage of the cell changes within the stable window of the electrolyte, accompanied by changes in the chemical potentials. Besides the cathode and anode that determine the fundamental redox reactions, the properties and performance of a lithium battery are also influenced by the inactive components, such as separators, current collectors, and conducting additives, which are discussed in the following sections Evaluation Criteria The pursuit of high-performance, affordable, and durable batteries has been a long-lasting goal. The following five metrics for the evaluation of lithium batteries are discussed: 1) energy density, 2) power density, 3) cycle life, 4) safety, and 5) economic cost Energy Density The need for high energy densities has been the primary driving force for the development of battery technologies. The amount of electrical energy, based on either per unit weight or per unit volume, is expressed as gravimetric energy density (Wh kg 1 ) and volumetric energy density (Wh L 1 ), which are directly associated with the intrinsic properties of the cathode and anode of the battery. The gravimetric energy density can be calculated from Equation (1), where Q is the charge capacity delivered during the electrochemical reactions, V is the cell voltage, determined by the difference between the redox potentials of the cathode Figure 1. a) Schematic of a lithium battery. b) Illustration of the energy levels involved in a lithium cell (3 of 22)

4 and the anode, and m c and m a are the respective weights of the cathode and the anode. The charge capacity delivered at the cathode and the anode are equivalent, and can be calculated based on the specific capacity and the mass of the cathode and the anode, respectively (Equation (2), where C c and C a represent the specific capacities of the cathode and the anode) [25] Q * V E = m + m c a Q = C * m = C * m (2) c c a a E c * a * = C C V C + C c a Equation (3) can be obtained from Equations (1) and (2) and clearly indicates that the energy density of a lithium battery is determined by the specific capacities and redox potentials of the cathode and the anode. The calculation of volumetric energy density can be performed in the same way, with the gravimetric specific capacities replaced by volumetric specific capacities. Therefore, it is concluded that high capacities of both the cathode and the anode as well as a large potential difference between the cathode and the anode are essential to achieve a high energy density. It is worth noting that, in order to better analyze the influence of the electrodes, the energy density is calculated based on the mass of electrodes, while in practical applications the mass of the whole battery, including inactive components such as electrolytes, separators, and packaging, needs to be considered. In this respect, electrodes with a low mass loading of active materials, which may give a high energy density according to Equation (3), often lead to a low practical energy density of the cell due to a relatively high proportion of inactive components. Therefore, besides the requirements for the specific capacities and potentials of the cathode and anode, designing electrodes with a high mass loading of active materials is also essential for achieving a high energy density. [26] (1) (3) Cycle Life For large-scale applications of rechargeable batteries, the requirements for long-term stability are stringent, and the cycle life is a very important metric for their evaluation. It has been indicated that, the main technological challenge, associated with many post-li ion battery systems including Li S batteries, is overcoming their inferior reversibility, which is often the main cause of a short cycle life. [28] Generally, the cycle life is associated with the stability of the electrodes during repeated cycling. [5] Mechanical instability such as volumetric expansion often leads to structural degradation of the electrode, resulting in fast capacity decay due to particle fracture and electrical isolation of the active materials. For example, a silicon anode and a sulfur cathode, known to have high theoretical specific capacities, suffer from large volumetric expansions up to 300% and 80%, respectively, during lithiation, which lead to poor cycle life and greatly prevent their practical use. [29 31] Uncontrolled reactions at the electrode/ electrolyte interface have also long been identified as a primary reason for the capacity fade in a lithium battery. [32,33] Especially for nanosize active materials, an unstable solid electrolyte interface (SEI) film formed on the surface due to the decomposition of organic electrolytes causes a continuous irreversible consumption of lithium ions, and the capacity decays gradually as the SEI film grows uncontrollably. [12,34] Therefore, both electroactive materials and electrode structure need to be developed and optimized to maintain the stability of the electrodes, and thus, improve the durability of the battery. In addition, it is worth noting that, for Li S batteries, the severe solubility of polysulfide intermediates in organic electrolytes is one of the most important issues for the deterioration of cycle life. The migrating polysulfides, either dissolved and retained in the electrolytes or reduced by a lithium anode, give rise to a continuous and irreversible loss of active materials that results in fast capacity decay, especially in the initial cycles. [35] In this regard, strategies for the effective encapsulation of polysulfides within the cathode are vital to achieve the long-term stability of Li S batteries Power Density The power density of a lithium battery indicates how it operates at a large current, which is a measure of rate capability. Taking an electric vehicle as an example, the energy density determines its driving range, while the power density determines its accelerating speed, i.e., how fast it can move. However, it is known that a high power density, corresponding to rapid energy storage, often leads to a compromise in energy density. [5] This can be ascribed to sluggish reaction kinetics at a high charge/ discharge rate, which lowers both the charge capacity and the output voltage of the battery. Recently, there has been a growing need for energy storage devices with a combination of high energy density and high power density. A possible solution is to develop materials that can store a large number of lithium ions and an electrode structure that provides sufficient paths for fast lithium ion and electron transport in a short charge/discharge duration. [27] Safety Undoubtedly, safety is one of the most basic requirements that must be guaranteed before a lithium battery can be certified for use. [36,37] Apart from the fact that lithium batteries contain highly oxidizing and reducing materials, the use of organic electrolytes has the inherent drawback of poor heat dissipation. Any abuse including overcharging and external short circuiting can trigger spontaneous heat-evolving reactions, which can lead to fire or explosion. [36,38] Therefore, the safe operation of lithium batteries needs to be fully guaranteed. From the perspective of energy density, lithium metal is the most ideal anode for lithium batteries owing to its high theoretical specific capacity (3860 mah g 1 ) and low redox potential ( 3.04 V vs the standard hydrogen electrode). [39 41] For Li S batteries, the use of a lithium metal anode is indispensable for its high energy density. However, uncontrollable lithium dendrite growth that can penetrate the separator and cause (4 of 22)

5 short circuiting of a working cell produces safety issues, and this is the most fatal obstacle to the practical applications of a lithium metal anode. Recently, the development of lithium anodes has become a necessity to satisfy ever-increasing high energy demands, and various approaches have been explored to address the safety concerns to make the lithium anode a viable technology. [42] Economic Cost Besides the measures of performance discussed above, the economic cost of lithium batteries, which is often ignored in fundamental research, is an important aspect that needs to be taken into consideration in industrial production. [43] In the emerging electric vehicle market, two primary concerns of customers are driving range (corresponding to the energy density) and price, and improvements in the driving range are unlikely to achieve substantial success in the electrical vehicle industry unless the added cost is low. [28] Therefore, any unnecessary increase in synthesis complexity and cost must be avoided in the design of lithium batteries, and simple electrodes using low-cost and commercially available materials as well as easy fabrication processes are necessary. Particularly for the Li S battery, the natural abundance and low cost of sulfur are among its prime advantages, which must not be offset by complicated fabrication processes resulting in a significantly increased cost of the sulfur cathode. Based on the above discussion of various evaluation criteria, it can be concluded that a good lithium battery should have the advantages of high energy density, high power density, long cycle life, high safety, and low economic cost. Nevertheless, there will be unavoidably tradeoffs among these requirements, and it is necessary to determine the detailed performance requirements for different applications of lithium batteries. 3. The Regulating Role of CNTs and Graphene in Li Ion and Li S Batteries With regard to the above-mentioned metrics for the evaluation of lithium batteries, the intriguing features of CNTs and graphene, including structural diversity and tailorability as well as their outstanding electronic, thermal, and mechanical properties, [13,14,44] enable them to be a highly versatile platform for building better lithium batteries. As discussed earlier, although CNTs and graphene are mostly not a protagonist (electroactive materials), their regulating role exerts a decisive influence on the properties and performance of lithium batteries. In order to guide and further widen the applications of CNTs and graphene, it is of great significance to have a general and objective understanding of their regulating role during the operation of lithium batteries. In this section, the role of CNTs and graphene in lithium batteries is comprehensively considered from the viewpoint of fundamental electrochemical reactions and electroactive materials to electrode structure and integral cell design (Figure 2). The following four aspects are included: 1) modulation of electroactive materials, corresponding to fundamental electrochemical reactions and electroactive materials; Figure 2. Illustration of the role of CNTs and graphene in lithium batteries. 2) construction of internal electrode circuitry, corresponding to electrode structure; 3) rebirth of lithium metal anode, corresponding to electrode structure; and 4) design of integrated cells, corresponding to integral cell design Modulation of Electroactive Materials It is known that the fundamental electrochemistry of a lithium battery is determined by electroactive materials with intrinsic lithium storage properties. As electrochemical reactions (M + xli + + xe Li x M) involve electrons and lithium ions, the materials need to be assembled with electron- and ion -conducting phases for fast reaction kinetics. In addition, structural instability issues, such as volume expansion, also need to be addressed to achieve long-term cycling stability. [29] Therefore, modifying the electroactive materials, either to enable fast charge transfer or to maintain structural integrity, is essential for improving their electrochemical performance. This means that the structure, morphology, composition, ionic diffusion kinetics, electronic conductivity, as well as surface characteristics of the materials need to be systematically addressed. In the following sections, the roles of CNTs and graphene in modifying the electroactive materials are discussed from three aspects: 1) electron- and ion-transport facilitators, 2) immobilization sites, and 3) volume expansion buffering. The first and third aspects involve both Li ion and Li S batteries, and the second aspect involves only Li S batteries Electron- and Ion-Transport Facilitators As discussed above, the transport parameters of electrons and lithium ions play a significant role in the implementation of electrochemical reactions. Generally, a transport process consists of two steps, the diffusion of lithium ions and electrons through the corresponding electron- and ion-conducting phases to reach the bulk electroactive materials followed by their solidstate transport within the materials. The electron- and ion-conducting phases should not only have high electronic and ionic conductivity, but also be continuous to guarantee contact with the active phases. However, the kinetics of the solid-state charge diffusion depend not only on the electronic and ionic conductivities but also on the structure and morphology of the active phase. In most cases, the diffusion time (τ d ) is theoretically (5 of 22)

6 determined by the diffusion length (L) and the effective chemical diffusivity of lithium in the electroactive material (D Li ) according to the relationship τ d L 2 /D Li. [45] Owing to the quadratic dependence of τ on L, shortening the diffusion path is expected to be effective in realizing fast reaction kinetics despite the fact that the D Li values of most electroactive materials, such as LiFePO 4, [46] LiMn 2 O 4, [47] LiNi 0.5 Mn 1.5 O 4, [48] and sulfur, [49] are fairly low (generally lower than 10 9 cm 2 s 1 ). Therefore, assembling electroactive materials with a continuous and percolating electron- and ion-transport facilitator is essential to achieve a high active material utilization with optimized reaction kinetics, especially for those with intrinsically poor electronic and ionic conductivities. In this respect, integrating the active phases with CNTs and graphene to form hybrid structures has proved to be a promising approach because it tends to form a robust and interpenetrating conductive network for fast charge transport with a shortened lithium diffusion length. Electroactive materials, either anchored, wrapped, or encapsulated by the CNTs and graphene, have been reported to exhibit significantly improved electrochemical performance. [15,19] Metal oxide anodes, which have a high theoretical capacity but suffer from sluggish charge transfer kinetics, are a typical example. Zhou et al. [50] reported the growth of NiO nanosheets on graphene for improving the lithium storage capacity (Figure 3a,b). The 2D sheet-on-sheet structure with negligible agglomerations of both NiO and graphene guarantees good electrical contact and sufficient accessible surface for electrolyte penetration, which allows fast electron and ion transport. Moreover, it was found that the NiO nanosheets are bonded strongly to graphene through oxygen bridges (C O Ni) originating from the pinning of hydroxyl/epoxy groups on the graphene to the Ni atoms of NiO nanosheets. The capacity of the NiO/graphene hybrid is much higher than the total sum of the individual capacities of NiO and graphene (Figure 3c), indicating a synergistic effect between them due to the existence of the oxygen bridges. In order to further understand the origin of this performance improvement, in situ transmission electron microscopy (TEM) was used to visualize the role of graphene in the NiO/graphene hybrid during lithiation. [51] According to the statistical data obtained, the presence of graphene leads to an increase in the Li + diffusion rate by two orders of magnitude (137 nm s 1 for the NiO/graphene hybrid and 4 nm s 1 for the pure NiO), indicating that graphene provides a path for easy Li + diffusion. This can be attributed to the strong interfacial interaction between NiO and graphene that ensures abundant interfacial Li + diffusion paths. Moreover, Li + reaction kinetics with NiO is significantly improved in the NiO/graphene hybrid (Figure 3d,e). The average reaction time for the lithiation of NiO in the NiO/graphene hybrid is calculated to be 5 s, which is 15.4 times faster than that for the pure NiO (77 s). Note that the lithiation of the second NiO nanosheet in the pure NiO can start only after the first nanosheet is fully lithiated (Figure 3e), and as the reaction proceeds, the time required for the full lithiation of a NiO nanosheet becomes longer due to the difficulty of transporting Li + over a long distance. Therefore, the presence of graphene provides a large number of interfacial Li + diffusion paths, which accounts for the fast reaction kinetics. Similarly, CNTs have proved to be highly effective as electron- and ion-transport facilitators. Lou et al. [52] designed a MoS 2 tubular structure internally wired with CNTs as an embedded conductive scaffold (Figure 4a,b). The CNTs serve as conductive paths for fast lithium ion transport and the porous tubular structure provides a short diffusion distance for fast Li+ ion diffusion. The MoS 2 /CNT hybrid delivered a reversible specific capacity of 1100 mah g 1 with no significant capacity decay after 200 cycles together with exceptional rate capabilities Figure 3. a,b) Scanning electron microscopy (SEM) images of a NiO/graphene hybrid. c) First reversible specific capacity of graphene, NiO nanosheets, a NiO/graphene hybrid, a NiO graphene mixture, and the calculated specific capacity based on experimental values. Snapshots from in situ transmission electron microscopy observations of the Li + reaction with d) NiO in a NiO/graphene hybrid and e) pure NiO. a,b) Reproduced with permission. [50] Copyright 2012, American Chemical Society. d,e) Reproduced with permission. [51] Copyright 2014, The Royal Society of Chemistry (6 of 22)

7 Figure 4. a) Schematic of the transport paths for Li + ions and electrons in a MoS 2 /CNT tubular hybrid structure. b) SEM image of the MoS 2 /CNT hybrid. c) Cycling performance and rate capabilities (inset) of the MoS 2 /CNT hybrid. d) Schematic of a SWCNT network with sulfur impregnation. e) TEM image of sulfur-coated SWCNTs. a c) Reproduced with permission. [52] Copyright 2016, American Association for the Advancement of Science. d,e) Reproduced with permission. [53] Copyright 2017, Elsevier. (Figure 4c), indicating significantly facilitated charge diffusion within the hybrid structure. In one of our recent studies, a single-wall CNT (SWCNT) conductive network was constructed for sulfur impregnation (Figure 4d). [53] Sulfur with a thickness of around 6 nm uniformly surrounds the SWCNTs (Figure 4e), which shortens the lithium diffusion paths. However, the interwoven SWCNTs in the network provide abundant paths for electron and lithium ion transport, which are favorable for high sulfur utilization. As a result, a high areal capacity of 8.63 ma h cm 2 was obtained (corresponding to a specific capacity of 1200 mah g 1 ), with a high sulfur content of 95 wt% and a high areal sulfur loading of 7.2 mg cm 2, much higher than that of Li ion batteries (4 ma h cm 2 ), indicating the high conduction efficiency of percolating CNT networks. The performance of various CNT- or graphene-based hybrid electrode materials for Li ion and Li S batteries is summarized in Tables 1 and 2 to illustrate the remarkable improvement produced by adding CNTs and graphene. [52 67,90,117,122, ] Representative electroactive materials for Li ion batteries with different electrochemical mechanisms are included, including (1) alloying and dealloying reaction-based electrodes such as Si, [54,55] Sn, [56] and SnO 2 ; [57] (2) conversion reaction-based electrodes such as metal oxides and sulfides; [52,58 62] (3) intercalation/deintercalation reaction-based electrodes such as LiFePO 4, [63,64] Li 4 Ti 5 O 12, [65] and TiO 2. [66,67] Immobilization Sites The severe dissolution and migration of polysulfide intermediates in electrolytes are among the most challenging issues that need to be addressed for the long-term stability of Li S batteries. Therefore, it is essential to include polysulfide immobilization sites in a sulfur cathode, with the specific aim of alleviating the dissolution and suppressing the migration of polysulfide intermediates. Considering the different chemical bonding natures of polysulfides (polar) and carbon (nonpolar), [68,69] physically confining polysulfides within the pore structure of carbon is insufficient to immobilize the migrating polysulfides due to their poor affinity with the conductive carbon surface, while chemical adsorption of polysulfides through polar polar interactions is considered effective in enhancing the affinity of polysulfides with the cathode. Graphene oxide (GO), with abundant oxygen-containing functional groups on the surface, has proved to show a strong adsorbing ability to anchor polysulfides and to effectively prevent them from dissolving in electrolytes during cycling (Figure 5a). Ji et al. [70] performed ab initio calculations to clarify the role of functional groups on graphene oxide in immobilizing polysulfides. The results indicated that both epoxy and hydroxyl groups increase the binding between sulfur atoms in polysulfides and carbon atoms in graphene due to the induced ripples by the functional groups, which then serve to improve the interactions between polysulfides and graphene and result in good polysulfide immobilization. Another surface chemistry strategy for anchoring polysulfides is to dope the CNTs or graphene with heteroatoms with different electronegativities. Nitrogen-doping (N-doping) has been the most popular model for calculation and experimental assessment [68,71 73] because nitrogen is more electronegative than carbon, which increases the electronegativity of the carbon surface to facilitate the chemical adsorption of polysulfides. Hou et al. [74] proposed a Li bond theory, which is an analog to the known hydrogen bond, to interpret the interfacial interactions between lithium polysulfides and N-doped carbon. It was found that, particularly for pyridinic N, the electronrich donor N atoms interact with the Li cations in lithium (7 of 22)

8 Table 1. Summary of the performance of various CNT- or graphene-based hybrid electrode materials for Li ion batteries. Materials Structure Initial discharge capacity Cyclability [62] CNTs/Co 3 O 4 Co 3 O 4 nanoparticles penetrated by CNTs 1171 mah g 1 (100 ma g 1 ) 100 cycles, 813 mah g 1 CNTs/CoS [61] CoS nanoparticles wrapped by CNTs 2083 mah g 1 (200 ma g 1 ) 100 cycles, 1668 mah g 1 [136] CNTs/GaS x GaS x nanofilms coating on CNTs 2118 mah g 1 (120 ma g 1 ) 100 cycles, 575 mah g 1 [63] CNTs/LiFePO 4 LiFePO 4 nanoparticles anchored to CNTs 115 mah g 1 (1700 ma g 1 ) 1000 cycles, 113 mah g 1 [137] CNTs/LiNi 0.5 Mn 1.5 O 4 LiNi 0.5 Mn 1.5 O 4 particles embedded in CNTs 140 mah g 1 (70 ma g 1 ) 100 cycles, 135 mah g 1 [65] CNTs/Li 4 Ti 5 O 12 Li 4 Ti 5 O 12 nanoparticles anchored to CNTs 130 mah g 1 (ma g 1 ) 1000 cycles, 118mAh g 1 [138] CNTs/Mn 3 O 4 Mn 3 O 4 surrounded by CNTs 1030 mah g 1 (468 ma g 1 ) 400 cycles, mah g 1 [139] CNTs/MoO 3 MoO 3 nanosheets bridged by CNTs 883 mah g 1 (100 ma g 1 ) 100 cycles, 865 mah g 1 [52] CNTs/MoS 2 MoS 2 tubular structure wired by CNTs 1850 mah g 1 (500 ma g 1 ) 200 cycles, 1100 mah g 1 [140] CNTs/Nb 2 O 5 Nanocable-like Nb 2 O 5 coating on CNTs 487 mah g 1 (40 ma g 1 ) 100 cycles, 441 mah g 1 [141] CNTs/LiNi 0.8 Co 0.15 Al 0.05 O 2 Mechanical Composite 189 mah g 1 (50 ma g 1 ) 60 cycles, 181 mah g 1 CNTs/Si [55] Si nanoparticles coating on CNTs 1629 mah g 1 (200 ma g 1 ) 200 cycles, 916 mah g 1 [66] CNTs/TiO 2 TiO 2 spheres penetrated by CNTs 316 mah g 1 (66 ma g 1 ) 100 cycles, 306 mah g 1 [142] CNTs/V 2 O 5 V 2 O 5 encapsulated in CNTs 298 mah g 1 (150 ma g 1 ) 200 cycles, 211 mah g 1 CNTs/ZnO [143] ZnO anchored to CNTs 1298 mah g 1 (100 ma g 1 ) 200 cycles, 850 mah g 1 [58] G/Co 3 O 4 Atomic layer-by-layer Co 3 O 4 /G composites 1134 mah g 1 (2000 ma g 1 ) 2000 cycles, 900 mah g 1 G/CoO [59] CoO nanoparticles anchored on G layers 810 mah g 1 (1000 ma g 1 ) 5000 cycles, 604 mah g 1 [144] G/Fe 2 O 3 Fe 2 O 3 nanoframeworks encapsulated with in G 1870 mah g 1 (200 ma g 1 ) 130 cycles, 1129 mah g 1 [64] G/LiFePO 4 LiFePO 4 encapsulated within G 123 mah g 1 (1700 ma g 1 ) 1000 cycles, 110 mah g 1 [145] G/Li 3 VO 4 Li 3 VO 4 nanobox wrapped with G nanosheets 220 mah g 1 (4000 ma g 1 ) 500 cycles, 220 mah g 1 [146] G/MnCO 3 Orderly packed MnCO 3 and G 1850 mah g 1 (500 ma g 1 ) 500 cycles, 1395 mah g 1 [147] G/MnO 2 G-wrapped MnO 2 coating on G nanoribbons 571 mah g 1 (400 ma g 1 ) 250 cycles, 612 mah g 1 [60] G/MoS 2 MoS 2 nanoplate wrapped by G nanocables 1150 mah g 1 (500 ma g 1 ) 160 cycles, 1150 mah g 1 G/Si [54] Si nanoparticles embedded in G 3500 mah g 1 (12600 ma g 1 ) 300 cycles, 750 mah g 1 G/Sn [56] Sn nanoparticles anchored on G 682 mah g 1 (2000 ma g 1 ) 1000 cycles, 657 mah g 1 [57] G/SnO 2 SnO 2 nanocrystals coating on G 1865 mah g 1 (500 ma g 1 ) 500 cycles, 1346 mah g 1 [67] G/TiO 2 TiO 2 quantum-dot anchored on G 272 mah g 1 (168 ma g 1 ) 100 cycles, 190 mah g 1 [148] G/WS 2 WS 2 nanotubes wrapped with G 886 mah g 1 (1000 ma g 1 ) 500 cycles, 319 mah g 1 polysulfides to form N Li 2 S x interactions using N lone-pair electrons, which can be theoretically regarded as a dipole dipole interaction (Figure 5b). The Li bond is expected to facilitate electrochemical contact of polysulfides with the conductive carbon surfaces and serve to localize polysulfides within the cathode scaffold. Moreover, it has been theoretically predicted that N-doped graphene with clustered pyridinic N-dopants binds lithium polysulfides more strongly than the electrolyte solvents do, indicating an effective anchoring effect of soluble polysulfides. [75] Therefore, the tunable surface chemical properties of CNTs and graphene can provide sufficient immobilization sites that chemically interact with the polysulfides, and thus contribute to improve the cycling stability of Li S batteries Volume Expansion Buffering Severe volumetric changes in electroactive materials upon lithium insertion and extraction, which are known to have a deleterious effect on electrochemical behavior, have been a critical challenge toward their use. [5] For example, sulfur undergoes a volume expansion of 80%, [76] and silicon up to 300% during lithiation. [77] Metal oxides, such as NiO, [50] Co 3 O 4, [62] and V 3 O 7, [78] are also found to show obvious volume changes during cycling, and the volume expansion of NiO on lithiation can be clearly visualized in Figure 3e. Repeated volume expansion and contraction directly lead to pulverization of the electrode and electrical isolation of the electroactive materials, which is among the primary reasons for permanent capacity decay. When the active materials expand and contract, the SEI film deforms and breaks, and the formation of a new SEI film on the freshly exposed surface results in a poor Coulombic efficiency due to the irreversible consumption of lithium ions. [29] Moreover, the accumulation of SEI films leads to an increase in the internal impedance and has a negative effect on the electrochemical reactivity of the active materials due to blocked lithium ion paths. [77] These problems originating from volume changes are considered to be intimately associated with the failure of lithium batteries and must be addressed in order to secure the stability of electrodes and thereby improve battery performance (8 of 22)

9 Table 2. Summary of the performance of various CNT- or graphene-based hybrid electrode materials for Li S batteries. Materials Structure Initial discharge capacity Cyclability CNTs/S [149] S embedded in aligned MWCNTs 736 mah g 1 (167.2 ma g 1 ) 85 cycles, 450 mah g 1 CNTs/S [122] S coating on the MWCNTs paper 995 mah g 1 (83.7 ma g 1 ) 150 cycles, 597 mah g 1 CNTs/S [150] S-encapsulated CNT-based hybrids 622 mah g 1 (1675 ma g 1 ) 600 cycles, 407 mah g 1 CNTs/S [151] S coating on hydroxylated CNTs 1274 mah g 1 (167.5 ma g 1 ) 100 cycles, 721 mah g 1 CNTs/S [152] S wrapped on CNT array 1092 mah g 1 (837.5 ma g 1 ) 50 cycles, 700 mah g 1 CNTs/S [53] S coating on SWCNTs 1280 mah g 1 (250 ma g 1 ) 300 cycles, 908 mah g 1 CNTs/S [153] S wrapped on CNTs 1065 mah g 1 (837.5 ma g 1 ) 300 cycles, 817 mah g 1 CNTs/S [154] S wrapped on CNT-based hybrid 1045 mah g 1 (167.5 ma g 1 ) 60 cycles, 958 mah g 1 CNTs/S [155] S encapsulated in spherical CNTs particles 1544 mah g 1 (837.5 ma g 1 ) 100 cycles, 901 mah g 1 CNTs/S [156] S encapsulated in CNTs 1138 mah g 1 (3350 ma g 1 ) 400 cycles, 683 mah g 1 CNTs/S [157] S embedded in CNT foam 1379 mah g 1 (335 ma g 1 ) 200 cycles, 1046 mah g 1 G/S [90] G S G Sandwich Structure 1000 mah g 1 (1500 ma g 1 ) 300 cycles, 689 mah g 1 G/S [117] S embedded in G foam 1550 mah g 1 (1500 ma g 1 ) 1000 cycles, 527 mah g 1 G/S [158] S embedded in hydroxylated G 1136 mah g 1 (1675 ma g 1 ) 100 cycles, 955 mah g 1 G/S [159] S core/go shell particles 900 mah g 1 (1000 ma g 1 ) 1000 cycles, 800 mah g 1 G/S [160] S/N-doped G 1102 mah g 1 (1675 ma g 1 ) 500 cycles, 628 mah g 1 G/S [161] S encapsulated in hierarchical porous G 1053 mah g 1 (836 ma g 1 ) 150 cycles, 879 mah g 1 G/S [162] S/G nanosheets 1047 mah g 1 (837.5 ma g 1 ) 70 cycles, 701 mah g 1 G/S [163] S embedded in 3D G 1260 mah g 1 (167.5 ma g 1 ) 100 cycles, 700 mah g 1 G/Li 2 S [164] Li 2 S-coated G aerogel 1072 mah g 1 (837.5 ma g 1 ) 300 cycles, 657 mah g 1 G/S [165] S embedded in activated G 955 mah g 1 (800 ma g 1 ) 1000 cycles, 426 mah g 1 G/S [166] S encapsulated in N-doped nanohollows 648 mah g 1 (1675 ma g 1 ) 140 cycles, 576 mah g 1 Figure 5. a) Schematic of graphene oxide immobilizing S atoms in polysulfides. Yellow, red, and white balls denote S, O, and H atoms, respectively, while the others are C atoms. b) Schematic diagrams of the interactions between polysulfides and pyridinic nitrogen. Mai and co-workers [78] designed a V 3 O 7 /graphene hybrid with V 3 O 7 nanowires wrapped by semihollow graphene scrolls (Figure 6a). The channels in the graphene scrolls provide internal void space for the free volume expansion of the V 3 O 7 nanowires during lithiation and effectively inhibit their agglomeration, which guarantees good structural stability of the hybrid with continuous paths for electron and Li + transport during cycling (Figure 6b). As a result, Li ion battery cathodes made of the V 3 O 7 /graphene hybrid had a high reversible capacity of 321 mah g 1 at 100 ma g 1 with an 87.3% capacity retention after 400 cycles at 2000 ma g 1. Yu et al. [79] prepared a Si nanoparticle-filled CNT material, which was used as the working electrode of a nanobattery for in situ TEM observations, aimed at directly investigating the structural change of the Si nanoparticles and the confinement of the CNTs during lithiation/delithiation. It was found that the volume expansion of the lithiated Si is restricted by the walls of the CNTs and that the CNT can accommodate this volume expansion without breaking its tubular structure. Jin et al. [80] reported a tube-in-tube structure with smalldiameter CNTs ( 20 nm) with sulfur-filled large-diameter CNTs ( 200 nm) as a sulfur cathode (Figure 6c,d). This structure had the dual function of buffering the volume expansion of sulfur during lithiation, both in the pores between the small-diameter CNTs and the void space in the large-diameter CNTs (Figure 6e). A large discharge capacity of 1193 mah g 1 was retained at 0.1 C after 100 cycles. Moreover, large discharge capacities of 1146, 1121, and 954 mah g 1 were also observed after 150 cycles at large current rates of 1, 2, and 5 C, respectively. Therefore, the (9 of 22)

10 Figure 6. a) TEM images of a V 3 O 7 /graphene hybrid (the inset gives an HRTEM image of a V 3 O 7 nanowire in graphene scrolls). b) Schematic of the V 3 O 7 /graphene hybrid structure with internal void space for free volume expansion of the V 3 O 7 nanowires during lithiation. c) TEM image of a tubein-tube structure with small-diameter CNTs inside a large-diameter CNT. d) TEM image of the CNT-based tube-in-tube structure after sulfur impregnation. e) Schematic of a reversible electrochemical reaction mechanism in the CNT-based tube-in-tube structure. a,b) Reproduced with permission. [78] Copyright 2013, American Chemical Society. c e) Reproduced with permission. [80] Copyright 2016, American Chemical Society. high flexibility and tailorability of CNTs and graphene provide multiple opportunities for changing electroactive materials with specified requirements for high-performance lithium batteries Construction of Electrode Internal Circuitry According to the discussion in the previous section, there is no doubt about the decisive role of electroactive materials in determining the performance of lithium batteries. Nevertheless, it is worth noting that a lithium battery is a complex device and its performance is sensitive to a variety of factors, not only involving the electroactive materials, but also any inactive components. Generally, the electroactive materials need to be assembled into a suitable electrode structure for performance evaluation. The optional inactive components in the electrode, such as the current collector and conducting additives, serve to construct the internal circuitry of the electrode, which plays a significant role in the final performance. In the following, the role of CNTs and graphene in optimizing the inactive components for the construction of electrode internal circuitry is discussed. The three aspects considered are as follows: 1) conducting additives, 2) current collectors, and 3) conductive interlayers. The first and second aspects involve both Li ion and Li S batteries, and the third aspect involves only Li S batteries Conducting Additives During electrode fabrication, conducting additives, mainly carbon black and graphite powder, are often added to improve the electronic conductivity of the electrode, which is especially necessary for electroactive materials with low electronic conductivities, such as LiMn 2 O 4 (10 4 S cm 1 ), LiFePO 4 and (10 9 S cm 1 ), and sulfur (10 18 S cm 1 ). [49,81] Especially in the case of high-power lithium batteries, the role of conducting additives becomes even more important due to the fast charge transfer requirement. CNTs and graphene have been investigated as conducting additives due to their advantages of high electronic conductivities and their effectiveness in constructing nanoscale circuitry throughout the electrode structure. [19,82] From the geometrical viewpoint, 0D carbon black particles follows a point-to-point conducting mode, in which a high mass loading with intimately contacting particles is necessary to form effective electron transport paths. In comparison, CNTs and graphene follow a line-to-point and plane-to-point conducting mode, respectively, which can provide long-range connectivity for electron transfer. This suggests that the high conduction efficiencies of CNTs and graphene allow the construction of an effective conducting network with little additional weight, equivalent to a relative increase in the active material content, which produces an increase in electrode capacity. In addition, the good thermal conductivities of CNTs and graphene enable effective heat dissipation within the electrode, potentially improving safety concerning thermal runaway problems. Landi et al. [83] reported that a 1 wt% SWCNT conducting additive dispersed in a LiNi 0.8 Co 0.2 O 2 cathode were able to create an improved percolation network over that using 4 wt% carbon black. At a high current density of 16.4 ma cm 2, the cathode with the SWCNT additive delivered a specific capacity more than three times that with the carbon black additive (10 of 22)

11 Figure 7. a) Cycle performance and b) charge/discharge profiles of two 2.0 Ah Li ion batteries using i) 2 wt% graphene and ii) commercial conducting additives. c) Cycle performance and d) charge/discharge profiles of two 2.6 Ah Li ion batteries using i) 1 wt% graphene plus 1 wt% carbon black and ii) commercial conducting additives. Reproduced with permission. [84] Copyright 2012, Elsevier. Moreover, the SWCNT additive increased the thermal stability of the electrode with a 40% reduction of the cathode exothermic energy released during delithiation. The situation is more complicated when graphene is used as a conducting additive. Yang and co-workers [84] investigated the role of a graphene additive using a commercial soft-packaged battery pack as a model system. Compared with the cells using a commercial conducting additive (7 wt% carbon black and 3 wt% conductive graphite), a 2 Ah LiFePO 4 cell using a 2 wt% graphene additive showed a higher capacity (Figure 7a) due to the construction of effective electronic conductive networks. However, it can be seen from the corresponding charge/ discharge profiles that the battery with the graphene additive exhibited a higher polarization despite its higher capacity (Figure 7b). This was attributed to the fact that the lithium ion transport paths are obstructed to some extent by the planar structure of the graphene sheets, which leads to an insufficient lithium ion flux instantly required for the electrochemical reactions. Generally, electrons and lithium ions need to reach or leave the reaction point of the active material to ensure desirable reaction kinetics. In this case, although fast electron transport can be guaranteed by the conductive graphene network, the impeded lithium ion transport leads to a negative effect on the reaction kinetics and thus results in a higher polarization. Nevertheless, by replacing half the graphene additive with carbon black, the polarization issue caused by the steric effect of graphene can be effectively alleviated (Figure 7c,d), indicating improved lithium ion transport behavior by a collaboration between the graphene sheets and carbon black particles. Therefore, both the electronic and ionic conductivity need to be considered and optimized to enable the real commercial application of graphene as a conducting additive in lithium batteries. From the viewpoint of practical applications, it is necessary to evaluate the possibility and feasibility of CNT and graphene conducting additives toward scale-up. During the past decades, related industries have shown great interests and made significant progress in the production and application of CNTs and graphene, and various industrial production processes have been built. A multiwalled carbon nanotube (MWCNT) capacity around 300 tons per year was reported in 2006, [85] and a production line with an annual output of 300 tons of multilayer graphene sheets was opened in [86] Moreover, it has been estimated from the announcements given by various companies that the market growth of both CNTs and graphene as conducting additives in lithium ion batteries is very rapid in recent years, [87,88] which indicates a promising future trend for their large-scale applications Current Collectors Current collectors are a necessary component in lithium batteries because they provide essential electrical connections between the electrodes and the external circuit and substantially influence the overall performance. Aluminum and copper foils are the most widely used current collectors for electrodes in lithium batteries. However, three issues need to be addressed regarding these conventional metal current collectors: first, their large weight increases the proportion of inactive components and leads to a decrease in energy density; second, (11 of 22)

12 Figure 8. a) Schematic of the procedure for making a flexible electrode with a CNT film functioning as a lightweight and thin current collector. b) Cycling and c) rate performances of the graphite electrodes using CNT and copper current collectors. Reproduced with permission. [92] Copyright 2013, Wiley-VCH. poor interfacial properties due to weak adhesion between active materials and the smooth metal surface lead to a potential risk of active material peeling away from the current collector; third, the stability of metals in organic electrolytes remains a problem, and they suffer severe electrochemical corrosion during long-term cycling. [89] CNTs and graphene can easily be assembled into freestanding films with the advantages of lightweight, mechanical durability, and chemical stability, which are considered promising for use as current collectors. [90,91] Wang et al. [92] reported the use of ultralight CNT current collectors (areal density: 0.04 mg cm 2 ) for the fabrication of freestanding electrodes: first, a CNT film was stacked on a glass substrate, then an electrode slurry was coated on top of the CNT film, and finally the electrode with the CNT current collector was easily separated from the glass substrate after drying (Figure 8a). Compared with conventional metal current collectors, better wetting, stronger adhesion, greater mechanical durability, and lower contact resistance were demonstrated at the electrode/ CNT interface, resulting in an improvement in cycling stability, rate capability, and gravimetric energy density for the graphite electrode (Figure 8b,c). Shi et al. [93] reported a highly conductive graphene membrane as a current collector for both a LiFePO 4 cathode and a Li 4 Ti 5 O 12 anode. A full Li ion battery assembled using these graphene-based electrodes showed a higher specific capacity and more stable cycling stability than one with conventional metal current collectors, especially at high current densities. In addition, CNT- and graphene-based current collectors showed an additional advantage when used in Li S batteries: they serve as a porous reservoir to accommodate polysulfides during cycling and contribute to an improved cycling stability. Moreover, it is worth noting that the use of CNT/graphene current collectors often gives the electrode flexibility, which is obviously desirable for the design of flexible devices Conductive Interlayers As discussed above, polysulfide dissolution and migration in electrolytes lead to a severe irreversible loss of active materials during cycling, and this is one of the most serious challenges for Li S batteries. Introducing a conductive interlayer between a sulfur cathode and a polymer separator has proved to be an effective approach to address these issues. [94] The interlayer functions as an embedded conductive network to intercept the migrating polysulfides and also enables their reutilization, resulting in high sulfur utilization and cycling stability. Su and Manthiram [95] introduced a conductive MWCNT interlayer in a Li S cell (Figure 9a,b). The MWCNT interlayer forms an additional circuit in the electrode, which serves to reduce the internal resistance for fast electron transport and alleviate the unwanted diffusion of solvated polysulfides to the anode. Benefiting from this MWCNT interlayer, significantly improved electrochemical performance with high capacities and good cycling stabilities were demonstrated (Figure 9c). Zhou et al. [90] designed a composite separator with highly conductive graphene interlayer coating on a conventional polymer separator, which is a simple and effective strategy for obtaining high-performance Li S batteries with pure sulfur as the active material. In our recent work, a partially oxygenated graphene interlayer was used for polysulfide interception, [96] which not only captured the migrating polysulfides but also immobilized them by strong chemical interactions (as mentioned in section 3.1.2). Therefore, the high electrical conductivity and the tunable pore structure and surface properties of CNTs and graphene provide opportunities for the design of conductive interlayers with multiple functions, which contribute to an improved Li S battery performance (12 of 22)

13 Figure 9. Schematic of cell configurations of Li S batteries: a) with and b) without a MWCNT interlayer. c) Cycling performance and Coulombic efficiency of Li S cells with and without a MWCNT interlayer. Reproduced with permission. [95] Copyright 2012, The Royal Society of Chemistry Rebirth of Lithium Metal Anode Lithium metal is the ultimate choice for the anode in lithium batteries due to its high theoretical capacity and low electrochemical potential among all anode candidates. [39,40] Moreover, a lithium metal anode is indispensable for the realization of high energy-density Li S batteries. However, the practical use of lithium metal anode has been halted for decades since its first introduction by Whittingham in the 1970s mainly because of safety concerns regarding the formation of lithium dendrites. Besides, severe volume changes and unstable SEI films lead to problems of low Coulombic efficiency, high overpotential, and short cycle life, resulting in a rapid performance degradation of the lithium anode. [97] In order to meet the growing high-energydensity demands for next-generation energy storage applications, tremendous efforts have recently been made to revive the lithium anode. Various strategies, regarding the optimization of electrolytes, [98,99] the construction of an artificial SEI, [100] and the design of a lithium host matrix, [101,102] have been extensively developed, aimed at addressing the dendrite issue and achieve a safe and stable lithium metal anode. The nucleation and growth of lithium dendrites is a complex dynamic process that is influenced by many factors, such as local current density, mechanical strength of SEI films, lithium nucleation location, electron/ion diffusion coefficient, etc. In the following discussion, the role of CNTs and graphene in influencing the growth behavior of lithium dendrites is discussed from three aspects: 1) change of local current density, 2) control of lithium nucleation, and 3) accommodation of lithium deposition Change of Local Current Density Sand s time is a widely accepted model to measure the time to initiate dendrite growth, [103] which is expressed as follows 0 τ = πd ec 2J 2 µ a + µ c µ a 2 where τ is the time when a lithium dendrite starts to grow, e is the electronic charge, C 0 is the initial concentration of lithium salt, J is the effective current density, and µ a and µ c are the cation and anion mobilities, respectively. D is the ambipolar diffusion coefficient: D = (µ a D c + µ c D a )/(µ a + µ c ), where D c and (4) D a are the cation and anion diffusion coefficients, respectively. The Sand s time model provides a quantitative understanding to describe Li dendrite formation and indicates that the time needed for dendrite growth is proportional to J 2. Moreover, Monroe and Newman [104] developed a comprehensive model to predict the tip growth rate (y tip ) of a dendrite, once initiated v tip = JV F where J is the effective current density normal to the dendrite tip, V is the molar volume of lithium, and F is Faraday s constant. This equation indicates that the dendrite tip growth rate would be slowed down with a reduced current density. Therefore, the electrode current density plays a crucial role in regulating the growth behavior of lithium dendrites, and a reduced local current density can significantly reduce or eliminate the possibility of lithium dendrite formation. In this respect, CNTs and graphene with high specific surface areas and a superior electrical conductivity can be used to change the local current density to a reliable value, and thus effectively suppress the dendrite growth. Zhamu et al. [105] reported the use of a graphene surfacesupported lithium metal anode to increase the anode surface area to reduce the anode current density, which dramatically prolongs the dendrite initiation time and decreases its growth rate. A copper metal based and a graphene-based electrode, with specific surface areas of and 890 m 2 g 1, respectively, were investigated for comparison. The effective current density J (ma cm 2 ) can be reduced by a factor of 10 4 with the use of a graphene-based electrode. According to Equations (4) and (5), the theoretical dendrite initiation time can be increased by a factor of 10 8, and the dendrite tip propagation speed can be reduced by a factor of 10 4, contributing to effective dendrite suppression. Zhang et al. [106] fabricated an unstacked graphene framework with a very high specific surface area (1666 m 2 g 1 ), pore volume (1.65 cm 3 g 1 ), and electrical conductivity (435 S cm 1 ) for lithium deposition. The high surface area of the graphene effectively reduces the local current density and inhibits the formation of lithium dendrites by controlling the lithium depositing morphology. During charging, lithium ions migrate through the SEI film and are homogeneously deposited on the graphene due to the ultralow local current density, forming (5) (13 of 22)

14 Figure 10. a) Schematic of a Li deposition/stripping process on one graphene flake. SEM images of a graphene-based anode b) before cycling, c) after lithium deposition, and d) after lithium stripping at a current density of 0.5 ma cm 2. Reproduced with permission. [106] Copyright 2016, Wiley-VCH. a sandwich-like core shell structure. During subsequent discharging, the deposited lithium strips from the interspace of the graphene and SEI, and a sandwich-like SEI-coveringgraphene structure remains (as illustrated in Figure 10a). SEM images of the graphene-based anode before cycling, after lithium deposition, and after lithium stripping at a current density of 0.5 ma cm 2 indicate that the unstacked graphene framework produces a dendrite-free morphology after Li deposition and a stable SEI layer after Li stripping (Figure 10b-d). At a high lithiation capacity of 5.0 ma h cm 2 and a high current density of 2.0 ma cm 2, a stable deposition/stripping morphology is maintained over 800 cycles Control of Lithium Nucleation Lithium dendrite growth is usually associated with uncontrolled lithium deposition, which is mainly induced by an inhomogeneous distribution of lithium ions on the electrode surface. Therefore, controlling the lithium nucleation behavior is essential for uniform lithium deposition which can help prevent dendrite growth. In this respect, building a lithium deposition matrix with active sites that have a good affinity for lithium ions may be a way to guide uniform lithium nucleation by chemical binding interaction, which is desirable for achieving a dendrite-free morphology. Mukherjee et al. [107] demonstrated a mechanism for the defect-induced plating behavior of metallic lithium inside a porous graphene network. Ab initio modelling was used to understand the role of defects in the graphene lattice, and the result showed that the defects act as seeding points to control uniform lithium nucleation, and the deposited lithium adsorbs strongly to the defects in stable configurations due to this effect. This entrapment of lithium metal results in uniform lithium deposition and enables the construction of highly stable anodes free from dendrite-related problems. Lin et al. [108] produced a layered Li-reduced graphene oxide (Li-rGO) electrode by molten lithium infusion into an rgo film with uniform nanogaps (Figure 11a). After lithium infusion, the layered structure of the rgo was maintained (Figure 11b,c), indicating good lithium wettability of rgo. Due to the existence of abundant nucleation sites for lithium in the rgo, the Li-rGO film remains smooth without observable cracks or dendrites on the surface after ten stripping/plating cycles at a relatively high current density of 3 ma cm 2, indicating homogeneous lithium nucleation and deposition. These characteristics give rise to stable cycling without lithium dendrite formation both in a symmetric-cell and full-cell configurations. Therefore, tunable surface functionalization of CNTs and graphene provides many opportunities for the design of a lithiophilic matrix for uniform lithium nucleation, which is effective in preventing the initiation of dendrite growth Accommodation of Lithium Deposition For a planar lithium metal anode, the initial plating of lithium on the planar substrate often results in inhomogeneous lithium particle deposition, and the inhomogeneities then serve as the nuclei tips for the growth of lithium dendrites. [101,109] In this regard, designing a matrix with abundant pores to accommodate the lithium deposition can minimize the possibility of undesired dendrite growth. In addition, the large volume change during lithium plating and stripping can also be relieved by the presence of a porous matrix. Jin et al. [110] reported a 3D current collector composed of CNT bundles hundreds of micrometers in length to accommodate the lithium deposition. The lithium metal deposition/ dissolution takes place in the spaces in the 3D CNT network rather than on the surface, resulting in suppressed lithium dendrite formation. Cheng et al. [111] synthesized a macroscopic graphene (14 of 22)

15 Figure 11. a) Schematic of the fabrication of a layered Li rgo hybrid film. SEM images of b) a layered rgo film, c) a layered Li rgo film, and d) the surface of a Li rgo film after 10 galvanostatic cycles at a current density of 3 ma cm 2. Reproduced with permission. [108] Copyright 2016, Macmillan Publishers Limited. framework with deposited lithium accommodated in the pores (Figure 12). The graphene framework is highly porous with a pore volume of 1.6 cm 3 g 1 and an average pore size of 10 nm. After lithium deposition, the pores between the graphene nanoflakes are filled by metallic lithium without any formation of lithium dendrites on the surface. After subsequent lithium dissolution, the nanoporous graphene framework is recovered with the 2D graphene partially exposed. It should be noted that the chemically derived graphene framework has a high density of structural defects, which is beneficial for uniform lithium nucleation as discussed in Section The graphene-based anode demonstrates a superior dendrite-inhibition behavior during 70 h of lithiation, while the cell with a copper foil-based metal anode was short-circuited after only 4 h of lithiation at 0.5 ma cm 2. Based on the above discussion, the advantages of a high specific surface area, high pore volume, and tunable surface Figure 12. Schematic of lithium deposition and dissolution in a graphene framework and the corresponding SEM images. Reproduced with permission. [111] Copyright 2015, American Chemical Society. functionality gives CNTs and graphene versatile functions for the rebirth of a lithium metal anode, which, also provide novel insight and understanding of the mechanism of the growth behavior of lithium dendrites Design of Integrated Cells Bendable and stretchable devices have emerged as an important branch of modern electronics. [14,25] A wide variety of applications, including implantable devices, touch screens, electronic skins, smart cloths, and stretchable displays, have been demonstrated. As a result, the corresponding powersupply systems in these flexible devices are also required to be flexible. It is worth noting that flexible electrodes require their components, including active materials and current collector to perform well without structural failure under deformation. However, conventional lithium battery configurations often involve brittle materials and metal current collectors, which cannot easily be assembled as flexible devices. Therefore, practical realization of flexible lithium batteries remains at an early stage. CNTs and graphene have remarkable advantages for building flexible lithium batteries due to their excellent mechanical properties and unique 1D and 2D structural characteristics. [14] First, the 1D and 2D nanostructures of CNTs and graphene allow them to have an extremely small radius of curvature, which gives high durability under bending conditions. Second, CNTs and graphene can be easily assembled into freestanding films or 3D structures, which provide many ways to fabricate different flexible composite electrodes. Third, the strong interactions between CNT bundles and graphene sheets can effectively maintain integrity during repeated bending or stretching conditions. They have therefore been widely studied for use in flexible lithium batteries. Generally, the role of CNTs and graphene for flexible lithium batteries has three aspects: a conductive phase, an independent current collector, and a flexible building unit, which will be discussed below (15 of 22)

16 Conductive Phases in Nonconductive Substrates Polymer substrates, such as polydimethylsiloxane (PDMS), [112] polyethylene terephthalate (PET), [113] polyvinylidene fluoride (PVDF), [114] cellulose paper, [115] and polyester fabrics, [116] are among the most widely used flexible supports for flexible devices due to their high elasticity. For example, PDMS has been reported to withstand elastic deformations up to 50%. However, polymer substrates are usually insulators and often need to be integrated with conducting additives to lower the resistance. CNTs and graphene with high conductivities and intrinsically good flexibilities have been incorporated into polymer substrates by many different methods, such as spraying, sputtering, and vacuum infiltration, with the aim of improving the conductivity of the flexible substrates and enhancing their electrochemical performance. [14] Lee et al. [112] reported the fabrication and characterization of highly porous, stretchable, and conductive polymer nanocomposites with embedded CNTs for use in flexible lithium ion batteries, and these are capable of good electrochemical performance with mechanical flexibility. However, the compatibility of most polymers with organic electrolytes may lead to the degradation of the polymer substrates, which remains a challenge and needs to be further addressed Independent Current Collectors It has been widely recognized that the use of metal current collectors is a major limiting factor in the production of flexible lithium batteries. As discussed in Section 3.2.2, CNTs and graphene-based current collectors have the advantages of high stability, low density, strong adhesion, as well as good flexibility, and have been used in both flexible electrodes and flexible full batteries. [92,93,117] For example, Li et al. [118] reported a thin, lightweight, and flexible LiFePO 4 /Li 4 Ti 5 O 12 full battery using 3D interconnected graphene foam current collectors that showed excellent flexibility with no structural failure after repeated bending to a radius of <5 mm (Figure 13a), and was able to power a red light-emitting diode when bent (Figure 13b). Moreover, the flexible battery showed good cycling stability between the straight and bent states, with negligible differences in the charge/ discharge curves (Figure 13c,d), indicating the high durability resulting from the 3D graphene foam current collector Flexible Building Units in Freestanding Composite Electrodes The preparation of conventional electrodes for lithium batteries involves a slurry-coating process using a mixture of Figure 13. Photographs of a) a bent LiFePO 4 /Li 4 Ti 5 O 12 full battery using 3D graphene foam current collectors and b) lighting a red LED device under bending. c) Galvanostatic charge/ discharge curves and d) Cycling performance of the full battery in straight and bent states. Reproduced with permission. [118] Copyright 2012, National Academy of Sciences of the USA. active materials, conducting additives and binders cast on current collectors. This configuration cannot really be bent due to the potential problems of cracks in the electrode layer and it peeling-off the current collector, especially when a metal current collector is used. Besides being used as current collectors, CNTs and graphene have been widely investigated as a building unit for freestanding composite electrodes that incorporate the active materials. [17, ] The high flexibility of CNTs and graphene enables the construction of a mechanically robust skeleton with high durability. In addition, this approach eliminates the use of current collectors, and conducting additives and binders, which results in a significantly decreased content of inactive components and contributes to the increase of energy density. Yuan et al. [122] reported a CNT-based freestanding sulfur electrode, with short multiwalled CNTs as a short-range electrical conductive framework for sulfur accommodation and superlong CNTs serving as both a long-range conductive network and an interlinked mechanical scaffold (Figure 14a) which gives it good flexibility (Figure 14b,c). With a moderate sulfur content of 54 wt% and a high sulfur loading of 6.3 mg cm 2, the freestanding CNT-based electrode retained a reversible capacity of 700 mah g 1 after 150 cycles at a current density of 0.38 ma cm 2. This indicates the dual role of CNTs in the construction of both an effective conducting network and a flexible building unit. While the intrinsic merits of CNTs and graphene in electrical, mechanical, and thermal properties make them compelling for use in flexible lithium batteries, further development is still needed with regard to improving their mechanical performance, processing properties, and integration within various flexible devices (16 of 22)

17 Figure 14. a) Schematic of the fabrication of a CNT-based freestanding sulfur electrode. b) Photographs of the CNT-based freestanding sulfur electrode in the flat and bent states. c) SEM image of the CNT-based freestanding sulfur electrode. Reproduced with permission. [122] Copyright 2014, Wiley-VCH. In this section, we have discussed how the regulating role of CNTs and graphene performs to influence the properties and performance of lithium batteries from the perspectives of fundamental electrochemical processes, electrode construction, and flexibility. The many compelling advantages of CNTs and graphene provide many ways to build better lithium batteries. It is worth noting that, in many cases, CNTs and graphene play multiple roles at the same time and the following are some examples. a) CNTs can provide abundant paths for both electron and lithium ion transport, while at the same time buffering the volume expansion of electroactive materials during cycling in the spaces between them. b) Functionalized graphene can be used as an interlayer to accommodate polysulfides and can also provide abundant sites for their immobilization 151. c) 3D CNT/ graphene foams with high specific surface areas not only serve to decrease the local current density, but also accommodate lithium deposition and alleviate dendrite growth. [123] d) When serving as a flexible building unit in freestanding composite electrodes, CNTs and graphene can also serve as electron- and lithium ion-transport media. Therefore, comprehensive consideration is needed to take full advantage of CNTs and graphene for future improved lithium battery performance. 4. Comparisons of CNTs or Graphene CNTs and graphene are both important allotropes of carbon and are quite similar in that they have high conductivity and excellent flexibility, due to the same basic structural unit: a hexagonal honeycomb lattice of carbon. Both have proven to play a significant regulating role in the properties and performance of lithium batteries. Nevertheless, different synthesis routes often lead to their having different chemical and physical properties with specific pros and cons for different requirements in lithium batteries. Their differences in the following aspects should be noted: 1) Basic Structures: CNTs and graphene have different geometries of 1D tubular and 2D planar structures, respectively. The 1D nanotubes and 2D nanosheets can both form interconnected structures that provides abundant electron paths. However, for lithium ion transport, they show remarkable differences. As discussed in Section 3.2.1, the planar structure of graphene impedes the passing of lithium ion through the nanosheet, especially at a high current density. [84,124] This leads to a negative effect on the reaction kinetics and thus results in a higher polarization, making graphene with a large planar sheets inappropriate for use as a conducting additive. Full graphene wrapping with the size of the graphene sheet larger than the particle surface area of the active material leads to steric hindrance for ion diffusion, while partial graphene wrapping of the active material provides a balance between increased electron transport and fast ion diffusion. This principle must be taken into consideration when considering the advantage of the high electronic conductivity of graphene. However, interconnected CNTs with large aspect (17 of 22)

18 ratios can facilitate fast transport of both electrons and lithium ions regardless of the size of the active material. 2) Surface Properties: Graphene tends to have many more surface functional groups and defects compared to CNTs, especially for those obtained from the exfoliation of graphite oxide. This provides graphene with a hydrophilic surface with abundant active sites, which has the following advantages: 1) providing sufficient immobilization sites for polysulfides because of strong polar polar surface binding; 2) enabling the nucleation and growth of active materials on the functionalized surface, which facilitates fast electron and ion transport due to strong interfacial binding; [125] and 3) regulating lithium nucleation behavior during the lithium plating process. However, the hydrophilic surface of graphene may have problems matching with organic electrolytes due to the risk of dissolution. [126] For CNTs, chemical vapor deposition is the dominant production method, and the obtained CNTs often have high purity and few surface functional groups. [85] 3) Surface Area: Graphene has a high theoretical specific surface area of 2630 m 2 g 1 and tends to have abundant pore structures compared to CNTs. A high surface area as well as abundant pore structures serves to facilitate the penetration of electrolytes, leading to improved lithium ion transport properties during electrochemical processes. For Li S batteries, a high surface area provides more active sites for polysulfide adsorption, which is desirable for improved cycling stability. 4) Electronic Conductivities: CNTs usually have a bigger advantage for situations that require high conductivities, such as conducting additives and current collectors. A large number of surface functional groups inevitably lead to a compromise in the electronic conductivity of graphene. For example, GOs are known to be insulators due to the presence of a large number of oxygenated functional groups, making CNTs preferable for the construction of a more efficient conducting network. Based on the above discussions, Figure 15 summarizes the comparisons of CNTs and graphene in basic properties and suitability for various applications in lithium batteries. The applications in conducting additives, current collectors, composite materials, and flexible devices involve both Li ion and Li S batteries, and the applications in polysulfide immobilization and interlayers involve only Li S batteries. It is worth noting that, instead of considering CNTs and graphene as competitors, their combined use sometimes shows advantageous synergistic effects. CNTs usually prefer to form bundles due to van der Waals interactions, while graphene tends to stack, both of which limit the extent to which their intrinsic properties and performance can be taken advantage of in lithium batteries. In this respect, constructing CNT graphene hybrid structures with CNTs serving as pillars within the graphene is an effective approach to make full use of their structural advantages. Indeed, CNT graphene hybrids have been effective in significantly improving the performance of both Li S and Li ion batteries. [ ] 5. Challenges and Prospects The use of CNTs and graphene as regulators in Li ion and Li S batteries has attracted tremendous research interest and the large number of papers on the topic signifies their importance. Their excellent properties (structural, electrical, thermal, and mechanical) show great promise for building better lithium batteries with high energy density, high power density, long cycle life, and high safety for next generation electrochemical energy storage devices. Although their high-performance characteristics far exceed those of commercially available lithium batteries, breakthroughs toward practical use have been rare owing to the lack of feasible techniques for large-scale production. More significance should therefore be given to filling the gap between laboratoryscale research and practical applications. Future directions and existing challenges for research are summarized as follows: 5.1. Reliable Mass Production Technologies As discussed in Section 2.2.5, economic cost is an important factor that needs to be taken into consideration in industrial production. The cost for the large-scale production of CNTs and graphene is still far from competing with commercially available carbons, [130] which presents a significant barrier to their large-scale implementation. In addition, the reduction of GO is one of the most widely used synthesis methods for graphene, which involves complicated procedures with the use of toxic and corrosive chemicals, leading to environmental concerns. [131,132] It is therefore essential to develop reliable mass production technologies for CNTs and graphene that are low in cost and environmentally friendly. Recently, Pei et al. [132] reported a scalable, safe, and green method to synthesize graphene oxide with Figure 15. Comparisons of CNTs and graphene in a) basic properties and b) suitability for various applications in lithium batteries. Blue represents CNTs and green represents graphene (18 of 22)