Graphene-based nano-materials for lithium–sulfur battery and sodium-ion battery

Author's Accepted Manuscript

Graphene-based Nano-materials for lithiumsulfur battery and sodium-ion battery Songping Wu, Rongyun Ge, Mingjia Lu, Rui Xu, Zhen Zhang

www.elsevier.com/nanoenergy

PII: DOI: Reference:

S2211-2855(15)00199-8 http://dx.doi.org/10.1016/j.nanoen.2015.04.032 NANOEN825

To appear in:

Nano Energy

Received date: 5 February 2015 Revised date: 16 April 2015 Accepted date: 28 April 2015 Cite this article as: Songping Wu, Rongyun Ge, Mingjia Lu, Rui Xu, Zhen Zhang, Graphene-based Nano-materials for lithium-sulfur battery and sodium-ion battery, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.04.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphene-based Nano-materials for Lithium-sulfur battery and Sodium-ion battery

Songping Wu,* Rongyun Ge, Mingjia Lu, Rui Xu, and Zhen Zhang

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510641, China. * To whom correspondence should be addressed. email: [email protected], Tel

&Fax: +86 -20- 87112897 Abstract Graphene-based nano-materials have provided an opportunity for next-generation energy storage device, particularly for lithium-sulfur battery and sodium-ion battery (SIB), due to their unique properties. This review comprehensively summarizes the present achievements and the latest progress of inorganic nano-materials/graphene composites as the electrode materials for Li-S battery and SIBs. Electrochemical principles, performances and key obstructions of graphene-based materials in the actual application are considered. This review gathers and classifies updated knowledge about Li-S battery and SIB nanomaterials related to graphene, with the aim of offering a wide view of those systems. It is concluded that cost-effective SIBs and Li-S battery are promising next-generation battery candidates in the near future, but require further investigation and improvement to deal with some critical scientific issues. Keywords: Graphene, Nano-materials, Lithium-sulfur battery, Sodium-ion battery 1

1. Introduction Sustainable development has been becoming a globally competitive new industry due to the urgent reality on unsustainable energy consumption, environmental degradation, and especially, the increasing appeal for the improvement of the living quality. As the hottest fields of investment, solar energy, geothermal energy, wind energy, tidal power and rechargeable chemical power source et al. have attracted great attention and funding. Among them, rechargeable lithium-ion batteries have established themselves as a dominant role for portable electronic devices as agile and universal power sources, and moreover, are widespread utilized in the small electronic equipment. In fact, three, based on lithium, are receiving extensive interest at the present time: rechargeable LIBs, Li-air (Li-O2 as O2 is the fuel) and Li-S batteries (Figure 1). Of particular note is that the electrical vehicles are now being commercialized. As important components of the electrical vehicles, power battery pack is the core of electric vehicles, and directly related to the cost and crucial technologies such as life and safety of electric vehicles, therefore, is the key of the industrial chain. Despite the great achievements, lithium ion battery, a widely used power in the present stage, still faces two long-term, fundamental challenges, those are: a low energy density, limited resources and high cost. (1) Lithium-ion batteries unfortunately remain the low energy density of 120 Wh kg-1,[1], [2] thus largely limits the overall improvement of battery life, weakens the competitiveness of the electric vehicle and reduces the purchase desire of consumers. Therefore, large energy battery become highly desirable due to 2

the extremely urgent requirement in high-power applications; (2) With the coming of the era of electric vehicles and smart grids, global lithium resources will not be able to effectively meet the huge demand of power LIBs. In 2012, JCESR (US Joint Center for Energy Storage Research) stated an “impossible” and “very aggressive” goal. That is that cell, when scaled up to the sort of commercial battery packs used in electric vehicles, would reach a target of 400 watt-hours (Wh kg-1) per kilogram as of 2017.[3] As a result, several new-concept batteries have been emerging for achieving the grand prospect, for example, flow battery [4] that packs a high energy density with no need for the expensive metals found in other models, vanadium flow batteries,[5] Li-S and sodium-ion batteries (as will be discussed in the article), Li-O batteries,[6] [7], thermal batteries, [8] sodium-sulfur batteries,[9] sodium breathing batteries (Na2O batteries),[10] magnesium ion batteries,[11] calcium-ion batteries (CIBs),[12] and high-energy supercapacitor[13] and so on. Among them, Li-S and sodium ion batteries are being commercialized, and establishing themselves as the most promising next-generation storage

device

due

to

high

energy

density,

good versatility,

reliability and cost advantages. Based on above opinion, we concentrated our interest on the LiS battery and sodium-ion battery in this review. As a two-dimensional sheet of sp2-hybridized carbon material,[14] rather than carbon,[15] graphene has been confirmed as excellent matrix to support oxides nanoparticles for LiBs.[16-20] Of late years, the enormous achievements in synthesis of defectless graphene make them easier to be commercialized,[21] [22] inevitably 3

producing a profound impact on the development of the next generation battery. In addition, a recent review summed up six different structure models of graphene and inorganic nanoparticles composites (Figure 2), and showed the channels for Li insertion/de-insertion, as a consequence, provided an important reference for future work. [23] Of special note is that, when the nanomaterials are employed in both of Li-S battery materials or SIBs materials, they possess the analogous shortcomings, i.e. low electronic

conductivity and large

volume

expansion

during

Li

(or

Na)

insertion/de-insertion, even though they each still have their own unique problems. Graphene can effectively improve the conductivity of nanomaterials, accommodate volume expansion and prevent nanoparticles from aggregating, enabling excellent electrochemical performances. As a result, graphene-based nanocomposite has been accepted as high performance electrode materials. Recently, several reviews on graphene-based materials on advanced energy have been published.[24-29] However, the article, referring to the special field in Li-S and sodium ion batteries, is still similarly lack to date. Herein, we provide a timely, well-organized and informative review on graphene-nanoparticle composite electrodes for Li-S and SIBs, covering reaction mechanism, structural configurations and electrochemical performances. 2. Graphene-based nanomaterials for Li-S battery Li-S battery is an “ancient” battery and has been studied since the 1940s; the problems in Li-S battery are rough, and arduous efforts have been made to address 4

them over 70 years. As the limitations of expensive, low-capacity LIBs become ever more apparent, lately, Li-S battery for storage devices is garnering a fresh surge of interest as scientists and engineers look for ways to extend the range of electric vehicles [30] owing to the simplest configuration, an extremely high capacity of 1675 mAh g −1, an impressive energy density of 2600 Wh kg-1 [31] [32] and abundantly and environmentally friendly raw materials, i.e. sulfur (Figure 3). In August, 2008, as the advanced rechargeable power, Li-S batteries were deployed in the longest and highest-altitude solar-powered airplane flight. A company, Sion Power, allows commercial Li-S battery to afford a specific energy of over 350 Wh kg-1 so far. Over 600 Wh kg-1 in specific energy is believed to achieve in the foreseeable future. Generally speaking, Li–S batteries utilize moderate weight of sulfur as the cathode and low atomic weight of lithium as the anode, thus, are comparatively light. During discharging, metal Li is electrochemically oxidized to Li ion at the anode, subsequently, the Li ions move through the electrolyte to react with reduced sulfur at the cathode, yielding the discharged cathode product of Li2S; while the reversible chemical reaction occurred during charging. Li–S batteries operate in the voltage range of 2.5–1.7 V vs. Li+/Li. As a “non-mainstream” Li-S battery, Li2S/graphene [33] and mesoporous graphene-silica composites[34] also currently received much attention. Of late, well-operated Li-S batteries offer energy density on the order of 500 Wh kg-1, significantly higher than commercial LIBs (150~ 200 Wh kg-1). Although not yet commercially available on a large scale as of now, we optimistically believed that the 5

practical application of the Li–S battery is on the way due to the inspiring achievement in Li-S battery (Figure 4 and Figure 5).[35] [36-38] However, several difficulties, i.e. inherit challenges of poor kinetics, large volume expansion and dissolution of polysulfides in the electrolyte, need to be tackled for the commercialization of Li-S batteries. Taken together, these challenging problems produce severe capacity fading and terrible cycling performances. It is worth noting that several reviews on rechargeable chemical power materials for energy storage and conversion have been lately published. [24], [25] [26, 27], [39], [40] As well-known, several strategies have been designed to overcome those obstacles. The promising physical restraint methods, which incorporate sulfur into mesoporous host structures, such as mesoporous carbon,[41] [42] [43] [44] [45] [46] multi-walled carbon nanotubes@meso carbon, [47] porous carbon aerogels [48] and carbon nano, [49] mesoporous metal organic framework@reduced graphene oxide (rGO)/S composite,[50] seem to be effective. Herein, we want to pay more attention on the graphene because of its excellent electron mobility, high surface area, and a flexible two-dimensional sheet morphology, which

can

accommodate

volume

expansion

derived

from

sulfur.

As

a

two-dimensional sheet of sp2-hybridized carbon material,[14] graphene are often selected as a matrix to support Li-S materials. Particularly, graphene itself possessed a high capacity. Recently, three-dimensional topological porous unstacked double-layer templated graphene via template-directed chemical vapour deposition have aroused

6

the interest of researcher. High reversible capacities of ca. 530 mAh g-1 and 380 mAh g-1 are retained at 5 C and 10 C, respectively, after 1,000 cycles.[37] 2.1 Core-shell structure (Encapsulated model) [23] It is strongly believed that the highly conductive flexible graphene-based carbon will exert a tangible influence on the sulfur anchored/coated into the its interior for a core-shell structure, and prevent sulfur from dissolving and expanding during discharge–charge. The unique 1D texture and tremendous specific area readily allow the graphene-based carbon to coat or wrap on the surface of nanoparticles, consequently, achieve a core-shell 3D structure via a facile and effective wet chemical approach. In the early work, the Li cycling feature of graphene-sulfur composites (GSC) at small current density was investigated, i.e. an initial discharge capacity of 705 mAh g-1 at a C/5 rate, and 8% capacity fading over the first 15 cycles. Analogous results were recently available from various groups. Take two examples as below: a core-shell structured S@rGO composite (with sulfur content of about 85%), via a reduction reaction by Zhao et al., [51] displayed a capacity retention of about 980 mAh g-1 at 0.05 C up to 200 cycles. In the composite, rGO was homo geneously coated on the surface of S, therefore, could effectively alleviate the shuttle phenomenon of polysulfides in organic electrolyte (Figure 6). In another instance, the core-shell structured graphene/sulfur composite, through a scalable wet chemical-reduction process, offered a reversible capacity of 808 mAh g−1 at a rate of

7

210 mA g−1 and an average columbic efficiency of ∼98.3% over 100 cycles (Figure 7). [52] The good Li storage capability of the structured core/shell graphene/sulfur composite under high current density offered a great potential for practical application. [53],[54] The rGO enveloped the sulfur particles (GSC) via solution reaction enable the hybrid material to possess a specific capacity of 1064 mAh g-1 and 37% decay over 200 cycles at 0.95 C. [53] In another example on graphene-encapsulated sulphur (GES) composites by Xu et al., [54] the GES composite cathode owned the discharge capacity of 915 mAh g-1 (at the rate of 0.75 C) after the initial two cycles of activation at 0.1C, with the reversible capacity of 788 mAh g-1 and 86% capacity retention after 160 cycles. Of special note is that the higher capacity still retained at 800 mAh g-1 under 1000 mA g-1 and 1000 cycles for core-shell sulfur/graphene oxide via ionic solution method (Figure 8) . [55] Addition research on sulfur@GO structure, by means of electrostatic interaction by Xiao et al., [56] was proposed to exhibit a cycling stability with 81% capacity maintenance (308 mAh g-1) over 210 cycles as the cathode for Li–S batteries. Several graphene-free carbon composites were selectively provided for a constructive comparison in the work. Many types of carbon materials have been also explored as the matrix for carbon/sulfur (C/S) composites, such as carbon nanotube,[57] carbon fibre, [58] porous carbon, [48] _ENREF_22_ENREF_10carbon spheres,[59] and mesoporous carbon.[41, 45, 60] As expressed in the aforementioned investigation, it is explicitly that porous carbon/sulfur composites, especially for 8

mesoporous carbon, could afford the enhanced electrochemical activity and cycling stability. Therefore, dual protections (i.e. mesoporous carbon and graphene) for sulfur were also logically considered to yield more stable and higher properties. Of late years, the combination of mesoporous carbon (CMK) and graphene received people’s attention. [61, 62] It is believed that mesopores carbon can accommodate volume change of sulfur and provide efficient diffusion channel to Li ion; meanwhile, the conductive rGO coating skin should prevent the polysulfides from dissolving. The

rGO@CMK-3/sulfur composites, via a self-assembly route followed by

thermal vaporization by Zhao et al., [61] offered a specific discharge capacity up to 650 mAh g−1 over 100 cycles at 0.1 C due to trapping of the polysulfides and relief of the dissolution of polysulfides. In another example, rGO@CMK-3/S (53.14 wt% sulfur), via a scalable wet chemical route by Zhou et al.,[62] delivered a similar reversible discharge capacity of ~ 734 mA h g-1 after 100 cycles at 0.5 C. (Figure 9) The hierarchical porous graphene (HPG)[63] also aroused interest as it could effectively entrap the sulfur by virtue of the artificially designed porous cage structure (Figure 10) and the robust binding of S to the C-C bonds by the functional groups. The nanocomposite electrode exhibited admirable discharging capacitance of 1068 and 543 mAh g-1 at current density of 0.5 and 10 C (1 C= 1672 mA g-1), respectively. Intriguingly, the discharging capacity of 386 mAh g-1 at -40

℃

suggested the

possible application as energy storage device under extreme situation such as

9

ultra-low temperature. More importantly, a discharging capacity of 140 mAh g-1 after 80 cycles was achieved when the working temperature was -40

℃ at 1672 mA g . -1

In another recent instance, Xie and coworker [64] incorporated the merits of the multi-walled carbon nanotubes webs (MWCNTs-W) and rGO to design a three-dimensional hybrid nanostructure, where MWCNTs-W/S composites (S was introduced through heat treatment) were tightly covered by the graphene thin film. The remarkable electrochemical performances for the rGO@MWCNTs-W/S composites have been generated, i.e. a discharge capacity as high as 620 mAh g-1 after 200 cycles, and an average Coulombic efficiency of 96% even at an ultrahigh rate of 5 C. Recently, super-aligned carbon nanotube/graphene hybrid materials as a framework for sulfur cathodes were reported by Sun et al..[65] Recently, additional references on sandwich-like hierarchically porous carbon/graphene (G@HPC) composites using a facile ionothermal process by Yan et al.,[66] graphene/sulfur hybrid via reduction by H2S by Zhang et al.,[67] sulfur/carbon nanospheres/graphene sheets composites (64.2 wt% S and giving rise to a discharge capacity of 1394 mAh g-1 at 0.1 C) by Wang et al., [68] sulfur-carbon nanocomposites via infiltrating into 3D graphene-like materials (GlM) with hierarchical pores by Li et al., [69] graphene/pure sulfur sandwich structure,[70] and sulfur/graphene composite (S@3D-graphene) with 73 wt % sulfur,[71] were also published for more experience and revelation. In brief summary, the combination of mesoporous carbon (CMK) and graphene seems to be a facile and robust strategy to improve the electrochemical performance. 10

Compared with the carbon nanotubes, [64] [65]mesoporous carbon has huge advantages: i.e. simple process and low cost; more importantly, mesoporous carbon can form a tight coating on the surface of sulfur particles. As a result, combination of mesoporous carbon and graphene will effectively enlarge the accommodation and protection effect of dots (mesoporous carbon) and planes (graphene) on the sulfur particles embeded into their interior, leading to high capacity (1068 mAh g-1 at 836 mA g-1[62, 63] and 1394mAh g-1 at 0.1C [68]) and excellent cycling stability.

2.2 Functionalization of graphene-based carbon (Anchored model) [12] A promising route to improving Li cycling performance of Li-S battery is functionalization of graphene-based carbon, which can expectedly yield a strong interaction between S and functional group in graphene, as a consequence, allow to immobilize the sulfur and lithium polysulfides into 2D or 3D structure. To achieve the aim, three following strategies have been adopted (Figure 11): (a) introduction of polymer to sulfur particles; (b) immobilization of sulfur on N-doped graphene and ( c) sulfur particles directly anchored on graphene. 2.2.1 The introduction of polymer to sulfur particles: The introduction of polymer was a viable way to decorate sulfur particles. In general, the graphene could provide electrical conducting paths to the S particles and entrap the polysulfides; meanwhile, the polymer can insert a molecular cushion between the S particles and graphene wrapping (Figure 11 a), leading to an excellent accommodation for volume expansion. 11

As a semicrystalline organic polymer resin, polyacrylonitrile arouse much interest of researchers. A dual-mode polyacrylonitrile (pPAN)@rGO@S (with 65.1 wt% sulfur), offering a reversible capacity of ca. 650 mA h g-1 after 10 cycles at 0.1 C, was reported by Yin et al., [72] who also exploited another in-situ anchored PAN/graphene/S composite (~47 wt% sulfur) with a reversible capacity of ca. 700 mA h g-1 even up to 0.1 C after 100 cycles. Inspired by above achievements, Wang et al [73] inspected a intriguingly hierarchical pPAN–S@GNS composite sulfur-based by spray drying followed by sulfur vaporing. 3D spherical GNS architecture framework enable pPAN–S@GNS composite to possess an admirable electrochemical properties: a reversible capacity of 681.2 mAh g-1 in the second cycle and 88.8% (605 mAh g-1) capacity retention after 300 cycles at a 0.2 C. More importantly, a remarkable capacity of near 329 mAh g-1 is still retained even at a high discharge rate of 10 C. The results logically defined them as promising candidate for high energy storage. Conducting polymer (such as PANI [74] and PPy [75] et al.) was also considered due to the conductive character. Polyaniline (PANI) is a conducting polymer of the semi-flexible rod polymer family, and is a frequently-used polymer to decorate sulfur nanoparticles. Liu et al.[74] reported that a sandwich-structured nanosulfur@polyaniline/graphene (nano S@ PANI/G) composite delivered a reversible capacity of 600 mAh g-1 after 100 cycles at 0.1 C (1C= 1670 mA g-1). And curiously, CTAB-modified PANI/graphene oxide (GO)/sulphur, affording a comparable Li storage capability of 970 mAh g-1 at 0.2 C after 300 cycles and 12

capacity decay of 0.036% per cycle at 0.5 C, was recently covered by Qiu et al.. [76] In 2014, the group of Li reported a hierarchical nanocomposite (Figure 12), [77] comprising of graphene nanoribbons, polyaniline and sulfur, which offered a specific capacity of 688 mAh g-1 at the 80th cycle at 0.1 C. Very recently, Wang et al. reported that the poly(3,4-ethylenedioxythiophene) (PEDOT)/GO and polyaniline (PANI)/GO composites were prepared by interface polymerization of monomers on the surface of GO sheets. The PEDOT/GO@S composites with the sulfur content of 66.2 wt% exhibited a reversible discharge capacity of 800.2 mAh g-1 after 200 cycles at 0.5 C, which was much higher than that of PANI/GO@S composites (599.1mAh g-1) and PANI@S (407.2mAh g-1).[78] Another conducting polymer of polypyrrole (PPy) was considered to yield S-PPy/graphene composites by in-situ chemical oxidation polymerization followed by heat vaporizing.[75] Such a composite (sulfur content of 50 wt%) presented a good rate capability, for example, the discharge capacity of 833 mAh g−1 (based on sulfur) at 0.1 C rate after 33 cycles. Although the Li-storage performances of sulfure functionalized by conductive polymer were fairly stable, partially due to the conductive nature of polymer. However, conductive polymers are, in general, rather costly, therefore, obviously make their industrical applications impossible. Polyethylene glycol is an environmentally friendly polymer. Wang [79, 80] exploited a facile solution method to wrap PEG-coated sulfur particles with graphene decorated by carbon black nanoparticles. The resultant graphene/sulfur composite 13

possessed a specific capacity up to ~ 600 mAh g-1 over more than 100 cycles at C/2 rate with ~10% decay per100 cycles.[80] In another work, [81] the introduction of ferric cation double layer (FCDL) into three dimensional net of rGO and PEG enable the resultant rGO–S–PEG composite (44.6% of S) to afford a capacity of 570 mA h g-1 after 60 cycles at 1 C (1670 mA g-1). Intriguingly, sulfur, rather than N, hydroxyl as the functionalized element, was orderly deposited on the surface of graphene, yielding a continuous sulfur film coating on rGO. Additional recent researches on amylopectin-GO/S,[82] polydopamine-rGO/S [83] and polyDADMAC-KBC/S [84] were also demonstrated here for referencing. The detailed descriptions were discussed as follows: Zhou et al [82] introduced amylopectin, a natural soluble polysaccharide polymer, into homogeneous sulfur-carbon black composite to give a 3- dimensionally cross- linked structure through the interaction between hydroxyl in amylopectin and hydroxyl or carboxyl in graphene oxide. The unique structure enables the GO-S-Amy electrode to emerge a discharge capacity of 441 mAh g-1 at 5C/16 after 175 cycles. More interestingly, stable cycling performance of 430 mAh g-1 was observed at C/2 after 100 cycles. As reported by Wang, [83] the grafting of a functional polydopamine (PD) buffer to the rGO/S allows the resulting composite to get a discharge capacity of 728 mAh g-1 after 500 cycles at 0.5 A g-1. Two factors have been believed to contribute cycling performance: namely polydopamine coating on rGO/S composite and cross-link reaction built between buffer and binder. PolyDADMAC, a high charge density

14

cationic polymer, was also utilized to synthesize g-KBC/S, affording a stabilized fade rate of 0.026% or 1.1 mAh g-1 per cycle over 200 cycles. The salient features are discussed here: (1) In general, the Li-cycling properties of sulfur/graphene structure are fairly stable (600-700 mAh g-1 at 0.1~ 0.2 C at various cycles) when the polymer was used; and (2) even though dual protection (i.e. graphene and introduced polymer or non-graphene-based carbon) seems to be feasible for relieving the dissolution of sulfur, the addition of polymer (with high molecular weight) inevitably reduced the content of sulfur, leading to the low reversiable capacity, as compared with mesoporous carbon/graphene/S core-shell structure. Therefore, the rationally designed 3D structure with high sulfur content maybe the next goal to be solved. Next, another approach is investigated whereby the sulfur was immmobilizd by means of the element nitrogen on the graphene. 2.2.2 Immobilization of sulfur on N-doped graphene: To improve the content of active materials in the electrode, and increase the capacity of corresponding electrode, the immobilization of sulfur on graphene via the chemical bonding seems to be more practicable. Intriguingly, more and more attention has been concentrated on N-doped graphene (Figure 11b) due to the facilitative rapid transport for electrons and Li ions across the doped graphene; meanwhile, strong chemical bonding between S and the nearby N atoms in graphene enable sulfur to immobilize on the surface of graphene. N-doping is a commonly-used strategy to functionalization of graphene. Li/S@ N-doped graphene (60% S), delivering specific discharge capacities of 978 mAh g-1 at 15

0.2 C after 150 cycles, was recently reported by Qiu et al..[85] (Figure 13) Additional two analogous results were received by different research groups. Wang et al.[86] reported that a porous three-dimensional N-doped graphene (3D-NG) via solvothermal process emerged a discharge specific capacity of 792 mA h g-1 after 145 cycles at 600 mA g-1 and the low capacity fading rate (0.05% per cycle); fascinatingly, the composites still retained a capacity of 671 mA h g-1 after 200 cycles even at a high rate of 1500 mA g-1 (Figure 14). Another example of N-doped graphene/sulfur (NGS/S) composites with 80 wt% of sulfur was observed by Wang et al..[87] The NGS/S possessed a reversible capacity of 1356.8 mAh g-1 at 0.1 C and long cycle stability of 578.5 mAh g-1 at 1 C up to 500 cycles. Wang et al.[88] recently took consideration of strongly covalent stabilization of sulfur and its discharge products on aminofunctionalized rGO that enables stable capacity of 650 mAh g-1 (retention of 80%) for 350 cycles with good high-rate response up to 4 C. Obviously, N-doped graphene/sulfur composites retain fairly high reversible capacity and rate performance; However, how to introduce robust binding among sulfur, nitrogen and graphene still remains a challenge. 2.2.3 Sulfur particles directly anchored on graphene: To date, much interest has been also devoted to directly anchored sulfur on the surface of graphene (Figure 11c) due to the highest content of active materials. Ji et al.[89] immobilized sulfur and lithium polysulfides via the reactive functional groups on graphene oxide by means of integrating emulsion method and thermal sulfur 16

vaporing strategy. Strong interaction between graphene oxide and sulfur or polysulfides could be responsible for the higher electrochemical performance, i.e. reversible capacity of 954 mAh g-1 for more than 50 cycles at 0.1 C (1 C = 1675 mA g-1). Aroused by the above advancement, Song et al. [90] designed a cetyltrimethyl ammonium bromide (CTAB)-modified sulfur-graphene oxide (S−GO) structure, in which CTAB modified sulfur was anchored on the functional groups of GO, producing a dual protect for sulfur from dissolving. Such a composite cathode has been confirmed to possess an impressive Li cyclability properties: a specific capacity (∼800 mAh g-1), a long cycle life exceeding 1500 cycles and a low decay rate (0.039% per cycle), as discharged at rates of 6C (1 C = 1.675 A g-1) and charged at 3 C. The performance explicitly suggested that Li/S cells can be invoked as high-power devices after being commercialized. Recent references on Li cycling of amorphous sulfur mesoporous graphene composites, activated with KOH by Ding et al, [91] amorphous sulfur-graphene composite by Wang et al, [92] [93] porous S/GO composite by Zhang et al. [94] were also taken into account. Additional interesting works operated by various research teams include: Nano S@G composite via hydrothermal method followed by thermal mixing (delivering a capacity of 720 mAh g-1 over 100 cycles at 335 mA g-1), [95] graphene/sulfur/porous carbon (Figure 15),[96] graphene/chromium-MOF(MIL-101) composite by Zhao et al.,[97] and sulfur/graphene composite cathode with gel polymer electrolyte (GPE battery) by Zhang et al..[98] 17

Additional widespread studies include but not limited to: activation graphene/sulfur (AG/S) complex (65wt% sulfur ), [99] graphene sheets via hydrothermal method by Wei et al., [100] graphene uniformly dispersed with sulfur by Sun et al.,[101] and sulfur/graphene composite. [102] These investigations have also provided much significant information on Li cyclability of Li-S battery based on various graphene/S nanomaterials with different morphologies, functional groups and microstructure through distinctive methodologies and unique technique. As exhibited above, the direct immobilization of sulfur and lithium polysulfides on the surface of graphene is a practical route because the graphene can provide a lot of functional groups to react with the sulfur and lithium polysulfides. There are two distinct features for the approach, i.e. simple process and large content of active materials, making it as a most promising route for anchoring sulfur and polysulfides. In brief summary, as a strong competitor to the above-mentioned core-shell structure, LiS cathode materials based functionalization of graphene are promising due to their unique space structure, simple configuration (particlarly N-doped and S-anchored graphene) and excellent electrochemical performances. However, how to form an effective bonding betweem sulfur and graphene is of great importance. 2.3 Freestanding flexible Li-S cathodes To explore the more practicable approach to accommodate the volume expansion and relieve the shuttle effect, fabrication of more novel graphene-based architecture has drawn considerable attention so far. As a general knowledge, the commercialized battery textures consist of the binder (for adhesion in the collector) and additive (for 18

increasing the conductivity), unavoidable lowering the operational capacity; therefore, the freestanding flexible electrode is eagerly expected. In fact, flexible Li batteries have gained great achievements under continuous efforts, as reviewed by Cheng et al., [103] for example flexible LIBs (Figure 16(a)) [104] and flexible Li-S/CNT battery (Figure 16(b)) . [105] Noticeably, graphene-based flexible freestanding Li-S cathodes is still a curiosity, as compared with the S-CNTs.[105] [103] The group of Huang [106] have created the mesoporous graphene–sulfur papers for Li-S batteries, where amorphous S was homogeneously introduced into mesoporous graphene by heating process. The cut specimens can be directly utilized as working electrodes in Li–S batteries, achieving a high discharge capacity of 1393 mA h g-1 with an enhanced cycling stability and good rate performance. Lu et al. [107] embedded a 3D electrode of sulfur into porous graphene sponges by a heat treatment. This 3D architecture electrode with 80 wt% sulfur demonstrated a high areal specific capacitance of 6.0 mAh cm-2 at 11th cycle, and retained 4.2 mAh cm-2 after 300 charge-discharge cycles at a rate of 0.1C, yielding a low decay rate (0.08% per cycle after 300 cycles). As reported by Zhou et al., [108] graphene-sulfur (G-S) self-supporting electrode, in which sulfur nanocrystals were anchored on interconnected fibrous graphene, possessed a Li-cycling capacity of ∼700 mA h g-1 after 50 cycles for the G-S63 hybrid due to strong binding between the functional groups on the graphene and sulfur/polysulfides, which alleviated the irreversible capacity loss of sulfur during discharge/charge. Very recently, Wang et al. reported a slurryless Li2S/rGO cathode paper for Li−S batteries, 19

which afforded a excellent cycling life and rate capability with a reversible discharge capacity of 816.1 mAh g−1 after 150 cycles at 0.1 C, and 597 mAh g−1 even at 7 C.[109] An intriguing structure is sandwich type graphene-sulfur for composite as potential lithium–sulfur cathodes. Song et al. [110] reported a flexible freestanding sandwich-structured sulfur cathodes, which afforded a high areal capacity of 4mA h cm-2 with high sulfur loading (~ 4 mg S cm-2) (Figure 17). Another meaningful research is the graphene/single-walled carbon nanotube hybrids contributed by Zhao et al. [111] In-situ deposition of both SWCNTs and graphene was achieved by means of a high-temperature CVD process; as a consequence, SWCNTs grew on the graphene plane to bring about G/SWCNT hybrids, which gathered a Li cycling capacity of 650 mAh g-1 after 100 cycles at 5 C. Recently, Yu et al [112] made use of the atomic layer deposition (ALD) to achieve a ultrathin and homogeneous ALD-Al2O3 artificial layer on the surface of graphene-sulfur composite, delivering a specific capacity of 646 mA h g-1 after 100 charge–discharge cycles at 0.5 C, reasonably ascribed to restraint of polysulfide dissolution and alleviation of the shuttle effect. In addition, a flexible, self-standing graphene-Se@CNT composite film [113] and graphene/cellulose composites [114] also aroused some concern. In summary, the physical properties and electrochemical Li cycling data of typical Li-S battery materials were listed in Table 1. As discussed above, on one hand, the easy functionalization of graphene makes the graphene-sulfur hybrids present different characteristics, and on the other hand, chemical exfoliated graphene is 20

relatively cheap and can be yielded on a large scale. Above-mentioned two advantages allow us to manufacture inexpensive graphene-based flexible Li-S battery with high energy density and low cost. As of now, flexible graphene-based LIBs have been extensively explored and achieved great progress, as commented in the newly published literatures.[103, 115] Currently, the targeting markets of Li-S battery are the electric vehicle, defence, and solar storage battery. Compared with flexible LIBs, the research on flexible Li-S battery is scare. We predict that more attention will be focused on graphene-based lithium-sulfur batteries as flexible power sources used in powerful flexible electronic devices, including a flexible computer, iWatch device, a bendable iPhone and wearable consumer electronic devices to name only a few.

3

Graphene-based nanomaterials for Na ion battery Li-based batteries have been widely used in many aspects in the past few years

and well-known for perfect performance in comparison with other now-available rechargeable cells in terms of energy density. Unfortunately, Lithium is scarce in earth; therefore, many researchers devote themselves to exploring new abundant and low-cost alternatives on the earth such as sodium-ion batteries (SIBs) and calcium-ion batteries (CIBs).[12] Now, SIBs are rapidly developing, and have been emerging to be a cheaper way to store energy than commonly-used LIBs. They will be suitable for most kinds of energy storage applications, but particularly the so-called “large format” applications, 21

including stationary energy storage such as grid storage, renewable energy storage as wind and solar power, backup systems as uninterrupted power supplies, and automotive. As of 2014, Aquion energy, one newly established company in Pennsylvania, has a commercially available SIBs (using manganese oxide spinel structure host as cathode and NaTi2(PO4)3/activated carbon as anode) and with cost/kWh capacity similar to a nickel-iron battery for use as a backup power source in electricity micro-grids, that can operate independently of the centralized grid. Another company, the UK-based Faradion, declared that they have already developed a series of SIBs materials with energy densities exceeding that of the popular Li-ion material (i.e. LiFePO4), dispelling the misconception held by some that sodium-ion materials will be unable to achieve high energy densities (Figure 18). In 2014, OXIS energy, based on the culham science centre in Oxfordshire, offered the high energy density of 300 Wh kg-1 for SIBs, which is expected to reach 400 Wh kg-1 as of 2016. More detailed facts on energy density of SIBs can refer to the review by Yabuuchi et al..[116] 3.1 Anode materials 3.1.1

Graphene as the active materials

From the viewpoint of technology, SIBs experience similar problems like LIBs, mainly volume expansion during Na cycling, when the nanomaterials were employed as electrode materials, even more severely due to the large radii of sodium ion. Graphene,

instead

of

graphite,[117]

carbon

22

microspheres[118]

or

carbon

nanofibers,[119] is still believed as a promising matrix for SIBs with high capacity due to its unique characters, as expressed in LIBs. It is interesting to note that graphene itself possesses a much high capacity. Influences of defect and/or distort of graphene on adsorption and cyclability of Na or Ca were investigated. [120, 121] The Group of Datta [120] discovered that Na and Ca could be absorbed in the defective graphene rather than pristine graphene. The capacities of Na+ and Ca2+ batteries reached 1450 and 2900 mAh g-1 when the divacancy defect density reached the limit. Wang et al. [122] explored the Na cyclability of rGO in SIBs. Good electrical conductivity, active character, large interlayer distances, and the disordered structure of rGO made it store more Na ions, leading to a reversible capacity of 174.3 mAh g-1 at 0.2 C (40 mA g-1) , so far as to reach 93.3 mAh g-1 at 1 C (200 mA g-1) after 250 cycles. rGO could maintain a capacity of 141 mAh g-1 (40 mA g-1) even circulated 1000 cycles at the rate of 0.2 C. Research on the porous carbon/graphene was also significant. In general, porous carbon on both sides of the graphene facilitates Na+ diffusion, leading to high specific capacity, long cycle life, and high rate capability.[123] Ding et al. meticulously designed a 3-dimensional well-ordered macroporous networks of carbon nanosheets with a highly broad graphene interfloor distance (0.388 nm), which facilitated large amount of Na intercalation even below 0.2 V vs. Na/Na+. It showed a rate capacity of 255 mAh g-1 at the 210th cycle and stable capacity of 203 mAh g-1 at 500 mA g-1. [124]

23

Recent investigation on expanded graphite also received people’s interest.[125] The expanded graphite can deliver a reversible capacity of 284 mAh g-1 at 20 mAg-1, maintain a capacity of 184 mAh g-1 at 100 mA g-1, and retain 73.92% of its capacity after 2,000 cycles (Figure 19). In addition, Na cyclability of a free-standing FLG (few-layered graphene) monolithic network foam cathode, demonstrating electrochemical stability and rate discharge capacity retention for up to 400 discharge/charge cycles at 3200 mA g-1, were also reported. [126] Another research on boron-doped graphene, offering a capacity of 762 mA h g-1 in SIBs according to 1st-principles, was noted by Ling et al.,[127]. Even though an inspiring development in graphene or graphene/carbon, the composites of nanoparticles and graphene may be more attractive due to huge theoretical capacity and low cost. Noticeably, several reviews on SIBs for energy storage and conversion have been lately published.[12, 128-134] [116] Similarly to LIBs, graphene-based composites allow SIBs to possess higher specific reversible capacity, better rate capability and long-cycle life than bare graphene. [23, 135, 136] Next, we turn our attention into the graphene-based composite anode materials. 3.1.2 Graphene-based Anode materials Graphite is the commonly-used anode electrode material in current LIBs [137] or SIBs [117] technologies. However, only a limited number of Na can be stored in graphite (~NaC70),[138]

petroleum (NaC30 ) or Shawinigan (NaC15),[129] hindering

the overall improvement of SIBs capacity. The first principles calculations displayed 24

that it is difficult to yield the Na-intercalated graphite compounds as compared to other alkali metals.[139] Therefore, novel anode materials need to be developed for satisfying the immediate requirement of SIBs. Recently, significant efforts have been concentrated on searching the alternatives (i.e., sulfide, transition metal oxides, Si, Sn et al.) to current anode materials. However, as illuminated by Kim et al.,[129] the Na alloying compounds should experience about 150% of volume expansion to reach the capacity of conventional graphite in LIBs. How to restrain the volume expansion is becoming a crucial issue for alternative anode materials used in SIBs. As a logical choice, flexible 2D graphene should play an important role to accommodate volume expansion of non-carbon anode materials. Actually, compared with Li cycling in LIBs, active materials in SIBs electrode adopt the analogous reaction mechanism with Na, and allow us to have an insight into the electrochemical essence of Na-cyclability during change/discharge by use of an analogy strategy. The reaction mechanisms of Na-cycling during change/discharge were summarized for further clarification, as depicted in Figure 20. (1) Intercalation− deintercalation mechanism; (2) Alloying mechanism, in which elements (A) or metals (M) can form alloys with Li metal (exactly, intermetallic compounds); (3) The so-called redox or “conversion” reaction. In general, graphene can act as a support for electroactive nanomaterials, and hinder their re-stacking by lowering the van der Waals forces among the layers. Moreover, the extensive, elastic and highly conductive graphene improves the electrical conductivity 25

of the composite and buffers volume expansion of electrode materials during cycling.[23] To highlight the correlation among Li-cycling mechanism, crystal structure and electrochemical performances, we classified our data by the reaction mechanism.

3.1.2.1 Anode based on intercalation−deintercalation reaction:Titanate/graphene composites Of late years, three-phase storage mechanism in Li4Ti5O12 anodes for room-temperature SIBs [140] and a zero-strain layered metal oxide, i.e. P2-type Na0.66[Li0.22Ti0.78]O2 [141] have aroused strong concern. As typical Ti-based compounds, the structure of TiO2 and NASICON-type NaTi2(PO4)3 were depicted in Figure 21; subsequently, graphene-based titanium compounds as SIBs anode were summarized as follows. As proposed by Cha et al., [142] TiO2 nanoparticles anchored on nitrogen-doped graphene showed, as anode materials for SIBs, a reversible capacity of 405 mAh g-1 at 50 mA g-1 and a cycle stability with a capacity of 250 mAh g-1 over 100 cycling at 100 mA g-1 due to the facilitated ion diffusion by their open pore channels and the promoted electron transfer in electrochemical reactions. Recently, microsphere C-TiO2 anode delivered a capacity of 149 mAh g-1 at 1 C, nearly 100% retention during 50 cycles, as reported by Oh et al..[143] TiO2 experiences the Na-ion intercalation/deintercalation reaction during charging and discharging.[144, 145]

26

NASICON-type NaTi2(PO4)3 can be utilized as an anode material for SIBs. In fact, it has been commercialized by Aquion energy. Delmas et al.[146] first reported reversible Na cycling of NASICON-type NaTi2(PO4)3. TiO6 octahedra are isolated from each other by corner-shared PO4, forming the open structure with some sets of vacant sites for alkali ions. As to NaTi2(PO4)3, 2 mol of sodium ions are electrochemically and reversibly inserted based on a Ti3+/Ti4+ redox couple.

Li et al.[147] synthesized NaTi2(PO4)3/graphene composite by the sol-gel followed by heat-treatment approach. The resultant NaTi2(PO4)3 containing 6.84% graphene delivered a capacity of approximately 63.5 mAh g−1 at 20 C, decaying 29% of the initial capacity after 2000 cycles. Analogous Na cyclability performances, i.e. a specific capacity of 65, 40 mAh g-1 at 20 C and 90% retention after 100 cycles at 2 C for NaTi2(PO4)3-graphene by solvothermal process, were observed by Pang et al..[148] The NaTi2(PO4)3 nanoparticles were uniformly attached on the surface of conductive graphene nanosheets and thus formed 2D structure. Recently, Na2Li2Ti6O14 has been examined as a new kind of anode material, which possesses a lower insertion potential plateau (about 1.25 V). To improve the electrochemical performance of Na2Li2Ti6O14, carbon black (CB), graphene (GN) or carbon nanotube (CNT) is introduced by a solid state method.[149] The reversible charge capacity keeps 104.4 mAh g-1 for Na2Li2Ti6O14/GN cycles.

27

at 100 mA g-1 after 50

3.1.2.2 Anode based on alloying/de-alloying reaction:Metal/graphene composites As noted by Reddy,[25] elements like Si, Sb, Sn, In, and Cd can arouse Li storage and cycling behaviour via alloying-dealloying reactions at potentials V ≤ 1 V vs Li metal. It is understandable to consider them (including alloys such as ternary Sn-Ge-Sb alloys[150]) as possible anode materials in SIBs. As opposed to carbon anodes, which merely provide organic complexes for the storage of Na+ ions, metals form inorganic complexes with the Na+ ions such as Na3Sb, Na3Sn, Na15Sn4[151] and Na3P. This capability allows alloyed anodes to offer a larger theoretical capacity than carbon. That is, from an energy point of view, sodium can form alloys with Sn (847 mAh g-1 ), Sb (664 mAh g-1 ), Pb (484 mAh g-1 ), and P (2596 mAh g-1 ) to electrochemically store Na ions as the anode in SIBs (Figure 22).[152] [116] Some interesting exploitations concerning graphene have been lately developed. Zhang et al.[94] yielded Sb/graphene nanocompound via in- situ solvothermal route. Na cyclability of Sb/G manifested the first charge and discharge capacities of 380 and 742 mAh g-1 at 20 mA g-1 between 0.005 and 1.5 V (vs. Na/Na+), respectively, while those of bare Sb were 126 and 684 mAh g-1, respectively. In another in-depth research, [153] the group of Nithya explored the mechanism of storing Na+ in the rGO/Sb composite, suggesting that, during galvanostatic cycling, transformation of nano rGO sheets to nanoribbons contributed to high capacity and capacity retention: a relative charge capacity of 641 mA h g-1 with good capacity retention (93%) and well work at high rate cycling (>6600 mA g-1). As an intriguing comparison, antimony Sb nanoparticles decorated N-rich porous carbon nanosheets were prepared through a 28

sol-gel route. The composite displayed a specific capacity of 220 mA h g-1 at charge-discharge rate of 2 A g-1 after 180 cycles for room temperature SIBs.[154] Alloys, such as Sn4+xP3@amorphous Sn-P composite, have been referred to as anodes for SIBs.[155] As reported by Li,[156] SnSb/CNT@GS electrode could retain a capacity of 360 mAh g-1 for up to 100 cycles, which is 71% of the theoretical capacity. Additional reference on graphene oxide wrapped CADS (with a capacity of 293 mAh g-1) was also noted. [157] In brief summary, Si/C composite was believed as the most promising LIBs anode candidate as of now; however, for SIBs, Sn/C composite may be promising. 3.1.2.3 Anode

based

on conversion reaction:

oxides

(sulfides)/graphene

composites (a) Oxides/graphene composites: Several other oxides, following the conversion mechanism for Na-cycling, have also drawn great concern to date. [158] [159] [160] In 2014, the group of Jian [158] took notice of Fe2O3@GNS for SIBs, which delivered a Na cycling performance of 400 mA h g-1 over 200 cycles at 100 mA g-1. Noticeably, 1D nanostructured sodium vanadium oxide[159] and FeWO4/graphene mesoporous composite[160] alos adopt the conversion reaction for Na cyclability. (b) Sulfide/graphene composites: Recently, sulfides have attracted great interests due to their unique chemical/physical characters and the potiential application in energy storage/ energy conversion.[161] _ENREF_16[162] [163]. In a recent article, WS2 nanocrystals with a 29

layered structure were uniformly distributed on the graphene nanosheets to gain WS2@graphene nanocomposites via a simple hydrothermal process.[164] When used as anodes in SIBs, the nanocomposites owned a reversible specific capacity of nearly 594 mA h g-1 and a retention capability of 283 mA h g-1 at 40 mA g-1 after 500 cycles. The reversible reaction could be established in Eq. (1):

MS2 + Na + + e − ↔ M + NaS2 ( 2 ) ( M = W , Ni )

(1)

As to NiS, a 3D conducting network allows rGO to alleviate volume changes of NiS, and emerge a charge capacity of 160 mA h g−1 after 10 cycles at 200 mA g−1 for NiS nanorods/rGO reported by Pan et al..[165] As a sharp contrast, NiS nanorods only owned 77 mA h g−1 after 10 cycles.

3.1.2.4 Anode based on intercalation/conversion reaction-MoS2/graphene composites Molybdenum disulfide is classified as a metal dichalcogenide, where each Mo (IV) center occupies a trigonal prismatic coordination sphere that is bound to six sulfide ligands. Each sulfur centre is pyramidal and is connected to three Mo centres. The trigonal prisms are interconnected to form a layered structure, locating molybdenum atoms between layers of sulfur atoms. Recently, zigzag single-walled MoS2 nanotubes was confirmed to permit a hole mobility of 740.93 cm2 V-1S-1 at room temperature, which is about six times of the electron mobility. [166] Similar to the reaction mechanism of Li with MoS2,[167] [168] [169]Na insertion and extraction into an idealized MoS2/rGO free-standing 30

composite paper electrode could be depicted (Figure 23). As a graphene-like structure, monolayer MoS2 could achieve a theoretical capacity of 335 mAh g−1 by double-side Na adsorption.[170] The group of David [169] fabricated a flexible anode composing of acid-exfoliated few-layer MoS2 and rGO in SIBs (used Na foil as counter electrode) by means of vacuum filtration. The crumpled hybrid afforded a Na cycling property, i.e. charge capacity of about 230 mAh g-1 while total weight of the electrode with Coulombic efficiency up to nearly 99%. Of special interest is the admirable mechanical properties of the composites: i.e. static uniaxial tension strength of ~ 2-3 MPa and failure strain of ~2%. In 2014, several researches into MoS2-rGO composites have been carried out. The exfoliated MoS2-rGO composite, affording a capacity of 165 mAh g−1 at 20 mA g−1 after 50 cycles, was also mentioned.[171] Another interesting result was available by the group of Wang.[172] The graphene-based SIBs and SICs composites (MoS2/G) offered a stable capacity of ~350 mAh g-1 over 200 cycles at 0.25 C for SIBs, and a capacitance of 50 F g-1 over 2000 cycles at 1.5 C in pseudocapacitors (SICs) with a wide voltage window of 0.1-2.5 V. And a surprise capacity of 930 mAh g-1 over 50 cycles at 100 mA g-1 for petal-shaped PVP-MoS2/rGO via hydrothermal route was noted by Zhang et al.[173] Not long ago, Qin et al.[174] reported that the MoS2-rGO composites exhibited a reversible specific capacity of about 305 mAh g-1 at a current density of 100 mA g-1 after 50 cycles and excellent rate performance. In

2013,

Chhowalla [162] pointed out that layered transition

metal

dichalcogenides (TMDs), such as MoS2 and WS2, are suitable for anode materials for 31

recharging battery. The interlayer spacing of TMDs provides a convenient environment for the accommodation of a variety of guest species, for example Li+ or Na+ ions. The electrical conductivity of TMDs is too low for their effective implementation as electrodes; in addition, the cycling stability of the TMD electrodes remains challenging; More importantly, capacity loss that occurs during the first cycle is still the major issue for TMD-based anodes, and must be minimized if TMD nanosheets are to become useful for energy storage applications. The recent researches suggest that the MoS2/rGO heterointerfaces can significantly increase the electronic conductivity of MoS2, store more Na ions, while maintaining the high diffusion mobility of Na atoms on MoS2 surface and high electron transfer efficiency from Na to MoS2.[175] 3.1.2.5 Anode based on conversion and alloying:oxides (sulfides)/graphene composites (a) Oxides/graphene composites: As a typical oxides, sodiation of Sb2O4 leads to conversion followed by alloying during cycling.[176] Sb2O4 has a high specific capacity (theoretical capacity = 1227 mAh g − 1 ) because a total of 14 Na ions can be stored per formula unit of Sb2O4 by the following reactions: Sb 2O 4 + 8Na + + 8e− → 2Sb + 4Na 2O (2)

2Sb + 6 Na + + 6e− ↔ 2 Na3Sb (3) As reported by Zhou et al., [177] SbOx/rGO composite through a wet-milling approach demonstrated a reversible capacity of 352 mAh g-1 at 5 A g-1 and more than

32

95% capacity retention (409 mAh g-1) after 100 cycles at 1 A g-1 when used in SIBs. SbOx/rGO adopts the analogous reaction mechanism with Sb2O4 during Na cycling. The mineral form of SnO2 is called cassiterite, and adopts the rutile structure, wherein the tin atoms are six coordinate and the oxygen atoms three coordinate. SnO or SnO2 has been considered as an “ancient” and extensively studied anode materials.[178] [179] [180] Na cyclability of ultrafine SnO2–rGO nanocomposite was recently reported by Wang et al.[181] This composite with SnO2 nanoparticles attaching to a rGO framework generated a Na-storage capacity of 330 mA h g-1 with a capacity retention of 81.3% over 150 cycles at 100 mA g-1. Another comparable outcome, i.e. a reversible capacity of above 508 mAh g-1 after 100 cycles at 80 mA g-1 for a 3D octahedral-shaped SnO2@graphene nanocomposite via situ hydrothermal method, was recently investigated by Su et al..[182] A facile and up-scalable wet-mechanochemical process is recently designed to fabricate ultra-fine SnO2 nanoparticles anchored on graphene networks for anode materials in SIBs.[183] As recently reported by Zhang et al.,[184] ultrafine SnO2/ rGO by a facile hydrothermal route delivered a high charge capacity of 369 mAh g-1 after 100 cycles at 100 mA g-1. Just recently, Liu et al. synthesized mesoporous Co3O4 sheets/3D graphene networks nanohybrids for high-performance sodium-ion battery anode.[185] Co3O4 MNSs/3DGNs nanohybrids offered a high sodium storage capacity of ~523.5 mAh g-1 at rate of 25 mA g-1 after 50 cycles. As reported by Zhang et al., [186] the Fe3O4/PMAA-PTMP electrodes showed superior cycling performance with a reversible Na-storage capacity of 204 mA h g-1 and outstanding cycling stability (i.e. 33

98% capacity was retained after 200 cycles). In another literature,[187] Fe2O3/rGO nanocomposites were utilized as anode materials in sodium-ion batteries, which delivered capacity of ~310 mAh g-1 after 150 cycles at 100 mA g-1. Obviously, due to their high theoretical capacity, low cost and abundant source, graphene-based oxides/sulfide nanomaterials have been presented as the great potential of sodium ion battery anode materials. Very significant improvements in sodium cycling performance of oxides/rGO composite have been reported to date. Among them, as far as we can see, SnO2 is the most promising candidate for SIBs application before other materials with low-cost and high Na cyclability are exploited.

(b) sulfide/graphene composites: Another layered sulphide, namely SnS2, has a CdI2-type of layered structure (a = 0.3648 nm, c = 0.5899 nm, space group P3m1) comprising of a layer of tin atoms located between two layers of hexagonally close packed sulphur atoms. The large interlayer spacing in layered structure allows the insertion and extraction of guest species (such as Li ion or Na ion) and adapts more easily to the volume changes in the host during cycling.[136] In a recent work, [188] rhombohedral SnS2/graphene oxide sheets, via deposition followed by heating sulfidization, afforded >610 mAh g-1 charge capacity at 50 mA g-1 (with >99.6% charging efficiency) between 0 and 2 V vs. Na/Na+ electrode, high cycling stability for over 150 cycles and very good rate performance, >320 mAh g-1 at 2000 mA g-1. Higher Na cyclying capacity was available in a layered SnS2-rGO composites in SIBs via hydrothermal route, i.e. a charge specific capacity of 630 mAh g-1 at 0.2 A g-1 coupled to a good rate performance of 544 mAh g-1 at 2 A g-1 and long 34

cycle-life of 500 mAh g-1 at 1 A g-1 for 400 cycles.[136] Another reversible specific sodium-ion storage capacity of 725 mAh g-1 was noted by Xie et al..[189] As to SnS, a recent research on Na cyclability of SnS electrode used in SIBs exhibited a sodium reversible capacity of 500 mAh g-1 at a discharge rate of 125 mA g-1.[190] However, another work offered a surprise capacity, [191] i.e. a specific capacity of 940 mAh g-1, and rate capability of 492 and 308 mAh g-1 after 250 cycles at 810 and 7290 mA g-1. Herein, orthorhombic-SnS phase was yielded by structural phase transitions from hexagonal-SnS2, and the lesser structural changes of SnS during the conversion resulted in a good structural stability and excellent Na cycling performance. Sb2S3/rGO composites was also surveyed, and offered a reversible Na cyclability capacity of about 700 mAh g-1 at 50 mA g-1 with negligible capacity fading over 50 cycles (Figure 24).[135] The reaction mechanism was expected as follows:

Sb 2S3 + 6Na + + 6e − → 2Sb + 3Na 2S (4) 2Sb + 6 Na + + 6e − ↔ 2 Na3Sb

(5)

Obviously, sulfides have emerged great potentials as anode materials in SIBs due to their high capacity, low cost and abundant sources. However, the researches on sulfides, including controllable synthesis, Na cycling performance and Na reaction mechanism, are fairly lacking as of now. In summary, even though great efforts have been devoted to SIBs anode materials, the study on graphene-based nanomaterials is quite lack so far, mainly for the following two aspects: (1) structural designing of nano-materials/graphene composites 35

and correlation between structure and performance. In existing literatures, some nanomaterials with special structure, such as hollow, flower, etc., have been synthesized; however, we need design more elaborate artificial structure in order to realize the handling of nano-structure; subsequently, influence of gradual evolution of the structure on the Li-cycling characters will be investigated to reveal the correlation between microstructure and electrochemical performances; As a consequence, how to acheieve the controllable synthesis of graphene-based nanomaterials is a cruical challenge; (2) Research on Na cycling mechanism during charging and discharging is unsufficient. As previously discussed, Na cycling mechanism of MoS2 was generally accepted as intercalation/deintercalaton; however, investigations into other compounds on Na-cyclability are very less, let alone mechanism exploration. Now, there are some problems difficult to explain. For example, as layered disulfide, why MoS2 follows the intercalation/conversion

mechanism,

whereas

SnS2

experiences

a

conversion/alloying reaction ? How to solve these disputes remanis a huge challenge.

3.2 Cathode materials In general, materials with high voltages ( >2 V vs. Na) are defined as cathode for SIBs.[134],[192] Representative cathode materials in SIBs include: Manganese oxides,[193] ,[194] layered oxides,[195],[196],[197] such as the recently hot materials of P2-NaxVO2[198] and

P2-NaxCoO2[199] as well as P2-Nax[Fe1/2Mn1/2]O2, [200]

Nasicon-type materials,[201] olivines,[202-204] sodium vanadium fluorophosphates (Na3V2(PO4)2F3),[205]

layered

sodium 36

iron

fluorophosphates,[206,

207]

alluaudite-type sulphate framework (Na2Fe2(SO4)3),[208] registering the highest-ever Fe3+/Fe2+ redox potential at 3.8 V (vs. Na/Na+, and hence 4.1 V vs. Li/Li+) and Tavorite sodium iron fluorosulfate.[209, 210] In this article, we draw our attention to the graphene-based cathode materials. As a typical high potential materials (4V), Na3V2O2(PO4)2F/rGO sandwich structure has been synthesized via solvothermal method by Xu et al..[211] The structure afforded a reversible capacity of 120 mA h g-1 at C/20 with a capacity retention of 100.4 mA h g-1 at 1 C and an excellent cyclic retention of 91.4% after the 200th cycle at C/10 when used as a sodium-insertion cathode material. Young et al. [212] synthesized a sub-micron sized hybrid of Na3V2(PO4)3 adhering to graphene by sol–gel and solid-state reaction. Na cycling properties of Na3V2(PO4)3 manifested a reversible capacity of 86 mA h g-1 at 5 C with polarization (0.15 V) , as opposite to only 43 mA h g-1 of bare NVP. Prominent electronic conductivity of 3.2 S cm−1 occurred in FePO4/graphene as cathode in SIBs,[213] which retained a reversible capacity of 112 mAh g-1 in 1 st cycle at 0.2 C and then stabilized at ~90 mAh g-1; more importantly, the cycle retention of

~60 mAh g−1 at 1 C and ~30 mAh g−1 at 4 C. Very recently,

Liu et al.[214] reported FePO4/rGO nanosheet by a microemulsion technique afforded 153.4 mAh g-1 after the 70th cycle at 0.1 C. As an interesting cathode material, graphene-based (NH4)0.19V2O5·0.44H2O composites, via a hydrothermal route followed by heat treatment by Fei et al.,[215] delivered a discharge capacity of 141.5 mA h g-1 after 40 cycles at 20 mA g-1 in a 1.5-3.4 V voltage. In a recent literature, [216] the VO2 arrays/graphene electrode delivered a Na storage capacity of 306 mAh 37

g-1 at 100 mA g-1, and a capacity of more than 110 mAh g-1 after 1500 cycles at 18 A g-1. P2-type Na2/3[Ni1/3Mn2/3]O2 experiences Na-cycling at a long voltage plateau of 4.2 vs. Na+/Na. As reported by Yang et al.,[217] a new, flexible and binder-free rGO/Na2/3[Ni1/3Mn2/3]O2 composite electrode (GNNM), by means of ultrasound dispersing and filtrating under vacuum pressure, delivered a capacity of 83 mA h g-1 at 0.1 C rate and retained 80 mA h g-1 after 200 cycles. The group of Zhu[218] took note of a hybrid of Na2/3Fe1/2Mn1/2O2 and graphene via an environmentally friendly water-based route. Of special interest is that the total electrical conductivities of the cell increased with the temperature increasing, for example 1.47 × 10−5 and 2.15 × 10−2 S cm-1 at 25 and 300

℃, respectively. The electrode offered a reversible capacity

of 156 mAh g-1 with high Coulombic efficiency. As noted above, the investigation into cathode materials for SIBs was continuously and fruitfully. Numerous promising materials have been exploiting. Among them, Manganese oxides and layered oxides have drawn more and more researchers’ attention and have shown great potential due to the low cost and excellent behaviors. Of special note is the Manganese oxides-based SIBs are commercially available as of 2014. It releases a great signal about the development tactics and investigative breakthrough of SIBs. As to graphene, the addition of them may produce the higher reversible capacity than that of active materials owing to the inherent properties of active materials in cathode electrodes. From the current study, main contribution of graphene for composite in SIBs is to increase the electrical 38

conductivity and prevent active substances from reuniting. More importantly, graphene will play a major role in flexible SIBs for wearable and other next-generation electronic devices in the near future.

5

Conclusions and outlook Graphene not only can be directly used as Li-S battery or SIBs electrode

materials, but also recombined with other nanomaterials in order to improve the comprehensive performance of electrode materials. Due to separation from each other, the nanoparticles can powerfully prevent graphene from reuniting; simultaneously, the aggregation of nanoparticles can be also effectively reduced by graphene. Such a synergistic effect, flexible characteristic as well as excellent conductivity offer excellent cycle performance and outstanding high-rate performance of the composite as electrode materials. Of special note is that graphene also provides a unique opportunity to form flexible battery through a low-cost technique. In addition to the fore-mentioned advantages, graphene maintains following features for nanomaterials in battery. As to the Li-S materials, there exist three critical difficulties to be dealt with: (1) deposition of solid Li2S2 and Li2S on the cathode inevitably causes the severe loss of active material and block the migration of lithium ion, further lowering the electrode conductivity; (2) more severely, sulfur possess an very low electronic conductivity, 5 × 10 −30 S cm −1 at room temperature, and is inherently insulation to both electrons and lithium ions. It reflects the main hindrance for utilizing sulfur; (3) large volume

39

expansion (80%) during lithiation and delithiation, resulting in the damage of the electrode. The dissolution of polysulfides in the electrolyte, low electronic conductivity and large volume expansion have been relieved by rationally designed architecture. The elastic graphene sheets, well-designed microstructure and robust chemical binding between sulfur and graphene explicitly exert influences on the chemical activation of sulfur and immediate polysulfides during charge–discharge through preventing them from expanding (sulfur) and dissolving (polysulfides). In addition, the high conductive graphene is of great assistanc for improving the electronic conductivity of sulfur. As to SIB materials, there also exist following critical difficulties to be tackled: (1) low energy density due to the nature of SIBs; (2) Searching for anode electrode materials for Na-ion batteries with high energy and power density, but with low volume change (or low-strain) during cycling remains a great challenge; (3) Cycle life is also another important factor to reduce the total cost. After 2010, the cyclability of the electrode materials in Na cells was significantly improved. However, the Coulombic efficiency per each cycle still needs to be improved.[116] The elastic graphene sheets can accommodate the volume change of anode materials during cycling, as expressed in LIBs, resulting in the high performance. On the other hand, graphene can partially enhance the cyclability and rate performance of cathode electrode in SIBs. Such a synoptically issues have been repeatedly confirmed by the previous works. Therefore, the nanomaterials/graphene composites as electrode 40

materials in Li-S battery or SIBs have achieved a great success, particularly in the high capacity and large rate properties by comparison with pristine inorganic nanoparticles. Tremendous advancements in graphene-based composites for the Li-S batteries or SIBs have been achieved under the strenuous effort devoted by researchers from the whole world. However, in order to satisfy the actual need in battery cycle life, rapid large current charging/discharging and high specific capacity, the further works should be strengthened in following aspects: (1) to develop a low-cost scalable synthesis route of graphene/nanoparticles to reduce commercial barriers to entry; (2) to improve cycling property and safety of graphene-based composite electrode at high rate to meet immediate requirement of the commercial application, particularly stationary energy storage and smart grids; (3) more importantly, the essential interaction between graphene and nanomaterials, such as enhanced coupling effect existing in inorganic/graphene, finite size effect and origin of chemical bond, still need to be profoundly revealed. The comprehensive research into structural design and correlation between electrochemical performance and structure, including space structure, chemical structure, microstructure (such as defects and pore size), will forcefully push the progress in graphene-based advanced battery, and generate a profound influence for energy storage device. (4) unique 2 D structure, large specific area and strong bonding capability allow a number of complicated chemical reaction to occur on the surface of graphene when active materials encounter electrolyte during

41

cycling; as a consequence, an in-depth study on interface chemistry is urgently needed to understand the nature of Li or Na ion cycling.

Acknowledgements

S. Wu gratefully acknowledge South China University of Technology

Notes and references School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510641, China. E-mail: [email protected].

[1] Q. Wang, Z.H. Wen, J.H. Li, Adv. Funct. Mater., 16 (2006) 2141. [2] LiFeng Cui, Yuan Yang, C. Hsu, Y. Cui, Nano lett., 9 (2009) 3370. [3] R.V. Noorden, Nature, 507 (2014) 26. [4] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X.D. Chen, A. Aspuru-Guzik, R.G. Gordon, M.J. Aziz, Nature, 505 (2014) 195. [5] R.F. Service, Science, 344 (2014) 352. [6] M.M. Ottakam Thotiyl, S.A. Freunberger, Z. Peng, Y. Chen, Z. Liu, P.G. Bruce, Nat Mater, 12 (2013) 1050. [7] Z.Q. Peng, S.A. Freunberger, Y.H. Chen, P.G. Bruce, Science, 337 (2012) 563. [8] I. Gur, K. Sawyer, R. Prasher, Science, 335 (2012) 1454. [9] B. Dunn, H. Kamath, J.M. Tarascon, Science, 334 (2011) 928. [10] P. Hartmann, C.L. Bender, M. Vračar, A.K. Dürr, A. Garsuch, J. Janek, P. Adelhelm, Nat Mater, 12 (2013) 228. [11] R.E. Doe, R. Han, J. Hwang, A.J. Gmitter, I. Shterenberg, H.D. Yoo, N. Pour, D. Aurbach, Chemical Communications, 50 (2014) 243. [12] H.N. He, H.Y. Wang, Y.G. Tang, Y.N. Liu, Progress in Chemistry, 26 (2014) 572. [13] P. Simon, Y. Gogotsi, B. Dunn, Science, 343 (2014) 1210. [14] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Electrochim. Acta, 55 (2010) 3909. [15] C. Wang, L. Yin, D. Xiang, Y. Qi, ACS Applied Materials & Interfaces, 4 (2012) 1636. [16] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature, 407 (2000) 496. [17] P.L. Taberna, S. Mitra, P. Poizot, P. Simon, J.M. Tarascon, Nat Mater, 5 (2006) 567. [18] A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin, Nat Mater, 9 (2010) 353. [19] S. Wu, R. Wang, Z. Wang, Z. Lin, Nanoscale, 6 (2014) 8350. [20] R. Wang, S. Wu, Y. Lv, Z. Lin, Langmuir, 30 (2014) 8215. [21] J.M. Tour, Nat Mater, 13 (2014) 545. [22] J.A. Torres, R.B. Kaner, Nat Mater, 13 (2014) 328. 42

[23] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat Mater, advance online publication (2014). [24] N. Mahmood, C.Z. Zhang, H. Yin, Y.L. Hou, Journal of Materials Chemistry A, 2 (2014) 15. [25] M.V. Reddy, G.V.S. Rao, B.V.R. Chowdari, Chemical Reviews, 113 (2013) 5364. [26] J.X. Zhu, D. Yang, Z.Y. Yin, Q.Y. Yan, H. Zhang, Small, 10 (2014) 3480. [27] D. Chen, W. Chen, L. Ma, G. Ji, K. Chang, J.Y. Lee, Materials Today, 17 (2014) 184. [28] N. Mahmood, C. Zhang, H. Yin, Y. Hou, Journal of Materials Chemistry A, 2 (2014) 15. [29] W.W. Sun, Y. Wang, Nanoscale, 6 (2014) 11528. [30] R.V. Noorden, Nature, 498 (2014) 416. [31] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nature Materials, 11 (2012) 19. [32] X.L. Ji, K.T. Lee, L.F. Nazar, Nature Materials, 8 (2009) 500. [33] K. Han, J.M. Shen, C.M. Hayner, H.Q. Ye, M.C. Kung, H.H. Kung, Journal of Power Sources, 251 (2014) 331. [34] K.H. Kim, Y.S. Jun, J.A. Gerbec, K.A. See, G.D. Stucky, H.T. Jung, Carbon, 69 (2014) 543. [35] Y.-S. Su, Y. Fu, T. Cochell, A. Manthiram, Nat Commun, 4 (2013). [36] Y.-S. Su, A. Manthiram, Nat Commun, 3 (2012) 1166. [37] M.Q. Zhao, Q. Zhang, J.Q. Huang, G.L. Tian, J.Q. Nie, H.J. Peng, F. Wei, Nature Communications, 5 (2014). [38] J. Li, Y.B. Guan, K. Fu, Y.F. Su, L.Y. Bao, F. Wu, Progress in Chemistry, 26 (2014) 1233. [39] J.W. Kim, J.D. Ocon, D.W. Park, J. Lee, Chemsuschem, 7 (2014) 1265. [40] D.-W. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H.-M. Cheng, I.R. Gentle, G.Q.M. Lu, Journal of Materials Chemistry A, 1 (2013) 9382. [41] F.G. Sun, J.T. Wang, H.C. Chen, W.C. Li, W.M. Qiao, D.H. Long, L.C. Ling, ACS Applied Materials & Interfaces, 5 (2013) 5630. [42] G.Y. Xu, B. Ding, P. Nie, L.F. Shen, H. Dou, X.G. Zhang, ACS Applied Materials & Interfaces, 6 (2014) 194. [43] J.X. Song, T. Xu, M.L. Gordin, P.Y. Zhu, D.P. Lv, Y.B. Jiang, Y.S. Chen, Y.H. Duan, D.H. Wang, Advanced Functional Materials, 24 (2014) 1243. [44] J.T. Lee, Y.Y. Zhao, H. Kim, W.I. Cho, G. Yushin, Journal of Power Sources, 248 (2014) 752. [45] W. Weng, V.G. Pol, K. Amine, Advanced Materials, 25 (2013) 1608. [46] D. Li, F. Han, S. Wang, F. Cheng, Q. Sun, W.C. Li, ACS Applied Materials & Interfaces, 5 (2013) 2208. [47] D. Wang, Y. Yu, W. Zhou, H. Chen, F.J. DiSalvo, D.A. Muller, H.D. Abruna, Physical Chemistry Chemical Physics, 15 (2013) 9051. [48] Z.W. Zhang, Z.Q. Li, F.B. Hao, X.K. Wang, Q. Li, Y.X. Qi, R.H. Fan, L.W. Yin, Advanced Functional Materials, 24 (2014) 2500. [49] Q.C. Zeng, D.W. Wang, K.H. Wu, Y. Li, F.C. de Godoi, I.R. Gentle, Journal of Materials Chemistry A, 2 (2014) 6439. [50] W.Z. Bao, Z.A. Zhang, Y.H. Qu, C.K. Zhou, X.W. Wang, J. Li, Journal of Alloys and Compounds, 582 (2014) 334. [51] H.B. Zhao, Z.H. Peng, W.J. Wang, X.K. Chen, J.H. Fang, J.Q. Xu, Journal of Power Sources, 245 (2014) 529. [52] X.F. Gao, J.Y. Li, D.S. Guan, C. Yuan, ACS Applied Materials & Interfaces, 6 (2014) 4154. [53] N. Li, M. Zheng, H. Lu, Z. Hu, C. Shen, X. Chang, G. Ji, J. Cao, Y. Shi, Chemical Communications, 48 (2012) 4106. 43

[54] H. Xu, Y. Deng, Z. Shi, Y. Qian, Y. Meng, G. Chen, Journal of Materials Chemistry A, 1 (2013) 15142. [55] J.P. Rong, M.Y. Ge, X. Fang, C.W. Zhou, Nano Letters, 14 (2014) 473. [56] M. Xiao, M. Huang, S.S. Zeng, D.M. Han, S.J. Wang, L.Y. Sun, Y.Z. Meng, RSC Advances, 3 (2013) 4914. [57] J.Q. Huang, Q. Zhang, S.M. Zhang, X.F. Liu, W.C. Zhu, W.Z. Qian, F. Wei, Carbon, 58 (2013) 99. [58] Y.H. Wu, M.X. Gao, X. Li, Y.F. Liu, H.G. Pan, Journal of Alloys and Compounds, 608 (2014) 220. [59] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat Mater, 14 (2015) 271. [60] W.Z. Bao, Z.A. Zhang, W. Chen, C.K. Zhou, Y.Q. Lai, J. Li, Electrochimica Acta, 127 (2014) 342. [61] X.Y. Zhao, J.P. Tu, Y. Lu, J.B. Cai, Y.J. Zhang, X.L. Wang, C.D. Gu, Electrochimica Acta, 113 (2013) 256. [62] X.Y. Zhou, J. Xie, J. Yang, Y.L. Zou, J.J. Tang, S.C. Wang, L.L. Ma, Q.C. Liao, Journal of Power Sources, 243 (2013) 993. [63] J.Q. Huang, X.F. Liu, Q. Zhang, C.M. Chen, M.Q. Zhao, S.M. Zhang, W.C. Zhu, W.Z. Qian, F. Wei, Nano Energy, 2 (2013) 314. [64] J. Xie, J. Yang, X.Y. Zhou, Y.L. Zou, J.J. Tang, S.C. Wang, F. Chen, Journal of Power Sources, 253 (2014) 55. [65] L. Sun, W. Kong, Y. Jiang, H. Wu, K. Jiang, J. Wang, S. Fan, Journal of Materials Chemistry A, 3 (2015) 5305. [66] Y. Yan, Y.X. Yin, Y.G. Guo, L.J. Wan, Advanced Energy Materials, 4 (2014). [67] C. Zhang, W. Lv, W.G. Zhang, X.Y. Zheng, M.B. Wu, W. Wei, Y. Tao, Z.J. Li, Q.H. Yang, Advanced Energy Materials, 4 (2014). [68] B. Wang, Y.F. Wen, D.L. Ye, H. Yu, B. Sun, G.X. Wang, D. Hulicova-Jurcakova, L.Z. Wang, Chemistry-a European Journal, 20 (2014) 5224. [69] Y.Y. Li, Z.S. Li, Q.W. Zhang, P.K. Shen, Journal of Materials Chemistry A, 2 (2014) 4528. [70] G.M. Zhou, S.F. Pei, L. Li, D.W. Wang, S.G. Wang, K. Huang, L.C. Yin, F. Li, H.M. Cheng, Advanced Materials, 26 (2014) 625. [71] C. Xu, Y. Wu, X. Zhao, X. Wang, G. Du, J. Zhang, J. Tu, Journal of Power Sources, 275 (2015) 22. [72] L. Yin, J. Wang, X. Yu, C.W. Monroe, Y. NuLi, J. Yang, Chemical Communications, 48 (2012) 7868. [73] J.L. Wang, L.C. Yin, H. Jia, H.T. Yu, Y.S. He, J. Yang, C.W. Monroe, Chemsuschem, 7 (2014) 563. [74] Y. Liu, J. Zhang, X.C. Liu, J.X. Guo, L.F. Pan, H.F. Wang, Q.M. Su, G.H. Du, Materials Letters, 133 (2014) 193. [75] W. Wang, G.C. Li, Q. Wang, G.R. Li, S.H. Ye, X.P. Gao, Journal of The Electrochemical Society, 160 (2013) A805. [76] Y.C. Qiu, W.F. Li, G.Z. Li, Y. Hou, L.S. Zhou, H.F. Li, M.N. Liu, F.M. Ye, X.W. Yang, Y.G. Zhang, Nano Research, 7 (2014) 1355. [77] L. Li, G.D. Ruan, Z.W. Peng, Y. Yang, H.L. Fei, A.R.O. Raji, E.L.G. Samuel, J.M. Tour, ACS Applied Materials & Interfaces, 6 (2014) 15033. 44

[78] X. Wang, Z. Zhang, X. Yan, Y. Qu, Y. Lai, J. Li, Electrochimica Acta, 155 (2015) 54. [79] H. Wang, H. Dai, Chemical Society Reviews, 42 (2013) 3088. [80] H. Wang, Y. Yang, Y. Liang, J.T. Robinson, Y. Li, A. Jackson, Y. Cui, H. Dai, Nano Letters, 11 (2011) 2644. [81] L.Q. Lu, L.J. Lu, Y. Wang, Journal of Materials Chemistry A, 1 (2013) 9173. [82] W.D. Zhou, H. Chen, Y.C. Yu, D.L. Wang, Z.M. Cui, F.J. DiSalvo, H.D. Abruna, ACS Nano, 7 (2013) 8801. [83] L. Wang, D. Wang, F.X. Zhang, J. Jin, Nano Letters, 13 (2013) 4206. [84] G. He, C.J. Hart, X. Liang, A. Garsuch, L.F. Nazar, ACS Applied Materials & Interfaces, 6 (2014) 10917. [85] Y.C. Qiu, W.F. Li, W. Zhao, G.Z. Li, Y. Hou, M.N. Liu, L.S. Zhou, F.M. Ye, H.F. Li, Z.H. Wei, S.H. Yang, W.H. Duan, Y.F. Ye, J.H. Guo, Y.G. Zhang, Nano Letters, 14 (2014) 4821. [86] C. Wang, K. Su, W. Wan, H. Guo, H.H. Zhou, J.T. Chen, X.X. Zhang, Y.H. Huang, Journal of Materials Chemistry A, 2 (2014) 5018. [87] X.W. Wang, Z. Zhang, Y.H. Qu, Y.Q. Lai, J. Li, Journal of Power Sources, 256 (2014) 361. [88] Z.Y. Wang, Y.F. Dong, H.J. Li, Z.B. Zhao, H.B. Wu, C. Hao, S.H. Liu, J.S. Qiu, X.W. Lou, Nature Communications, 5 (2014). [89] L. Ji, M. Rao, H. Zheng, L. Zhang, Y. Li, W. Duan, J. Guo, E.J. Cairns, Y. Zhang, Journal of the American Chemical Society, 133 (2011) 18522. [90] M.K. Song, Y.G. Zhang, E.J. Cairns, Nano Letters, 13 (2013) 5891. [91] B. Ding, C. Yuan, L. Shen, G. Xu, P. Nie, Q. Lai, X. Zhang, Journal of Materials Chemistry A, 1 (2013) 1096. [92] J.-Z. Wang, L. Lu, M. Choucair, J.A. Stride, X. Xu, H.-K. Liu, Journal of Power Sources, 196 (2011) 7030. [93] B. Wang, K. Li, D. Su, H. Ahn, G. Wang, Chemistry – An Asian Journal, 7 (2012) 1637. [94] Y.G. Zhang, Y. Zhao, Z. Bakenov, Ionics, 20 (2014) 1047. [95] J. Zhang, Z.M. Dong, X.L. Wang, X.Y. Zhao, J.P. Tu, Q.M. Su, G.H. Du, Journal of Power Sources, 270 (2014) 1. [96] X. Yang, L. Zhang, F. Zhang, Y. Huang, Y.S. Chen, ACS Nano, 8 (2014) 5208. [97] Z.X. Zhao, S. Wang, R. Liang, Z. Li, Z.C. Shi, G.H. Chen, Journal of Materials Chemistry A, 2 (2014) 13509. [98] Y.G. Zhang, Y. Zhao, Z. Bakenov, Nanoscale Research Letters, 9 (2014) 137. [99] F.B. Chen, Y.N. Wang, B.R. Wu, Y.K. Xiong, W.L. Liao, F. Wu, Z. Sun, Journal of Inorganic Materials, 29 (2014) 627. [100] Z.-k. Wei, J.-j. Chen, L.-l. Qin, A.-w. Nemage, M.-s. Zheng, Q.-f. Dong, Journal of The Electrochemical Society, 159 (2012) A1236. [101] H. Sun, G.-L. Xu, Y.-F. Xu, S.-G. Sun, X. Zhang, Y. Qiu, S. Yang, Nano Research, 5 (2012) 726. [102] Z. Feifei, D. Yunhui, H. Yun, H. Gang, Z. Xinbo, W. Limin, Journal of Physics: Conference Series, 339 (2012) 012003. [103] G.M. Zhou, F. Li, H.M. Cheng, Energy & Environmental Science, 7 (2014) 1307. [104] M. Koo, K.-I. Park, S.H. Lee, M. Suh, D.Y. Jeon, J.W. Choi, K. Kang, K.J. Lee, Nano Letters, 12 (2012) 4810.

45

[105] G. Zhou, D.-W. Wang, F. Li, P.-X. Hou, L. Yin, C. Liu, G.Q. Lu, I.R. Gentle, H.-M. Cheng, Energy & Environmental Science, 5 (2012) 8901. [106] X. Huang, B. Sun, K. Li, S. Chen, G. Wang, Journal of Materials Chemistry A, 1 (2013) 13484. [107] S.T. Lu, Y. Chen, X.H. Wu, Z.D. Wang, Y. Li, Scientific Reports, 4 (2014). [108] G.M. Zhou, L.C. Yin, D.W. Wang, L. Li, S.F. Pei, I.R. Gentle, F. Li, H.M. Cheng, ACS Nano, 7 (2013) 5367. [109] C. Wang, X. Wang, Y. Yang, A. Kushima, J. Chen, Y. Huang, J. Li, Nano Letters, 15 (2015) 1796. [110] J.X. Song, Z.X. Yu, T. Xu, S.R. Chen, H. Sohn, M. Regula, D.H. Wang, Journal of Materials Chemistry A, 2 (2014) 8623. [111] M.-Q. Zhao, X.-F. Liu, Q. Zhang, G.-L. Tian, J.-Q. Huang, W. Zhu, F. Wei, ACS Nano, 6 (2012) 10759. [112] M.P. Yu, W.J. Yuan, C. Li, J.D. Hong, G.Q. Shi, Journal of Materials Chemistry A, 2 (2014) 7360. [113] K. Han, Z. Liu, H.Q. Ye, F. Dai, Journal of Power Sources, 263 (2014) 85. [114] M.U.M. Patel, N.D. Luong, J. Seppala, E. Tchernychova, R. Dominko, Journal of Power Sources, 254 (2014) 55. [115] H. Gwon, J. Hong, H. Kim, D.H. Seo, S. Jeon, K. Kang, Energy & Environmental Science, 7 (2014) 538. [116] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chemical Reviews, 114 (2014) 11636. [117] E.M. Lotfabad, J. Ding, K. Cui, A. Kohandehghan, W.P. Kalisvaart, M. Hazelton, D. Mitlin, ACS Nano, 8 (2014) 7115. [118] T.Q. Chen, L.K. Pan, T. Lu, C.L. Fu, D.H.C. Chua, Z. Sun, Journal of Materials Chemistry A, 2 (2014) 1263. [119] T.Q. Chen, Y. Liu, L.K. Pan, T. Lu, Y.F. Yao, Z. Sun, D.H.C. Chua, Q. Chen, Journal of Materials Chemistry A, 2 (2014) 4117. [120] D. Datta, J.W. Li, V.B. Shenoy, ACS Applied Materials & Interfaces, 6 (2014) 1788. [121] L.-H. Yao, M.-S. Cao, H.-J. Yang, X.-J. Liu, X.-Y. Fang, J. Yuan, Computational Materials Science, 85 (2014) 179. [122] Y.X. Wang, S.L. Chou, H.K. Liu, S.X. Dou, Carbon, 57 (2013) 202. [123] Y. Yang, Y. Ya-Xia, G. Yu-Guo, W. Li-Jun, Advanced Energy Materials, 4 (2014) 1301584. [124] J. Ding, H.L. Wang, Z. Li, A. Kohandehghan, K. Cui, Z.W. Xu, B. Zahiri, X.H. Tan, E.M. Lotfabad, B.C. Olsen, D. Mitlin, ACS Nano, 7 (2013) 11004. [125] Y. Wen, K. He, Y. Zhu, F. Han, Y. Xu, I. Matsuda, Y. Ishii, J. Cumings, C. Wang, Nat Commun, 5 (2014). [126] K. Xi, P.R. Kidambi, R.J. Chen, C.L. Gao, X.Y. Peng, C. Ducati, S. Hofmann, R.V. Kumar, Nanoscale, 6 (2014) 5746. [127] C. Ling, F. Mizuno, Physical Chemistry Chemical Physics, 16 (2014) 10419. [128] B.C. Melot, J.M. Tarascon, Accounts of Chemical Research, 46 (2013) 1226. [129] S.W. Kim, D.H. Seo, X.H. Ma, G. Ceder, K. Kang, Advanced Energy Materials, 2 (2012) 710. [130] K.B. Hueso, M. Armand, T. Rojo, Energy & Environmental Science, 6 (2013) 734. [131] Y. Kim, K.H. Ha, S.M. Oh, K.T. Lee, Chemistry-a European Journal, 20 (2014) 11980. [132] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Energy & Environmental Science, 5 (2012) 5884. 46

[133] B.L. Ellis, L.F. Nazar, Current Opinion in Solid State & Materials Science, 16 (2012) 168. [134] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Advanced Functional Materials, 23 (2013) 947. [135] D.Y.W. Yu, P.V. Prikhodchenko, C.W. Mason, S.K. Batabyal, J. Gun, S. Sladkevich, A.G. Medvedev, O. Lev, Nat Commun, 4 (2013). [136] B.H. Qu, C.Z. Ma, G. Ji, C.H. Xu, J. Xu, Y.S. Meng, T.H. Wang, J.Y. Lee, Advanced Materials, 26 (2014) 3854. [137] C.K. Chan, X.F. Zhang, Y. Cui, Nano lett., 8 (2008) 307.

–30, Part 2 (1988) 1172.

[138] P. Ge, M. Fouletier, Solid State Ionics, 28

[139] D. DiVincenzo, E. Mele, Physical Review B, 32 (1985) 2538. [140] Y. Sun, L. Zhao, H. Pan, X. Lu, L. Gu, Y.-S. Hu, H. Li, M. Armand, Y. Ikuhara, L. Chen, X. Huang, Nat Commun, 4 (2013) 1870. [141] Y. Wang, X. Yu, S. Xu, J. Bai, R. Xiao, Y.-S. Hu, H. Li, X.-Q. Yang, L. Chen, X. Huang, Nat Commun, 4 (2013). [142] H.A. Cha, H.M. Jeong, J.K. Kang, Journal of Materials Chemistry A, 2 (2014) 5182. [143] S.M. Oh, J.Y. Hwang, C.S. Yoon, J. Lu, K. Amine, I. Belharouak, Y.K. Sun, ACS Applied Materials & Interfaces, 6 (2014) 11295. [144] H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, Journal of Physical Chemistry Letters, 2 (2011) 2560. [145] Y. Xu, E. Memarzadeh Lotfabad, H. Wang, B. Farbod, Z. Xu, A. Kohandehghan, D. Mitlin, Chemical Communications, 49 (2013) 8973. [146] P. Senguttuvan, G. Rousse, M. de Dompablo, H. Vezin, J.M. Tarascon, M.R. Palacin, Journal of the American Chemical Society, 135 (2013) 3897. [147] X.N. Li, X.B. Zhu, J.W. Liang, Z.G. Hou, Y. Wang, N. Lin, Y.C. Zhu, Y.T. Qian, Journal of The Electrochemical Society, 161 (2014) A1181. [148] G. Pang, C.Z. Yuan, P. Nie, B. Ding, J.J. Zhu, X.G. Zhang, Nanoscale, 6 (2014) 6328. [149] K.G. Wu, J. Shu, X.T. Lin, L.Y. Shao, M.M. Lao, M. Shui, P. Li, N.B. Long, D.J. Wang, Journal of Power Sources, 272 (2014) 283. [150] B. Farbod, K. Cui, W.P. Kalisvaart, M. Kupsta, B. Zahiri, A. Kohandehghan, E.M. Lotfabad, Z. Li, E.J. Luber, D. Mitlin, ACS Nano, 8 (2014) 4415. [151] P.R. Abel, M.G. Fields, A. Heller, C.B. Mullins, ACS Applied Materials & Interfaces, 6 (2014) 15860. [152] D.A. Stevens, J.R. Dahn, Journal of The Electrochemical Society, 147 (2000) 1271. [153] C. Nithya, S. Gopukumar, Journal of Materials Chemistry A, 2 (2014) 10516. [154] X. Zhou, Y. Zhong, M. Yang, M. Hu, J. Wei, Z. Zhou, Chemical Communications, 50 (2014) 12888. [155] W.J. Li, S.L. Chou, J.Z. Wang, J.H. Kim, H.K. Liu, S.X. Dou, Advanced Materials, 26 (2014) 4037. [156] L. Li, K.H. Seng, D. Li, Y. Xia, H.K. Liu, Z. Guo, Nano Research, 7 (2014) 1466. [157] C. Luo, Y.J. Zhu, Y.H. Xu, Y.H. Liu, T. Gao, J. Wang, C.S. Wang, Journal of Power Sources, 250 (2014) 372. [158] Z.L. Jian, B. Zhao, P. Liu, F.J. Li, M.B. Zheng, M.W. Chen, Y. Shi, H.S. Zhou, Chemical Communications, 50 (2014) 1215. [159] C. Deng, S. Zhang, Z. Dong, Y. Shang, Nano Energy, 4 (2014) 49.

47

[160] W. Wang, L.W. Hu, J.B. Ge, Z.Q. Hu, H.B. Sun, H. Sun, H.Q. Zhang, H.M. Zhu, S.Q. Jiao, Chemistry of Materials, 26 (2014) 3721. [161] W. Wu, L. Wang, Y. Li, F. Zhang, L. Lin, S. Niu, D. Chenet, X. Zhang, Y. Hao, T.F. Heinz, J. Hone, Z.L. Wang, Nature, 514 (2014) 470. [162] M. Chhowalla, H.S. Shin, G. Eda, L.-J. Li, K.P. Loh, H. Zhang, Nat Chem, 5 (2013) 263. [163] J. Kibsgaard, Z. Chen, B.N. Reinecke, T.F. Jaramillo, Nat Mater, 11 (2012) 963. [164] D.W. Su, S.X. Dou, G.X. Wang, Chemical Communications, 50 (2014) 4192. [165] Q. Pan, J. Xie, T.J. Zhu, G.S. Cao, X.B. Zhao, S.C. Zhang, Inorganic Chemistry, 53 (2014) 3511. [166] J. Xiao, M.Q. Long, X.M. Li, H. Xu, H. Huang, Y.L. Gao, Scientific Reports, 4 (2014). [167] R. Wang, C. Xu, J. Sun, Y. Liu, L. Gao, H. Yao, C. Lin, Nano Energy, 8 (2014) 183. [168] C.B. Zhu, X.K. Mu, P.A. van Aken, Y. Yu, J. Maier, Angewandte Chemie-International Edition, 53 (2014) 2152. [169] L. David, R. Bhandavat, G. Singh, ACS Nano, 8 (2014) 1759. [170] J.C. Su, Y. Pei, Z.H. Yang, X.Y. Wang, RSC Advances, 4 (2014) 43183. [171] G.S. Bang, K.W. Nam, J.Y. Kim, J. Shin, J.W. Choi, S.Y. Choi, ACS Applied Materials & Interfaces, 6 (2014) 7084. [172] Y.X. Wang, S.L. Chou, D. Wexler, H.K. Liu, S.X. Dou, Chemistry-a European Journal, 20 (2014) 9607. [173] C.X. Zhang, X.X. Zhang, H.J. Tao, Acta Physico-Chimica Sinica, 30 (2014) 1963. [174] W. Qin, T.Q. Chen, L.K. Pan, L.Y. Niu, B.W. Hu, D.S. Li, J.L. Li, Z. Sun, Electrochimica Acta, 153 (2015) 55. [175] X.Q. Xie, Z.M. Ao, D.W. Su, J.Q. Zhang, G.X. Wang, Advanced Functional Materials, 25 (2015) 1393. [176] Q. Sun, Q.-Q. Ren, H. Li, Z.-W. Fu, Electrochemistry Communications, 13 (2011) 1462. [177] X.S. Zhou, X. Liu, Y. Xu, Y.X. Liu, Z.H. Dai, J.C. Bao, Journal of Physical Chemistry C, 118 (2014) 23527. [178] D. Aurbach, A. Nimberger, B. Markovsky, E. Levi, E. Sominski, A. Gedanken, Chemistry of Materials, 14 (2002) 4155. [179] G.F. Ortiz, I. Hanzu, P. Lavela, P. Knauth, J.L. Tirado, T. Djenizian, Chemistry of Materials, 22 (2010) 1926. [180] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, A. Subramanian, H. Fan, L. Qi, A. Kushima, J. Li, Science, 330 (2010) 1515. [181] Y.X. Wang, Y.G. Lim, M.S. Park, S.L. Chou, J.H. Kim, H.K. Liu, S.X. Dou, Y.J. Kim, Journal of Materials Chemistry A, 2 (2014) 529. [182] D.W. Su, H.J. Ahn, G.X. Wang, Chemical Communications, 49 (2013) 3131. [183] S. Li, Y.Z. Wang, J.X. Qiu, M. Ling, H.H. Wang, W. Martens, S.Q. Zhang, RSC Advances, 4 (2014) 50148. [184] Y.D. Zhang, J. Xie, S.C. Zhang, P.Y. Zhu, G.S. Cao, X.B. Zhao, Electrochimica Acta, 151 (2015) 8. [185] Y.G. Liu, Z.Y. Cheng, H.Y. Sun, H. Arandiyan, J.P. Li, M. Ahmad, Journal of Power Sources, 273 (2015) 878. [186] S.H. Zhang, W.J. Li, B.E. Tan, S.L. Chou, Z. Li, S.X. Dou, Journal of Materials Chemistry A, 3 (2015) 4793.

48

[187] Z.-J. Zhang, Y.-X. Wang, S.-L. Chou, H.-J. Li, H.-K. Liu, J.-Z. Wang, Journal of Power Sources, 280 (2015) 107. [188] P.V. Prikhodchenko, D.Y.W. Yu, S.K. Batabyal, V. Uvarov, J. Gun, S. Sladkevich, A.A. Mikhaylov, A.G. Medvedev, O. Lev, Journal of Materials Chemistry A, 2 (2014) 8431. [189] X.Q. Xie, D.W. Su, S.Q. Chen, J.Q. Zhang, S.X. Dou, G.X. Wang, Chemistry-an Asian Journal, 9 (2014) 1611. [190] P.K. Dutta, U.K. Sen, S. Mitra, RSC Advances, 4 (2014) 43155. [191] T.F. Zhou, W.K. Pang, C.F. Zhang, J.P. Yang, Z.X. Chen, H.K. Liu, Z.P. Guo, ACS Nano, 8 (2014) 8323. [192] P. Senguttuvan, G. Rousse, V. Seznec, J.-M. Tarascon, M.R. Palacín, Chemistry of Materials, 23 (2011) 4109. [193] Y.H. Liu, T. Takasaki, K. Nishimura, S. Katsura, M. Yanagida, T. Sakai, Journal of The Electrochemical Society, 161 (2014) A1194. [194] X. Li, X. Ma, D. Su, L. Liu, R. Chisnell, S.P. Ong, H. Chen, A. Toumar, J.-C. Idrobo, Y. Lei, J. Bai, F. Wang, J.W. Lynn, Y.S. Lee, G. Ceder, Nat Mater, 13 (2014) 586. [195] A.M. Abakumov, A.A. Tsirlin, I. Bakaimi, G. Van Tendeloo, A. Lappas, Chemistry of Materials, 26 (2014) 3306. [196] X.F. Wang, G.D. Liu, T. Iwao, M. Okubo, A. Yamada, Journal of Physical Chemistry C, 118 (2014) 2970. [197] F.R. Beck, Y.Q. Cheng, Z.H. Bi, M. Feygenson, C.A. Bridges, Z. Moorhead-Rosenberg, A. Manthiram, J.B. Goodenough, M.P. Paranthaman, A. Manivannan, Journal of The Electrochemical Society, 161 (2014) A961. [198] M. Guignard, C. Didier, J. Darriet, P. Bordet, E. Elkaïm, C. Delmas, Nat Mater, 12 (2013) 74. [199] R. Berthelot, D. Carlier, C. Delmas, Nat Mater, 10 (2011) 74. [200] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat Mater, 11 (2012) 512. [201] S. Kajiyama, J. Kikkawa, J. Hoshino, M. Okubo, E. Hosono, Chemistry-a European Journal, 20 (2014) 12636. [202] N. Wongittharom, C.H. Wang, Y.C. Wang, C.H. Yang, J.K. Chang, ACS Applied Materials & Interfaces, 6 (2014) 17564. [203] M. Galceran, V. Roddatis, F.J. Zuniga, J.M. Perez-Mato, B. Acebedo, R. Arenal, I. Peral, T. Rojo, M. Casas-Cabanas, Chemistry of Materials, 26 (2014) 3289. [204] N. Wongittharom, T.C. Lee, C.H. Wang, Y.C. Wang, J.K. Chang, Journal of Materials Chemistry A, 2 (2014) 5655. [205] P. Serras, V. Palomares, A. Goni, P. Kubiak, T. Rojo, Journal of Power Sources, 241 (2013) 56. [206] R. Tripathi, S.M. Wood, M.S. Islam, L.F. Nazar, Energy & Environmental Science, 6 (2013) 2257. [207] T. Ramireddy, M.M. Rahman, N. Sharma, A.M. Glushenkov, Y. Chen, Journal of Power Sources, 271 (2014) 497. [208] P. Barpanda, G. Oyama, S.-i. Nishimura, S.-C. Chung, A. Yamada, Nat Commun, 5 (2014). [209] R. Tripathi, G.R. Gardiner, M.S. Islam, L.F. Nazar, Chemistry of Materials, 23 (2011) 2278. [210] P. Barpanda, J.N. Chotard, N. Recham, C. Delacourt, M. Ati, L. Dupont, M. Armand, J.M. Tarascon, Inorganic Chemistry, 49 (2010) 7401.

49

[211] M.W. Xu, L. Wang, X. Zhao, J. Song, H. Xie, Y.H. Lu, J.B. Goodenough, Physical Chemistry Chemical Physics, 15 (2013) 13032. [212] Y.H. Jung, C.H. Lim, D.K. Kim, Journal of Materials Chemistry A, 1 (2013) 11350. [213] Q. Fan, L.X. Lei, G. Yin, Y.F. Chen, Y.M. Sun, Electrochemistry Communications, 38 (2014) 120. [214] Y. Liu, S.J. Xu, S.M. Zhang, J.X. Zhang, J.C. Fan, Y.R. Zhou, Journal of Materials Chemistry A, 3 (2015) 5501. [215] H.L. Fei, H. Li, Z.W. Li, W.J. Feng, X. Liu, M.D. Wei, Dalton Transactions, 43 (2014) 16522. [216] D.L. Chao, C.R. Zhu, X.H. Xia, J.L. Liu, X. Zhang, J. Wang, P. Liang, J.Y. Lin, H. Zhang, Z.X. Shen, H.J. Fan, Nano Letters, 15 (2015) 565. [217] D.Z. Yang, X.Z. Liao, J.F. Shen, Y.S. He, Z.F. Ma, Journal of Materials Chemistry A, 2 (2014) 6723. [218] H.L. Zhu, K.T. Lee, G.T. Hitz, X.G. Han, Y.Y. Li, J.Y. Wan, S. Lacey, A.V. Cresce, K. Xu, E. Wachsman, L.B. Hu, ACS Applied Materials & Interfaces, 6 (2014) 4242.

Songping Wu obtained his Ph.D at South China University of Technology, China. 2002. For the past 10 years, he has been working as an associate professor (2004- ) at South China University of Technology. He once worked as a visiting scholar at The Hong Kong Polytechnic University (2006-2007) and Georgia Institute of Technology (2013-2014). He has been working on advanced functional ceramic materials and Li ion battery materials. He has published more than 50 peer-reviewed papers in various international journals. His current research interests are focused on graphene-based materials for lithium ion batteries.

Rongyun Ge completed her bachelor studies at Henan University of Technology in 2012. She has been engaging in the synthesis and application of graphene-based anode materials for lithium ion batteries as a master in South China University of Technology since Sep. 2013.

50

Mingjia Lu received her Bachelor’s degree from Institute of Physical Chemistry Henan Polytechnic University in 2012. She is currently pursuing her Master degree at School of Chemistry and Chemical Engineering, South China University of Technology. Her current research interests are focused on the synthesis and application of graphene-based anode materials for lithium ion batteries.

Rui Xu received her Bachelor’s degree from Institute of Chemical and Materials Engineering at Hubei Polytechnic University in 2012. She is currently pursuing her Master degree at School of Chemistry and Chemical Engineering, South China University of Technology. Her current research interests are focused on the synthesis and application of graphene-based anode materials for lithium ion batteries.

Zhen Zhang currently is a Professor at South China University of Technology, China. He serves as director of physical chemistry course. He received his M.S. in Applied Chemistry from South China University of Technology. His current research interests include the anode material for lithium-ion batteries, copper electroplating (electrolytic copper foil) and electrochemical degradation of wastewater. He has co-authored 4 books/textbooks and produced more then 40 papers. His research and technical contributions has been recognized with scientific and technological progress awards from Ministry of Education, Guangzhou Municipal Government and China Petroleum and Chemical Industry Federation respectively.

51

Figures and Figure Captions

Figure 1 Schematic representations of Li-ion, non-aqueous and aqueous Li–O2 and Li–S cells. Reproduced with permission from ref. [31] Copyright 2012 Nature.

52

Figure 2 Structural models and a possible drawback of graphene composites. a, Schematic of the different structures of graphene composite electrode materials. All models (except where specifically indicated) refer to composites in which graphene and the active material are synthesized through one-pot processes. Encapsulated: Single active-material particles are encapsulated by graphene, which acts as either an active (for example, LIB anodes) or an inactive (for example, LIB cathodes) component. Mixed: Graphene and active materials are synthesized separately and mixed mechanically during the electrode preparation. In this structure, graphene may serve as an inactive conductive matrix (for example, LIB cathodes) or an active material (for example, LIB anodes). Wrapped: The active-material particles are wrapped by multiple graphene sheets. This structure well-represents pseudocapacitor electrodes, in which graphene is the active material, as well as metal-ion battery cathodes, where graphene is an inactive component. Anchored: This is the most common structure for graphene composites, in which electroactive nanoparticles are anchored to the graphene surface. This structure is very relevant for metal-ion battery anodes and pseudocapacitors, where graphene serves as an active material, as well as for metal-ion battery cathodes and in LSBs, where graphene acts as an inactive component. Sandwich-like model: Graphene is used as a template to generate active material/graphene sandwich structures. This graphene-composite model, although not widespread, is used for LIB cathodes. Layered model: Active-material nanoparticles are alternated with graphene sheets to form a composite layered structure, which has been proposed for use in metal-ion battery anodes and cathodes. b, Li+ paths in carbon black- (left) and graphene- (right) based electrodes in the mixed structural model. The figure highlights a possible drawback of graphene in terms of Li+ mobility. Reproduced with permission from ref. [23] Copyright 2014 Nature.

53

Figure 3 Practical specific energies for some rechargeable batteries, along with estimated driving distances and pack prices. For future technologies, a range of anticipated specific energies are given as shown by the lighter shaded region on the bars in the chart for rechargeable batteries under development and R&D. Reproduced with permission from ref. [31] Copyright 2012 Nature.

54

Figure 4 Recharge strategy for elongating the cycle life of Li-S batteries. (a) Typical discharge profile of the Li-S battery showing an upper plateau and a lower plateau. (b) The strategic approach to recharge Li-S batteries. A full initial discharge followed by recharging and cycling within the lower plateau region. The lower plateau possesses a theoretical capacity of 1256 mAh g-1. Reproduced with permission from ref. [35] Copyright 2014 Nature.

55

Figure 5 Scheme for the synthesis of unstacked DTG. (a) Charge–discharge curves of the DTG/S cathode. (b) Long cycle performance of the DTG/S cathode at 1.0, 5.0 and 10.0 C. Reproduced with permission from ref.[37] Copyright 2014 Nature.

56

Figure 6 (a)(b)(c) Scheme of synthesizing S@rGO composite and possible reaction mechanism of GO reduction by hydrogen iodide; (d) Electrochemical performance of S@GO and S@rGO electrodes, a: cycling performance at 0.05C. Inset: C-rate performance. Reproduced with permission from ref. [51] Copyright 2014 Elsevier.

57

Figure 7

(a) Cycle performance at a constant current rate of 210 mA g−1 after initial

activation processes at 42 mA g−1 for two cycles; (b) Rate performance of GWS composite electrode. Reproduced with permission from ref. [52] Copyright 2014 The American Chemical Society.

58

Figure 8

(a) Digital camera images of (upper) GO dispersed in different solutions at

the beginning. (middle) GO dispersion after 12 h and (low) after adding sulfur particles to GO dispersion; (b) Electrochemical measurements of bare sulfur particles and sulfur/GO: Galvanic charge-discharge performance and Coulombic efficiency of sulfur/GO at 1 A g-1 for 1000 cycles. Discharge specific capacity calculated based on weight of sulfur only (green dotted line) and total weight of sulfur/GO are plotted. Reproduced with permission from ref. [55] Copyright 2014 American Chemical Society.

59

Figure 9

(a) Cyclic voltammograms of the RGO@CMK-3/S cathode in the potential

window from 1.6 to 3.0 V (versus Li/Li+) at the scan rate of 0.2 mV S-1, (b) Cycling performances of RGO@CMK-3/S composite and CMK-3/S composite at 0.2 C, (c) Cycling performances of RGO@CMK-3/S composite at 0.5 C and 1 C, (d) Rate capability of RGO@CMK-3/S composite, (e) Scheme of RGO@CMK-3/S composite for improving the cathode performance. Reproduced with permission from ref. [62] Copyright 2013 Elsevier.

60

Figure 10

Schematic illustration of entrapment of sulfur in graphene support for

lithium–sulfur batteries during the charging–discharging process. Reproduced with permission from ref. [63] Copyright 2013 Elsevier.

61

Figure 11 Schematic illustration of the combination strategy between sulfur and functional graphene-based carbon. (a) introduction of polymer to sulfur particles; (b) immobilization of sulfur on N-doped graphene and (c) sulfur particles directly anchored on graphene.

62

Figure 12 (a) Schematic illustration of the synthesis of SPGs; (b): (1) Rate performance of SPGs at various rates from 0.1 to 1.0 C with respect to cycle numbers. (2) Cycling performance of SPGs at a rate of 0.4 C. Reproduced with permission from ref. [77] Copyright 2014 The American Chemical Society.

63

Figure 13 (a) Proposed synthesis route for creating S@NG nanocomposite and N functional groups for trapping Li2Sx. (b) The rate performance comparisons between the S@NG electrode with and without Li2S8 additive and (c) its corresponding voltage-capacity profiles of the S@NG electrode with Li2S8 additive at various discharge/charge current rates. (d) Cycling performance comparisons between the 60% (S/total electrode weight) S@NG electrode with and without the Li2S8 additive, and 70% (S/total electrode weight) S@NG electrode with the Li2S8 additive cycled at 0.2 C. Reproduced with permission from ref. [85] Copyright 2014 The American Chemical Society.

64

Figure 14 Electrochemical performance of the 3D-NGS composite. Cycling stability and corresponding coulombic efficiency of the 3DNGS at a current density of 100 mA g-1 for the first two cycles and 600 mA g-1 (a) and 1500 mA g-1 (d) for the following cycles. (b) EIS of the GO-S and 3D-NGS composites with an applied sinusoidal excitation voltage of 5 mV in the frequency range from 100 kHz to 0.1 Hz. (c) Rate capability of the 3D-NGS with current densities ranging from 100 mA g-1 to 1500 mA g-1 and the first discharge-charge profiles at different current densities.. Reproduced with permission from ref.[86] Copyright 2014 The Royal Society of Chemistry.

65

Figure 15 (a) A schematic showing the preparation of L-GPC material, Electrochemical characterization of L-GPCS-68% material as the cathode of Li-S battery. (b) Cyclic voltammetry (CV) measured between 1.5 and 3 V at a sweep rate of 0.1 mV S-1 for the first, second, and third cycles. (c) Discharge/charge voltage profiles at various rates from 0.25 to 5 C (1 C = 1675 mA g-1). (d) Discharge/charge capacity cycled at various rates from 0.25 to 5 C. (e) Capacity retention of L-GPCS-68% material cycled at 0.5 C, in comparison with GS-63% material. Specific capacity values were all calculated based on the mass of sulfur. Reproduced with permission from ref.[96] Copyright 2014 The American Chemical Society.

66

Figure 16 (a) Photograph of a bendable LIB turning on a blue LED in the bent condition. The inset shows stacked layers in the flexible LIB. Reproduced with permission from ref.[104] Copyright 2012 The American Chemical Society. (b) A model and structure of the S-CNT cathode. Reproduced with permission from ref.[105] Copyright 2012 The Royal Society of Chemistry.

67

Figure 17 (a) Schematic illustration of a Li–S cell with sandwich-structured cathode. (b) Photo images of sandwich-structured electrodes at titled (left) and bended state (right). (c)The rate capability of sandwich-structured sulfur and C–S cathodes; (d) the specific capacity and areal capacity of sandwich structured sulfur cathode as function of cycling with a sulfur loading of 4.0 mg cm-2. Reproduced with permission from ref.[110] Copyright 2014 The The Royal Society of Chemistry.

68

Figure 18 Graph showing the specific energies (based on discharge 1) of sodium ion materials achieved in sodium ion cells. Insets: electric vehicle (left) and A 48 cell battery pack design by Williams Advanced Engineering, incorporating Faradion’s 3 Ah cells (right).The Graph and battery pack are from the http://www.faradion.co.uk.

69

Figure 19 Schematic illustration of sodium storage in graphite-based materials. (a) Na+ cannot be electrochemically intercalated into graphite because of the small interlayer spacing. (b) Electrochemical intercalation of Na+ into GO is enabled by the enlarged interlayer distance because of oxidation. However, the intercalation is limited by steric hindering from large amounts of oxygen-containing groups. (c) A significant amount of Nat can be electrochemically intercalated into EG owing to suitable interlayer distance and reduced oxygen-containing groups in the interlayers. Reproduced with permission from ref. [125] Copyright 2014 Nature.

70

Figure 20 Classification of anode materials based on the reversible Li insertion and extraction process: Intercalation-deintercalation, alloying-dealloying, and conversion (redox) reaction. In favorable cases, they can act synergistically to yield to large and stable capacities. Selected examples are given. Schematic diagram of the process is shown.

71

Figure 21

Schematic illustrations of (a) anatase-type TiO2, (b) NaTi2(PO4)3.

Reproduced with permission from ref.[116] Copyright 2014 The American Chemical Society.

72

Figure 22 (a) Elements of groups 14 and 15 in the Periodic Table to form binary compounds with Na, and schematic illustrations of the most Na-rich phases for Si and Ge (b), Sn and Pb (c), and P, As, and Sb (d). Reproduced with permission from ref.[116] Copyright 2014 The American Chemical Society.

73

Figure 23

(a) Voltage profile of 60MoS2 free-standing electrode along with its

corresponding structure, (b) Digital picture showing large area composite paper prepared through vacuum filtration. Inset marked by arrows: Schematic representation showing the predicted mechanism for Na insertion and extraction into an idealized MoS2/rGO free-standing composite paper electrode. Reproduced with permission from ref. [169]Copyright 2014 The American Chemical Society.

74

Figure 24 Electrochemical performance of bulk Sb2S3 and rGO/ Sb2S3. (a) First cycle charge–discharge profiles at 50 mA g-1, (b) Cycle performance at 50 mA g-1, (c) Cyclic voltammogram of bulk Sb2S3 and (d) Cyclic voltammogram of rGO/Sb2S3. Reproduced with permission from ref. [135] Copyright 2014 Nature.

75

Table 1. Physical properties and electrochemical Li cycling data of typical Li-S materials S percentage(by Morphology

Synthesis approach

Cycle performance (mAh g-1)

Electrolyte/voltage window

ref.

800 mAh g-1 at 1 A g-1 after

Ionic solutions, 1.9- 2.6 V

[55]

698 mA h g-1/210 mA g-1/ over

1 M LiTFSI/DOL-DME (1:1)1

[52]

15 cycles

wt% LiNO3, 1.5- 3.0 V

788 mA h g-1 (at 0.75C) after

1 M LiTFSI/DOL-DME (1:1),

160 cycles

1.5-3.0 V

650 mA h g-1 (at 0.1C) after

1 M LiTFSI/DOL-DME (1:1),

100 cycles

1.5-3.0 V

140 mA h g-1 (at 1672 mA g-1 )

1 mol Kg-1

after 80 cycles(-40 )

LiTFSI/DOL-DME(1 : 1), 1.5-

weight) Sulfur/graphene oxide

Ionic solutions

50%

core-shell particles Graphene-wrapped S NPs

1000 cycles (based on sulfur) Wet chemical reduction

Graphene-encapsulated

Chemical reduction

sulphur

method

Graphene-coated mesoporous

Self-assembly approach

carbon/ S

followed by heat treatment

Hierarchical porous

Thermal exploitation

graphene/S

followed by heat treatment

80%

83.30%

75%

66%

℃

[54]

[61]

[63]

3.0 V Graphene/S/carbon

Heat treatment followed

nanospheres

with chemical reduction

pPAN-S/mGO-S

Heating treatment and

-1

64.20%

Emulsion and precipitation

65.1%

black

route

Amylopectin wrapped GO/S

Chemical precipitation

CTAB-modified

Chemical reduction

sulfur/graphene oxide

followed by heat treatment

70%

1 M LiTFSI/DOL-DME(1 : 1),

100 cycles

0.1M LiNO3, 1.0- 3.0 V

-1

650 mA h g (at 0.1C ) after

1 M LiTFSI/DOL-DME(1 : 1),

10 cycles

1.0- 3.0 V

600 mA h g-1 (at 0.2C) after 10

1 M LiTFSI/DOL-DME (1:1),

cycles

1.7-2.5 V

441 mAh g-1 at 5C/16 after

1 M LiTFSI/DOL-DME (1:1),

175 cycles

1.5-2.8 V

800 mA h g-1/6C after 150

PYR14TFSI/DOL/DME (2:1:1)

cycles

containing 1 M LiTFSI, 0.1M

chemical reaction PEG modified GO-carbon

815 mA h g (at 0.1 C ) after

56%

80%

[68]

[72]

[80]

[82]

[90]

LiNO3,1.5- 2.8 V KOH activated graphene

High temperature

-1

67%

volatilization Porous structure of S/GO

Wet mixing followed by

Wet chemical reduction

63%

Hydrothermal followed by

63.60%

Sulfur melt approach

63%

60%

1.0- 3.0 V

591 mA h g /0.1C after 100

1 M LiCF3SO3/DOL-DME

cycles

(1:1)1.0- 3.0 V

804 mA h g /0.186C after 80

1 M LiTFSI/DOL-DME

cycles

(1:1),1.5-3.0 V

-1

freeze drying Graphene–SWCNT hybrid

cycles

-1

graphene sheets Graphene–sulfur hybrid

1 M LiTFSI/DOL-DME(1 : 1),

-1

heat treatment S NPs dispersed on the

1007 mA h g /0.2C/60th

-1

700 mA h g (at 0.3 A g )

1 M LiTFSI/DOL-DME(1 : 1),

after 50 cycles

0.5 wt% LiNO3, 1.5- 2.8 V

-1

530 mA h g (at 1 C) after 100

1 M LiTFSI/DOL-DME(1 : 1),

cycles

1.5- 3.0 V

76

[91]

[94]

[101]

[108]

[111]

Highlights 1.

Graphene-based nanomaterials for energy-storage device (Li-S battery and SIBs).

2.

Electrochemical principles and performances of graphene-based materials are considered

3.

key obstructions and future development of novel materials were reviewed.

77

*Graphical Abstract

TOC Graphic

Graphene-based nano-materials have aroused considerable interest as electrode materials for lithium-sulfur battery and sodium-ion battery (SIBs) due to their unique properties. The excellent electrochemical performances of graphene-based nanomaterials suggested that they are promising candidates for electrode materials in next-generation energy storage device. This article is a timely and powerful report, which comprehensively elaborates the present achievements, the latest progress and future advancement of graphene-based electrode materials for Li-S battery and SIBs.