Scholarly article on topic 'Novel Graphene-Based Composite as Binder-Free High-Performance Electrodes for Energy Storage Systems'

Novel Graphene-Based Composite as Binder-Free High-Performance Electrodes for Energy Storage Systems Academic research paper on "Nano-technology"

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{"Energy storage" / Free-standing / Graphene / "Lithium battery" / Nanostructure / Supercapacitor}

Abstract of research paper on Nano-technology, author of scientific article — Qingyu Liao, Shuaixing Jin, Chengxin Wang

Abstract To meet the power demands of various electronic devices, high-performance electrodes have been designed for use with multiple energy storage devices, from lithium batteries to supercapacitors. Graphene has many unique morphological and structural features and is important for developing highly effective devices because of its use for electrode fabrication and as a backbone for hosting other electroactive materials. Therefore, we constructed two types of novel graphene electrodes with different structures that both can be used to make binder-free electrodes: vertically aligned graphene nanosheets and freestanding, flexible, transparent graphene paper. The focus of this review is on the synthesis and properties of these graphenes and the application of related hybrid structure electrodes by our research group. The performances of these electrodes indicate that they have unlimited potential for application in the next generation of electrochemical storage devices.

Academic research paper on topic "Novel Graphene-Based Composite as Binder-Free High-Performance Electrodes for Energy Storage Systems"

Accepted Manuscript

Novel Graphene-Based Composite as Binder-Free High-Performance Electrodes for Energy Storage Systems

Qingyu Liao, Shuaixing Jin, Chengxin Wang

Materiomics

PII: S2352-8478(16)30008-9

DOI: 10.1016/j.jmat.2016.09.002

Reference: JMAT 72

To appear in: Journal of Materiomics

Received Date: 29 February 2016 Accepted Date: 5 September 2016

Please cite this article as: Liao Q, Jin S, Wang C, Novel Graphene-Based Composite as Binder-Free High-Performance Electrodes for Energy Storage Systems, Journal of Materiomics (2016), doi: 10.1016/ j.jmat.2016.09.002.

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Graphical Abstract_Review Article

Novel Graphene-Based Composite as Binder-Free High-Performance Electrodes for Energy Storage Systems

x # X # x . #

Qingyu Liao,'' Shuaixing Jin,'' and Chengxin Wang*' '

xThe Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, *State Key Laboratory of Optoelectronic Materials and Technologies, #School of Physics Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People's Republic of China

We review the synthesis and properties of the vertically aligned graphene nanosheet and freestanding, flexible and transparent graphene paper, highlight their applications in Li-ion batteries, electrochemical double layer capacitors and pseudocapacitors.

Novel Graphene-Based Composite as Binder-Free High-Performance Electrodes for Energy Storage Systems

x # x # x . #

Qingyu Liao,'' Shuaixing Jin,'' and Chengxin Wang*' ' 'i> xThe Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, *State Key Laboratory of Optoelectronic Materials and Technologies, #School of Physics Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People's Republic of China

Abstract

To meet the power demands of various electronic devices, high-performance electrodes have been designed for use with multiple energy storage devices, from lithium batteries to supercapacitors. Graphene has many unique morphological and structural features and is important for developing highly effective devices because of its use for electrode fabrication and as a backbone for hosting other electroactive materials. Therefore, we constructed two types of novel graphene electrodes with different structures that both can be used to make binder-free electrodes: vertically aligned graphene nanosheets and freestanding, flexible, transparent graphene paper. The focus of this review is on the synthesis and properties of these graphenes and the application of related hybrid structure electrodes by our research group. The performances of these electrodes indicate that they have unlimited potential for application in the next generation of electrochemical storage devices.

Correspondence and requests for materials should be addressed to C. X. Wang.

Tel & Fax: +86-20-84113901 E-mail: wchengx@mail.sysu.edu.cn

1. Introduction

The demand for power sources of portable electronic devices is increasing due to the extensive use of e-readers, tablets, and smartphones in recent years.[1] To meet this need, small, light and efficient energy storage systems are required in daily life.[2] Among the numerous electric energy storage techniques, electrochemical devices, including lithium-ion batteries (LIBs) and supercapacitor (SCs), stand out due to their low cost, long lifespans, high energy-power density and good reversibility. [2] First produced by Sony in 1991, LIBs have been widely used in electronic devices, from mobile phones to personal computers. Known as the "rock chair" concept, the operation of LIBs involves the intercalation-deintercalation of Li ions to and from the electrodes to store and deliver electrochemical energy during the charge and discharge processes, respectively.[3] LIBs provide high energy density, enough to drive a vehicle such as the Model S by Tesla. However, SCs, also called electrochemical capacitors, excel in applications that require high power density, for example, in the emergency exit doors of the Airbus A380. Based on energy storage mechanisms, there are two types of SCs: electrical double-layer capacitors (EDLCs) and pseudocapacitors. EDLSs store electrical energy via the electrostatic accumulation of charges in the electric double-layer near electrode/electrolyte interfaces.[4] In contrast, pseudocapacitors take advantage of the reversible faradic reactions that occur at the electrode surface to store energy.[4] However, in recent years, LIBs with higher power densities than SCs and SCs with higher energy densities than LIBs have been reported.[1, 5] This indicates that the LIBs and SCs may have more potential applications than expected.

Due to its high conductivity, large surface area and flexibility,[6] graphene is a

promising material for building electrochemical electrodes.[7-9] Graphene can be

used as an active material alone[8, 10-12] or to construct hybrid electrodes with other

active materials in energy storage systems[1, 6, 13, 14]. However, in many cases,

polymer binders must be used to hold the electrodes together.[15-17] These binders,

without electrochemical activity, lower the specific capacitance of the electrode,

which is calculated based on the mass of the entire electrode. Moreover, binders affect

the physical contact and charge transfer between graphene and other active materials in the hybrid electrode, resulting in inferior performance.[18] In addition, the electroactive material can be directly attached to the conductive substrate after removing the binder; thus, the poor conductivity can be easily overcome.[19, 20] Therefore, binder-free electrodes based on graphene become very popular in recent years.[21-24]

To produce binder-free electrodes, two methods have been intensely studied. The first method is to directly deposit or grow material on the current collector. [19, 23, 25]. The current collector, or the substrate, is responsible for bringing the active materials together. The active material always contacts the current collector or the substrate tight enough such that they cannot be separated. The second method is to fabricate a free-standing electrode.[24, 26] The electrodes are designed as a free-standing network[14, 24] or as thin hybrid composite films that are usually fabricated via vacuum filtration.[18] Herein, we present an overview of two types of graphene that can be used to build binder-free electrodes: vertically aligned graphene nanosheets (VAGNs) and freestanding, flexible and transparent graphene paper (FFT-GP), combined with progress made by our research group for applications in the LIBs and SCs. VAGNs directly grow on the substrate, whereas the FFT-GP is a freestanding network. These two graphene materials serve not only as ion hosts alone but also as a backbone to hold other active materials for LIBs and SCs. We explore how these unique graphene nanostructures can improve the electrochemical performances of electrodes. We believe this work provides a foundation for the future evolution of the next generation of high performance electrochemical storage devices.

2. Graphene material

Although characterized by many advantages, conventional graphene has various limitations regarding its applications for energy storage. For example, normal graphene nanosheets (GNS) prepared by conventional methods tend to restack themselves during preparation and subsequent procedures due to the strong n-n interactions and van der Waals forces between the planar basal planes.[27] When

typical GNS are developed into hybrid electrodes with another electrode material (Si, SnO, MnO2, etc.), the stacking of graphene results in an irregular arrangement of the active materials, thus reducing the effective surface area of the composites and leading to a decrease in electrochemical performance, especially the specific capacity and capacitance. Therefore, non-restacking graphene-based electrodes aimed at high capacity and cycle stability must be built to overcome these limitations.[5, 27] However, the demand for transparent stretchable electronics is becoming more and more important for self-powered wearable optoelectronics.[9] These types of electronics do not require a capacity as high as conventional devices used to power smart phones, but do not function properly if they are not transparent and easily break if they are not flexible. However, most candidates from the graphene family do not meet this specific demand, such as VAGNs, which offer a high energy density but are opaque. Thus, a transparent and stretchable graphene material with enough capacity must be developed, and different graphene structures must be fabricated to meet different needs.

2.1. VAGN

To overcome the limitations of restacking conventional graphene, a class of graphene with networks of carbon nanosheets that are typically aligned vertically on the substrate have been considered for energy storage applications.[28] In this review, we refer to these nanosheets as VAGNs, which are also called vertically oriented graphene,[29] carbon/graphene nanowalls,[30, 31] and carbon/graphene nanoflakes[32]. First discovered in 1997,[33] most VAGNs are fabricated using a plasma-enhanced chemical vapor deposition system (PECVD).[28] Various plasma recourses have been employed, such as microwave plasma,[34] dc plasma,[35] and radio frequency plasma,[36] which all share a similar growth mechanism.[28] We used Li's work as an example to illustrate the typical monopoly and synthesizing process of VAGNs.[5] In Li's work, Cu foil was selected as the substrate, and CH4 was used as the carbon resource in the microwave enhanced plasma chemical vapor deposition(MPECVD) system. As shown in the scanning electron microscopy (SEM)

images in Figure 1a, the graphene sheet stood and formed an aligned vertically shape, with the bottom fixed to the surface of the substrate. The sheets were spaced at distances of approximately 1 |im, which was wide enough to prevent restacking. The transmission electron microscopy (TEM) images in Figure 1b and 1c show three to five layers of graphene sheets with an intermediate distance of 0.364 nm. The Raman spectrum inset in Figure 1a shows a single, sharp 2D peak at 2700 cm-1 with a graphitic G-band at 1583 cm-1 and a D-band at 1353 cm-1. The ID/IG intensity ratio of the D-band and G-band (ID/IG), which is widely used to analyze graphitic structures, was calculated as 0.98, indicating that the VAGNs possessed a nanocrystalline structure.[37] The size of the nanosheet, the space between each nanosheet, the number of the graphene layers and the graphitic structures of the VAGNs can all be controlled by the conditions of the synthesizing process, such as pressure, temperature and flow velocity.[28]

The VAGN growth mechanism is believed to have three steps,[38-40] as shown in Figure 2. First, a graphite base layer is formed parallel to the substrate surface. This layer may have irregular cracks and dangling bonds, which can serve as nucleation sites for VAGN growth. Second, influenced by stress or localized electric fields, graphene nanosheets grow vertically on the substrate with carbon atoms continuously incorporated into open edges. Third, the growth of the VAGN stops with closure of the open edges, as shown in Figure 2c and d. Notably, VAGNs can be synthesized on various substrates, such as Ni foam[27] and carbon fabric[1], which have analogous morphologies. This characteristic allows VAGNs to have a wide range of applications beyond energy storage, such as field emitters[41-43] and methanol oxidation.[44]

Most processes used to obtain VAGNs from plasma systems are far more complex than the processes used to obtain graphene oxide produced via the modified Hummers method, which can readily be purchased.[15, 45] This may limit the industrial applications of VAGNs. Recently, Zhu et. al. reported a novel and versatile salt-templating strategy for the synthesis of VAGNs without the aid of PECVD system.[46] In Zhu's work, 1-ethyl-3-methylimidazolium dicyanamide (Emimdca) was mixed with a salt mixture of ZnCl2/KCl and then the slurry was coated on a

nickel foam substrate, followed by heat treatment under an Ar atmosphere. The as-made VAGNs revealed thin nanosheets with lateral dimensions of 100-200 nm. This type of VAGN demonstrates has a morphology that is similar to graphene produced via PECVD system. The production method can easily be explored for manufacturing because only a heat treatment is required, representing a new route for producing VAGNs in addition to the PECVD system.

2.2. FFT-GP

Transparent and stretchable electronics are critical components for power-integrated stretchable systems, ranging from self-powered rolled-up displays to wearable optoelectronics. Moreover, materials in the graphene family can have more potential if freestanding, which indicates that they could easily be transferred to various substrates for numerous applications. Much effort has been made to reach these goals.[47-50] For example, Chen et al. made flexible and freestanding graphene foam via template-directed chemical vapor deposition that were neither transparent nor stretchable[24]. Kim et al. synthesized large-scale stretchable transparent graphene films, but a substrate was needed to support the films.[22] Nevertheless, these methods do not produce films that meet the required freestanding, transparent and stretchable characteristics. Recently, Li et al. of our group made an FFT-GP that met these requirements by using prism-like graphene (PLG) as a building block.[9] This new freestanding carbon material is expected to be suitable for fabricating transparent and stretchable supercapacitors with outstanding electronic and mechanical properties. The FFT-GP synthesis process is illustrated in Figure 3a-e. Similar to VAGNs, FFT-GP were fabricated using an MPECVD system. As shown in Figures 3 c and d, the NaCl powder melted and recrystallized on the Si substrate, forming a prism-like structure by controlling the direct current (DC) bias and the plasma in the chamber. Graphene blocks synthesized on these unique NaCl crystals and formed a similar prism-like structure. Then, these prism-like graphene blocks grew to form a graphene film on the substrate, as shown in Figure 3e. After the

formation of PLG, the NaCl was etched by the plasma, leading to a final product

without NaCl. The SEM images (Figures 3h and i) of the FFT-GP demonstrated that the PLG building blocks were connected face to face and were homogeneously aligned on the silicon substrate. The FFT-GP can be fabricated as large as 10*85 mm , as shown in the photograph in Figure 3f. Figure 3g shows an example of the transplant and stretchable characteristics of the FFT-GP. Notably, the FFT-GP is a free-standing film that can be used to fabricate a binder-free electrode, in contrast with other graphene-block network structures.[51]

3. Applications in SCs

As mentioned above, SCs have been divided into EDLCs and pseudocapacitors by using different mechanisms. Thus, we discuss the applications of VAGNs and FFT-GP in two sections.

3.1 Applications of EDLCs

The large specific surface area (SSA) of EDLCs is essential because the surface area in contact with the electrolyte determines the amount of charge that can be stored.[52, 53] Thus, graphene is considered a perfect candidate for EDLCs due to its theoretical SSA of 2,630 m /g and theoretical specific capacitance of 550 F/g.[53-55] Because VAGNs and FFT-GP have SSAs of ~1100 m2/g[56] and 909 m2/g[9], respectively, they are both good candidates for use as electrodes in EDLCs.

3.1.1 VAGNs for EDLCs

Seo et al. fabricated VAGN electrodes that exhibited a high specific capacitance of 230 F/g at a scan rate of 10 mV/s and maintained over 99% of the initial capacitance after 1,500 charge/discharge cycles at a high current density.[57] This result matches the previously reported values of 205 F/g for graphene oxide[58] and 264 F/g for exfoliated graphite oxide[59] but is still much lower than the theoretical value of 550 F/g. This discrepancy could be ascribed to the unique structure of the VAGNs and involves the calculation of the theoretical specific capacitance of graphene. The value of 550 F/g was obtained using the following equation [53-55]:

Ct=SSA*Cs (1)

where Ct is the theoretical capacitance per gram of graphene, SSA is the specific surface area, and Cs is the theoretical capacitance of graphene per square centimeter.

Cs was determined as 21 |iF/cm based on quantum theoretical measurements in an aqueous system where one side of a graphene sheet was exposed to electrolyte.[55, 60] When the SSA is 2630 m2/g, the Ct is ~550 F/g. If each standing sheet of VAGN is a monolayer, the SSA of the VAGN may be doubled, resulting in a Ct of ~1100 F/g. In addition, the VAGN has many edge planes, which may induce extra capacitance.[56, 57] However, not all of the VAGN is a monolayer, and the planar graphene at the bottom that is used to connect each sheet must be calculated.[38] Thus, the SSA of the VAGN is ~1100 m /g, which is much lower than the theoretical value of 2630 m /g for planar graphene. As a result, the theoretical capacitance of the VAGN can be approximated as ~230 F/g, indicating that Seo 's work made full use of the VAGN.

Significant effort has been paid to increasing the capacitance of the VAGN electrode. Yen et al. doped nitrogen into VAGN-like graphene to build an electrode with a specific capacitance of 991.6 F/g, which was higher than the capacitance of conventional VAGN.[23] The graphene used in their work differed from that used in the VAGNs discussed here because epitaxial growth was used to fabricate vertically aligned SiC nanosheets with a similar morphology to VAGNs. The authors also noted that even though both electrodes had similar SSAs, the N-doped graphene had a specific capacitance that was more than three times greater than the specific capacitance of pristine graphene without N-doping. We believe their work provides evidence that authentic VAGNs could have higher capacitances when they are doped with other atoms.

3.1. 2FFT-GP for EDLCs

To fabricate transparent, stretchable electronics, Li et al. of our group assembled

two pieces of FFT-GP electrodes to create all-solid-state binder-free EDLCs, as

demonstrated in Figures 4a and b.[9] The device maintained a transmittance of 46.5%

at a wavelength of 550 nm, as shown in Figure 4c. The specific capacitance was 3.3

mF/cm at 0.02 mA/cm , with a volumetric specific capacitance of up to 3.096 F/cm . These values are all higher than previous reports of transparent, flexible supercapacitors based on carbon materials.[10, 61, 62] The device also exhibited good stability under long-term cycles, with 95.4% retention of the initial capacity after 20,000 cycles at 0.3 mA/cm (Figure 4f). Notably, the device demonstrated stability when stretched or bent. The galvanostatic charging/discharging curves remained almost the same when the devices were stretched from 0% to 38% strain (Figure 4f). The capacitances remained nearly unchanged after hundreds of cycles of stretching up to a fixed deformation with 20% strain, as shown in Figure 4f. As Figures 4e and f, the device responded very little to bending angles from 30° to 180° in the bending test and only lost 8% of its initial capacity after 1200 bending cycles at 30°. In summary, the freestanding FFT-GP-based EDLCs device exhibited outstanding electrochemical performance for maintaining stable transparency, stretchability, and bendability,

3.2 Applications of pseudocapacitors

Pseudocapacitors can take advantage of the reversible faradic reactions that occur at electrode surfaces and offer better electrochemical performances than EDLCs, especially specific capacitance.[4] For example, the theoretical specific capacitance of Co3O4 is ~3560 F/g, which is much higher than that of graphene of 550 F/g.[1] Many transition metal oxides (TMOs), such as MnO2,[63] RuO[64] and NiO,[65] have been used to build pseudocapacitors. However, in many cases, the poor conductivities of TMOs limit electrode performance.[14, 66] Thus, hybrid electrodes combing TMOs with a highly conductive substrate, such as carbon nanotubes or graphene, offer an effective solution to tackle this problem.[67-69]

Therefore, VAGNs have been used to build binder-free electrodes for pseudocapacitors. In this case, the VAGN serves as the backbone to hold the TMO. Liao et al. of our group fabricated a hybrid binder-free electrode with manganese monoxide (MnO) and VAGN.[27] In this report, MnO nanoparticles with a grain size of ~20 nm were deposited on the VAGN using a facile hydrothermal method and

manganese acetate (Mn(CH3COO)2). As the SEM images show in Figure 5c, the unique architecture constructed by the VAGNs were not broken after the hydrothermal procedure, and the VAGN/MnO composites manifested an analogous morphology with the VAGN. As shown in Figure 5d, electrodes with MnO mass contents of 37% 80% and 90% were fabricated, which exhibited high Csp values of up to 790 F/g, 381 F/g and 286 F/g at 2 mV/s, respectively. The high MnO mass loading content of the 80% electrode achieved a good capacitance retention of 80% after 4000 cycles at 10 A/g, as shown in Figure 5e. The above specific capacitances were all higher than the literature values at similar mass contents and scan rates.[14]

The outstanding electrode performance can be ascribed to the unique structure of VAGNs. As illustrated in Figures 5a and b, this method differed from the conventional method using planar graphene with a two-dimensional (2D) or paper-like structure because the composites of the MnO and VAGN provided high surface/body ratios and facilitated H+ transfer. Moreover, the VAGN was able to hold more MnO particles due to its two exterior surfaces on both sides of each sheet. Thus, electrodes with high MnO mass loading exhibit good performance, illustrating that VAGNs serve as excellent backbones for collecting electrons.

We mentioned that VAGNs can grow on many substrates with similar morphologies, thus new characteristics can be introduced to VAGN-based electrodes created using various substrates. Recently, carbon fabric- or carbon fiber (CF)-based electrodes have been intensively used to build supercapacitors, which can be easily bent or twisted.[70-73] In addition, conventional supercapacitors with a liquid electrolyte, such as Na2SO4, H2SO4 or KOH solutions, are not feasible for flexible electronic or wearable electronic devices. In this type of supercapacitor, the electrodes are directly immersed in solution, making it very difficult to shelter the other elements from the liquid electrolyte.[8] Therefore, an all-solid-state supercapacitor device was introduced for flexible and wearable electronic devices.[8, 73, 74] The electrolyte used in all-solid-state capacitors was based on polyvinyl alcohol (PVA), which is not as mobile as the solution and is flexible after drying.[75]

Liao et al. also fabricated a VAGN/Co3O4/CF hybrid supercapacitor electrode

that achieved both high capacity and flexibility. [1] At the first step, the VAGNs were synthesized on the surfaces of each carbon fiber, as shown in Figures 6a and d. Second, Co3O4 nanoparticles were deposited on the VAGN via a facile hydrothermal method, as shown in Figures 6c and e. The TEM image shown in Figure 6f reveals that the Co3O4 nanoparticles at a size of ~5 nm were anchored to both sides of the VAGNs and that the graphene sheet exhibited a small amount of deformation among the crowded nanoparticles, indicating good connection between the nanoparticles and the graphene sheets. Three electrodes with low, median and high Co3O4 mass loading contents were made, achieving the highest Csp values of 3482 F/g, 1828 F/g and 1330 F/g at 1 mV/s, respectively (Figure 6g). The Csp of 3482 F/g was near the theoretical value (3560 F/g) and was higher than the values reported in previous reports,[76-78] demonstrating that the VAGN with the carbon fabric served as an excellent backbone and current collector for the faradic reactions.

For further investigation, an all-solid-state symmetric supercapacitor device was fabricated by assembling two pieces of the as-made VAGN/Co3O4/CF electrodes, which illuminate a light-emitting diode (LED) after charging to 2 V, as shown in Figures 7a and b. The device delivered high Csp values of 580, 450, 350, 300, 238, and 196 F/g at discharge current densities of 1, 2, 5, 10, 20 and 40 A/g, respectively, as listed in Figure 7e. These specific capacitance values are higher than values reported in the literature for symmetric all-solid-state supercapacitors.[75, 79] The device reached various energy densities for various power densities of 33 Wh/kg at 10 kW/kg, 41.6 Wh/kg at 5 kW/kg and 48.6 Wh/kg at 2.5 kW/kg. A maximum energy density of 80 Wh/kg was achieved at a power density of 0.5 kW/kg, and the highest power density of 20 kW/kg was achieved at an energy density of 27 Wh/kg. These values are superior to the previously reported symmetric system [14, 73, 79].

The device exhibited a capacitance retention of 86.3% after 20,000 cycles at 20

A/g, as shown in Figure 7d. Moreover, as a characteristic of carbon fabric, the

supercapacitors can be easily bent or twisted. As Figure 7c indicates, the device

demonstrated excellent flexibility without a significant sacrifice in electrochemical

performance when bent to 150 degrees. In short, due to the architecture of the VAGN

and the bendable characteristics of the CF-based all-solid-state supercapacitor, the C03O4/VAGN/CF hybrid structure electrode has outstanding electrochemical performance and great potential for applications in energy management for flexible and lightweight electronics.

4. Applications in LIBs

It is well known that graphene-based hybrid anodes can improve the performance of LIBs. However, it is unclear which hybrid nanostructures incorporate graphene most efficiently. We believe VAGNs have the potential to yield the best electrode. As mentioned above, based on the conventional method, the anode material and graphene are mixed with a binder, which is not an active ingredient in LIBs. Moreover, the materials that are mixed with graphene suffer severe pulverization due to the high volumetric change during the lithiation and delithiation process, which leads to particle pulverization and destabilization of the solid electrolyte interphase (SEI) film.[80, 81] VAGNs represent a new route for overcoming these limitations due to their unique dispersed stand-up nanosheets. In this section, we review step-by-step VAGN-based LIBs regarding various anode materials with four sections that are based on the complexity of the nanostructure, and we discuss how VAGNs take advantage of their nanostructure for LIBs by comparing them with other conventional binder-on electrodes with analogous nanostructures. We denote the letter "M" as the non-carbon electroactive material in the hybrid electrode, such as Sn, Si and ZnO2.

4.1 VAGN as the Li ion host alone

The carbon materials can be used to build LIB electrodes without other active

materials[82, 83], which indicates that the VAGNs can serve as the Li ion host

without another material. Xiao et al. used VAGNs with nickel foil as the substrate for

LIBs.[84] A reversible capacity of ~380 mAh/g could be reached and maintained for

150 cycles. However, because a single layer of graphene can host two times the

amount of Li ions than conventional methods, graphene is believed to have a

theoretical capacity of 744 mAh/g, which is twice the capacity of graphite (372 mAh/g)[12, 53, 85]. Xiao et al. achieved a capacity near the theoretical capacity of graphite but far lower than the theoretical capacity of graphene. This may be because the VAGN was not a monolayer, which is also why VAGNs cannot approach the theoretical specific capacitance in EDLC applications. In light of the capacity performance, VAGNs are more like perfect graphite than perfect graphene. Recently, carbon-based LIBs with much higher capacities, even greater than 744 mAh/g, have be reported. Wu et al. fabricated nitrogen- and boron-doped graphene, which both exhibited high reversible capacities of more than 1040 mAh/g at a low rate of 50 mA/g[86]. This work inspires us to further study VAGNs as a single Li ion host. Thus, VAGNs may exhibit better performance if the graphene is doped or decorated.

4.2 VAGN/M hybrid electrode

Group IV elements (Si, Ge and Sn) and transition metal, such as Mn and Zn, and their oxides have been introduced for constructing electrodes for large-capacity LIBs because these materials have higher specific capacities than carbon materials.[80, 87-89] For example, Si has a theatrical specific capacity of 4200 mAh/g, which is ten times higher than the capacity of graphitic carbon (372 mAh/g).[90] However, these materials suffer severe pulverization, which leads to inferior cycling stability[80]. Thus, significant effort has been dedicated to developing hybrid electrodes.[15, 91-93] The most facile way to make a hybrid electrode is to mix M and carbon materials together with a binder.[94-96] VAGNs can accomplish this more efficiently to build binder-free electrodes. As illustrated in Figure 8b, the VAGN-based anodes allow for more rapid ion and electron transport pathways compared to the conventional structures shown in Figure 8a. Moreover, the rate capacity can be enhanced by overcoming the restacking of graphene planes. Based on this method, Li et al. of our group deposited SnO2 and ZnO nanoparticles onto VAGN electrodes, yielding specific capacities of 1055 mAh/g and 810 mAh/g at 80 mA/g[5, 97]. Here we use the SnO2/VAGN as an example to illustrate this kind of hybrid electrode. As shown in Figures 8c and d, SnO2 nanoparticles 2-5 nm in diameter were deposited on the both

sides of VAGNs via a hydrothermal treatment. Due to the unique structure, this electrode exhibited excellent cycling stability, with a capacity loss of 5.2% after 120 cycles, and retained a reversible capacity of 210 mAh/g at 9 A/g after 5,000 cycles.

Not only the nanoparticles, but also the nanoflakes can be deposited on to VAGN. Jin et al. (in our group) prepared unique VAGN@amorphous GeOx sandwich nanoflakes electrode that exhibited excellent electrochemical performance. [98] A layer of amorphous GeOx with a thickness of 5-6 nm was deposited on both sides of the prepared VAGNs via a thermal Ge/Sn co-evaporation method, as shown in Figure 9a. As shown in Figure 9c, the GeOx was more like a thin film that wrapped both sides of the VAGN. The VAGN separated the active materials and prevented aggregation, as expected. The isotropic amorphous GeOx layer (Figure 9b) released the stress and absorbed the volume changes during the cycles. As a result, the VAGN@amorphous GeOx electrode demonstrated a stable capacity of 1008 mAh/g at C/3 for 100 cycles, with a retention capacity of 96%.

4.3 VAGN/M@Graphene hybrid electrode

To eliminate the effect of volume change, many works have developed LIBs with core-shell structures.[99-105] In these reports, the active material is always coated with carbon, which forms a core-shell structure. Thus, the electrolyte cannot contact the active material directly, and the Li-ion is transferred though the carbon coat/shell. This core-shell suppresses aggregation and buffers volume expansion.[101] Jin and Li built VAGN hybrid core-shell electrodes with Ge and Sn nanoparticles, both with high reversible capacities of ~1000 mAh/g.[106, 107] We use Jin's work as an example to illustrate how to take advantage of this unique structure. Jin et al. synthesized a Ge@graphene/VAGN electrode with a core-shell structure via two steps.[106] First, a layer of GeO2 film was deposited on the VAGN by using a thermal GeO2/Sn co-evaporation method. Second, GeO2 was reduced to Ge by H2 in the MPECVD chamber, followed by the deposition of graphene by CH4. As Figures 10a and b show, Ge nanoparticles with an average size of 42 nm were wrapped in 2~3 layers of graphene and highly dispersed across the surface of the VAGN. This electrode

exhibited a high capacity of 1014 mAh/g, nearly no capacity loss in 90 cycles and a good performance rate of 420 mAh/g at 13 A/g. To investigate the structure changes upon the cycling test, Jin observed the electrode in a fully delithiated state. As Figures 10c and d show, the Ge@graphene/VAGN was kept, but the single crystalline Ge nanoparticle was changed to a polycrystalline structure with the diameter increased from 42 to 51 nm. Figure 10d demonstrates that the integrity of the electrode was maintained and the graphene coating remained closely wrapped around the Ge nanoparticles, indicating that the coating layer synchronously expanded/shrank with Ge expansion/contraction and successfully accommodated the particle volume. Therefore, this structure leads to excellent LIB performance.

Li et al. of our group built a VAGN-based electrode with a more complicated nanostructure than in Jin's work: Si nanoparticles@graphene nanosheets-graphene trees (SiNPs@GNS-GrTr). [34] As illustrated in Figures 11a and c, the VAGN backbone was further developed into graphene trees, and the Si@graphene core-shell units were loaded onto the graphene trees and connected to each other. The synthetic process of this complex structure is facile and occurs during the MPECVD system. After the synthesis of VAGNs on Cu foil, the silicon nanoparticles and the graphene trees were deposited on the VAGNs in turns with the aid of SiH4 and CH4, respectively. Three rounds of deposition allowed the graphene to form a tree-like structure, and the SiNPs were loaded onto both sides of the VAGNs with a thin layer of graphene nanosheet encapsulation, as shown in Figure 11b. Compared to Jin's work of Ge@graphene/VAGN, [ 106] this SiNPs@GNS-GrTr structure clearly holds more active material per unit area, and the connection between each core-shell is far more efficient in the three dimensional continuous conductive network of the graphene leaves, which shortens the electron transport path and facilitates Li ion diffusion. As a result, the composite exhibited a high capacity (1528 mAh/g at 150 mA/g), good cycle stability (88.6% retention after 50 cycles) and a fast charge/discharge rate (412 mAh/g at 8 A/g).

Notably, Li gives more details of the change in the nanostructure during cycling. First, a thin SEI film formed on the GNS coatings due to the decomposition of the

electrolyte during the cycling process. Then, the SiNPs@GNS structure remained integrated for at least 10 cycles, as shown in Figure 12a. As the 50th cycle approached, the SiNPs cores had pulverized into smaller cores but remained in the GNS shells, as shown in the TEM images in Figure 12b. The pulverization with volume expansion created some void space in the core-shell structure, and the GNS shells were gradually enlarged by the expansion of the SiNP cores, eventually partly detaching from the graphene sheet. Even at this moment, the initial GNS shell was still well preserved. Finally, the GNS shells enlarged and were broken by the 100th cycle (Figure 12c). Thus, the small pulverized SiNPs from different core-shells united and aggregated, resulting in the degradation of cycle performance after 100 cycles.

4.4 VAGN/M@ carbon nanotubes

Li's work on SiNPs@GNS-GrTr[34] showed that the graphene shell may not maintain its shape during long-term cycling. The volume change and pulverization finally break the shell, even though the graphene coat synchronously expands/shrinks. Thus, many studies have focused on creating a void space inside the shell for the volume variation of the active material in LIBs, such as the yolk-shell structure.[108-111] Li et al. used carbon nanotubes (CNTs) and VAGNs to build a yolk-shell-like nanostructure with enough space for Sn nanoparticle expansion[38]. As Figure 13a illustrates, SnO2 nanoparticles were first deposited on VAGNs via a hydrothermal process. Second, the SnO2 were reduced completely to Sn by H2 in the MPECVD system. Third, the CNTs were grown via the catalysis of the Sn nanoparticles, and finally, many Sn@CNTs composites were dispersed onto the VAGN surface (Figure 13b and c). The Sn nanoparticles filled 40%-50% of the space inside the CNTs on average, as shown in Figure 13d. This novel Sn@CNTs-VAGN electrode exhibited a high reversible capacity of 1026 mAh/g at 0.25 C for more than 280 cycles and a capacity of 140 mAh/g at a discharge time of 12 s.

Li further studied the electrode after 280 cycles at a fully charged state, as shown

in Figure 14. The Sn cores were pulverized into smaller particles with a volume

expansion but remained encapsulated in the CNT shell. The SEI layer formed in the

expanding state remained unbroken, serving as a protective membrane to buffer the large volume change during cycling. The biggest difference with the SiNPs@GNS-GrTr structure is that the initial Sn@CNT structure was well-preserved and appeared to remain in good contact with the VAGN. This phenomenon showed that the Sn core did not enlarge to break the CNTs shell due to the space in the CNTs even after 280 cycles, leading to better stability during the charging and discharging process.

4.5. Brief Summarization for LIBs

To mitigate the high volumetric change during the lithium insertion/extraction process, VAGNs were introduced to create a hybrid structure of the LIBs binder-free anode. The VAGNs remained standing and separated during the cycling process, thus preventing the aggregation of active materials. The graphene nanosheets tightly contact the current collector and align in a unified orientation, which shortens the electron transport and facilitates the diffusion of Li ions.

Other methods have been developed to avoid capacity fading. By depositing nanosized particles, electrodes can alleviate mechanical strain. By forming an amorphous layer to release stress isotropically, the volume changes can be absorbed. A more effective method is to encapsulate the electroactive material with a carbon layer. The carbon layer has enough elasticity to provide good contact with the anode material during the volume change, thus maintain the mechanical integrity of the composite electrode. However, this layer may be broken by repeated expanding/shrinking during long term cycling. Thus, a void space is needed for volume expansion. This void space, which may be constructed by CNTs, accommodates volume change and protects the electrode structure from breaking, leading to better stability during the charging and discharging process.

Additionally, we list the performances of VAGN-based electrodes in Table 1 compared with the previous reports, which revealed electrodes that share similar nanostructures without VAGNs. The VAGN-based electrodes offer better properties than their counterparts for most of the electrochemical performance tests. Notably, the

comparison reveals the progress made in the field of nanoscience rather than simply competing electrodes. After years of research, new designs and materials are sure to be discovered to replace former designs and materials, and each step is based on previous work. Only in this way can we move closer to the ideal and perfect nanostructured material for energy storage applications.

5. Conclusion

In summary, graphene-based binder-free electrodes exhibit remarkable performance for energy storage systems. VAGNs serve as an excellent backbone for constructing hybrid electrodes. First, VAGNs maintain their unique 3D structure during both the synthesis and test processes, thus preventing the aggregation of active materials. Second, the tight contact between VAGNs and the current collector shortens the electron transport and facilitates the diffusion of ions, whether in LIBs or SCs. Moreover, VAGNs have more surface active sites than conventional planar graphene due to the two exterior surfaces on both sides of each sheet. This makes it possible to hold more electro-active particles without increasing thickness, which always leads to a decrease in electrochemical activity. In addition, FFT-GB-based EDLC devices maintain transparency, stretchability and bendability without sacrificing electrochemical performance. Overall, high-performance novel graphene-based binder-free electrodes have great potential in energy storage system applications, ranging from conventional high-capacity devices with long life cycles to next-generation flexible, lightweight and transparency electronics.

A cknowledgments

This work was financially supported by the National Natural Science Foundation of

China (No. U1401241 and No. 11274392).

Table captions

Table 1 Comparison of electrochemical performance of VAGN-based electrodes

Structure Main electro active material Highest capacitances obtained Capacity retention Rate performance Ref

VAGN Graphene 380 mAh/g at C/3 [84]

M/VAGN / J

ZnO-VAGN ZnO NPs with diameters of 3~6 810 mAh/g at 80 94% after 105 cycles 201 mAh /g" at 6.4 A/g [112]

nm mAh/g

* Sandwich-structured ZnO NPs with diameters of 5~10 556 mAh/g after 100 250 mAh/g at 2 C [113]

graphene-ZnO nm cycles

SnO2-VAGN. SnO2 NPs in diameters of 2~5 1055 mAh/g at 94.8% after 120 210 mAh/g at 9 A/g [5]

nm 80 mA/g, cycles

*SnO2-GO SnO2 nanocrystals smaller than 5 800 mAh/g at more than 90% 80% retention of initial [15]

nm 100 mA/g retention after 200 capacity at 10 A/g after

cycles 1000 cycles

VAG@ amorphous GeOx amorphous GeOx with 1008 mAh/g at 96% after 100 cycles 545 mAh/g at 15 C [98]

thicknesses of 5~6 nm 0.33 C

* Bean-like GeO2 particles with lengths of 1021 mAh/g 94.3% after 200 730 mAh/g at 5 C [114]

GeO2/graphene 400 to 500 nm and diameters of cycles at 0.2 C

200 to 300 nm

VAGN/M@graphene

Ge@ graphene/VAGN Ge NPs with an average 1014 mAh/g at 97% in 90 cycle 420 mAh/g at 13 A/g [106]

diameter of 42 nm 260 mA/g

*Ge@C/RGO Ge NPs wrapped within carbon -938 m A h/g at retain ~940 mAh/g at 380 mAh/g after 50 cycles [102]

shells of 10-15 nm 100 mA/g 50 mA/g after 50 cycles at 3.6 A/g

Sn-NP@GS-VAGN Sn NPs with a size of -25 nm 1005 m Ah/gat 0.25 C 95% after 130 cycles 510 mAhg/at 3 C and 450 mAh/g at 6 C [107]

*Sn@C-GNs Sn nanoparticles with diameters from 50 nm to 2 |j,m with a carbon shell of -10 nm 1069 m Ah/gat 75 mA/g 566 mAh/g after 100 cycles 286 mAh/g at 3750 mA/g [115]

SiNPs@GNS-GrTr Si NPs with a size of -15 nm 1528 mAh/gat 150 mA/g 88.6% after 50 cycles 412 mAh/g at 8 A/g [34]

*Silicon@carbon Silicon with sizes of 20-180 nm 1290 mAh/g at 50 mA/g 97% after 30 cycles 450 mAh/g at 1000 mA/g [116]

M®,CNTs/VAGN

Sn@CNTs-VAGN Sn core with diameters from 30 to 50 nm 1026 mAh/gat 0.25 C 280 cycles 164 mAh/g at 300 C [38]

*Sn@CNTs Sn-CNT nanocapsules drop from 850 mAh/g to 600 mAh/g during 10 cycles [117]

The reports with * are used for comparison with similar nanostructures but do not incorporate VAGNs and thus a binder is needed in these reports.

Figure caution

Figure 1. Morphological and structural characterization of pure VAGN: (a) low-resolution SEM image of graphene nanosheets grown on Cu copper substrate, (b) low resolution TEM image of an intact graphene sheet, and (c)-(d) HRTEM images of a single graphene sheet. The inset in (a) presents the Raman spectrum of pure VAGN. Adapted with permission from ref [5]. Copyright 2014, Elsevier.

Figure 2. SEM analysis of the growth mechanism of VAGN. (a) In a first step, a graphite base layer is deposited parallel to the defect-rich substrate surface. Ridges formed a net on the thin graphite film surface. Some defects were formed due to the H impinging. (b) The upward curling ridges and defects are the nucleation sites for the VAGN in the second step. The growth direction is radically changed from parallel to vertical to the substrate surface. (c)- (d) The third step includes the accumulation and incorporation of carbon radicals at the edges and sides of the nanosheets. Adapted from ref [38] with permission from The Royal Society of Chemistry.

Figure 3 Scheme for the synthesis of freestanding, flexible, transparent graphene paper (FFT-GP) . (a) Si substrate, with a bulk NaCl polycrystal in the center, is placed into the chamber. (b) Bulk NaCl gradually melts, and NaCl ions diffuse into the mixed plasma. A graphene base layer rapidly forms on the silicon substrate. (c) DC bias is applied, causing the NaCl to completely collapse and form a thin wet layer on the graphene base layer film. (d) DC bias is turned off, and prismlike graphene (PLG) begins to form along with the recrystallization of wet NaCl layer into crystal grains. (e) Final product, FFT-GP, forms as a coating on the Si substrate. (f) Photograph of a selfsupporting, 1.2 cm wide, 8.2 cm long graphene sheet that has been hand drawn from a graphene film on a silicon substrate. (g) Photographs, acquired with a 60° angle between the FFT-GP and the underlying logo, illustrating the transparency of the sheet from various angles by demonstrating the visibility of the Sun Yat-Sen University logo placed behind the graphene sheet. (h) Low-resolution SEM micrograph of an FFT-GP sheet. (i) SEM micrograph showing the cross section of an FFT-GP sheet. The region marked in red is the reconstructed topology of the corresponding connections of the PLG cells in the FFT-GP. Adapted with permission from ref [9]. Copyright 2015 American Chemical Society.

Figure 4 (a) Scheme for the fabrication of a flexible solid-state supercapacitor based on FFT-GP. (b) Photograph of a transparent supercapacitor based on FFT-GP. (c) Transmittance of pure PDMS, FFTGP on PDMS and an assembled supercapacitor. (d) Galvanostatic charging-discharging curves of the supercapacitors with different tensile strains at 0.1 mA cm- . (e) Galvanostatic charging-discharging curves of the

supercapacitors with different curvatures at 0.1 mA cm- . (f) Cycle stability of the

devices at a current density of 0.3 mA cm- for alternating releasing, bending and stretching states during the cycling process. Adapted with permission from ref [9]. Copyright 2015 American Chemical Society.

Figure 5 Schematic illustration of the fabrication process and architecture of (a) an electrode made from planar graphene sheets and MnO nanoparticles, and (b) an electrode made from the VAGN and MnO nanoparticles. (c) low- and high resolution SEM images of MnO-90%. (d) Specific capacitance vs. scan rate for VAGN and various VAGN/MnO composites. (e) Cycling stability of MnO-80% and MnO-90% at 10 A g-1. Adapted from ref [27] with permission from The Royal Society of Chemistry.

Figure 6 Schematic illustration of the fabrication process and architecture of (a) carbon fabric, (b) the deposition of VAGN onto the carbon fabric, and (c) the deposition of Co3O4 nanoparticles onto the carbon fabric with VAGN. (d) Low-resolution SEM images of the VAGN, (e) high-resolution SEM image of Co3O4-H. (f) high-resolution TEM images of the VAGN and Co3O4 particles. (g) specific capacitance vs scan rate for various Co3O4 composites. Adapted with permission from ref [1]. Copyright 2015 American Chemical Society.

Figure 7 (a) Optical photographs of the fabricated solid-state supercapacitor device. The square in the left image indicates the capacitance region. Candle oil is used to prevent the electrolyte from immersing the electrode beyond the capacitance region. The right image demonstrates the flexibility of the device. (b) Four supercapacitor units are connected to light a LED light. (c) CVs for the device at various bending angles. (d) Cycling stability of the device at 20 A/g. The inset shows the galvanostatic charge/discharge curves of the last 10 cycles. (e) Specific capacitance vs current densities for the supercapacitor device. Adapted with permission from ref [1]. Copyright 2015 American Chemical Society.

Figure 8. Schematic depiction of insertion/extraction principle of Li+ in cells with (a) SnO2-GNS anode produced by conventional brushing process and (b) SnO2-VAGN anode directly grown on current collectors. (c)low resolution SEM image of SnO2-VAGN grown on Cu copper substrate. (d)HRTEM images of a single SnO2 -graphene layer. Adapted with permission from ref [5]. Copyright 2014 Elsevier.

Figure 9 (a) High-magnification SEM image of the VAG@GeOx; (b) High-resolution TEM image of the GeOx sediments on graphene surface, inset is the corresponding SAED image; (c) HRTEM image of the flake edge, clearly shows the sandwich structure of graphene wrapped by GeOx. Adapted with permission from ref [98]. Copyright 2013 Elsevier.

Figure 10 (a) Panoramic view of the as-prepared Ge@graphene/VAGN; (b) HRTEM image of a single Ge NPs, the graphene layers are calculated to 3; Postcycling morphologies of the Ge@graphene/VAGN electrode (c) Low-magnification SEM image;. (d) HRTEM of a particle, inset is the fast Fourier transform. Adapted with permission from ref [106]. Copyright 2014 American Chemical Society.

Figure 11 (a) Schemes for the volume changes of the SiNPs@GNS-GrTr anode during the cycling process. (b) The high resolution TEM images of SiNPs encapsulated in a few-layer graphene shell. (c) low resolution SEM image of SiNPs@GNS-GrTr. Adapted with permission from ref [34]. Copyright 2014 Elsevier.

Figure 12 Morphology and structure analysis of SiNPs@GNS-GrTr post cycling. (a)-(c) TEM images of some typical area with a few SiNPs@GNS particles anchored on VAGN at the fully charged state (delithiation) after 10 cycles. (b) and (c) TEM images of typical single SiNPs@GNS particles acquire at the fully charged state (delithiation) after 50 cycles. The initial Sn@GB structure was well-preserved, silicon particles pulverized into smaller ones which were still encapsuled in the graphene wraps. (d) and (e) Low and high resolution TEM images of some typical SiNPs@GNS particles anchored on GrTr at the fully charged (delithiation) state post 100 cycles. The elastic graphene shell was enlarged and the pulverized SiNPs were gradually aggregated. Adapted with permission from ref [34]. Copyright 2014 Elsevier.

Figure 13 (a) Model of Sn@CNT-VAGN growth process. (b) low resolution TEM images of a graphene nanosheet anchored with Sn@CNT-VAGN composites, the inset (c) HRTEM images shows the Sn@CNT composite grown on VAGN surface. (d) High resolution TEM images of Sn@CNT. Adapted from ref [38] with permission from The Royal Society of Chemistry.

Figure 14 (a) Model of Sn@CNT-VAGN cycling process (b) Structure analysis of Sn@CNT-VAGN post cycling: (a) low resolution TEM image of a typical Sn@CNT composite structure anchored on VAGN at a fully charged (delithiation) state. The initial CNT shell remained intact except for the enlarged size, while the Sn core pulverized into smaller ones which were stillencapsulatedd in the CNT shell. (c) a typical TEM image of Sn@CNT composite structure. The CNT shell, Sn core and newly generated SEI film are all presented. Adapted from ref [38] with permission from The Royal Society of Chemistry.

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Author Profile

Qingyu Liao: received his Bachelor's degree and PhD degree in School of Physics Science and Engineering from Sun Yat-Sen University in 2010 and 2016 respectively. He is now a postdoc with Prof. Chengxin Wang at Sun Yat-Sen University, majoring in Materials Physics and Chemistry. His research focuses on the applications of nanomaterials in energy storage devices.

Shuaixing Jin: received his Bachelor's degree and PhD degree in School of Physics Science and Engineering from Sun Yat-Sen University in 2010 and 2015 respectively. His research interests mainly focus on the synthesis of nanomaterials and their applications in energy storage.

Chengxin Wang: is currently a professor in the School of Physics Science and Engineering, Sun Yat-Sen University. He received his BSc, MSc and PhD degree from Jilin University in 1992, 1999 and 2002 respectively. From 2003 to 2005 he conducted postdoctoral research in the School of Physics and Engineering, Sun Yat-Sen University. Subsequently, he was awarded a JSPS fellowship by the Japan Society for the Promotion of Science (JSPS) in 2005.

In 2007 he came back to work in China. He obtained Distinguished Young Scientist of National Natural Sciences Foundation of China in 2011. He was granted the second prize of National Natural Science Award in 2011. Dr. Wang's research interest lies in developing both theory and experiment investigation on the nanomaterials of IV main group element and related compound. His current research focuses on nano energy materials.