Carbon nanotube-graphene nanosheet conductive framework supported SnO2 aerogel as a high performance anode for lithium ion battery

Electrochimica Acta 240 (2017) 7–15

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Carbon nanotube-graphene nanosheet conductive framework supported SnO2 aerogel as a high performance anode for lithium ion battery Ming-Shan Wanga,* , Zhi-Qiang Wanga , Zhen-Liang Yangb , Yun Huanga , Jianming Zhengc,* , Xing Lia,* a The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, 610500, Sichuan, PR China b Institute of Materials, China Academy of Engineering Physics, Mianyang, 621908, Sichuan, PR China c Energy and Environmental Directorate Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99354, USA

A R T I C L E I N F O

Article history: Received 2 March 2017 Received in revised form 3 April 2017 Accepted 6 April 2017 Available online 8 April 2017 Keywords: Tin dioxide graphene carbon nanotube aerogel lithium ion battery

A B S T R A C T

Tin oxide (SnO2) based materials are considered promising anodes for high-energy lithium ion batteries (LIBs). However, significant challenges including low initial coulombic efficiency, poor cycling stability and low rate capability are still hindering their practical applications. Effectively constructing a conductive material structure plays a vital role in improving the electrochemical performance of tin based composite anodes for LIBs. In this work, we utilize carbon nanotube-graphene nanosheet with 3D conductive framework to fabricate a SnO2/carbon nanotube-graphene nanosheet (SnO2/CNT-GN) aerogel composite, in which a small amount of carbon nanotube is introduced to increase the electronic transportation by interconnecting the independent porous graphene structure. In addition, the synergistic interplay between high mechanical property of CNT and flexibility of graphene significantly enhance the stability of SnO2/CNT-GN composite. As a result, the SnO2/CNT-GN composite exhibit a very decent cycling stability, retaining a stable specific capacity of 809 mAh g
1 (87% capacity retention) after 100 cycles at 0.2 A g
1, as well as an excellent rate capability, delivering 787 mAh g
1 even at a high current density of 5A g
1. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction During the past decades, a large amount of effort has been devoted to developing rechargeable lithium-ion batteries (LIBs) with high energy density, high safety and long cycling performance for various applications such as smart electronics, electric vehicles, and large scale energy storage system [1,2]. However, rapid development of market, especially the increasing demand for highenergy-density LIBs to be deployed in hybrid electric vehicles has prompted the consideration of electrode material candidates for LIBs [3,4]. Among those anode materials under being explored, tin oxide (SnO2) based material is regarded as a promising anode candidate due to its high theoretical reversible Li+ storage capacity (782 mAh g
1), low cost, and facile synthesis technology [5,6].

* Corresponding authors. Tel.:+ +86 28 83037409. E-mail addresses: [email protected] (M.-S. Wang), [email protected] (J. Zheng), [email protected] (X. Li). http://dx.doi.org/10.1016/j.electacta.2017.04.031 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

However, the practical use of tin based materials is greatly hampered by its huge volume change (300%) during the lithiation of Sn, leading to pulverization of particles as well as exfoliation of electrode materials. It is recently reported that designing the tin based materials of nanostructures, such as 1D tin dioxide nanowires [7], nanotubes [8,9], 2D tin nanosheets [10], 3D hollow sphere [11], hollow submicroboxes [12], or porous nanosphere [13], is proved to be an effective strategy to enhance the capacity of tin based anode. However, some technical challenges still exist in pure tin based anode for LIBs. For example, the volume expansion of electrode materials cannot be effectively suppressed by some nanostructure design. In addition, the large surface area of nanoparticles gives rise to serious parasitic side reactions, and aggravates the formation and fracture of unstable solid electrolyte interphase (SEI) film, resulting in continuing capacity degradation. To address those issues, some positive methods have been made to introduce carbon buffer matrix for tin based anodes to improve their electrochemical properties in LIBs [14–18]. Among those carbonaceous materials, graphene exhibiting several

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inherent advantages, such as excellent conductivity, superior flexibility and chemical stability, has been investigated as a carbon matrix to fabricate of SnO2/graphene hybrid anode materials [19– 23]. Firstly, graphene play an important role in promoting the electron transfer during the lithiation and delithiation process. Moreover, the flexibility of graphene can help release the residual stress arising from the volume change of SnO2 particles. Owing to these merits, various preparation methods have been conducted to construct SnO2/graphene composite, including solution-based synthesis [24–26], hydrothermal synthesis [27,28], self-assembly synthesis [29,30] and so on. A representative example can be found in anchoring SnO2 nanocrystals into 3D macroscopic frameworks built up by graphene, which can provide abundant interconnected porous structure with large surface area [27,31]. These 3D frameworks avoid the p
p restacking of graphene nanosheets, significantly enhancing the accessible surface area and strengthening the electrochemical reaction kinetics. Therefore, improved capacity of SnO2/graphene composites has been achieved. However, the low electrical conductivity in the graphene-based aerogel structures may result in low rate capability (below 500 mAh g
1 at 2 A g
1) [32–34], which is primarily due to the inadequate conductive frameworks established in these porous structures. Therefore, it is greatly important to effectively construct a conductive structure as a bridge to connect the relatively independent porous graphene aerogel. To realize these aims, different kinds of tin based/graphene composites for LIBs have been focused on adopting a second carbon precursor, such as glucose, dopamine, polyvinyl alcohol to form a supporting layer on the surface of tin oxide [35–37]. The double conductive layer can offer fast electron transportation and enough Li+ ion diffusion pathway, which enables improved rate performance for LIBs [38]. However, it is still a big challenge to control the uniformity of coating layer on the tin matrix and the optimal thickness of coating layer for those tin dioxide/carbon-graphene composites. In this study, we have fabricated carbon nanotube-graphene nanosheet (CNT-GN) conductive framework supported SnO2 aerogel composites by one-pot hydrothermal process. CNT is chosen as a conductive additive agent to build interconnected conductive framework. A systematic electrochemical analysis discloses that the 3D CNT-GN framework could suppress the volume expansion/shrinkage due to the synergistic effect of high mechanical property of CNT and flexibility of graphene, which increase the reversibility and kinetic behaviors of the composite. Thus, the as-prepared tin oxide/carbon nanotube-graphene nanosheet (SnO2/CNT-GN) composite exhibits a higher reversible capacity, better cycling stability and significantly enhanced rate capability as compared to its SnO2/GN counterpart. 2. Experimental 2.1. Materials Graphene oxide (GO) (GO, 99 wt%; Thinkness, 0.55–3.58 nm; layer number <10) and carbon nanotube (CNT, >95 wt%; OD, >50 nm; Length, 10–20 um;
OH content, 0.71 wt%) were purchased from Daying Juneng Technology and Development Co., Ltd without further purification. Stannic chloride pentahydrate (SnCl45H2O >99.0%) and polyvinylpyrrolidone K30 (PVP) were purchased from Chengdu Kelong Chemical Reagent Factory (China). 2.2. Synthesis of SnO2/CNT-GN composite The SnO2/CNT-GN composite was prepared by a facile hydrothermal process followed by the freeze drying and pyrolyzation. First, a uniform GO solution was prepared by dispersing

2.5 g freeze dried GO powder in 500 mL deionized water under sonication for 8 h. Then, 10 mg (0 g were also investigated) CNT and 10 mg polyvinylpyrrolidone (PVP) were carefully added in 200 mL as-prepared GO aqueous suspension (5 mg ml
1) followed by being sonicated for 30 minutes. After that, 2.5 g SnCl45H2O was added to the above aqueous solution under constant stirring. After stirring and sonication, the solution was transferred into a 250 mL Teflonlined stainless steel autoclave, which was kept at 180  C for 12 h in an oven and naturally cooled down to room temperature. Then the as prepared hydrogel was immersed in DI water for three times to remove excess acid. After that, the hydrogel was further transferred into the freezer dryer for 48 h to remove the residual water. Finally, the SnO2/CNT-GN composite was obtained by annealing at 550  C under an argon atmosphere for 5 h with a heating rate of 5  C min
1. For comparison, SnO2 nanocrystals are prepared by addition of 2.5 g SnCl45H2O in 200 mL DI water and then keeping in Teflon-lined stainless steel autoclave for 12 h at 180  C. After that, the white precipitation was freeze dried followed by heat treatment at 550  C for 5 h. The composite without adding CNT is also prepared in the same manner, which is labeled as SnO2/ GN. 2.3. Characterization The crystal structures of samples were characterized by X-ray diffraction (XRD) using XPERT-PRO diffractometer with Cu Ka radiation (l = 1.5406 Å) in the 2u range of 10–90 at 8 min
1. The content of carbon was calculated by thermogravimetric analysis (TGA) using TGA/SDTA851 system (Mettler-Toledo) in air from 25 to 900  C at a heating rate of 20  C min
1. Fourier transform infrared (FT-IR) spectra and Raman spectra were recorded using a Nicolet 6700 FT-IR Spectrometer (Thermo Scientific, USA) and ID Raman micro IM-52 (Oceanoptics) confocal Raman spectrometer, respectively. The pore structure and Brunauer
Emmett
Teller (BET) surface area were measured by V-Sorb 2800P analyzer at 77 K after being degassed at 180  C for at least 3 h. The morphology of composites was investigated by field-emission scanning electron microscopy (FESEM, FEI INSPECT-F, 20 kV) and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, 200 kV). 2.4. Electrochemical measurements The electrochemical tests were carried out with the CR2032 coin-type cells by using 1 M LiPF6 dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC) (1:1:1 by volume) as electrolyte. Firstly, 80 wt% active material was mixed with 10 wt% acetylene black and 10 wt% LA-132 binder to form a slurry and coated onto a copper foil, followed by being dried at 70  C under vacuum. The pure lithium foil was used as negative electrode and Celgard 2400 was used as separator. The galvanostatic charge/discharge tests were carried out at 200 mA g
1 in the voltage range of 0.01-3.0 V versus Li/Li+ using automatic battery measurement system (Neware BTS, Shenzhen, China). The electrochemical impedance spectroscopy (EIS) measurement was performed on electrochemical working station (Shanghai, Chenhua CHI660C). 3. Results and discussion Fig. 1 illustrates the synthesis procedure of the SnO2/CNT-GN composites. The SnO2/CNT-GN composites were fabricated by one pot hydrothermal route. CNT was homogeneously distributed in a solution of graphene oxide (GO) by the assistance of PVP as surfactant. During the hydrothermal process, Sn4+ ions can be anchored onto the surface of GO as the function of electrostatic attraction by the abundant functional groups such as hydroxyl,

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Fig 1. Schematic illustration showing the procedure for the preparation of 3D of SnO2/CNT-GN composite, and the advantage of using this material as an anode for LIBs.

carboxyl, and epoxy groups. Meanwhile, the GN anchored with SnO2 nanoparticles can be self-assembled into a 3D graphene hydrogels by the restoring of p-conjugation in the polar solvent [39]. In the 3D SnO2/CNT-GN framework, the physical cross-links between graphene sheets can be further strengthened by the conjunction of CNT. Therefore, the macroscopic framework of CNTGN aerogel is apparent different with GN aerogel. With freezedrying process, the macroscopic 3D structure of SnO2/CNT-GN can be well retained. The interconnected 3D CNT-GN framework not only enhances the structural integrity of SnO2 composite anode, but also could facilitate the prompt transportation of electrons and Li+ ions during the charge/discharge of SnO2 composite anode, improving the long-term cycling performance as well as rate performance. Fig. 2a displays the XRD patterns for SnO2/GN and SnO2/CNTGN composites. The main peaks of SnO2 in SnO2/GN and SnO2/CNTGN are in good agreement with tetragonal rutile-like SnO2 (JCPDF card no. 41-1445). In addition, the peaks of SnO2 are relative broad, indicating the nano size of particles. In all cases, the peak for GO at 11.4 could not be identified, which indicates that the disordered stacking characteristics of graphene after hydrothermal process. The (002) peak of CNT at 26 could not be detected either due to the limited content of CNT in composites and/or the overlapping with peak of SnO2. According to the TGA shown in Fig. 2b, the sharp weight loss of SnO2/GN and SnO2/CNT-GN occurs between 300  C and 600  C, which indicate the decomposition of carbon. Based on the weight loss of the composites, the carbon contents in SnO2/GN and SnO2/CNT-GN are calculated to be approximately 37.9% and 39.1%. Due to the presence of additive amount (1.2%) of CNT, SnO2/CNT-GN composite shows slightly higher weight loss during heating in air. The reduction of graphene oxide of the SnO2/GN and SnO2/CNTGN composites were characteristic by FT-IR and Raman spectroscopy. As shown in Fig. 2c, both SnO2/GN and SnO2/CNT-GN display the similar stretching vibration band. After hydrothermal and carbonization process, most of the oxygen-containing groups on GO (see Fig. S1), such as C

O at 1050 cm
1, C

OH at 1220 cm
1,
1
1 O

H at 1400 cm ,C

O at 1620 cm , and

COOH at 1730 cm
1 were eliminated, which indicates that GO has been reduced to graphene. Furthermore, the strong stretching vibrations of C¼C at

1562 cm
1 is attributed to the stretching vibration of the quinoid ring. It can also be found that stretching vibrations of C

O at 1181 cm
1 still exist in both SnO2/GN and SnO2/CNT-GN, which means a residual oxygen-containing group by thermal reduction reaction. The strong transmission from 560 cm
1 and 630 cm
1 could be assigned as the vibration of Sn-OH terminal bonds and the O

Sn

O stretching mode, respectively [40]. The effect of CNT on the graphitic nature of graphene layers was examined by Raman spectroscopy. Fig. 2d compares the characteristic peaks of D band and G band for GO, SnO2/GN and SnO2/ CNT-GN. The D band and G band represent the sp3- and sp2hybridized carbons (the k-point phonons of A1g symmetry and the E2g phonon of sp2-bonded carbon atoms), respectively. In the Raman spectrum of GO, the characteristic peaks for D band and G band are observed at about 1357 cm
1 and 1598 cm
1, respectively. The relative intensities of D to G band (ID/IG) value is about 0.747, implying the higher mode of sp2-bonded carbon atoms for GO because of the p-p stacking by the Van der Waals force. With reduction of GO to form GN hydrogel through hydrothermal process, the graphene nanosheets are reconstructed by reducing the oxygen groups, which results in the increasing of disorder degree of SnO2/GN. This may be the reason as to why higher ID/IG is detected for SnO2/GN (1.415). It is noticeable that the ID/IG of SnO2/ CNT-GN (1.276) becomes lower than that of SnO2/GN. This phenomenon indicates that the number of sp3 amorphous carbon is decreased by the introduction of CNT, which to some extent prevents the reconstruction of disorder carbon framework and thereby reduce the disorder degree of GN. The porous structures of SnO2/GN and SnO2/CNT-GN were further characterized by nitrogen isothermal adsorption/desorption measurements. As shown in Fig. 2e, both SnO2/GN and SnO2/ CNT-GN exhibit the typical type IV nitrogen adsorption branch, evidencing the mesoporous feature of composites. Compared with SnO2/GN, the N2 hysteresis loop for SnO2/CNT-GN is much lower, indicating a relatively small pore volume of SnO2/CNT-GN. The pore size distribution in Fig. 2f further confirms that both the SnO2/ GN and SnO2/CNT-GN display the similar pore size around  2– 4 nm. However, the BET surface area (see Table 1) of SnO2/CNT-GN (175.92 m2g
1) is much smaller than that of SnO2/GN (303.24 m2g
1). This result shows that the introduction of CNT

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Fig. 2. (a) XRD patterns of SnO2/GN and SnO2/CNT-GN; (b) TGA curve of the SnO2/GN and SnO2/CNT-GN composite recorded at a ramp rate of 10 min
1 under ambient atmosphere. (c) FT-IR spectra of SnO2/GN and SnO2/CNT-GN; (d) Raman spectra of GO, SnO2/GN, and SnO2/CNT-GN. (e) N2 adsorption
desorption isotherms and (f) pore size distribution of SnO2/GN and SnO2/CNT-GN.

into GN did not influence the pore structure of SnO2/CNT-GN composite, but could significantly increase the conjunction of individual graphene sheets, giving rise to the formation of more densely packed SnO2/CNT-GN composite. The morphologies of SnO2/GN and SnO2/CNT-GN were scrutinized by SEM and TEM in Fig. 3. The 3D porous structure can be obviously observed in both SnO2/GN (Fig. 3a–c) and SnO2/CNT-GN (Fig. 3d–f) composites. However, the surface of SnO2/GN exhibits larger wave and wrinkle in comparison with the surface of SnO2/ CNT-GN. Meanwhile, most of the porous structure is generated from the bending and pleat of the graphene nanosheet surface. In contrast, the pore morphology of SnO2/CNT-GN is apparently different with that of SnO2/GN, the surface of the CNT-GN framework (Fig. 3e) is relative smooth, which means the pore structure are originated from the cross linking between graphene

nanosheets. The pore structure of SnO2/CNT-GN may be more beneficial for linking the disordered graphene nanosheets to build interconnected conductive framework. Both the high-resolution Table 1 BET data and pore size data for SnO2/GN and SnO2/CNT-GN composites.

SnO2/GN SnO2/CNT-GN a

SBET (m2 g
1)a

Smeso (m2 g
1)b

Vtotal (cm3 g
1)c

APD (nm)d

284.63 169.81

303.24 175.92

0.218 0.147

3.070 3.452

The BET surface areas (SBET) are derived using multipoint BET method. Mesopore areas (Smeso) are calculated by BJH method. c Total pore volume (Vtotal) was estimated from the amount adsorbed at a relative pressure P/P0 of 0.99. d APD: average pore diameters are calculated by using BJH model. b

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Fig. 3. The SEM of (a–c) SnO2/GN and (d–f) SnO2/CNT-GN; (g-i) the TEM of SnO2/CNT-GN.

TEM images of SnO2/GN (Fig. S2) and SnO2/CNT-GN (Fig. 3g–i) demonstrate that SnO2 nanocrystals are distributed on the surface of graphene with the particle sizes of ca. 5 nm. The distribution of SnO2 nanocrystal is homogeneous without obvious agglomeration, which can be further confirmed by the elemental mapping images of the SnO2/CNT-GN (see Fig. S3). Meanwhile, CNT can be clearly observed to connect with graphene nanosheet and form an interconnected web-like structure, which is favorable for enhancing the electrochemical properties and is discussed as follows. The charge/discharge voltage profiles of the SnO2, SnO2/GN, and SnO2/CNT-GN at a current density of 200 mA g
1 between 0.01 and 3.0 V (vs. Li/Li+) are displayed in Fig. 4. The typical charge/discharge curves of SnO2 is shown in Fig. 4a, which exhibit two discharge voltage plateaus at approximately 0.8 V and 0.2 V, respectively. The short plateau at 0.8 V and the longer plateau at 0.2 V can be attributed to the conversion of SnO2 to Sn and the subsequent alloying reaction of Sn with lithium, respectively. The SnO2 electrode shows initial charge (delithiation) and discharge (lithiation) capacity of 442 mAh g
1 and 1219 mAh g
1 with initial coulombic efficiency of only 36.2%. The large capacity loss in the first cycle may result from irreversible decomposition of SnO2 to Sn and the consumption of Li+ by the formation of SEI layer. Moreover,

the specific capacity of SnO2 decreases rapidly after the first cycle, which only delivers 118 mAh g
1 at the 50th cycle and 74 mAh g
1 at the 100th cycle. By contrast, anchoring SnO2 on the graphene largely enhances the cycling stability of SnO2 (Fig. 4b). The SnO2/ GN composite displays similar initial discharge capacity (1220 mAh g
1) but much higher charge capacity (728 mAh g
1) than that of pure SnO2 electrode. However, the capacity still degrades slowly owing to the cracking and pulverization of SnO2 nanocrystals after repeated charge/discharge process. It delivers a limited discharge capacity of 480 mAh g
1 after 100 cycles, with only 65% of capacity retention. As expected, the SnO2/CNT-GN electrodes shows significant improvement in cycling performance compared with both SnO2 and SnO2/GN composite (Fig. 4d). The SnO2/CNT-GN composite displays a high initial charge/discharge capacity of 924/1797 mAh g
1 with an initial coulombic efficiency of 51.4% (Fig. 4c). The discharge capacity is much higher than the theoretical specific capacity of SnO2 (782 mAh g
1). The larger discharging capacity of tin based/carbon composite has been reported by other literature [41], which are attributed to the generating from the reaction (SnO2 + 4Li+ + 4e
! Sn + 2Li2O). Meanwhile, the SEI layer formation, side reactions and the lithium ion reactions with high surface area graphene and CNT would also

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Fig. 4. Charge/discharge voltage profiles as a function of cycle number of the (a)SnO2, (b) SnO2/GN, and (c) SnO2/CNT-GN electrodes. (d) Cycling performance of electrodes made from SnO2, SnO2/GN, and SnO2/CNT-GN at a current density of 200 mA g
1 (the capacity is calculated on the weight of the composite).

contribute to the initial lithium ion consumption. However, the high initial charge capacity of SnO2/CNT-GN composite can aslo be delivered, demonstrating that the effective electrical transfer and ionic transport of CNT-GN framework could stimulate the electrochemical activity of composite between SnO2 particles and Li+ [42]. As a conductive assistant, CNT significantly increases the specific capacity of SnO2/CNT-GN composite due to its positive effect in enhancing the electrical conduction enabled by interconnecting the graphene nanosheets. As a result, SnO2/CNT-GN shows a very decent long-term cycling stability, which is able to deliver 883 mAh g
1 at 50th cycle (96% capacity retention) and 808 mAh g
1 at 100th cycle (87% of capacity retention), which are much superior over those of SnO2 and SnO2/GN composite. To further gain insight into the different kinetics behaviors between the electrodes, the rate capability of SnO2/GN and SnO2/ CNT-GN composite was investigated and the result is displayed in

Fig. 5a. Besides its improved capacity and stable cycling performance, the SnO2/CNT-GN composite also exhibits superior rate capability. The SnO2/CNT-GN electrode is able to deliver reversible capacities of 1033, 970, 887, and 787 mAh g
1 at rising current densities of 0.5, 1, 2, and 5 A g
1, respectively, which significantly outperforms the SnO2/GN electrode at different current rates. In particular, the capacity can be recovered for SnO2/CNT-GN electrode when the current density value is switched back to 100 mA g
1, and a high capacity of 1223 mAh g
1 is still retained after 100 cycles. Fig. 5b compares several typical SnO2/graphene based composites published by other research groups. The result demonstrates that the rate capacity of this work is well comparable with or even better than other SnO2/graphene based composites with various hierarchical structures, e. g. MQDC-SnO2/RGO [43]; DF-SnO2/G@Pani nanosheets [40]; GNRs/SnO2 [44]; CoSnO3pGN [45]; SnO2/GFs [31]; SnO2 QDs@GO [46]; SnO2NC@SGH [47].

Fig. 5. (a) Rate performance of SnO2/GN and SnO2/CNT-GN composites at 0.1, 0.2, 0.5, 1, 2, 5 and 0.1 A g
1, respectively; (b) Comparison of capacity at different current densities between SnO2/CNT-GN electrode and those reported for other SnO2/graphene based composites.

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Table 2 Kinetic parameters of SnO2/GN and SnO2/CNT-GN electrodes after 100 cycles.

SnO2/GN SnO2/CNT-GN

Fig. 6. Nyquist plots of the SnO2/GN and SnO2/CNT-GN electrodes after 100 cycles in the delithiated state.

To further understand the superior electrochemical properties of SnO2/CNT-GN, the EIS measurements were performed after 100 cycles in the delithiated state. Fig. 6 displays the Nyquist plots for the SnO2/GN and SnO2/CNT-GN electrodes. The equivalent circuit that is used for fitting the impedance spectra is inset in Fig. 6. Both the SnO2/GN and SnO2/CNT-GN electrodes display a small depressed semicircle in the high-frequency range followed by a long steep sloping line in the low-frequency range. The semicircle at the high-frequency region is ascribed to the surface film resistance and charge transfer resistance, while the slope line in the low-frequency region is associated with the Li+ ion diffusion process in the solid electrode [48]. The diameter of the semicircle for SnO2/CNT-GN electrodes is smaller than that of SnO2/GN electrode, which suggests that SnO2/CNT-GN possess lower SEI film resistance and interfacial charge transfer reaction resistance. The simulated result indicates that Li||SnO2/CNT-GN cell shows lower electrolyte resistance (Re) and SEI layer resistance (Rsf) as compared to those for Li||SnO2/GN cell (as shown in Table 2). This suggests that a more robust SEI layer formed on the surface of SnO2/CNT-GN composite electrode, which in turn avoids the consumption of electrolyte. Additionally, SnO2/CNT-GN shows a much smaller interfacial charge transfer resistance (26.93 V), which his only about half of that found for SnO2/GN electrode

Re (V)

Rsf (V)

Rct (V)

Wo (V s
1/2)

CPE1 (F)

CPE2 (F)

8.69 6.06

5.70 4.83

53.1 26.93

0.72 0.39

3.55E-6 3.58E-5

0.98E-3 0.63E-3

(53.1 V). This indicates that the CNT-GN interconnected network structure provide more charge transfer pathways at the SnO2 interface, enhancing the lithiation/delithiation kinetics and hence ensuring better electrochemical reversibility of the SnO2/CNT-GN electrode. To better visually understand the superior electrochemical performance of SnO2/CNT-GN electrode, the morphology of SnO2/ GN and SnO2/CNT-GN electrode after 100 cycles at 0.2 A g
1 and after different current density test were compared in Fig. 7. The morphologies of SnO2/GN and SnO2/CNT-GN electrodes after long term cycling are different. A lot of obvious cracks can be found on the electrode for SnO2/GN (Fig. 7a and b). This means the electrode suffered repeated volume change, which results in structure fracture by the weak connection between graphene nanosheets. On the contrary, it can be easily seen that there is no serious crack on the surface of SnO2/CNT-GN electrode owing to the supporting of CNT (Fig. 7c and d). For further understand the aforesaid viewpoint, the TEM image of SnO2/CNT-GN after 100 charge/ discharge cycles at 200 mA g
1 (see Fig. S4) have been taken. The TEM result is well corresponding to the above-mentioned viewpoint. This phenomenon further confirms that the improved reversible capacity is originated from the effective connection of framework, which guarantees the integrity of electrode and efficient electronic transportation. The morphology of electrode after different current density test for SnO2/GN (Fig. 7e and f) electrode suggests that non-uniform SEI layers are formed on the surface of SnO2/GN electrode, resulting in the loss of electrochemical activity for composites. For comparison, SnO2/CNT-GN electrode exhibits more smooth SEI layer on the electrode and greatly improved structural integrity without obvious electrode cracking observed (Fig. 7 g and h). These observations substantiate that the interconnected 3D framework of SnO2/CNT-GN composite could effectively buffer the volume changes during lithiation/ delithiation of the composite anode and maintain a better

Fig. 7. (a) and (b) the SEM images of SnO2/GN electrode after 100 cycles; (c) and (d) the SEM images of SnO2/CNT-GN electrode after 100 cycles; (e) and (f) the SEM images of SnO2/GN electrode after rate test; (g) and (h) the SEM images of SnO2/CNT-GN electrode after rate test.

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electrical connection network, enabling the largely improved longterm cyclability and rate capability. 4. Conclusions CNT-GN 3D conductive framework supported SnO2 aerogel composites have been successfully synthesized by one-pot hydrothermal treatment followed by freeze drying and pyrolysis process. The design sufficiently takes advantages of porous structure by reconstructing graphene nanosheet, where a large amount of SnO2 nanocrystals are anchored. The introduction of CNT increases the connection of relatively independent graphene sheets, which provide much faster charge transfer pathways at the SnO2 interface. In addition, owing to the mechanical property of CNT and the flexibility of GN, the interconnected conductive framework can be maintained during repeated charge/discharge cycling. Therefore, the SnO2/CNT-GN electrode exhibits higher reversible capacity and rate capability than those of the SnO2/GN electrode. The fundamental findings of this work provide deep insights for the design of SnO2 based anode of hierarchical structure for the development of high performance lithium ion batteries. Acknowledgements This work was supported by the National Natural Science Foundation of China (grant No. 51502250, 51604250, 51474196, 51302232), the Science & Technology Department of Sichuan Province (grant no. 2015JY0089, 2016RZ0071), and Education Department of Sichuan Province (No. 16ZB0085). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.04.031.

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