Controllable synthesis of rod-like SnO₂ nanoparticles with tunable length anchored onto graphene nanosheets for improved lithium storage capability

Abstract

In our work, rod-like SnO₂ nanoparticles with tunable length have been successfully anchored onto graphene nanosheets through a simple and in situ hydrothermal strategy under acidic conditions. The SEM and TEM images demonstrate that the unique rod-like SnO₂ nanoparticles with a diameter of 10–15 nm and length of 18–34 nm are uniformly anchored onto the surface of graphene nanosheets. Moreover, the particle sizes of rod-like SnO₂ nanoparticles can be readily adjusted by simply varying the reaction temperature. Interestingly, with the increase of reaction temperature from 120 to 160 °C, the rod length of SnO₂ nanoparticles significantly increased. More importantly, the SnO₂@graphene products exhibit a very high specific surface area, which played a key role in maintaining the structural stability against the irreversible volume change during Li⁺ insertion/extraction. The nanocomposites show an extremely high lithium storage capability and an excellent cycling performance. The initial discharge capacities are 1284 mA h g⁻¹ at current densities of 200 mA g⁻¹. After 100 cycles, the discharge capacity still remains as high as 981 mA h g⁻¹, indicating a superior retention capacity.

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1. Introduction

As the dominant power source, rechargeable lithium-ion batteries (LIBs) have been extensively applied in various fields because of their well-known advantages including high energy intensity, long cycle life and no memory effect. However, the current LIBs technology can't meet the ever-growing demands for large-scale applications such as electric vehicles and scalable electricity storage. Consequently, advanced electrode materials with high energy density, long cycle life, and good rate performances are highly desirable.^1–5 Among the potential anode materials for LIBs, a lot of metals, metal oxides, and metal sulfides have attracted more and more attention because their theoretical capacities are higher than the commercial graphite anodes (372 mA h g⁻¹). In the case, various materials with different morphologies have been thoroughly developed for a wide range of applications such as zero-dimensional (0D) nanoparticle,^6,7 one dimensional (1D) nanorod/nanofibre/nanotube,^8,9 two dimensional (2D) nanosheet/nanoplate,^10,11 and three dimensional (3D) architectures.¹² Particularly, SnO₂ nanomaterial has been regarded as one of the most promising candidates for alternative anodes of Li-ion batteries due to its high theoretical gravimetric lithium storage capacity of 782 mA h g⁻¹ and low potential of lithium-ion intercalation.^13,14 Nevertheless, the practical application of SnO₂ anodes is still restricted due to their large volume change (200–300%)¹⁵ and the bad aggregation during cycles, resulting in a rapid capacity fading and short cycle life.¹⁶ Therefore, much attention should be paid to explore suitable electrode materials with good electrochemical cyclic stability and simple fabrication techniques.

Graphene, as a sparkling star, has been served as an intriguing substrate to build up 2D nanohybrids for energy storage and conversions,^17–19 owing to its two dimensional (2D) structure,^20,21 high specific surface area,²² excellent charge carrier mobility²³ and good mechanical properties.²⁴ Firstly, the graphene nanosheets can provide an elastic buffer space to accommodate the volume expansion/contraction of the supported materials during the Li insertion/extraction process. Secondly, the 2D structure of graphene can act as a support to immobilize the supported materials upon repetitive cycling and prevent their agglomeration. Furthermore, the porosity and defects generated in the composite can increase the electrode/electrolyte contact area and provide more active sites for lithium reaction and storage.²⁵ Hence, the excellent properties could effectively improve the electrochemical performances during cycling. Despite the great progress has been made for the construction of SnO₂@graphene composites with versatile nanostructures including nanoparticles, nanosheets, nanoflowers and nanospheres.^26–30 Nevertheless, the destruction of the desirable morphologies for electrodes could not be truly avoided especially at long-term cycles and high rates.

Herein, we present an environment-friendly, one-pot and in situ hydrothermal route for fabrication of SnO₂@graphene nanocomposites starting from SnCl₂·2H₂O and graphene with the assistance of cetyltrimethylammoniumbromide (CTAB). The structure and composition of the products are analyzed in detail by SEM, TEM, XRD, and BET techniques. The resulting products show that the rod-like SnO₂ nanoparticles are homogeneously anchored onto the graphene matrix. The excellent electrochemical performance is mainly attributed to the conducting, buffering and protective effects of graphene nanosheets onto the rod-like SnO₂ nanoparticles, as well as the uniform morphology and high specific surface area. We expect that the prepared materials will provide more potential applications in various fields including the dye-sensitized solar cell, sensor and optical devices.

2. Experimental section

2.1 Materials

All the chemical reagents in the experiments were used without further purification. Stannous chloride dihydrate (SnCl₂·2H₂O), natural graphite powder, phosphorus-pentoxide (P₂O₅), potassium hypermanganate (KMnO₄), hydrogen-peroxide (H₂O₂), potassium persulfate (K₂S₂O₈), sulfuric acid (H₂SO₄), cetyltrimethylammonium bromide (CTAB), hydrochloric acid (HCl, 37 wt%) and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2 Synthesis of SnO₂@graphene nanocomposites

Graphene oxide (GO) sheets were synthesized from natural graphite powder by a modified Hummers' method.³¹ Here, the reduced graphene oxide (RGO) was further obtained by a thermal reduction of the as-synthesized GO. The details was described in a previous work.³² SnO₂@graphene nanocomposites were prepared by a simple hydrothermal route. In a typical synthesis, 50 mg of graphene nanosheets (RGO) was dispersed into 60 mL of ethanol/water (2 : 4, in volume) mixed solutions by ultrasonication for 2 h to get a suspension. Then, 0.55 g of cetyltrimethylammonium bromide (CTAB) and 2 mL of HCl solution (37 wt%) were added the above suspension under stirring for 30 min. Afterwards, 50 mg of SnCl₂·2H₂O was added to the above mixture with magnetic stirring for 1 h. Subsequently, the mixture was transferred to 150 mL Teflon-lined stainless steel autoclave and maintained at 120–160 °C for 24 h. After cooling to room temperature naturally, the precipitate was collected by centrifuge and repeatedly washed with ethanol and deionized water, and then dried at 60 °C overnight. The resultant composites were obtained, in which the SnO₂@graphene was denoted as the SG-120, SG-140, and SG-160, respectively, corresponding to the hydrothermal reaction temperate. For comparison, pure SnO₂ nanoparticles were also prepared according to our previous publication work.³³

2.3 Characterizations

X-ray powder diffraction (XRD) patterns were obtained on the Japan Rigaku D/max-2550 instrument using CuKα radiation. FT-IR spectrum was recorded on a Nicolet AVATAR 370 Fourier transform infrared spectrometer using KBr pellets. Raman spectra were collected using a Renishaw inVia system with an excitation wavelength of 514 nm. The structure and morphology of the samples were investigated by transmission electron microscopy (TEM, JEOL 200CX), scanning electron microscopy (SEM, JEOL JSM-6700F), and high resolution transmission electron microscopy (HRTEM, JSM-2010F). Elemental qualitative analysis was performed by the energy-dispersive X-ray spectroscopy (EDX, OXFORD INCA). X-ray photoelectron spectra (XPS) were detected with a Kratos Analytical Axis UltraDLD spectrometer with Mg Kα X-ray radiation. The C1s line (284.6 eV) was taken as a reference to calibrate the shift of binding energy due to electrostatic charging. N₂ sorption isotherms were measured on a QUADRASORB SI surface area & pore size analyzer at 77 K, after the products were degassed in vacuum at 150 °C for 6 h. The Brunauer–Emmett–Teller (BET) specific surface area and the pore size distribution were calculated by using the desorption data. Thermogravimetric (TG) analysis was provided with a Netzsch TG 209F1 system in a flow of air.

2.4 Electrochemical measurements

The working electrodes were fabricated as follows: 80 wt% of the active material (SG-120, SG-140, SG-160, and pure SnO₂, respectively), 10 wt% of the conductive agent (acetylene black), and 10 wt% of the binder (poly(vinylidene difluoride), PVDF) were homogeneously mixed in N-methyl-2-pyrrolidinone (NMP) and pasted on a copper foil. The electrochemical properties of the working electrodes were measured using two electrodes CR2016 coin-type cells with lithium foil serving as both counter and reference electrodes under ambient temperature. The electrolyte was 1 M LiPF₆ solution in EC/DMC/EMC (ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate, 1/1/1, w/w/w, Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd). The galvanostatic discharge-charge tests were carried out using a LAND-CT2001A test system. The coin cells were discharged and charged at different current densities (50–2000 mA g⁻¹) in the fixed voltage range of 0–3.0 V. The first cycle discharging was kept at 50 mA g⁻¹. The electrochemical measurement of pure graphene sheets (RGO) was followed the same procedure except without addition of carbon black, in which the weight ratio of graphene sheets to PTFE was at 80 : 20.

2.5 Structure and morphology

As shown in Scheme 1, the SnO₂@graphene nanocomposites are prepared by a simple hydrothermal method, in which the length of SnO₂ nanoparticles decorated into the graphene can be well adjusted by easily altering the reaction temperature. Firstly, GO nanosheets with abundant functional groups (e.g. carboxyl and hydroxyl) are prepared through the modified Hummers method. Subsequently, the reduced GO nanosheets (RGO) are obtained by annealing GO at 500 °C for 2 h in nitrogen atmosphere and the prepared RGO nanosheets are uniformly dispersed in the ethanol/water solutions. Afterwards, SnCl₂·2H₂O is added to the CTAB/HCl mixed solutions under stirring, in which Sn²⁺ ions are hydrolyzed to SnO₂ nanoparticles and firmly attach onto the surface of graphene nanosheets. At the end, the rod-like SnO₂ nanoparticles are uniformly dispersed onto the surface of RGO. During the process, the defects in RGO serve as docks in favor of Sn²⁺ ions to be adsorbed on the surface of graphene. It should be noted that the size of SnO₂ nanoparticles could be easily tuned by only changing the reaction temperature.

Scheme 1 Schematic illustration for the formation of rod-like SnO₂@graphene nanocomposites.

The morphology and structure of SnO₂@graphene nanocomposites are examined by electron microscopy technique. TEM images (Fig. 1(a), (d) and (g)) reveal that a lot of uniform SnO₂ nanoparticles have been deposited onto the high quality graphene substrate, regardless of the reaction temperature. No free particles or unloaded graphene sheets could be obviously seen. The magnified TEM images demonstrate that the nanoparticles are rod-like morphology with a diameter of 10–15 nm and a length of 18–34 nm (Fig. 1(b), (e) and (h)). Interestingly, with the increase of reaction temperature from 120 to 160 °C, the rod length of SnO₂ nanoparticles significantly increases. However, the change of the diameter is very little. The particle size distributions further confirm the fact, in which the length of the SnO₂ nanoparticles is 18.2, 28.8, 33.8 nm, respectively, but little change in diameter, about 10.5, 13.9, 14.2 nm (Fig. 1(c), (f) and (i)) corresponding to the reaction temperature from 120 to 160 °C. Usually, a crystal prefers to expose crystal planes with low surface energy in the growth process. For example, both natural and synthetic forms of SnO₂ crystals are generally enclosed by (110), (101), or (100) facets with low surface energy.³⁴ In present SnO₂@graphene nanocomposite, the crystal growth of the rod-like SnO₂ nanoparticles seems to also follow the trend. In addition, it should be mentioned that without CTAB in the precursor, some SnO₂ nanoparticles are not fully loaded onto the RGO (not shown). As a result, it can be concluded that the addition of CTAB played a key role for the formation of the desired SnO₂@graphene nanocomposites. Certainly, the reaction temperature is also a very important parameter.

Fig. 1 TEM images with different magnification and the corresponding particle size distributions of various SnO₂@graphene nanocomposites: (a–c) SG-120, (d–f) SG-140, (g–i) SG-160.

Fig. 2 shows the low-magnification SEM images and the corresponding EDX patterns of various SnO₂@graphene nanocomposites. Seen from the pictures (Fig. 2(a), (c) and (e)), all products synthesized at the range of 120–160 °C have homogeneous morphology, in which a number of SnO₂ nanoparticles grow onto the graphene matrix, in agreement with the TEM analysis. Only Sn, C and O elements are detected in the corresponding EDX patterns (Fig. 2(b), (d) and (f)), no matter the reaction temperature, suggesting a high purity of all products. While a small amount of Si element is observed, origining from the Si substrate used to disperse SEM samples. Meanwhile, that will avoid the carbon effect from the common carbon substrate.

Fig. 2 SEM images and the corresponding EDX patterns of various SnO₂@graphene nanocomposites: (a and b) SG-120, (c and d) SG-140, (e and f) SG-160.

The crystal structure of SnO₂@graphene nanocomposites is investigated by XRD technique. As can be seen from the Fig. 3, an obvious diffraction peak at 2θ = 10.3° in GO nanosheet is observed, owing to the (001) crystal plane of GO, indicating that GO has been successfully obtained via the chemical oxidation of graphite powder. The absence of this peak in the SnO₂@graphene nanocomposites indicates that the SnO₂ nanoparticles effectively inhibit the re-stacking of graphene nanosheets, in other words, that limits constructive diffractions from ordered graphene sheets, indirectly confirming the formation of SnO₂@graphene nanocomposites. The four main diffraction peaks corresponding to the (110), (101), (211), and (310) are attributed to the tetragonal SnO₂ with cassiterite structure (JCPDS card no. 41-1445).^35,36 Compared with bare SnO₂ nanoparticle, the intensity of the diffraction peaks in the all SnO₂@graphene nanocomposites is lower and wider. That suggests the influence of thermal reduction of graphene oxide on the growth of SnO₂ nanoparticles.

Fig. 3 XRD patterns of various products.

Fig. 4(a) shows the FT-IR spectra of SnO₂@graphene nanocomposites. All samples display strong absorption assigned to the carbon oxygen bonds from GO in the wavenumber region of 900–1800 cm⁻¹.³⁷ The peaks at 1721 cm⁻¹ and 1680 cm⁻¹ are due to the characteristic absorption of –COOH and C=C, respectively.³⁸ The broad absorption band located at 3421 cm⁻¹ is ascribed to the stretching vibration of –OH. However, the characteristic peaks of GO in SnO₂@graphene become weaker at the wavenumber region of 900–1800 cm⁻¹. The absorption located at 1721 cm⁻¹ even disappears.^39,40 These results demonstrate the successful reduction of the oxygen-containing functional groups of GO during the hydrothermal treatment. The broad absorption band located at 673 cm⁻¹ comes from the stretching vibration of O–Sn–O, which can be seen in the three SnO₂@graphene nanocomposites.³²

Fig. 4 (a) FT-IR spectra, (b) Raman spectra of various products.

Raman spectra are further characterized the carbon framework. As illustrated in Fig. 4(b), the peak at 1590 cm⁻¹ (G band) in the Raman spectra of the SnO₂@graphene composites is usually assigned to the E_2g phonon of C sp² atoms, while the peak at 1350 cm⁻¹ (D band) is the breathing mode of k-point phonons of A_1g symmetry.^41,42 The intensity ratio (I_D/I_G) of D band to G band is related to the extent of p-conjugation and concentration of defects, which is 0.78, 0.93, 0.95 and 0.92, respectively, for GO, SG-120, SG-140 and SG-160. The difference in I_D/I_G value reveals the electronic interactions between rod-like SnO₂ nanoparticles and graphene substrate, which is beneficial to electron transport between SnO₂ and graphene and the formation of the three-dimensional network, thus also desirable for lithium storage.

N₂ adsorption–desorption isotherm analysis is employed to further investigate the textural properties of the products. As presented in Fig. 5, all samples are of type IV isotherm curves with a distinct hysteresis loop at the relatively pressure p/p₀ = 0.4–1.0, suggesting their mesoporous structures. And all products exhibit the high specific surface areas. The specific surface area of SG-140 is calculated to be 544.2 m² g⁻¹, much higher than that of SG-160 (511.6 m² g⁻¹), lower than of that of SG-120 (610.5 m² g⁻¹). The high specific surface area should be mainly attributed to the contribution of graphene architecture. The corresponding pore size distribution plots calculated from the desorption isotherm using the Barrett–Joyner–Halenda (BJH) method are shown in Fig. 5(b), (d) and (f). The results show that the three samples all possess small mesoporous mainly peaked at 3.9 nm, which is mainly ascribed to interparticle aggregation of adjacent SnO₂ nanoparticles. As a consequence, such a high specific surface area and narrow pore size distribution in SnO₂@graphene nanocomposites are expected to exhibit superior performance as anode materials for lithium-ion storage.

Fig. 5 N₂ adsorption–desorption isotherms and the corresponding pore size distribution plots of various SnO₂@graphene nanocomposites: (a and b) SG-120, (c and d) SG-140, (e and f) SG-160.

In order to better understand the surface chemical compositions of the SnO₂@graphene nanocomposites, XPS spectra are further measured, as shown in Fig. 6. According to Fig. 6(a), only Sn, O and C elements are detected in the sample, indicating its high chemical purities. C1s of SG-140 is deconvolved into peaks related to different oxygen containing functional groups (Fig. 6(b)). The main peak located at about 284.5 eV is ascribed to non-oxygenated ring carbon atoms. While the other peaks at 286.2, 287.8 and 289.1 eV are assigned to the oxygen-containing groups (C–OH), (C=O) and (O=C–OH), respectively. These changes indicate that most of the oxygenated functional groups are successfully reduced and graphene is formed accordingly.⁴³ O1s of SG-140 (Fig. 6(c)) is divided into two peaks located at 531.2 eV and 533.4 eV, which is due to the existence of the crystal lattice O²⁻ and O_(OH) (hydroxyl ions) species in the SnO₂@graphene composites, respectively.⁴⁴ Fig. 6(d) presents the separated XPS spectrum of Sn3d peaks. The binding energy of Sn3d_3/2 and Sn3d_5/2 are found at 487.9 eV and 495.9 eV, respectively, which belongs to the characteristics of SnO₂ compounds.^45–47

Fig. 6 (a) survey scans spectra of SG-140, (b) separated spectra of C1s region, (c) separated spectra of O1s region, (d) spectra of Sn3d.

Fig. 7(a) shows the HRTEM image of SG-140 and the corresponding SAED pattern. The crystal lattice fringe of SnO₂ with the interlayer distances of 0.334 nm is attributed to the spacing of the (110) planes of monoclinic SnO₂ crystals in the nanocomposites.^48,49 In order to further investigate the distribution and composition of the SG-140, the STEM image and the corresponding elemental mapping analysis are adopted. Fig. 7(b)–(d) show that Sn and O elements are homogeneously distributed throughout the samples, confirming the SnO₂ nanocrystals are well distributed in the graphene matrix, in accordance with the TEM results.

Fig. 7 (a) HRTEM image (inset is the corresponding SAED pattern), (b) STEM image of SG-140, (c) and (d) the corresponding elemental mapping images.

TG curves of the three SnO₂@graphene nanocomposites are displayed in Fig. 8. The weight loss at first is due to the absorbed water in the SnO₂@graphene nanocomposites. All samples have a significant weight loss between 450 and 700 °C, because of the decomposition of graphene nanosheets. By calculation, the amount of SnO₂ in the SG-120, SG-140, SG-160 nanocomposites is about 27.3%, 25.7%, 23.1%, respectively, as listed in Table 1.

Fig. 8 TG curves of various SnO₂@graphene nanocomposites: (a) SG-120, (b) SG-140, (c) SG-160.

Table 1 The physicochemical properties of various SnO₂@graphene nanocomposites synthesized at different reaction temperature

Sample	SBET^a (m^2 g^−1)	Weight (%)	Length^b (nm)	Charge (mA h g^−1)	Discharge (mA h g^−1)
a The specific surface area calculated by the (Brunauer–Emmett–Teller) BET method.b The size of SnO2@graphene estimated from TEM image.
SG-120	610.5	27.3	18.7	980.4	1085.7
SG-140	544.2	25.7	28.4	1270.1	1284.9
SG-160	511.6	23.1	33.8	675.0	732.1
SnO2	6.6		20.3	5.2	5.2

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2.6 Electrochemical performance

Fig. 9 displays the charge/discharge curves of (a) SG-120, (b) SG-140 and (c) SG-160 for the 1st, 10th, 50th and 100th cycles at a rate of 200 mA g⁻¹, respectively. The discharge and charge capacity profiles of the same cell at various cycles are illustrated. With increasing cycles, the discharge capacity and the charge capacity of our materials decrease. The cell is first cycled at a current density of 200 mA g⁻¹ for SG-140, a very high specific capacity of 1608 and 1024 mA h g⁻¹ based on the total mass of the composites is seen, as depicted in Fig. 9(a) (red curves). These are larger than the theoretical capacity of SnO₂ (782 mA h g⁻¹) and our SnO₂@graphene composites (C_theo. = C_SnO2 × %_SnO2 + C_graphene × %_graphene = 782 × 25.7% + 774 × 74.3% = 776.1 mA h g⁻¹; here, we regard the theoretical capacity of single layer graphene as 774 mA h g⁻¹, which is related to the electrochemical reaction of Li₂C₆ = C₆ + 2Li⁺ + 2e).⁵⁰ Particularly, the capacity of the SnO₂@graphene is more than the theoretical capacity even at a current density of 200 mA g⁻¹ after 50 cycles. When the cycle increases to 100 cycles, our materials still could deliver a discharge capacity of 981 mA h g⁻¹. Moreover, the first discharge and charge capacities of others are 732 and 675 mA h g⁻¹ for SG-160, 1085 and 980 mA h g⁻¹ for SG-120. It should be noteworthy that the discharge capacities of three SnO₂@graphene nanocomposites are higher during the first cycle. After that, during the discharge process, SG-120, SG-140 and SG-160 samples still retain less capacities of the initial capacity, respectively. The capacity loss could be attributed to the irreversibility resulting from SnO₂ reduction and the formation of solid electrolyte on the surface of the active material.

Fig. 9 The charge–discharge curves of various SnO₂@graphene nanocomposites: (a) SG-120, (b) SG-140, (c) SG-160 at a current density of 200 mA g⁻¹.

As we known, the main reason for the rapidly fading of SnO₂ electrode is the large volume expansion of the SnO₂ material occurs during cycling, leading to the pulverization of the electrode.^51,52 Fig. 10(a) shows the cycle performance of the three SnO₂@graphene nanocomposites at a current density of 200 mA g⁻¹. As expected, the SnO₂@graphene nanocomposites exhibited a good cycling stability. Significantly, the SG-140 still exhibits a discharge capacity as high as 981 mA h g⁻¹ after 100 cycles, higher than those of SG-120 (530 mA h g⁻¹), SG-160 (377 mA h g⁻¹), GO and pure SnO₂. The superior cycling performance is mainly due to the rational growth of SnO₂ nanoparticles onto the graphene matrix, which serves as a spacer that keeps the neighboring graphene separated. Moreover, the graphene could effectively prevent the volume expansion/contraction and aggregation of SnO₂ nanoparticles during Li ions charge/discharge process.

Fig. 10 (a) Cycle performance of various SnO₂@graphene nanocomposites at a current density of 200 mA g⁻¹, (b) the rate capabilities of SnO₂@graphene nanocomposites various current rates from 50 to 2000 mA g⁻¹.

To investigate the rate capability, the SnO₂@graphene nanocomposites are discharged and charged at various current rates from 50 to 2000 mA g⁻¹. From the Fig. 9(b), the capacity of the SnO₂@graphene nanocomposites decreases with improving the current density. Impressively, the rate capability of SG-140 is far better than that of others. Even at a high current density of 2000 mA g⁻¹, the capacity of the SG-140 is still much higher than those of SG-120 and SG-160 up to approximately 186 mA h g⁻¹. Furthermore, the reversible capacity can be recovered to a high value when the current density is returned to 50 mA g⁻¹. The lithium storage properties of the hybrid electrodes are superior to the self-assembly of ordered, SnO₂@graphene nanoporous composites prepared by reassembled process and in situ chemical methods. On one hand, the graphene with a good electrical conductivity can improve the electrical conductivity of the whole electrode, thus enhancing the rate capability.^53,54 On the other hand, the relatively high specific BET surface area can increase the electrolyte–electrode interface and make the electrochemical reaction to easily carry out. In addition, the high reversible discharge capacity compared with the other SnO₂ composites may also be due to the topological defects of our materials. As presented in Table 1, SG-140 material has a high I_D/I_G in Raman spectra. A large number of surface defects are introduced onto the graphene nanosheets during chemical exfoliation process, which leads to the formation of disordered carbon structure and further increases the active sites for lithium storage, further improving lithium intercalation properties. Besides, the rod-like SnO₂ nanoparticles are homogeneously anchored onto the surface of graphene nanosheets, which could also provide parallel channels for Li⁺ ions intercalation through the surface with the least resistance to some extent.

Scheme 2 presents the illustration for bare SnO₂ nanoparticle and the present SnO₂@graphene composite electrodes during the insertion/extraction processes of lithium ions. The bare SnO₂ nanoparticles undergo a large volumetric expansion during the lithiation process and cause pulverization of the electrode materials (Scheme 2a), thus resulting in capacity decay. While the volume expansion of the SnO₂@graphene composites could be effectively restricted by the wrapped graphene substrate, Li⁺ ions insert into SnO₂ nanoparticles (Scheme 2b). According to the BET data in Table 1, even though the volume expansion still happens, the graphene layers could facilitate the formation of 3D electronic conductive networks, provide a large number of active sites for Li-ion migration, enhance the conductivity and shorten the electronic transport route, and expand the gap between graphene layers, thus resulting in an easier migration for Li-ions. Finally, the short transport distance of lithium ions results in a reversible reaction of Li₂O and Sn to yield SnO₂. Consequently, the obtained SnO₂@graphene nanocomposites demonstrate an excellent electrochemical property including a high lithium storage capability, good rate performance and long cycle life during the cycling.

Scheme 2 Diagram for the lithiation process of electrodes after many cycles: (a) bare SnO₂, (b) SnO₂@graphene nanocomposite.

3. Conclusions

In conclusion, we provide a facile hydrothermal method for fabrication of rod-like SnO₂@graphene nanocomposites with large surface areas and well-defined morphology. The resulting rod-like SnO₂ nanoparticles are homogeneously anchored onto the surface of graphene nanosheets. Additionally, the length of rod-like SnO₂ nanoparticles can be readily adjusted by simply varying the reaction temperature. The electrochemical results indicate that the SnO₂@graphene nanocomposites exhibit superior lithium ion battery performance with a large reversible capacity, excellent cyclic performance, and good rate capability. In particular, the obtained SnO₂@graphene nanocomposites at 140 °C have a reversible discharge capacity as high as 981 mA h g⁻¹ after 100 cycles at a current density of 200 mA g⁻¹. The results indicate that the structure and morphology of SnO₂ have a significant influence on its electrochemical performance. The present method can provide an alternative route for the design and fabrication of morphology-controlled other nanocrystals onto the graphene, which will extend their applications in the energy storage.

Acknowledgements

The work was financially supported by the National Natural Science Foundation of China (11275121, 21471096, 21371116), and Program for Innovative Research Team in University (IRT13078).

Notes and references

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