Nano Si embedded SiOx-Nb2O5-C composite as reversible lithium storage materials

Journal of Alloys and Compounds 699 (2017) 351e357

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Nano Si embedded SiOx-Nb2O5-C composite as reversible lithium storage materials Moon-Soo Kim a, Kyungbae Kim a, Pil-Ryung Cha a, Hee-Kook Kang b, Sang-Gil Woo b, Jae-Hun Kim a, * a b

School of Advanced Materials Engineering, Kookmin University, Seoul, 02707, Republic of Korea Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi, 13509, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 December 2016 Received in revised form 2 January 2017 Accepted 3 January 2017 Available online 4 January 2017

This study reports a simple and effective method for preparing nanocrystalline Si embedded carbon composites as a high-capacity anode material for Li-ion batteries. Micron-sized Si and Nb2O5 powders were used as starting materials for high-energy mechanical milling (HEMM) process. During the HEMM operation, Si particles obtained oxygen from Nb2O5; the nanocrystalline Si embedded SiOx phase was simultaneously created; and commmercial monoclinic Nb2O5 was also transformed to orthorhombic Nb2O5 with partial reduction. Results show that a nano Si embedded SiOx-niobium oxides-carbon composite was successfully synthesized. Material characterization of the composite was performed by X-ray diffraction analysis, X-ray photoelectron spectroscopy, and electron microscopies. Electrochemical test results demonstrate that the composite electrode has a greatly enhanced performance with a reversible capacity of about 800 mAh g
1 and excellent capacity retention of up to 200 cycles. This improvement can be attributed to the in-situ formation of the nanocrytalline Si embedded silicon suboxide phase with niobium oxides, which is dispersed in carbon matrix. It is considered that this finding will be useful in the preparation of robust Si-based anode materials for high-energy density Liion batteries. © 2017 Elsevier B.V. All rights reserved.

Keywords: Composite materials Electrode materials Energy storage materials Nanostructured materials Mechanochemical processing

1. Introduction With the rapid development of mobile electronic devices and electric vehicles there is an increasing demand for improvements in rechargeable batteries. Out of the various battery systems, Li-ion batteries (LIBs) continue to dominate the market because of their relative high-energy density, good cycle life, and reasonable cost. Although considerable attention has recently been paid to nextgeneration batteries, such as Li-air and Li-S systems, the development of advanced LIBs with enhanced energy density is also significant because this is a more feasible option for use in the near future. To meet ever-growing energy demands made on rechargeable batteries, the use of high-capacity electrode materials is strongly required. Currently, graphite materials are widely adopted as anode materials of LIBs due to their moderate capacity (theoretically 372 mAh g
1), low operating potential, and excellent cycle

* Corresponding author. E-mail address: [email protected] (J.-H. Kim). http://dx.doi.org/10.1016/j.jallcom.2017.01.024 0925-8388/© 2017 Elsevier B.V. All rights reserved.

performance. However, the capacity of graphite anodes is intrinsically limited because of their use of Li intercalation chemistry. Li-alloy systems are an alternative to graphite anodes and have attracted considerable attention because of their chemical ability to electrochemically store Li ions by making Li-metal alloys. Among the Li-alloying materials, Si is the most popular because it electrochemically delivers a high specific capacity of 3580 mAh g
1 for the Li15Si4 phase, as well as being abundant and environmentally benign [1e4]. However, the Si anode suffers poor cycle performance owing to its reaction mechanism with Li during cycling. Lialloying materials, including Si, generally experience mechanical failure because of the large volume change during Liþ ion insertion and extraction processes. Therefore, to improve the cycling stability of Si electrodes by alleviation of volume change during repeated cycling, efficient strategies such as nanoengineering and carbon incorporation have been widely demonstrated [5e14]. In addition, a further element or compound has been added to carbon to enhance the structural stability of Si active materials; these elements and compounds include metals such as Cu [15], Ni [16,17], and Fe [18], and compounds such as Al2O3 [19], TiN/TiB2 [20], NiSi2

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[21,22], TiO2 [23,24], and SiOx [25e29]. Inactive phases such as Cu, Ni, Al2O3, which do not react electrochemically with Liþ ions, mainly play the role of alleviating the volume change and/or assisting with the increase of electrical conductivity. Conversely, active phases such as TiO2 and SiOx are able to store the Li ion and alleviate the volume change because the materials themselves do not undergo extensive volume changes [30e32]. These materials provide a synergistic effect to Si, by improving both capacity and structural stability. In this study, we introduce niobium oxide into Si-C composites. Nanosized Si embedded SiOx-niobium oxides-carbon composites were prepared using a high-energy mechanical milling (HEMM) process. Nb2O5 materials have recently attracted renewed interest as Liþ storage electrodes. More than 30 years ago, it was discovered that Nb2O5 could store Liþ ions, and it was thus considered as a cathode material for 2V Li secondary batteries [33e35]. However, orthorhombic Nb2O5 materials have recently been reported to exhibit fast Li storage and excellent cycling stability with moderate capacities of 150e200 mAh g
1, and have also been actively evaluated as hybrid supercapacitor electrodes [36e41] although they have a limited capacity for use in high-energy density LIBs. Therefore, in this study, Nb2O5 is incorporated with high-capacity Si in a carbon matrix for use in anode materials of LIBs, with the aim of achieving high-capacity, fast rate capability, and an excellent cycle performance. To achieve this, micron-sized Si and commercial monoclinic Nb2O5 powders were simply ball-milled and consequently, nanocrystalline Si embedded SiOx and orthorhombic Nb2O5 were dispersed in carbon matrix. In this HEMM process, Nb2O5 was found to play an important role in the formation of nanocrystalline Si. The materials and electrochemical properties of the prepared composites were then thoroughly investigated using various analytical methods.

electrodes were pressed and dried under vacuum at 80  C for 12 h, and then cut into 12 mm-diameter discs, where the mass of the active material in each disc was approximately 2 mg cm
2. The CR2032 coin type half-cells were then assembled using a prepared working electrode, a porous polypropylene separator (Asahi Kasei Chemicals), and a lithium foil counter/reference electrode. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC)/fluoroethylene carbonate (FEC) (5:70:25 vol ratio, PANAX Etec). Cells were galvanostatically tested at constant currents of 200e1000 mA g
1 (1C ¼ 1000 mA g
1), and the dischargecharge (i.e., Li insertion-extraction) tests were carried out within a voltage window of 0.001e3.0 V vs. Liþ/Li. All electrochemical experiments were conducted in an argon-filled glovebox. 3. Results and discussion Fig. 1 shows the XRD patterns of 6 h-milled Nb2O5 powder and 5 h-milled Si-Nb2O5-C composite. The crystallographic phase of 6 hmilled Nb2O5 powder was an orthorhombic phase (T-Nb2O5, ICDDJCPDS No. 30-0873). XRD patterns of the as-received and Nb2O5 powders milled from 3 to 24 h are presented in Fig. S1. The starting commercial powder confirms that the Nb2O5 phase was monoclinic (H-Nb2O5, ICDD-JCPDS No. 37-1468). After milling for 3 h, peaks appeared in relation to the orthorhombic phase; after milling for 6 h this monoclinic phase was completely transformed to an orthorhombic phase; when increased to 12 h, the orthorhombic phase coexisted with a tetragonal NbO2 phase (ICDD-JCPDS No. 431043); and at 24 h only the NbO2 phase was observed. From these results, it was found that a specific milling time (6 h) is required to obtain orthorhombic Nb2O5, which is beneficial for fast Liþ ion

2. Experimental 2.1. Material preparation Nano Si embedded SiOx-niobium oxides-carbon composite was prepared from pure Si (3 mm, 99% purity, Kojundo), Nb2O5 (100 mesh, 99.5% purity, Alfa Aesar), and activated carbon (Super P) powders using the HEMM process. Si, Nb2O5, and Super P powders (2:4:4 in weight ratio) were placed in a steel vial (65 cm3) with steel balls measuring 9.53 mm at a ball to powder ratio of 30:1. The powders were placed into the vial under an ambient Ar atmosphere; the steel vial filled with the starting materials was then subjected to the HEMM process for different lengths of time. 2.2. Material characterization The crystal structure of the composite was investigated by X-ray diffraction (XRD, Rigaku D/MAX-2500V). The morphology and microstructure were observed using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7000F) and a highresolution transmission electron microscope (HR-TEM, JEOL ARM200F) with an attached energy dispersive spectroscope (EDS), and the latter was used to perform elemental mapping. In addition, Xray photoelectron spectroscopy (XPS, Thermo Scientific Ka) was used to examine the chemical state of the composites. 2.3. Electrochemical measurement For cell tests, the composite active material (70 wt%), a conducting agent (Ketchen black, 15 wt%), and a binder (polyacrylic acid, 15 wt%) were dissolved in de-ionized water; the slurry was then coated onto copper foil substrates. After coating, the

Fig. 1. XRD patterns of 6 h-milled Nb2O5 and 5 h-milled Si-Nb2O5-C composite samples.

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storage [37e39]. To incorporate the orthorhombic Nb2O5 powder in Si-C composite, micron-sized pure Si and the commercial Nb2O5, and activated carbon (Super P) were ball-milled to obtain composite materials. The constituent phases of the composites varied with an increase in the milling time; the XRD patterns are shown in Fig. S2. From the patterns, it can be seen that the 5 h-milled composite consisted of Si and orthorhombic Nb2O5 phases, as presented in Fig. 1. In the powder XRD patterns, the obvious diffraction peaks were assigned only to the Si and Nb2O5 phases; however, the broad nature of the peaks and backgrounds indicates that the prepared composite may be composed of nanocrystalline and/or amorphous phases that could not be identified in the XRD patterns. To examine the chemical state of each element in the composite material, XPS analyses were performed on the 5 h-milled composite sample. Fig. 2a shows the Si 2p core level spectrum with deconvoluted profiles for each valence state; the valance state of Si can be assigned to 0, þ1, þ2, þ3, and þ4, as reported in the literature [42e44]. Because the XPS profiles provide information pertaining to surface chemical states of materials, oxidized states of Si can be detected even on pure Si particles, due to the surface native oxide film. However, a considerable amount of oxidized Si (SiþeSi4þ) is observed in the composite, which indicates that a certain amount of Si was oxidized during the HEMM operation. The Nb 3d core level XPS spectrum for the composite is shown in Fig. 2b and the peaks at 207.6 and 210.3 eV can be assigned to Nb2O5 [45e48]. Although the profiles for Nb2O5 (Nb5þ) appear dominant, peaks related to NbO2 (Nb4þ) are also found at 205.8 and 208.8 eV [45e48]. As demonstrated in the literature, the surface of NbO2 is easily oxidized to Nb2O5 in an ambient atmosphere. Accordingly, the small amount of the deconvoluted NbO2 profiles

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may imply that a considerable amount of NbO2 exists in the composite (which is consistent with results of the deconvoluted Si 2p profiles) and that the initial Si particles take O atoms from Nb2O5 during the HEMM process. Fig. 2c shows the O 1s XPS spectrum, which can be deconvoluted into 4 profiles; the sub-profiles correspond to four different states of the O atom in Nb2O5, NbO2, SieO, and CeO [48,49], respectively, agrees well with the profile analysis results of both Si 2p and Nb 3d. The C 1s core-level spectrum was then deconvoluted into 4 profiles, which can be easily assigned to CeC bonding and oxygen-containing functional groups in the carbon composites (Fig. 2d) [49,50]. Electron microscopies were used to analyze the morphology and microstructure of the 5 h-milled composite. Fig. 3a shows an FESEM image of the composite powder, showing that the secondary particle sizes ranged from a few hundred nm to 1 mm after HEMM. The low-magnification TEM image in Fig. 3b shows that the composite powder consists of nanocrystallites and primary particles on a scale of a few to a several tens of nanometers. Additional TEM images are given in Fig. S3, where the size of nanocrystallites can be confirmed. Fig. 3c shows a high-magnification TEM image; a specific part on the surface was then further enlarged and the corresponding HR-TEM image is shown in Fig. 3d. From the HR-TEM image, it can be seen that Si nanocrystallites measuring a few to a few tens nanometers are embedded in the composite. Furthermore, T-Nb2O5 nanocrystallites also coexist with the Si nanocrystallites (Fig. S3). Fig. 3e and f shows fast Fourier transform (FFT) patterns corresponding to the marked areas in Fig. 3d, from which Si and TNb2O5 phases are confirmed. On the basis of TEM analysis combined with the above XPS results, it can be concluded that nanocrystalline Si embedded silicon suboxide-nanocrystalline and amorphous (reduced) niobium

Fig. 2. XPS spectra of the Si-SiOx-Nb2O5-C composite: (a) Si 2p, (b) Nb 3d, (c) O 1s, and (d) C 1s profiles.

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Fig. 3. Electron microscopy analyses of Si-SiOx-Nb2O5-C composite: (a) FE-SEM image, (b) low-magnification TEM image, (c) high-magnification TEM image, (d) HR-TEM image, and (eef) FFT patterns for selected areas in the HR-TEM image.

oxides-carbon composite was prepared using the HEMM process. The starting materials were micron-sized Si and Nb2O5 powders, and together with the activated carbon powder these materials were mechanically ball-milled. During the high energy milling process, Si particles took O atoms from Nb2O5 and the particle size was decreased. Simultaneously, nanocrystalline Si was embedded in the amorphous SiOx with niobium oxides, and as a result, Si and T-Nb2O5 nanocrystallites were dispersed in the carbon matrix of the composite. To verify the distribution of Si, Nb, O, and C elements, EDS elemental mapping using scanning transmission electron microscopy (STEM) was conducted on the composite; the results are shown in Fig. 4. It can be clearly observed that the C element was uniformly distributed over the entire range of the material, which indicates that the C element is a matrix for the composite (Fig. 4c). Nb and O elements were relatively well dispersed, although the distribution of the two elements was different, indicating the O element has bonding Si as well as Nb (Fig. 4d and e). Analysis of the XPS Si 2p profile supports the observed existence of amorphous Si suboxides. The Si map shows strong spots on a few tens of nanometers scale map, which reveals that nanocrystalline Si measuring a few to a few tens nanometers is embedded in the composite (Fig. 4f). These results are consistent with the HR-TEM analysis. Fig. 5a shows the voltage profiles of the 5 h-milled composite electrode for the first cycle at a constant current of 200 mA g
1 (0.2C), and the profiles for the Si-Nb2O5-C (2:4:4 in weight ratio) mixture electrode are shown for comparison (where commercial Si and 6 h-milled T-Nb2O5, and Super P powders are used). The electrochemical performance results of a Si-C mixture electrode (2:8 in weight ratio) are also displayed in Fig. 4S. The discharge (Liþ insertion) and charge (Liþ extraction) capacities of the composite electrode are 1425 and 1045 mAh g
1, respectively, with an initial coulombic efficiency of 73.3%. However, the discharge and charge capacities of the mixture electrode are 996 and 603 mAh g
1, respectively, with a coulombic efficiency of 60.5%. In the mixture

electrode, pure Si particles suffer from large volume changes during the first cycle, and thus the reversible capacity and the initial coulombic efficiency are relatively low. Conversely, the composite electrode exhibits a relatively high reversible capacity and coulombic efficiency, due to the microstructure where nanocrystalline Si is dispersed in the carbon matrix. To compare the Liþ insertion-extraction behavior of the electrodes, differential capacity plots (DCPs) are shown in Fig. 5b. The mixture electrode exhibits a broad shape during the first Liþ insertion, which corresponds to the sloping voltage profiles from 3.0 to 0.1 V vs. Liþ/Li. In this region, an amount of Liþ was inserted into T-Nb2O5 and Super P carbon, and another amount of Liþ was irreversibly consumed to form solid electrolyte interphase (SEI) layer films. A sharp peak was observed below 0.1 V vs. Liþ/Li, which is typical for Si electrodes. In this region, Liþ reacted with Si to form amorphous Li-Si alloy and a final crystalline Li15Si4 phase, as reported in the literature [51]. However, a different behavior was observed in the composite electrode until 0.5 V vs. Liþ/Li, where Liþ was inserted into niobium oxide and carbon matrix, and some Liþ was consumed to form the SEI films. Below 0.5 V vs. Liþ/Li, broad peaks were found, and these can be attributed to Li insertion into nanocrystalline Si and amorphous SiOx phases. These shape profiles can be observed when Liþ is inserted into nanocrystalline and/or amorphous Si, as reported in the literature [1e4]. In addition, the amorphous SiOx phase also can react with Liþ, forming Li2O and/or lithium silicate (Li4SiO4) [52e56]. Similar profiles were observed for both electrodes during the Li extraction process; between 0.0 and 1.0 V vs. Liþ/Li the peaks are mainly related to the de-alloying reaction, which means that Liþ is extracted from Li-Si and/or Li-Si-O phases. However, over 1.0 V vs. Liþ/Li, the profiles correspond to Li extraction reactions from the Li alloy phases, niobium oxide, and carbon materials. A cycle performance of the mixture and 5 h-milled composite electrodes is compared in Fig. 5c. The mixture electrode showed a drastic capacity fading in the first 50 cycles, which can be ascribed

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Fig. 4. EDS elemental mapping results for Si-SiOx-Nb2O5-C composite: (a) HR-TEM image, (b) layered image, (c) C, (d) Nb, (e) O, and (f) Si.

Fig. 5. Electrochemical properties of Si-SiOx-Nb2O5-C composite: (a) voltage profiles, (b) differential capacity plots, (c) cycle performance, and (d) rate performance.

to the mechanical failure of pure Si particles by large volume changes during Liþ insertion/extraction reactions upon cycling. The capacity retention was then stabilized by Nb2O5 and Super P carbon; the cycling stability of T-Nb2O5 is reported to be excellent because of the Liþ intercalation chemistry in T-Nb2O5 materials. The

composite electrode showed an excellent cycling stability up to 200 cycles without capacity fading at a constant current of 500 mA g
1 (0.5 C). In addition, a high capacity of approximately 800 mA g
1 was retained after 200 cycles. The cycle performance of the composite electrode at different current densities is shown in Fig. 5S.

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This performance can be attributed to the microstructure of the composite. As previously mentioned, the starting materials were micron-sized Si and Nb2O5, and during the HEMM operation Si particles take O from Nb2O5. With the assistance of high-energy ball milling, nanocrystalline Si is then embedded in silicon suboxide and niobium oxide; as a result, nano Si embedded SiOxniobium oxides-carbon composite is synthesized. In this preparation process, the role of Nb2O is essential. The Nb2O5 material itself has very good cycling stability and rate performance, and thus it is beneficial for electrochemical properties of the composite. In addition, it provides oxygen to Si, resulting in the formation of nanocomposites. Within this composite, the volume change of Si during cycling can be alleviated by the role of SiOx, niobium oxides, and the carbon matrix, and thus an excellent cycle performance was achieved with a high capacity of about 800 mA g
1 at a high rate of 500 mA g
1. Fig. 5d shows the rate performance of the composite electrode. For the test, the discharge current was fixed at 200 mA g
1 and the charge (Li extraction) capacities were measured at various constant-current values. The reversible capacity was well maintained, even at a high current density of 10 A g
1 (10C). This excellent rate capability can be ascribed to the nanostructured composite consisting of nanocrystalline Si, silicon suboxides, and niobium oxides in carbon matrix, which enables the fast Liþ diffusion from active material to the electrolyte. 4. Conclusions Nanocrystalline Si embedded SiOx-niobium oxides-carbon composite was successfully prepared by using the HEMM process. The starting materials were micron-sized Si and Nb2O5 particles, and during the HEMM operation Si particles took oxygen from Nb2O5 and a nanocrystalline Si embedded silicon suboxide phase was simultaneously produced. Commmercial monoclinic Nb2O5 was also transformed to orthorhombic Nb2O5 with partial reduction. XPS analysis confirmed the formation of silicon suboxide and reduced niobium oxide phases in the composite, and nanocrystalline Si embedded phase and its distribution were verified by HRTEM and EDS analyses. The resulting nano Si embedded SiOxniobium oxides-carbon composite exhibited excellent electrochemical properties with a reversible capacity of about 800 mAh g
1 and remarkable capacity retention up to 200 cycles. This improvement can be attributed to the microstructure of nanocrytalline Si embedded SiOx-niobium oxides-carbon composites. Here, the Nb2O5 material itself is helpful for enhancement of electrochemical properties of the composite. In addition, it gives oxygen to Si, resulting in the formation of nanocomposites. It is considered that this simple and cost-effective method can provide an insight for the preparation of high-capacity Si-based anode materials of LIBs. Acknowledgements This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (2009-0093814, 2015R1A5A7037615, and 2016R1C1B1014015). This work was also supported by the Korea Evaluation Institute of Industrial Technology, which is funded by the Ministry of Trade, Industry & Energy, Republic of Korea (No. 10053711). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.01.024.

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