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Jan 17, 2019 - Division of Physics, Department of Mathematical Sciences, Xi'an Jiaotong-Liverpool University, 111 Ren'ai Road, Suzhou Dushu...
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Nano-SnO2/Carbon Nanotube Hairball Composite as a High-Capacity Anode Material for Lithium Ion Batteries Min Liu, Shuo Zhang, Houcai Dong, Xi Chen, Shan Gao, Yiping Sun, Weihong Li, Jiaqiang Xu, Liwei Chen, Anbao Yuan, and Wei Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05869 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Nano-SnO2/Carbon Nanotube Hairball Composite as a High-Capacity Anode Material for Lithium Ion Batteries

Min Liu,†‡ Shuo Zhang,‡ Houcai Dong,‡ Xi Chen,§ Shan Gao,‡ Yiping Sun,‡ Weihong Li,‡ Jiaqiang Xu,† Liwei Chen,‡ Anbao Yuan,*† Wei Lu*‡

†Department

of Chemistry, College of Sciences, Shanghai University, 99 Shangda

Road, Baoshan District, Shanghai 200444, China ‡i-Lab,

CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech

and Nano-Bionics (SINANO), Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industrial Park, Suzhou 215123, China §Division

of Physics, Department of Mathematical Sciences, Xi'an Jiaotong-Liverpool

University, 111 Ren'ai Road, Suzhou Dushu Lake Science and Education Innovation District, Suzhou Industrial Park, Suzhou 215123, China

*Corresponding author. E-mail address: [email protected] (A. Yuan) E-mail address: [email protected] (W. Lu)

Keywords: Lithium ion battery; Anode; Tin dioxide; Multi-walled carbon nanotube hairball; Spray drying; Solvothermal method 1

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ABSTRACT: An ultrafine (6−7 nm) and well dispersed nano-SnO2/carbon nanotube hairball (SnO2/CNTH) composite material with a three-dimensional (3D) hierarchical structure is prepared by spray drying and solvothermal method. This composite material demonstrates much superior electrochemical performance over the bare SnO2 and an obviously improved Li-storage performance over the SnO2/CNT composite in respect to specific capacity, rate performance and cycling stability. It exhibits a high reversible capacity of 1109.5 mAh g−1 at 0.1 A g−1, achieves a maximum reversible capacity of 1090.6 mAh g1 when continuously cycled at 0.2 A g−1, and remains a capacity of 809.2 mAh g1 after 100 cycles with the capacity retention of 74.2%. The improved electrochemical performance is attributed to the increased conductivity and hence the enhanced electrode reactivity as well as the electrode stability due to the particular 3D hierarchical structure of the SnO2/CNTH composite. This structure can also address the large volume change upon cycling.

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INTRODUCTION To fulfill the rigorous requirements of desirable energy density, power density and cycle life, more improvements need to be made in the performance of the next generation lithium ion batteries (LIBs). Conversion-type electrode materials with high theoretical specific capacities are promising anode materials for LIBs. However, their applications in practical LIBs are severely hampered by the unsatisfactory cyclability resulting from their inherent low conductivity and large volume change. Thus, many attempts have been taken so that properties of conversion-type anode materials can be improved.13 SnO2 has ever been explored as an anode material for LIBs. In the early days, SnO2 was employed as a typical alloying-type electrode with higher specific capacity over commercial graphite anode. As early as in 1997, the group of Dahn reported a SnO2 anode with a

capacity of 300 mAh g−1 through 25 cycles at 37.2 mA g−1 within the

range of 0.31.0 V (vs. Li+/Li) based on the alloying/dealloying reaction of Li with Sn.4 However, serious capacity decline was observed when upper cutoff potential reached 1.3 V. Retoux et al. found that during the first cycle the SnO2 crystallites were decomposed into Sn grains of 10−50 nm encircled by an amorphous ring composed of Sn and O (5−10 nm wide). They also found that after 500 cycles the size of Sn crystallites was increased, and the structure of the amorphous compound surrounding the Sn particles was changed as well.5 Moreover, the upper cutoff potential of SnO2 can be extended to 3 V when both the alloying reaction and the conversion one were considered, thus obtaining a higher specific capacity.6 3

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Large volume changes commonly occur in these host materials during lithium insertion and deinsertion, resulting in pulverization and/or aggregation of the material, and generation of undesirable and unstable solid electrolyte interphase (SEI), and thus leading to a rapid decline in capacity. The volume expansion could be reduced via decreasing the size of SnO2 particles. Compared with bulk SnO2, nanostructure SnO2 with high specific surface area has shorter diffusion path and better electrolyte permeability, which has attracted researchers' attention. In addition, the nanostructure can also reduce the strain caused by lithiation, thus improving the cycling performance.1 Zhu et al. have studied the lithium cycling behavior. The first-cycle reversible capacity of SnO2 nanoparticles in the range of 3−62 nm has been reported to be 300−800 mAh g−1 at 0.2 mA cm−2 when cycled between 0 V and 1.3 V.7 The group of Pol reported an ordered network nanostructure of SnO2 that can exhibit substantially high discharge capacity of 778 mAh g−1 at 0.1 C rate when cycled between 0.3 V and 1.5 V.8 A stable reversible capacity of 740 mAh g−1 (at 60 mA g−1 within 0−1.2 V up to 60 cycles) was observed by Kim et al. for the SnO2 nanoparticles (3 nm), but the one with relatively larger particle size (4 or 8 nm) showed capacity fading under the similar cycling conditions.9 To further promote the practical application of SnO2 in lithium ion batteries, nanoSnO2 based composites have been extensively studied in recent years.6,1014 It has been agreed to be a good direction to fabricate the anode material by combining the high specific capacity of SnO2 and advantages of carbon nanotubes (CNT), such as high conductivity and aspect ratio, large specific surface area and long cycle life.6,10,11,1619 4

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Here, we report a strategy to achieve a coral-like 3D heterostructure that incorporates SnO2 nanoparticles with multi-walled carbon nanotube hairballs (SnO2/CNTH) as a composite anode for LIBs. Firstly, the carbon nanotube hairballs (CNTHs) were constructed by spray drying of CNT suspension, and then the SnO2 nanoparticles were incorporated into the CNTH framework by solvothermal method using tin dichloride (SnCl2·2H2O) as the Sn source. The SnO2/CNTH composite material exhibits improved Li-storage performance over the bare SnO2 and the SnO2/CNT composite materials prepared under the similar conditions.

EXPERIMENTAL SECTION Preparation of CNTH. Firstly, 30 g multi-walled carbon nanotubes (MWCNTs) were added into a solution containing deionized water of 900 mL and ethanol of 100 mL, and then the mixture (suspension) was dispersed by ultrasonic treatment for 30 min. Finally, the sample of carbon nanotube hairballs (CNTHs) was obtained by spray drying of the ultrasonically treated suspension. Synthesis of SnO2/CNTH and SnO2/CNT Composites and Bare SnO2. The SnO2/CNTH composite was prepared as follows. First, 5.64 g of SnCl2·2H2O (Sigma– Aldrich) was dissolved in 25 mL ethanol, followed with adding 1.00 g CNTHs so that they are just submerged in the solution. After magnetic stirring for 60 min, the resulting suspension was transferred into an autoclave, heated to 150 °C and kept at this temperature for 10 h in a drying box. Thereafter, the resulting product was washed with deionized water, dried and then heat-treated in a muffle furnace at 360 °C in air atmosphere for 10 min to further oxidize the possibly available SnO to SnO2. After 5

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cooling down naturally to ambient temperature, the SnO2/CNTH composite was obtained. The preparation procedure of the composite is shown in Scheme 1, while the procedure for preparation of SnO2/CNT composite is similar, except for changing CNTH to CNT. The SnO2 contents for the SnO2/CNTH and SnO2/CNT composites were 57.6 wt% and 52.2 wt%, respectively, which were determined based on the thermogravimetric analysis results (Figure S1, Supporting Information (SI)). The bare SnO2 sample was prepared under the similar conditions without adding either CNTH or CNT.

Scheme 1. Schematic diagram illustrating the procedure for preparation of SnO2/CNTH composite.

RESULTS AND DISCUSSION Structural and Morphological Analysis. XRD patterns of the SnO2, CNTH, SnO2/CNT and SnO2/CNTH composites are shown in Figure 1. The CNTH shows an intensive peak at 2θ = 26.2°, which is the characteristic (002) peak of graphitized carbon. For the bare SnO2, the clearly observed diffraction peaks at 2θ = 26.6°, 33.9° and 51.8° correspond to the (110), (101) and (211) reflections, respectively, of the 6

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standard SnO2 (PDF 721147, space group P42/mnm). For both the SnO2/CNT and SnO2/CNTH samples, the XRD characteristics are essentially similar to those of the bare SnO2. As can be seen, the (110) peak for both of the SnO2/CNT and SnO2/CNTH becomes intensive and moves slightly to smaller 2θ angles compared to that of the bare SnO2 due to the overlap between the (110) peak of SnO2 and the (002) peak of CNTH. Based on these XRD results, it can be demonstrated that the bare SnO2, SnO2/CNT and SnO2/CNTH composites are successfully synthesized by solvothermal method.

Figure 1. XRD patterns of CNTH, SnO2, SnO2/CNT and SnO2/CNTH composites.

Figure 2 shows the SEM images of the SnO2, CNTH, SnO2/CNT and SnO2/CNTH composites. As shown in Figure 2a, the SnO2 particles are aggregated together in micron-scale compact clumps. It can be observed in Figure 2b that the CNTHs are hairball-like aggregates of 510 m in size gathered from CNTs with rich mesopores around 30 nm in diameter (Figure S2). The SnO2/CNT composite exhibits the morphology of irregular aggregates with larger sizes (Figure 2c). Whereas, the 7

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SnO2/CNTH shows the morphology of hairball-like aggregates with relatively smaller sizes analogous to the CNTHs (Figure 2d). The hairball-like morphology with the hierarchic structure may lead to a high packing density.20 The Sn, O and C elements in the SnO2/CNTH composite are shown to be homogeneously distributed in the composite particles by the EDS mappings (Figure 3). By contrast, for the SnO2/CNT composite, the C element is unevenly distributed in the relatively larger SnO2/CNT particle (Figure S3), suggesting an uneven distribution of CNTs in the SnO2/CNT composite.

Figure 2. SEM images of (a) SnO2, (b) CNTH, (c) SnO2/CNT and (d) SnO2/ CNTH composites.

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Figure 3. (a) SEM image of SnO2/CNTH and corresponding EDS mapping of (b) Sn, (c) O and (d) C elements.

As depicted in Figure 4, the SnO2/CNTH and SnO2/CNT composites were also probed by high resolution transmission electron microscopy (HRTEM). It is evident from Figure 4a that the SnO2 nanoparticles are well dotted on cross-linked CNTs surface. The nano-dispersed SnO2 particles with the hierarchic structure would provide more electrochemical reaction sites and disperse the strain generated during the electrochemical reaction. Moreover, this structure can facilitate electrolyte diffusion and penetration through the electrode, and maintain good contact between the SnO2 and the conductive CNTs, which will do favor to the rapid transport of electrons and lithium ions.3 The diffraction rings are observed in the SAED pattern of SnO2/CNTH (Inset of Figure 4a), revealing the polycrystalline nature of the SnO2 particles. As 9

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shown in Figure 4b, the SnO2 crystallites (nanograins) are interspersed uniformly on the surface of cross-linked CNTs and interconnected each other. The lattice fringe spacing of the crystallites (0.33 and 0.26 nm) matches the (110) and (101) planes of SnO2 (Figure 1). Apparently, the SnO2 crystallites are randomly oriented, and so, the polycrystalline rings are observed in the SAED pattern. Figure 4c and 4d are the HRTEM images of the SnO2/CNT, which are essentially similar to those of the SnO2/CNTH, but with a major difference. As can be seen, the SnO2 nanograins (crystallites) are interspersed unevenly on the surface of CNTs, which is different from the case of SnO2/CNTH. This may be related to the “cage effect” (or confinement effect) of the hairball-like CNTHs during the solvothermal reaction, wherein the Sn2+ ions may be captured and locally range-limited in the empty space of CNTHs, leading to the formation of uniformly interspersed SnO2 nanograins on the surface of cross-linked CNTs. Figure S4 also presents the HRTEM images of the bare SnO2 sample with different magnitudes, showing that the sample is constructed from aggregates of SnO2 nanoparticles with good crystallinity.

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Figure 4. HRTEM images of (a, b) SnO2/CNTH and (c, d) SnO2/CNT. Insets of Figure 4a and 4c are corresponding SAED patterns showing the polycrystalline nature of aggregates of SnO2 nanoparticles.

As shown in Figure 5a, the nitrogen absorption of the bare SnO2 sample could be rarely seen when the relative pressure is less than 0.95, and there is just a slight increase in its quantity when relative pressure is close to 1.0. In contrast, the adsorption quantities for the CNTH, SnO2/CNT and SnO2/CNTH samples rise gradually as the relative pressure increases, which are considerably larger than that of the bare SnO2 sample. Moreover, there are obvious hysteresis loops between 0.8 and 1.0, especially for the CNTH sample. The specific surface areas (SSA) of the SnO2, CNTH, SnO2/CNT and SnO2/CNTH were measured to be 22.37, 252.92, 197.68 and 176.01 m2 g−1, respectively. This low SSA of the bare SnO2 should be related to its 11

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morphology of compact aggregates (Figure 2a). As shown in Figure 5b, the CNTH, SnO2/CNT and SnO2/CNTH materials are essentially mesoporous with a bimodal pore size distribution, but are dominated by the pores of ca. 10−80 nm in diameter.

Figure 5. (a) Nitrogen adsorption−desorption isotherms and (b) pore-size distributions of SnO2, CNTH, SnO2/CNT and SnO2/ CNTH. Inset of Figure 5b shows the pore-size distributions of the materials in the pore diameter range of 2−7 nm.

Electrochemical Performance. The cyclic voltammograms (CVs) for the first three cycles of the SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes are displayed in Figure 6a−d, respectively. For the SnO2 electrode (Figure 6a), the obscure peak around 1.2 V and the strong peak at ca. 0.75 V in the first cathodic scan correspond to the conversion reaction of SnO2 with Li+ ions (formation of Sn and Li2O), and the electrolyte decomposition (formation of a SEI), as described in Eq. (1) and (2).6 Li+ + e + electrolyte → SEI

(1)

SnO2 + 4Li+ + 4e → Sn + 2Li2O

(2)

Sn + xLi+ + xe ↔ LixSn

(3)

(0 ≤ x ≤ 4.4)

SnO + 2Li+ + 2e ↔ Sn + Li2O

(4) 12

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C + xLi+ + xe ↔ LixC

(5)

SnO2 + 2Li+ + 2e ↔ SnO + 2Li2O

(6)

The sharp reduction peak observed between 00.3 V results from the alloying reaction of Sn with Li (formation of LixSn) as explained in Eq. (3),6,10,21,22 which is often thought to be reversible and contributes to a high theoretical capacity of 782 mAh g1 (based on Li4.4Sn).23 In the first anodic scan process, the sharp peak at ca. 0.74 V is ascribed to the de-alloying reaction (Eq. (3), reverse reaction),6, 21−23 while the broad peak around 1.3 V is often observed and is commonly attributed to Sn partial oxidation into SnO2.10,2123 In our opinion, it should be attributed to Sn oxidation (or partial oxidation) into SnO (Eq. (4), reverse reaction). From the second cycle on, the cathodic peak observed at ca. 0.75 V in the first scan is positively moved to ca. 1.0 V, which should be assigned to reduction of SnO (Eq. (4)). Besides, the intensity of the anodic peak at ca. 0.74 V representing de-alloying reaction increases gradually, indicating an activation process upon the CV cycling. For the CNTH electrode (Figure 6b), a strong peak appears at ca. 0.5 V in the first cathodic scan, but vanishes during the following cycles. This peak results from the decomposition of electrolyte and the generation of SEI.6 The sharp peak approaching 0 V and the corresponding anodic current response should be owed to the reversible lithium insertion/deinsertion reaction of graphitized carbon material as described in Eq. (5).6

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Figure 6. CVs for the first three cycles of (a) SnO2, (b) CNTH, (c) SnO2/CNT and (d) SnO2/CNTH at a scan rate of 0.5 mV s−1.

For the SnO2/CNT electrode (Figure 6c), the cathodic peak around 1.7 V for the first scan can be assigned to the reduction of SnO2 to SnO (Eq. (6)), and the sharp peak at 0.75 V appears when SnO further reduces into Sn (Eq. (4)) along with electrolyte decomposition (Eq. (1)). By contrast, the sharp peak approaching 0 V should be attributed to the alloying reaction (Eq. (3)) coupled with lithium intercalation into carbon (Eq. (5)).22,2426 The anodic peak at 0.6 V at the first scan corresponds to the de-alloying of LixSn, and the broad peak around 1.3 V is commonly ascribed to Sn partial oxidation into SnO2,10,2124,26 as stated before. In addition, the rarely observed oxidation peak at a higher potential of ca. 2.5 V is possibly a result of the 14

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decomposition and oxidization of the previously formed SEI or extraction of Li+ from LixC.21,2426 In fact, as can be seen clearly, in the following cycles, there are three anodic peaks and four cathodic peaks, which is very similar to the reported CV of the 3D dual-doped porous carbon/SnO2 quantum dots (3−5 nm) composite ([email protected]).21 Here, we think that the three pairs of reduction/oxidation peaks located at ca. 1.5/2.5, 0.7/1.3 and 0/0.6 V may be attributed to the reversible (or partial reversible) reactions described in Eq. (6), Eq. (4) and Eq. (3). In order to know the oxidation states of the bare SnO2, SnO2/CNT and SnO2/CNTH electrodes upon charging to a high potential, the SnO2, SnO2/CNT and SnO2/CNTH electrodes after the first charging to 3 V were separated from the coin-type cells and analyzed by XPS. Binding energies of Sn3d5/2 and Sn3d3/2 excitations of the charged SnO2 electrode are measured to be ca. 485.2 and 493.5 eV, while those of the charged SnO2/CNT and SnO2/CNTH electrodes are ca. 485.9 and 494.3 eV, respectively (Figure S5), suggesting a higher Sn oxidation state of the SnO2/CNT and SnO2/CNTH active materials upon charging to the high potential of 3 V. The wide but unconspicuous cathodic peak around 2.3 V may be resulted from the partial reduction of SnO2 to an intermediate tin oxide (with Sn valence between 4 and 2). Yet, further research is needed to elucidate the detailed mechanisms of the electrochemical reaction. The CV characteristics of SnO2/CNTH (Figure 6d) are essentially similar to those of the SnO2/CNT, which are quite different from those of the bare SnO2 (Figure 6a). We think that the conductive CNT network along with the nanoscale dispersion of SnO2 grains on the surface of CNTs could increase conductivity of the composite and 15

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hence significantly enhance the activity of the SnO2, resulting in deep oxidation of the active material during recharging process. This extended redox reaction taken place at higher potentials will lead to much higher specific capacities. The CVs of the 2nd and 3rd cycles of the SnO2/CNT and SnO2/CNTH are almost identical, especially for the SnO2/CNTH, indicating a good reversibility of the composites. Nevertheless, there are still some differences between the two composites. The anodic peak intensity of SnO2/CNTH at the higher potential of 2.5 V is weaker than that of the SnO2/CNT, but the intensities of the lower-potential redox peaks at 0 and 0.6 V of the SnO2/CNTH are obviously stronger than those of the SnO2/CNT. This result suggests that the alloying/de-alloying reaction of the SnO2/CNTH is more active than that of the SnO2/CNT. Galvanostatic discharge/charge curves for the first three cycles of SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes at 0.1 A g1 are displayed in Figure 7a–d, with the corresponding specific charge capacities listed in Table 1 respectively. The characteristics of discharge and charge curves for the first three cycles of the four electrodes are essentially consistent with the corresponding CV features (Figure 6a– d). For each electrode, the coulombic efficiency increases with the increasing cycle number, especially for the change of coulombic efficiency from the first cycle to the second cycle. The first-cycle coulombic efficiencies follow the order of CNTH  SnO2/CNT  SnO2/CNTH  SnO2. The lower first-cycle coulombic efficiency of CNTH electrode is related to the significant electrolyte decomposition and SEI generation (Figure 6b). For this reason, the first-cycle coulombic efficiency of both 16

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SnO2/CNT and SnO2/CNTH are lower than that of the SnO2. The maximum reversible specific capacities of the SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes are 442.9, 351.0, 975.0 and 1109.5 mAh g1, respectively. In other words, the SnO2/CNT and SnO2/CNTH composite electrodes exhibit much higher specific capacities over the bare SnO2 and CNTH electrodes as a result of the effective combination of SnO2 with CNTs. Furthermore, the SnO2/CNTH electrode shows higher specific capacity than the SnO2/CNT electrode and the best cycling stability among the four electrodes, as can be seen from Figure 7 and Table 1.

Figure 7. Galvanostatic discharge/charge curves for the first three cycles of (a) SnO2, (b) CNTH, (c) SnO2/CNT and (d) SnO2/CNTH at the current density of 0.1 A g1.

Table 1. Specific charge capacities (in mAh g1) of SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes for the 1st, 2nd and 3rd cycles and corresponding coulombic 17

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efficiencies. Sample

1st cycle

2nd cycle

3rd cycle

SnO2

414.8/54.0%

442.9/87.3%

432.9/93.2%

CNTH

351.0/25.1%

319.2/75.3%

302.2/81.0%

SnO2/CNT

944.9/45.2%

975.0/87.0%

951.4/90.9%

SnO2/CNTH

1098.3/48.7%

1106.2/85.5%

1109.5/88.8%

By contrast with the SnO2/CNT electrode, the higher specific capacity and better cycling stability of the SnO2/CNTH electrode could be related to its higher content of SnO2 as well as its special 3D hierarchical structure with well dispersed nano-SnO2 grains. The theoretical capacity of SnO2 calculated based on the conversion reactions (Eq. (4) and (6)) is 711 mAh g1, and that based on the alloying/de-alloying reaction (Eq. (3), Li4.4Sn) is 782 mAh g1. It follows that the theoretical capacity of SnO2 should be 1493 mAh g1. The SnO2/CNT composite contains 52.2 wt% SnO2 and 47.8 wt% CNTs (Figure S1). Thus, its theoretical capacity should be 1493×0.522 + 372×0.478 = 957.2 mAh g1 (assuming that the theoretical capacity of CNTs is 372 mAh g1), and the theoretical capacity of SnO2/CNTH composite should be 1493×0.576 + 372×0.424 = 1017.7 mAh g1, close to the experimental capacities presented in Table 1. However, the actual capacities of the two electrodes are somewhat higher than the corresponding theoretical capacities. This phenomenon is commonly found for many conversion-type anode materials. The detailed cause is not very clear up to now. The rate performances of the SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes 18

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are depicted in Figure 8. The maximum charge specific capacities of SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes at 0.1 A g1 are 442.9, 351.0, 975.0 and 1107.9 mAh g1, respectively; while the current density is raised to 2 A g1, the maximum delivered specific capacities decrease to 9.4, 110.5, 376.3 and 512.7 mAh g1, respectively, giving capacity retentions of 2.1%, 31.5%, 38.6 and 46.3%, respectively. Thus, the bare SnO2 electrode exhibits a poor cycling stability, and its low initial specific capacity and cycling stability should be related to its poor conductivity. The relatively lower capacity retention of the CNTH electrode is mainly related to its fast drop of capacity during the initial cycling at 0.1 A g1 (Figure 8). The SnO2/CNT electrode also shows rapid capacity decline when cycling at 0.1 and 0.2 A g1. Nevertheless, the SnO2/CNTH electrode exhibits a high cycling stability, especially for cycling at  0.5 A g1. Besides, when the current density is decreased back to 0.1 A g1, the reversible capacity of SnO2/CNTH electrode is recovered to a high value of 1062.5 mAh g1. Overall, the SnO2/CNTH electrode shows the best rate performance and cycling performance.

Figure 8. Rate performances of SnO2, CNTH, SnO2/CNT and SnO2/ CNTH. 19

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After the rate performance tests, the SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes are subjected to electrochemical impedance spectroscopy (EIS) measurements conducted under charged state (charged to 3 V) within the range of 100 kHz − 3 mHz. The results are displayed in Figure 9 (the insets show the close-up view of the EISs in the high-frequency regions). Judging from the real parts of the Nyquist plots, the real part of impedance for the SnO2 is very large and that for the CNTH is small, while that for the SnO2/CNT or SnO2/CNTH is between the two. It can be seen from Figure 9 and the insets that the ohmic resistances (indicated by the highfrequency intercepts on the real axis) of the four electrodes are ca. 0.032, 0.005, 0.007 and 0.006 ohm g, respectively. Namely, the SnO2 electrode has a much larger ohmic resistance, and the ohmic resistances of both SnO2/CNT and SnO2/CNTH are slightly larger than that of the CNTH, suggesting a significantly improved conductivity of the composite materials. The small ohmic resistances of SnO2/CNT and SnO2/CNTH result from the enhancement of electric conductivity due to the combination of CNTs.27 The electrical conductivities of SnO2/CNT and SnO2/CNTH materials were also measured by four-point probe system and proved to be 2.500 and 3.333 S cm−1, respectively, which further verifies the above standpoint. A high frequency semicircle, an intermediate frequency semicircle and a low frequency sloping line can be clearly seen in the EIS of the SnO2 electrode, whilst the EISs of other electrodes consist of partially overlapped semicircles and a line. These three features in the EIS are generally considered to be related to the migration of Li+ 20

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ions, the charge transfer resistance at the interface and Li+ ion diffusion in the bulk electrodes.10,17,25,28 According to the size of the high- and middle-frequency arcs, both the SEI-related resistance and the charge transfer resistance of SnO2 electrode are significantly higher than those of the other electrodes, especially the charge transfer one. Since the electrode kinetics of SnO2 electrode is mainly controlled by charge transfer process, the result is in consistent with the CV result and can be explained as follows. The EISs were measured at open circuit potentials under charged state. As shown in Figure 6a, almost no electrochemical reaction takes place at the higher potential range for the SnO2 electrode, which is quite different from the case for SnO2/CNT and SnO2/CNTH electrodes. Therefore, the charge transfer resistance for either SnO2/CNT electrode or SnO2/CNTH electrode is far lower than that for SnO2 electrode. Nevertheless, the charge transfer resistance for SnO2/CNTH is slightly higher than that for SnO2/CNT owing to the slightly smaller peak current response of the former at higher potential range (Figure 6c and 6d). However, the overall impedance of the former is slightly lower than that of the later. These EIS results should be responsible for the rate performance of the electrodes observed in Figure 8.

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Figure 9. EISs of SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes measured under charged state (charged to 3 V) in the frequency range of 100 kHz − 3 mHz after the rate performance tests. Insets show the close-up view of EISs in the high frequency region.

Figure 10 depicts the cycling performances of the SnO2, CNTH, SnO2/CNT and SnO2/CNTH electrodes cycled at 0.2 A g–1 between 0.01–3 V (vs. Li+/Li). The SnO2/CNTH electrode shows the largest maximum charge capacity with the value of 1090.6 mAh g1 achieved in the 3rd cycle, compared to those of SnO2 (818.5 mAh g1 in the 6th cycle), CNTH (339.3 mAh g1 in the 1st cycle) and SnO2/CNT (726.1 mAh g1 in the 2nd cycle). Moreover, a capacity of 809.2 mAh g1 can still be retained in the SnO2/CNTH electrode after 100 cycles, giving capacity retention of 74.2%, which is also the highest among those of the four electrodes examined (SnO2: 244.8 mAh g1 and 29.9%; CNTH: 230–240 mAh g1 and 69.3%; SnO2/CNT: 488.5 mAh g1 and 22

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67.3%). Therefore, the SnO2/CNTH composite exhibits not only the largest specific capacity but also the most excellent cycling performance among the three investigated SnO2-based materials cycled at 0.2 A g1. Both the maximum recharge capacity and cycling stability follow the order of SnO2/CNTH  SnO2/CNT  SnO2.

Figure 10. Cycling performances of SnO2, CNTH, SnO2/CNT and SnO2/CNTH at a current density of 0.2 A g1.

After the cycling performance tests, the cycled SnO2, SnO2/CNT and SnO2/CNTH electrodes were subjected to EIS and CV measurements and HRTEM observation, and the results are shown in Figure S68. It can be found by comparing Figure S6 with Figure 9 that the ohmic resistance of the cycled SnO2 electrode is obviously increased. By contrast, that of the cycled SnO2/CNT or SnO2/CNTH electrode changes only a little. By comparing Figure S7 with Figure 6, we can find that both of the redox current peak intensity and enclosed area of all the electrodes obviously decrease after 100 cycles, especially for the SnO2 electrode. Nevertheless, the CV features in the high potential range of the cycled SnO2/CNT and SnO2/CNTH electrodes are still visible, 23

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especially for the cycled SnO2/CNTH electrode. What’s more, the morphology of the cycled SnO2/CNTH and SnO2/CNT has hardly changed compared with the asprepared materials (Figure S8), and the active nanograins are still adhered well to the conductive CNTs, especially for the cycled SnO2/CNTH composite (Figure 4). Whereas, the active material of the cycled bare SnO2 electrode shows the morphology of severe aggregates compared with the as-prepared SnO2 material (Figure S4). The high Li-storage capacity of SnO2/CNTH composite in this work is primarily related to the 3D hierarchical heterostructure of nanoscaled SnO2 well dispersed on the surface of CNTs that construct the 3D framework of CNTH, as verified by the TEM image. The interconnected CNTs in the framework can act as the mini current collectors,29 while the rich mesopores in the composite can offer easy accessibility of lithium ions, resulting in improved accommodation behavior for lithium and improved kinetics of the electrode.30 This structure shortens the electronic and ionic transport lengths, facilitates fast electron and ion transport, and realizes high reactivity of the SnO2 active material,19 thus promoting the oxidation reaction upon charging the electrode to a higher potential. On the other hand, an elastic matrix is provided by such a 3D hierarchical structure which helps to tolerate the reversible volume change, disperse the internal stress, and suppress the aggregation of SnO2 nanoparticles during the lithiation/delithiation process, hence leading to improved discharge/charge cyclability.31,32 CONCLUSIONS In conclusion, a SnO2/CNTH composite was successfully fabricated by solvothermal 24

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method with SnCl22H2O and CNTs applied as starting materials. SEM and TEM experiments confirmed that the composite material has a 3D hierarchical heterostructure with nano-SnO2 particles uniformly loaded on the surface of crosslinked CNTs in the CNTHs. Electrochemical results demonstrated that the SnO2/CNTH composite can exhibit much superior Li-storage performance over the bare SnO2 and also superior to the SnO2/CNT composite regarding to reversible specific capacity, rate performance and cycling performance. A high charge specific capacity of 1109.5 mAh g−1 is achieved at 0.1 A g−1. A capacity of 512.7 mAh g1 is delivered as the current density is increased to 2 A g−1. When it is repeatedly cycled at 0.2 A g−1, a maximum charge capacity of 1090.6 mAh g1 is achieved in the 3rd cycle. After 100 cycles, a capacity of 809.2 mAh g1 is still retained, giving capacity retention of 74.2%. The high capacity is mainly due to the enhancement of composite conductivity thanks to the cross-linked conductive CNTs available in the form of CNTHs, which can promote the electrode reaction upon charging the electrode to higher potentials. Meanwhile, the particular 3D hierarchical heterostructure of the composite should be responsible for the improved cycling stability.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssusche-meng.xxxxxxx.

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Description of instruments and methods for physical characterizations and electrochemical measurements, TG, N2 adsorption analysis, SEM, EDS, HRTEM, XPS, EIS and CV of some samples (PDF) AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] *E-mail address: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21773290, 21625304, 21473242, 21733012) and the ‘‘Strategic Priority Research Program’’ of the Chinese Academy of Sciences (Grant No. XDA09010600 and XDA09010303).

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Table of Contents/Abstract Graphic

Synopsis Facile spray drying and solvothermal method were employed to construct nanoSnO2/multi-walled carbon nanotube hairball composite with high capacity for Li-ion battery anodes.

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