SnO₂ nanorods encapsulated within a 3D interconnected graphene network architecture as high-performance lithium-ion battery anodes

Abstract

SnO₂ nanorods (NRs) have been demonstrated as one of the potential candidates for high-performance lithium-ion battery anodes due to their unique structural features, high theoretical capacity, natural abundance and low cost of fabrication. However, they still suffer from the problem that their direct exposure to the electrolyte leads to the instability of the SEI layer, causing low coulombic efficiency, high ionic resistance and low electronic conductivity. In this study, SnO₂ NRs were synthesized by a hydrothermal method, and then spatially confined within graphene sheets by a facile freeze-drying process. The as-fabricated architecture exhibits a full encapsulation arrangement with graphene sheets interlaced into an interconnected macroporous network, serving not only as a robust framework with accessible space for the electrolyte but also a physical barrier layer to prevent the SnO₂ NRs from direct exposure to the electrolyte. Moreover, the SnO₂ NRs can function as pillars to prevent the graphene sheets from restacking while preserving the highly robust structure and efficient electron and ion transport channels. Benefiting from the admirable synergistic effect between SnO₂ NRs and graphene, the assembled electrode shows excellent cycle performance (1179.2 mA h g⁻¹ after 400 cycles at 1.0 A g⁻¹) and rate capabilities (624.2 mA h g⁻¹ at 8.0 A g⁻¹; 458.4 mA h g⁻¹ at 16.0 A g⁻¹).

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1. Introduction

To meet the ever-growing demand for portable electronics and large-scale energy storage devices, lithium-ion batteries (LIBs) with high energy density and power density have been the subject of intense research.¹ A variety of emerging anode materials such as tin-based and silicon-based anodes as well as transition metal compounds have attracted enormous attention.^2–6 Among them, SnO₂ is regarded as one of the most promising anode materials for next generation LIBs due to its high theoretical capacity (782 mA h g⁻¹), abundant resources, inexpensive preparation and environmental benignity.^7,8 However, SnO₂ suffers from not only the poor electrical conductivity but also severe pulverization caused by the large volume change during charge/discharge processes.⁹ Consequently, electrical disconnection between active materials and the electrode framework may occur due to cracking; meanwhile the solid electrolyte interface (SEI) layer is repeatedly formed on the fresh surface. The capacity of SnO₂ anodes hence fades rapidly upon cycling.¹⁰

The use of nanostructured SnO₂ is a popular approach to partially solve the above issues; especially one-dimensional SnO₂ nanorods (NRs) present a special geometry that can offer direct channels for efficient electron transport while sharing similar advantages to other nanostructures.¹¹ For instance, Liu et al.¹² synthesized SnO₂ NR arrays on large-area flexible metallic substrates, delivering significantly improved electrochemical performance compared to SnO₂ nanoparticles and yielding a high reversible capacity of 580 mA h g⁻¹ after 100 cycles at 0.1C (1C = 782 mA g⁻¹) and excellent rate capability (350 mA h g⁻¹ at 5C). He et al.¹³ developed a facile strategy for integrating SnO₂ NRs into a polypyrrole film, exhibiting good cycling stability (701 mA h g⁻¹ after 300 cycles at 0.2 A g⁻¹) and high rate capability (512 mA h g⁻¹ at 3.0 A g⁻¹).

Although the above-mentioned SnO₂ NRs have shown improved electrochemical performance, their direct exposure to the electrolyte can lead to the instability of the SEI layer, causing low coulombic efficiency, high ionic resistance and low electronic conductivity.¹⁴ Therefore, capacity decay remains unavoidable when the electrodes are operated at a high rate and over a long-term cycle. In this regard, an electrolyte barrier layer should be engineered in the electrode structure. Recently, graphene has been considered as an ideal electrolyte barrier layer owing to its large surface area, strong adsorption capacity, excellent chemical stability, and good mechanical properties, as well as high electrical conductivity.¹⁵ Sun et al.¹⁶ developed a facile one-step hydrothermal procedure to prepare a SnO₂ NRs/graphene composite. Compared with pure SnO₂ NRs, the composite shows a high reversible specific capacity and improved cycling stability (710 mA h g⁻¹ after 50 cycles at 0.1 A g⁻¹). Zhang et al.¹⁷ reported a controllable synthesis of SnO₂ NRs with a tunable length anchored onto graphene nanosheets, showing high lithium storage capability and good cycling performance. However, simple decoration of the SnO₂ NRs on graphene sheets is not welcomed because the weak combination between SnO₂ and graphene usually causes the detachment of SnO₂ NRs from graphene sheets, resulting in capacity decay.¹⁸

In this study, we designed and fabricated a 3D interconnected graphene network (GN) architecture for encapsulating SnO₂ NRs within graphene sheets by a facile freeze-drying process. The full encapsulation of SnO₂ NRs into graphene can not only prevent them from direct exposure to the electrolyte and detachment from graphene sheets, but also accommodate the large volume change of the SnO₂ NRs during charge/discharge processes and enhance the electronic conductivity of the electrode. Meanwhile, the pillar effect of SnO₂ NRs prevents the graphene sheets from restacking while preserving the highly robust structure and efficient electron/ion transport channels. As a result of the synergistic interplay between the interconnected GN and the well-encapsulated SnO₂ NRs, the composite shows a significant improvement in cycle performance (1179.2 mA h g⁻¹ after 400 cycles at 1.0 A g⁻¹) and rate capabilities (624.2 mA h g⁻¹ at 8.0 A g⁻¹; 458.4 mA h g⁻¹ at 16.0 A g⁻¹).

2. Experimental

2.1. Synthesis of SnO₂ NRs

The SnO₂ NRs were synthesized by a facile hydrothermal method. In brief, 0.6 g of SnCl₄·5H₂O was added to a mixed solvent of ethanol (30 mL) and water (30 mL) with stirring to allow for complete dissolution. Then NaOH was added to control the pH value of the solution around 10. The solution was transferred to a 100 mL polyphenylene (PPL) autoclave, then sealed and heated at 220 °C, 240 °C, and 260 °C for 24 h, respectively. The precipitates were centrifuged, washed thoroughly with ethanol and then dried at 80 °C for 12 h under vacuum. The resultant products were named SnO₂-220, SnO₂-240 and SnO₂-260 according to the reaction temperatures.

2.2. Synthesis of SnO₂ NRs@GN composites

Graphene oxide (GO) was synthesized by a modified Hummers method.¹⁹ The SnO₂ NRs@GN composites were prepared by a freeze-drying method with subsequent calcination. Typically, a predetermined amount of SnO₂-240 was added to a mixed solvent of ethanol (4 mL) and water (6 mL), and ultrasonically treated for 1 h. Then, 17.5 mg of sodium carboxymethyl cellulose and 25 mg of poly(vinyl alcohol) were added to the mixed solvent in sequence. After 10 min of stirring, 50 mL of GO suspension (2 mg mL⁻¹) was poured into the above solution and kept at 50 °C under stirring for 12 h. The suspension was placed in a cylindrical mold and fully frozen by using liquid nitrogen, followed by being taken out from the mold and dried thoroughly at −50 °C. The as-obtained monoliths were calcined at 550 °C in an Ar atmosphere for 2 h. The resultant products were named SnO₂@GN-1, SnO₂@GN-2 and SnO₂@GN-3 according to the weight ratio of SnO₂-240 to GO controlled as 1 : 1, 1 : 2, and 1 : 3 in the precursor suspensions, respectively.

2.3. Materials characterization

The as-obtained samples were initially characterized by X-ray diffraction (XRD) using an X-ray diffraction analyzer with Cu-Kα radiation (D8-Discover, Bruker). The morphology of the samples was examined using a field emission scanning electron microscope (FESEM, Sirion, FEI) and transmission electron microscope (TEM, Tecnai F20, FEI). Raman analysis was performed using a Renishaw Invia (Thermo Fisher) with a laser wavelength of 532 nm. Thermogravimetric analysis (TGA) was performed using an STA449 F3 thermal analyzer with a heating rate of 10 °C min⁻¹ over the range 90–800 °C in an air atmosphere.

2.4. Electrochemical measurements

The electrochemical performance of the samples was evaluated using a CR2032-type coin cell. The working electrodes were fabricated by mixing the active materials with polyvinylidene difluoride (PVDF) and super P in a weight ratio of 80 : 10 : 10. Then, an appropriate amount of N-methylpyrrolidone (NMP) solvent was slowly added to the mixture to produce a slurry. The uniform slurry was coated onto a copper foil and dried at 120 °C for 24 h under vacuum. These electrodes were punched into the form of 12 mm diameter disks and assembled into half cells in an Ar-filled glovebox. The mass loading of each electrode was calculated to be ∼1.5 mg (∼1.2 mg for active materials) by weighing the mass discrepancy of the electrode and copper foil. Lithium foils were used as the counter electrodes, and the Celgard 2400 films were used as the separators. The electrolyte employed was 1.0 M LiPF₆ in 1 : 1 w/w ethylene carbonate (EC) and diethyl carbonate (DEC) with 2 wt% vinylene carbonate. The galvanostatic charge/discharge measurements were conducted using a LAND battery test system in the voltage range of 0.01 to 3.0 V (vs. Li/Li⁺) at different current densities. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660e electrochemical workstation. The EIS data were collected before cycles, and the weights and thicknesses of the tested electrodes are similar.

3. Results and discussion

3.1. Characterization and electrochemical performance of SnO₂ NRs

The SnO₂ NRs (SnO₂-220, SnO₂-240 and SnO₂-260) were synthesized by a hydrothermal method at different reaction temperatures, and their XRD patterns are shown in Fig. 1. Clearly, all diffraction peaks correspond to the (110), (101), (200), (211), (220), (002), (310), (112) and (301) planes of tetragonal SnO₂ (JCPDS no. 41-1445),²⁰ indicating high purity and crystallization in the samples. Moreover, it is found that the diffraction peaks from SnO₂-220 to SnO₂-260 become stronger and sharper, suggesting that the grain size becomes larger with the increase of reaction temperature.²¹ On the basis of the Scherrer equation, the average grain sizes of SnO₂-220, SnO₂-240 and SnO₂-260 are evaluated to be 11.9, 15.7 and 49.2 nm, respectively.

Fig. 1 XRD patterns of SnO₂-220, SnO₂-240 and SnO₂-260.

The TEM images of SnO₂-220, SnO₂-240 and SnO₂-260 are shown in Fig. 2. The three samples exhibit a similar rod-like morphology but obvious difference in size and uniformity. With the increase of reaction temperature from 220 to 260 °C, the length of the SnO₂ NRs obviously increases while the diameter change is slight. Among them, the SnO₂-240 exhibits the best size uniformity with a length of 20–25 nm and a diameter of ∼7 nm, corresponding to the aspect ratio of 2.9–3.6, as observed in Fig. 2c and d. By contrast, the SnO₂-220 has a smaller size with a length of 5–15 nm and a diameter of ∼5 nm (the aspect ratio is 1.0–3.0, Fig. 2a and b), while the SnO₂-260 consists of not only larger size NRs with a length of 20–100 nm and a diameter of ∼9 nm (the aspect ratio is 2.2–11.1), but also some ultrafine nanopowders (<1 nm) that aggregate around the NRs (Fig. 2e and f). The observed and calculated d-spacings from the HRTEM images are ∼0.33 nm for all samples (the insets in Fig. 2b, d and f), corresponding to the d-spacing of the (110) planes of SnO₂, suggesting well-crystallized SnO₂ NRs in the samples.²² Note that the exposure of (110) planes and the preferred growth direction along [001] are the results of minimizing surface energy.^23,24 The theoretical calculations^25,26 revealed that in the order of increasing energy the surfaces follow the sequence (110) < (100) < (101) < (001). In addition, the HRTEM results show that the size of SnO₂ NRs increases with the increase in hydrothermal temperature, indicating that higher reaction temperature accelerates the growth process and promotes the formation of longer 1D structures.

Fig. 2 TEM images of (a and b) SnO₂-220, (c and d) SnO₂-240 and (e and f) SnO₂-260. The insets in (b), (d) and (f) show the HRTEM images of relevant samples.

It is well known that the morphology of electrode materials has profound effects on their electrochemical performance. Fig. 3a shows the cycle profiles of the three samples at a current density of 0.5 A g⁻¹. It is found that the SnO₂-240 exhibits the best cycle performance and retains a discharge capacity up to 488.5 mA h g⁻¹ after 50 cycles, which is much higher than that of SnO₂-220 (408.5 mA h g⁻¹) and SnO₂-260 (402.6 mA h g⁻¹). To evaluate the kinetics of electrochemical reactions on the electrode, electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 100 kHz to 0.01 Hz. As shown in Fig. 3b, the Nyquist plots of all the samples possess the same features with a semicircle in the high frequency region and a line in the low frequency region, corresponding to charge-transfer resistance and the diffusion of lithium ions in the electrodes, respectively.²⁷ It can be seen that the SnO₂-240 has a smaller semicircle diameter than the SnO₂-220 and SnO₂-260, suggesting a smaller charge-transfer resistance in SnO₂-240.²⁸ This may be because the SnO₂-240 features a better size uniformity which may provide faster lithium ion transfer along the length direction.²⁹ However, it should be noted that the capacities of these pure SnO₂ NRs decrease gradually upon cycling, which can be attributed to the large volume change of SnO₂ during charge/discharge processes, leading to electrode structure degradation and instability of the SEI layer.

Fig. 3 (a) Cycle performance of SnO₂-220, SnO₂-240 and SnO₂-260 at a current density of 0.5 A g⁻¹, and (b) electrochemical impedance spectra of the samples in the frequency range of 100 kHz to 0.01 Hz.

3.2. Characterization and electrochemical performance of SnO₂ NRs@GN composites

In order to improve the electrochemical performance of the SnO₂ NRs, the 3D interconnected GN architecture is engineered to fully encapsulate the SnO₂ NRs by a facile freeze-drying method. The SEM images of the as-fabricated SnO₂ NRs@GN composites (SnO₂@GN-1, SnO₂@GN-2 and SnO₂@GN-3) are shown in Fig. 4a–c, showing that the graphene sheets interlace into interconnected macroporous networks. The high magnification SEM image of SnO₂@GN-2 (Fig. 4b) reveals that the SnO₂ NRs are fully encapsulated within graphene sheets. By contrast, the composite with a lower SnO₂ loading ratio (SnO₂@GN-1) shows an inadequate SnO₂ NR encapsulation amount (Fig. 4a), while the composite with a higher SnO₂ loading ratio (SnO₂@GN-3) exposes a part of the SnO₂ NRs outside the graphene sheets due to the excessive SnO₂ amount in the composite (Fig. 4c). The TEM image of the SnO₂@GN-2 shows that the SnO₂ NRs are closely wrapped by graphene sheets (Fig. 4d), which not only is beneficial to the electronic transportation but also accommodates the large volume change of the SnO₂ NRs during charge/discharge processes, as well as prevents the aggregation of the SnO₂ NRs.³⁰

Fig. 4 (a–c) SEM images of SnO₂@GN-1, SnO₂@GN-2 and SnO₂@GN-3, respectively, (d) TEM image of SnO₂@GN-2. The insets in (a), (b) and (c) exhibit the HRSEM images of relevant samples.

Fig. 5a exhibits the XRD patterns of the SnO₂ NRs@GN composites. All diffraction peaks correspond to those of pure SnO₂-240 NRs (Fig. 1), implying that the crystal structure of SnO₂ NRs remains intact after hybridization with graphene. Note that the characteristic stacking peak of graphene at around 26° may be overlapped with the (110) peak of SnO₂ in the composites. Raman spectra of all three samples show two broad bands at around 1345 and 1585 cm⁻¹ (Fig. 5b). The D band at around 1345 cm⁻¹ is induced by defects or disorder while the G band at around 1585 cm⁻¹ corresponds to the sp²-type ordered graphitic carbon.³¹ The intensity ratio of D and G bands (I_d/I_g) is related to the extent of disorder degree and average size of the sp² domains.³² It is found that the ratio of I_d/I_g increases from 0.92 for SnO₂@GN-1 to 0.94 for SnO₂@GN-2 and 0.97 for SnO₂@GN-3, suggesting the increase of defects and disordered structures with the increase of the SnO₂ loading ratio in the composites. Fig. 5c shows the TGA curves of SnO₂@GN-1, SnO₂@GN-2 and SnO₂@GN-3. The weight fractions of SnO₂ NRs in the composites were calculated to be about 68.1% for SnO₂@GN-1, 76.2% for SnO₂@GN-2 and 81.7% for SnO₂@GN-3 according to the TGA results.

Fig. 5 (a) XRD patterns, (b) Raman spectra and (c) TGA curves of SnO₂@GN-1, SnO₂@GN-2 and SnO₂@GN-3.

To demonstrate the potential application of the SnO₂ NRs@GN composites as LIB anode materials, we consider the SnO₂@GN-2 as a representative to first investigate the electrochemical behaviors through CV measurements at a scan rate of 0.2 mV s⁻¹ in the potential range of 0.01–3.0 V (vs. Li/Li⁺), as shown in Fig. 6a. In the first cycle, the cathodic peak at ∼0.80 V corresponds to the initial reduction of SnO₂ to Sn (SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O) and the formation of the SEI layer.³³ Two anodic peaks at ∼1.30 and 1.90 V are attributed to the reverse process from Sn to SnO₂. Another strong cathodic peak at ∼0.12 V is assigned to the alloying process (Sn + 4.4Li⁺ + 4.4e⁻ → Li_4.4Sn), while the corresponding anodic peak at ∼0.58 V corresponds to the dealloying process.³⁴ In the subsequent cycles, the peak potential and intensity remain similar, suggesting good reversibility and reproducibility for the SnO₂@GN-2 electrode.

Fig. 6 (a) Cyclic voltammograms of SnO₂@GN-2 at a scan rate of 0.2 mV s⁻¹, (b) galvanostatic charge/discharge curves of SnO₂@GN-2 at a current density of 0.5 A g⁻¹, (c) cycle performance of the SnO₂ NRs@GN composites at 0.5 A g⁻¹, (d) galvanostatic charge curves of SnO₂@GN-2 in the potential range of 1.0–3.0 V (vs. Li/Li⁺), (e) rate capability of the composites and pure SnO₂-240 NRs at various current densities, and (f) comparison of the rate capability of the SnO₂@GN-2 with those of other recently reported SnO₂-based electrodes.

The typical galvanostatic charge/discharge curves of SnO₂@GN-2 at a current density of 0.5 A g⁻¹ are shown in Fig. 6b. In accordance with the CV results in Fig. 6a, two voltage plateaus are observed during the first discharge process. The plateau at ∼0.8 V is attributed to the reduction of SnO₂ to Sn, accompanied by the decomposition of the electrolyte to form a SEI layer, and the following slope plateau at 0.5 to 0.01 V is attributed to the formation of the Li_xSn.³⁵ These plateaus are still observable in the subsequent cycles, suggesting the stable electrochemical process during cycles. The first discharge and charge capacities are 1640.8 and 1010.6 mA h g⁻¹, yielding a coulombic efficiency (CE) of 61.6%. The initial large capacity loss is attributed to the irreversible conversion reaction and the formation of the SEI layer as well as the Li⁺ insertion into the defects in graphene.³⁶ However, the CE of SnO₂@GN-2 increases to 94.1% in the 2nd cycle, 98.8% in the 10th cycle and then remains ∼99% in the subsequent cycles. By contrast, the SnO₂-240 delivers an initial CE of 45.2% which is much lower than that of SnO₂@GN-2. The improved CE of SnO₂@GN-2 is attributed to the existence of the graphene sheath in the composite, avoiding direct exposure of the SnO₂ NRs to the electrolyte and thus facilitating the formation of a stable SEI layer.

Fig. 6c exhibits the cycle profiles of the composites at a current density of 0.5 A g⁻¹. It is found that the capacities of all samples decline more slowly than those of pure SnO₂ NRs (Fig. 3a), especially the SnO₂@GN-2 even shows a gradually increasing capacity since the 20th cycle, and a capacity of 953.0 mA h g⁻¹ is retained after 100 cycles, much higher than that of the SnO₂@GN-1 (531.6 mA h g⁻¹) and SnO₂@GN-3 (768.6 mA h g⁻¹), as well as the theoretical values of SnO₂ (782 mA h g⁻¹). The additional capacity is often attributed to the initial only partially reversible conversion reaction that becomes progressively more reversible with increasing cycle number.¹⁸ To clarify this point, the galvanostatic charge curves of the SnO₂@GN-2 electrode between 1.0 and 3.0 V (vs. Li/Li⁺), corresponding to the potential range of the reverse conversion process (Sn + 2Li₂O → SnO₂ + 4Li⁺ + 4e⁻), are shown in Fig. 6d. The capacity contribution from this reverse conversion process can be calculated according to the difference between the intercepts at 3.0 and 1.0 V. It is clear that the difference between the intercepts becomes larger upon cycling, and the capacities of 395.3, 420.3 and 478.9 mA h g⁻¹ are achieved in the 20th, 60th, and 100th charge processes, respectively, demonstrating that the conversion reaction becomes more reversible with cycles.

Fig. 6e shows the rate capability of the composites and pure SnO₂-240 NRs. Compared with other samples, the SnO₂@GN-2 exhibits the best rate capability, even at the high rates of 8.0 A g⁻¹ and 16.0 A g⁻¹, delivering ultrahigh average reversible capacities of 624.2 mA h g⁻¹ and 458.4 mA h g⁻¹, respectively. In contrast, the SnO₂@GN-1 and SnO₂@GN-3 show lower reversible capacities of 241.5 mA h g⁻¹ and 219.5 mA h g⁻¹ at 16.0 A g⁻¹, respectively, while the reversible capacity of the pure SnO₂-240 NRs decreases dramatically with the increase of current density to as low as 142.1 mA h g⁻¹ at only 4.0 A g⁻¹. The remarkable rate capability of SnO₂@GN-2 can be attributed to the encapsulation arrangement with the interconnected GN which can provide fast electron and lithium ion transport pathways through the overall electrode, significantly enhancing electrical conductivity and lithium insertion/extraction kinetics. To be noted, the rate capability of SnO₂@GN-2 is compared with those of other recently reported SnO₂-based anodes in Fig. 6f, including SnO₂/graphene aerogels,³⁴ SnO₂/graphene microspheres,³⁷ graphene/SnO₂/polyaniline composites,³⁸ carbon-coated flower-like SnO₂ spheres,³⁹ boron-doped C/SnO₂/graphene nanosheets,⁴⁰ and graphene-wrapped SnO₂ quantum-dot clusters.⁴¹ It is clear that the present electrode possesses ultrahigh capacities and remarkable rate capability at various rates.

The long-term cycle performance of the SnO₂@GN-2 at a high current density of 1.0 A g⁻¹ was further investigated, as shown in Fig. 7. It is found that the specific capacity also declines first and then increases gradually to as high as 1179.2 mA h g⁻¹ after 400 cycles, and the capacity retention ratio reaches up to 167% (based on the first charge capacity). The corresponding CE is held close to 99% after the initial several cycles.

Fig. 7 Long-term cycle performance of SnO₂@GN-2 at 1.0 A g⁻¹ and the corresponding coulombic efficiency.

As mentioned above, the increasing capacity of SnO₂@GN-2 is mainly attributed to partially reversible conversion of Sn to SnO₂. It is well known that the lithiation of SnO₂ involves both the conversion reaction (SnO₂ + 4Li⁺ + 4e⁻ ↔ Sn + 2Li₂O) and the alloying reaction (Sn + 4.4Li⁺ + 4.4e⁻ ↔ Li_4.4Sn).⁴² Theoretically, as much as total 8.4 Li⁺ ions can be utilized during the first discharge process. Unfortunately, approximately 4 Li⁺ ions became inactivated in the initial several cycles owing to the irreversible conversion reaction. This will lead to capacity decay and low CE in the initial several cycles, which is in good agreement with our results. However, in this study, it is important to note that the CE is significantly improved, and the capacity also gradually increases upon cycling, suggesting that the conversion reaction becomes reversible. That is to say, these inserted Li⁺ ions can be returned adequately in the subsequent cycles, thus achieving the high CE.

To substantiate the correlation between the structural features and the cycling stability of the electrode materials, the morphology of SnO₂@GN-2 and SnO₂-240 electrodes after 100 cycles was examined. As shown in Fig. 8, the SnO₂@GN-2 electrode exhibits a close-packed surface morphology with tiny cracks while the SnO₂-240 electrode shows serious collapse and large cracks. These results indicate that the encapsulation of SnO₂ NRs into graphene is an effective strategy to keep the structural and electrical integrity of the electrode.

Fig. 8 SEM images of (a) SnO₂@GN-2 composite and (b) pure SnO₂-240 electrodes after 100 cycles.

In light of the above analyses, we believe that the ultrahigh capacity, and extraordinary cycle and rate performance of the SnO₂@GN-2 are related to its unique structural features, such as full encapsulation arrangement, highly oriented macropores, interconnected conductive network, and electrolyte barrier layer, as well as the pillar effect of SnO₂ NRs in between the graphene sheets. The synergistic coupling of these features not only effectively accommodates the large volume change of SnO₂ NRs during charge/discharge processes, but also provides fast electron and lithium ion transport pathways through the overall electrode, thus achieving fast and robust lithium storage.

4. Conclusions

We fabricated a 3D interconnected GN architecture where SnO₂ NRs were fully encapsulated within graphene sheets by a facile freeze-drying strategy. The as-built electrode shows extraordinary cycle performance (1179.2 mA h g⁻¹ after 400 cycles at 1.0 A g⁻¹) and remarkable rate capabilities (624.2 mA h g⁻¹ at 8.0 A g⁻¹; 458.4 mA h g⁻¹ at 16.0 A g⁻¹). The unprecedentedly favorable performance is attributed to the following factors: (1) the 3D interconnected GN architecture provides continuous conductive paths and extensive diffusion channels for electron and lithium ion transport, significantly enhancing the electrode reaction kinetics; (2) the elastic graphene accommodates the large volume change of SnO₂ NRs during charge/discharge processes, keeping the cycling stability of the electrode; (3) the full encapsulation of SnO₂ NRs in graphene sheets enables the SEI layer to be formed around the outer surface instead of on the SnO₂ NRs, ensuring the SEI stability; (4) the SnO₂ NRs function as pillars to prevent the graphene sheets from restacking while preserving the highly robust structure. It is believed that the novel SnO₂@GN-2 composite can be a promising new anode material for high energy density and power density LIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the MOST (grant number: 2017YFA0205800), NSFC (grant number: 11472080 and 51731004), NSF of Jiangsu Province (grant number: BK20141336), Jiangsu Key Laboratory for Advanced Metallic Materials (grant number: BM2007204) and Fundamental Research Funds for the Central Universities.

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