Introduction

As one of the most promising electrochemical energy conversion and memory devices, lithium-ion batteries (LIBs) have been widely used in electric vehicles, biomedical, military, and aerospace fields due to their high specific energy, low price, no memory effect, and long cycle life [1,2,3,4,5]. Unfortunately, commercial graphite is currently the main anode material for LIBs [6, 7], but it can hardly meet the growing market demand due to its defects such as low theoretical capacity (372 mAhg−1) and easy formation of lithium dendrites [8,9,10,11]. Therefore, the development of electrode materials with higher energy density and good cycling stability has become one of the most attractive topics [12].

Among the many anode materials for LIBs, SnO2 has been intensively studied owing to its abundant reserves, low price, simple synthesis, and high theoretical specific capacity (783 mAhg−1) [13,14,15,16,17]. However, SnO2 has some intrinsic disadvantages that limit their commercial application in LIBs, for example, the large volume expansion effect (300%) during charge/discharge leads to electrode pulverization and poor electronic conductivity during the lithiation/delithiation process [18,19,20].

In this context, much effort has been devoted to solve these problems [21,22,23,24]. The most common solution is to form composites with carbon materials [25, 26]. These carbon matrices not only increase the electrical conductivity of the negative electrode material but also effectively prevent the volume expansion of the electrode material, which greatly improves the cycling performance [27, 28]. Abouali [29] proved that ultrafine SnO2 particles encapsulated in hollow nanochannels in an ordered mesoporous carbon (OMC) framework performed excellent cycling and multiplicity performance as anode materials for LIBs. Carbon nanofiber (CNF) and SnO2 nanorod composites used as anode materials for lithium-ion batteries exhibited good electrochemical properties as reported by Abe [30]. Wang [16] found the coating of MnO2 nanosheets on SnO2@C hollow nanospheres to obtain SnO2@C@MnO2 hierarchical hollow hybrid nanospheres, which exhibited excellent electrochemical performance (644.5 mAhg−1 after 200 cycles). The above researches show that the carbon-coated SnO2 can improve the electrochemical properties of the material.

In this work, carbon-coated SnO2 nanocomposites were synthesized by hydrothermal method. Here, we report the use of carbon-coated SnO2 nanocomposites as anode for LIBs, which exhibit good electrochemical properties.

Experimental

Synthesis of SnO2/C nanospheres

First, 4 mmoL SnCl2ꞏ2H2O, 5 mmoL Na3C6H5O3ꞏ2H2O, and 2 mmoL NaOH were dispersed in the mixture of ethanol (20 mL) and water (20 mL). Then, 2 mmoL PVP and amount of glucose were added to the above suspension under magnetic stirring. Afterwards, the mixed solution was stirred at 25 °C for 30 min. The suspension was subsequently transferred to a 100 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 12 h and then cooled to room temperature naturally. The as-synthesized samples were collected and washed with ethanol and deionized water several times. After that, the samples were dried at 70 °C for 10 h. Then, heat treated in argon flow at 500 °C for 3 h. Under the doping ratio of glucose and tin dioxide to 0wt%, 5wt%, 15wt%, 30wt%, and 50wt%, samples with different carbon coating contents have been prepared and recorded as SnO2/C-0, SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50, respectively.

Materials characterization

The crystallographic characteristics of the SnO2/C were characterized by powder X-ray diffraction (XRD) from a Cu Kα radiation source; the 2-theta values were scanned in the range of 20–80° at a rate of 10° min−1 with a step size of 0.02°. Morphological features of the samples were observed by a JSM-2010 transmission electron microscope (TEM) operating at 200 kV accelerated voltage and a JSM-6700F cold field emission scanning electron microscope (SEM) at 5 kV. X-ray photoelectron spectroscopy (XPS) was performed on a spectrometer (Thermo Escalab 250Xi), and the binding energy scale was calibrated using a C 1 s peak at 284.8 eV.

Materials characterization

Electrochemical performance measurements were performed by assembling CR2023-type coin cells. The electrodes consist of 70 wt% active material, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF). Lithium metal was served as the counter electrode. The electrolyte was 1 M LiPF6 in 1:1:1 in volume ratio of ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC). The cells were assembled in an Ar-filled glovebox. Cyclic voltammetry (CV) was performed on an electrochemical workstation (CHI 660E) with a potential range of 0.01 ~ 3.00 V and a scan rate of 0.1 mVs−1. Electrochemical impedance spectroscopy (EIS) measurements were conducted on a CHI 660E electrochemical workstation. The galvanostatic charge/discharge test was performed on the LAND CT2001A battery tester.

Results and discussions

Figure 1 displays the XRD patterns for the five samples. All diffraction peaks of SnO2/C-0, SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50 samples are matched well with the standard diffraction peaks of tetragonal rutile SnO2 (JCPDS No. 41–1445) [31]. The diffraction peaks at 26.6°, 33.9°, 38.9°, 51.8°, 54.7°, 57.8°, 61.8°, 64.7°, 66.0°, 71.3°, and 78.7° were assigned to the (110), (101), (200), (211), (220), (002), (221), (112), (301), (202), and (321) crystal planes, respectively, and the sharp diffraction peaks indicate the strong crystallinity of SnO2 nanoparticles. For carbon coating of SnO2 nanospheres, no clear crystalline carbon phase can be detected in Fig. 1, indicating that the carbon layer is amorphous [32].

Fig. 1
figure 1

XRD patterns of SnO2/C-0, SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50

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In order to examine the effect of the variation of carbon cladding content on the product morphology, five samples of SnO2/C-0, SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50 were observed using SEM, and the results are shown in Fig. 2. According to the figure, it can be found that the change of carbon coating content during the synthesis process has a significant effect on the morphology of the final spherical SnO2/C. A slight agglomeration was found in the SnO2/C-5 sample in Fig. 2a, and the particles were not uniform in size and poorly dispersed. As shown in Fig. 2b, the SEM image of the SnO2/C-15 sample reveals that the product size is homogeneous and well dispersed, indicating that the morphology of SnO2/C-15 was best. However, Fig. 2c, d shows SEM images of SnO2/C-30 and SnO2/C-50 with poor dispersion and inhomogeneous size of the products. Furthermore, it is observed that agglomeration of the material increases with the increase of the carbon cladding content. Thus, we can see that the SnO2/C-15 sample has a better morphological structure than the electrode materials prepared with other carbon cladding contents. Figure 2f is the elemental image of SnO2/C-15, showing that the spherical material is mainly composed of the three elements C, O, and Sn.

Fig. 2
figure 2

SEM images of a SnO2/C-5, b SnO2/C-15, c SnO2/C-30, d SnO2/C-50, e SnO2/C-0, and f EDS elemental mapping of SnO2/C-15

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More close observation was performed by TEM. Figure 3 is the TEM images of the SnO2/C-15 complex. Figure 3a shows the TEM bright-field image of SnO2/C-15. It could be observed that the structure of the product is a large spherical structure of about 500 nm, which is aggregated from some small-size nanospheres with more complete spherical structure. Figure 3b shows a magnified image of the spherical edge, and it can be seen that the small-size nanospheres have a smooth surface with high integrity. The clear crystalline streaks in the small nanospheres with about 5–10 nm in diameter can be observed in Fig. 3c, which further demonstrates the well-crystallized structure of these SnO2/C. The crystalline lattice spacing of 0.33 nm is identified corresponding to the (110) plane of SnO2. The selected area electron diffraction (SEAD) pattern shows five different diffraction rings, as shown in Fig. 3d, corresponding to the (110), (101), (200), (211), and (220) planes, and further confirms the correctness of the XRD analysis [33].

Fig. 3
figure 3

TEM images of SnO2/C-15 composite a and b TEM, c HRTEM, d SAED

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Figure 4 shows the Raman spectra of SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50. The Raman peaks identified at 478 and 621 cm−1 were assigned to the Eg and A1g modes of anatase phase of SnO2, respectively [34]. From the diagram, the peaks appearing near 1365 cm−1 and 1595 cm−1 correspond to the D-peak and G-peak of the samples, respectively. The ratio of the intensity of the D and G peaks (ID/IG) reflects the degree of disorder in the material; the higher values indicate a higher degree of disorder and the greater number of defects, and therefore, there are a greater number of active sites available for Li+.

Fig. 4
figure 4

Raman spectra of SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50

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Figure 5a displays the XPS survey spectra of SnO2/C-15. The peaks located at 284, 486, and 530 eV can be attributed to C1s, Sn 3d, and O1s spectra, respectively. Figure 5b shows a typical high-resolution XPS spectrum of Sn 3d. The two characteristic peaks appearing at 485.7 eV and 494.2 eV correspond to Sn 3d5/2 and Sn 3d3/2, respectively [22]. The high-resolution C1s spectrum in Fig. 5c is deconvoluted into three peaks at 284.55 eV, 283.65 eV, and 283 eV, indicating the presence of C = O, C-O, and C–C bonds, respectively. In Fig. 5d, the high-resolution spectrum of O1s shows three chemical states: Sn–O (530.55 eV), O = C (531.5 eV), and O = C (532.5 eV) [35]. The XPS results indicate that the SnO2/C-15 material consists of three elements, O, Sn, and C, and no other impurities are present, which corroborates with other characterizations.

Fig. 5
figure 5

a XPS spectra of SnO2/C-15, and the corresponding high-resolution spectra of b Sn 3d, c C 1 s, and d O 1 s

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Figure 6 displays the cyclic voltammograms (CVs) curves of the SnO2/C-15 electrode in the voltage range of 0.01–3.0 V (for Li/Li+) for the first three cycles at a scan rate of 0.1 mVs−1. During the first cycle, a clear reduction peak can be seen at about 0.8 V. This is mainly due to the irreversible origination of the SEI film and partially reversible reaction between the lithium-ions and SnO2(SnO2 + 4Li+ + 4e− → Sn + 2Li2O) [36]. The reduction peak near 0.1 V and the oxidation peak near 0.58 V correspond to the occurrence of reversible reactions between lithium ions and Sn (xLi+ + xe− + Sn ↔ LixSn(0 ≤ x ≤ 4.4)). The broad oxidation peak located near 1.28 V is attributed to the occurrence of a partially reversible oxidation reaction between Sn and Li2O [37]. In the following two cycles, the curves overlapped with each other well, indicating an excellent cycle stability and reversibility of SnO2/C-15.

Fig. 6
figure 6

CVs curves of SnO2/C-15 at a scan rate of 0.1 mVs.−1

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Figure 7a compares the cycling performance of SnO2/C with different carbon cladding contents at the current density of 0.1C, and Fig. 7b compares the cycling performance of SnO2/C-0 and SnO2/C-15 at the current density of 1C. In Fig. 7a, the initial discharge capacity was found to be 1359.2, 1626.8, 2124.9, 1525.8, and 1349.4 mAhg−1 for SnO2/C-0, SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50, respectively. However the initial discharge capacity was found to be 1348.4 and 2025.7 for SnO2/C-0 and SnO2/C-15, as Fig. 7b. It can be seen that the SnO2/C samples show higher capacity and better cycling stability than the original samples SnO2/C-0, which indicates that the presence of carbon reduces the problem of crushing due to volume expansion during electrochemical processes [34]. Although the product capacity decayed rapidly within the first 20 cycles, the capacity of the samples gradually leveled off in the following cycles. After 200 cycles, SnO2/C-15 showed a higher discharge capacity (361.4 mAhg−1). These results suggest that the proper carbon-coated content is crucial to enhance the electrochemical performance, especially the SnO2/C-15 exhibits good cycling stability.

Fig. 7
figure 7

a Cycling performances of SnO2/C-0, SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50 at the current density of 0.1C. b Cycling performances of SnO2/C-0 and SnO2/C-15 at the current density of 1C

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The rate performance of the SnO2/C electrode was investigated by discharging/charging at different current densities from 0.1C to 2C and back to 0.1C, as shown in Fig. 8. It is obvious that the capacity of SnO2/C decreases gradually with the increase of the rate. For the sample of SnO2/C-15, discharge capacity decreased from 1221.66 to 888.89, 728.81, 575.5, and 459.22 mAhg−1 when the current density was increased from 0.1 to 0.2, 0.5, 1.0, and 2.0C, respectively, which is higher than the other samples. Then, the high discharge capacity of 547.32 mAhg−1 can be regained as the current density was switched back to 0.1C, which indicates the high stability and excellent reversibility of the electrode.

Fig. 8
figure 8

Rate performances of SnO2/C-0, SnO2/C-5, SnO2/C-15, SnO2/C-30, and SnO2/C-50 at different current densities ranging from 0.1C to 2C

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The EIS profiles of the SnO2/C-0 and SnO2/C-15 are shown in Fig. 9. It can be seen from the figure that a semicircle in the high-frequency region and a linear in the low-frequency region were observed for all materials [38]. The semicircle in the high-frequency region can be attributed to the charge transfer resistance, while the straight line in the low-frequency region corresponds to the diffusion resistance of Li+ in the electrode [39]. Comparing the electrochemical impedance spectroscopy of SnO2/C-0 and SnO2/C-15, it was found that the charge transfer resistance and the diffusion resistance of SnO2/C-15 were smaller than that of SnO2/C-0. The results show that the carbon coating can effectively reduce the Li+ transport distance and increase the transport rate, which can play a positive role in improving the electrochemical performance of SnO2 electrode materials [40].

Fig. 9
figure 9

Electrochemical impedance spectra (EIS) of the SnO2/C-0 and SnO2/C-15

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Conclusions

A series of SnO2/C nanocomposite electrode materials have been successfully prepared. Owing to the introduction of carbon, the SnO2/C nanospheres show excellent electrochemical properties. Notably, the nanospheres size of SnO2/C-15 is only about 5–10 nm, and the first discharge specific capacity of it reached 2124.9 mAhg−1 and still maintained discharge specific capacity of 361.4 mAhg−1 after 200 cycles. The carbon layer presence leads to an effective improvement of electron conductivity and lithium ion transport capabilities. Meanwhile, the small size of nanospheres effectively reduces the lithium-ion transport distance, improves its transport rate, and reduces the volume expansion of SnO2 during the cycling process. The method provides an effective preparation route for the practical application of SnO2 in lithium-ion batteries.