In contrast to other power sources,LIBs have attracted much attention for various applications,from portable devices to electric cars,due to their low-cost,long cycle life,high stability and power density. The electrode materials of LIBs are important components to decide the overall performance of the LIBs. Generally,graphite is commonly used as the anode for commercial LIBs. However,the low specific capacity of graphite (theoretically 372 mAh g^-1) hinders the development and application of LIBs ^{[1, 2]}. Therefore, it is essential to develop new electrode materials for the fabrication of high-performance LIBs.

As an important n-type band gap semiconductor,tin dioxide (SnO₂) has been considered as an ideal material to replace graphite due to its high specific capacity (782 mAh g^-1) and safe and easy preparation. However,SnO₂ electrodes usually come across the problem of poor cycle performance due to large volume expansion that produces large internal stress and leads to electrode pulverization during the charge-discharge process ^{[3, 4]}. Currently, there are three primary methods to improve the cycle performance of SnO₂ as LIB anode materials: First the design of nanoscale materials that can accommodate large volume changes. In this regard,SnO₂ nanoparticles (NPs) with different sizes using various approaches as anode materials for LIBs have been widely reported ^{[5, 6, 7, 8, 9, 10, 11]}. Second is the fabrication of unique structures to improve cyclic capacity retention. For example,void spaces in hollow and mesoporous nanostructures can inhibit the volume change and thus elevate the cyclic capacity ^{[12, 13, 14, 15, 16, 17]}. Third is the synthesis of core-shell heterostructure materials to elevate the stability of core materials. For instance,combining SnO₂ with other materials such as carbon and graphene to form SnO₂/C heterostructures ^[18]. For example,Lou et al. have prepared hollow SnO₂ nanostructures (500 nm) with 500 mAh g^-1 cycle capacity over 40 cycles at 0.2 C current density ^[19]. The as-prepared SnO₂ NPs cannot support the high current density which results in the poor cycling performance during the charge-discharge process. Subsequently,Lou et al. synthesized a SnO₂@C hollow composite (200 nm) for application in LIBs. The product showed better cycling performance (600 mAh g^-1) than pure SnO₂ nanospheres (300 mAh g^-1) ^[20]. Carbon decoration for anode and cathode materials has been proven to improve the electronic conductivity of electrodes ^{[21, 22]}. Indeed,composite nanomaterials have been used as electrode materials in LIBs due to enhanced electrical conductivity and reduced internal stress,which both improve the total electrical performance of LIBs ^{[18, 23, 24, 25, 26, 27, 28]}. Therefore,it is better to combine the small SnO₂ NPs with other materials to protect the inner SnO₂ NPs. However,the large size of SnO₂ would cause lower cycling performance at high current density.

Herein,a well-defined sub-100 nm carbon coated hollow SnO₂ nanoparticle has been designed and fabricated by a facile two-step hydrothermal method without the presence of any surfactants. As shown in Scheme 1,carbon coating is realized via hydrothermal treatment of low-cost glucose,and the whole synthesis process is environmentally benign. As-synthesized hollow SnO₂@C NPs satisfy the aforementioned advantages to improve the cycle performance. The as-obtained sub-100 nm hollow SnO₂@C NPs have been applied as anode materials for LIBs. The sub-100 nm hollow SnO₂@C nanoparticle is a promising candidate for the fabrication of high capacity,low cost,and environmentally safe anode materials for LIBs.

Scheme. 1

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Scheme. 1.The synthesis process of sub-100 nm hollow SnO2@C NPs.

Potassium stannate trihydrate (K₂SnO₃.3H₂O) was purchased from Shanghai Jingchun Chemical Reagent Co.,Ltd. Ethanol (C₂H₅OH),urea (CO(NH₂)₂),ethylene glycol (EG),and glucose (C₆H₁₂O₆) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the materials were used as received and without further purification in the whole experimental process.

According to our previous report ^[29],typically,0.6 g of potassium stannate trihydrate were dissolved in 50 mL of EG through the ultrasonic method,and then 30 mL of urea aqueous solution (0.4 g of urea) was added. Then,the mixture was transferred into a Teflon-lined stainless steel autoclave (100 mL) and heated at 170 ℃ in a furnace for 24 h. After the autoclave cools down to room temperature,the products were collected via centrifugation and washed several times by deionized water and ethanol,and then dried at 80 ℃ under vacuum. In a single batch of experiments,more than 0.2 g of hollow SnO₂ nanoparticles could be prepared.

Synthesis of hollow SnO₂@C nanoparticles: briefly,0.4 g of asprepared hollow SnO₂ NPs and 4 g of glucose were re-dispersed in ethanol/H2O solution. After stirring,the solution was transferred into a Teflon-lined stainless steel autoclave (100 mL) and heated at 180 ℃ for 8 h. The as-prepared products were collected via centrifugation and washed several times by H₂O and ethanol,and then dried at 80 ℃ in air. Subsequently,the products were annealed at 450 ℃ for 4 h under argon with a heating rate of 5 ℃ min^-1.

Transmission electron microscopy (TEM),selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) were carried out on a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. Raman measurement was performed by a HORIBA Jobin Yvon LabRAM HR spectroscope using 488 nm excitation wavelength. Powder X-ray diffraction (XRD) was recorded on a D/ruanx2550PC (Japan) using Cu Ka radiation (l = 0.1542 nm) operated at 40 kV and 40 mA and at a scan rate of 0.058 s^-1. Thermo gravimetric analysis was performed on an SNTQ600 thermo gravimetric analyzer.

In preparation of the coin cell battery,N-methyl pyrrolidinone (NMP) slurry consisting of 80 wt% hollow SnO₂ or SnO₂@C powders mixed with conductive agents and binder was coated onto a Cu foil. Pure lithium foil was used as the counter electrode,and the electrolyte used was LiPF₆. Cell assembly was implemented in a recirculating argon glove box. Electrochemical measurements were executed by a LAHE Battery Tester under different conditions.

Fig. 1a shows representative XRD patterns of as-prepared hollow SnO₂ and SnO₂@C NPs. The diffraction peaks in both curves are well indexed to the tetragonal rutile phase of SnO₂ (PDF#41- 1445). The result reveals that the structure of the SnO₂ is not changed by the carbon coating process. Furthermore,Raman spectroscopy was used to confirm the generation of the carbon layer on the as-prepared hollow SnO₂ seeds. As shown in Fig. 1b, two peaks of 1585 cm^-1 and 1360 cm^-1 can be observed in the hollow SnO₂@C sample,which can be attributed to E2g vibration mode of ordered C (G band) and A1g vibration mode of disordered C (D band),respectively. The peak at 560 cm^-1 can be indexed to the Sn-O surface vibrations,the peak at 629 cm^-1 is attributed to the A1 g mode of SnO₂. The above results demonstrate that carbon has been successfully coated on the surface of SnO₂ NPs,and the structure of SnO₂ is not changed.

Fig. 1

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Fig. 1.XRD patterns (a) and Raman spectra (b) of as-obtained hollow SnO2 and hollow SnO2@C NPs.

Fig. 2 shows the element valence state and the composition of the as-obtained hollow SnO₂ NPs before and after carbon coating. The full-scan XPS spectra of the as-obtained hollow SnO₂ and SnO₂@C NPs are shown in Fig. S1 in Supporting Information. The core-level spectra of Sn 3d peaks are investigated and the results are shown in Fig. 2b. The peaks at 486.58 and 487.18 eV belong to the Sn 3d5/2,495.08 and 495.48 eV belong to Sn 3d3/2,respectively. The result illustrates the characteristics of Sn⁴⁺ in the as-prepared hollow SnO₂ and SnO₂@C NPs,and the result further confirms that the heat treatment did not cause the generation of metal tin. Obviously,the carbon coating reduces the peak strength of Sn 3d5/2 and 3d3/2,compared with hollow SnO₂ and SnO₂@C NPs. As shown in Fig. 2a,the peaks at 287.88 and 287.78 eV are assigned to C 1s. The carbon coating enhances the peak strength of C 1s,compared to the uncoated hollow SnO₂ NPs. In fact,several reports show that XPS signal intensity of the core becomes weak after coating in a core-shell structure. Meanwhile,the XPS signal intensity of the shell becomes predominant ^{[30, 31, 32]}. Clearly,the above results confirm the formation of the carbon coated SnO₂ core-shell NPs. The morphology and microstructure of the as-obtained hollow SnO₂ NPs before and after carbon coating are investigated by TEM and HRTEM. As shown in Fig. 3a,the as-prepared samples mainly consist of uniform flower-like NPs with a mean size of ~50 nm (the histogram size distribution of SnO₂ is 50.9 nm,as shown in Fig. S3a in Supporting Information). The clear contrast between the edge and the center of the particle suggests the hollow nature of NPs. The bright rings in the SAED pattern corresponds to the rutile phase of SnO₂. As shown in Fig. 3b,it can be seen that as-obtained SnO₂ NPs consist of clusters of SnO₂ crystals and the surface of SnO₂ particles are rough. There are two kinds of lattice spacings of 0.264 and 0.334 nm,which can be indexed to (1 0 1) and (0 ^-1 1) planes,respectively. As shown in Fig. 3c,it can be seen that the hollow SnO₂@C NPs remain uniform in size and include a thin layer of carbon on the surface. As shown in Fig. 3d,the corresponding HRTEM image shows a smooth and continuous carbon layer with the thickness of about 1-2 nm (the histogram size distribution of SnO₂@C is 52.2 nm,as shown in Fig. S3b). There are two kinds of lattice fringes with interplanar spacings of 0.334 and 0.264 nm, which are attributed to the (1 1 0) and (1 0 1) planes of SnO₂, respectively. The above results confirm the successful coating of carbon,and the hollow nature of SnO₂ NPs that is well maintained after the coating and annealing process. The thermal gravimetric results (Fig. S2 in Supporting Information) illustrate that about 17.3% of carbon has been coated on the hollow SnO₂ NPs.

Fig. 2

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Fig. 2.XPS core peaks of C 1s (a) and Sn 3d (b) for the as-prepared hollow SnO2 NPs before and after carbon coating.

Fig. 3

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Fig. 3.TEM and HRTEM images of as-obtained hollow SnO2 (a, b, the inset scale bar is 10 nm) and SnO2@C NPs (c, d).

In the electrochemical measurements,the size of the electrode plate is Ø12 mm (the corresponding area is 1.1304 cm²),meanwhile, the active materials quality coating on the electrode plate is 1.5 mg. Therefore,we calculate that the mass loading is 1.327 mg cm^{-2 [33]}. To study the electrochemical performance of the as-prepared hollow SnO₂@C NPs electrodes,cyclic voltammetry (CV) analysis was performed at a scan rate of 0.5 mV s^-1 and the potential range was 3.0-0.01 V (vs. Li/Li⁺). Fig. 4a displays the first three cycles of CV curves. A clear difference between the 1st and subsequent cycles could be observed. In the 1st cycle,a typical reduction peak occurs at ~0.7 V,suggesting the generation of solid electrolyte films and the reduction of tin oxide to metallic tin. The proposed reaction is as follows:

Fig. 4

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Fig. 4.The cyclic voltammograms curves of the hollow SnO2@C NPs (a); the charge–discharge curves of hollow SnO2@C NPs between 0.01 V and 3 V (V vs. Li/Li+) at 5 C (b), the cycle performance of hollow SnO2 NPs before and after carbon coating (c); the rate performances of hollow SnO2@C nanoparticle at variable rates (0.2–5 C, 1 C = 782 mAh g-1)(d).

During the negative scan,two clear peaks at 0.1 and 0.6 V could be observed,which are attributed to the generation of reversible alloy-dealloy (RAD) reaction as follows:

and the RAD reaction is mainly responsible for lithium storage capacity. In the 2nd and 3rd cycles,the CV curves are almost stable suggesting that the electrochemical performance becomes steady with reversible RAD processes. By comparison,the reduction peak at 0.7 V in the first cycle shifts to 1.01 V in the 2nd and 3rd scanning cycles,which reveals the irreversibility of the reaction during the first cycle ^[34] with an initial irreversible capacity loss. Another peak at 1.25 V in all the cycles correspond to the oxidation of Sn to SnO or SnO₂,suggesting that the Reaction 1 (R1) is partially reversible. The electrochemistry mechanism of the as-obtained hollow SnO₂@C NPs is well consistent with that of SnO₂ anode materials ^[35].

Fig. 4b shows the 1st,2nd,100th,200th,300th and 400th cycles of charge-discharge curves of as-obtained hollow SnO₂@C NPs at a current density of 3910 mA g^-1 (5 C). The as-obtained SnO₂@C NPs exhibit extremely high discharge capacity with the value of 1860 mAh g^-1 in the 1st cycle,and the corresponding charge capacity is 1216 mAh g^-1,suggesting that the initial irreversible capacity loss is about 34.6%. The decomposition of tin dioxide Reaction 1 (R1) and other possible irreversible processes result in the initial capacity loss. This phenomenon is in good agreement with SnO₂-based anode materials. In the 100th,200th,300th and 400th cycles,the discharge/charge profiles show no significant degradation,which reveals that Li-insertion/extraction performance is quite stable and the discharge capacity is about 370 mA g^-1. Fig. 4c shows the cycle performance of hollow SnO₂ and SnO₂@C NPs at the current densities of 782 mA g^-1 (1 C) and 3910 mA g^-1 (5 C) between 3 V and 5 mV,respectively. Clearly, hollow SnO₂@C NPs demonstrate much higher capacity and better cycle stability than the hollow SnO₂ NPs. For hollow SnO₂@C NPs, after 573 cycles,the capacity is still up to 370 mAh g^-1 at 5 C. This stable cycling is credited to the accommodating ability of SnO₂ volume change during the charge-discharge process by the combining effect of nanosized SnO₂,unique hollow structure and the carbon coated hollow SnO₂ NPs surface. For a comparison, the hollow SnO₂ NPs demonstrates a high initial-capacity but it falls off rapidly with charge/discharge cycling at 1 C. Although the bare SnO₂ NPs consist of nanosized SnO₂ grains and have the unique hollow structure,it cannot completely restrain the irreversible capacity loss when there is a huge volume expansion during the charge-discharge process. The cycle performance of hollow SnO₂@C NPs at 1 C is shown in Fig. S3. The rate performances of as-prepared hollow SnO₂@C NPs at various charge and discharge rates (from 0.2 C to 5 C) are shown in Fig. 4d. With increasing current,the discharge capacity decreased slowly. The reversible capacity is retained about 700 mAh g^-1, even at high rates of 5 C. When the current returns to 0.2 C again,a high reversible capacity of ~1200 mAh g^-1 is recovered. Compare with previous literature reports,the as-prepared sub-100 nm hollow SnO₂@C NPs show significant enhancement in lithium storage capacity and cycle stability,because of the unique hollow structure and thin carbon layer coated on the surface of hollow SnO₂ NPs,which exhibited improved cycle performance compared to the literature ^{[36, 37, 38]}.

In summary,we prepared sub-100 nm hollow SnO₂@C nanoparticles in a large-scale via a facile hydrothermal approach. As an anode nanomaterial for lithium ion batteries,the as-obtained hollow SnO₂@C nanoparticles exhibit superior capacity and stability owing to their nanosize,unique hollow structure,and the carbon coating on the surface of hollow SnO₂ nanoparticles. The thin carbon coated layer and hollow structure effectively inhibit the volume change of SnO₂ during the alloy-dealloy process. The results reveal that the as-prepared sub-100 nm hollow SnO₂@C nanoparticle is a promising candidate to develop high-capacity anode materials of LIBs. The present synthesis strategy could also extend to the fabrication of other electrode materials systems.

We greatly acknowledge partial financial support by the Program for the NSFC (Nos. 51302325,51201115,51471121), New Century Excellent Talents in University (No. NCET-12-0553), Program for Shenghua Overseas Talent (No. 1681-7607030005) from Central South University,Hubei Provincial Natural Science Foundation (No. 2014CFB261),and the partial financial support from the Open-End Fund for the Valuable and Precision Instruments of Central South University (No. CSUZC2014032),Fundamental Research Funds for the Central Universities (No. 2042015kf0184) and Wuhan University.

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.05.051.