Octahedral Tin Dioxide Nanocrystals Anchored on Vertically Aligned Carbon Aerogels as High Capacity Anode Materials for Lithium-Ion Batteries

A novel binder-free graphene - carbon nanotubes - SnO2 (GCNT-SnO2) aerogel with vertically aligned pores was prepared via a simple and efficient directional freezing method. SnO2 octahedrons exposed of {221} high energy facets were uniformly distributed and tightly anchored on multidimensional graphene/carbon nanotube (GCNT) composites. Vertically aligned pores can effectively prevent the emersion of “closed” pores which cannot load the active SnO2 nanoparticles, further ensure quick immersion of electrolyte throughout the aerogel, and can largely shorten the transport distance between lithium ions and active sites of SnO2. Especially, excellent electrical conductivity of GCNT-SnO2 aerogel was achieved as a result of good interconnected networks of graphene and CNTs. Furthermore, meso- and macroporous structures with large surface area created by the vertically aligned pores can provide great benefit to the favorable transport kinetics for both lithium ion and electrons and afford sufficient space for volume expansion of SnO2. Due to the well-designed architecture of GCNT-SnO2 aerogel, a high specific capacity of 1190 mAh/g with good long-term cycling stability up to 1000 times was achieved. This work provides a promising strategy for preparing free-standing and binder-free active electrode materials with high performance for lithium ion batteries and other energy storage devices.

Scientific RepoRts | 6:31496 | DOI: 10.1038/srep31496 However, SnO 2 octahedral nanocrystals encounter similar disadvantages of poor recyclability with the common SnO 2 nanomaterials due to their drastic volume expansion/shrinkage during the alloying reaction with lithium ions 19,20 . This phenomenon is believed to be the result of the pulverization of active materials, which can further block the electrical contact pathways between adjacent particles and fatigue failure and disintegration of the active SnO 2 -based electrode materials 21,22 . Hybridizing SnO 2 nanomaterials with carbon nanomaterials, especially carbon nanotubes (CNTs) and graphene sheets, and nanostructured engineering of SnO 2 with various morphologies seem to be an effective way to improve their electrochemical performance 20,23 . Particularly, many groups have developed graphene-SnO 2 electrodes for LIBs with promising electrochemical performance due to the excellent electronic conductivity and superior mechanical flexibility of graphene sheets 24,25 . However, hybridizing SnO 2 with graphene or CNTs by a simple mixing method cannot realize uniform distribution of SnO 2 nanoparticles on the carbonic matrix due to their high surface energy, and cannot afford efficient space for volume expansion of SnO 2 . To overcome these problems, fabricating sandwich-structured graphene-SnO 2 nanomaterials with good porous structures and excellent dispersion of SnO 2 nanoparticles seems to be an effective method to exploit the superior performance of SnO 2 , because good distribution of SnO 2 nanoparticles can achieve the full utilization of their active sites, and the porous graphene matrix can accelerate the electron transport, as well as provide sufficient expansion volume for the lithiation of SnO 2 . Thus, developing a versatile method for preparing three dimensional (3D) porous SnO 2 -based active materials by hybridizing unique SnO 2 octahedrons with excellent conductive matrix with excellent distribution is of great importance for the promotion of active electrode materials based on metal oxide or other highly active materials for energy storage applications.
In addition, binder and additional carbon fillers were widely used for preparing active electrodes for LIBs, with the objective of pasting the active materials on the collectors and accelerating the transport kinetics of electrons inside the electrodes 26,27 . However, binder materials (e.g. polyvinylidene fluoride, PVDF) have not any ability for storing lithium ion, but damage the conductive networks of the active materials. Additional carbon fillers with barely any lithium ion storage ability were mass employed (10-20 wt%), resulting in the decreasing of the energy density and specific capacity of assembled LIBs. Therefore, developing high active electrodes without any utilization of binder and additional carbon fillers is of great importance for the solid progress of LIB scientific research systems.
In this work, we report a simple and effective strategy to fabricate 3D giant graphene sheets-carbon nanotubes (GCNT)-SnO 2 octahedrons (GCNT-SnO 2 ) aerogels, in which octahedral SnO 2 nanoparticles exposed of high-energy {221} facets were tightly anchored on the surface of GCNT. Importantly, there are vertically aligned pores inside the GCNT-SnO 2 aerogels which can efficiently prevent the emersion of "closed" pores, but can provide sufficient expansion space for SnO 2 octahedrons during the long-term cycling. Meanwhile, the high-energy facets of SnO 2 can be fully exposed to the lithium ions due to their perfect interfacial distribution on GCNT matrix. Benefiting from the good immersion of electrolyte, superior electrical conductivity of GCNT matrix, full utilization of SnO 2 octahedrons, and the synergistic effect of GCNT and SnO 2 , the resultant GCNT-SnO 2 aerogels achieve a rapid insertion/extraction of lithium ions (as illustrated in Fig. 1), and exhibit an ultrahigh specific capacity (up to 1190 mAh/g), excellent rate capability, as well as highly reversible capacity (80% retention after 1000 cycles), demonstrating their great potential prospects as electrode materials for LIBs.

Results and Discussion
The preparation process for GCNT-SnO 2 aerogel with aligned pores is schematically illustrated in Figure S1. Herein, pristine CNTs with bundle morphology can be homogeneously dispersed by graphene oxide sheets under strong sonication, according to the intense interfacial interactions including van der Waals and π -π stacking 28 . The prepared SnO 2 octahedrons can be uniformly dispersed on the surface of CNT/graphene oxide composite with the assistance of 2-[2-(2-Methoxyethoxy)ethoxy]acetic acid (MEEAA). Low weight percent of poly(amic acid) (PAA) (0.5 wt%) was introduced in the CNT/graphene oxide/SnO 2 composite solution in order to induce their aligned arrangement during directional freezing process. The vertically aligned pores were produced by the vertically aligned ice pillars formed in the directional freezing process (Fig. 2), following by the treatment of freeze-drying and high temperature pyrolysis.
Detailed structural information of SnO 2 octahedrons is provided by the transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED), as seen in Fig. 3. TEM image of several SnO 2 octahedrons ( Fig. 3a) with random configuration indicates the uniform size of prepared octahedrons. Figure 3b shows the TEM image of single octahedron projected along the {110} direction with corresponding SAED pattern (inset). The single-crystalline characteristics of SnO 2 octahedrons can be indexed by the {110} zone axis in the SAED pattern 29 . Schematic model of octahedron (Fig. 3c) enclosed by {221} facets exhibits the same apex angle of 65.7 o as that of SnO 2 particle in Fig. 3b. The same SnO 2 particle was rotated to the {111} zone axis (Fig. 3d), and both the outline and the apex angle of the particle still corresponded well with the octahedral model enclosed by {221} facets (Fig. 3e). High-resolution TEM (HRTEM) image taken from the top apex of SnO 2 octahedron exhibits lattice fringes of 0.333 and 0.315 nm, corresponding to the {110} and {001} planes of SnO 2 octahedron. Based on these TEM observations and structural analysis, it can be concluded that the as-prepared SnO 2 particles are exposed with the {221} high-energy facets with uniform size.
The morphology of SnO 2 octahedrons and G/CNT-SnO 2 aerogels were characterized by scanning electron microscopy (SEM), as seen in Fig. 4. Figure 4a shows that the SnO 2 octahedrons consist of high-purity particles with smooth surfaces and edges, and the inset image confirms the well-defined octahedron-shaped morphology of the obtained SnO 2 particles. Figure 4b,c present the GCNT-SnO 2 aerogel with vertically aligned pores at low and high magnifications. These aligned pores can effectively connect the holes inside GCNT-SnO 2 aerogel, and further prevent the emergence of "closed" pores. The vertically aligned pores can ensure the thorough immersion of electrolyte but can also expose all their porous structures and active sites to lithium ions contained in electrolyte. Inset in Fig. 4b exhibits the optical image of GCNT-SnO 2 aerogel with free-standing architecture. Figure 4d shows the composite of CNTs and graphene sheets. It can be seen that the CNTs are thoroughly dispersed and tightly bonded on the surface of graphene sheets. Interestingly, the introduced CNTs on the surface of graphene sheets can act as skeletons between different graphene sheets to greatly decrease their tightly interfacial stacking. Furthermore, CNTs up to several micrometers in length can bridge different graphene sheets as a connecting conductive pathway. SEM image of GCNT-SnO 2 (1) aerogel was presented in Fig. 4e, and the SnO 2 particles were homogenously dispersed on the surface of GCNT composite without any aggregation. The good dispersion of SnO 2 octahedrons and their perfect interfacial contacting with GCNT, coupling with the good permeability of vertically aligned pores, can achieve excellent synergistic effect in lithium ion storage application. Figure 4f-h present the SEM images of GCNT-SnO 2 (2), GCNT-SnO 2 (3), and GCNT-SnO 2 (4) aerogels, respectively, which were prepared by increasing the amount of SnO 2 octahedrons by 2, 3, and 4 times in the resulted GCNT-SnO 2 aerogels. Interestingly, the good interfacial distribution of SnO 2 octahedrons on GCNT surface was not affected by their increased content, even up to four times weight of GCNT, as seen in the enlarged picture of GCNT-SnO 2 (4) aerogel at high magnification ( Fig. 4i). TEM images were used to further analyze the morphology of GCNT-SnO 2 aerogels ( Figure S2), and no aggregation was observed in both GCNT-SnO 2 (1) and GCNT-SnO 2 (3) samples. Especially, several SnO 2 octahedrons with ambiguous edges or frames due to the coverage effect of graphene sheets confirm that the SnO 2 particles were deposited on both sides of GCNT composite sheets. The good dispersion of SnO 2 on GCNT can be further confirmed by the Energy disperse spectroscopy (EDS) mapping detection, as seen in Figure S3. Sn ( Figure S3c) and O ( Figure S3d) elements can be distinguished as observed on the carbon layer ( Figure S3b), agreeing well with the SEM image of GCNT-SnO 2 (3) aerogel ( Figure S3a). The weight percent of SnO 2 in GCNT-SnO 2 (1), GCNT-SnO 2 (2), GCNT-SnO 2 (3) and GCNT-SnO 2 (4) aerogels are about 43%, 57%, 72% and 80%, respectively, which were tested by thermogravimetric analysis (TGA) ( Figure S4). These results verified the rational and credible design of SnO 2 /GCNT ratio in GCNT-SnO 2 aerogels. The crystalline structures of SnO 2 octahedrons and GCNT-SnO 2 (3) aerogels were investigated by X-ray diffraction (XRD), as seen in Fig. 5a. For pure SnO 2 octahedrons, all the peaks can be readily indexed to the rutile phase SnO 2 (JCPDS no. 41-1445) 30,31 . The XRD pattern of GCNT-SnO 2 (3) aerogel shows similar diffraction peaks with the SnO 2 octahedrons, indicating the crystalline morphology of SnO 2 was not eroded after the introduction of GCNT matrix. The appearance of a broadened peak at 2θ = 26.1° corresponding to the (002) of graphite indicates the existence of graphene and CNTs. The vertically aligned pores inside GCNT-SnO 2 aerogels coupled with the good distribution of SnO 2 particles can positively contribute to the increase of their specific surface area. Figure 5b shows the nitrogen isothermal adsorption/desorption result and the corresponding pore size distribution of GCNT-SnO 2 (3) aerogel. High BET surface area of 344 m 2 /g was observed, which is much larger than 34 m 2 /g of pure SnO 2 octahedrons ( Figure S5). In addition, based on the Barrett-Joyner-Halenda (BJH) model (inset in Fig. 5b), the pore size of GCNT-SnO 2 (3) aerogel is centered at ~4 nm. The greatly enhanced surface area of GCNT-SnO 2 (3) aerogel associated with the meso-and macroporous structures is favorable for the electrolyte accessibility and fast lithium ion diffusion 32,33 . To further confirm the chemical compositions of GCNT-SnO 2 aerogel, X-ray photoelectron spectroscopy (XPS) measurements were performed on GCNT-SnO 2 (3) aerogel in the range of 0 -800 eV, as seen in Fig. 5c. The peaks located at the C, O, and Sn core level regions can be assigned as C 1 s, O 1 s, Sn 3p, Sn 3d, and Sn 4d, respectively. Two peaks centered at 496.9 and 488.0 eV can be attributed to the Sn 3d 3/2 and Sn 3d 5/2 (Fig. 5d) 34,35 , and the barely detected C = O and C-O-C peaks in the C 1 s region ( Figure S6) confirm the good chemical reduction effect of hydrazine vapor. Moreover, sheet resistance of prepared samples was detected based on a four-probe method. As seen in Table S1, GCNT-SnO 2 aerogels exhibit low sheet resistance from 79.4 to 105.9 Ω sq −1 , which is comparable with the ITO and commonly used graphene sheets 36,37 .
The free-standing GCNT-SnO 2 (3) aerogel with film architecture (Fig. 6a) ensures them to be directly used without any binder or additional carbon fillers. Interestingly, GCNT-SnO 2 aerogel can be used in a closed circuit as a substitution of copper wire (Fig. 6b), and the high brightness of green light emitting diodes (LEDs) confirms the good electrical conductivity of the prepared GCNT-SnO 2 aerogels. These results permit the GCNT-SnO 2 aerogels to be utilized as promising candidate as electrode materials in LIBs.
The electrochemical performance of GCNT, GCNT-SnO 2 aerogels and pure SnO 2 octahedrons acting as electrode materials for LIBs was investigated. Figure 7a shows the typical cyclic voltammogram (CV) curves of GCNT-SnO 2 (3) aerogel as electrode materials for LIBs over a voltage range of 0.01~2.5 V vs. Li/Li + . In the first cycle, an irreversible reduction peak with a maximum value at 0.57 V was emerged, which can be attributed to the formation of a solid electrolyte interface (SEI) layer, as well as the reduction of SnO 2 to amorphous lithium oxide and metallic Sn (equation 1) 38 . The cathodic peak closed to 0 V can be attributed to the lithium alloying reaction with Sn (equation 2), and the oxidation peak at 0.6 V for all the following cycles can be ascribed to the corresponding dealloying reaction 39 . The oxidation peaks observed at 1.30 V and 1.84 V are resulted from the partially reversible reactions of formation of SEI layer and SnO 2 40 . In addition, an obvious oxidation peak around 0.14 V in the anodic process represents the lithium extraction from GCNT matrix (equation 3). Compared with the indistinctive oxidation/reduction peaks of GCNT ( Figure S7a), GCNT-SnO 2 aerogels with other mass ratios exhibit apparent oxidation/reduction peaks ( Figure S7b-S7e) as that of SnO 2 octahedrons, confirming the efficient incorporation of SnO 2 with the GCNT matrix. Sample GCNT-SnO 2 (3) aerogel exhibits the largest anodic/cathodic current density compared with the others, indicating its largest specific capacity as a result of the synergistic effect of conductive GCNT matrix and SnO 2 octahedrons exposed of high-energy facets. Interestingly, CV curves of GCNT-SnO 2 (3) aerogel up to 60 cycles ( Figure S7f) exhibit similar oxidation/reduction peaks with nearly undiminished current intensity as before, which demonstrates its good endurance property upon long term cycling.
x Figure 7b shows the charge/discharge curves of GCNT-SnO 2 (3) aerogel on the 1 st , 2 nd , and 5 th cycles at a rate of 0.1 A/g. Voltage plateaus observed on the charge/discharge curves corresponding to the oxidation/reduction peaks in the CV curves can be ascribed to the lithium ion insertion/extraction reactions. Comparatively, sample GCNT does not show any potential plateaus during the charge/discharge process (Figure S8a), as a result of the   Similarly, samples of GCNT-SnO 2 (1), GCNT-SnO 2 (2) and GCNT-SnO 2 (4) aerogels ( Figure S8b-S8d) exhibiting alike lithium ion insertion/extraction voltage plateaus as that of GCNT-SnO 2 (3) aerogel also undergo conspicuous capacity loss during the first charge/discharge cycle. The huge capacity loss of the prepared samples can be ascribed to the irreversibility resulting from SnO 2 reduction and the formation of SEI layer on the surface of active materials 42,43 . Charge/discharge curves from the 6 th to 100 th cycles of GCNT-SnO 2 (3) aerogel were recorded, as seen in Figure S9f. The insignificant decline upon the 100 cycles indeed confirms the good cycling stability of GCNT-SnO 2 (3) aerogel. Specific capacities of the prepared samples calculated from the discharge curves on the 5 th cycle were compared (Fig. 7c). GCNT-SnO 2 (1), GCNT-SnO 2 (2), GCNT-SnO 2 (3), GCNT-SnO 2 (4) aerogels exhibit greatly enhanced specific capacity of 720, 1027, 1190 and 1034 mAh/g, respectively, compared to 402 mAh/g of GCNT and 688 mAh/g of pure SnO 2 octahedrons, due to the synergistic effect of GCNT and SnO 2 for lithium ion storage. Here, the GCNT-SnO 2 (4) aerogel with higher content of SnO 2 shows a little lower specific capacity compared with GCNT-SnO 2 (3), which may be resulted from the excessive loading of SnO 2 octahedrons that the utilization of active sites of SnO 2 was not so effectively as before. The rate capabilities of GCNT-SnO 2 aerogels compared with GCNT and SnO 2 octahedrons were also investigated from current densities from 0.1 to 2 A/g, as seen in Fig. 7d. The GCNT-SnO 2 (3) aerogel displays excellent rate capabilities and delivers rate capacities of 1190, 1095, 974, 875, 735 mAh/g at current densities of 0.1, 0.2, 0.5, 1, and 2 A/g, respectively. Clearly, GCNT-SnO 2 (3) aerogel exhibits much higher capacity compared with GCNT (165 mAh/g) and pure SnO 2 octahedrons (296 mAh/g) at high current density of 2 A/g. It should be noted that, GCNT-SnO 2 (3) aerogel delivers a comparable specific capacity of 1143 mAh/g as before when the current density returns to 0.1 A/g, and also exhibits good cycling performance in the following 35 cycles. In addition, GCNT-SnO 2 (1), GCNT-SnO 2 (2) and GCNT-SnO 2 (4) aerogels also exhibit good rate performances under different current densities, as seen in Figure S9. The superior rate performance of GCNT-SnO 2 aerogels clearly demonstrates that the successful hybridization of SnO 2 octahedrons on GCNT matrix endows them with perfect tolerance to varied discharge current densities, and good prospect in high power LIBs. The excellent rate capabilities of GCNT-SnO 2 aerogels were probably rooted in high energy facets exposed by SnO 2 octahedrons, coupling with the good distribution of SnO 2 on the conductive GCNT matrix, as well as the highly porous structures of GCNT-SnO 2 aerogels. Figure 7e shows the relative cyclic performance of the GCNT-SnO 2 (3) aerogel, GCNT and SnO 2 octahedrons at 0.1 A/g. SnO 2 octahedrons exhibit a high specific capacity of 688 mAh/g, but possess a very poor recyclability with dramatic decrease of the capacity to 175 mAh/g only after 40 cycles. This poor cycling performance of pure SnO 2 octahedrons was caused by the large volume expansion taking place during the rapid lithium ion insertion/ extraction process, which further deteriorates the intimate contact between active SnO 2 particles and the current collector. GCNT material displays an excellent cycling stability upon 100 cycles, but exhibits a low specific capacity of 404 mAh/g, due to lack of sufficient active sites. GCNT-SnO 2 (3) aerogel exhibits good cycling stability with a capacity retention of 83% after 100 cycles, and also possesses a high Coulombic efficiency up to 99% after the first five cycles. GCNT-SnO 2 aerogels with other contents of SnO 2 also exhibit good cycling performance and high Coulombic efficiency under a current density of 0.1 A/g, as seen in Figure S10. The much better cycling stability of GCNT-SnO 2 aerogels compared to the result of pure SnO 2 octahedrons was offered by the synergistic effect from the efficient combination strategy. Particularly, SnO 2 octahedrons with high-energy facets provide sufficient active sites for lithium ions, good interfacial contact between GCNT and SnO 2 particles offers excellent transport of electrons, and the vertically aligned pores inside GCNT-SnO 2 aerogels ensure thorough immersion of electrolyte throughout the electrodes. Moreover, GCNT-SnO 2 aerogels can be directly used as electrode materials without any binder, conductive additives or current collectors, which further contributes to their superior electrochemical performance. In addition, the morphologies of GCNT-SnO 2 (3) sample after the 1 st cycle ( Figure S11) and the 100 th cycles ( Figure 12) were provided. As can be seen, the octahedron morphology of SnO 2 was integrally maintained after the 1 st cycle. After 100 cycles, the SnO 2 nanoparticles are still homogeneously dispersed on the surface of GCNT, but the octahedron morphology of SnO 2 becomes ambiguous ( Figure S12a,S12b), which is consistent with the TEM observation of GCNT-SnO 2 (3) (Figure S12c). HRTEM image of single SnO 2 octahedron separated from the GCNT-SnO 2 (3) aerogel after 100 cycles confirms its good interplanar spacing, which can further confirm the good cycling stability of the GCNT-SnO 2 (3) aerogel.
Electrochemical impedance spectra (EIS) of the prepared samples were recorded in order to deeply understand their different performances as electrodes in LIBs. Figure 7f shows the EIS curves of GCNT, GCNT-SnO 2 (3) aerogel and pure SnO 2 octahedrons. Typically, each of these three EIS curves exhibits a semicircle in the high frequency range and a sloping straight line in the low frequency range. EIS curve of GCNT-SnO 2 (3) aerogel, coupling with the results of GCNT-SnO 2 (1), GCNT-SnO 2 (2) and GCNT-SnO 2 (4) aerogels ( Figure S13), show much smaller radii than that of pure SnO 2 octahedrons. Solution resistance (R s ) and Warburg impedance (Z w ) of these electrodes were also recorded according to the equivalent circuit (inset in Fig. 7f). Resistance values calculated based on the equivalent circuit were listed in Table S2. R ct values of GCNT-SnO 2 aerogels (101.3~119.5 Ω) were greatly decreased compared with the result (264.8 Ω) of pure SnO 2 octahedrons, confirming that the charge transfer resistance of GCNT-SnO 2 aerogel electrode was greatly decreased with the assistance of GCNT conductive matrix. EIS curves of GCNT-SnO 2 (3) aerogel electrode after the 1 st cycle and the 100 th cycle were recorded, as seen in Figure S14. The two impedance spectra exhibit similar semicircle shape in high frequency and straight line in low frequency range, indicating their excellent stability of interfacial transfer of ions and electrons. And the slightly decreased R s and R ct values (inset in Figure S14) of GCNT-SnO 2 (3) aerogel electrode after the 100 th cycle compared with the values after the 1 st cycle can be ascribed to the activation effect of the multiple cycles of charge/discharge. Interestingly, with GCNT-SnO 2 (3) aerogel as working electrode without any binder and conductive additives, the assembled lithium ion battery can be steadily cycled for 1000 cycles at 2 A/g, achieving a promising capacity retention of 80% (Fig. 8a). The assembled batteries with GCNT-SnO 2 (3) aerogel electrodes can be successfully used to light up the LED light up to 420 min (Fig. 8b). Furthermore, due to the well-designed architectural morphologies, especially the vertically aligned pores throughout the aerogels, the prepared GCNT-SnO 2 (3) aerogel acting as anode material for LIBs without any binders or conductive additives exhibits comparable or much higher electrochemical properties with other SnO 2 based active materials 30,25,26,39 , as seen in Fig. 9.

Conclusions
In summary, a novel GCNT-SnO 2 aerogel film with vertically aligned pores was prepared by integrating SnO 2 octahedrons and multidimensional carbon nanomaterials with a directional freezing method. The designed architecture shows excellent electrochemical performance with the largest specific capacity of 1190 mAh/g, as well as long-term cycling stability up to 1000 times. Of more importance, the vertically aligned pores can effectively prevent the emersion of "closed" pores which cannot load the active SnO 2 nanoparticles, further ensure adequate immersion of electrolyte throughout the aerogel, and largely shorten the transport distance between lithium ion and active sites of SnO 2 . Especially, vertically aligned pores inside GCNT-SnO 2 aerogel create meso-and macroporous structures with large surface area and excellent electrical conductivity, achieving great benefit to the favorable transport kinetics for both lithium ion and electrons. Any binder or additional carbon fillers were not employed in the GCNT-SnO 2 aerogel electrode that greatly simplifies the electrode preparation process, and on the other hand, efficiently enhances the energy density and specific capacity of the prepared electrodes. Therefore, this work provides a general and effective approach to prepare active electrodes beyond the SnO 2 materials for lithium ion batteries.  Typically, SnCl 4 ·5H 2 O (2 mmol), HCl (36.5%, 1.2 mL) and poly(vinyl pyrrolidone) (PVP, 0.012 mmol) were sequentially dispersed into ethanol/ultrapure water (12 mL, 1/1 v/v) under intense sonication. The resulting solution was transferred to a Teflon-lined stainless steel autoclave (50 mL) and maintained at 200 °C for 12 h. The obtained products were collected after being washed with ultrapure water and ethanol for several times.
Preparation of GCNT-SnO 2 aerogels. CNTs were purchased from Sigma-Aldrich (30~50 nm, ~10 μ m in length). Graphene oxide (GO) was prepared according to a modified Hummers' method 44 . Pristine CNTs (100 mg) with bundle morphology can be uniformly dispersed by GO solution (150 mL, 2 mg/mL) under sonication 44 . SnO 2 octahedrons (300 mg, 530 mg, 1030 mg and 1600 mg) were dispersed into the GO/CNT suspension with the assistance of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEAA), and the mixed materials were washed by ultrapure water for several times to remove the additional MEEAA. And a little amount (0.5 wt%) of PAA was added into the above hybrid solution with assistance of triethylamine. Then, the prepared composite solution was directionally frozen by dipping into liquid nitrogen under a constant speed of 2 mm/min. Freeze-drying treatment (less than 30 Pa) was utilized to completely remove the ice pillars meanwhile retain the aligned pores inside the product. The obtained bulk hybrid materials were then treated with pyrolysis at 350 °C for 2 h in air and then at 800 °C for 2 h in Ar, resulting in the formation of GCNT-SnO 2 (1), GCNT-SnO 2 (2), GCNT-SnO 2 (3) and GCNT-SnO 2 (4) aerogels. The residual oxygen containing groups introduced on the surface of GO sheets were removed under high temperature pyrolysis. The ultrathin PAA film was conducted with imidization and carbonization treatments under 350 °C and 800 °C, achieving the formation of ultrathin carbonic film on the surface of GCNT-SnO 2 aerogel.
Characterizations. The structures and morphologies of the samples were studied with a field-emission SEM (Hitachi S-4800). EDS was conducted on an Oxford instrument (X-Max 50). XRD patterns were conducted on a Bruker D8 GADDS X-ray diffractometer with Cu Kα radiation. TEM and HRTEM investigations were carried out with a Tecnai G2 F20 microscope (FEI). Nitrogen adsorption/desorption isotherms were measured on an Auto sorb-1 Quantachrome Instruments at 77 K. TGA was conducted in air at a heating rate of 5 °C/min. XPS measurements were carried out on a Thermo ESCALAB 250Xi spectrometer with an Al Kα X-ray source (1486.6 eV), X-ray radiation (15 kV and 10 mA) and hemispherical electron energy analyzer.
Electrochemical measurements. The electrochemical tests were performed in a two electrode system of columnar mold, in which the aerogel can be directly used as cathode electrode without any pressure or extrusion, and pure lithium foils were used as counter and reference electrodes. Here, GCNT-SnO 2 aerogels were used as working electrode without any binder and current collector. 1 M LiPF6 in ethylene carbonate-dimethyl carbonate-diethyl carbonate (1:1:1, weight percent) was taken as electrolyte. Celgard 2400 microporous polypropylene membrane was used as a separator. The LIBs were assembled in an Ar filled glovebox with oxygen and water contents of less than 1 ppm. Cyclic voltammograms were recorded from 0 to 2.5 V on ARBIN electrochemical working station (MSTAT-10 V/10 mA/48Ch) at a scan rate of 0.1 mV/s. Charge/discharge curves, rate performance, and long-term cycling tests were recorded on LAND 2001 A testing systems. Electrochemical impedance measurements were carried out on a Solartron electrochemical interface analysis system (SI 1260, SI 1287). And the Nyquist plots were recorded potentiostatically by applying an AC voltage of 10 mV from 100 KHz to 0.01 Hz.