C composite with dual interface as Li-ion battery anode material

Journal of Alloys and Compounds 802 (2019) 704e711

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Synthesis of SiOx/C composite with dual interface as Li-ion battery anode material Yanxia Liu a, b, *, 1, Jingjing Ruan a, 1, Fan Liu a, Yameng Fan a, Pu Wang a a

Zhengzhou Key Laboratory of Energy Storage Science and Technology, Zhengzhou Institute of Emerging Industrial Technology, Zhengzhou, 45000, China Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 February 2019 Received in revised form 13 May 2019 Accepted 5 June 2019 Available online 6 June 2019

A dual interface SiOx/C composite is prepared through a novel, facial, one-step route by redox reaction of organic carbon with silica precursor, using tetraethyl orthosilicate (TEOS) and sucrose as raw material, in which sucrose acts as both a reductant and coated carbon. HRTEM indicates that the composite has a dual interface structure with carbon coating layer and intermediate layer. Crystalline Si approximately 3 nme20 nm in size is dispersed in amorphous silicon oxide matrix. The SiOx/C is utilized in LIB as anode material and exhibits a high reversible specific capacity of 755 mA h g
1 after 300 cycles at 100 mA g
1 with capacity retention about 91%. Such outstanding cycling stability can be ascribed to the intermediate layer and carbon scaffold, which serve as a buffering to relieve volume change of produced Si upon cycles. © 2019 Published by Elsevier B.V.

Keywords: Silicon oxide anode Dual interface structure Carbon Lithium ion batteries

1. Introduction Compared with other types of secondary batteries, lithium-ion batteries show advantages of high operating voltage, high energy density, low memory effect, less environmental pollution, and low maintenance requirements [1e3]. However, subject to the specific capacity of conventional positive and negative active materials, currently quotient industrialized lithium-ion batteries can't meet the needs of higher energy density [4e6]. Silicon has been considered to be the most promising anode material for nextgeneration LIB, mainly due to its abundant reserves, relatively low discharge potential and high specific capacity [7e9]. However, Liþ embedded crystalline silicon exists in various alloy forms, such as Li12Si7, Li13Si14, Li7Si3, Li22Si5, etc, whose unit cell volume is larger than the unit cell volume of crystalline silicon [10,11]. During the lithium-ion insertion/extraction, Si experiences a large volume change more than 300%, causing the structure of active material to pulverize and deviate from the current collector to lose activity and repeated destructions/reconstructions of an unstable solid

* Corresponding author. Zhengzhou Key Laboratory of Energy Storage Science and Technology, Zhengzhou Institute of Emerging Industrial Technology, Zhengzhou, 45000, China. E-mail address: [email protected] (Y. Liu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.06.072 0925-8388/© 2019 Published by Elsevier B.V.

electrolyte interface (SEI) film, which results in poor cycling stability [12e15]. Researchers have made unremitting efforts to solve the problem of volume expansion and poor conductivity of silicon materials [16e18]. Studies have shown that silicon material nanocrystallization has achieved superior cycling stability and reversible capacity than bulk silicon materials [19,20]. Although nanomaterials have higher specific surface area, they can reduce the volume expansion and improve diffusion speed of Liþ in active material, excessively high specific surface also means that a large area of SEI film is formed in the first cycle, and nanometer materials are still easy to agglomeration [21e23]. Recently, SiOx has received special interest because of its high theoretical capacity, low price and less volume expansion [24]. As a result of the introduction of oxygen, inert substances Li2O and Li4SiO4 can be produced in the initial discharge. Li2O and Li4SiO4 are electrochemically inert materials, but they can relieve the volume change and improve the cycling performance of SiOx [25,26]. Although the volume change of SiOx during charge and discharge is smaller than Si, there is still a change of about 200%, and SiOx also has the problem of poor electronic conductivity [27]. Studies have shown that SiOx-C composite can significantly improve cycling performance and electronic conductivity of bare SiOx [28e30]. However, the commercial SiOx is produced by vacuum evaporation of Si and/or SiO2 with elevated temperature (over 1500  C), which is not easy to prepare in the laboratory. While, it is

Y. Liu et al. / Journal of Alloys and Compounds 802 (2019) 704e711

found that SiOx can be obtained by wet chemistry routes [31e40]. The silicon-containing precursor is obtained by a wet chemistry method using siloxanes as silicon source such as tetraethyl orthosilicate (TEOS), followed by heat treatment to prepare SiOx or SiOx/C composite. The structure, morphology and ratio of silicon to oxygen can be effectively regulated by selecting a suitable silicon source, carbon source, sol-gel process and templating agent. Wu et al. €ber synthesized SiOx/C with hierarchical structure by a modified Sto method, which presented a high capacity of 674.8 mA h g
1 over 100 cycles (capacity preservation 83.5%). However, the preparation process of this experimental was quite complex and tooks a long time to get the final product. Specifically, in order to get a hierarchical structure, the carbon sources should be added in a two-step way, in addition, it needed 10 h for TEOS to hydrolysis, 24 h for forming a gel precursor, 5 h for mixing dry gel with carbon source (excepting the time of gel drying) [32]. Similarly, Li et al. fabricated € ber method, using CTAB, graSiOx-C/GNPs composite also by a Sto phene nano-platelets, C2H5Si(OC2H5)3 and ammonium hydroxide as starting materials, which showed a stable capacity of about 630 mA h g
1 over 250 cycles. In the preparation process, besides long time for silicon source to hydrolysis, the carbonization temperature was as high as 1000  C [35]. Dai et al. prepared Si/SiOx/ SiO2 nanocomposite by wet chemistry routes, which possessed excellent cycling capacity (600 mA h g
1 over 350 cycles). This experiment needed to be completed in glove box and involved reflux, washing procedure etc [40]. Unfortunately, most of these experiment processes not only are very complicated, but also usually take 2e3 days to hydrolyze for TEOS and need higher temperature more than 1000  C or need hydrothermal reaction. In this study, a dual interface SiOx/C composite is designed through a facial, one-pot synthesis method assistant redox reaction of organic carbon with silica precursor, in which organic carbon acts as both a reductant and carbon source. It takes only 5 h to complete the experiment before calcination. A good coating carbon effect is achieved by adding organic carbon source during the preparation of silica precursor. Moreover, by adding acidic catalyst, we control the colloidal process of TEOS solution, which helps to enhance the ability of carbothermal reduction reaction during heat treatment, therefore, carbothermal reduction can occur at lower temperature. Structure and electrochemical performance of SiOx/C dual interface composite are studied. The as-prepared SiOx/C shows high capacity and outstanding cycling performance. 2. Experiment 2.1. Materials Tetraethyl orthosilicate (TEOS, 99.0 wt %) and sucrose (C12H22O11, analytical purity) were purchased in Tianjin Komiou Chemical Reagent Co., Ltd. Hydrochloric acid (HCl, 36.0e38 wt %) and hydrogen peroxide (H2O2, 30.0 wt %) were purchased in Sinopharm Chemical Reagent Co., Ltd. Ammonia (NH3$H2O, 25.0e28.0 wt %) was purchased in Xiqiao Science Co., Ltd. Ethanol (C2H5OH, 99.0 wt %) was purchased in Dongda Chemical Reagent Co., Ltd. Deionized water was obtained from Milli-Q Dierct 8 pure water equipment. 2.2. Synthesis The fabrication of dual interface SiOx/C composite was prepared with a sol-gel process, which was schematically illustrated in Fig. 1. First, 10 ml deionized water, 2 ml hydrochloric acid, 2 ml hydrogen peroxide and 2 g sucrose were sequentially added to a homogeneous mixture TEOS and ethanol (in a mass ratio of 4:5) under magnetic stirring for 3 h. Then, the temperature rose to 90  C under

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magnetic stirring until the excess solvent was completely volatilized to form a transparent gel. Finally, the gel was manually crushed and heat-treated at 900  C for 1 h under N2 atmosphere by a rate of 3  C/min. As comparison, SiOx/C excluding hydrogen peroxide (labeled as SiOx/C-1), equivalent amount of ammonia water substituted for hydrogen peroxide (labeled as SiOx/C-2), sucrose carbon (labeled as free C) and SiOx without sucrose carbon (labeled as single SiOx) were also obtained respectively with the same condition. 2.3. Material characterization X-ray diffraction (XRD, D8 Advance, Germany) was carried out by using a Cu Ka (l ¼ 1.54178 Å) in the 2q ranging of 5-80 . Fourier transform infrared spectroscope (FTIR, Bruker Tensor II, America) was collected from 400 to 4000 cm
1. The elemental composition of synthesized composite was determined with CarboneSulfur analyzer (EA, CS230, America) and NitrogeneOxygen analyzer (EA, LECO TC300, America). Morphology and microstructure of synthesized composites were examined by scanning electron microscope (SEM, Zeiss Auriga FIB SEM, Germany) and highresolution transmission electron microscopy (HR-TEM, FEI Talos F200S, America). The particle size distribution was measured by Malvern laser particle size analyzer (Mastersizer 3000, England). The amount of C in the SiOx/C was confirmed by thermogravimetric analysis (TGA2, Mettler Toledo, America) under air. The surface chemistry of SiOx/C was investigated by X-ray photoelectron spectroscope (XPS, AXIS Supra, England). 2.4. Electrochemical measurement The working electrode was fabricated via pasting slurries of 80 wt% active material, 10 wt% conductive material (acetylene black) and 10 wt% binder (poly acrylic acid) on a thin copper foil. After drying and pressing, the electrode was stamped into a disk with a diameter of 14 mm. Coin cell CR-2025 was finally assembled using Li foil as counter electrode and Celgard 2400 as separator in a glove box filled with argon. Electrolyte consisted of 1 M LiPF6 in a mixed solvent with EC/DEC/EMC ¼ 1:1:1 by volume. Charge/ discharge measurements were performed by Land CT 2100 A (Wuhan, China) battery-test system in the voltage of 0.005e2 V. Cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy (EIS) were conducted by an CHI660E (Chenhua, Shanghai, China). The voltage window for CV was 0.005e2.0 V at different scan rates. EIS measurements were performed with a signal amplitude of 5 mV at frequency from 106 to 10
2 Hz. 3. Results and discussion 3.1. Materials characterization XRD patterns of SiOx/C-1, SiOx/C-2 and SiOx/C composite are compared in Fig. 2(a). Three samples show similar XRD spectra and have no distinct diffraction peaks, indicating that the composites synthesized by this method are amorphous and have the same phase structure. The broad peak located in 20 e25 region corresponds to SiOx [41], also the main peak of 26 in the graphite type overlaps with SiOx peak. The small peak around 43 represents the amorphous carbon (JCPDS NO. 26-1081) [42]. FTIR spectra of synthesized SiOx/C, accompanying with that of SiOx/C-1 and SiOx/C-2, are exhibited in Fig. 2(b). Samples have same absorption peak on FTIR spectra. Peaks around 1100 cm
1, 800 cm
1 and 473 cm
1 are ascribed to three different structural vibrations: asymmetric stretching vibration (asymmetric SieOeSi bond stretching) and symmetric stretching vibration (SiO4 tetrahedral ring), bending

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Fig. 1. Schematic illustration for dual interface SiOx/C composite.

Fig. 2. (a) XRD and (b) FT-IR patterns of SiOx/C-1, SiOx/C-2 and SiOx/C.

vibration (SieOeSi bond angle deformation), which are represented to SiOx [26]. Peak around 1593 cm
1 stands for C]C vibration [43], meaning that sucrose has been carbonized during heat treatment. In order to confirm the atom ratio of O/Si in SiOx/C-1, SiOx/C-2 and SiOx/C composites, element analysis test is employed, The atomic ratios of oxygen to silicon in SiOx/C-1, SiOx/C-2 and SiOx/ C composites are 1.14, 0.89 and 0.78 respectively (Table S1). Afterwards, the particle size distributions of SiOx/C-1, SiOx/C-2 and SiOx/ C composites are also measured by Malvern laser particle size analyzer (Fig. S1), the D50 of SiOx/C-1, SiOx/C-2 and SiOx/C are about 28.5 mm, 24.5 mm, 20.5 mm respectively. Results of above characterizations indicate that the presence of NH3$H2O or H2O2 can't affect phase structure of obtained composites from the carbon reduction silica precursor. Therefore, we only perform structural characterization of TGA and XPS on SiOx/C. Fig. 3(a) displays TGA curve of SiOx/C performed under air atmosphere from 25 to 900  C at a heating rate of 5  C/min. In TGA curve, weight loss mainly occurs between 600  C and 750  C. According to amorphous C generally begins to react with air at 600  C to produce CO2, it can be calculated that mass fraction of carbon is about 12%, and mass fraction of SiOx in the composite is about 88%. XPS is utilized to confirm the chemical characteristics of Si in SiOx/ C. Full-range XPS survey, shown in Fig. 3(b), reveals that asprepared composite consists of C, Si and O elements. Highresolution spectra of C 1s spectrum in Fig. 3(c) displays three peaks situated at 284.7, 285.8 and 288.8 eV, which respectively

assign to CeC bond in graphite carbon, CeO bond and C]O bond [44]. Peak area of CeC bond is the largest among three peaks of C1s, which indicates that sucrose has a higher degree of carbonization during heat treatment, and this is good for providing good electrical conductivity. Fig. 3(d) shows the Gaussian fit of Si 2p spectrum. It can be seen that Si 2p peak can be split into five peaks, which correspond to different spins of Si element in p orbital under different oxidation states. Peaks at 103.8, 102.8, 102.1, 101.3 and 99.9, eV are referred to Si4þ, Si3þ, Si2þ, Siþ and Si0, respectively [45]. The results indicate that Si element is reduced to mixed valence state by carbothermal reduction reaction between SiO2 and amorphous C. The proportion of these valence states of Si are estimated to be 4.91%, 37.37%, 40.60%, 14.46% and 2.66%, respectively, according to the fitted peak areas of Fig. 3(d). With regard to lithium-ion battery electrode material, the microstructure of material has a very important influence on electrochemical performance. The synthesized comparative SiOx/C1, SiOx/C-2, SiOx/C composites are characterized by SEM and the results are shown in Fig. 4. In Fig. 4(a), it demonstrates that asprepared comparative SiOx/C-1 is composed of glass-like material with micron size from 1 mm to 5 mm. Most of particles are dense and have some wrinkles in surface. This is because SiOx and C are formed in situ during heat treatment. The two phases are uniformly dispersed at nanometer scale, and further densification occurs during heat treatment to form a dense particle. Fig. 4(b) is SEM of comparative SiOx/C-2. It can be seen that there is still a large piece

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Fig. 3. (a) TGA curve of SiOx/C and XPS spectra (b, c, d) of SiOx/C, (b) Survey spectrum, (c) C 1s, (d) Si 2p.

Fig. 4. SEM image of (a) SiOx/C-1, (b) SiOx/C-2, (c) SiOx/C and (d) EDS mapping images of the SiOx/C.

of glass-like, but unlike the structure of SiOx/C-1, the glass-like structure is covered with small agglomerated particles. This may be that during hydrolysis of TEOS, when NH3$H2O is added, a large amount of silica gel is formed instantaneously, which is harmful for

substances to be uniformly dispersed and affects subsequent carbothermal reduction process. SEM image of SiOx/C is exhibited in Fig. 4(c) and different magnifications from 5000X to 100000X are shown in Fig. S2. Compared with SiOx/C-2, its particles not only are

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well dispersed, but also are filled with pore channels. This is because the solution is always in a clear and transparent state during hydrolysis of TEOS. At the same time, due to the presence of hydrogen peroxide, it will decompose to generate a small amount of gas during heating process, which will further prevent agglomeration between particles. To further illuminate the homogeneity of SiOx/C, EDS mapping is performed. As represented in Fig. 4(d), the elemental mapping images display that Si, O and C have a relatively homogeneous distribution. In order to further acquaintance inner structure of SiOx/C, TEM and HRTEM characterizations are employed. Fig. 5(a) exhibits obvious comparison between C and SiOx, where the dark nuclei denote SiOx granules. SiOx granules are completely surrounded by amorphous carbon, indicating good contact between SiOx granules and C. Fig. 5(b)-5(d) are the local enlargement (marked white dotted circle) of Fig. 5(a). As can be observed from locally magnified image of edge, in Fig. 5(b), it shows that the surface of SiOx granules are covered with amorphous C with 3e15 nm thickness, which not only provides an overall conductive network, but also has the ability to ensure uniform expansion of all parts of the material and avoid direct contact between silicon material and electrolyte. From HRTEM image in Fig. 5(c), the thickness of middle layer is about 10 nme20 nm, and its microstructure is obviously different from C layer and SiOx layer. Scanning transmission electron microscopeenergy dispersive X-ray spectroscopy (STEM-EDAX) analyzer is also performed to confirm the composition of intermediate layer (marked orange crosses) for SiOx/C in Table S2. It can be seen that from region 1 to region 3, carbon content decreases sharply (from 15.51% to 6.00%), while silicon content increases sharply (from 44.41% to 57.32%) and oxygen content decreases slightly (from 40.08% to 36.68%). This maybe that part of sucrose change into carbon-coated, and the other part of sucrose undergoes a carbothermal reduction reaction during heat treatment process. While the reduction reaction is more likely to happen on the SiOx surface relative to interior, resulting in a different microstructure between

SiOx surface and internal. It is worth noting that lattice fringes with size 3-20 nm can be observed in amorphous SiOx region. As shown in Fig. 5(d) and Fig. S3, it is estimated that the interplanar spacing of these nano-crystallites are 0.313 nm and 0.198 nm, respectively, which correspond well to the crystalline planes of Si (111) and (220) (JCPDS NO. 77-2107) [46]. The appearance of nano-silicon microcrystalline regions confirms the formation of nano-silicon, which can well correspond to XPS test results. 3.2. Electrochemical performances Cycling stability tests are carried out on SiOx/C and comparative experiments SiOx/C-1, SiOx/C-2 composites using charging/discharging test in the potential interval of 0.005e2.0 V at 100 mA g
1 current density. In Fig. 6(a), cycling stability of SiOx/C-2 is the worst and its capacity is only 572 mA h g
1 after 100 cycles. This is because carbon and silica precursors are not completely mixed due to the addition of ammonia. It can also be seen from SEM that particle agglomeration on glass-like structure is serious, leading to an increased resistance in the Liþ deintercalation process. Cycling stability of SiOx/C-1 is improved compared with SiOx/C-2 and capacity maintains 640 mA h g
1 after 100 cycles. Since carbon source and silicon source are uniformly dispersed at molecular scale during hydrolysis of TEOS, which is favorable for promoting the in-situ carbothermal reduction reaction in heating treatment process to obtain a lower Si/O value. Cycling stability of SiOx/C cycle is the best, capacity maintains above 753 mA h g
1 after 100 cycles, showing excellent structural stability. The Li storage behavior of SiOx/C composite is analyzed by using CR-2025 coin cells. Fig. 6(b) shows CV curves of SiOx/C composite at 0.1 mV s
1 from 0.001 to 2 V. In the first cycle, reduction peaks around 1.6 and 0.75 V correspond to decomposition of electrolyte and formation of a solid electrolyte interface film [40,47]. Furthermore, these reduction peaks vanish in the following cycles, indicating the formation of stable SEI. Additionally, from the fourth

Fig. 5. (a) TEM image of SiOx/C. (b, c, d) the local enlargement (marked white dotted circle) of Fig. 5a. (b) HRTEM image of C-coated SiOx/C, (c) HRTEM image of the double layer interface structure, (d) HRTEM image of some crystalline silicon distributed in the SiOx matrix.

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Fig. 6. (a) Cycling stability comparison of SiOx/C-1, SiOx/C-2 and SiOx/C at 100 mA g
1; (b) Cyclic voltammogram curves of SiOx/C at 0.1 mV s
1; (c) Charge-discharge curves of SiOx/C at 100 mA g
1; (d) Rate capability of SiOx/C at various current densities; (e) Cycling stability comparison of single SiOx, free C and SiOx/C at 100 mA g
1; (f) The long-term cycling stability of SiOx/C.

cycle, reduction peak around 0.37 V and oxidation peak around 0.45 V represent process of the formation of LieSi alloy and the LieSi alloy to Si [48]. As can be seen from CV curves, intensity of peaks enhance slowly during scanning, as a result of the gradual activation process of material. To further understand the Li storage mechanism of SiOx/C material, CV measurements at various scan rates from 0.1 to 3 mV s
1 are also conducted and shown in Fig. S4. The nearly linear relationship between peak currents and square root of sweep rate indicates the b-value of ca. 0.5, which means charge storage behavior is controlled by diffusion - process [49e51]. Fig. 6(c) shows charge/discharge curves from 0.005 to 2.0 V vs Li/ Liþ at 100 mA g
1 for the 1st, 2 nd, 10 th 50 th, 100 th and 200 th cycles. Charge and discharge curves almost overlap after the first cycle, indicating that SiOx/C electrode has good reversibility. It exhibits an initial charge/discharge capacity of 830/1284 mA h g
1 (the capacity is in line with the basis of gross mass of composite). Initial irreversible capacity loss comes from the formation of SEI and the generation of inert Li2O and Li4SiO4. Meanwhile, coulombic efficiency of SiOx/C quickly reaches to 90% at the second cycle and continuously promotes to 98% after a few cycles. Rate capability of SiOx/C is conducted at different current densities from 100 to 1000 mA g
1. In Fig. 6(d), SiOx/C presents excellent stable rate capability with capacities of approximately 748, 692, 624, 506, 423 mA h g
1 at 100, 200, 400, 800 and 1000 mA g
1, respectively. When current density is recovered from 1000 mA g
1 to 100 mA g
1, the capacity rises again to 743 mA h g
1, which demonstrates that material has good structural stability and capacity recovery performance. Good rate capability benefits from carbon scaffold that enhances electric conductivity and dual interface structure that provides a good buffering. To study the effect of free carbon and single SiOx on capacity of SiOx/C, electrochemical performance of free carbon and single SiOx are also studied in this work. Unfortunately, Single SiOx possesses a very low capacity (only about 50 mA h g
1), which indicates that final product is inactive silica in the absence of sucrose and its contribution to capacitance is limited. Fig. 6(e) shows cycling stability

comparison of SiOx/C and free C at 100 mA g
1. Although free carbon from sucrose exhibits excellent cycling performance, its stable capacity is only around 200 mA h g
1. Compared with that of free carbon, both capacity and cycling stability of SiOx/C are improved obviously, which attribute to synergetic effect of SiOx, carbon, crystalline silicon and robust dual interface structure. The longterm cycling stability of SiOx/C is tested in the potential interval of 0.005e2.0 V at 100 mA g
1. In Fig. 6(f), SiOx/C can still deliver a high capacity of 755 mA h g
1 over 300 cycles with capacity maintenance of about 91%. The dual interface structure of SiOx/C is advantageous to adapt to volume change and maintain integral morphology during cycles. In this work, the mass loading of SiOx/C in the electrode is 0.95 mg cm
2. The loading mass of electrode is an important factor for practical application [52,53], therefore, we discuss the effect of loading mass (0.42 mg cm
2-4.31 mg cm
2) on rate performance and cycling stability of SiOx/C in Fig. S5, which indicates that high mass loading should be controlled between 2.33 and 3.2 mg cm
2. Table 1 displays the comparison of cycling performance of SiOx/C composites synthesized by wet chemistry routes. Fig. 7 shows EIS change of SiOx/C before and after charge/ discharge cycles. Each Nyquist plot presents a compressed semicircle at high-medium frequency region and a straight line at low frequency region. The former corresponds to parallel combination of capacitance with charge-transfer resistance (Rct) and/or resistance from SEI layer (RSEI), while the latter reflects Liþ diffusion resistance into electrode material [54]. Compared with depressed semicircle of SiOx/C before cycles, a downward trend is noticed for the follow-up curves. This decrease in resistance can be attributed to the activation of SiOx/C as well as the steady dual interface structure and carbon scaffold improve electronic/ionic conductivity during cycles. Fig. 8 depicts equivalent circuit model for further analysis resistance of SiOx/C, in which Re refers to electrolyte resistance. The values for RSEI, Rct and n (the constant phase element) are shown in Table 2. Obviously, the values of Rct reduce after cycles, and this phenomenon is related to the result of CV. In Fig. 6(b), the peaks of reduction and oxidation become enhance along with the increasing of cycle number, meaning gradually

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Table 1 Comparison of cycling performance of SiOx/C composites synthesized by wet chemistry routes. Materials

Current density

Specific capacity

Ref

meso-porous SiOx/rGO composite Hollow nanotubular SiOx templated by cellulose fibers Amorphous Si/SiOx/SiO2 nanocomposites sugar apple-shaped SiOx@C nanocomposite SiOx-C/GNPs composite SiOx@C composite nanorods carbon fiber-supported SiOx hierarchical structured SiOx/C composite Nano-sized SiOx/C composite This work

100 mA g
1 100 mA g
1 200 mA g
1 50 mA g
1 100 mA g
1 100 mA g
1 250 mA g
1 100 mA g
1 100 mA g
1 100 mA g
1

580 mAh g
1 after 200 cycles 940 mAh g
1 after 50 cycles 600 mAh g
1 after 350 cycles 630 mAh g
1 after 150 cycles 630 mAh g
1 after 250 cycles 720 mAh g
1 after 350 cycles 481 mAh g
1after 150 cycles 674.8 mAh g
1 after 100 cycles 800 mAh g
1 after 50 cycles 755 mAh g
1 after 300 cycles

33 37 40 34 35 39 38 32 36

Fig. 7. EIS of SiOx/C before cycle (black), after 50 cycles (red) and after 200 cycles (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

structure is successfully synthesized using a facial route by modified sol-gel method. By controlling the process of hydrolysis of TEOS, combined with uniformly mixing the carbon source and the silicon source, the ability of carbothermal reduction during heat treatment is enhanced greatly. Therefore, we can achieve dual interface structure with good coating effect through one-step method. It can be seen from HRTEM that SiOx/C has a dual interface structure with a carbon layer thickness of 3 nme15 nm and an intermediate layer thickness of about 10 nme20 nm. As active material, crystalline Si approximately 3 nme20 nm in size is dispersed into an amorphous SiOx matrix. Carbon scaffold enhances electric conductivity and dual interface structure provides a strong buffering for volume change of SiOx/C. As a result, SiOx/C anode exhibits an excellent reversible specific capacity of 755 mA h g
1 after 300 cycles with less than 9% degradation at 100 mA g
1. This method is facial, low-cost and the preparation process does not involve acid washing, centrifugation, which may provide a means for developing anode materials for high capacity lithium ion batteries. Acknowledgement This work was financially supported by Major Science and Technology Projects of Zhengzhou (project No. 174PZDZX570) and Science and Technology Planning Project of Henan (No. 182106000022 and No. 182102310802).

Fig. 8. The equivalent circuit of SiOx/C.

Table 2 The change in values of RSEI, Rct and n for SiOx/C.

before cycle after 50 cycles after 200 cycles

Appendix A. Supplementary data

Rct (ohm)

RSEI (ohm)

n

86.36 34.86 24.79

/ 11.4 11.68

/ 0.62 0.59

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.072. References

activation of SiOx/C. Values of RSEI after the 50th and 200th are similar, which indicates that the formed SEI film is stable even after 200 cycles. That is to say the dual interface structure of SiOx/C can effectively restrain volume expansion during cycles. The constant n ~0.5 reveals the Warburg characteristic with diffusion controlled electrochemical behavior. According to Liþ diffusion coefficient (DLi) corresponds to Warburg impedance [49,55], the DLi of SiOx/C after 50 cycles and 200 cycles are calculated to be 2.74  10
13 cm2 s
1 and 1.06  10
13 cm2 s
1 respectively (Fig. S6). EIS result indicates that SiOx/C has a robust structure during cycles, which further reveals that long-cycling stability and high-rate capability of SiOx/C. 4. Conclusion In conclusion, a unique SiOx/C composite with dual interface

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