Emergence of graphene as a promising anode

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Materials Today Chemistry 11 (2019) 225e243

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Emergence of graphene as a promising anode material for rechargeable batteries: a review M.R. Al Hassan*, A. Sen, T. Zaman, M.S. Mostari Department of Glass & Ceramic Engineering, Rajshahi University of Engineering & Technology, Bangladesh

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 July 2018 Received in revised form 10 October 2018 Accepted 14 November 2018

Very recently, graphene is extensively investigated as anode material for rechargeable lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) because of its amazing superlative properties. With the nanostructural evolution of graphene, its electrochemical performances as well as other properties enhance to a new degree. The authors introduce the individual component, i.e. graphene as anode in LIB and SIB, and thereon evaluate how advanced nanostructures elevate synergistic effect when graphene and metal oxides/sulfides put together to develop nanocomposite and nanohybrid structure. At last, this review aims to extract out the most promising nanostructured anodes as well as pointing out the challenges to make it scalable. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Lithium-ion battery Sodium-ion battery Electrochemistry Electrode materials

1. Introduction The energy and storage sector of today's world, despite remarkable advances, is consistently facing challenges in terms of the performance, functionality, and durability of the fundamental materials. Researchers are relentlessly working to find the possible solutions for these challenges. To approach the demand of electrochemical energy storage, especially for portable devices, Lithium (Li)-ion rechargeable batteries were introduced into market since a long period. Sony for the first-time commercialized lithium-ion battery (LIB) in 1991 [1]. Devotion to rigorous efforts and researches enabled LIB to play a leading role in the portable secondary battery market. Also, sodium-ion battery (SIB), more lately under extensive research, is opening the possibilities of initiating its commercial journey as a substitution of LIB. Moreover, the emergence of carbon-based nanomaterials, such as carbon fiber, carbon nanotube (CNT), nanoribbons, graphene, etc., as battery electrode have advanced this research field toward studious stage [2]. Among them, graphene possesses most potentials to face the challenges of the energy and storage sector [3].

* Corresponding author. E-mail addresses: [email protected] (M.R. Al Hassan), senaungkan@gmail. com (A. Sen), [email protected] (T. Zaman), [email protected] (M.S. Mostari). https://doi.org/10.1016/j.mtchem.2018.11.006 2468-5194/© 2018 Elsevier Ltd. All rights reserved.

The wonder material graphene is actually one-atom thick sheet of carbon, arranged in a sp2-bonded hexagonal network (Fig. 1) [4]. Since its discovery in 2004 by the groundbreaking work of Nobel laureates Geim and Novoselov, numerous applications have been proposed depending on its wide range of properties [5]. Researches on graphene have been going on in all the fields because of to its outstanding mechanical [6], optical [7], electrical [8], thermal [9], and sensing [10] properties. Extraordinary electron mobility (2.5  105 cm2 V1 s1) and outstanding surface areas (2630 m2 g1) of graphene make it a key material for energy and storage sector [3,11,12]. The functionality of graphene has been checked by the researchers around the globe for versatile applications including its usage as electrode (anode/cathode) [13] and electrolyte [14] for rechargeable batteries. For enhanced battery performances, the design of promising anode material acts as a one of the key factors. Researches performed on employing graphenebased anode for LIBs and SIBs are arising the feasibility of using it in rechargeable batteries with enhanced performances. Despite having such potential, the future of commercial scale synthesis of graphene is still in a challenging stage [15,16]. A large of amount of high-quality graphene production is a hope for the future. Therefore, extensive studies are going on worldwide for the aid of commercializing this miracle material. We believe that future of the electrochemical performances of rechargeable batteries will be highly dependent on advanced electrode structure design as well as its collective properties. In this study, our goal is to look into the development of promising

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Fig. 1. Graphene: mother of all carbon nanostructures. (a) Wrapped up into OD fullerenes, (b) rolled into 2D CNTs, and (c) stacked into 3D graphite. OD, one-dimensional; 2D, twodimensional; 3D, three-dimensional; CNT, carbon nanotube. Reproduced from Ref. [4].

graphene-based LIB and SIB anode materials, i.e. graphene and modified graphene nanostructures, graphene nanocomposites, and graphene nanohybrids. Subsequently, we will evaluate how advanced nanostructures affect the electrochemical properties of anode. Finally, the authors would remark the most potential nanostructures of graphene-based anodes that might be scalable in future and point out the challenges that still persist to be exceeded reviewing relevant researches. 2. Graphene production methods at a glance It is a matter of fortune that the structure of graphene exists naturally in our surroundings, to be exact in graphite which is abundant around the globe [17,18]. Therefore, we do not need to create graphene structure, rather need to exfoliate it from graphite. Novoselov and Geim are the first to exfoliate graphene back in 2004. They used scotch tape method to peel a layer of graphene from highly ordered pyrolytic graphite sample [5]. Though this mechanical means of graphene production yield a product of highest quality but cannot be used commercially because of the limitation of bulk production [19]. Since then, various methods of graphene production have been discovered. In general, there are two approaches, i.e. top-down and bottom-up approaches. Topdown approach involves the exfoliation of graphene from a precursor material usually graphite. Mechanical method [19], chemical method [20], electrochemical method [21,22], etc. are various topdown approaches. Among them, chemical method gained huge attention because of its advantages of moderate quality and quantity production at low cost and ease in synthesis procedures. Usually in this method, graphite is oxidized to produce graphene oxide (GO) by chemical means and then subsequently reduced into

graphene [20]. This reduction can be done by various techniques, i.e. thermal [23], chemical [24], solvothermal [25], photocatalytic [26], etc. In recent years, several modifications have been done in other top-down approaches. Bottom-up approaches involve the growth of graphene on a precursor substrate. Chemical vapor deposition (CVD) [27], epitaxial growth on SiC [28], pyrolysis [29], etc. are the various bottom-up approaches of graphene synthesis. Very high-quality graphene layers can be found by the CVD and epitaxial growth methods, and therefore, these two methods are widely used in laboratory for graphene research purposes. There are some other unconventional techniques. Unzipping CNT [30], production using various hydrocarbon precursor [31], using food waste [32], electronic waste [33], etc. are some unconventional techniques of graphene production reported by the researchers around the world. 3. Applications in rechargeable batteries 3.1. Graphene as anode material in LIB Investigating advanced devices and related materials for storing and producing electricity is a key issue to meet the increasing global energy demand. Li-based rechargeable batteries are considered as a very effective representatives for its higher energy density, faster charge/discharge rate, and more durable cycling performance to meet this challenge [34]. Rechargeable LIBs have been used widely in portable electronics, such as cell phones, laptops, tablets, digital cameras, etc., and are believed to be promising choices as energy-effective and environment friendly devices. Generally, rechargeable LIB is composed of anode, electrolyte, and cathode as shown schematically in Fig. 2. On charging stage, Li

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Fig. 2. Schematic illustration of a rechargeable lithium battery composed of cathode, anode, and electrolyte. Reproduced from Ref. [35].

ions are extracted from the cathode material, passed through the electrolyte, and inserted into the anode material. Discharging is just the reverse process. As the further recharging is completed with the Li-ion insertion/extraction process in the electrodes, the nature of the two electrode materials is crucial to the performance of the battery [35]. The current electrode materials employed in LIB are Li intercalation compounds such as graphite anode and LiCoO2 cathode [36], as they show successful reversible charging/discharging under intercalation potentials. Though graphite as battery anode material shows better coulombic efficiency, it still lags behind in higher Li storage capacity (theoretical value: 372 mAh g1) to be compatible as anode in enhanced power storage devices [35]. Therefore, much higher charge storage capacity with promising cyclic stability and rate capability anode in LIB application research has drawn intensive attention. Here in this review, we will focus on the appearance of graphene-based anode material with its nanostructural evolution to be used as beneficial anode in rechargeable batteries. For a better comprehensive study, we have categorized the graphene-based anode materials as (i) graphene and modified graphene nanostructure anode, (ii) graphene-based nanocomposite anode, and (iii) graphene-based nanohybrid anode. The first category anode material covers the discussion of one or few atomic thick graphene sheets, helical tubes of sp2-bonded graphene nanosheet (GNS) also known as CNTs, GNS strips (width less than 50 nm) known as graphene nanoribbons (GNRs), defective GNSs, wrapped up zero-dimensional graphene sheets defined as fullerenes, etc. as anode application in LIB. The other two classes, graphene nanocomposites and nanohybrids, are used assumed as same type material and often used ambiguously in literature. To resolve this inconsistency, we have segregated them into two different categories, i.e. graphene-based nanocomposite and graphene-based nanohybrid. Graphene nanocomposite are those materials where atomic or molecular level mixture of graphene and other materials form these types of materials. These are multiphase materials where the properties of different phases are added up. On the other hand, graphene nanohybrid has been defined as atomic or molecular level mixture of graphene and other materials with chemical bonds existing between them [37]. The presence of

chemical bond offers better electrochemical functions in many cases over nanocomposite class, where simply mixture forms without chemical bonding. 3.1.1. Graphene materials as anode for LIBs Investigation of graphene as anode material is disclosing promising results with ahead of time. Some of its very much appealing properties such as superior electrical conductivity; high surface area (2620 m2 g1); high surface-to-volume ratio; ultrathin thickness, which can shorten the diffusion distance of ions; structural flexibility, that paves the way for constructing flexible electrodes; thermal and chemical stability, etc. guarantee its durability in harsh environments [38]. One of the pioneering works of increasing Li-ion storage capacity for LIB anode materials was done by Liu et al. [39]. They successfully accommodated two Li atoms on the both sides of graphene single layer maintaining stoichiometry of Li2C6 having specific capacity of 540 mAh g1 better than Li-intercalated graphite (LiC6-372 mAh g1). Enhancement of GNS’s specific capacity came from a later research by Yoo et al. [40]. The study showed the increment in specific capacity by GNSs interdistance change. By the addition of CNT and fullerene(C60) with GNS, the increased specific capacity of 730 mAh g1 & 784 mAh g1, respectively, was found [Fig. 3]. Besides the interlayer spacing, there are other key structural parameters, such as significant disorder/defects in GNS, high specific surface area and remaining surface functional groups, etc., that strongly affect the storage capacity of GNSs. Such type of structural parametersetuned GNSs were prepared via hydrazine reduction, low temperature pyrolysis, and electron beam irradiation by Pan et al. [41]. They declared that disordered GNS could be used for their enhanced reversible capacities (794e1054 mAh g1) in high-capacity LIBs because of additional reversible storage sites such as edges and other defects. But for better performance, specific capacity and cyclic stabilities are also needed to be considered. Hence the capacity increments further motivated Bhardwaj et al. Their notable study came later in 2010 [42], improved Li storage capacity using carbonaceous one-dimensional GNRs. They extracted GNRs by unzipping pristine multiwalled carbon nanotubes (MWCNTs). The authors showed oxidized GNSs outperformed in

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terms of energy density than all other materials tested (GNSs and MWCNTs). Oxidized GNRs had the first charge capacity of ~1400 mAh g1 with a low coulombic efficiency for the first cycle (~53%) and reversible capacity in the range of 800 mAh g1.

Fig. 3. Lithium insertion/extraction properties of the GNS families. (A) Charge/ discharge profiles of (a) graphite, (b) GNS, (c) GNS þ CNT, and (d) GNS þ C60 at a current density of 0.05 A/g. (B) Charge/discharge cycle performance of (a) graphite, (b) GNS, (c) GNS þ CNT and (d) GNS þ C60. GNS, graphene nanosheet; CNT, carbon nanotube; C60, fullerene. Reproduced from Ref. [40].

3.1.2. Graphene-based nanocomposite materials as anode for LIBs Graphene-based nanocomposite materials are multiphase materials where the graphene remains as matrix phase. Noticeable findings in terms of higher electrochemical characteristics have been reported exclusively in literature performing exploration on graphene composite materials. Most of the composite materials developed ever are integration of metal, metal oxide, and sulfide nanoparticles into graphene-based materials in which their electrochemical performance as anode materials for LIBs was considerably enhanced. Wu et al. in a detailed review article denoted the superiority (in terms of electrochemical features) of graphene/ metal oxides composites of versatile types, such as anchored, wrapped, encapsulated, sandwiched, layered, and mixed models, (Fig. 4) over the individual constituent [43]. For example, graphene anchored with Co3O4 nanoparticles [44], Si nanoparticles highly dispersed between graphene sheets [45], GNSs decorated with Fe3O4 particles [46], monodispersed SnO2 nanoparticles onto single-layer graphene sheets [47], graphene-wrapped sulfur particles [48], Si/graphene paper [49], CuO nanoparticles/reduced graphene oxide (rGO) sheets [50], anatase grade TiO2/graphene [51], 3D architecture MoS2/graphene [52], and nanostructured rGO, rGO/ Fe2O3 [53] are the few graphene-based composite materials already researched as anode of LIBs in the near past. All these and many others that are not reported here raised the feasibility of graphene composite to be an attractive LIB anode material for their excellent electrochemical performances. In this study, a focused view will be delivered on the significant graphene composite materials explored yet just to mark their superlative electrochemical features resulted from innovative structural design and synthesis. Let us have an interesting example here that is exploring layered MoS2/graphene composites by Chang and Chen [52]. They developed few graphene composite samples remarking as MoS2/G (1:1), MoS2/G (1:2), and MoS2/G (1:4) of

Fig. 4. Schematic of graphene/metal oxide composite's models: (a) anchored model, (b) wrapped model, (c) encapsulated model, (d) sandwich-like model, (e) layered model, and (f) mixed model. In the structures, red balls are metal oxide particles and blue scheme is graphene sheets. Reproduced from Ref. [43].

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Fig. 5. Microstructure of MoS2/G (1:2) composite: (a) SEM image, (b) TEM image, (c) cycling behavior of samples after annealing in H2/N2 at 800 C for 2 h at a current density of 100 mA/g: (1) MoS2; (2) MoS2/G (1:1); (3) MoS2/G (1:2); and (d) MoS2/G (1:4). (b) Rate capability of MoS2/G samples at different current densities: (1) MoS2/G (1:1); (2) MoS2/G (1:2); and (3) MoS2/G (1:4). Reproduced from Ref. [52].

varying Mo:C molar ratios of 1:1, 1:2, and 1:3 respectively. The study reported highest specific capacity of 1571 mAh g1 at a current of 100 mA/g measured from MoS2/G (1:2) because of its spherical 3D morphology (Fig. 5a), rendering larger interior spaces with very good cyclic stability and rate capability (Fig. 5c and d). Clearly, it is much better to substitute MoS2/G anode in lieu of MoS2 anode where specific capacity lowers down to 256 mAh g1 after 100 cycles and cyclic stability decreases because of MoS2 nanoparticles aggregation during lithiation process. Zhu et al. demonstrated a facile two-step synthesis technique of graphene/Fe2O3 composite by homogeneous precipitation of FeCl3 in a suspension of graphene oxide platelets with urea, with subsequent reduction of the graphene oxide with hydrazine to yield rGO platelets decorated with Fe2O3 nanoparticles [53]. They dispersed the Fe2O3 nanoparticles uniformly in rGO nanoplatelets that offer upper electrical conductivity and enough Li-ion insertion and extraction sites for superior performances. Raman spectrum of this composite revealed the well distribution of free Fe2O3 nanoparticles on RG-O surface. Higher specific capacity (1693/1227 mAh g1 first discharge/charge capacity at 100 mAh g1), good cyclic stability, and efficient capacity retention were observed of this composite in comparison to the individual pure rGO or Fe2O3 nanoparticle constituent (Fig. 6aec). Moreover, the intimate interactions between the Fe2O3 particles and rGO nanosheets are very important for structural integrity and cyclic stability. Mere physical mixture of Fe2O3 nanoparticles with as-prepared rGO results in lower stability than graphene/Fe2O3 composite (Fig. 6d). Other researches performed later also revealed similar findings that represents the property enrichment toward composite structure formation. For example, Mai et al. reported CuO/graphene composite prepared by in situ chemical synthesis approach by

dispersing CuO nanoparticles on hydrazine-treated graphene oxide [50]. In this composite, they found homogeneous distribution of CuO granules on GO sheets from scanning electron microscope (SEM) and detected lower crystallization of CuO nanoparticles than individually prepared CuO through X-ray diffraction (XRD) characterization. Here, the smaller size retention of CuO is relatively easy because of the high-density presence of oxygen functional groups, including carboxylic, hydroxyl, and epoxy groups on GO surface that hinder diffusion, crystallization, and growth of CuO grains. Therefore, the GO sheets with high electrical conductivity gives conducting network for fast electron transfer between the active materials and charge collector, as well as act as buffer to accommodate the volume expansion/contraction during discharge/ charge process. These synergistic effects improved electrochemical features showing initial coulombic efficiency (ICE) (68.7%) and reversible capacity of 583.5 mAh g1 with 75.5% retention of the reversible capacity after 50 cycles. Similarly, in 2014, Choi and Kang prepared crumpled graphene/MoO3 composite by spray pyrolysis and subsequent annealing at 300 C [54]. They concluded its performance as improved specific capacity (1490 mAh g1), higher coulombic efficiency, better cyclic stability, and increased rate capability in comparison with bare MoO3 powders (Fig. 7). As LIB anode material, Si nanoparticles alone also has been studied for its highest known theoretical charge capacity (4200 mAh g1) and low discharge potential (~0.5 V vs. Li/Liþ) [55,56]. Herein at the same manner, its effective application in LIB cannot be implemented because of its huge volumetric expansion (~270%) [57] from electrochemical reactions and rapid capacity fading. Therefore, few more works have been explored to show graphene incorporation as matrix phase with Si-based nanoparticles to overcome these shortcomings [49,58]. For instance,

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Fig. 6. Electrochemical performance of the rGO/Fe2O3 composite. (a) Discharge/charge profiles of rGO/Fe2O3 composite for the first cycle at the current density of 100 mA/g. (b) Cycling performance of rGO/Fe2O3 composite at the current density of 100 mA/g. (c) Rate capacity of rGO/Fe2O3. (d) Capacity retention of free Fe2O3 nanoparticles physically mixed with rGO platelets at a current density of 100 mA/g. rGO, reduced graphene oxide. Reproduced from Ref. [53].

Fig. 7. Electrochemical performances: (a) initial charge/discharge curves at a constant current density of 2 Ag1, (b) CV curves, (c) cycle performances at a constant current density of 2 Ag1, and (d) rate performances. Reproduced from Ref. [54]. CV, capacity vs voltage.

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Lee et al. designed Si nanoparticlesegraphene sheet (Si/GS) nanocomposite by graphene sheet sandwiching that forms conductive 3D graphite network with accommodation of welldispersed Si nanoparticles inside the graphene layers [45]. They assessed higher storage capacity ~2200 mAh g1 after 50 cycles and better cyclic stability ~1500 mAh g1 after 200 cycles. Modification of Si/GS structure came as report in 2016 by another group. They applied ex situ carbon coating gel and carbonization technique to thermally reduced 2D planer Si/GS structure and resolved three-dimensional Si/graphene sheet/carbon (Si/GS/C) nanocomposite with enough void space inside it [56]. The group showed superior specific capacity ~1350 mAh g1 and better cyclic stability (~600 mAh g1 after 200 cycles) than Si/GS in the basis of long cycle performances [Fig. 8]. Another very recent study, in 2017 by Yi et al. [59], showed effective design and synthesis of SiC/ graphene nanocomposite where SiC nanoparticles are homogeneously embedded in the GNSs by using soda papermaking block liquor raw material via in situ thermal chemical method at 800 C. They proposed easier SiC/graphene anode synthesis route with enhanced electrochemistry due to shortened Li-ion diffusion path. The report measured reversible capacity of 1044 mAh g1 at 100 mAg1, outstanding cycling stability of 230 mAh g1 after 1000 cycles at 1 Ag1, and high rate performance (Fig. 9). Another contemporary work by Shi et al. [60] exclusively developed vertical graphene-encapsulated SiO microparticles (d-SiO@vG) by direct CVD process [Fig. 10]. Their stable interconnected vertical graphene-conducting network formed through encapsulation notably improved the cycling stability in high mass loading SiO anodes as well as exhibited outstanding capacity of 1600 mAh g1 with 93% capacity retention after 100 cycles. Nanostructure amendment of Si/graphene nanocomposite by surficial carbon coating also found much attention from researchers. Li et al., in 2013, reported carbon-coated Si nanoparticle/ graphene composite (Si@C/G) synthesized (Fig. 11) by combining freeze-drying of an aqueous mixture of GO, sucrose, and Si nanoparticles with following thermal annealing [61]. Here, the carbon coating of Si nanoparticles reduces direct contact between Si and the electrolyte, and buffer volume changes during charging, leading to high coulombic efficiency and improved cycling stability; the well-dispersion of Si in graphene sheets facilitates the high electrical conductivity, resulting in high capacity, good cycling stability, and superior rate capability. Another study by Ma et al., in 2017 reported enhanced electrochemical performances exploring carbon nanofibers intertwined graphene/silicon (G/Si@CFs) structure [62]. In this study, initially prepared Si/graphene composite was coated by highly conductive carbon nanofibers networks represented in their SEM and transmission electron microscope (TEM) morphologies (Fig. 12). Consequent electrochemical features of this structure are initial discharge capacity of 1792.1 mAh g1, 86.5% capacity retention after 200 cycles, and high rate performance valued 543 mAh g1 capacity at the rate of 1000 mAg1. Therefore, effective Si/

Fig. 8. Cycling performances of Si/GS, Si/GS/C-P, and Si/GS/C-S. Reproduced from Ref. [56]. Si/GS, Si nanoparticlesegraphene sheet.

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Fig. 9. High rate capability of graphene/SiC@800 C between 0.01 V and 3.00 V at a current density of 1, 2, 5, 10, 12, 15, 20 A g1. Reproduced from Ref. [59].

graphene-based nanocomposite design might be those structures where graphene remains as continuous conductive network to deliver fluent passage of Li ions to active materials and buffer the volume change of Si-based materials. In turn, Si-based nanoparticles distributed homogeneously with high surface area should be served as host sites of Li insertion/de-insertion. It has been already highlighted from literature that the nanosized metallic oxides, sulfides, and carbides are superior to their bulk counterparts in terms of large electrode/electrolyte contact area, abundant Li-/ sodium (Na)-ion insertion/extraction sites, and short ions diffusion path [63]. Nevertheless, anode composed of these materials alone suffers from aggregation and pulverization results from volume changes during charge/discharge cycling phenomena as we mentioned before. Use of graphene as a matrix phase to disperse and confine such metallic oxides and sulfides active materials because of its 2D layered structures, large surface areas, and superior electrical conductivity is very much effective. Despite having these advantages, graphene matrix further faces the problem of layer restacking in cycling operation [64]. Significant mitigation of this constraint has been reduced by employing the concept of synthesizing 3D graphene/CNT nanocomposite. Shen et al., in 2011, constructed a hierarchical structure of TiO2GNSCNT nanocomposite by well-distribution of TiO2 on graphene and CNT surface (Fig. 13) following a facile solution-based route [65]. In this 3D structure, TiO2 nanoparticles serve as host for fast and efficient Li storage sites whereas the CNT embedded in graphene surface form 3D conductive networks beneficial for higher diffusion kinetics as well as high rate performance. Significantly stabilized 3D composite structure with reduced graphene layer and TiO2 nanoparticles aggregation shows enhanced cycling performance (8.7% capacity loss after 100 cycles). Later in 2015, Zhang et al. developed similar 3D hierarchical structure of graphene-CNT matrix where SnS2 nanosheets are dispersed and confined within the conductive grapheneCNT composite frameworks [64]. They explained the electrochemical outperformance of this nanocomposite anode in comparison with bare SnS2, likewise the former TiO2-GNSCNT nanocomposite anode. Therefore, reserving interior void space in nanocomposite structure to accommodate volume change and to deliver faster diffusion kinetics is a key factor to have ultralong life and superior rate capability. Zhao et al. designed graphene multiwalled CNTs (MWCNTs)@SnO2@C (Scheme Fig. 14), where mesopores (2 nme3.8 nm) and broad pores (4.3 nme100 nm) exist to lodge

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Fig. 10. (a,b) Schematic design of vertical graphene encapsulated silicon-based microparticles. (c) TEM image of connected particles of d-SiO@vG. (d) Magnified image of the selected area in white box of panel c. Reproduced from Ref. [60].

the SnO2 nanoparticles volume expansion [66]. According to the design expectation, this composite exhibited first discharge capacity of 1562 mAh g1 and long-term cyclic stability at a high rate of 1.6 Ag1 up to 1300 cycles. So, reasonably it can be said that the 3D graphene conductive network inheriting sufficient void space will be focused more in future research as LIB anode material to achieve better electrochemical performances specially for higher rate capability and ultralong cyclic stability. Hopefully, all these recent studies hurriedly drive the future toward industrially scalable routes and promising features of graphene-based LIB anode fabrication with low cost and application in next generation rechargeable batteries. 3.1.3. Graphene-based nanohybrid materials as anode for LIBs In graphene-based nanohybrid materials, where constituents combine chemical bonds generation in nanometer level results in newer properties not necessarily found in individual components. Many latest results have been outlined exploring graphene-based hybrid materials as LIB anode. Attempts taken to accommodate metallic oxides and sulfide-types nanoparticles in well-dispersed manner on GNSs with strong structural integrity got much attention for elevated electrochemistry. A notable study of Wang et al. [67], reported in 2010, experimented Mn3O4 nanoparticles/rGO hybrid as graphite substituted anode in LIB. In this nanohybrid, Mn3O4 nanoparticles anchored

covalently on reduced graphene oxide (rGO) to form a Mn3O4rGO hybrid material. In terms of electrochemical features, it showed stable specific capacity of ~900 mAh g1 higher than individual Mn3O4 nanoparticles anode. The intimate interaction between the graphene substrates and the Mn3O4 nanoparticles enabled in conducting charge carriers from and to Mn3O4 nanoparticles through the highly conducting 3D graphene network. Therefore, very good capacity, rate capability, and cycling stability could be obtained from it. The authors also experimented the physical mixture of free Mn3O4 nanoparticles with carbon black and reasonably found the worse performance than the Mn3O4/ (rGO) nanohybrid. Likewise, Mn3O4 nanoparticles- and SnO2 nanoparticles-based LIB anode cannot be utilized effectively because of its severe capacity fading, arising from the large volume change (>300%) and serious aggregation of tin particles formed during Li insertion as well as continual formation of a very thick solid-electrolyte interphase (SEI) during cycling, leading to pulverization of the electrodes and formation of electrochemically inactive Li2O as well as continual depletion of the electrolyte. Subsequently in 2013, SnO2/N-doped rGO nanohybrid anode superior electrochemical performances was reported by Zhou et al. [68]. They prepared the anode by in situ hydrazine monohydrate vapor reduction method. This method facilitated the binding of SnO2 nanocrystals in graphene sheets by Sn-N bonding to obtain the SnO2/nitrogen-doped rGO nanohybrid (denoted as SnO2

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Fig. 11. Schematic synthesis route for Si@C/G: aqueous mixture of Si nanoparticles, sucrose, and GO was (1) freeze-dried and then (2) annealed at N2 atmosphere. Reproduced from Ref. [61].

Fig. 12. (a) SEM image of G/Si particles. (b,c) HRTEM image of G/Si nanoparticles. (d,e) Low and high magnitude SEM images of G/Si@CFs. Reproduced from Ref. [62]. CF, carbon nanofiber.

NC@N-rGO). Homogenous distribution of SnO2 nanocrystals on GNSs is revealed in SEM and TEM micrographs [Fig. 15]. The research claimed low cost, abundance, environment-friendly benefits of the anode material and showed upgraded specific capacity (1352 mAh g1), excellent cyclic stability, and high rate capability. These electrochemical outperformances due to the formation of Sn-N bonds between SnO2 nanocrystals and graphene sheets effectively suppressed the aggregation of Sn nanoparticles during the lithiation process. It is the unchanged small size of the nanoparticles that retained the high storage capacity of Li in SnO2, as well as, the undisturbed electron supply

from N-rGO upgraded the Li electrochemical activity of SnO2 nanocrystals, resulting in improved reversibility of the conversion reaction of SnO2. Another group Zhang et al. designed and synthesized phosphorus (P)-bridged SnO2 and graphene nanohybrid (denoted as SnO2@P@GO) as well as SnO2/GO and P-mixed SnO2/P/GO nanocomposite [69]. Notably, the researchers ensured the existence of P-C and P-OSn covalent bonds in SnO2@P@GO nanohybrid and no presence of bond in SnO2/P/GO nanocomposite through Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) characterizations. Interestingly, these

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Fig. 13. Schematic illustration for the synthesis of 3D TiO2GNSCNT nanocomposite. Reproduced from Ref. [65]. GNS, graphene nanosheet; CNT, carbon nanotube; TBOT, tetrabutyl orthotitanate.

Fig. 14. Schematic illustration for the fabrication of MWNTs@SnO2@C composite. First, MWNTs were sequentially coated with thick SiO2, SnO2, thin SiO2, and carbon layers from steps 1 to 4. After the removal of SiO2 layers, an MWNTs@SnO2@C composite with large internal void space can be finally obtained, which could effectively accommodate the volume changes of SnO2 during Li þ or Na þ insertion/extraction cycles. Reproduced from Ref. [66].

covalent bonds enabled the nanohybrid anode to hinder SnO2 nanoparticles aggregation and maintain excellent structure integrity as well as electronic connection resulting in better cyclic stability and rate capability over SnO2/P/GO nanocomposite anode [Fig. 16]. Serious attention and effort are given to fabricate hybrid structures between nanosized metallic oxides of higher specific capacity and graphene to improve the electrochemical performance of anode in LIBs because simple decoration of metallic oxide nanoparticles on graphene sheets cannot offer a firm bonding that can impede the nanoparticles aggregation or peeling off from the substrate easily, leading to a bad performance of LIBs. Few more examples of such structured nanohybrids explored recently are SnO2/graphene-like partitioned pomegranate structure (SnO2@C@half-rGO) [70], tungsten-doped SnO2, and graphene nanohybrid formed through strong bonding of Sn-N-C [71], electrophoretically deposited Co3O4/graphene hybrid [72], Sb2O3 nanoparticles uniformly anchored on rGO sheets (Sb2O3/rGO), [73] etc. are investigated as high-performance anode materials for LIBs. In another contemporary research, seamless covalent bond has been formed between flat GNSs and vertically aligned CNT.

Germanium (Ge) dispersed on this hybrid structure (denoted as Ge/ GCNT) lessens mechanical strain between electrode and current collector, alleviates Ge pulverization, and facilitates electron transport from the Ge to current collector [74]. Exclusively, they experimented its high specific capacity of 1315 mAh g1 after 200 cycles at a current density of 0.5 Ag1 and a very high rate performance of 803 mAh g1 at 401 Ag. An overview of graphene use as LIB anode material has been represented in Table 1 to focus enhanced properties in recent works. In light of the literature discussed above, hopefully graphenebased anodeeintegrated LIB technology with outstanding properties will emerge as most promising one to fulfill the market demand of imminent future, and few more challenges are still to be exceeded to reach the industrially adaptable mass production goal. More electrochemical features such as initial columbic efficiency, volumetric energy density, and power density should be taken under due attention [63]. More importantly, designing novel structured graphene-based composite anodes to inherit high transfer speed of Liþ as well as high reversible capacity, cyclic stability, and rate performance are to be treated as the mission of next period.

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Fig. 15. (a) SEM image of SnO2 NC@N-rGO. (b) High-magnification SEM image of SnO2 NC@N-rGO. (c) TEM image of SnO2 NC@N-rGO. Inset: The corresponding SAED pattern. (d) HRTEM image of SnO2 NC@N-rGO. Reproduced from Ref. [68]. NC, nanocrystal; rGO, reduced graphene oxide; HRTEM, high resolution transmission electron microscope; SAED, selected-area electron diffraction.

3.2. Graphene as anode material in SIB At present, LIBs used for energy storage in portable electronics and automotive transportation as mentioned before may suffer the Li scarcity and its high cost problems in coming days. At the same time, these shortcomings restrict the use of LIBs in grid-scale energy storage applications [75]. Being concerned to these risks and to cope up with these emerging challenges, sustainable alternative technologies instead of Li-hosted battery technology are being explored extensively. Much efforts are being given to find out an exclusive substitute of Li that possesses promising electrochemical features similar to Li. However, on the basis of explored researches linked to this matter till date, SIBs have been focused as the most potential alternative of current LIBs. Recently, SIB has come to the focus of researches for several key issues such as similar intercalation principles of Na ion on electrode materials like Li ion, availability of Na resources, and their low cost [76]. Till date, various types of cathodic electrode materials of exclusive electrochemical features have been suggested through different works. For example, sodium manganese hexacyanoferrate [77], Na3V2(PO4)/carbon composite [78], and Na3V2(PO4)/G composite [79] are proposed as possible candidates. On the contrary, effective anodic electrode fabrication for SIBs is still in progress [80]. To find out advantages of SIB anode electrode for enhanced battery performances taking metal sulfides, metal oxides, carbonaceous materials, etc. have been reported as earlier stage researches [81e83]. But none of them are recognized as beneficiary anode candidate due to some drawbacks, such as poor structural stability for electrode pulverization, lower storage capacity, very low cyclic stability, and rate capability [84,85]. As of now, the novel

microstructure of graphene enables Na ion to intercalate easily into the graphene layer, and graphene-based SIB anode material research is rushed to a new degree in this period. Though graphene material as Na-ion intercalation compounds renders promising outcomes, there are many controversial explanations about Na storage mechanism in graphene. To develop a comprehensive understanding of Na storage mechanism in graphene, we have explored few reports and reached to the most possible mechanism. Na-ion storage in hard carbon (where roughly parallel few graphene sheets consists of hard carbon micro crystallites) has been presented by ‘house of cards model’ in various researches which is also known as ‘insertion adsorption’ mechanism. This model demonstrates that Na-ion insertion in graphene sheets occurs in two ways, i.e. Na ion intercalation between graphene sheets in the sloping voltage region of potentiogram and Na ions adsorption onto the nanoporous sites in the plateau region [86]. Bommier et al. suggested three types of Na storage mechanism: the first category Na storage is in surface defects sites at sloping voltage region, and it is supported by ex situ total neutron scattering/ associated pair distribution function studies. The next plateau region was broken down to two possible storage mechanisms; in the low voltage plateau region is due to the intercalation between graphene sheets and minor phenomenon of Na ion on the sp2configured pore surfaces observed from galvanostatic intermittent titration technique as illustrated in Fig. 17 [87]. Certainly, further studies are demanded to fully understand the mechanisms of Na storage in hard carbon, particularly for the plateau region. Na storage capacity can be farther enhanced by inducing void and pore defects onto graphene sheets through heteroatom doping [88]. Development of 2D porous graphene sheets results in high

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Fig. 16. (a) Charge/discharge potential profiles of the 15-SIO2@C-G at 100 mAg1. (b) Comparison of cycling stability of SnO2@P@GO with SnO2/P/GO at a current density of 100 mA g1. (c) Long-term cycling performances at 1000 mA g1. (d) Rate performances ranging from 0.1 A g1 to 1 A g1. (e) Nyquist plot of SnO2@P@GO and SnO2/P/GO at frequencies from 100 kHz to 0.01 Hz. Reproduced from Ref. [69].

electronic mobility, good Na mobility, and enhanced capacity, showing a great potential in Na storage. Beyond the intercalation of Na ion between graphene layers, other Na adsorption process onto empty pores enhance the Na storage capacity to a large extent. Reviewing related researches, here we shall highlight three categories graphene-based anode materials we discussed before in case of LIB anode material. Moreover, a summarizing study of outstanding performances SIB graphene-based anode will be represented focusing structure-property relationship, and the authors also look forward to their application potentiality in SIB. 3.2.1. Graphene materials as anode for SIBs As stated before, Li can readily be intercalated through graphite interlayer until the C:Li ratio reaches 6:1 (LiC6). But, graphite application as anode in SIBs does not allow Na ions to intercalate to any appreciable extent and is electrochemically irreversible [89]. It was calculated by Cao et al. that to attain better Naþ storage, the carbon interlayer spacing should be higher than 0.37 nm as they explored Na insertion in hollow carbon nanowires [90]. Interestingly, synthesis of nanostructured non-graphitic carbon materials, i.e. graphene and rGO, by various routes showed graphite interlayer distance expansion (from 0.34 nm to 0.37e0.38 nm) due to oxygen containing functional group and other type of defects presence [91]. Wang et al. for the first time enabled to insert Na ion in rGO sheets to a reasonable extent [92]. The group outlined the promising electrochemical behavior of Na ion stored rGO nanosheet

providing reversible capacity as high as 174.3 mAh g1 at current density of 40 mAg1. After 250 cycles, at current density of 200 mAg1, specific capacity was retained to 93.3 mAh g1. This process succeeded to increase rGO interlayer distance (0.37 nm) that is larger than graphite interlayer distance (0.34 nm) paving the way to accommodate large size Na ion. Also, they noted the trapping of Naþ ion at defective sites that cannot be extracted in the subsequent discharge process ultimately results in lower first cycle coulombic efficiency. In 2015, electrochemical Na-ion storage properties of GNSs were experimented by Luo et al. [93]. They synthesized GNSs (thickness around 4 nm comprising 3e5 graphene layers) by the reduction and exfoliation of graphite oxide by heating at 300 C. The study showed Na-ion intercalation in polarized electrode at 0.20 V with expanded graphene interlayer distance of 0.413 nm. Also, they found more Na storage capacity in GNSs with a comparison to CNT as more Na intercalation occurred in graphene interlayer. It was proved from XRD study that showed d002 plane interlayer distance expansion to a large extent in case of GNSs as illustrated in Fig. 18. After all, the GNSs anode showed promising reversible Na storage capacity of 220 mAh g1 at 30 mAg1 current density and better cyclic stability. 3.2.2. Graphene-based nanocomposite materials as anode for SIBs Getting inspired from reasonable electrochemical performance of graphene-based materials as anode of SIB, researches employing

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Table 1 Few graphene-based anode materials and their electrochemical performances for LIBs. Compound

Specific capacity and cyclic stability

Graphite

Reversible capacity 320 mAhg1 at 0.05 Ag1. After 20 cycles 240 mAhg1 Reversible capacity 540 mAhg1 at 0.05 Ag1. After 20 cycles 290 mAhg1 Reversible capacity 730 mAhg1 at 0.05 Ag1. After 20 cycles 480 mAhg1 Reversible capacity 784 mAhg1 at 0.05 Ag1. After 20 cycles 600 mAhg1 First discharge capacity 1400 mAhg1 at 0.05 Ag1. After 10 cycles 600 mAhg1 First discharge capacity 1571 mAhg1 at 100 mAg1. After 100 cycles 1187 mAhg1 First discharge capacity 1693 mAhg1 at 100 mAg1. After 50 cycles 1027 mAhg1 First discharge capacity 1490 mAhg1 at 2 Ag1. After 10 cycles 975 mAhg1 First discharge capacity 2158 mAhg1. After 30 cycles 1168 mAhg1 First discharge capacity 2246 mAhg1 at 100 mAg1. After 70 cycles 1000 mAhg1 First discharge capacity 1792 mAhg1 at 100 mAg1. After 200 cycles 897 mAhg1 First discharge capacity 1743 mAhg1 at 0.5 Ag1. After 1000 cycles 798 mAhg1 Specific capacity around 900 mAhg1 was obtained after first 5 cycles at 40 mAg1. First discharge capacity 1865 mAhg1 at 0.5 Ag1. After 500 cycles 1074 mAhg1 First discharge capacity 1278 mAhg1 at 100 mAg1. After 200 cycles 550 mAhg1 First discharge capacity 1240 mAhg1 at 0.1 Ag1. After 100 cycles 1100 mAhg1 First discharge capacity of 2260 mA g1 at 100 mAg1. After 120 cycles 808 mAhg1 First discharge capacity 1304 mAhg1 at 0.2 Ag1. After 100 cycles 1113 mAhg1

GNS GNS þ CNT GNS þ C60 Oxidized GNRs MoS2/graphene composite Fe2O3/rGO composite Crumpled graphene/MoO3 Si/G Si/G/carbon G/Si@CNFs SnS2/G-CNT Mn3O4/G SnO2/N-doped rGO SnO2@P@GO W-doped SnO2/G Sb2O3/rGO Co3O4/G

Rate capability

Synthesis technique

Year

Ref.

e

e

2008

[40]

e

Mechanical exfoliation

2008

[40]

e

e

2008

[40]

e

e

2008

[40]

e

Unzipping CNTs

2010

[42]

L-cysteine

2011

[52]

1031 mAhg1 at 100 mAg1, 900 mAhg1 at 1000 mAg1 1227 mAhg1 at 100 mAg1, 800 mAhg1 at 800 mAg1 1228 mAhg1 at 500 mAg1, 845 mAhg1 at 3000 mAg1 e 1

1

1350 mAhg at 100 mAg , 900 mAhg1 at 500 mAg1 974 mAhg1 at 100 mAg1, 543 mAhg1 at 1000 mAg1 1118 mAhg1 at 100 mAg1, 635 mAhg1 at 2000 mAg1 900 mAhg1 at 40 mAg1, 390 mAhg1 at 1600 mAg1 1074 mAhg1 at 500 mAg1, 417 mAhg1 at 2000 mAg1 574 mAhg1 at 100 mAg1, 419 mAhg1 at 1000 mAg1 1100 mAhg1 at 100 mAg1, 580 mAhg1 at 5000 mAg1 1025 mAhg1 at 200 mAg1, 496 mAhg1 at 2000 mAg1 994 mAhg1 at 100 mAg1, 688 mAhg1 at 5000 mAg1

assisted hydrothermal and annealing Sol-gel and hydrazine reduction of GO Spray pyrolysis

2011

[53]

2014

[54]

Solvothermal method

2009

[58]

Electrostatic self-assembly method CVD and electrospinning

2016

[61]

2017

[62]

Facile vacuum filtration and calcination Hydrothermal

2015

[64]

2010

[67]

Solution mixing and reduction

2013

[68]

Solvothermal

2017

[69]

Hydrothermal

2017

[70]

Solvothermal

2017

[73]

Electrophoretic deposition and annealing

2017

[72]

G, graphene; GNS, graphene nanosheet; CNT, carbon nanotube; C60, carbon 60; GNR, graphene nanoribbon; G, graphene; GNF, graphene nanofiber; N, nitrogen; W, tungsten; P, phosphorus; rGO, reduced graphene oxide; CVD, Chemical vapor deposition; CNF, carbon nanofibre.

graphene nanocomposite anode in SIB rushed to a remarkable state more in recent period. For example, in 2013, Su et al. [81] explored SnO2@graphene nanocomposite prepared by in situ hydrothermal synthesis where field emission scanning electron microscope (FESEM) and TEM morphology exposed uniform distribution of SnO2 nanocrystals on GNSs. They demonstrated remarkable first discharge capacity of 1942 mAh g1 and after first cycle capacity retained at an average of 700 mAh g1 within 100 cycles at 20 mA g1 much higher than both of bare graphene and bare SnO2

Fig. 17. A three-tiered mechanism for Na-ion storage in hard carbon anode. Reproduced from Ref. [87].

as illustrated in Fig. 19. The outstanding performance of SnO2@graphene nanocomposites was ascribed as to the unique 3D structure formation by SnO2 nanoparticles and GNSs where SnO2 nanoparticles are wrapped by conductive graphene matrix. Xie et al. in the following year synthesized SnS2 nanoplatelet@GNSs nanocomposite via hydrothermal route that demonstrates sheetlike 2D nanoarchitecture morphology revealed from SEM and TEM characterizations [94]. The individual SnS2 nanoplatelet is surrounded by GNSs offering the hindrance to SnS2 nanoplatelets agglomerations and specific surface area increment available for the reaction between Na ions and SnS2 compared with bare SnS2 nanoplatelets. They measured larger first discharge capacity of 1339 mAh g1 and 725 mAh g1 capacity retention after first cycle at 20 mA g1, good cyclability, and much better rate capability compared with bare SnS2 nanoparticles. Peng et al., in 2016, fabricated CoS@rGO nanocomposite via one-pot solvothermal route where well-dispersed CoS nanoplates are tightly anchored on rGO nanosheets with 2D architectureelike morphology [95]. The novel nanostructure with enhanced electronic conductivity and structural integrity results from the addition of rGO nanosheets to CoS nanoplates. Exclusive electrochemical performances such as high initial discharge/charge capacity of 581/540 mAh g 1 with a coulombic efficiency of 93%, while the value for bare CoS is only 550/492 mAh g 1 with a relatively low coulombic efficiency of 89%. Also, they assessed very promising cyclability: 420 mAh g1 at 1 A g1 after 1000 cycles due to strong structural integrity and outstanding rate performance and capacity fall from 636 mAh g1 to 306 mAh g1 even after 100-fold current density increment

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Fig. 18. Ex situ synchrotron XRD patterns of (a) GNS electrodes and (b) CNT electrodes prepolarized at various potentials for 1 h. Reproduced from Ref. [93]. GNS, graphene nanosheet; CNT, carbon nanotube.

Fig. 19. (a) 1st and 2nd cycles discharge and charge profiles of bare graphene, bare SnO2, and SnO2@graphene nanocomposites at 20 mAg1 current density. (b) Cycling performance of bare graphene, bare SnO2, and SnO2@graphene nanocomposites at 20 mAg1 current density. (c) Cycling performance of SnO2@graphene nanocomposites at current densities of 40, 80, 160, 320, and 640 mAg1. (d) Rate performance of SnO2@graphene nanocomposites. Reproduced from Ref. [81].

showing its efficiency to reduce the charge/discharge time in practical applications. Enhancement of graphene electrochemical characteristics, such as specific capacity, energy density, etc., by heteroatom doping more specifically by nitrogen doping has gained serious attention in recent period. Nitrogen-doping graphene is believed to increase electric conductivity by lowering the semiconducting gap, attracting a large number of positive ions, and introducing defects on the graphene sheets that facilitates faster Na-ion diffusion [96]. Zhang et al., in 2017, developed N-doped graphene aerogel that is capable of overcoming layer restacking and aggregation and offers initial capacity of 1013.8 mAh g1, but unsatisfactory poor capacities are observed in following cycles. Now to optimize 3D nitrogendoped nanostructure electrochemistry, further incorporation of

metal sulfides/oxides nanoparticles in this structure to build up nanocomposite would be more effective to enhance electrochemical performances. A novel work by Choi et al. developed 3D structured graphene microspheres (Scheme Fig. 20) using one-pot spray pyrolysis process those are divided into several tens of uniform nanospheres coated with few-layer MoS2 layers [97]. Graphene nanosphere void spaces lodged the huge volume change of MoS2 layers resulting in magnificent first discharge capacity of the composite microspheres of 797 mAh g1 at a current density of 0.2 A g1 and 322 mAh g1 after 600 cycles at a current density of 1.5 A g1. Another attracting study of enhanced cyclic performances SnS nanoparticles (SnS NPs) anchored on 3D N-doped graphene network (SnS/3DNG) recently demonstrated by Xiong et al. [98] in

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Fig. 20. Schematic diagram for the formation mechanism of the 3D MoS2egraphene composite microsphere by the one-pot spray pyrolysis and description of Naþ insertion process. Reproduced from Ref. [97].

2017. Exclusively, their 3D porous and interconnected microstructure provided efficient and fast pathways for both Naþ ion and electron transport, while preserving the integrity of electrode structure as well as resisting the electrode pulverization and aggregation of active material during cycling. Well-interconnected and porous framework with random open pores surrounded by graphene and well-crystallized SnS nanoparticles presence were confirmed from SEM and TEM microstructure as illustrated in Fig. 21aed. It outperformed by showing initial discharge and charge capacity of SnS/3DNG are 1099.8 and 890.5 mAh g1, with a high ICE of 80.1%, due to efficient ion and electron transport in this electrode. Stabilized capacity at 912.5 mAh g1 was also found for the following cycles (Fig. 21eeg). 3.2.3. Graphene-based nanohybrid materials as anode for SIBs Graphene-based nanohybrid which we have defined as an intimate mixture of graphene and metallic element/metallic oxides/sulfides/phosphides, etc. nanoparticles where graphene remains both as matrix or reinforced phase with prominent chemical bond existence. Outstanding breakthroughs in maximizing the electrochemical performances of SIBs was taken by exploring graphene-based hybrid anode materials. In 2014, Song et al. developed phosphorus/GNS hybrid anode where phosphorous nanoparticles are surrounded by GNSs to form C-P bond. The bond existence between GNSs and phosphorus particles was ensured by FTIR and XPS characterization. The bond facilitates robust and intimate contact between phosphorus and GNSs, and the graphene at the particle surfaces can help to maintain electrical contact and stabilize the solid electrolyte interphase upon the large volume change of phosphorus during cycling [99]. This chemical bonding

enables to bind graphene and phosphorus particles strongly, thereby helps to prevent the loss of electrical contact between phosphorus particles and the conducting network during electrochemical cycling. Therefore, it shows better discharge capacity of 2077 mAh g1 superior cyclic stability that has capacity retention of 95% relative to the second cycle after 60 cycles and very good rate capability with a high coulombic efficiency of ~99%. These results are more promising than previously studied bare phosphorous by Qian et al. [100] and phosphorous/carbon composite by Kim et al. [101]. So, P/G hybrid nanostructured anode has a great potential for practical application in high-performance SIB (see Fig. 22). Recently, in 2017, Zhan et al. fabricated CoS/rGO nanohybrid by microwave-assisted route [102]. They confirmed the presence of CS-C bond by XPS characterization technique of prepared CoS/rGO sample that demonstrates the strong interaction of CoS and rGO. The introduction of rGO to CoS nanoparticles form nanohybrid structure of porous 3D conductive network and increased specific surface area with intimate interaction between CoS and rGO, where enhanced mesopores act as active reaction sites for sodiationdesodiation process. This nanohybrid structure formation results in enhanced capacity (first discharge capacity 755.8 mAh g1 with reversible specific capacity of 426.2 mAh g1), cycling performance, and rate capability than bare CoS nanostructured anode. Rapid capacity and coulombic efficiency fading in case of CoS has been solved to a large extent by nanohybrid formation where CoS nanoparticles volume expansion is buffered by rGO porous matrix. One more contemporary study by Zeng et al. who developed nanohybrids coupling red phosphorus quantum dots and rGO (RPQDs/rGO) [103]. They showed the presence of P, C, N, and O elements those form RPQDs/rGO hybrid structure and mild

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Fig. 21. (a)e(b) SEM images of SnS/3DNG at different magnifications. (c) TEM and (d) HRTEM images of SnS/3DNG. (e) Rate performance of SnS/3DNG and SnS/3DG. (f) Discharge/ charge profiles of SnS/3DNG at different cycles at 2 Ag1. (g) Cycle stability of SnS/3DNG and SnS/3DG at 2 Ag1. Reproduced from Ref. [98]. 3DNG, 3D N-doped graphene.

oxidation of RPQD on its surface decided from XPS spectra result. The anode electrochemical performance was measured as initial specific capacity of 1161 mAh g1 and ultralong cyclic stability with very low capacity deterioration rate of less than 0.12% per cycle even after 250 cycles at a current density of 200 mAh g1. This excellent performance resulted because of several reasons such as phosphorus elevated theoretical capacity (~2600 mAh g1), outrageous electrical conductivity of graphene, buffering the huge volume change of phosphorus during charge-discharge, reduced chance of graphene layer restacking, etc. The group concluded by declaring it as a very promising anode for SIBs with outstanding Na storage capability as well as feasible technique in industrial scale associated with low cost advantage. Graphene layers sandwiched with other materials such as Ndoped carbon/graphene hybrid by Liu et al. [104], layered SnS2/

rGO hybrid by Zang et al. [105], and uniformly distributed In2S3 nanoparticles in graphene matrix In2S3/graphene (in 2017 capacity 620 mAh g1) by Wang et al. [106] are studied exclusively. Though they reported ultralong cyclic stability and superior rate capability, still these lag behind in Na storage capacity (