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Three-dimensional graphene/single-walled carbon nanotube aerogel anchored with SnO2 nanoparticles for high performance lithium storage Jing Wang, Fang Fang, Tao Yuan, Junhe Yang, Liang Chen, Chi Yao, Shiyou Zheng, and Dalin Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10807 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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ACS Applied Materials & Interfaces

Three-dimensional graphene/single-walled carbon nanotube aerogel anchored with SnO2 nanoparticles for high performance lithium storage Jing Wang †, Fang Fang †, Tao Yuan ‡, Junhe Yang ‡, Liang Chen §, Chi Yao §, Shiyou Zheng ‡*, Dalin Sun †* †

Department of Materials Science, Fudan University, Shanghai 200433, China

‡

School of Materials Science and Engineering, University of Shanghai for

Science & Technology, Shanghai 200093, China. §

Department of Chemistry, Fudan University, Shanghai 200433, China.

* Corresponding authors. Tel.: C86 21 6564 2874; fax: C86 21 6564 2873. E-mail addresses: [email protected] (S. Zheng), [email protected] (D. Sun).

ABSTRACT：： A unique 3D graphene-single wall carbon nanotube (G-SWNT) aerogel anchored with SnO2 nanoparticles (SnO2@G-SWCNT) is fabricated by hydrothermal self-assembly process. The influences of mass ratio of SWCNT to graphene on structure and electrochemical properties of SnO2@G-SWCNT are investigated systematically. The SnO2@G-SWCNT composites show excellent electrochemical performance in Li-ion batteries, for instance, at a current density of 100 mA g−1, a specific capacity of 758 mAh g−1 was obtained for the SnO2@G-SWCNT with 50 % SWCNT in G-SWCNT and the coulombic efficiency is close to 100 % after 200 cycles; even at current density of 1 A g−1, it can still maintain

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a stable specific capacity of 537 mAh g−1 after 300 cycles. It is believed that the 3D G-SWNT architecture provides a flexible conductive matrix for loading the SnO2, facilitating the electronic and ionic transportation and mitigating the volume variation of the SnO2 during lithiation/delithiation. This work also provides a facile and reasonable strategy to solve the pulverization and agglomeration problem of other transition metal oxides as electrode materials. KEYWORDS: Li-ion battery, anode, tin oxide, graphene, carbon nanotube 1. INTRODUCTION Lithium-ion batteries have been widely applied in 3C devices and more because of relatively high energy density, high working voltage and no memory effect.1 However, with the ever-increasing demands for electrical energy storage for electric vehicles, smart grid and rechargeable power applications, it is crucial to increase the capacity of batteries. Graphite as commercial anode material by an insertion reaction of lithium in current Li-ion batteries, is not suitable here due to its intrinsic insufficient theoretical capacity of 372 mAh g−1.2 To overcome the restriction, new concepts for the anode materials is one option, e.g., to substitute the Li/graphite interaction for reactions with lithium by either conversion or alloying mechanisms. A series of metal oxides, nitrides, sulfides and fluorides have been studied as conversion/alloying reaction anode material of Li-ion batteries.3−10 Among them, SnO2 draws much attention for Li-ion anode material because of its relatively high specific capacity of 782 mAh g−1,11 low working voltage platform, along with cost-effectiveness and environmental benignity.12 However, for practical application, the SnO2 anode suffers from severe

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capacity

fading

owing

to

large

volume

change

(~300

%)

during

lithiation/delithiation,13 which causes pulverization and detachment of active material. A variety of strategies have been pursued to circumvent these problems, including fabricating hollow SnO2 nanospheres,14−16 introducing SnO2 to porous conductive matrix,17,18 composing SnO2 with other metal oxides and so on.19,20 Among them, embedding SnO2 nanoparticles into carbonaceous materials (e.g. graphene (G), carbon nanotubes (CNT), and carbon nanofibers (CNF)) is considered as one of the most effective approaches. As compared with the pristine SnO2, a better lithium storage performance was demonstrated for SnO2/G,21−24 SnO2/CNT25−28 and SnO2/CNF composites,29−32 which is mainly ascribed to the carbon-based matrix providing good electronic conductivity for transportation of electrons and Li+, as well as accommodating the volume change of SnO2 during lithiation/delithiation. However, the fabrication of uniform SnO2/nano-carbon composite is complicated because of self-agglomerating propensity of the carbonaceous materials to form stacked graphene, tangled CNT or CNF. An alternative method is to build a 3D architecture carbon matrix by combing 2D graphene with 1D CNT.33−35 It is well known, as comparing with 0D, 1D and 2D carbon materials, the 3D carbonaceous matrix possess comprehensive advantages including relatively higher specific surface areas and porous volume, which facilitate to load much SnO2 inside; especially, the 3D conductive channels are favor to improve the contact between electrode and electrolyte, and enhance the diffusion of Li+ ion, and thus a superior conductivity can be promised.36 To form a uniform SnO2/nano-carbon composite，it is crucial for the

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dispersion of piled graphene and the untanglement of the interwined CNT through the introduction of CNT into graphene nanosheets. For instance, Zhang et al. reported that a capacity of 635 mAh g−1 can be delivered at 0.25 A g−1 after 80 cycles for SnO2–G– CNT composite fabricated by hydrolysis and calcination method.37 Chen et al. adopted one pot microwave method to synthesize a SnO2–reduced graphene oxide– CNT composite with a reversible capacity of 502 mAh g−1 at 0.1 A g−1.38 Zhang and co-workers fabricated G/CNT/SnO2 aerogel by one pot hydrothermal approach and presented a specific capacity of 842 mAh g−1 at 0.2 A g−1 after 40 cycles.39 Though the introduction of CNT into graphene is an effective strategy to improve the lithium storage performance for SnO2, there are still two key points that need to be further considered: i) the appropriate mass ratio between graphene and CNT, which can substantially affects the morphology of hybrid matrix and the sizes of SnO2 particles; ii) the interaction between graphene and CNT for forming a robust 3D structure, which is one of the indispensable factors for the high SnO2 loading and long cycle life. In order to construct the stable 3D carbonaceous interaction matrix, various approaches have recently been employed by decorating graphene layers with additive agent or by fabricating functional CNT.40−43 However, little work has been reported in integrating graphene layers with CNT through strong chemical interaction, meanwhile, it is generally complex and time-consuming to prepare these carbon/SnO2-based composite anode materials, which limits the practical applications in Li-ion batteries. In the present work, a unique engineered graphene and SWCNT (G-SWCNT) architecture anchoring with SnO2 as anodes for Li-ion battery was fabricated by a

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facile hydrothermal self-assembly approach, that is, designing carboxyl functional modified SWCNT enhancing the aqueous dispersion of SWCNT and improving the chemical self-assembly of graphene oxide to form well organized graphene-SWNT aerogel architecture. The 3D G-SWCNT matrix provides a flexible conductive matrix for loading the SnO2, facilitating the electronic and ionic transportation and mitigating the volume variation of the SnO2 during lithiation/delithiation, thus leads to enhanced electrochemical performance of the SnO2 anodes in Li-ion batteries. 2. EXPERIMENTAL SECTION 2.1.Synthesis of Graphene Nanosheets and Single Wall Carbon Nanotubes Aerogel（（G-SWCNT）） The graphene oxide (GO) was prepared from natural graphite flakes through a modified Hummers method. The single wall carbon nanotube (SWCNT) was carboxylic also via modified Hummers method.44 Aqueous suspension of GO (10 mg ml−1) and SWCNT (10 mg ml−1) were mixed in 20ml reagent bottle according to a certain mass ratios (3:1, 1:1, 1:2) and 5 wt% pyrrole were added into the bottle. After the mixture was mixed by ultrasound, the bottle was transferred into 100 ml Teflon-lined stainless steel autoclave for hydrothermal self-assembling at 180 °C for 12 h. Then, the water within the obtained aerogel was replaced by tert-butyl alcohol for three times with 8 h each time, and dried the aerogel by freezer dryer. Finally, the G-SWCNT sample was prepared by carbonization of the aerogel at 1000 °C for 2 h in a stream of argon, at a ramp rate of 5 °C min−1. In the present work, 25%, 50% and 67% mass ratios of SWCNT in G-SWCNT hybrid were prepared and referred as

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G-SWCNT25, G-SWCNT50 and G-SWCNT67, respectively. 2.2. Preparation of SnO2@G-SWCNT Certain amount of the G-SWCNT and SnCl2·2H2O were mixed in 50 ml teflon lining and dispersed in 20 ml ethanol and water (1:1) solvent by ultrasound for 30 min, then put the container into autoclave and hydrolyzed at 200 °C for 16 h. Finally, the resultant precipitate (SnO2@G-SWCNT) was washed with deionized water and ethanol for several times, and dried at 80 °C for overnight. 2.3. Materials Characterization X-ray diffraction (XRD) patterns of the samples were collected using Germany Bruker D8A-Advance with Cu Ka (l50.154 nm) radiation at 40 kV and 40 mA. Thermogravimetric analysis (TGA) was conducted with America TA-Q500 instrument under air at a heating rate of 5 °C min–1 within a temperature range of 30-800 °C. Scanning electron microscopy (SEM) studies were carried out with a Holland Philips XL30-FEG. Transmission electron microscopy (TEM) observation was acquired with Japan JEOL JEM-2100F field emission transmission electron microscope operated at accelerating voltage of 200 kV. 2.4. Electrochemical Measurements Electrochemical analyses were carried out using CR2032-type coin cells. The working electrodes were prepared by uniformly painting homemade slurry on a copper foil and dried in a vacuum oven at 80 °C overnight. The slurry contains 80 wt% active materials, 10 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) solvent. The coin cells were assembled in a glove box

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filled with high pure argon. Celgard 2500 polypropylene was used as separator and the counter electrode was lithium metal foil. The electrolyte was 1 M LiPF6 dissolved in a volume ratio of 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) mixture. Galvanostatic charge and discharge performance were conducted to test the electrochemical capacity and cycle stability of the electrodes at various current densities (in the voltage range from 0.001 to 3.0 V) using battery test system (CT2001A, LAND, China) at room temperature. The measurement of cyclic voltammetry (CV) were performed from 0.001 to 3.0 V at a scan rate of 0.1 mV s−1 and electrochemical impedance spectroscopy (EIS) measurements over a frequency range from 100 kHz to 0.01 Hz were carried out using a Gamry Reference-6000 electrochemical workstation.

3. RESULTS AND DISCUSSION 3.1. Structure Characterization and Morphology The synthesis process of SnO2@G-SWCNT composite is schematically shown in Figure 1. The carboxyl functional modified SWCNT and pyrrole were added into GO suspension. During hydrothermal reaction, the carboxyled SWCNT might be wrapped by GO, attached on the surface of GO and interconnected with GO, in this way, the space between the graphene sheets were enlarged owing to the intercalation of SWCNT; meanwhile, the functional groups of GO and SWCNT could experience the esterification reaction between hydroxyl and carboxyl, along with π-π interactions, thus the 3D structural G-SWCNT were self-assembled with hand of these interaction

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force. By the hydrolysis of bivalent tin salt, the SnO2 nanoparticles were uniformly anchored on the 3D networks.

Figure 1. Schematic of the synthesis of SnO2@G-SWCNT composites.

The phase structures of G-SWCNT and SnO2@G-SWCNT composites were characterized by XRD. As shown in Figure 2a, a broad diffraction peak at around 26° was observed for G-SWCNT25，which is ascribed to the (002) lattice plane of graphene. With the increase of SWCNT mass content, the peak becomes unclear in G-SWCNT50 and G-SWCNT67 patterns, indicating that the graphene nanosheets are further dispersed by SWCNT.45 In addition, this peak slightly shifts to lower angle when increasing the mass proportion of SWCNT, which suggests that the intercalation of SWCNTs enlarge the interlayer distance of graphene nanosheets according to the Braggs law (2dsinθ=nλ). The XRD patterns of SnO2@G-SWCNT samples indict the existence

of

cassiterite

phase

of

SnO2

(JCPDS

No.41-1445).

The

SnO2@G-SWCNT50 composite shows relatively better crystallinity as compared with SnO2@G-SWCNT25 and SnO2@G-SWCNT67, according with the fact that the 8


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nanoparticles commonly tend to well crystal growth in orderly organized carbonaceous matrix. Thus, the crystallinity of SnO2@G-SWCNT67 is relatively weaker, which may be related to less graphene dispersing in SWCNT for the G-SWCNT67 matrix and disadvantage for the crystal growth. The morphologies of the G-SWCNT and SnO2@G-SWCNT samples are further investigated by SEM and TEM. Figure 2b−d are SEM images of G-SWCNT samples. It is clear that these G-SWCNT hybrids are composed of crumped graphene nanosheets and outstretched SWCNTs, which are interconnected to form 3D networks. For the G-SWCNT25 sample (Figure 2b), it seems that almost all SWCNTs are attached on the surface of graphene nanosheets and dense packed graphene nanosheets. With the increase of the proportion of SWCNT, the graphene layers in G-SWCNT50 and G-SWCNT67 samples are opened wider by SWCNTs obviously (Figure 2c, d), suggesting the alleviation of agglomeration of graphene nanosheets, which is consistent with the XRD results. However, in the 3D structure of G-SWCNT67, it is obvious that the one-dimensional SWCNTs are much more than the two-dimensional graphene. After anchoring SnO2 particles in G-SWCNT materials, the 3D SnO2@G-SWCNT composites remains integrated (Figure S1), but the SnO2 nanoparticles are too small to be clearly identified in the SEM images. Therefore, TEM observation is further conducted to reveal more clear and distinguished structural information of SnO2@G-SWCNT (Figure 3).

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Figure 2. (a) XRD patterns of G-SWCNT25, G-SWCNT50, G-SWCNT67, SnO2@G-SWCNT25, SnO2@G-SWCNT50, and SnO2@G-SWCNT67. SEM images of (b) G-SWCNT50, (c) G-SWCNT25, and (d) G-SWCNT67.

As shown in Figure 3, the SnO2@G-SWCNT samples show different microstructure due to the different architectures of G-SWCNT precursors. The SnO2 nanoparticles

are

somewhat

reunited

in

the

SnO2@G-SWCNT25

and

SnO2@G-SWCNT67 samples (Figure 3a, e). For the SnO2@G-SWCNT25 sample, the stacked graphene lacks of effective dispersion space for SnO2 nanoparticles. While for the SnO2@G-SWCNT67 sample, too much one-dimensional SWCNTs may hinder the dispersion of SnO2 in graphene sheets. However, the SnO2@G-SWCNT50 sample, by contrast, shows better dispersion of SnO2 in Figure 10


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3c. Therefore, evenly scattered graphene and SWCNT in right proportion can effectively prevent the aggregation of SnO2 nanoparticles. A mesoporous interlinked network was formed consisting of few thin layers of graphene and around 3~5 nm in diameters of SWCNT, which facilitates the penetration of electrolyte into the 3D structure for enhancing Li+ ions transport. The SnO2 nanoparticles synthesized by the hydrolysis of bivalent tin salts are mostly “block-shaped” nanoparticles with size around 6~8 nm and the growth of these “block-shaped” nanoparticles are subject to the G-SWCNT structures. It can be found that the crystals prefer to grow along the (110) facets and inhibit the growth of [001] direction as the nanoparticles uniformly anchored on well-organized matrix.46, 47 Meanwhile, the relative intensities of X-ray diffraction peaks of SnO2@G-SWCNT are also compared with the standard card of cassiterite SnO2 (Table S1). The SnO2@G-SWCNT50 has the highest relative intensity of the (110) phase among the three samples, and the growth of its other faces decrease at a large extent, conforming to the results of TEM images. According to

the

electron

diffraction

SnO2@G-SWCNT50

possess

patterns the

(insets best

of

Figure

crystallization

3b,

d,

property

f),

the

among

SnO2@G-SWCNT composites and the four distinct diffraction rings are assigned to cassiterite phase SnO2 (110), (101), (200) and (301). Furthermore, these results prove that well dispersed structure not only can induce uniform growth of nanoparticles, but can also improve its degree of crystallization as well. Also, we conducted the Nitrogen adsorption and desorption tests of the G-SWCNT and SnO2@G-SWCNT samples. It can be found the specific surface area and pore

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volume are decreased as the SnO2 loaded, for instance, the specific surface area and pore volume of G-SWCNT50 are 437 m²/g and 0.842 cm³/g, respectively. After loading with SnO2, the corresponding values are decreased to 299 m²/g and 0.547 cm³/g (See Figure S2). The Brunauer−Emmett−Teller (BET) analysis results suggest that the SnO2 nanoparticles are indeed confined within the porous 3D G-SWCNT matrix.

Figure 3. TEM images of (a, b) SnO2@G-SWCNT25, (c, d) SnO2@G-SWCNT50, 12


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and (e, f) SnO2@G-SWCNT67.

TGA

is

used

to

investigate

quantitatively

the

carbon

contents

of

SnO2@G-SWCNT composites. From Figure 4a, three SnO2@G-SWCNT samples exhibit a significantly weigh loss approximately between 500 and 650 °C. It is no doubt that the weight decline of the composites in TGA is assigned to the combustion of carbon in air. For comparison, we also prepared the SnO2@G composite with the same ratio but without SWCNT participation. Its TGA curve is inserted in Figure 4a. As shown in the inset of Figure 4a, the turning point of the TGA curve occurs at around 380 °C, which is much lower than that of SnO2@G-SWCNT samples. Therefore, SnO2@G-SWCNT composites possess better thermal stability than SnO2@G material, confirming that the SWCNTs have strong bonds with graphene nanosheets which results in the robust SnO2@G-SWCNT composites with 3D networks. The contents of G-SWCNT in SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 samples are calculated to be 46.9 %, 44.7 %, and 47.8 %, respectively. The Raman spectra of G-SWCNT25, G-SWCNT50 and G-SWCNT67 are shown in Figure 4b. There are two peaks located around 1340 cm−1 (D band) and 1580 cm−1 (G band), which correspond to the defects such as vacancies, grain boundaries among the carbonaceous materials and the vibration of sp2 (or double bonded) carbon atoms, respectively.48 The intensity ratio of the D band to the G band commonly used as indicator as the graphitization degree of carbonaceous materials.49 As shown in Figure

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4b, the value of ID/IG of G-SWCNT25, G-SWCNT50 and G-SWCNT67 are 1.04, 0.64 and 0.72, respectively. And the G-SWCNT50 sample has the lowest ID/IG value, demonstrating the orderly dispersed 3D structure of G-SWCNT50 possess higher degree of graphitization with better electronic conductivity.

Figure 4. (a) TGA curves of SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 composites in air, and (b) Raman spectra of G-SWCNT25, G-SWCNT50, G-SWCNT67.

The X-ray photoelectron spectroscopy (XPS) was used to characterize the covalent bond between graphene and SWCNT for G-SWCNT50 composites. The general XPS spectrum of G-SWCNT50 reveals that the percentages of carbon, oxygen and nitrogen elements are 93.4, 3.55 and 3.05 %, respectively. Figure 5b present the spectra of C 1s, four distinct divided peaks around 284.8, 285.6, 287.2 and 290.4 eV, which can be associated with the C=C, C-O, C=O and O-C=O bonds, respectively.50 The strong peak of O-C=O bond certify that by carboxylic functional SWCNT and GO one can successfully create covalent connected structure. Furthermore, the 14


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oxygen element is also examined to prove the existence of ester group as shown in Figure 5c, the three oxygen regions at about 531.6, 532.5 and 533.4ev are attributed to C-O, C=O and O-C=O bonds, respectively (Figure 5c).51 Because of the strong chemical bonds, the G-SWCNT can be thermally stable under elevated temperatures. In addition, there are three types of nitrogen bonds as shown in Figure 5d, the peaks at around 398.5, 401.2 and 403.4 eV related to pyridinc N, pyrrolic N and graphitc N, respectively,52 indicating the nitrogen element as dopant in graphene and SWCNT, which may positively modified the electronic property of the G-SWCNT hybrid.

Figure 5. (a) XPS spectra of G-SWCNT50, (b) C 1s XPS spectra of G-SWCNT50, (c) O 1s XPS spectra of G-SWCNT50, and (d) N 1s XPS spectra of G-SWCNT50.

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3.2. Electrochemical Performance The electrochemical performance of the SnO2@G-SWCNT composites were studied using half-cell system. The galvanostatic charge and discharge profile of SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 at 100 mA g−1 current density are presented in Figure S3a, Figure 6a, and Figure S3c, respectively. A lithiation plateau at 0.86 V is observed in the first discharge cycle, and turns into a slope plateau at 0.9~0.6 V in following cycles. The initial specific capacities of SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 are 1964, 1900 and 1230 mAh g−1 with the capacity retentions of 39.0, 53.2 and 53.5 %, respectively, demonstrating that all SnO2@G-SWCNT composites experience irreversible capacity loss during the first cycle. This is a common issue for high-capacity anodes such as Si, Sn, SnO2 and so on. However, with the development of Li-ion battery technology, this problem can be resolved by pre-lithiation strategy. The cycle voltammetry analysis was conducted to examine the electrochemical conversion of the SnO2@G-SWCNT electrodes in the voltage range of 3.00~0.001 V vs. Li+/Li (Figure S3b, 6b, S3d). There are two strong peaks centered at 0.86 and 0.02 V in the first discharge cycle. The peak at 0.86 V corresponds to the conversion reaction between SnO2 and Li+ ions and the formation of Sn and Li2O (Eq.(1)), and the peak situated on 0.02 V is assigned to Sn further reduced by Li+ ions forming LixSn alloy (Eq.(2)), and the intercalation of lithium into the carbonaceous materials (Eq.(3)).53 As for the first anodic cycle, two peaks located at around 0.16 and 0.56 V can be accounted to the extraction of lithium from carbonaceous materials54 and the dealloying of LixSn, respectively. Besides, an

16


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anodic peak at 1.28 V is attributed to Sn partial oxidation into SnO2. From the second cycle on, there are significant change in lithiation process, the peak at 0.86 V is disappeared and replaced by a broad cathodic shoulder at 0.7~1.2 V, which is in accord with the charge and discharge profiles. The irreversible capacity loss during the first cycle is generally due to the irreversible reaction of SEI formation and the partially irreversible conversion reaction of SnO2 into Sn.55 Consequently, the electrochemical process of the electrodes can be described by the three primary reactions:

SnO2 + 4Li+ + 4e− ⇌ Sn + 2Li2O (1) Sn + xLi+ + xe− ⇌ LixSn

(2)

C + xLi+ + xe− ⇌ LixC

(3)

Figure 6. (a) Charge/discharge profile and (b) cycle voltammetry curve of SnO2@G-SWCNT50.

The

cycling

performance

and

electrochemical

rate

performance

of 17



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SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 electrodes were investigated. The results show that SnO2@G-SWCNT50 composite has advantage in reversible lithium ion storage performance. At low current density of 100 mA g−1 after 100 cycles, the SnO2@G-SWCNT50 contained higher capacity of 785 mAh g−1 compared with 612 mAh g−1 for SnO2@G-SWCNT25 and 576 mAh g−1 for SnO2@G-SWCNT67 (Figure 7a). Electrochemical rate performance of SnO2@G-SWCNT50 is remarkably excellent at various current densities, which holds average capacities of 864, 760, 663, 575, 510, 426 mAh g−1, at current densities of 50, 100, 200, 500, 1000, 2000 mA g−1, respectively. Even after cycling at higher current densities, the special capacity is able to recover back to 754 mAh g−1 at 100 mA g−1. In particular, the difference of electrochemical performance becomes significant with the increasing of the current density. For instance, the specific capacity of SnO2@G-SWCNT50

is around 1.2 (or 1.6) times higher than that of

SnO2@G-SWCNT25 (or SnO2@G-SWCNT67) at 200 mA g−1, while at 2 A g−1 current density the value can reach approximately 2 (or 4) times high. The impressive rate capability of SnO2@G-SWCNT50, demonstrate that a superior 3D conductive network structure has been successful built by optimizing the incorporation of SWCNT.

The

long

cycling

performance

and

coulombic

efficiency

of

SnO2@G-SWCNT50 were shown in Figure 7c and d. The discharge specific capacity at 100 mA g−1 after 200 cycles still remained as high as 758 mAh g−1 and the coulombic efficiency was kept above 95 % after the 10th cycle. In addition, the SnO2@G-SWCNT50 apparently possesses overwhelming advantage in long cycle life

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and fast reversible lithium ion storage. Though the electrode experience a gradually capacity decay at the first 25 cycles, it can keep a stable coulombic efficiency close to 100% after that, and the specific capacity sustained 537 mAh g−1 after as long as 300 cycles even at 1 A g−1 current density. It is believed that it is because of the strong covalent bond between the graphene and SWCNT, which is essential for keeping the 3D network stable. For evaluating the improvement of SnO2 electrode, the electrochemical performance of the original 3D carbonaceous materials were also tested (Figure S5). The specific capacities of G-SWCNNT50, G-SWCNNT25 and G-SWCNNT67 at 100 mA g−1 after 100 cycles are 580, 510 and 487 mAh g−1, respectively (Figure S5). The SnO2 electrode capacity is calculated according to this equation: CSnO2 = [ Ctotal − CG-SWCNT*WG-SWCNT%] / WSnO2%, Consequently, we obtained an experimental special capacity of 939 mAh g − 1 based on the mass percentage of SnO2 in SnO2@G-SWCNT50 composite, this value is much higher than the theoretical capacity of 782 mAh g − 1, attributing to the improvement of the partially reversible conversion from Sn to SnO2. Meanwhile, the results indicate superior synergetic effect between 3D structure and N doping which makes homogeneous distribution and confinement of SnO2 nanoparticles in 3D structure possible.

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Figure 7. (a) Galvanostatic cycling performance and (b) electrochemical rate performance of SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67. Cycling property of SnO2@G-SWCNT50 at (c) 100 mA g−1, (d) 1 A g−1 current density.

To better elucidate their electrochemical properties, the electrochemical impedance spectra (EIS) of SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 electrodes were compared in Figure 8. According to the plots, from high to medium frequency regions were presented the compressed semicircle and followed by a straight line in the low frequency region. It can be connected with the SEI film resistance (RSEI) for the semicircle at high frequency, and at medium frequency belongs to the charge transfer resistance (Rct) at the electrode-electrolyte 20


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interface or internal charge transfer resistance of the electrodes.56 The linear region represents the diffusion of the Li+ ions in electrode.57 The SnO2@G-SWCNT50 electrode has the smallest diameter of the semicircle in the plots. Moreover, the linear slope of SnO2@G-SWCNT50 is distinctly more abrupt than SnO2@G-SWCNT25 and SnO2@G-SWCNT67, indicating SnO2@G-SWCNT50 has relatively smaller charge transfer impedance and faster Li+ ion transmission channel, which is favorable for Li+ ions insertion and extraction into the anode materials. TEM was also employed to analyze the structural properties of the SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 samples after undergoing 100 cycling, as shown in Figure S6. As for the cycling SnO2@G-SWCNT25 and SnO2@G-SWCNT67 electrodes, the pulverization and aggregation of SnO2 nanoparticles can be distinctly observed. While for the SnO2@G-SWCNT50 electrode, the porous and interconnected structure is still maintained and the SnO2 nanoparticles are kept to anchor on the 3D networks integrally, leading to relatively better cycling performance. The 3D structure offering highly mesoporous structure and large contact area with electrolyte, and short diffusion path of Li+ ions, is essential to the electrochemical performance of SnO2@G-SWCNT50. In addition, well dispersion of G-SWCNT50 promises uniformly anchored SnO2 nanoparticles and flexible conductive matrix, which are critical for the electrode to endure volume change and prevent pulverization during lithiation/delithiation process.

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Figure

8.

Electrochemical

impedance

spectra

of

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SnO2@G-SWCNT25,

SnO2@G-SWCNT50and SnO2@G-SWCNT67.

4. CONCLUSION A uniformly dispersed SnO2 nanoparticles embedded in 3D graphene interconnected SWCNT aerogel was fabricated by a facile hydrothermal approach. The influences of mass ratio between SWCNT and graphene on structure and electrochemical properties of SnO2@G-SWCNT are investigated systematically. By adjusting the content of SWCNT, we obtain uniformly dispersed 3D structures with negligible agglomeration of SnO2 nanoparticles, thus resulting in excellent Li-ion storage performance. The SnO2@G-SWCNT50 anodes exhibit stable, high and reversible capacities together with good rate and cycling capabilities in Li-ion batteries. This advancement can be ascribed to the unique G-SWCNT structure, which provides: i) large wetting area for electrode contacting with electrolyte and accelerating the movement of Li+ ions; ii) 3D electronic transfer channel and good electronic conductivity; iii) producing more porous network in electrode material and accommodating huge volume change during lithiation/delithiation. In this study, we 22


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mainly focus on the strategy to improve cycling performance for SnO2 anode material, and also we acknowledge that the common low initial Coulombic efficiency problem for SnO2-based material is not so well settled. Therefore, it’s our next effort to improve the initial Coulombic efficiency and make the SnO2@G-SWCNT anode material more practical, such as research on pre-lithiation and SEI films.

ASSOCIATED CONTENT

Supporting Information

SEM images of SnO2@G-SWCNT composites; Charge/discharge profile and cycle voltammetry of SnO2@G-SWCNT25 and SnO2@G-SWCNT67; Cycle voltammetry of G-SWCNT hybrids and SnO2; Cycling performance of G-SWCNT hybrids at 100mAg-1 current density. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author

* Tel.: C86 21 6564 2874; fax: C86 21 6564 2873. E-mail addresses: [email protected] (S. Zheng), [email protected] (D. Sun).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of the National Natural Science Foundation of China (21403139, 51472161, 51472160, 51671135, 51272157), the Program of Shanghai Subject Chief Scientist, the Key Program for the Fundamental Research

of

the Science and Technology Commission of Shanghai Municipality

(15JC1490800, 12JC1406900) and the International Cooperation Program of the Science and Technology Commission of Shanghai Municipality

(14520721700).

We acknowledge the support of the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (TP2014048). REFERENCES (1) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (2) Martin Winter, J. O. B. M. Insertion Electrode Materials for Rechargeable Lithium Batteries. Adv. Mater. 1998, 10, 725-763. (3) Li, W.; Wang, F.; Liu, Y.; Wang, J.; Yang, J.; Zhang, L.; Elzatahry, A. A.; Al-Dahyan, D.; Xia, Y.; Zhao, D. General Strategy to Synthesize Uniform Mesoporous TiO2/Graphene/Mesoporous TiO2 Sandwich-Like Nanosheets for Highly Reversible Lithium Storage. Nano Lett. 2015, 15, 2186−2193. (4) Jiao, Y.; Han, D.; Ding, Y.; Zhang, X.; Guo, G.; Hu, J.; Yang, D.; Dong, A. Fabrication of Three-Dimensionally Interconnected Nanoparticle Superlattices and Their Lithium-Ion Storage

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Table of Contents Graphic

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Figure 1. Schematic of the synthesis of SnO2@G-SWCNT composites. 70x33mm (300 x 300 DPI)



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Figure 2. a) XRD patterns of G-SWCNT25, G-SWCNT50, G-SWCNT67, SnO2@G-SWCNT25, SnO2@GSWCNT50, and SnO2@G-SWCNT67. SEM images of b) G-SWCNT50, c) G-SWCNT25, and d) G-SWCNT67. 114x89mm (300 x 300 DPI)


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Figure 3. TEM images of a, b) SnO2@G-SWCNT25, c, d) SnO2@G-SWCNT50, and e, f) SnO2@G-SWCNT67. 170x197mm (300 x 300 DPI)



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Figure 4. a) TGA curves of SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67 composites in air, and b) Raman spectra of G-SWCNT25, G-SWCNT50, G-SWCNT67. 57x22mm (300 x 300 DPI)


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Figure 5. a) General XPS spectra of G-SWCNT50, b) C 1s XPS spectra of G-SWCNT50, c) O 1s XPS spectra of G-SWCNT50, and d) N 1s XPS spectra of G-SWCNT50. 116x93mm (300 x 300 DPI)



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Figure 6. a) Charge/discharge profile and b) cycle voltammetry curve of SnO2@G-SWCNT50. 55x21mm (300 x 300 DPI)


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Figure 7. a) Galvanostatic cycling performance and b) electrochemical rate performance of SnO2@GSWCNT25, SnO2@G-SWCNT50 and SnO2@G-SWCNT67. Cycling property of SnO2@G-SWCNT50 at c) 100mA g−1, d) 1A g−1 current density. 111x84mm (300 x 300 DPI)



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Figure 8. Electrochemical impedance spectra of SnO2@G-SWCNT25, SnO2@G-SWCNT50 and SnO2@GSWCNT67. 55x42mm (300 x 300 DPI)


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Table of Contents Graphic 35x15mm (300 x 300 DPI)


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