Integrating in situ solvothermal approach synthesized nanostructured tin anchored on graphene sheets into film anodes for sodium-ion batteries

Accepted Manuscript Title: Integrating in situ solvothermal approach synthesized nanostructured tin anchored on graphene sheets into film anodes for sodium-ion batteries Author: Fei Pan Weimin Zhang Jingjing Ma Ningna Yao Li Xu Yu-Shi He Xiaowei Yang Zi-Feng Ma PII: DOI: Reference:

S0013-4686(16)30505-9 http://dx.doi.org/doi:10.1016/j.electacta.2016.02.204 EA 26821

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

9-12-2015 21-2-2016 28-2-2016

Please cite this article as: Fei Pan, Weimin Zhang, Jingjing Ma, Ningna Yao, Li Xu, Yu-Shi He, Xiaowei Yang, Zi-Feng Ma, Integrating in situ solvothermal approach synthesized nanostructured tin anchored on graphene sheets into film anodes for sodiumion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.02.204 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 proof before it is published in its final 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.

Integrating in situ solvothermal approach synthesized nanostructured tin anchored on graphene sheets into film anodes for sodium-ion batteries Fei Pan,†, ┴ Weimin Zhang,†, ┴ Jingjing Ma,† Ningna Yao, ‡ Li Xu, ‡ Yu-Shi He,† * Xiaowei Yang,§ Zi-Feng Ma† †

Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China ‡ State Grid Smart Grid Research Institute, Changping District, Beijing, 102209, China § School of Materials Science and Engineering, Tongji University, Shanghai, 201804 , China ┴These

authors contributed equally to this work.

Corresponding Author *Tel.:

+86-21-54744533.

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+86-21-54747717.

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[email protected]

Graphical abstract Self-supporting and binder-free Sn/GS nanocomposite films as anode materials for SIBs were synthesized through a facile in situ solvothermal assisted thermal reduction route

ABSTRACT We demonstrate here an effective strategy to construct three-dimensional (3D) metal-graphene based film electrodes. The build blocks are tin nanopartilces well anchored on graphene sheets which were synthesized by a feasible and controllable in situ solvothermal method and a subsequent thermal reduction process. In this study, we found the solvent type is the key and only with organic solvent (ethanol) the desired 3D graphene sheets anchored tin nanoparticles (Sn/GS) films can be obtained. The graphene oxide (GO), which is the original source of conductive GS network, can also serve as a surfactant to inhibit the secondary aggregation of nanoparticles. Accordingly, the homogeneous Sn nanoparticles (~10 nm) can evenly distribute into the GS matrix. The as-obtained self-supporting Sn/GS films, when were used as anode materials for sodium-ion batteries (SIBs), show promising electrochemical performances. This method not only blazes a new trial for synthesizing Sn/GS nanocomposite films, but also can be extended to develop other free-standing non-noble metal (or alloy) nanoparticles/GS films for wide applications. KEYWORDS: tin nanoparticles, graphene, film electrodes, in situ solvothermal reaction, sodium-ion battery

1. INTRODUCTION Non-noble metal/graphene nanocomposites have attracted extensive attention due to their unique properties in many areas, such as energy storage [1-3], catalysis [4,5], electronics [6], sensors [7] and environmental applications [8]. In order to adequately harness the synergistic effect between GS and non-noble metal nanoparticles, the key

issue in the construction of non-noble metal nanoparticles anchored on GS composites rests with how to effectively control the size of the nanoparticles, uniformly disperse the nanoparticles on the GS network and restrain the restacking of GS.

Recent efforts have been made to develop the above graphene-based nanocomposites by diverse methods including potentiostatic electrodeposition [7], inkjet printing [9], electrospinning [10] and hydrothermal [2] or solvothermal processes [5]. In comparison with the conventional hydrothermal synthesis, in situ non-aqueous solvothermal techniques have exhibited their exceptional advantages in fabricating three dimensional (3D) graphene-based materials in our previous works [11,12]. The as-prepared 3D graphene-based aerogels can maintain the macroporous structure of original 3D GS-based organogels through the solvethermal treatment and sequent freeze-drying process. Moreover, non-aqueous solvothermal conditions possess their own particular properties, including high self-generated pressure inside the sealed autoclave, favorable reduction ability of some solvents containing hydroxyl/amino groups, and relative low dielectric constant and solvent surface energy [13,14].

Recently metallic tin has been regarded as a promising anode material for SIBs due to its abundant sources, environmental benignancy and high theoretical capacity (847 mA h g−1 corresponding to Na15Sn4) [15-21]. However, the drastic volume change of Sn during the Na ion insertion/extraction will result in the pulverization of the electrode, loss of electrical contact and finally a rapid capacity decay, which is similar

to the phenomenon emerged in lithium-ion batteries (LIBs) [20,21]. The nanonization (especially within 10 nm) of Sn particle has been reported as a desirable alternative to address the above issues [18,22]. Nevertheless, the aggregation of nano-Sn during sodiation and desodiation process will still lead to inferior electrochemical performance. Therefore, to effectively suppress the aggregation of nano-Sn particles and improve the electrical conductivity, the introduction of conductive porous carbon-based matrices, such as graphene, is quite necessary. Furthermore, flexible and bendable self-supporting electrodes are also vital for the potential applications of SIBs in soft portable electronics, such as artificial electronic skins and wearable devices [23-27].

In this work, we report an organic-based sovothermal approach assisted reduction process which is facile to realize the synthesis of Sn nanoparticles and the simultaneous deposition on GS. The as-synthesized tin/graphene nanocomposite can be easily used to construct robust 3D metal anchored on graphene film electrodes by a simple mechanical press procedure. When directly employed as an anode without using binder and current collector for SIBs, the Sn/GS film with an appropriate thickness delivers a superior electrochemical performance. What is far more important is that such an in situ solvothermal assisted thermal reduction strategy can also be adopted to develop a range of non-noble metal (or alloy) nanoparticles anchored on graphene sheets films for various applications.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Self-supporting Sn/GS Films. Sn/GS films were prepared by a solvothermal assisted thermal treatment method. Briefly, graphite oxide was synthesized from natural graphite powder (Grade 230, Asbury Carbons) by a modified Hummer’s method. The graphite oxide (30 mg) was exfoliated into 30 mL ethanol under sonication to form a graphene oxide (GO) dispersion. Then, several drops of deionized water as surfactant were added to wet the surface of the graphite oxide, which is necessary to achieve a well dispersion of the graphite oxide in ethanol solvent. A certain amount of SnCl4·5H2O was added into the dispersed GO solution by a weight ratio of 10:1 (SnCl4·5H2O: GO). The as-derived mixture was transferred into a Teflon-lined autoclave and heated at 180 oC for 12 h. After cooling naturally to room temperature, the as-formed black cylindrical gel was immersed in deionized water for 10 h to remove the residual ions and further freeze-dried to keep the three dimensional (3D) porous structure, which was labelled as SnO2/rGO. Thereafter, the as-synthesized 3D SnO2/rGO aerogel was calcined and reduced under an Ar/H2 (5% H2 in volume) atmosphere at 600 °C for 2 h to convert into 3D porous Sn/GS aerogel. Finally, the 3D Sn/GS samples with different thicknesses were pressed under a pressure of about 1.5 MPa to produce the corresponding Sn/GS films.

2.2. Structural and Morphological Characterization. X-ray diffraction (XRD) measurements were conducted using a Rigaku D/MAX-2200/PC X-ray diffractometer at 40 kV and 20 mA, with a Cu Kα radiation (λ=1.5406 Å). Raman spectra were measured using a Bruker Optic SENTERRA (R-200L) with the laser wavelength of

633 nm. The surface chemical composition was characterized by an X-ray photoelectron spectrometer (XPS, Kratos Axis Ultra DLD). Field emission scanning electron microscopy (FESEM) images were performed on a Nova NanoSEM 450 scanning electron microscope (FEI, USA) equipped with energy dispersive X-ray spectroscopy (EDS, Apollo X, EDAX Inc.). Transmission election microscopy (TEM) were carried out using an instrument (JEM-2100, JEOL Ltd., Japan) equipped with EDS (INCA-IET200, Oxford Instruments) at an operating voltage of 200 kV.

2.3. Electrochemical Measurements. The free-standing and binder-free Sn/GS films with a diameter of about 1.4 cm were directly applied as working electrodes without the use of current collector. The electrodes were assembled into CR2016-type coin cells for the electrochemical characterizations which were constructed and handled in an argon-filled glove box with sodium metal as the counter electrode and a glass fiber filter (GA 55, Advantec, Japan) as the separator. The electrolyte was a solution containing 1.0 M NaClO4 in polypropylene carbonate (PC) and fluoroethylene carbonate (FEC) mixed solvent with a volume ratio of 9 : 1. The galvanostatic charge/discharge tests of the half-cells were performed using a LAND CT2001A model battery test system (Wuhan Jinnuo Electronics,Ltd.) with a cut-off potential window of 0.01-0.75 V vs. Na/Na+ at ambient conditions. The specific charge/discharge capacities were calculated based on the weight of Sn/GS films. Cyclic voltammetry (CV) measurements were conducted using a CHI instrument (CHI660) at a scan rate of 0.5 mV·s-1.

3. RESULTS AND DISCUSSION Figure 1 schematically illustrates the typical process for the formation of the Sn/GS film. Firstly, SnCl4·5H2O was uniformly dispersed in GO ethanol suspension (Figure 1a). The negative charged oxygenated functional groups on GO sheets can adsorb Sn4+ ions due to the mutual electrostatic interactions. Afterwards, the above mixture was in situ self-assembled under the ethanol solvothermal reaction to obtain a columniform ethanol gel of 3D SnO2/rGO (Figure 1b). During the solvothermal process, Sn4+ ions anchored on GO sheets were heterogeneously nucleated to generate SnO2 nanocrystals via both hydrolysis and esterification reactions under high pressure and temperature [28-30]. Meanwhile, GO was self-assembled and reduced to form 3D rGO framework in ethanol medium [11]. In order to elucidate the special characteristics of ethanol solvothermal synthesis, SnO2/rGO samples in three different solvents (deionized water, ethanol and water/ethanol (1:1 in volume ratio)) at the same GO concentration (1 mg mL-1) were prepared (Figure S1) at the identical conditions. However, 3D SnO2/rGO can be obtained only when ethanol was used as solvent. The 3D SnO2/rGO ethanol gel was freeze-dried in order to retain the 3D monolithic structure and achieve 3D SnO2/rGO aerogel (Figure 1c). After the subsequent calcination and reduction at 600 °C for 2 h in an Ar/H2 atmosphere and following compression treatment, Sn/GS films with different thicknesses were obtained (Figure 1c). To measure the weight ratio of GS in the Sn/GS composite, we dissolved the tin nanoparticles on graphene by a diluted hydrochloric acid solution and determined the residual mass ratio corresponding to graphene in the Sn/GS

composite was about 29.1 wt% [1].

We carried out XRD in order to characterize the crystalline properties for the samples. Figure 2a shows the XRD patterns of the intermediate product SnO2/rGO and the sample Sn/GS. All the diffraction peaks for SnO2/rGO and Sn/GS are in good agreement with the standard cards of SnO2 (JCPDS card no. 41-1445) and Sn (JCPDS card no. 65-0296), respectively. The broad peaks centered at 2θ values of 26.6o, 33.9o, and 51.8o are indicative of the formation of tin oxide nanocrystals [31,32]. As no impurity peak was observed in the XRD pattern of the Sn/GS aerogel, it can confirm that tin oxide anchored on the graphene sheets were successfully reduced to pure tin nanoparticles after calcination in H2/Ar atmosphere. In order to figure out an optimized reduction temperature for the formation of the Sn/GS aerogel, the corresponding comparative experiments have been fulfilled at 200 oC and 400 oC with the same heating rate. But as shown in Figure S2, the signals from tin oxide in the samples calcined at above two temperatures can still be detected in their XRD spectra. The Sn/GS, SnO2/rGO and GO samples were also investigated by Raman spectroscopy and the results are displayed in Figure S3. The Raman spectrum of GO contains the disordered (D) band (point phonons of A1g symmetry) and graphitic (G) band (the E2g mode of C sp2 atoms) of carbon materials in the vicinity of 1350.2 cm-1 and 1596.8 cm-1, respectively [33]. Both D and G bands were observed in the Raman spectra of SnO2/rGO and Sn/GS, however, with an increased D/G intensity ratio compared with that for GO. These changes can be due to a decrease in the average size of the sp2 domains on account of removal of the functional groups in GO and the

enhancement of the disordered carbon content arising from the partial insertion of tin nanoparticles into graphene layers, which is consistent with the previous reports [34,35]. It is noteworthy that the characteristic peak of tin oxide at about 612 cm-1 in the spectrum for SnO2/rGO disappeared in that for the Sn/GS after the calcination procedure [36,37]. The aforementioned XRD and Raman characterizations indicate the reduction process for the tin oxide to the tin metal element.

XPS was conducted at a range of 0-1100 eV to study the chemical states and surface composition of the Sn/GS composite. Figure 2b displays the survey XPS spectrum which reveals the presence of C, O, and Sn elements. And the high-resolution spectra of C 1s and Sn 3d were also recorded as shown in Figure 2c,d. The C 1s XPS spectrum (Figure 2c) of the Sn/GS composite was used to evaluate the reduction degree of GO. The spectrum can be deconvoluted into four peaks which are corresponding to C-C/C=C bonds (284.8 eV), C-O bonds (286.1 eV), C=O bonds (287.1 eV), and O-C=O bonds (289.8 eV), respectively [38]. The content percentage of carbon–carbon bonding is about 74.61%, which indicates that a majority of the oxygenated functional groups from the GO have been efficiently de-functionalized through the solvothermal reaction and subsequent annealing process. In Figure 2d, the Sn 3d XPS spectrum was also separated and fitted on the basis of valence state. The peaks located at ca. 485.2 eV and 493.6 eV can be attributed to metallic Sn in the zero-valence state (Sn(0)) [39,40]. The other two distinct peaks at ca. 487.1 eV and 495.6 eV corresponds to the characteristic peaks of Sn(IV) species, suggesting that a

trace amount of Sn nanoparticles anchored on the surface of GS were oxidized to SnO2 [41,42].

The surface morphology of the Sn/GS aerogel was observed by FESEM. The SEM images in Figure 3a,b and Figure 3c,d are corresponding to the top-view and cross-section view for the morphology of Sn/GS, respectively. It can be seen that the Sn/GS aerogel possesses a fully interconnected porous 3D honeycomb structure without any Sn particles obviously attached on the surface of GS. The pore sizes of the GS matrix range from submicrometer (GS surface) to dozens of micrometers (interlayer spacing between GS). The construction of the 3D honeycomb structure can be ascribed to the overlapping or coalescing of flexible GS via π–π stacking interactions during the in situ solvothermal reaction. Moreover, according to the elemental mapping images and EDS spectrum of the Sn/GS aerogel in Figure S4, Sn particles were homogeneously distributed in the 3D porous GS framework. To further understand the reduction process from SnO2/rGO to Sn/GS, TEM and high-resolution TEM (HRTEM) images of the SnO2/rGO aerogel were taken and shown in Figure S5 which demonstrate rice-shaped SnO2 nanoparticles with a length of about 8 nm evenly distributed on the crumpled and overlapped rGO matrix. In Figure S5d, the regular and clear lattice fringes with interplanar distances of 0.2620 nm and 0.1760 nm can be assigned to the (110) plane and (211) plane of rutile SnO2, respectively [33]. The inset fast Fourier transform (FFT) pattern display the crystal structure of a single grain of SnO2. The microstructure of the Sn/GS aerogel was examined by TEM (Figure 3e) and HRTEM (Figure 3f). A low magnification TEM image displayed in Figure 3e

reveals a morphology which consists of spherical Sn nanoparticles featured a size of about 10 nm and GS. The morphology of Sn nanoparticles is different from that of SnO2 nanoparticles, which can be ascribed that rice-shaped SnO2 nanoparticles melted and reassembled to form Sn nanoparticles with spherical structure during calcination procedure at high temperature. A HRTEM image of Sn/GS exhibits distinct crystal lattice in Figure 3f and the typical regular lattice fringes of Sn nanocrystals were measured to obtain a d spacing of 0.2775 nm by FFT pattern, corresponding to the (101) planes of Sn in the XRD spectrum of the Sn/GS composite [18]. In addition, it is notable that the uniform immobilization of Sn nanoparticles into the GS matrix can effectively suppress the aggregation of Sn nanoparticles and provide void space to buffer the volume expansion of Sn during charging and discharging process.

Three Sn/GS films with different thicknesses were pressed and cut as free-standing electrodes without using current collector and binder, named as Sn/GS-I (~100 μm), Sn/GS-II (~200 μm) and Sn/GS-III (~380 μm) (Figure 4 and S5). The free-standing Sn/GS film electrode is bendable, which is shown in the inset of Figure 4b. And the distance between layers manifests a decreasing trend along with the increase of the thickness from Sn/GS-I to Sn/GS-III (Figure 4d-f).

Figure 5 depicts the electrochemical performance of the Sn/GS film electrodes for SIBs. To illuminate the mechanism of the redox reactions, the CV curves of the Sn/GS-II film at the first, second and fifth cycles were recorded at a scanning rate of

0.5 mV·s-1 in Figure 5a. The first cathodic scan shows a broad irreversible reduction peak between 0.4 and 1.2 V, corresponding to the formation of solid-electrolyte interface (SEI) film and the insertion of Na into the Sn/GS composite [17]. Meanwhile, the oxidation peaks at about 0.19, 0.63 and 0.73 V in the anodic scans can be ascribed to the desodiation of Na15Sn4, NaSn, NaSn5, respectively [16,18-20]. In the subsequent cycles, the CV curves show good reproducibility, implying the good stability and reversibility of Sn/GS-II film. For the free-standing Sn/GS electrode, the reversible reaction process of Na with tin in SIBs can be summarized as follows: Sn  xNa   xe  Nax Sn( x  3.75)

(1)

Figure 5b shows the typical discharge (sodiation) and charge (desodiation) potential profiles of the Sn/GS-II electrode at a current density of 50 mA g-1 over a potential range of 0.01–0.75 V vs. Na/Na+. The multi-step potential plateaus between 0.0 and 0.7 V are observed in the first desodiation process, which is the characteristic of dealloying NaxSn (x≤3.5) [15,43]. This is in good agreement with the above CV result. Furthermore, the first discharge and charge capacities are around 853 and 366 mA h g-1, respectively. It is noted that the initial coulombic efficiency (CE) is only about 43%, which still needs to be improved by presodiation or optimizing the electrolyte. The irreversible capacity loss in the first cycle can be owing to the formation of SEI film arisen from the decomposition of electrolyte.

The discharge-charge cycling performance of the three film electrodes with different thicknesses was investigated at a current density of 50 mA g-1 (Figure 5c). The

cycling abilities of Sn/GS-I and Sn/GS-II are clearly superior to that of Sn/GS-III and the reversible capacities can still maintain at 234 and 324 mA h g-1 after 30 cycles, respectively. As a contrast, after 30cycles, Sn/GS-III can only deliver a reversible capacities of 137 mA h g-1. Figure 5d exhibits the rate performance of the Sn/GS films at different current densities increasing from 50 to 400 mA g-1. In comparison with Sn/GS-III, Sn/GS-I and Sn/GS-II possess relatively stable and high specific capacities at various current densities, which may be ascribed to the swelling effect between the electrolyte and electrodes and the impedance change resulted from the thickness of the films. The film with a high thickness may hamper the permeation of electrolyte into the electrode material and lead to the decrease of ionic conductivity, resulting in the decay of the electrochemical performance. When the current rate was set back to 50 mA g-1, it can still returned to a reversible capacity of about 323 mA h g-1 (95.5% of the initial reversible capacity) after the 50th cycle, suggesting superior structural stability and reversibility.

4. CONCLUSION In summary, self-supporting and binder-free Sn/GS nanocomposite films as anode materials for SIBs have been successfully synthesized through a facile but effective in situ solvothermal assisted thermal reduction method. In the synthesis process, the in situ heterogeneous nucleation and growth of SnO2 nanoparticles on the surface of rGO were accompanied with the simultaneous reduction of GO and formation of 3D rGO framework. Subsequent annealing under reducing atmosphere enables SnO2

nanoparticles (~8 nm in length) to be reduce to Sn nanoparticles (~10 nm) which were uniformly embedded within the GS network. When directly pressed and applied as self-supporting and binder-free anode for SIBs without using current collector, the as-synthesized Sn/GS film electrode with a suitable thickness exhibits promising electrochemical performances. Most importantly, the size of as-obtained non-noble metal (or alloy) particles within the GS matrix and layer number of graphene can be further modified by controlling the concentration of reactants, reaction time and temperature. We believe that this study can provide a basis for developing various free-standing graphene anchored non-noble metal (or alloy) nanoparticles films for other applications, such as catalysts and sensors.

Conflict of interest The authors declare no competing financial interest.

Acknowledgements We are grateful for financial support for this work from the National Basic Research Program of China (2014CB239700), the National Natural Science Foundation of China (21336003, 21006063, 21573147, 21333007, 21303251), the Shanghai Natural Science Foundation (15ZR1422300), and the Science and Technology Commission of Shanghai Municipality (14DZ2250800).

Appendix A. Supplementary data Additional XRD patterns, Raman spectra, EDS mapping and TEM images. This

material is available via the Internet at http://dx.doi.org/xx.xx/j. electacta. xxxx. xx. xxx.

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CAPTIONS Figure 1. Schematic illustration for the formation process of free-standing Sn/GS films.

Figure 2. (a) XRD patterns of SnO2/rGO and Sn/GS. XPS spectra for the Sn/GS composite: (b) survey spectrum and high-resolution; (c) C 1s; and (d) Sn 3d spectra.

Figure 3. Typical FESEM images of 3D Sn/GS aerogel: (a) and (b) top-view images; (c) and (d) cross-sectional images. (e) TEM and (f) HRTEM images of the Sn/GS composite. The inset in (f) shows a corresponding fast Fourier transform (FFT) pattern.

Figure 4. Sn/GS films with different thickness obtained under the same pressure of 1.5 MPa: ((a), (b) and (c)) cross-sectional images of the films after compression and ((d), (e) and (f)) corresponding images of interlayer distances.

Figure 5. (a) CV curves of Sn/GS-II film in the potential range from 0.01 to 2.6 V at a scan rate of 0.5 mV s–1. (b) Charge–discharge curves of Sn/GS-II film at a current density of 50 mA g–1 in a potential range of 0.01–0.75 V vs. Na/Na+. (c) Cycling behavior of the Sn/GS films at a current density of 50 mA g–1. (d) Rate performance of the Sn/GS films at different current densities.

GO sheets

rGO sheets

graphene sheets

Ethanol solvothermal

Sn4+

Freeze-drying

SnO2

Sn

H2/Ar reduction

Self-assembly

Compression 1.4 cm

a

b

c

d

Figure 1. Schematic illustration for the formation process of free-standing Sn/GS films.

(b)

Sn/GS

C 1s

(a) 200 101

SnO2/rGO

O 1s Sn 3d

Intensity(a.u.)

Intensity(a.u.)

Sn JCPDS 65-0296

211 220

110 101

SnO2 JCPDS 41-1445

211 200

20

30

40

50

60

70

80

1000

800

2-Theta

600

400

(c)

(d)

292

290

288

0

5/2

Sn3d 3/2

Sn3d

Sn(IV)

Intensity(a.u.)

Intensity(a.u.)

raw fitted C-C/C=C(284.8)74.61% C-O(286.1)11.51% C=O(287.1)7.63% O-C=O(289.8)5.88% background

294

200

Binding Energy (eV)

286

Binding Energy (eV)

284

282

280

Sn(0)

500

498

496

494

492

490

488

486

484

482

480

Binding Energy (eV)

Figure 2. (a) XRD patterns of SnO2/rGO and Sn/GS. XPS spectra for the Sn/GS composite: (b) survey spectrum and high-resolution; (c) C 1s; and (d) Sn 3d spectra.

(a)

(b)

(c)

(d)

(e)

(f) d(101)=0.2775 nm

Figure 3. Typical FESEM images of 3D Sn/GS aerogel: (a) and (b) top-view images; (c) and (d) cross-sectional images. (e) TEM and (f) HRTEM images of the Sn/GS composite. The inset in (f) shows a corresponding fast Fourier transform (FFT) pattern.

(a)

(b)

(c)

500 μm

(d)

(e)

(f)

Figure 4. Sn/GS films with different thickness obtained under the same pressure of 1.5 MPa: ((a), (b) and (c)) cross-sectional images of the films after compression and ((d), (e) and (f)) corresponding images of interlayer distances.

(a)

(b)

1st 2nd 5th

0.6

Potential VS. Na/Na (V)

0.4

Current (mA)

+

0.2 0.0 -0.2 -0.4 -0.6

2.0

1st 2nd 30th 1.5

1.0

0.5

-0.8 -1.0

0.0 0.5

1.0

1.5

2.0

2.5

3.0

0

200

Voltage (V)

900

(d)

800

700

700

600

discharge of Sn/GS-I discharge of Sn/GS-II discharge of Sn/GS-III

500 400 300 200

15

20

25

coulombic efficiency of Sn/GS-II

80

charge charge charge

60

30

discharge of Sn/GS-I discharge of Sn/GS-II discharge of Sn/GS-III

40 20

-1

50 mA g

-1

0 300

100 mA g

-1

200

0

Cycle number

50 mA g

400

0 10

100

500

100

5

120

600

100

0

800

-1

charge charge charge

600

900

800

Specific capacity (mA h g )

-1

Specific capacity (mA h g )

(c)

400

Specific capacity (mAh/g)

-20

200 mA g

-1

400 mA g

Coulombic efficiency (%)

0.0

-40

-1

-60 0

10

20

30

40

50

Cycle number

Figure 5. (a) CV curves of Sn/GS-II film in the potential range from 0.01 to 2.6 V at a scan rate of 0.5 mV s–1. (b) Charge–discharge curves of Sn/GS-II film at a current density of 50 mA g–1 in a potential range of 0.01–0.75 V vs. Na/Na+. (c) Cycling behavior of the Sn/GS films at a current density of 50 mA g–1. (d) Rate performance of the Sn/GS films at different current densities.