nitrogen doped carbon composite and its enhanced lithium storage capability

Materials Chemistry and Physics 201 (2017) 302e310

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Fabrication of porous Si/nitrogen doped carbon composite and its enhanced lithium storage capability Xiaosong Zhang a, Mengyi Huang a, Chaofan Yang a, Guoliang Dai b, Junjie Huang a, * a

College of Chemistry & Chemical Engineering, Shaoxing University, Shaoxing 312000, PR China Jiangsu Key Laboratory for Environment Functional Materials, School of Chemistry Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou 215009, PR China

b

h i g h l i g h t s
Si/nitrogen doped carbon composite with porous structure has been obtained.
NBO analysis shows a strong electronic interaction exists between Si and NDC.
Porous Si/NDC composite exhibits excellent Li storage performance.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 March 2017 Received in revised form 19 July 2017 Accepted 14 August 2017 Available online 29 August 2017

With the aim to stabilize the structure of Si then improve the lithium storage performance, in this study a facile way is applied to deposit a layer of nitrogen-doped-carbon (NDC) on porous Si surface by carbonizing melamine, and the porous Si is easily prepared by etching SiAl alloy with HCl solution. After NDC coating, the cycling performance has been improved from 585.3 mAh g1 for pure porous Si in the 100th cycle at 200 mA g1 to 1496 mAh g1 for Si/NDC composite, and the rate capability is also enhanced from 47.1 mAh g1 for pure porous Si at 3200 mA g1 to 499.6 mAh g1 for Si/NDC composite. TEM images show the coating layer helps to stabilize the porous structure of Si during cycling. XPS analysis indicate Si has chemically acted with N and C during the calcination step, and the theoretical calculation based on density function theory indicates these chemical interactions benefit for the electron transferring between Si and the coating layer of NDC, thus enhancing the rate capability. © 2017 Elsevier B.V. All rights reserved.

Keywords: Silicon Nitrogen doped carbon Anode Lithium ion battery

1. Introduction The high requirements of rechargeable batteries in electric vehicle and renewable energy storage have stimulated the development of lithium ion batteries with high energy density and high power performance. The commercialized anode materials, graphite based materials, are difficult to satisfy the high requirements in the new applications due to the low theoretical capacity (372 mAh g1) and poor rate performance [1e4]. Si can alloy with Li to form Li22Si5 with a theoretical capacity of 4200 mAh g1, which is ten times higher than that of graphite, therefore Si has been viewed as a promising candidate of graphite [5e7]. Except the high specific capacity, Si also has the advantages of low delithiation potential (~0.5 V versus Li/Liþ), environmental benignity, safety and natural

* Corresponding author. E-mail address: [email protected] (J. Huang). http://dx.doi.org/10.1016/j.matchemphys.2017.08.026 0254-0584/© 2017 Elsevier B.V. All rights reserved.

abundance [8e10]. However, volume changed greatly (around 300%) during the lithiation and delithiation process, which makes Si experience structure fracture and pulverization, even damages the mechanical integrity of electrode, resulting in a rapid capacity fading upon cycling [10e12]. Another issue that contributes to the fast anode degradation is the solid-electrolyte interface (SEI), which forms on the surface of Si during the first discharge process. Since silicon constantly cracks or pulverize almost in each cycle, so many new surfaces were produced, which needs continuous formation of SEI film by decomposing the liquid electrolyte, leading in the deterioration of liquid electrolyte and the thicker SEI layer in turn, thus the capacity dropped constantly [13e17]. Tremendous efforts have been devoted to deal with the capacity fading. One way is to decrease the size of Si down to the nano range [18e22]. Since nano-Si has a short distance for Li diffusing and facilitates to relax the strain, the electrochemical performance of rate capability and cycling stability can be improved greatly.

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However, the nanoscaled Si particles is easy to coaggregate, which is not only worsen its electrochemical performance, but also don't benefit to its application. Another way is to fabricate Si-based composite (such as Si-C and Si-Cu etc.) to prevent the pulverization of Si and improve the electronic conductivity [23e29]. But it is almost impossible for the implemented materials to endure the huge volume change, the fracture of Si’ structure is difficult to be avoided over charge and discharge cycling; therefore, most of the Si-based composites fail to perform a stable charge/discharge cycling. Recently, porous Si and porous Si-based composites have attracted more and more attentions [30e33]. Si based materials with porosity possess sufficient voids to absorb huge volume expansions and eliminate the internal stress, which will alleviate the pulverization, so the cycling stability can be enhanced. Moreover, the porous structure allows liquid electrolyte to wet Si in a greater degree, facilitating the fast transport of Liþ and improving the highrate performance. Liang prepared nano porous Si through the air oxidation demagnesiation of Mg2Si [30], the nano porous Si has a capacity of 3335 mAh g1. Wang applied the magnesiothermic reduction method to synthesize porous Si, which deliver a high capacity of 3105 mAh g1 [31]. Recently, Jiang introduced an easy to synthesize porous Si with micrometer size by etching Al-Si alloy with HCl solution [32], Al-Si alloy powder is widely used in powder metallurgy and casting industries, and the obtained porous Si also presented the excellent Li storage performance. With the aim to improve the cycling stability, nano Cu was deposited on the surface of the porous Si to stabilize the structure and increase the electron conductivity [34], and an improved charge and discharge performance was obtained. How to stabilize the structure of porous is a key issue for the practical application of porous Si prepared by the method of etching Al-Si alloy. Nitrogen doped carbon (NDC) has been reported to have a good electronic conductivity and a strong interaction with Li ions, which making NDC to be a promising material to modify the electrode materials like Li4Ti5O12 [35,36], LiFePO4 [37,38], SiOx [39] and Si [40,41] to improve their electrochemical performance. Fabricating NDC on the Si surface is helpful to increase the electronic conductivity, stabilize the structure of Si and form a stable SEI film due to the core/shell structure, therefore the higher coulombic efficiency and better cycle stability can be obtained. M.G. Jeong used ethylenediamine as the source of NDC to coat the surface of porous Si [40], L. Shi applied gelatin as the source of NDC to form a porous NDC to load Si on the porous structure [41], and an improved electrochemical performance of Si was obtained by them. But, ethylenediamine is a flammable and poisonous compound, and the method of using gelatin as the source of NDC is complicated, which are not suitable to be used in large-scale production. In this study, a facial way was introduced to fabricate NDC on the surface of porous Si by pyrolysis of melamine, in which the porous Si is prepared through an easy and economy route of acid-etching Al-Si alloy powder. Si/NDC composite presents an excellent cycling performance and rate performance even with high Si loading of 1 mg cm2. Here, a half-cell had been studied as a demonstration. 2. Experimental 2.1. Preparation of porous Si Firstly, a 5% (wt %) dispersion was obtained by dispersing Al-Si (80:20 in weight) alloy powder (Changsha Tianjiu metal material company, China) in water. Then diluted HCl (2 mol L1) solution was dropped into the dispersion under stirring for two days, and the amount of HCl in molar was added with the amount of about 4.5 times Al. Finally, the dispersion was filtered and washed by water to remove Al completely, and the obtained powder was dried

303

at 60  C in vacuum. 2.2. Preparation of porous Si/NDC composite 150 mg porous Si powder and 300 mg melamine were mixed well in agate mortar by grinding. Then the mixture was wrapped by Cu foil, and this package was calcinated at 550  C for 5 h under N2 atmosphere with a ramp of 10  C min1. When the temperature cooled down to room temperature, an earthy yellow powder can be obtained, which is the porous Si/NDC composite. 2.3. Structural characterizations The crystal structures of porous Si and Si/NDC composite were analyzed by XRD (X-ray diffraction) on the equipment of Bruker D8 advanced diffractometer with Cu Kɑ radiation. The morphologies were characterized by SEM (JEOL JSM-6360LV) and TEM (JEOL JEM1011) and the elemental distributions were characterized by EDS (energy dispersive X-ray spectrometry) (JEOL JSM-6360LV). The structure information was also analyzed by Raman spectra (Horiba HR Evolution with a laser of 632.8 nm). XPS (AXIS Ultra DLD) was conducted to characterize the surface composite and the elemental valence. The specific surface areas of Si and Si/NDC composite particles were measured with the Brunauere Emmete Teller (BET) method by nitrogen adsorption isotherms collected at 77 K using a Tristar ǁ 3020 analyzer. Element analysis equipment (EA3000, Euro Vector Company) was applied to test the contents of carbon and nitrogen in samples. 2.4. Electrochemical measurements The slurry was made by mixing the active material (Porous Si or porous Si/NDC), conductive carbon and binder with a ratio of 5:3:2 in weight, the binder is a mixture of SBR (styrene butadiene rubber) and CMC with a ratio of 50:50 in weight. Then, the slurry was spread on the surface of copper foil and dried at 90  C for 10 h under vacuum. The electrode was cut in a circle shape with a diameter of 14 mm, and the loading of Si on the circle electrode is about 1 mg. The test coin cell was assembled in an argon glove box (Lab2000, Etelux, China), which consisted of Si working electrode, counter electrode of a metal lithium foil and a separator of Celgard 2300 membrane. The electrolyte was 1 M LiPF6 in EC/DMC (1:1, v/v) with additive 15% FEC (BASF, America). Galvanostatic charge/ discharge processes were carried out by the battery tester (LANDCT2001, Wuhan Land electro-chemical equipment company, China), and the cut-off voltages were between 1.5 and 0.005 V. Electrochemical impedance spectroscopy was conducted on the electrochemical equipment (Zahner Ennium, Germany). The cyclic voltammogram (CV) test was performed on CHI600B (ShangHai, China) electrochemistry workstation between 0.01 and 1.60 V vs. Liþ/Li. All the tests were conducted under 25  C. 3. Results and discussion Fig. 1 shows the SEM images of SiAl, Si and Si/NDC. SiAl alloy with spherical shape can be observed clearly (Fig. 1a) and the surfaces of the SiAl alloy spheres are smooth (Fig. 1b). After SiAl alloy was etched by HCl, the obtained Si presents a spherical morphology like SiAl alloy (Fig. 1c). Different from SiAl alloy spheres, many pores were formed in Si spheres because of the removal of aluminum from SiAl alloy as shown in Fig. 1d. When the porous Si was mixed with melamine then calcinated at high temperature, the pore on the surface of Si sphere can't be seen clearly (Fig. 1f), which is caused by NDC formation on the surface of Si. Moreover, many small NDC chips were formed among Si spheres

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Fig. 1. SEM images of SiAl alloy (a, b), Si (c, d), Si/NDC (e, f).

(Fig. 1d), which benefit to the transfer of electron and alleviate the internal strain produced by the volume expansion. The fabrication process of Si/NDC was illustrated in Fig. 2. In the first step aluminum was removed from SiAl alloy by HCl etching, leaving many pores in Si spheres; in the second step, melamine will gasify as temperature increasing, and some of melamine gas can be absorbed into the pores of Si or exist among Si spheres, finally they were transferred into nitrogen doped carbon, resulting in Si/NDC composite. Fig. 3 shows the TEM images of Si and Si/NDC (Fig. 3a and b), Si

Fig. 2. Schematic illustration of Si/NDC fabrication process.

and Si/NDC are constructed by many intertwined nano fibers with diameters lower than 50 nm. Different from Si, the Si fibers in Si/ NDC composite are coated by a layer of NDC marked by the red lines. Fig. 3c is the elemental mapping images of Si, O, C and N in Si/ NDC sample, it can be observed clearly that Si located in the core of the fiber, and the coating layer is only consisted with the elements of C, O and N, indicating Si fibers are coated by NDC, which is agreeable with the result of SEM. Meanwhile, the average thickness of NDC layer is about 10 nm. The formation of NDC in porous Si may block some small pores, resulting in the smaller pore volume, which was proved by the results of N2 adsorption/desorption isotherms. Fig. 4a displays the N2 adsorption/desorption isotherms and corresponding pore size distributions of Si and Si/NDC samples. The curves of Si and Si/NDC are typical of type IV. In the relative pressure range of 0.6e1.0 a clear hysteresis loop can be found, and a rapid capillary condensation step occurred in the region of P/ Po > 0.9 in the adsorption branches of the isotherm, indicating the presence of mesopore [42e46]. Estimated from adsorption data using the Barrett-Joyner-Halenda (BJH) method, there exist two types of pores with the sizes of about 40 and 130 nm in Si and Si/ NDC samples, and the pore with the size of about 8 nm existed in porous Si sample can't be found in porous Si/NDC composite, which is possibly due to some small pores in Si were blocked by NDC. Accordingly, the pore volume decreased from 0.295 cm3 g1 of Si to

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Fig. 3. TEM images of Si (a) and Si/NDC (b), EDS elemental mapping of Si, O, C and N (c).

0.136 cm3 g1 of Si/NDC composite, and the specific surface areas decreased from 91.92 m2 g1 of Si to 45.39 m2 g1 of Si/NDC composite. The coating of NDC on the surface of Si fibers is helpful to stabilize the structure of Si during the charging and discharging process. The XRD patterns of Si and Si/NDC are shown in Fig. 4b. Si/NDC exhibits almost the same curve as Si with the peaks located at 28.4 , 47, 56 , 69 and 76 , which can be indexed to (111), (220), (311), (400) and (331) planes of crystalline silicon (JCPDS 27-1402) [47]. Almost the same diffraction peaks in porous Si and Si/NDC implies the crystal structure of Si was not influenced by the heat-treatment and the carbonization of melamine. Different from Si, a broad peak appeared at 20.5e26.5 can be observed in Si/NDC as marked by blue line, which is related to amorphous carbon formed by the carbonization of melamine [48]. Raman spectra of porous Si and Si/ NDC samples were illustrated in Fig. 4c. The Raman peaks at 479 and 517 cm1 is the characteristic peak of Si in Si and Si/NDC samples [48e50]. Different from Si sample, two new peaks at 1350 and 1580 cm1 can be observed in Si/NDC sample, which are indexed to the D band and the G band of carbon, respectively, indicating carbon has been fabricated in porous Si. The XPS spectra of Si and Si/NDC are shown in Fig. 5. Three sharp peaks at 103.0, 155.0 and 533.0 eV can be observed clearly in Si and Si/NDC samples, which can be indexed to Si2p, Si2s and O1s [39,41], respectively, indicating the existence of Si and O. Another two peaks at 285.0 eV and 400.0 eV can be found in Si/NDC sample but not in Si sample, the first peak can be assigned to C1s and the second peak is related to N1s, which is from NDC. The C1s and N1s

peaks were deconvoluted into Gaussian-Lorenzian shapes. The C1s peak was deconvoluted into two peaks centered at 284.5 and 285.4 eV, the sharp peak at 284.5 eV is typically indexed to graphitic C¼C in carbon matrix, and the peak at 285.4 eV is assigned to the C atoms with sp2 hybrid bonded to N inside the aromatic structure [51,52]. And the N1s peak was deconvoluted into three peaks centered at 398.3, 400.5 and 401.4 eV, which are attributed to pyridinic N, pyrrolic N and quaternary N respectively [39,53]. Pyrrolic N is located in the five-membered ring and contributes two electrons to the p system. Pyridinic N is the N atom substituting the carbon atom in the C6 ring and bonds with two sp2 carbon atoms. Quaternary N is graphitic N, which is in the graphitic carbon plane and bonds with three sp2 carbon atoms. The doping of nitrogen in carbon structure has been reported to provide more active sites for Li storage and facilitates the transfer of Li ions and electrons in the electrode [54]. The high-resolution spectrum of Si2p is shown in Fig. 5b. There exist elemental Si at 99.2 eV and oxidized Si at 103.3 eV in both Si and Si/NDC samples [55]. As shown in Table 1, the peak area ratios of elemental Si to oxidized Si is 54.9:45.1 in Si sample and 17.3:82.7 in Si/NDC sample, the higher peak area ratio in Si/NDC demonstrates a higher content of oxidized Si in the surface layer due to the chemical reaction between Si and C, N in NDC structure. To better understand the electron interaction between Si and NDC, density function theory calculations (DFT) were conducted by using B3LYP/ 6-311þþg** level of theory embedded in the Gaussian 09 program, and C3N4 was used as a model compound of nitrogen doped carbon. The C3N4 sheet model composed of 18 carbon, 30 nitrogen and 18

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Fig. 5. XPS spectra of Si and Si/NDC (a), the corresponding expansion of the Si 2p (b) in Si and Si/NDC, C1s (c) and N1s (d) peaks in Si/NDC.

Table 1 The ratio of Si and Si-O in Si and Si/NDC composite, respectively, calculated from Si 2p spectra.

Fig. 4. N2 adsorption-desorption isotherms and pore size distributions (the inset) (a), XRD patterns of Si and Si/NDC(b), and Raman spectra of Si and Si/NDC (c).

hydrogen atoms (Fig. 6). The calculated bond lengths of Si-doped sheet, denoted by Si1-C4, Si1-N2, and Si3-N3, were estimated to be 1.9098, 1.9826, and 1.9826 Å, respectively. The calculated adsorption energy of Si atom over the vacancy site of C3N4 is 207.6 kJ mol1, indicating a strong interaction between Si and C3N4. The calculated electronic structures of the molecular orbits are shown in Fig. 6c and d. The highest occupied molecular orbits (HOMO) of Si-doped C3N4 are mainly located on the Si atom and the atoms of carbon and nitrogen bonded with Si atom, while the

Samples

Si (%)

Oxidized Si (%)

Si Si/NDC

54.9 17.3

45.1 82.7

electron clouds of the lowest unoccupied molecular orbits (LUMO) for the compound is mainly located on the C3N4 substrate, indicates substantial charge transfer between the dopant atom and surface moiety when molecules are excited. To further analyze the intermolecular interactions, the geometry of Si-doped C3N4 obtained from B3LYP/6-311þþG** was used to perform NBO analysis. The result shows that about 1.076e is transferred from Si atom to its adjacent atoms due to the electronegativity difference between Si and the atoms of C and N, leading to the strong bonding between Si atom and its neighbors. The theoretical calculation results based on Si and C3N4 is very useful to understand the electron interaction between Si and nitrogen doped carbon compound, implying there exists a strong electron interaction between Si and NDC. The strong electron interaction facilitates electron transferring in Si/NDC composite, resulting in a decrease in impedance, which is agreeable with the EIS results as shown in Fig. 7d. Fig. 7 (a, b) shows the cyclic voltammogram curves of Si and Si/ NDC. It can be observed that two peaks at about 1.1 V and 0.6 V appear only in the first cathodic scanning cycle, the first peak is attributed to the formation of SEI film, and the second peak is due to the reduction of the oxidized Si produced in the fabrication process, which are agreeable with literature results [56e60]. The cathodic peak below 0.1 V is related to the Li alloying process from crystalline Si into amorphous LixSi [61], and the cathodic peak at about 0.2 V appeared from the second cycle corresponds to the lithiation process of amorphous Si [62,63]. The anodic peaks at about 0.35 V and 0.52 V can be attributed to the phase transition from amorphous LixSi to amorphous Si [48,64]. As can be seen, all the currents of anodic peaks become bigger as the cycles increasing, which is due to the activation of Si [47]. When the potential is higher than 0.7 V, the gap between cathodic current and anodic current in Si/NDC electrode is bigger than that in Si electrode, which is possibly due to the Li storage ability of nitrogen doped carbon, resulting in an extra capacity.

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Fig. 6. The Optimized structures of C3N4 (a) and Si-doped C3N4 (b) (distances in angstroms), and frontier molecular orbitals for Si-doped C3N4 calculated at the B3LYP/6-311þþG** level of theory, HOMO (c) and LUMO (d).

The charge and discharge curves of Si and Si/NDC electrodes for the first two cycles at the current density of 200 mA g1 are shown in Fig. 7b. A long flat plateau below 0.1 V in the first discharge (lithiation) curve corresponds to Li-alloying process, in which crystalline Si was transferred to amorphous LixSi [65]. After that, all curves presented the characteristic charge and discharge behavior of amorphous Si. Porous Si delivers a capacity of 3666.9 and 2374.8 mAh g1 for the first discharge and charge with a Coulombic efficiency of 64.8%, while that for Si/NDC electrode are 3012.5, 2181.2 mAh g1 and 72.4% respectively. Because the content of nitrogen doped carbon is 9.81% tested by element analysis equipment, the real capacities of Si in Si/NDC for the first discharge and charge are about 3303.2.1 and 2376.1 mAh g1 respectively, this charge capacity is almost the same as that of pure Si electrode, and the lower discharge capacity compared to Si sample is due to NDC, which helps to inhibit the decomposition of electrolyte on Si, resulting in a smaller irreversible discharge capacity in the first cycle, therefore the initial columbic efficiency of Si/NDC (72.4%) is higher than that of Si (64.8%). The irreversible capacity loss is mainly due to the formation of SEI film, the decomposition of electrolyte and the reduction of the oxidized Si [58]. The 2nd charge curve for Si/NDC is almost the same as the 1st charge curve, indicating an excellent structure stability of Si/NDC. Fig. 7d is the cycling performance of Si and Si/NDC electrodes at the current density of 200 mA g1. The porous Si electrode presents the highest initial capacity of 2374.8 mAh g1. However, the capacity drops quickly and its capacity retention at the 100th cycle is only 24.6% (585.3 mAh g1). The capacity loss is mainly due to the pulverization of Si particles and electric disconnection of the material resulting from the large volume change during the cycling [65]. Under the same condition, porous Si/NDC displays a high

electrochemical reversibility with the capacity retention of 68.6% (1496 mAh g1), and all columbic efficiencies are approaching 99%. It means that NDC coating is very useful to maintain the cycling stability. Compared with other Si/C composites containing nitrogen, the porous Si/NDC composite in this study showed a superior electrochemical performance. Si/nitrogen-rich porous carbon composite prepared by the co-assembly of gelatin presents a specific capacity of 1103 mAh g1 after 100 cycles at 100 mA g1 [41], and the N-containing core-shell Si/C nanocomposite synthesized by carbonization of polyaniline delivers the specific capacity of 795 mAh g1 after 50 cycles at 100 mA g1 [66]. The good cycling performance of porous Si/NDC is ascribed to the porous structure of Si with large void space to buffer volume change, and the coating layer of NDC with the ability to stabilize the structure of Si. The rate capability of Si and Si/NDC electrodes were shown in Fig. 7c, which was conducted at the current densities from 200 mA g1 to 3200 mA g1. Both Si sample and Si/NDC sample exhibit a gradual decline of capacity as the charge/discharge current density increasing, which is mainly caused by the low electronic conductivity of Si. Si/NDC sample shows a superior rate performance in comparison with Si sample, at 3200 mA g1 the charge capacity of Si/NDC is 499.6 mAh g1, but that of Si sample is only 41.7 mAh g1. The improved rate capability of Si/NDC is mainly due to the coating layer of NDC, which facilitates the transfer of Li ions and electrons in the electrode [54], leading to the low electrochemical impedance. The electrochemical impedance spectra of Si and Si/NDC are illustrated in Fig. 7d. All plots present two typical features with the semicircle in the middle frequency region and the straight line in the low frequency. The semicircle is related to charge transfer resistance, and the straight line corresponds to the capacitive behavior of the electrodes. The semicircle diameter of Si/NDC is

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Fig. 7. CV of Si and Si/NDC from 0.01 to 1.60 V at a scanning rate of 0.1 mV s1 (a and b), The charge/discharge profiles of Si and Si/NDC electrodes for the first and second cycle at a current density of 200 mA g1 (c), Cycling performance of Si and Si/NDC electrodes at a current density of 200 mA g1 (d), Rate performance of Si and Si/NDC electrodes at various current densities (e), and Nyquist plots of Si and Si/NDC composite (f).

Fig. 8. TEM images of Si and Si/NDC after 50 cycles.

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smaller than that of Si, indicating the impedance for charge transfer in porous Si/NDC composite is smaller than that in porous Si sample, which is due to the coating layer of NDC has an excellent transferring behavior of Li ions and electrons. The low electrochemical impedance in Si/NDC benefits to the charge and discharge performance at high current density. The stability of porous structure is very important to the charge/ discharge performance of Si, with the aim to investigate the morphology change of porous Si and porous Si/NDC during cycling, the coin cells were disassembled in an argon-filled glove box after 50 cycles, and the electrodes were washed with DMC solvent to remove LiPF6. The morphologies of porous Si and porous Si/NDC after 50 cycles were shown in Fig. 8, the fiber morphology for Si and Si/NDC samples also can be distinguished, but the fiber diameters become bigger in comparison with that before charge and discharge cycles as shown in Fig. 1, which is mainly caused by the pulverization of Si due to the large volume change during cycling. After 50 cycles, almost no pore can be observed among the fibers in Si sample, different from that the pores in Si/NDC sample are kept well, and the fiber diameter in Si/NDC sample is much smaller than that in Si sample, which indicates NDC is helpful to stabilize the porous structure of Si. The improved stability of porous structure facilitates to the charge and discharge performance of Si/NDC. 4. Conclusions NDC was fabricated on the porous Si surface by pyrolysis of melamine, forming porous Si/NDC composite, which displayed an excellent cycling stability and rate capability. The improved charge/ discharge performance of porous Si/NDC composite is attributed to the porous structure and NDC, the porous structure can absorb the large volume expansion, and NDC shell facilitate to stabilize the porous structure, therefore, the structure destroy and pulverization of Si can be greatly alleviated. Moreover, NDC has a good electronic conductivity and a strong interaction with Li ions due to the N atom incorporated into carbon networks, resulting in low electrochemical impedance. NBO analysis based on the results of density function theory calculations indicates the strong electron interaction exists between Si and NDC, which facilitate the transfer of electron in Si/NDC composite, leading to a decrease in the electrochemical impedance. All the above-mentioned reasons contribute to the excellent chemical performance of porous Si/NDC composite.

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Acknowledgements Financial supports from the program of science technology bureau of Shaoxing (2014B70016), Zhejiang provincial key research project (2015C01002) and Jiangsu Overseas Visiting Scholar program for University Prominent Young and Middle-aged Teachers and Presidents are gratefully acknowledged.

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