carbon-coated SnO2 nanoparticle

Materials and Design 114 (2017) 234–242

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Materials and Design journal homepage: www.elsevier.com/locate/matdes

Cotton/rGO/carbon-coated SnO2 nanoparticle-composites as superior anode for Lithium ion battery Xueqian Zhang a, Xiaoxiao Huang a,⁎, Xiaodong Zhang a, Long Xia b, Bo Zhong b, Tao Zhang b, Guangwu Wen a,b,⁎ a b

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, WeiHai 264209, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The rGO/SnO2-C composites were designed and synthesized at a low cost. • The flexible anode was prepared without any additives. • Amorphous carbon covering on the SnO2 nanoparticles leads to the high capacity retention and coulombic efficient.

a r t i c l e

i n f o

Article history: Received 17 September 2016 Received in revised form 20 November 2016 Accepted 21 November 2016 Available online 24 November 2016 Keywords: Amorphous carbon rGO Flexible Lithium ion battery

a b s t r a c t The three-dimensional carbonized cotton framework covered by reduced graphene oxide (rGO) was uniformly decorated by the SnO2 nanoparticles encapsulated by a layer of the amorphous carbon and was directly used in the anode materials for the lithium ion battery. The carbonized frameworks covered by rGO are interconnected and the pores are enriched, facilitating the diffusion of electron and releasing the strain of lithium ions insertion and extraction. The rGO sheets with better mechanical and foldable characterization increase the conductive and buffer the electrode expansion during the cycling process. Furthermore, the amorphous carbon coating effectively prevents the direct contact between the SnO2 nanoparticles and the electrolyte, which can format a stable solid electrolyte interphase and greatly reduce the irreversible reaction. As a result, the capacity of composite is as high as 496.3 mAh g−1 (1.72 mAh cm−2) after 200 cycles at current density of 100 mA g−1, which means that the product as free-standing and binder-free electrode exhibits the stable cycle performance and will be a promising material for the application of lithium ion battery. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Among variety of the secondary batteries, lithium ion battery (LIB) is demonstrated as one of the most hopeful products for large scale extension, due to their high energy, high power densities, long lifespan, and environmental consideration [1–4]. In recent years, such new methods and structure designs have been developed to enhance the performance ⁎ Corresponding authors. E-mail addresses: [email protected] (X. Huang), [email protected] (G. Wen).

http://dx.doi.org/10.1016/j.matdes.2016.11.081 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

of LIB anodes. Although encouraging improvements have been achieved, many studies only focus on the active materials themselves [5,6]. In most researches, the electrode materials inevitably need to mix active materials with conductive additives and polymer binders, then paste onto the metal current collector [7,8]. Nevertheless, this method not only complex, but also bring some unwillable negative effects. Due to low capacity, the conductive additives have little contributes to lithium storages [9]. The insulating and electrochemical inactive polymer binders greatly decrease the electrical conductive and inevitably increase the polarization of electrodes [9]. Additionally, conductive

X. Zhang et al. / Materials and Design 114 (2017) 234–242

additives, polymer binders, and metal current collectors increase the whole weight of electrodes, which decrease the specific capacity of total electrode. In the same time, with the development of wearable electronic devices, such as smart watches and folding mobile phones, a flexible electronic system requires that the electrodes can be able to bend or crimp [10]. Thus, the large-scale fabrication flexible electrode, without using conductive additives, binders, and current collector is still a great challenge for LIB. Recently, the free-standing and binder-free three-dimensional (3D) electrode with flexible ability is an extraordinary concept, which not only avoids the use of the conductive additives, binders and current collectors, but also reduces the cumbersome process of traditional methods and ensures the stability of the electrode preparation process and of the performance [11]. As flexible substrate, commercial available cotton has the advantages of large-scale, cheap, easy weave and bendable, which has been carried out related researches on flexible energy storage [12,13]. Graphene, with intrinsically excellent electrical conductivity, high surface area and outstanding mechanical flexibility, has been integrated with other active materials to enhance cycling stability and improved rate capacities [14,15]. Our previous researches on the cotton covered by graphene composite (CGN) have shown their free-standing and binder-free characteristics [16]. However, It is still deserved to further develop for the utilization of CGN as free-standing, binder-free, and flexible 3D framework composite with high specific capacity active material due to the growing demands. As one of the most promising anode materials for LIB, SnO2 has attracted increasing attention due to its chemical stability, abundance, low cost, friendly environmental, and high theoretical specific capacity (782 mAh g−1) [1,17]. Although the specific capacity of SnO2 is much larger than that of graphite (372 mA g−1), the low conductivity and dramatic volume change upon lithium ions insertion/extraction easily lead to the structural damage. The apparent capacity loss of the electrodes after an amount of the repeated cycling hinders the commercialization for LIB [1,18]. To circumvent these challenges, two strategies could effectively solve those problems. The first strategy is to increase the effective void spaces to buffer the volume change and alleviate the diffusion-induced strains during cycles, resulting in designing the unique morphology of SnO2 with zero dimensional (0D) nanoparticle [7], one dimensional (1D) nanorod [19], nanowire [20], and nanotube [21], two dimensional (2D) nanosheet [22], and three-dimensional (3D) hollow nanosphere [23], and so on. The other one is to compose with carbonaceous materials, which based on reducing volume change and increasing conductivity with the assistance of the carbon materials. In particular, the high reversible stability and rate capacities performance of the graphene/SnO2 composites are attributed to high electrical conductive, efficient electrolyte penetration way, and the smaller volume change [24,25]. In addition, the graphene with high area surface is powerful in preventing the aggregation of SnO2 nanoparticles during the cycle processes [26]. However, there still have a critical issue to affect the cycling performance, the unstable solid electrolyte interphase (SEI) layer caused by the SnO2 direct contact with electrolyte. Among the available techniques, amorphous carbon coatings seem to be a promising prospect, and to introduce amorphous carbon could be an effective way to avoid the SnO2 nanoparticles to direct contact with electrolyte. In this study, a novel 3D carbonized cotton covered by rGO/amorphous carbon coating on SnO2 nanoparticles (CGN/SnO2-C) composite is step-wise conducted by in situ and hydrothermal method, then followed with thermal annealing process. The unique 3D flexible architecture can not only prevent the SnO2 direct contact with electrolyte, but also provide accommodate spaces to release the volume change during cycling, which is essential to improve the cyclic stability and rate capacity for LIB. As a result, the CGN/SnO2-C can be directly applied as the LIB anode, without any conductive additive, polymer binder, and current collector, delivering a high capacity of 496.3 mAh g−1 (1.72 mAh cm−2) after 200 cycles. The easy facile and low cost make it very suitable for large-scale production.

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2. Experimental 2.1. Materials synthesis Typically, graphene oxide (GO) was synthesized from natural graphite flakes using a modified Hummer's method, as our previous reported [16]. The commercial non-woven cotton piece was immersed into the GO (7 mg ml−1) suspension, soaked for 30 min at room temperature to prepare cotton covered by graphene oxide (CGO). Then the CGO composites were frozen at − 50 °C in cryogenic refrigerator for 24 h, and then freeze-drying for 48 h. Like other reports [27,28], the non-woven cotton at inert atmosphere under high temperature was named carbonized cotton. The carbonized cotton covered by rGO/amorphous carbon coating on SnO2 nanoparticles (CGN/SnO2-C) process as follows: 3 g SnCl2·2H2O was dispersed in 100 ml ethanol and stirred for 4 h at 60 °C. Then the CGO composite was immersed in the above solution for 24 h at room temperature. Afterwards, the as-prepared precipitate was transferred into a 100 ml Teflonlined stainless steel autoclave, with 80 ml of glucose (10 mg ml−1) and CO(NH2)2 (10 mg ml−1) mix solution and kept at 180 °C for 10 h. The product was then washed with distilled water several times and dried at 60 °C overnight. Afterwards, the dried product was thermal annealing at 500 °C under N2 atmosphere for 1 h to obtain CGN/SnO2-C composite. For comparison, the sample without amorphous carbon coating (without hydrothermal process) assigned as CGN/SnO2. In addition, the cotton and CGO were thermal annealing at 500 °C under N2 atmosphere for 1 h to prepare the carbonized cotton and carbonized cotton covered by rGO (CGN). 2.2. Materials test Crystallographic information of the as-prepared composites was characterized by X-ray diffraction (XRD) machine (D&A25ADVANCE, BRUKER, Germany), using Cu Kα radiation (λ = 0.15418 nm). The microstructures of the as-prepared samples were observed by scanning electron microscope (SUPRA™55, ZEISS, United Kingdom). Transmission electron microscopy (TEM) images and the corresponding selected-area electron diffraction (SAED) patterns were captured on a microscope (Tecnai G2 F30, FEI, America). The microstructures of the samples were investigated by Raman spectroscopy (inVia, Renishaw, England) with an excitation wavelength of 532 nm from 800 to 3200 cm−1 wave number. The electrical resistance was analyzed using a Semiconductor analyzer (4200-SCS, Keithley, America). 2.3. Electrochemical test CGN/SnO2-C and CGN/SnO2 were first cut into small disks with the diameter of 10 mm (thickness of 140–160 μm). The disks were directly used as free-standing and binder-free anodes for electrochemical measurements toward the storage of Li+, without any conductive agent, binder and current collector. A piece of lithium foil was used as the counter-electrode and reference electrode in the half cells. The electrolyte was LiPF6 in a mixture of ethylene carbonate (EC)/diethyl carbonate (DMC) = 1:1. The discharge-charge cycling performance of the as-prepared samples were investigated using a cell test system (LAND CT2001A model, Wuhan SHENGLAN, China), while the specific capacities of the composites were based on their total mass. The cells were tested in the voltage window between 0.001 and 3 V at room temperature. The cyclic voltammetry (CV) was recorded on a CHI760E electrochemical work station. 3. Results and discussion As shown in Fig. S1(a), the XRD spectrum of graphite exhibits a strong characteristic (002) peak at 26.4°. After the oxidation and exfoliation, this peak disappears and a new peak is observed at 11.5°,

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corresponding to the lattice of GO, which indicates the graphite is converted into GO [29,30]. From calculating, the interlayer spacing of GO (0.77 nm) is larger than that of graphite (0.34 nm), due to oxygen-containing groups formed on the layers. The GO sheets are dispersed in deionized water for a uniform GO suspension (7 mg mL− 1), without precipitate and stable for more than two weeks (Fig. S1(b)), indicating that GO has a good solubility in water. The non-woven cotton is used as the flexible framework to designed and synthesized CGN/SnO2-C composite. The formation process is mechanism illustrated in Fig. 1. Firstly, the non-woven cotton is immersed into GO suspensions from the electrostatic interaction, van der Waals' and hydrogen bond between the oxygen groups on GO and cotton fibers, and the GO is absorbed on the cotton fibers to form the composite with the cotton covered by graphene oxide (CGO) [13,31]. Fig. 2(a) shows the typical optical image of white non-woven cotton. After immersing in GO suspension and freeze-drying, the color is changed from white to brown (CGO), as shown in Fig. 2(b). Secondly, the CGO is then stepped in SnCl2 solution. Because the oxygen functional groups exist on the cotton fibers and GO sheets can chemically bond with the metals ions, Sn2+ ions (from SnCl2) are likely to be absorbed by electrostatic interaction, then SnO2 nanoparticles would form by a situ way [32], and at the same time the GO is partly reduced by Sn2+ to form rGO. Subsequently, the amorphous carbon coats on the surface of SnO2 by hydrothermal method. Finally, the non-woven cotton is carbonized, which will facilitate the electron and lithium ions transport. It can be found the color of CGN/SnO2-C is black (Fig.2(c)), and the composite can be bent without any detachment of any debris (inset in Fig. 2(c)). The strong mechanical strength and flexibility of CGN/SnO2-C would be very helpful for using directly as an electrode without any post-treatment. The XRD patterns of the prepared CGN/SnO2 and CGN/SnO2-C are shown in Fig. 2(d). The diffraction peaks located at 2Θ value of 26.4, 33.5, 37.6, 62.3 and 65.6° can be attributed to the (110), (101), (200), (211), (310) and (301) planes of rutile SnO2 (JCPDS card NO. 411445). There are no other forms of tin oxide with different oxidation states detected, which indicates that highly phase-pure SnO2 nanoparticles can be produced in the experiment process. The size of SnO2 crystallites is estimated from the widths of distance of the (110) plane using Scherrer's formula, which indicates that the CGN/SnO2-C (2.6 nm) is smaller than that of CGN/SnO2 (3.9 nm). The smaller crystallite size of SnO2 coated by amorphous carbon compared to that of bare SnO2 particles suggests effective stabilization of nucleate and grow particles by the amorphous carbon. The non-woven cotton, as a self-supporting skeleton, mainly consists of flower-shaped hub cotton fibers with diameters ranging from 10 to 20 μm (Fig. S2). After a series synthesis process to prepare CGN, the rGO covers on non-woven cotton fibers surface (Fig. S3(a)). It

worth to note there are some large multilayer rGO sheets connecting neighbor fibers with each other, which facilitates the electron transport between the fibers. The high magnification shows the rGO with fold morphology covering on the carbonized cotton fibers (Fig. S3(b)), which can be useful for facilitating ion migration and accommodating volume change during cycle. Besides, the high conductive rGO sheets would support an effective way for electron fast transport, which is helpful for anode performance at high rate. As shown in Fig. 3 and S4, the CGN/SnO2 and CGN/SnO2-C composites have the similar morphology and microstructure, combining with CGN. Fig. 3(c)–(d) and Fig. S4(c)–(d) show the CGN/SnO2-C and CGN/SnO2 composites EDS elemental mappings of C, Sn, and O elements. As observed, the C, Sn, and O elements are uniformly distributed in the selected area. From XRD analysis, the diffraction peaks of SnO2 can be found only, there is no other tin oxide and Sn can be detected, indicating the Sn element should come from SnO2. Those results imply the SnO2 nanoparticles well distributing on CGN framework. Fig. S5 shows the cross section of the CGN/SnO2-C, in which the rGO covers on flower-shaped hub of cotton fibers, and the rGO sheets are separated by SnO2 nanoparticles. The space between rGO sheets is produced by layer-by-layer packing of SnO2 nanoparticles, which can buffer the volume changes during cycling. At the same time, the uniform decorating of the SnO2 nanoparticles on rGO sheets can enhance their interface contact areas, thus makes it promising for enhancing the structure stability and high electrochemical activity. Furthermore, the aggregation and restacking problems of the rGO sheets during the reduction process can be effectively prevented, which maintenances the high surface area of rGO. N2 adsorption and desorption isotherm of CGN/SnO2-C was measured by Brunauer-Emmett-Teller (BET) method (Fig. S6). Specific surface area of the CGN/SnO2-C is to be 17.5 m2 g−1. Although the surface area of CGN/SnO2-C is not high enough, the hundreds of micrometers free space among carbonized cotton fibers would be beneficial for the diffusion of the electrolyte into the electrode, leading to reduced diffusion length of lithium ion. After a strong ultrasonic treatment during preparation of the TEM samples process, the SnO2 nanoparticles are still firmly anchored on the CGN framework. The selected area electron diffraction (SEAD) images (inset images of Fig. 4(a) and (c)) of CGN/SnO2 and CGN/SnO2-C show the characteristic diffraction rings, which are assigned to the (110), (100), (200), (211), and (211) crystal planes of SnO2. The size of SnO2 nanoparticles in the CGN/SnO2-C is smaller than that of CGN/ SnO2, indicating that the amorphous carbon can effective hinder the SnO2 aggregation during the thermal annealing process. The ultrasmall SnO2 particles are significantly attractive for short diffusion distances for lithium ions insertion/extraction for LIB application. At high magnification (Fig. 4(b) and (d)), it is found that both CNG/SnO2 and CGN/

Fig. 1. Schematic illustration of the preparation process of CGN/SnO2-C. (I) non-woven cotton covered by graphene oxide through immersing and freeze-drying process; (II) in situ growth of SnO2 nanoparticles; (III) glucose coat on SnO2 nanoparticles through simple hydrothermal approach; (IV) carbonized of glucose and non-woven cotton in N2 atmosphere.


Fig. 2. Photographs of non-woven cotton (a), CGO (b) and CGN/SnO2-C (c); XRD patterns of CGN/SnO2 and CGN/SnO2-C (d).

Fig. 3. SEM images of CGN/SnO2-C low (a) and high (b) magnification, EDS mapping images of C (e), Sn (f), and O (g) elements.

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Fig. 4. TEM images of CGN/SnO2 (a), and CGN/SnO2-C (c), HRTEM images of CGN/SnO2 (b), and CGN/SnO2-C (d).

SnO2-C composites have rGO sheets inside. Besides, the SnO2 particles of the CGN/SnO2-C composite in comparison to CGN/SnO2 is betterdefined, and the size distribution is narrowed into the range of 3– 5 nm. Because both rGO and amorphous carbon are composed of carbon atoms, it is hard to identify the amorphous layer from CGN/SnO2-C. However, it is worth to note the much smoother SnO2 nanoparticles surfaces of CGN/SnO2-C exist, comparing that of CGN/SnO2, indicating the SnO2 nanoparticles have been fully encapsulated by the amorphous carbon layer. The surface chemistry of the CGN/SnO2 and CGN/SnO2-C composites were obtained from the XPS. The high-resolution Sn3d spectrum in Fig. 5(a) shows clear Sn3d5/2 (596.0 eV) and Sn3d3/2 (487.5 eV) peaks in the composites, suggesting that the Sn species exist in the state of SnO2 [33]. Fig. 5(b) and (c) show the XPS profiles of C1s from the CGN/SnO2 and CGN/SnO2-C. The C1s peak in the XPS profile can be attributed to sp2bonded carbon (C\\C) at 284.8 eV, epoxy and alkoxy groups (C\\O) at 285.6 eV, and carbonyl and carboxylic (C_O) groups at 286.6 eV [31, 33]. The relative sp2-bonded carbon content of CGN/SnO2 (73.6%) and CGN/SnO2-C (71.7%) is much higher than that of CGO (48.6%, as shown in Fig. S7), indicating that the most of oxygen functionalities are removed by the thermal annealing process. Furthermore, the FWHM of C1s component peak of CGN/SnO2-C (~1.68 eV) is much bigger than that of CGN/SnO2 (~0.91). The broadening of interfacial peak might suggest that it contains more than one component compare with CGN/SnO2 [34,35]. This constitutes further evidence for the absence of amorphous material in the CGN/SnO2-C, further demonstrate amorphous carbon coating on SnO2 nanoparticles. Raman spectroscopy is the most useful and nondestructive technique to characterize the structure and graphitization quality of carbon materials [36–38]. The Raman spectrum is shown in Fig. 5(d) for the CGN/SnO2 and CGN/

SnO2-C composites, which exhibits two prominent peaks are observed: bands at around 1350 and 1580 cm− 1 corresponding to the welldefined D band and G band, respectively. The D band is associated with structural defects and partially disordered structures as A1g mode, and the G band corresponds to the E2g mode of sp2 bonded graphitic carbons [23,39,40]. The ratio of the peak area of these two bands (ID/IG) is usually used in judging the samples degree of graphitization. The ID/IG in the CGN/SnO2-C is lower than that in the CGN/ SnO2, which indicates the CGN/SnO2-C has less defect in CGN framework compare with CGN/SnO2. Some reactions between SnO2 and CGN would be weaken caused by amorphous carbon coating, which decreases the defects of CGN carbon framework. In the same time, the defects of aromatic structures may be recovered by carbon source from glucose repairing in the hydrothermal process. The resistance of electrode is a great factor to influence the LIB systems. To investigate electrical properties of composites, cutting the samples for chips, the schematic graph as shown in Fig. 6(a). According to the equation ρ = RS/L (ρ is resistivity, R is resistance, L is the depth of chip, and S is the area of the chip). From Fig. 6(b), the resistance of CGN (2890 Ω cm) is lower than carbonized cotton (23,002 Ω cm), implying the conductive rGO can great improve the electronic conductivity. The resistance of CGN/SnO2-C (318 Ω cm) is lower than CGN/SnO2 (331 Ω cm), which indicates that the amorphous carbon can improve the electronic conductivity of the composite. The high conductive pathway for electrons transport can help to achieve good rate capability and activate electrode materials. The previous mix method for preparing the electrode, the mixed composite paste lacks well interconnected pore structure for electrolyte, and the electrical insulating polymer binder may disrupt the long-rage electron transfer through the electrodes. Although the CGN/SnO2-C materials resistance is not much low, however,


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Fig. 5. XPS Sn3d spectra of CGN/SnO2 and CGN/SnO2-C (a), C1s spectra of CGN/SnO2 (b), and CGN/SnO2-C (c); Raman spectra of CGN/SnO2 and CGN/SnO2-C (d).

the electrode is free-standing and binder-free, avoiding the unfavorable factors influenced by mix method, which would have better electrochemical performance for lithium storage. Lithium storage properties of CGN/SnO2 and CGN/SnO2 as LIB electrode are investigated by assembling half-cell. Fig. 7 shows the first five CV cycles in the voltage window of 0.001 to 3.0 V at a scan rate of 0.5 mV s−1. In the first cycle of CGN/SnO2, two well-defined cathodic peaks are observed, an obvious one at around 0 and a tiny one around 0.6 V. The tiny one is attributed to the irreversible formation of a SEI layer, the electrolyte decomposition, and the Li2O (SnO2 + 4Li + 4e−= Sn + 2Li2O) formation. The obvious one can be assigned to the formation of a LixSn alloy (Sn + xLi+ + xe−= LixSn, 0 ≤ x ≤ 4.4) and lithium reaction with carbon framework (xLi+ + C + xe− = LixC). A similar peak is also observed in CGN/SnO2-C (Fig. 6(b)), but the tiny peak intensity of CGN/SnO2-C is much smaller (nearly unseen) than that of the CGN/SnO2 electrode, which is caused by the introduction of amorphous carbon to reduce the irreversible reaction. In the anodic scans, the two electrodes show three peaks, the peak around 0.70 V corresponds to the Li dealloying from LixSn, the oxidation of metallic Sn0 into SnO and further SnO2 occurring at the potential around 1.25 V and 2.05 V. The CGN/SnO2-C oxidation of metallic Sn0 into SnO peak is obvious comparing with CGN/SnO2, suggesting that amorphous carbon coating could increase the reversibility from Sn to SnO. From second cycle onwards, the CV curves are almost same in sequential cycles, indicating that the binder-free electrode could serve as a very stable anode material for LIB. Fig. 8(a) shows the initial discharge/charge profiles of CGN/SnO2 and CGN/SnO2-C electrodes between 0.001 and 3.0 V under a current density of 100 mA g−1, and the specific capacity is calculated based on total weight of electrode. The initial irreversible capacity is notably reduced from 902.4 mAh g−1 to 417.0 mAh g−1 for the CGN/SnO2 at first cycle with the coulombic efficiency is 46.2%, which should be associated

with the SEI formation, and decomposition of the electrolyte [3,41]. The CGN/SnO2-C composite delivered a higher discharge (1165.1 mAh g−1) than CGN/SnO2. This is caused by the smaller particle size of CGN/SnO2C than that of CGN/SnO2, which provides more active points for lithium storage. The SEI formation on the smaller particle would consume more electrolyte and lithium due to the much higher interfacial area between electrode and electrolyte, which leads the low initial coulombic efficiency. According to the above analysis, the CGN/SnO2-C has smaller particle size than CGN/SnO2, which means the CGN/SnO2-C has more contact interface with electrolyte than that of CGN/SnO2. However, the first coulombic efficiency of the CGN/SnO2-C composite with the value of 62.6% is higher than that of CGN/SnO2. This phenomenon is caused by the amorphous carbon coating which inhibits the direct contact between electrolyte and SnO2 nanoparticles, reducing the irreversible SEI layers formation. Furthermore, the amorphous carbon could form stable SEI layer, which effectively reduces the transport resistance of lithium ions and prevents the new SEI layer formation during cycling process [42]. Fig. 8(b) exhibits the galvanostatic discharge/charge behaviours of CGN/SnO2 and CGN/SnO2-C, together with their coulombic efficient. The CGN/SnO2-C anode exhibits much higher capacity and improved capacity retention than that of CGN/SnO2. After 200 cycles, the CGN/SnO2C exhibits a high discharge capacity of 496.3 mAh g−1; this value is much higher than 80% of CGN/SnO2 (274.6 mAh g−1). In addition, the average capacity fading rates of the CGN/SnO2-C and CGN/SnO2 are 0.15% and 0.20% per cycle, which suggests that the CGN/SnO2-C has the higher capacity and cycling stability. This undoubtedly demonstrates the benefits of amorphous carbon coating increase the electrochemical property. The SnO2 is protected by amorphous carbon with a better electronic conductivity and a smaller volume change during cycle process. According to the fabrication of traditional LIB anode, mixing active material (80 wt% slurry) with carbon black (10 wt%

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Fig. 6. Schematic of resistance test system (a), resistance of carbonized cotton, CGN, CGN/ SnO2 and CGN/SnO2-C (b). Fig. 7. CV curves of CGN/SnO2 (a), and CGN/SnO2-C (b).

slurry) and binder (10 wt% slurry), and then coating the slurry on a Cu foil (area density ≥ 13 mg cm−2) [9,43,44], the active material is even lower than 20 wt% of total electrode mass [5]. The CGN/SnO2–C composite is a free-standing and binder-free, without current collectors. The specific capacity of 496.3 mAh g−1 is equivalent to conventional electrodes with the specific capacity of 2481.5 mAh g−1 or even more. The areal storage capacity is an important technological challenge for development of satisfied commercial anodes. The reversible areal capacity of CGN/SnO2-C is calculated to be 1.72 mAh cm−2 after 200 cycles, which is higher than materials reported before, such as SnO2@Si nanowire/ carbon cloth (1.39 mAh cm−2 after 50 cycles) [45], Cu-Sn nanowires (0.07 mAh cm− 2 after 100 cycles) [46], Si-CNT nanocomposites (1 mAh cm− 2 after 50 cycles) [47], CoNiO nanowire/TiO2 nanotubes (0.36 mAh cm− 2 after 40 cycles) [48], SnO2/Fe2O3 nanotubes (0.73 mAh cm−2 after 50 cycles) [4], and carbon-coated porous silicon anodes (0.18 mAh cm−2 after 70 rate cycles) [49]. In addition, the electrochemical impedance spectroscopy was measured to further study the mechanism of electrochemical performance in CGN/SnO2 and CGN/SnO2-C. Fig. 8(c) displays the Nyquist plots of the CGN/SnO2 and CGN/SnO2-C samples after 200 cycles (the inset is the equivalent circuit). All those curves are two semicircles and a slope line. The intercept at the Zreal at high-frequency (RΩ) represents the total resistance of the electrolyte, separator, and electrical contacts [50]. The high-frequency and medium-frequency semicircles reflect the resistances of SEI film (RSEI) and charge-transfer (Rct), respectively [51]. The slope region in the low-frequency indicates the Warburg impedance [50]. To compare in more detail, the CGN/SnO2-C has the smaller RSEI and Rct values (50.0 and 87.2 Ω) comparing with that of CGN/ SnO2 (272.8 and 155.0 Ω). The small RSEI indicates the carbon coating avoids the electrode direct contact with electrolyte, decreases the SEI

product during the cycle. In addition, the small Rct reflects carbon coating actives and improves kinetics of the electrochemical reaction. Fig. 8(c) presents the rate capability of CGN/SnO2 and CGN/SnO2-C at different current rates ranging from 50 to 800 mA g− 1. It can be observed that the CGN/SnO2-C delivers high capacities of 686.5, 604.8, 500.4, 361.1, and 270.2 mAh g− 1 at 50, 100, 200, 400, and 800 mA g−1, respectively. In comparison, the CGN/SnO2 electrode delivers much lower rate capacities of only 393.4, 340.7, 292.7, 228.3, and 140.4 mAh g−1 at 50, 100, 200, 400, and 800 mA g−1. More importantly, the capacity of CGN/SnO2-C can recover to 590 mAh g−1 upon the current density back to 100 mA g−1. The free-standing and binder-free CGN/SnO2-C electrode displays a superior capacity retention as well as excellent capacity recovery performance. The superior cycling and rate performance of CGN/SnO2-C composite can be explained by the following considerations. First, the freestanding, binder-free, without current collector, the CGN/SnO2-C electrode maintains good electrical contact during the lithiation and delithiation processes, and decreases the polarization. Second, the flexible of foldable rGO sheets provides the buffer space for whole electrode volume expansion during cycling process, which suppresses the mechanical degradation of electrode. Third, the amorphous carbon coating on the SnO2 nanoparticles directly grows on electrically connected CGN framework as an effective way to achieve fast electron/ion transport. Fourth, the ultra-small SnO2 nanoparticles not only shorten the distance for lithium ion diffusion but also offer a large electrode/electrolyte contact area for lithium ions cross the interface. Last, the elastic amorphous carbon layers can restrict the severe volume change of SnO2


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Fig. 8. First discharge-charge curves (a), cycling performance (b), EIS spectrum (c), and rate tests (d) of CGN/SnO2 and CGN/SnO2-C.

nanoparticles, and form the stable SEI layer during cycling process, thus improve the structural stability. 4. Conclusion In summary, the flexible CGN/SnO2-C composites via a facile and low cost fabrication process were design and synthesize. As a free-standing and binder-free anode, the CGN/SnO2-C composites exhibit the excellent electrochemical performance, in terms of high initial specific capacity, high first coulombic efficiency, stable cycling performance, and excellent rate ability. The improved capacity is found to be relevant to the unique structure. The CGN framework is decorated with amorphous carbon coating on SnO2 nanoparticles, which as an integrate electrode increases the conductive and eliminates the negative effective caused by conductive additive, polymer binder, and current collector append. The fold rGO sheets not only increase the contact area with electrolyte, but also buffer the volume change during cycling process. Moreover, the amorphous carbon prevents SnO2 to direct contact with electrolyte, which forms the stable SEI layer and limits SnO2 expansion. The current design strategy has great potential and can be applied for the active energy storage materials in energy storage and conversion systems. Acknowledgements This project was supported by the National Natural Science Foundation of China (51172050, 51102060, 51102063, 51302050, and 51372052), Shandong Province Young and Middle-Aged Scientists Research Awards Fund (BS2013CL003), and the Fundamental Research Funds for the Central Universities (HIT. ICRST. 2010009). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.matdes.2016.11.081.

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