Ultrafine Sn nanoparticles embedded in shell of N-doped hollow carbon spheres as high rate anode for lithium-ion batteries

Ultrafine Sn nanoparticles embedded in shell of N-doped hollow carbon spheres as high rate anode for lithium-ion batteries

Applied Surface Science 404 (2017) 342–349 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 404 (2017) 342–349

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Ultrafine Sn nanoparticles embedded in shell of N-doped hollow carbon spheres as high rate anode for lithium-ion batteries Peng Dou a , Zhenzhen Cao a , Chao Wang a , Jiao Zheng a , Xinhua Xu a,b,∗ a b

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, PR China

a r t i c l e

i n f o

Article history: Received 13 December 2016 Received in revised form 22 January 2017 Accepted 24 January 2017 Available online 26 January 2017 Keywords: Metal-organic nanoparticles N-doped carbon Sn nanoparticles Hollow structure High rate

a b s t r a c t A novel reversible interaction in polymeric nanoparticles is used to induce hollow Sn4+ -MOPs. Then ultrafine Sn nanoparticles uniformly embedded in shell of N-doped hollow carbon spheres is successfully synthesized by pyrolysis of the Sn4+ -MOPs precursor. In this architecture, the N-doped carbon shells can effectively avoid the direct exposure of embedded Sn nanoparticles to the electrolyte and efficiently accommodate the volume change of Sn nanoparticles. Furthermore, the hollow structure of carbon sphere can prevent Sn nanoparticles aggregation over repeated cycling and shorten the diffusion path of both electrons and ions. As a consequence, this N-doped hollow Sn/C anode delivers a reversible capacity of 606 mA h g−1 at a current density of 0.2 A g−1 after 250 cycles and a reversible capacity of 221 mA h g−1 even at a much higher current density of 10 A g−1 , which are much better than those of pure Sn nanoparticles. The desirable cyclic stability and rate capability were attributed to the unique architecture that provided fast pathway for electron transport and simultaneously solved the major issues of Sn-based anodes, such as pulverization, aggregation and loss of electrical contact. © 2017 Published by Elsevier B.V.

1. Introduction With the development of electronic vehicle and intelligent electronic equipment in recent years, traditional commercial lithium-ion batteries (LIBs) can no longer fulfill the ever-growing demand for high energy and power density applications [1–3]. Anode material is the key component of LIBs, which determines the success or failure of full cells. Therefore, researches are fastened on developing new high performance anode materials with high capacity, high rate and long cycle life. Metallic tin (Sn) should be an attractive anode material for the next generation LIBs for its high theoretical capacity of 994 mA h g−1 , which is calculated from Li4.4 Sn and a relatively low discharge potential versus Li/Li+ [4–6]. Nevertheless, the Sn anode undergoes a rapid capacity loss and limited cycle life, mainly caused by its severe volume expansion during Li+ insertion/extraction process, and subsequent the pulverization of electrodes, uncontrolled formation of solid electrolyte interface (SEI), loss of electrical conductivity [7–10]. Especially, the Sn nanoparticles tend to aggregate upon cycling, resulting in poor capacity retention [8]. In order to address all these problems at

∗ Corresponding author at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (X. Xu). http://dx.doi.org/10.1016/j.apsusc.2017.01.253 0169-4332/© 2017 Published by Elsevier B.V.

once and maintain the integrity of electrode material, various Sn/C composites with different carbon matrices have been synthesized, presenting great improvements in electrochemical performance than that of pure Sn [8,10,11–18]. For instance, Noh and co-workers synthesized amorphous carbon-coated Sn particles as anode materials for LIBs by hydrothermal method, which could provide a stable capacity of 664 mA h g−1 at a current density of 400 mA g−1 [12]. This carbon coating layer not only acted as a buffer to accommodate the volume expansion, but also prevented the aggregation of Sn nanoparticles. It is reported that one of the most effective ways is to disperse the nanometer-sized Sn into a carbon matrix, like Xu’s group fabricated Sn nanoparticles uniformly dispersed in a spherical carbon matrix by a spray pyrolysis technique, showing a capacity of 710 mA h g−1 after 130 cycles at 0.2 A g−1 [17]. The significant performance can be attributed to the small particle size and continuous path for Li+ and electrons inside the nano-Sn/C composites. Moreover, another way to improve the electrochemical performance is to fabricate hollow Sn-based composites, since the internal void can alleviate the volume expansion during Li insertion/desertion, shorten the diffusion distance of Li+ and prevent the pulverization and aggregation of the electrode material [14,19–22]. At present, template method [23], galvanic replacement [22], Ostwald ripening [24], Kirkendall effect [25] and self-assembly [26] have been widely investigated to construct desirable hollow

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structure. Among them, the template method is believed to be the most versatile and most immediate way to control the morphology and size with high uniformity. In the past decades, metal organic frameworks (MOFs), which are composed of metal ions and organic ligands, have been widely used as effective templates to prepare various porous materials and have gained wide attention in many fields including energy storage, gas adsorption, sensors, drug delivery and catalysis [27–31]. Moreover, based on the large specific surface areas, high porosity and diversity of structures and functions, MOFs have been considered as sacrificial templates or precursors to construct novel electrodes for LIBs [32,33]. With the continuous development of MOFs, there is a recent surge in strategies to fabricate hybrid metal-organic coordination materials constructed from metal ions and organic ligands [34–37]. For instance, Li and colleagues developed a novel, simple and versatile approach to generate hollow metal-organic nanoparticles (MOPs) [37]. To make full use of the reversible interactions and abundant nitrogen element in polymers, we provide a new idea to synthesis of N-doped hollow Sn/C composites for us. It is believed that nitrogendoped (N-doped) carbon materials are conducive to the insertion of Li ions and can enhance the electrical conductivity, resulting in higher capacities and better cycling stability in comparison with non-doped carbon materials [18,39]. In this study, we have successfully synthesized ultrafine Sn nanoparticles uniformly embedded in shell of N-doped hollow carbon spheres using a novel reversible interaction in polymeric nanoparticles followed by an in situ reduction and carbonizing Sn4+ -MOPs. The p-phenylenediamine of Sn4+ -MOPs provided nitrogen sources, so we got N-doped Sn/C composites easily. In contrast with previous methods that need mixing Sn with carbon source, this method make the Sn is uniformly distributed in the organic framework at molecular dimension. As a result, small Sn nanoparticles with a diameter of about 20 nm were dispersed in the shell of nitrogen-doped hollow carbon spheres uniformly. In this particular structure, the uniform dispersion of Sn nanoparticles in the carbon matrix can effectively avoid the direct exposure of embedded Sn nanoparticles to the electrolyte and efficiently accommodate the volume change of Sn nanoparticles. The hollow structure of carbon sphere can shorten the diffusion path for both electrons and ions, and prevent particle aggregation and accommodate the large volume change of Sn nanoparticles over repeated cycling, maintaining the structural integrity. Furthermore, N-doping can enhance the electrical conductivity of carbon materials. As a result, we believe the obtained N-doped hollow Sn/C electrode can enhance the electrochemical performance of LIBs effectively compared with that of pure Sn nanoparticles.

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2.2. Synthesis of N-doped hollow Sn/C composites Sn4+ -MOPs were heated at 300 ◦ C for 1.5 h in a muffle furnace and then the resultant solid was placed in a tubular furnace at 650 ◦ C for 2 h under argon and hydrogen atmosphere, and N-doped hollow Sn/C composites were finally got. Similarly, N-doped solid Sn/C composites were made by the same method. The N-doped carbon nanoparticles were synthesized by pyrolysis of BNPs in a tubular furnace at 650 ◦ C for 2 h under argon atmosphere. 2.3. Material characterization The crystalline structures of the products were characterized by X-ray powder diffraction (XRD) using an automated D/max-2500 diffractometer with monochromatic Cu K␣ radiation at room temperature. Raman spectra were carried out with a laser excitation at 532 nm. Morphologies and structure features of the samples were investigated by a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM) with energy dispersive spectrometer (EDS) and a high-resolution transmission electron microscope (HR-TEM, JEM-2100F) operated at 200 kV. 2.4. Electrochemical measurements Electrochemical measurements of the products were carried out by CR2032 coin-type test cells. The working electrodes were prepared by coating the slurry made by mixing the pure Sn, N-doped carbon nanoparticles or N-doped Sn/C nanoparticles, acetylene black, and polyvinylidene fluoride (8:1:1 by weight) in n-methyl pyrrolidinone onto Cu foil substrates, following by drying at 90 ◦ C for 12 h and pressing at 10 MPa. Li metal foil was utilized as the counter/reference electrode; a polypropylene membrane (Celgard 2400) was used as the separator, and 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) was used as the electrolyte. The cell assembly was performed in an Ar-filled glove box and each cell was aged for 24 h at room temperature before the electrochemical tests. Then the assembled cells were galvanostatically tested (discharging, charging, and cycling) within a voltage range of 0.01-2.00 V (vs. Li/Li+ ) at room temperature by a LAND battery system. Cyclic voltammetry (CV) was performed on a CHI 660E electrochemical workstation in the voltage range of 3.0-0.01 V (vs. Li/Li+ ) at a scan rate of 0.1 mV s−1 . Electrochemical impedance spectroscopy (EIS) was measured with a frequency range of from 1 × 106 to 1 Hz by using coin-type half batteries on the same electrochemical workstation. 3. Results and discussion

2. Experimental 2.1. Synthesis of Sn4+ -MOPs In a typical synthesis, 10 mL 3 mg/mL bisimido boronic acid (Im-BA) methanol solution and 10 mL 3 mg/mL bisimido catechol (Im-Ca) methanol solution were separately prepared with the help of heating. After cooling down to room temperature, 10 mL ImCa solution was added dropwise into 10 mL Im-BA solution, the mixture became deep orange and the suspension of BNP was got [38]. Then 10 mL methanolic solution of SnCl4 with the same concentration was added to the above suspension once. After 6 h of stirring, the resultant particles were washed with methanol, collected by centrifugation and dried in vacuum to afford Sn4+ -MOPs. The preparations of Im-BA and Im-Ca were described in Supporting Information (Fig. S1).

The overall synthetic procedures leading to uniformly embedding ultrafine Sn nanoparticles in shell of N-doped hollow carbon spheres are illustrated in Fig. 1. In briefly, BNPs were firstly synthesized through condensation-driven cooperative polymerization of Im-BA molecule and Im-Ca molecule in methanol, and then used as precursor for preparing hollow Sn4+ -MOPs. During the polymerization, the dative bond between the nitrogen in the imine building blocks and the boron in the boronate ester makes the N-E polymerization happen, so we got BNPs with solid sphere structure (Fig. S5). After that, due to the boronate ester bond of BNPs can be rendered reversible under some conditions. So when adding of Sn4+ , the metal ions could form stable coordination complexes with Im-Ca molecules, meanwhile the depolymerization reaction of BNP also occurs. Of particular interest, Owing to the diffusion rate of Sn4+ into the interior of the BNP is slower than outwards diffusion of the boronate ester polymer, so the as-prepared Sn4+ -MOPs have a hollow structure (Fig. 1c). If increasing the concentration of Sn4+ , the diffusion rate of Sn4+ into the interior of the BNP will

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Fig. 1. Schematic illustration of the synthesis of BNP, Sn4+ -MOP, and N-doped hollow Sn/C composites.

be faster than outwards diffusion of the boronate ester polymer, so the as-prepared Sn4+ -MOPs can also be a completely solid structure. Finally, the N-doped Sn/C composites were prepared by in-situ reducing as-prepared Sn4+ -MOPs. The Sn4+ -MOPs are heated in air to form tin dioxide (SnO2 ) nanocrystallites, which are dispersed in the precursors (XRD pattern is displayed in Fig. S7). The resultant solid containing SnO2 is then reduced and carbonized to generate N-doped Sn/C composites and Sn should be homogeneously distributed in the N-doped carbon spheres. First of all, the structure of the composites is studied carefully with XRD and Raman spectrum. The XRD patterns of the N-doped Sn/C composites compared with Sn nanoparticles are shown in Fig. 2a. All diffraction peaks can be easily corresponded with crystalline Sn (JCPDS No. 04-0673) and peaks associated with SnO2

are disappeared after heating to 650 ◦ C, demonstrating SnO2 has been reduced to Sn completely. The diffraction peaks at 30.6◦ , 32.0◦ , 43.9◦ , 44.9◦ , 55.3◦ , 62.5◦ , 63.8◦ , 64.6◦ , 72.4◦ , 73.2◦ and 79.5◦ can be well-ascribed to the (200), (101), (220), (211), (301), (112), (400), (321), (420), (411) and (312) planes of Sn with tetragonal structure, respectively. Besides, no peaks belonging to carbon are detected, showing the amorphous nature of carbon [40]. This is also supported by the Raman spectrum of the N-doped hollow Sn/C sample (Fig. 2b), in which two broad peaks located at 1371 and 1597 cm−1 can be assigned to typical D and G bands of amorphous carbon, respectively [41]. In general, amorphous carbon possesses larger interlayer space, abundant defects and vacancies, which is not only conducive to the diffusion of lithium ions, but also offers much more active sites to help enhance the overall capacity [11,42].

Fig. 2. (a) XRD patterns of N-doped hollow Sn/C composites and pure Sn nanoparticles; (b) Raman spectrum of N-doped hollow Sn/C composites.

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Fig. 3. (a) SEM image of Sn4+ -MOPs; (b) SEM image of after heating to 300 ◦ C; (c) SEM image of N-doped hollow Sn/C composites; (d) EDS spectrum of N-doped hollow Sn/C composites.

In order to reveal the microstructure of the N-doped Sn/C composites, SEM and TEM are utilized to describe the detailed morphology and structure. Fig. 3a shows the SEM image of Sn4+ -MOPs, from which it can be seen that the sample is sphere-shaped, with an average diameter of about 450 nm and a smooth surface, the same with those of BNPs, demonstrating the addition of Sn4+ has no effect on the particle size and surface morphology of BNPs. However, it is worth noting that a broken nanoparticle can be observed in Fig. 3a, which confirms the successful synthesis of hollow structures. After heat treatment process, the surfaces become rough, however, the particles are still in monodisperse and the size of particles is maintained at 450 nm (Fig. 3b and c). Fig. 3d shows the EDS spectrum of the as-prepared sample, showing the weight percentage of C, N and Sn in N-doped hollow Sn/C composites is 41.85, 12.41 and 45.74%, respectively. The TEM image of hollow N-doped Sn/C composites (Fig. 4a) confirms the Sn nanoparticles with a typical size of about 20 nm (black spots) are uniformly dispersed in the hollow carbon matrix (gray matrix), whose shell thickness is 35 nm. What we should pay special attention to is that the uniform Sn nanoparticles do not aggregate at their formation temperature, benefiting from its inhibition by the N-doped carbon matrix [43]. The marked lattice spacings of 0.292 and 0.280 nm in the high-resolution TEM image

(Fig. 4b) correspond well to the (200) and (101) planes of tetragonal Sn, respectively, being consistent with the XRD results. To further distinguish the structural feature, the EDS mapping study is carried out and presented in Fig. 4c–e. From the mappings, it is found that Sn, C and N are detected and Sn and N are homogeneously dispersed in the carbon matrix as expected, which gives direct proof for the uniform distribution of Sn in the N-doped carbon matrix. It is believed that N-doping can increase the electrical conductivity of carbon materials and thus enhance the electrochemical performance [39]. Also, the abundant defects and vacancies induced by N-doping promote more active sits for lithium storage, contributing to enhanced cycling performance [45]. On the other hand, the hollow carbon matrix can not only alleviate the volume expansion of Sn nanoparticles, but also prevent the agglomeration of them during cycling. Hence, we believe that the N-doped hollow Sn/C composites can enhance the electrochemical performance of Sn anode significantly. For comparison, TEM images of N-doped solid Sn/C composites are shown in Fig. S9. From the images, we can see that the distribution of Sn in solid carbon spheres is different from that in hollow spheres. Uneven distribution of Sn in solid carbon spheres is due to high concentration of Sn4+ in MOPs, which leads to agglomeration or precipitation of Sn during calcination process. The agglomeration of Sn may not be good for lithium ion inser-

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Fig. 4. TEM image and EDS mapping of N-doped hollow Sn/C composites.

tion and also will bring about some other problems, such as serious stress concentration. Precipitated Sn nanoparticles are more likely to pulverization upon cycling, resulting in poor capacity retention. The electrochemical properties of the N-doped hollow Sn/C composites have been investigated using coin-type cells, in which Li metal foil was utilized as the counter electrode. The cyclic voltammogram (CV) curves of the N-doped hollow Sn/C composites of the first three cycles at a scan rate of 0.1 mV s−1 in the voltage range of 0.01–3 V are shown in Fig. 5a. During the cathodic sweep, a broad reduction peak located at 1.15 V is observed, corresponding to the decomposition of the electrolyte on the electrode surface to form a SEI film [10]. Three small reduction peaks centered at 0.36, 0.53 and 0.61 V can be assigned to the formation of Lix Sn alloy [45]. In the anodic sweep, four oxidation peaks at 0.56, 0.64, 0.73 and 0.8 V can be found, being originated from the delithiation reation

of Lix Sn alloy to Sn [18]. The other broad anodic peak at 1.12 V represents lithium extraction from carbon [16]. The stability and repetition of all peaks, except the broad reduction peak at 1.15 V, implies the strong reversibility of the electrochemical reactions and good cycling stability of the N-doped hollow Sn/C electrode. The CV curves of the N-doped C nanoparticles are shown in Fig. S9. The CV curves are in good agreement with the previous reported amorphous carbon anodes. In the cathodic sweep, a broad reduction peak around 0.01-0.3 V can be ascribed to Li+ insertion into the microcrystalline graphite, vacancies and defects of the amorphous N-doped C nanoparticles. Subsequently, during the anodic sweep, two broad oxidation peaks at 0.2 and 1.2 V represent Li+ extraction from the N-doped C nanoparticles [12,15,48]. Fig. 5b shows the voltage profiles of N-doped hollow Sn/C electrode in a voltage range of 2.0-0.01 V at a current density of 0.2 A g−1 . Unless otherwise stated,

Fig. 5. (a) Cyclic voltammograms of the initial 3 cycles scanned between 0.01–3 V at a rate of 0.1 mV s−1 ; (b) Discharge-charge curves of N-doped hollow Sn/C electrode.

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Fig. 6. (a) Cycling performance and (b) coulombic efficiencies of N-doped hollow Sn/C and pure Sn electrodes.

all the calculated specific capacity values reported in this paper are based on the mass of Sn/C composites. The initial discharge and charge capacities of N-doped hollow Sn/C electrode are 1165 and 917 mA h g−1 , respectively, corresponding to a coulombic efficiency of 79%. The high CE demonstrates that the uniform embedment of Sn in the carbon matrix can prevent the harmful reactions between Sn and electrolyte to a great extent, consistent with the CV result. What’s more, the reversible capacity is stabled at 600 mA h g−1 even after 250 cycles. The shrinkage of charge/discharge curves after the first cycle is slow, illustrating good cycling stability of the N-doped hollow Sn/C composite. For comparison, the pure Sn nanoparticle anode is also investigated under the same conditions on the basis of the mass of Sn nanoparticles, whose initial discharge and charge capacities are 864 and 426 mA h g−1 with a CE of 49.2% (Fig. S8). Additionally, the Sn anode undergoes a rapid capacity loss in the first three cycles caused by its structural defects and uncontrolled formation of the SEI layers upon cycling, demonstrating the effective protection of carbon matrix on Sn particles in the structure of hollow Sn/C composites. Fig. 6a displays the charge/discharge cycling performance of pure Sn, N-doped C nanoparticles, N-doped solid Sn/C composite, and N-doped hollow Sn/C composite electrodes at a current density of 200 mA g−1 . Compared with pure Sn and N-doped solid Sn/C anode, the N-doped hollow Sn/C anode exhibits significantly improved cycling stability as expected. In detail, the pure Sn anode exhibits rapid capacity fading, providing only 90 mA h g−1 after 100 cycles. The N-doped C nanoparticles deliver a stable capacity of 348 mA h g−1 after 250 cycles. The greatly enhanced capacity is mainly ascribed to the microcrystalline graphite and a large number of additional reversible Li+ storage sites such as vacancies and other defects [48,49]. For the N-doped solid Sn/C anode, the capacity first increases and then decreases as the cycle numbers increases. The gradual increase of capacity is perhaps an activation process of Sn core in carbon spheres. This phenomenon has also been observed for previous Sn/C composites [47]. After 100 cycles, the capacity gradually decreases to 515 mA h g−1 . It should be mainly ascribed to pulverization of precipitated Sn nanoparticles and solid carbon spheres (Fig. S10). However, the N-doped hollow Sn/C anode delivers a stable capacity of 585 mA h g−1 after 100 cycles, which is nearly 7 times higher than the value of pure Sn anode and 2 times higher than N-doped C nanoparticles. Moreover, the capacity of Ndoped hollow Sn/C anode remains 606 mA h g−1 at the 250th cycle, beyond the maximum capacity of 454.3 mA h g−1 (According to the theoretical capacity of Sn, 994 mA h g−1 ) that Sn in the N-doped hollow Sn/C composite can contribute to because the Sn content in the composite is 45.7%. C&N in the in the N-doped hollow Sn/C

composites can contributed 188.6 mA h g−1 , corresponding with 348 mA h (g (C))−1 . This result suggests that the Sn in the N-doped hollow Sn/C composites is very close to the theoretical capacity. On the other hand, as shown in Fig. 6b, it should be pointed out that the CE of the N-doped hollow Sn/C anode gradually increases to 99% by the 10th cycle and remains greater than 99% in subsequent cycles, demonstrating the integrity of the formed SEI film and excellent reversibility of N-doped hollow Sn/C anode. The superior cycling performance can be attributed to the following merits. First, the uniform dispersion of Sn nanoparticles in the carbon matrix makes the generated stress upon cycling balanced over the whole composite and electrode and thus prevents local cracking. Second, the hollow structure of carbon matrix can not only shorten the diffusion path for both electrons and ions, but also prevent particle aggregation and accommodate the large volume change of Sn nanoparticles over repeated cycling, maintaining the structural integrity. Finally, N-doping can enhance the electrical conductivity of carbon materials and thus enhance the electrochemical performance. Taking advantages of these unique features, the N-doped hollow Sn/C anode also displays superior rate capability at various current densities from 0.2 to 10 A g−1 (Fig. 7). Along with the increasing of the current density, the N-doped hollow Sn/C anode delivers specific reversible capacities of 617, 400, 343, 304 and 269 mA h g−1 at the increasing current densities of 0.2, 0.5, 1, 2, and 5 A g−1 every 10 cycles, respectively. Even at a high current density of 10 A g−1 , the capacity is maintained at 221 mA h g−1 . Moreover, the capac-

Fig. 7. Rate capabilities of N-doped hollow Sn/C, N-doped C nanoparticles and pure Sn electrodes.

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Fig. 8. EIS spectra of N-doped hollow Sn/C and pure Sn electrodes after 5 cycles.

ity can increase to 609 mA h g−1 when the current density return back to 0.2 A g−1 , indicating an excellent high rate performance. This result compares favorably with previously reported Sn/C composites [8,10,11–19,40–43]. By comparison, pure Sn anode shows fast capacity fading, whose capacity is almost 0 mA h g−1 at the current density of 5 and 10 A g−1 , respectively. Also, the capacity retention rate is quite low when the current density is reduced to 0.2 A g−1 . The excellent rate performance can be ascribed to the synergistic effects between the high electric conductivity offered by N-doped carbon matrix and the short diffusion path for both electrons and ions provided by the hollow carbon structure, thus facilitates fast reversible insertion/extraction of Li ions [44]. Hence, the N-doped hollow Sn/C anode is desirable for high power applications of rechargeable batteries. In order to better understand the reasons for the improvement of electrochemical performance and investigate the reaction kinetics of the N-doped hollow Sn/C composite electrodes, EIS measurements are conducted on pure Sn and N-doped hollow Sn/C anodes after 5 cycles at 0.2 A g−1 to ensure the stable formation of the SEI films. As shown in Fig. 8, each Nyquist curve consists of a sloping line in low frequency region and a semicircle in high frequency region, representing the Warburg diffusion impedance and interfacial impedances including SEI film and charge-transfer resistance, respectively [46]. According to the simulation results, the charge transfer resistances of N-doped hollow Sn/C anode and pure Sn anode were 127 and 268 , respectively. The semicircle diameter of N-doped hollow Sn/C anode is much smaller than that

of pure Sn, which could be attributed to the thin SEI film and low charge-transfer resistance, indicating the stability of SEI film and structural integrity for the N-doped hollow Sn/C electrode, consequently, enhancing the cycling life and cycling stability. Besides, in low frequency region, the N-doped hollow Sn/C anode has a larger slope, illustrating faster Li+ transport, giving strong evidence why the N-doped hollow Sn/C anode has a superior rate performance. Studies have shown that stable electrode structure is a necessary precondition for the maintenance of good electrochemical performance of the anode materials for LIBs. Hence, it can be concluded from above results that the uniform dispersion of small size Sn nanoparticles in N-doped hollow carbon matrix can not only address the problems of Sn-based anodes during cycling, such as volume expansion and aggregation, but also maintain the integrity of the electrode structure and form a stable SEI film. To better understand the outstanding electrochemical performance of N-doped hollow Sn/C composites, TEM analysis has been carried out to observe the morphology evolution after 250 cycles at 0.2 A g−1 , as shown in Fig. 9. In comparison to the original appearance of the N-doped hollow Sn/C composites (as shown in Fig. 4a), only a slight increase in wall thickness can be observed even after long time cycling. Notably, a smooth and uniform SEI film with average thickness of 17 nm is formed on the surface of N-doped hollow Sn/C composites, which we can see from Fig. 9b. Moreover, it is clearly seen that the hollow structure of the composites is sustained as a whole integration structure, suggesting that the hollow carbon matrix can effectively accommodate the volume change of Sn nanoparticles and prevent the particle aggregation during the Li+ insertion/deinsertion reactions. Thus, integrate structure and stable SEI film have been achieved, leading to an excellent cycling performance. 4. Conclusions In summary, a novel Sn/C composite of ultrafine Sn nanoparticles uniformly embedded in shell of N-doped hollow carbon spheres is successfully synthesized by reversible covalent reaction and pyrolysis of Sn4+ -MOPs precursor. This unique architecture can effectively prevent the pulverization, aggregation and loss of electrical contact of Sn nanoparticles during the cycling. Furthermore, the hollow structure of N-doped carbon sphere can effectively avoid the direct exposure of embedded Sn nanoparticles to the electrolyte and provide fast pathway for both electrons and ions transport. Benefiting from the synergistic effects among these advantages, the N-doped hollow Sn/C anode maintains a reversible capacity of 606 mA h g−1 at a current density of 0.2 A g−1 after 250 cycles. Even at a much higher current density of 10 A g−1 , a reversible capac-

Fig. 9. (a) TEM and (b) HRTEM images of N-doped hollow Sn/C composites after 250 cycles.

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