A facile post-process method to enhance crystallinity and electrochemical properties of SnO₂/rGO composites with three-dimensional hierarchically porous structure

Abstract

In this work, three SnO₂/reduced graphene oxide (SnO₂/rGO) composites with a three-dimensional hierarchically porous structure were synthesized via freeze drying and different annealing temperatures in an air atmosphere. The results show that the crystallinity of SnO₂ can be improved with increasing annealing temperature in air atmosphere, but the crystallinity quality of rGO presents a volcanic type change with increasing annealing temperature. The cyclic voltammetry curves indicate that the reaction of Li⁺ intercalation into SnO₂ and rGO can only occur in the 120 °C sample, so that the cycling performance of the 120 °C sample is the best. Its initial discharge capacity is 1610 mA h g⁻¹ at current densities of 0.5 A g⁻¹. After 100 cycles, the specific discharge capacity is 429 mA h g⁻¹. The XPS results reveal that SnO₂ nanoparticles are anchored on the surface of rGO via Sn–O–C bonds. During the charge/discharge process, the volume expansion of SnO₂ nanoparticles can be prevented through enhancing the crystallinity of ultra-small SnO₂ nanoparticles (about 3.2 nm). This might pave the way for the commercial production of the SnO₂/rGO electrode because these post-processing technologies are more easily implemented in industrial production.

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1. Introduction

Lithium ion batteries (LIBs) with high capacities and long cycle lives have attracted considerable research activity due to the rapid development of electric vehicles (EVs) and portable electronic devices.^1,2 As important components, anode materials are crucial for the next generation of high-performance rechargeable LIBs.³ Although graphite is the earliest commercial anode material in LIBs, its theoretical specific capacity is only 372 mA h g⁻¹, which is insufficient to satisfy the increasing demand for batteries with higher capacity.^4,5 Tin oxide (SnO₂) based nanomaterials have many potential advantages as compared to graphite, such as high specific capacity (1494 mA h g⁻¹), low cost, abundance and environmental benignity, etc.⁶ However, when SnO₂ is employed as an anode material in LIBs, it will undergo an irreversible conversion and form a reversible Li_4.4Sn alloy.⁷ This process shows a volume change of about 300% during charge/discharge process,^8,9 which results in a poor cycling performance,¹⁰ rapid capacity decrease,¹¹ and hindering the practical applications of SnO₂ based anode materials in LIBs.¹²

To address these issues, one strategy is to design various SnO₂ nanostructures, such as nanotube,¹³ nanobox,¹⁴ nanosheet,¹⁵ and hollow sphere,¹⁶ etc. These nanoarchitectures can overcome the large volume change of SnO₂ through the space reserved and specific surface area. The other strategy is to minimize the physical strain through nanosized SnO₂.^17,18 Previous reports have indicated that SnO₂ nanoparticles of ∼3 nm size could not aggregate into larger Sn clusters, and have a superior capacity and cycling stability.¹⁹ However, it is difficult to synthesize the ultra-small size of SnO₂,¹⁷ and pure SnO₂ nanoparticles are easy to aggregate owing to high surface energy of nanoparticles.²⁰ Therefore, new approach is to integrate SnO₂ with carbonaceous materials, such as SnO₂/porous carbon,²¹ SnO₂/carbon nanotubes²² and SnO₂/reduced graphene oxide (SnO₂/rGO),²³ etc. The porous nature of carbonaceous materials is used to accommodate huge volume changes of SnO₂, and the electronic conductivity of the electrode can be enhanced by the carbonaceous materials,²⁴ so that SnO₂/carbonaceous composite materials have attracted considerable attentions for the applications of energy and sensors.^25,26

Among all carbonaceous materials, graphene has been regarded as a good matrix for SnO₂ anode since it has ultrathin graphitic layers, excellent electronic conductivity, highly specific surface areas (2600 m² g⁻¹), good mechanical properties, and good chemical stabilities.^27,28 Up to now, many chemical methods have been attempted to synthesize SnO₂/rGO, such as one-pot route,²⁹ electrostatic spray deposition,³⁰ atomic layer and in situ deposition,⁷ hydrothermal/solvothermal,^25,31 and so on. For the synthesis of SnO₂/rGO composites, it is a facile method that graphene oxide (GO) is reduced by stannous ions without any reductants. However, the crystallinity of SnO₂ and crystallization quality of rGO is poor in the SnO₂/rGO, so that the SnO₂/rGO composites remain highly resistive, and it is not optimal for energy storage applications.³² Although a high-temperature annealing process is frequently used to enhance the crystallinity of SnO₂ and crystallization quality of rGO, the protective atmosphere is essential to prevent the decomposition of rGO in the most of previous reports.³³ This high-temperature annealing process is a major disadvantage in the industrial production, because it needs a relatively high equipment requirements and operation condition as compared to low temperature and air atmosphere. Therefore, it is still important for how to improve the crystallinity of SnO₂ and crystallization quality of rGO in industrial production. In addition, the volume expansion and aggregation of SnO₂ nanoparticles can not be prevented by the SnO₂/rGO composites,⁷ and SnO₂ based nanoparticles could be easily peeled off from the graphene and agglomerate on the anode. All of these results show that SnO₂/rGO composites still suffer from the large contact resistance,³⁴ self-discharge, capacity loss and even electrode failure.²

Herein, we demonstrated that a facile method improve the crystallinity of SnO₂ and crystallization quality of rGO, which is performed by a low-temperature calcining process in air. Before calcining, Li⁺ can only intercalate into rGO. However, the crystallinity of SnO₂ and crystallization quality of rGO can be improved at 120 °C in air atmosphere. In this case, Li⁺ can intercalate into SnO₂ and rGO. With increasing temperature to 280 °C, although the crystallinity of SnO₂ can be improved, the crystallization quality of rGO decrease, and the reaction of the Li⁺ intercalation into rGO disappears. The morphology and structural characterization reveal that SnO₂ nanoparticles are anchored by Sn–O–C bonds on graphene skeleton, and three-dimensional (3D) hierarchically porous structure of SnO₂/rGO composites can be retained. Moreover, the volume expansion of SnO₂ can be prevented through enhancing crystallinity of ultra-small SnO₂ nanoparticles size (about 3.2 nm) at 120 °C. This approach could be an effective way to improve the electrochemical performances of SnO₂/rGO composites in the industrial production, because it is simple and low-cost.

2. Experimental section

Tin dichloride dihydrate (SnCl₂·2H₂O, analytical grade purity) was purchased from the Tianjin Deen Chemical Reagent Co., Ltd (China). All other chemicals were of analytical grade and used without further purification.

2.1. Synthesis of SnO₂/rGO composites

GO was synthesized from natural graphite powders by a modified Hummer's method.³⁵ Then the as-prepared GO was dispersed in deionized water through the aid of ultrasound, the concentration of GO suspension was controlled at 1.7 mg mL⁻¹. In a typical process, 2.0 g of SnCl₂·2H₂O and several tin metal grains were added to 200 mL deionized water under magnetic stirring for 15 min. Then, 200 mL of GO suspension was added to the above solution and stirred at room temperature for 12 h, and the black precipitate was obtained by centrifugation, washed with deionized water several times. The resultant product was collected after freeze-drying (−52 °C 24 h), and denoted as SnO₂/rGO. As a comparison, the SnO₂/rGO was calcined at 120 °C for 4 h in air, and named as SnO₂/rGO-120. The SnO₂/rGO was calcined at 280 °C for 2 h in air, and named as SnO₂/rGO-280.

2.2. Characterizations

The microstructures and morphologies were observed using a SUPRA-40VP field emission scanning electron microscopy (FESEM, ZEISS, Germany) and a JEM-2100 transmission electron microscope (TEM, JEOL, Japan). The X-ray diffraction (XRD) patterns were obtained on the Bruker advance-D8 XRD instrument using CuKα radiation at 40 kV and 100 mA. X-ray photoelectron spectra (XPS) was detected with a VG Scientific ESCALABMKLL spectrometer using Al Kα X-ray source (10 mA, 15 kV). Nitrogen adsorption/desorption isotherms were determined with an ASAP 2020 (Micromeritics Instruments). Surface-area determination and pore distribution were evaluated by using the Brunauer–Emmet–Teller (BET) method and the Barrett–Joyner–Halenda (BJH) method. Raman spectra was collected using an inVia (Renishaw, England) with an excitation wavelength of 532 nm. The thermogravimetric and differential thermal analysis (TG-DTA) were performed on a STA449C Analyzer (NETZSCH, Germany) with a heating rate of 2.4 °C min⁻¹ under air atmosphere. Fourier transform infrared spectrographs (FT-IR) were recorded on a Bio-Rad FTS-40 Fourier transform infrared spectrophotometer with KBr pellets.

2.3. Electrochemical measurements

The electrochemical properties were evaluated by coin-type cells (2016-type). Lithium metal foil was used as a counter electrode. The working electrodes were prepared by mixing 90 wt% of active materials (SnO₂/rGO) and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP) to form a slurry, which was then coated onto a copper foil, dried in air at 120 °C overnight. The cell was assembled in an Ar-filled glove box (O₂ and H₂O < 0.1 ppm). The electrolyte was 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 v/v). Cyclic voltammetry (CV) tests were performed using CHI660D Electrochemical Workstation at a scanning rate of 0.1 mV s⁻¹. The electrochemical impedance spectroscopy (EIS) was recorded on a CHI660D Electrochemical Workstation by applying a sine wave with amplitude of 5.0 mV over the frequency range from 100 kHz to 0.01 Hz. Galvanostatic charge/discharge measurement was carried out on a LAND CT2001A battery-testing instrument at various current rates. The voltage range was from 0.01 to 3.0 V for all tests.

3. Results and discussion

3.1. Morphology and structural characterization

The FESEM and TEM images of the three samples (SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280) are shown in Fig. 1. The FESEM images (Fig. 1a, e and i) of the three samples show a disordered highly opened macroporous structure, and these macroporous are formed by rGO thin films with dispersive SnO₂ nanoparticles. However, the SnO₂/rGO-280 (Fig. 1i) presents a collapse structure. According to the previous researches, these macroporous structure should come from the sublimation of the ice crystals,³⁶ and the collapse structure of SnO₂/rGO-280 should be due to the high calcinating temperature, so that 3D hierarchically porous structure was destroyed. The high-magnification FESEM images of SnO₂/rGO and SnO₂/rGO-120 (Fig. 1b and f) reveal that there are many blocks of 10–50 nm on thin films of rGO, and these blocks link each other to form mesopores with diameter of 10–50 nm. The thickness of SnO₂/rGO films ranges from 10 to 25 nm. The FESEM image of SnO₂/rGO-280 (Fig. 1j) is fuzzy, and the blocks of 10–50 nm disappear, but its lamellar and porous structure can still be observed explicitly.

Fig. 1 The FESEM (a, b, e, f, i, j), TEM (c, g, k) and HRTEM (d, h, l) images of SnO₂/rGO (a–d), SnO₂/rGO-120 (e–l) and SnO₂/rGO-280 (i–l), respectively. The insets in (d), (h), (l) are the SAED patterns of samples, respectively.

The TEM images (Fig. 1c, g and k) of the three samples show that the SnO₂/rGO (Fig. 1c) is a smooth sheet-like structure, but SnO₂/rGO-120 (Fig. 1g) is a wrinkled sheet-like structure, and SnO₂/rGO-280 (Fig. 1k) is stacked together. The HRTEM images (Fig. 1d, h and l) of the three samples show that SnO₂ nanoparticles were uniformly loaded on the surface of graphene layers, and their size is less than 5 nm nanocrystals. According to the HRTEM images, the grain size distributions of SnO₂ nanoparticles are shown in Fig. S1.† SnO₂/rGO and SnO₂/rGO-120 exhibited a relatively narrow grain size distribution. The average grain sizes of SnO₂ nanoparticles in the SnO₂/rGO, SnO₂/rGO-120, and SnO₂/rGO-280 composites are 2.9 ± 0.5 (Fig. S1a†), 3.2 ± 0.5 (Fig. S1b†), and 3.8 ± 0.5 nm (Fig. S1c†) respectively. Although the size of SnO₂ nanoparticles increased gradually according to the grain size distributions (Fig. S1†), considering the measure error with TEM, the SnO₂ nanoparticles size of the three samples should be in the same range. The selected-area electron diffraction (SAED, insets of Fig. 1d, h and l) patterns of the three samples contain a series of well-defined Debye–Scherrer rings, confirming their polycrystalline nature. The calculated d-spacing values of 3.3, 2.6 and 1.7 Å correspond to Miller indices (110), (101) and (211) of rutile SnO₂ (cassiterite).

The XRD patterns of the three samples are shown in Fig. 2. XRD patterns of SnO₂/rGO (Fig. 2a) and SnO₂/rGO-120 (Fig. 2b) exhibit lower the intensity of diffraction peaks, indicating the poor crystallinity of SnO₂. The strong diffraction peaks of SnO₂/rGO-280 (Fig. 2c) can be indexed to a tetragonal rutile SnO₂ (JCPDS no. 41-1445).

Fig. 2 XRD patterns of (a) SnO₂/rGO, (b) SnO₂/rGO-120 and (c) SnO₂/rGO-280, respectively.

To further verify the successful reduction of GO by Sn²⁺ at room temperature, the chemical composition and surface state of the three samples were discreetly investigated by XPS. The survey spectra of the three samples are shown in Fig. 3a, only Sn, O, and C elements were found. Sn 3d high-resolution spectra of the three samples are shown in Fig. 3b, the Sn 3d peaks of SnO₂/rGO could be fitted with two peaks centered at 495.3 (Sn 3d_3/2) and 486.9 eV (Sn 3d_5/2). For the SnO₂/rGO-120 and SnO₂/rGO-280, the Sn 3d_3/2/3d_5/2 were 495.7/487.3 eV and 495.6/487.2 eV, respectively. It can be seen that the peak-to-peak separations between the Sn 3d_3/2 and the Sn 3d_5/2 are 8.4 eV, indicating the Sn⁴⁺ oxidation state in the three samples.^15,37,38 Meanwhile, the peak shifting of Sn 3d should be attributed to the influences of different functional groups on the surface of rGO. The C 1s high resolution spectra of the three samples are shown in Fig. 3c, C 1s binding energies of SnO₂/rGO appear at 283.8 (C–H),³⁹ 284.2 (C–C),^5,40 284.6 (C=C),^41,42 285.1 (C–OH),⁵ 286.2 eV (C=O),^43,44 and 288.7 eV (O–C=O).^29,39 After calcinating at 120 °C, the C–H peaks (283.8 eV) disappear, and the peak area ratio of C–OH, C=O and O–C=O are decreased. However, the peak area ratio of C–OH and O–C=O is increased when the calcinating temperature reached to 280 °C. The O 1s peak of SnO₂/rGO can be resolved into six peaks that correspond to Sn–OH, C–OH, Sn–O–Sn, Sn–O–C, Sn=O and C=O with binding energies of 530.4, 530.8, 531.3, 531.8, 532.3, and 533.1 eV, respectively.^41,45–47 The Sn–OH peak (530.4 eV) disappeared after calcinating at 120 °C, but the peak area ratio of Sn–O–Sn, Sn–O–C, Sn=O (SnO₂/rGO-120 is 532.1 eV, SnO₂/rGO-280 is 532.2 eV) are gradually increase with increasing calcinating temperature. It means that there are some Sn–O–C and Sn–OH bonds in SnO₂ nanoparticles of SnO₂/rGO, and they may also result in the poor crystallinity of SnO₂ in three samples (Fig. 2). Meanwhile, it also indicates that SnO₂ nanoparticles could be anchored on the surface of rGO via the Sn–O–C bond, and the amount of Sn–O–C bond is gradually increased according to the peak area ratio of Sn–O–C.

Fig. 3 (a) XPS survey spectrum, (b) high resolution XPS spectra of Sn 3d, (c) C 1s, and (d) O 1s in the SnO₂/rGO, SnO₂/rGO-120, and SnO₂/rGO-280, respectively.

TG-DTA analysis was performed to determine the appropriate processing temperature and the mass percentage of SnO₂ nanoparticles in the SnO₂/rGO composite. Fig. 4 shows the TG-DTA curves of SnO₂/rGO in air atmosphere. The mass loss of about 7% before 120 °C is attributed to the removal of adsorbed water and crystal water in SnO₂/rGO composite.⁴ The mass loss of about 12% from 120 to 500 °C is attributed to the oxidation of rGO.⁴⁸ It also means that the mass percentage of SnO₂ and rGO are about 81% and 12% in SnO₂/rGO composite, respectively. After calcinating at 120 °C, the mass percentage of SnO₂ could reach to about 87% [percentage = 81%/(81% + 12%)]. In addition, there are two platforms in DTA curve (blue line). One is from 226 to 263 °C, and the other is from 315 to 345 °C. We speculate that the first platform may correspond to the crystal phase transition of SnO₂ or rGO, and the second platform may correspond to the oxidation of functional groups on the surface of rGO.

Fig. 4 TG-DTA curves of SnO₂/rGO composite in air atmosphere.

The changes in molecular structure can be revealed by the FT-IR spectra of SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280. As shown in Fig. 5, several absorption bands of oxygen functional groups are observed in the three samples, such as the absorption bands at about 3430 (O–H stretching vibrations), 1729 (C=O stretching), 1587 (O–H bending), 1227 (C–O stretching), 1044 (C–O–C stretching), and 565 cm⁻¹ (Sn–O–Sn stretching).^1,29,49 It suggests that there are some oxygen functional groups in the three samples, they should come from GO. However, for the intensity of absorption bands at 1729, 1587, and 1227–1044 cm⁻¹, the SnO₂/rGO-280 significantly enhance compared with SnO₂/rGO and SnO₂/rGO-120. The result is consistent with the XPS and TG-DTA analysis.

Fig. 5 FT-IR spectra of (a) SnO₂/rGO, (b) SnO₂/rGO-120 and (c) SnO₂/rGO-280.

The crystallinity of SnO₂ and crystallization quality of rGO have been investigated by Raman spectroscopy. As shown in Fig. 6, for all samples, two characteristic peaks can be observed at 1342 and 1601 cm⁻¹, corresponding to the D and G band of rGO.^5,43,47 The D band and G band in Raman spectra are related to the defects/disorder in the graphitic layers and stretching vibration of sp² C in a graphene layer, respectively.⁵⁰ The intensity ratios of D and G band (I_D/I_G) of three samples are 1.13, 1.20 and 1.04, respectively. It can be seen that the I_D/I_G of SnO₂/rGO-120 is the biggest, which was suggestive of a partial reduction of GO in SnO₂/rGO-120.⁵ With further increasing the calcinating temperature to 280 °C, the value of I_D/I_G decreases to 1.04, suggestive of further reduction of the rGO in SnO₂/rGO-280.²⁶ A possible reason for the higher I_D/I_G ratio of the graphene generated at 120 °C is that more oxygen-containing groups (e.g., C–OH, C=O, and O–C=O) decompose to CO₂ at 120 °C, leaving more defects in the graphene.^43,51 Meanwhile, the decomposing of oxygen-containing groups also generate more rGO in SnO₂/rGO-120. This speculation can be confirmed by the XPS analysis of C and O (Fig. 3c and d). As shown in Fig. 3, the peak area ratio of C–OH (285.1 eV), C=O (286.2 eV), and O–C=O (288.7 eV) decrease (Fig. 3c) after calcinating at 120 °C, and the C–H peaks (283.8 eV) disappear. Moreover, the amount of C–O–C band increase (Fig. 3d, SnO₂/rGO-120) in this process, which should arise from the esterification or condensation reaction of oxygen-containing groups on the surface of GO. However, the absorption bands intensity of C=O and C–O bonds (Fig. 5c) significantly enhance at SnO₂/rGO-280, which means that new and more oxygen-containing groups are again generated on the surface of rGO. It also suggests that the crystallization symmetrical hexagonal graphitic lattice may have been destroyed at 280 °C in air, so that crystallization quality of rGO decreases. Meanwhile, the SnO₂/rGO-120 and SnO₂/rGO-280 composites show a weak peak at ∼478 cm⁻¹ corresponding to the E_g vibrational mode of rutile SnO₂,^52–56 and this peak intensity is enhanced gradually with increasing the calcinating temperature. It means that the crystallinity of SnO₂ can be enhanced by increasing the calcinating temperature. This is also consistent with the XRD and XPS results. In addition, the peaks at ∼2674 and 2948 cm⁻¹ are attributed to the 2D and D + G band of single-layer graphene, respectively.

Fig. 6 Raman spectra of (a) SnO₂/rGO, (b) SnO₂/rGO-120, (c) SnO₂/rGO-280.

The porous nature of SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280 was demonstrated by BET measurement. The N₂ adsorption–desorption isotherms and the pore size distribution curves are shown in Fig. 7, and the results are listed in Table 1. The three samples show the typical IV adsorption isotherm (Fig. 7a), which indicates that all of samples contain mesoporous structure. The pore size distributes (Fig. 7b) demonstrate that the as-prepared samples have three kinds of pore structure in the range of 2–130 nm.

Fig. 7 (a) N₂ adsorption–desorption isotherms of three samples. (b) The pore size distribution calculated from the desorption branch.

Table 1 The specific surface area, pore volume, and average pore size of the products

Sample	Specific surface area [m^2 g^−1]	Pore volume [cm^3 g^−1]	Average pore size [nm]
SnO2/rGO	179	0.34	7.6
SnO2/rGO-120	183	0.32	7.2
SnO2/rGO-280	142	0.23	6.5

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The first is mesopore, the pore diameter centers are about 30, 37 and 18 nm in the SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280, respectively. The second is mesoporous with 3–5 nm, and the third is microporous with less than 2 nm. The mesopore mainly come from the cross-link between graphene layers. The 3D hierarchically porous structure was destroyed when the calcinating temperature reached to 280 °C (Fig. 1i), which result in the amount of mesopore is decreased (Fig. 7b). The mesoporous and microporous mainly come from the connection of SnO₂ nanoparticles and the defects of graphene layers. As shown in Table 1, because the collapse of highly opened macroporous structure (Fig. 1), the pore volume and average pore sizes of as-prepared samples decrease gradually with increasing the calcinating temperature.

The distribution of C, Sn and O elemental can be further revealed by element mapping of composite. For example, the element mapping images of SnO₂/rGO-120 are shown in Fig. S2,† it is clear that C, Sn and O are evenly distributed onto the surface of SnO₂/rGO-120. The C, O and Sn atomic percent in the SnO₂/rGO composite is about 29.6%, 54.17% and 16.23% according to the elemental analysis, respectively.

All the results indicate that SnO₂ nanoparticles could be anchored on graphene skeleton via Sn–O–C bonds. The crystallinity of SnO₂ and crystallization qualities of rGO can be enhanced through calcinating at 120 °C in air atmosphere, and the amount of C–OH, C=O and O–C=O are decreased (Fig. 3c). But rGO can also be decomposed with further increasing calcinating temperature to 280 °C. Therefore, the fabrication process can be illustrated in Scheme 1. It is an advantage for the industrial production of SnO₂/rGO composites as compared to a high-temperature annealing process in protective atmosphere, because it is not necessary for a relatively high equipment requirements and strict operation condition, and its cost is also relatively low at low temperature and air atmosphere.

Scheme 1 Scheme illustration for the fabrication of SnO₂/rGO composites at different calcinating temperature.

3.2. Electrochemical performance study

The electrochemical reactivity of the SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280 was further evaluated by cyclic voltammetry in lithium-ion half-cell. Fig. 8 displays the cyclic voltammetry curves (CVs) of three samples on the first 3 cycles, which were obtained between 0 and 3.0 V at a scanning a rates of 0.1 mV s⁻¹. The CVs of SnO₂/rGO are shown in Fig. 8a, in the first cycle, an irreversible broad peak at about 0.84 V is observed, which should be attributed to the formation of a solid electrolyte interphase and decomposition of SnO₂ to form Sn.^57–59 The subsequent CVs show that it (about 0.84 V) becomes reversible peak at about 0.94 V, indicating the formation of various Li_xSn species.⁵⁷ During the anodic process, two oxidation peaks at around 0.64 and 1.34 V can be attributed to the dealloying of Li_xSn and the partial conversion of Sn to SnO₂,^11,60–62 respectively. The electrochemical process of SnO₂/rGO electrode can be described by the following reactions:^28,29,63

Li⁺ + e⁻ + electrolyte → SEI (1)

SnO₂ + 4Li⁺ + 4e⁻ → 2Li₂O + Sn (2)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4) (3)

xLi⁺ + C (graphene) + xe⁻ ↔ Li_xC (4)

Fig. 8 Cyclic voltammetry (CVs) of the different SnO₂/rGO electrodes at a scan rate of 0.1 mV s⁻¹. (a) SnO₂/rGO, (b) SnO₂/rGO-120, (c) SnO₂/rGO-280.

As shown in Fig. 8a, the peaks of ∼0.01 V can be ascribed to Li⁺ intercalation into graphene to form LiC₆ (eqn (4)).^64,65 According to the previous reports, the formation of Li_xSn (eqn (3)) should form a peak at about 0.25 V,⁶⁵ but we can not find this peak in Fig. 8a. It means that the formation of alloy between Sn and Li was very weak owing to the poor crystallinity of SnO₂. As shown in Fig. 8b, the broad peak of SnO₂/rGO-120 has almost no change at about 0.84 V on the first 3 cycles. It suggests that the decomposition of SnO₂ is a stable process in the SnO₂/rGO-120 electrode. Meanwhile, the irreversible reduction peak at about 0.19 V was divided into two reversible peaks at 0.06 and 0.26 V, corresponding to Li intercalation into graphene (eqn (4)) and the formation of alloy between Sn and Li (eqn (3)).^64,65 Furthermore, the subsequent CVs show good reproducibility with several cathodic and anodic peak pairs, suggesting a very high degree of reversibility of the multistep conversion. It also suggests that both rGO and SnO₂ play the activity materials role in the SnO₂/rGO-120.

For the SnO₂/rGO-280 electrode (Fig. 8c), the peak position drift to about 0.90 V, and the intensity increase significantly in the subsequent CVs, it should be due to that more SnO₂ are decomposed into Sn (eqn (2)). Meanwhile, the reduction peak at about 0.19 V is very stable in the SnO₂/rGO-280 electrode. It suggests that the reaction of Li intercalation into graphene is a very weak process in the SnO₂/rGO-280 electrode. All these CVs results indicate that the intercalation object of Li⁺ is different in the SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280. It should come from the change of SnO₂ crystallinity and the amount of rGO.

The Nyquist plots of SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280 are shown in Fig. 9a. After fitting the Nyquist plots with the typical circuit model (inset of Fig. 9a),^66–68 the resistance parameters are listed in Table 2. The transfer resistances (R₃) of three samples are 490, 90.1, and 3.2 × 10⁻¹⁰ Ω, respectively, suggesting that the electron transference for the latter is prior to that of the former.⁶⁹ Especially SnO₂/rGO-280, the electron transference has almost no resistance. According to the XPS (Fig. 3d) and Raman spectroscopy (Fig. 6), the crystallinity of SnO₂ and the amount of Sn–O–C bonds are the highest in SnO₂/rGO-280. These factors should be advantage for enhancing the electron transference of composites, so that R₃ of SnO₂/rGO-280 is very small. When the crystallization quality of rGO is also considered, we speculate that the crystallization quality and the amount of rGO could be small in the as-prepared SnO₂/rGO-280 sample (Fig. 6), so the reaction of Li intercalation into graphene is a very weak process in the SnO₂/rGO-280 electrode. Therefore, the cycling capacities of SnO₂/rGO-280 should be the smallest in the three samples because only SnO₂ plays the active materials role in the charge–discharge process.

Fig. 9 (a) Nyquist plots of the different SnO₂/rGO electrodes. The inset shows equivalent circuit for the impedance spectrum, the solid lines are the fitted curves. (b) Charge–discharge profiles of the different SnO₂/rGO electrodes at a current rate of 0.2 A g⁻¹ for the first cycle. (c) Cycling performance of the different SnO₂/rGO electrodes at 0.5 A g⁻¹. (I)–(III) are corresponding to the SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280 in (a–c). Cycling performances of SnO₂/rGO (d), SnO₂/rGO-120 (e) and SnO₂/rGO-280 (f) at the different current densities. All of the measurements were conducted using a voltage window of 0.01–3.0 V.

Table 2 The circuit used to fit the EIS spectra and the fitted values for the corresponding components in the equivalent circuit

Samples	R1/Ω	R2/Ω	R3/Ω	Rcell/Ω
SnO2/rGO	19.5	437.1	490	946.6
SnO2/rGO-120	10.5	128.2	90.1	228.8
SnO2/rGO-280	13.3	53.4	3.2 × 10^−10	66.7

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The first discharge–charge curves of SnO₂/rGO, SnO₂/rGO-120 and SnO₂/rGO-280 are shown in Fig. 9b. For the three samples, the initial discharge capacity could reach to about 2046, 1947, and 1596 mA h g⁻¹ at current densities of 0.2 A g⁻¹, and the first charge capacity is about 1258, 1166, and 900 mA h g⁻¹, respectively. Thus their coulombic efficiency is 61%, 60%, and 56%, respectively. As shown in Fig. 9c–f, the charge–discharge capacities of the SnO₂/rGO and SnO₂/rGO-120 in the first 3 cycles is unstable at the different current densities. On the contrary, the SnO₂/rGO-280 is stable. In addition, the cycling performances of SnO₂/rGO-120 is the best in the three samples, such as at current densities of 0.5 A g⁻¹, the initial discharge capacity of three samples are 1194, 1610 and 1364 mA h g⁻¹, respectively. After 100 cycles, the specific discharge capacity of three samples are 250, 429 and 172 mA h g⁻¹, respectively. According to the structural characterization (Section 3.1), CVs (Fig. 8) and impedance analysis (Fig. 9a), good cycling performance and high rate capability of SnO₂/rGO-120 should be attributed to the appropriate calcinating temperature. On the one hand, it provides a novel approach to modify SnO₂/rGO composites. On the other hand, it decreases the resistance of SnO₂/rGO-120 electrode. Meanwhile, we note that the SnO₂/rGO is dried in the drying oven at 120 °C (electrode), and the SnO₂/rGO-120 is calcined in the muffle furnace, but they are difference in the electrochemical properties. It means that the effect of heat treatment mode is important for the rGO composites. However, many studies have ignored this in previous reports.

Fig. 10 shows the HRTEM images of three samples and the grain size distributions of SnO₂ nanoparticles after 100 cycles at current densities of 0.2 A g⁻¹. The HRTEM images (Fig. 10a–c) of three samples show that the dispersion of SnO₂ nanoparticles is still good. The grain size distributions of SnO₂ nanoparticles (Fig. 10d–f) show that the average sizes of SnO₂ nanoparticles are 4.9 ± 0.5, 3.2 ± 0.5 and 4.0 ± 0.5 nm in three samples, respectively. The volume expansion of SnO₂ is significant for SnO₂/rGO during the charge/discharge process. The reason is that the SnO₂ crystallinity is poor in SnO₂/rGO. On the contrary, with increasing the SnO₂ crystallinity, the volume expansion of SnO₂ can be prevented. The previous reports have indicated that the SnO₂ nanoparticle size is one of the key factors for the stable cycling performance of SnO₂, where smaller particle size could help to prevent gradual aggregation of Sn into large clusters.⁷⁰ In this work, the SnO₂ nanoparticles size of 3.2 nm demonstrates that it also help to prevent the volume expansion of SnO₂ during the charge/discharge process.

Fig. 10 The HRTEM images and the grain size distributions of SnO₂ nanoparticles in the three samples. (a, d) SnO₂/rGO, (b, e) SnO₂/rGO-120, (c, f) SnO₂/rGO-280. The samples were obtained after 100 discharge–charge cycles at a current rate of 0.2 A g⁻¹.

4. Conclusion

In summary, three 3D hierarchically porous SnO₂/rGO composite was synthesized through stannous ions reduced GO. A facile post-process approach can be used to improve the crystallinity of SnO₂ and crystallization quality of rGO in SnO₂/rGO composites, and SnO₂ NPS can be anchored on the surface of rGO substrate by Sn–O–C bond. The electrochemical performance results indicate that the crystallinity of SnO₂ and the crystallization quality of rGO can effect on the intercalation object of Li⁺, conductivity of composites, cycling performances and volume expansion of SnO₂, etc. Since this post-process approach can be performed at low temperature in air atmosphere rather than high temperature in protecting atmosphere, it is beneficial for the synthesis of large quantities of SnO₂/rGO composites and improving the electrochemical properties of SnO₂/rGO composites in LIBs applications.

Acknowledgements

This work was financially supported by Science and Technology Research Project of Henan Province (No. 152102310311, 152102210083 and 142102210455), the Key Scientific Research Project of Colleges and Universities in Henan (No. 16A150032, 15A150057) and Henan Normal University Innovation Funds for Postgraduate (No. YL201407, No. YL201512). We would also like to thank for the High Performance Computing Center of Henan Normal University, Engineering Technology Research Center of Motive Power and Key Materials for providing a LAND CT2001A battery-testing instrument. F. T. and X. W. contributed equally to this work.

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Footnote

† Electronic supplementary information (ESI) available: The grain size distributions of SnO₂ nanoparticles, elemental mapping images. See DOI: 10.1039/c6ra23236a

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