MoSi2

Jun 14, 2016 - Enhanced Electrochemical Performance of Heterogeneous Si/MoSi2 Anodes Prepared by a Magnesiothermic Reduction ... accumulation generate...
4 downloads 3 Views 6MB Size
Subscriber access provided by UNIV OF CAMBRIDGE

Article

Enhanced electrochemical performance of heterogeneous Si/MoSi2 anodes prepared by a magnesiothermic reduction Lili Wu, Juan Yang, Xiangyang Zhou, Jingjing Tang, Yongpeng Ren, and Yang Nie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04448 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 17, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Enhanced electrochemical performance of heterogeneous Si/MoSi2 anodes prepared by a magnesiothermic reduction Lili Wu, † Juan Yang, † Xiangyang Zhou, †* Jingjing Tang, ‡ Yongpeng Ren, † and Yang Nie†



School of Metallurgy and Environment, Central South University, Changsha 410083, China.



Department of Mechanical and Engineering, The Hong Kong Polytechnic University, Hong

Kong, China. *

Corresponding author. Tel. /fax: +86-073188836329.

E-mail address: [email protected]

Keywords:

Silicon;

Molybdenum

disilicide;

Magnesiothermic

reduction;

Local

heat

accumulation; Anodes

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

ABSTRACT This work explores facile synthesis of heterogeneous Si/MoSi2 nanocomposites via a one-step magnesiothermic reduction. The MoSi2 serves as a highly electrically conductive nanoparticle has several advantages of electrochemical properties, which is formed through the absorption of local heat accumulation generated by magnesiothermic reduction. As a result, the Si/MoSi2 nanocomposites exhibit an excellent electrochemical performance, showing initial charge capacity of 1933.9 mAh g-1 at a rate of 0.2 C and retain 85.2% after 150 cycles. This work using local heat accumulation generated by magnesiothermic reduction demonstrates a large-scale method for producing high performance Si-based anode materials, which could provide referential significances for other materials.

ACS Paragon Plus Environment

2

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

INTRODUCTION A rapidly developing market for portable electronics and electrical vehicles (EVs) requires an urgent supply of lithium ion batteries (LIBs) with high energy density, fast charge/discharge capability and long cycle life.1-2 At present, the lamellar structure of graphite has been widely used as anode for LIBs, due to its good cycle stability and safety. However, the specific capacity of current commercial graphite has almost reached its theoretical lithium storage capacity (372 mAh g-1), which can hardly meet the demand for high energy density batteries.3 Silicon as the second most abundant element alone constitute 25 wt.% of the mass of the earth's crust, has lately attracted broad attention as a promising anode material, due to its low discharge potential ( <0.5 V versus Li/Li+) and high theoretical capacity (4200 mAh g-1, Li4.4Si).4-6 Although silicon is quietly different from the commercial anode graphite, facing large volume changes during lithiation/delithiation and poor electric conductivity, which results in pulverization of silicon, fracture of solid–electrolyte interface (SEI) layer and poor rate capability, significant efforts have been put forward by designing the structure of electrode materials, improving the stability of the SEI film and developing an ideal coating layer.7-8 Decreasing the size of Si into nanoscale dimensions and coating with conducting layers are proved to be effective methods to buffer the large volume change and facilitate the fast transport of Li+, resulting in a significant improvement in cycling performance and rate capability.9-12 Nevertheless, silicon is rarely found in nature in its uncombined form, which is widely distributed in clays, sands, rocks and planets as various forms of silica or silicates. Successful implementation of silicon as anode material has been impeded by the scalable synthesis of nanosized silicon. Numerous methods for synthesizing nano Si materials have been described, including metal-assisted etching,13-14 thermal decomposition of SiH4,15-16 reduction of SiCl4,17-18

ACS Paragon Plus Environment

3

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

and magnesiothermic reduction of silica.19-20 Comparatively, magnesiothermic reduction is the most effective method due to its cost-effective, environmental friendly and facile synthesis process.21-25 Rice husks have been used as silica sources to produce nanostructured silicon, and silicon nanoparticles have been successfully synthesized from purified beach sands.23,

26

However, during the magnesiothermic reduction process, large amount of heat would be released, leading to the fusion of silicon and occurrence of side reaction, especially the formation of silicon carbide due to the carbon coating.26-28 Transition metal silicide is one of the most promising material for applications in electronics, due to its low electronic resistivity. Transition metal silicide is usually produced by hightemperature reaction between Si powder and metal powder in an inert atmosphere. To effectively utilize and gently absorb the massive heat released from the magnesiothermic reduction reaction. Herein, we propose a facile strategy to fabricate heterogeneous Si/MoSi2 nanocomposites, in which the molybdenum disilicide plays an important role in the rate capability of electrode. With the mixture of silica and MoO3 as starting material, a subsequent magnesiothermic reduction process was carried out, leading to the reduction of silica and formation of MoSi2. It is demonstrated that through absorption of heat generated by magnesiothermic reduction, molybdenum disilicide was successfully formed on the interfaces between Si nanoparticles. Such a facile design has multiple advantages: (1) the growth of the hybrid structure containing interconnected Si and MoSi2 nanoparticles; (2) the nanosized Si particles can deliver higher electrochemical reaction activity; (3) the MoSi2 framework can facilitate electron transport and act as a mechanical backbone;29-31 (4) the massive heat release from the magnesiothermic reduction reaction is effectively utilized for the processing of MoSi2. With this design, the

ACS Paragon Plus Environment

4

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

obtained Si/MoSi2 nanocomposites manifests superior cycling performance and rate capability compared with bare Si.

Experimental Section Synthesis of SiO2 nanoparticles SiO2 nanoparticles were synthesized via the modified Stöber method.32 In brief, 18 ml of NH3·H2O, 32 ml of absolute ethanol and 50 ml of deionized (DI) water were premixed together and stirred at room temperature to form a uniform solution A. 9 ml of TEOS was added into 91 ml of absolute ethanol to form a uniform solution B. Then solution B was added to the solution A immediately. After keep stirring for 2 h, SiO2 nanoparticles were collected by centrifugation at 9000 rpm and then washed with DI water and dried at 80 °C. Synthesis of Si/MoSi2 nanocomposites Typically, 0.1 g of (NH4)6Mo7O24·4H2O and 1 g of SiO2 was dissolved in DI water to obtain a uniform solution. The suspension was constantly stirred at 100 °C to remove water and the resulting powder was calcined at 400 °C for 2 h to obtain the SiO2/MoO3 powder. After that, SiO2/MoO3 was mixed with Mg powder (mass ratio=1:1) and sealed in a reactor filled with argon gas. The reactor was firstly heated to 400 °C, and then increased to 700 °C with a ramp rate of 2 °C min-1, after that the temperature was kept for 6 h. This process was carried out under Ar atmosphere in a tube furnace. The Si/MoSi2 was obtained after immersed the resulting powder in 2 M HCl solution for 6 h and then washed with DI water and vacuum dried at 60 °C. For comparison, bare Si was obtained through direct magnesiothermic reduction of SiO2.

ACS Paragon Plus Environment

5

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

Characterization X-ray diffraction (XRD) patterns were recorded on a Rigaku-TTRIII equipment with Cu Kα radiation at a scanning rate of 10° min-1. The X-ray photoelectron spectroscopy (XPS) analysis was characterized using an Al Kα XPS (ThermoFisher-VG Scientific). The microscopic morphologies and structures were obtained using field emission scanning electron microscopy (SEM, NOVA NANO SEM 230) and field emission transmission electron microscopy (TEM, JEOL JEM-2100F). Scanning transmission electron microscopy (STEM) and corresponding energy dispersive X-ray (EDX) elemental mapping were characterized by FEI Tecnai G2 F20. Electrochemical Characterization The electrochemical properties were tested using a CR2025 coin-type half-cell. To prepare the working electrodes, a slurry contains Si/MoSi2, carbon black and sodium alginate binder with a mass ratio of 6:2:2 dissolved in DI water was uniformly coated on a piece of copper foil. Considering the capacity contribution of carbon black, all capacities were calculated based on the total mass of active material (including Si, MoSi2 and carbon black), and the mass loading is around 0.46-0.54 mg cm-2. Before assembling the coin cells in an argon-filled glovebox, the electrodes were dried at 60 °C for at least 1 h. 1M LiPF6 dissolved into the mixture of ethylene carbonate and diethyl carbonate (EC:DEC, 1:1 in volume) with 10wt% fluoroethylene carbonate additives was used as the electrolyte. The cycling and rate performances were cycled between 0.01-1.2 V versus Li/Li+ using a LAND CT-2001A. The rate capability was calculated based on the theoretical capacity of Si (1C = 4200 mA g-1). Electrochemical impedance spectroscopy (EIS) measurements were operated on an electrochemical workstation (PARSTAT MC) by applying an amplitude of 5 mV over the frequency range of 100 kHz to 10 mHz.

ACS Paragon Plus Environment

6

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

RESULTS AND DISCUSSION Crystalline phases of the as-prepared samples were identified by XRD as displayed in Figure 1. The XRD pattern of SiO2 precursor reveals its amorphous structure, and the peaks of SiO2/MoO3 at around 12.8°, 23.3°, 25.7°,27.3° and 33.8° can be assigned to MoO3 (JCPDS, no. 76-1003), indicating that (NH4)6Mo7O24 was decomposed into MoO3 during the annealing process (Figure S1). After magnesiothermic reduction and pickling treatment, the broad SiO2 peak disappeared from the XRD spectrum, and the well resolved diffraction peaks located at 28.4°, 47.3°, 56.1°, 69.1° and 76.4°, indicates a well crystallized silicon phase (JCPDS, no. 772109). It should be noted that, some minor peaks were detected in the pattern of Si/MoSi2, which can be assigned to the crystal shape of α-MoSi2 (JCPDS, no. 41-0612) and β-MoSi2 (JCPDS, no. 17-0917). Different from Mg2Si, MoSi2 can be stabilized in HCl solution. Besides the phase of α-MoSi2 and β-MoSi2, a weak peak located around 40.5° can be observed from a fine XRD pattern of Si/MoSi2 nanocomposites between 38° and 44° (Figure S2). This peak can be assigned to the Mo (110) peak (JCPDS, no. 89-5156), which would be further discussed in the XPS section. The chemical composition of the surface of bare Si and Si/MoSi2 were characterized by XPS as shown in Figure 2. In the case of Si/MoSi2 nanocomposites, three elements (Si, O, Mo) were distinguished in the survey XPS spectrum. Generally, the surface analysis depth of XPS is around 5 nm. The presence of oxygen from XPS results indicates that the SiOx layer outside the Si nanoparticles is less than 5nm, which have been confirmed in previous literatures.33-35 As shown in Figure 2b, the deconvolution of the Mo3d spectrum results in four peaks centered at 227.4 eV, 228.3 eV, 231.2 eV and 234.7 eV corresponding to 3d5/2 of MoSi2, 3d5/2 of metallic Mo, 3d3/2 of MoSi2 and 3d3/2 of MoO3, respectively, which is in accordance with the previous

ACS Paragon Plus Environment

7

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

reported MoSi2 fabricated by a self-propagating high temperature synthesis (SHS) reaction from a mixture of Mo and Si powders.36 The existence of metallic Mo may be formed during the magnesiothermic reduction. To provide further information of the formation of MoSi2 during the reaction, SiO2 and (NH4)6Mo7O24·4H2O with a molar ratio of Si:Mo = 2:1 was carried out as precursors, and the final product was identified by XRD as shown in Figure S3. Si, α-MoSi2, βMoSi2 and metallic Mo can be detected from the XRD pattern. This result indicates that metallic Mo is a crucial and in-process product during the reaction. Furthermore, the formation of different phases of MoSi2 can be reproducible. SEM images are shown to illustrate the structural and morphological information of the assynthesized products. Images of the SiO2 precursor are shown in Figure S4, and the SiO2 exhibited nanosphere structures with a diameter of 300-400 nm. After magnesiothermic reduction and etching process, most of the bare Si and Si/MoSi2 maintained the original morphology and diameter (Figure 3a and b). From the low magnification SEM image (Figure 3c), the Si/MoSi2 primary particles combined with each other, possibly due to the agglomeration during the magnesiothermic reduction. EDX mapping in Figure S5 reveals the uniform distribution of Mo and Si elements in the Si/MoSi2 cluster. In addition, TEM measurements were used to reveal the microstructure of bare Si and Si/MoSi2. The size and shape were preserved after magnesiothermic reduction (Figure 3d and e). Although from the SEM images, it is obvious that the bare Si has obtained substantial porosity, while the Si/MoSi2 has a much smoother surface. TEM images reveal that the inner structure of Si/MoSi2 is similar to the bare Si. The smooth surface of Si/MoSi2 may be attributed to the small particles of MoSi2 outside the Si nanoparticles. The crystal structures of Si and MoSi2 nanoparticles are revealed by the high resolution TEM (HR-TEM) acquired from the edges of Si/MoSi2 as shown in Figure 3f, which is

ACS Paragon Plus Environment

8

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

in accordance with the XRD patterns. In addition, this HR-TEM image exhibits a clear grain boundary between Si and MoSi2, indicating that Si and MoSi2 are connected tightly. Further insights into the internal structure and elemental distribution of the Si/MoSi2 nanocomposites were extensively characterized using STEM and EDX mapping. High-angle annular dark field (HAADF) STEM images of the Si/MoSi2 nanoparticles are shown in Figure 4a. From the STEM image of Si/MoSi2 nanocomposites, a certain portion of white area corresponding to Mo can be observed. EDX mapping (Figure 4b and c) for Si and Mo presents the interconnected distribution of Si and Mo elements in the as-synthesized Si/MoSi2 nanocomposite, which is consistent with the result of HR-TEM. In addition, to quantify the mass contents of Si and Mo elements in the Si/MoSi2 nanocomposites, EDX analysis were carried out (Figure S6). From this result (Table S1), the weight ratio of Si and Mo was estimated to be 95.72% and 4.27% respectively. Based on the previous measurements, it is reasonable to speculate that the formation of Si/MoSi2 can be described as follows: (1) SiO2 is reduced to Si by the magnesiothermic reduction; (2) When temperature rise to the melting point of magnesium, MoO3 would be reduced by magnesium directly;37 (3) Local heat accumulation facilitate the formation of MoSi2. The overall reactions occur as below: SiO2 + 2Mg → Si + 2MgO

(1)

MoO3 + 3Mg → Mo + 3MgO

(2)

2Si + Mo → MoSi2

(3)

ACS Paragon Plus Environment

9

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

It is demonstrated that magnesiothermic reduction of silica is an exothermic reaction, and lots of heat would be released to the reaction system, leading to the fusion of Si nanoparticles and the occurrence of side reactions. As a result, the local heat accumulation would hinder the preparation of Si with controlled morphology. In this work, the released heat was utilized for the processing of the third process (Eq. (3)), which is reported for the first time. Compared with the previous molten-salt route for the synthesis of Si nanostructures,22-23, 38 no additive salts were involved in the process and a superior electrochemical performance can be obtained due to the electrically conductive MoSi2 nanoparticles, which will be discussed below. As anode materials for rechargeable LIBs, the voltage profiles at typical cycles of the bare Si and Si/MoSi2 nanocomposites electrodes tested at a rate of 0.2 C (1C=4200mA g-1) are displayed in Figure 5a and b. The shape of these profiles is similar to that of a typical Si anode. The discharge/charge capacities of bare Si and Si/MoSi2 for the first cycle are 2486.7/1949.2 and 2492.6/1933.9 mAh g−1, with initial coulombic efficiency of 78.4% and 77.6%, respectively. The irreversible capacity loss of the electrodes in the first cycle can be mainly ascribed to the formation of SEI films and the irreversible reaction between Li+ and surface oxide layer (SiOx) of the active materials.39-40 During the first cycle, distinct discharge plateau below 0.1 V is relating to the Li-alloying process of crystalline Si to form amorphous LixSi phase.15, 41 After the following cycles, the discharge/charge profiles exhibit an obvious characteristic of amorphous phase. The shape of the profile in 50th cycle is similar to that of 10th cycle, indicating a quite stable electrochemical behavior of Si/MoSi2 electrode. To further clarify the influence of MoSi2 particles on the discharge plateau of electrodes, a comparison between these two electrodes at typical cycles was provided in Figure S7. Although the Si/MoSi2 electrode shows a lower starting lithiation voltage of crystalline Si than that of bare Si during the first lithiation process,

ACS Paragon Plus Environment

10

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

the discharge plateau of Si/MoSi2 electrode increased significantly and is higher than that of bare Si at subsequent cycles. This phenomenon can be ascribed to an activation process during the first several cycles. In addition, the higher discharge plateau indicates the less polarization of Si/MoSi2 electrode.42-43 As a result, the existence of MoSi2 particles has positive effects on electrical conductivity of electrodes. Cycling performance of bare Si and Si/MoSi2 electrodes tested at a rate of 0.2C was provided in Figure 5e. Although the capacity of silicon is easily decreased due to the large volume change during the cycling, the Si/MoSi2 electrode exhibits a reversible capacity of 1647.3 mAh g-1 after 150 cycles, and the coulombic efficiency is above 98% after the 10th cycle and much higher than that of the bare one, indicating that the SEI layer remains intact. The capacity retention of bare Si and Si/MoSi2 after 150 cycles is 63.7% and 85.2%, respectively. Excellent cycling performance of the Si/MoSi2 electrode may be attributed to the efficient buffer network of MoSi2 and the void spaces between each particle. The rate capabilities of bare Si and Si/MoSi2 electrodes at various C rates were also investigated as shown in Figure 5f. With the increase of discharge/charge current density from 0.1C to 5C, the bare Si electrode shows low capacity at high rates. However in the first 5 cycles tested at 0.1C, the bare Si electrode (2200 mAh g-1) shows a higher average reversible capacity than that of Si/MoSi2 electrode (2100 mAh g-1). According to the literature, besides Mg2Si and CaSi2, most of silicides exhibit the reversible capacities below 200 mAh g-1.44-45 Courtel et al. have reported that the special capacity of MoSi2 is 135 mAh g-1 at a current density of 62 mA g-1.31 This inferior capacity value can be responsible for the low capacity of MoSi2 electrode. Although the low reactivity of MoSi2 with Li would be helpful to stabilize the cycling performance of electrode, the amount of MoSi2 should be suitable to guarantee the high energy density of LIBs. At the current density of 5C, the reversible capacity of the bare Si electrode drop to ~150 mAh g-1

ACS Paragon Plus Environment

11

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

directly, while the Si/MoSi2 electrode could still maintained a reversible capacity of ~550 mAh g-1. When the current density was returned back to 0.1C after high rate test, the capacities of both electrodes resumed. To further discern the enhanced electrochemical behavior of Si/MoSi2 electrode, EIS measurements were applied after three cycles at the fully delithiated state. As shown in Figure S8, two semicircles in the high-frequency region corresponding to the SEI resistance (RSEI) and charge transfer resistance (Rct) can be observed. Both of the RSEI and Rct values of bare Si electrode are obviously higher than that of the Si/MoSi2 electrode. The EIS result confirms that the Si/MoSi2 electrode has a higher electrical conductivity, which is consistent with the rate performances of these electrodes. For comparison, some typical Si-based anodes synthesized through magnesiothermic reduction in recent years have been summarized, and the cycling and rate performance of the as-prepared Si/MoSi2 nanocomposites are comparable or even higher than these reports (Table S2 and S3). To confirm that the Si/MoSi2 electrode has a better cycling performance compared with bare Si, the coin cells were dissembled after 50 cycles and washed with dimethylcarbonate (DMC) and ethanol to remove residual electrolyte. According to the SEM images (Figure 6a and b), most nanoparticles of Si/MoSi2 electrode could still maintain sphere structures. In addition, Si/MoSi2 nanocomposites are similar to that before cycling, while the bare one suffers disintegration and pulverization of Si nanoparticles (Figure 6c and d). These results suggest that the interconnected electrically conductive MoSi2 nanoparticles play an important role in preparing high performance anode materials, including a highly stable cycling and an excellent rate capability.

ACS Paragon Plus Environment

12

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

CONCLUSION In summary, we have successfully synthesized heterogeneous Si/MoSi2 nanocomposites via the facile utilization of released heat from magnesiothermic reduction process. The electrically conductive MoSi2 nanoparticles connect with Si nanoparticles tightly, providing electronic pathway between the Si nanoparticles. As a reference, the Si/MoSi2 electrode demonstrates a high reversible capacity of 1647.3 mAh g-1 at a rate of 0.2 C over 150 cycles. With further improvements, this synthetic route using local heat accumulation generated by magnesiothermic reduction could provide referential significances for the formation of other transition metal silicide, which could also facilitate electron transport. The as-prepared Si/MoSi2 material could provide more opportunities for scalable production of high-performance Si based anodes for LIBs.

ASSOCIATED CONTENT Supporting Information. XRD patterns, SEM image, EDX mappings and EDX elemental analyses of samples, discharge plateaus and EIS measurements of Si and Si/MoSi2 electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected]

ACS Paragon Plus Environment

13

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation of China (no. 51204209 and 51274240) and the Project of Innovation-driven Plan in Central South University.

ACS Paragon Plus Environment

14

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Figure 1. XRD patterns of bare Si and Si/MoSi2.

ACS Paragon Plus Environment

15

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

Figure 2. (a) Survey XPS spectra of bare Si and Si/MoSi2. (b) The Mo3d XPS spectra of the Si/MoSi2.

ACS Paragon Plus Environment

16

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Figure 3. SEM images of (a) bare Si and (b) Si/MoSi2 nanocomposites. (c) Low magnification SEM image of Si/MoSi2 cluster. TEM images of (d) bare Si and (e) Si/MoSi2. (f) HRTEM images of Si/MoSi2.

ACS Paragon Plus Environment

17

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

Figure 4. (a) HAADF STEM image and (b and c) EDX mapping analysis (Si and Mo) for Si/MoSi2 nanocomposites. Mapping images were taken from the red solid line rectangle.

ACS Paragon Plus Environment

18

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Figure 5. The discharge/charge profiles at typical cycles of (a) bare Si and (b) Si/MoSi2 electrodes and discharge/charge profiles of (c) bare Si and (d) Si/MoSi2 electrodes at various C rates. (e) The cycling performances of bare Si and Si/MoSi2 at a current density of 0.2C. (f) The rate performance of bare Si and Si/MoSi2.

ACS Paragon Plus Environment

19

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Figure 6. (a and b) SEM and (c and d) TEM images of bare Si and Si/MoSi2 electrodes after 50 cycles.

ACS Paragon Plus Environment

20

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

REFERENCES 1.

Tarascon, J. M.; Armand, M., Issues and Challenges Facing Rechargeable Lithium

Batteries. Nature 2001, 414 (6861), 359-367. 2.

Damen, L.; Lazzari, M.; Mastragostino, M., Safe Lithium-Ion Battery with Ionic Liquid-

Based Electrolyte for Hybrid Electric Vehicles. J Power Sources 2011, 196 (20), 8692-8695. 3.

Wang, G. X.; Shen, X. P.; Yao, J.; Park, J., Graphene Nanosheets for Enhanced Lithium

Storage in Lithium Ion Batteries. Carbon 2009, 47 (8), 2049-2053. 4.

Kasavajjula, U.; Wang, C. S.; Appleby, A. J., Nano- and Bulk-Silicon-Based Insertion

Anodes for Lithium-Ion Secondary Cells. J Power Sources 2007, 163 (2), 1003-1039. 5.

Szczech, J. R.; Jin, S., Nanostructured Silicon for High Capacity Lithium Battery Anodes.

Energ Environ Sci 2011, 4 (1), 56-72. 6.

Obrovac, M.; Christensen, L., Structural Changes in Silicon Anodes During Lithium

Insertion/Extraction. Electrochemical and Solid-State Letters 2004, 7 (5), A93-A96. 7.

Wu, H.; Cui, Y., Designing Nanostructured Si Anodes for High Energy Lithium Ion

Batteries. Nano Today 2012, 7 (5), 414-429. 8.

Lv, R. G.; Yang, J.; Wang, J. L.; NuLi, Y. N., Electrodeposited Porous-Microspheres Li-Si

Films as Negative Electrodes in Lithium-Ion Batteries. J Power Sources 2011, 196 (8), 38683873. 9.

Yu, Y.; Gu, L.; Zhu, C.; Tsukimoto, S.; van Aken, P. A.; Maier, J., Reversible Storage of

Lithium in Silver-Coated Three-Dimensional Macroporous Silicon. Adv Mater 2010, 22 (20), 2247-2250. 10.

Jia, H. P.; Gao, P. F.; Yang, J.; Wang, J. L.; Nuli, Y. N.; Yang, Z., Novel Three-

Dimensional Mesoporous Silicon for High Power Lithium-Ion Battery Anode Material. Adv

ACS Paragon Plus Environment

21

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

Energy Mater 2011, 1 (6), 1036-1039. 11.

Liu, N.; Wu, H.; McDowell, M. T.; Yao, Y.; Wang, C. M.; Cui, Y., A Yolk-Shell Design

for Stabilized and Scalable Li-Ion Battery Alloy Anodes. Nano Lett 2012, 12 (6), 3315-3321. 12.

Wu, L.; Yang, J.; Tang, J.; Ren, Y.; Nie, Y.; Zhou, X., Three-Dimensional Graphene

Nanosheets Loaded with Si Nanoparticles by in Situ Reduction of Sio 2 for Lithium Ion Batteries. Electrochim Acta 2016, 190, 628-635. 13.

Huang, Z. P.; Geyer, N.; Werner, P.; de Boor, J.; Gosele, U., Metal-Assisted Chemical

Etching of Silicon: A Review. Adv Mater 2011, 23 (2), 285-308. 14.

Li, X.; Xiao, Y.; Bang, J. H.; Lausch, D.; Meyer, S.; Miclea, P. T.; Jung, J. Y.; Schweizer,

S. L.; Lee, J. H.; Wehrspohn, R. B., Upgraded Silicon Nanowires by Metal‐Assisted Etching of Metallurgical Silicon: A New Route to Nanostructured Solar‐Grade Silicon. Adv Mater 2013, 25 (23), 3187-3191. 15.

Chan, C. K.; Peng, H. L.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y.,

High-Performance Lithium Battery Anodes Using Silicon Nanowires. Nat Nanotechnol 2008, 3 (1), 31-35. 16.

Sourice, J.; Quinsac, A.; Leconte, Y.; Sublemontier, O.; Porcher, W.; Haon, C.; Bordes,

A.; De Vito, E.; Boulineau, A.; Jouanneau Si Larbi, S. v., One-Step Synthesis of Si@ C Nanoparticles by Laser Pyrolysis: High-Capacity Anode Material for Lithium-Ion Batteries. Acs Appl Mater Inter 2015, 7 (12), 6637-6644. 17.

Kim, H.; Seo, M.; Park, M. H.; Cho, J., A Critical Size of Silicon Nano-Anodes for

Lithium Rechargeable Batteries. Angew Chem Int Edit 2010, 49 (12), 2146-2149. 18.

Lin, N.; Han, Y.; Wang, L.; Zhou, J.; Zhou, J.; Zhu, Y.; Qian, Y., Preparation of

Nanocrystalline Silicon from Sicl4 at 200° C in Molten Salt for High‐Performance Anodes for

ACS Paragon Plus Environment

22

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Lithium Ion Batteries. Angewandte Chemie International Edition 2015, 54 (12), 3822-3825. 19.

Bao, Z. H.; Weatherspoon, M. R.; Shian, S.; Cai, Y.; Graham, P. D.; Allan, S. M.; Ahmad,

G.; Dickerson, M. B.; Church, B. C.; Kang, Z. T.; Abernathy, H. W.; Summers, C. J.; Liu, M. L.; Sandhage, K. H., Chemical Reduction of Three-Dimensional Silica Micro-Assemblies into Microporous Silicon Replicas. Nature 2007, 446 (7132), 172-175. 20.

Zhang, R.; Du, Y.; Li, D.; Shen, D.; Yang, J.; Guo, Z.; Liu, H. K.; Elzatahry, A. A.; Zhao,

D., Highly Reversible and Large Lithium Storage in Mesoporous Si/C Nanocomposite Anodes with Silicon Nanoparticles Embedded in a Carbon Framework. Adv Mater 2014, 26 (39), 67496755. 21.

Fang, S.; Shen, L.; Tong, Z.; Zheng, H.; Zhang, F.; Zhang, X., Si Nanoparticles

Encapsulated in Elastic Hollow Carbon Fibres for Li-Ion Battery Anodes with High Structural Stability. Nanoscale 2015, 7 (16), 7409-7414. 22.

Wang, W.; Favors, Z.; Ionescu, R.; Ye, R.; Bay, H. H.; Ozkan, M.; Ozkan, C. S.,

Monodisperse Porous Silicon Spheres as Anode Materials for Lithium Ion Batteries. Sci Rep-Uk 2015, 5, 8781. 23.

Favors, Z.; Wang, W.; Bay, H. H.; Mutlu, Z.; Ahmed, K.; Liu, C.; Ozkan, M.; Ozkan, C.

S., Scalable Synthesis of Nano-Silicon from Beach Sand for Long Cycle Life Li-Ion Batteries. Sci Rep-Uk 2014, 4, 5623. 24.

Lee, D. J.; Lee, H.; Ryou, M.-H.; Han, G.-B.; Lee, J.-N.; Song, J.; Choi, J.; Cho, K. Y.;

Lee, Y. M.; Park, J.-K., Electrospun Three-Dimensional Mesoporous Silicon Nanofibers as an Anode Material for High-Performance Lithium Secondary Batteries. Acs Appl Mater Inter 2013, 5 (22), 12005-12010. 25.

Zhou, X.; Wu, L.; Yang, J.; Tang, J.; Xi, L.; Wang, B., Synthesis of Nano-Sized Silicon

ACS Paragon Plus Environment

23

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

from Natural Halloysite Clay and Its High Performance as Anode for Lithium-Ion Batteries. Journal of Power Sources 2016, 324, 33-40. 26.

Liu, N. A.; Huo, K. F.; McDowell, M. T.; Zhao, J.; Cui, Y., Rice Husks as a Sustainable

Source of Nanostructured Silicon for High Performance Li-Ion Battery Anodes. Sci Rep-Uk 2013, 3, 1919. 27.

Shi, Y. F.; Zhang, F.; Hu, Y. S.; Sun, X. H.; Zhang, Y. C.; Lee, H. I.; Chen, L. Q.; Stucky,

G. D., Low-Temperature Pseudomorphic Transformation of Ordered Hierarchical MacroMesoporous Sio2/C Nanocomposite to Sic Via Magnesiothermic Reduction. J Am Chem Soc 2010, 132 (16), 5552-5553. 28.

Zhu, S. M.; Zhu, C. L.; Ma, J.; Meng, Q.; Guo, Z. P.; Yu, Z. Y.; Lu, T.; Li, Y.; Zhang, D.;

Lau, W. M., Controlled Fabrication of Si Nanoparticles on Graphene Sheets for Li-Ion Batteries. Rsc Adv 2013, 3 (17), 6141-6146. 29.

Kobel, S.; Pluschke, J.; Vogt, U.; Graule, T. J., Mosi2-Al2o3 Electroconductive Ceramic

Composites. Ceram Int 2004, 30 (8), 2105-2110. 30.

Guo, Z. Q.; Blugan, G.; Graule, T.; Reece, M.; Kuebler, J., The Effect of Different

Sintering Additives on the Electrical and Oxidation Properties of Si3n4-Mosi2 Composites. J Eur Ceram Soc 2007, 27 (5), 2153-2161. 31.

Courtel, F. M.; Duguay, D.; Abu-Lebdeh, Y.; Davidson, I. J., Investigation of Crsi2 and

Mosi2 as Anode Materials for Lithium-Ion Batteries. J Power Sources 2012, 202, 269-275. 32.

Stöber, W.; Fink, A.; Bohn, E., Controlled Growth of Monodisperse Silica Spheres in the

Micron Size Range. Journal of colloid and interface science 1968, 26 (1), 62-69. 33.

Morita, M.; Ohmi, T.; Hasegawa, E.; Kawakami, M.; Ohwada, M., Growth of Native

Oxide on a Silicon Surface. Journal of Applied Physics 1990, 68 (3), 1272-1281.

ACS Paragon Plus Environment

24

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

34.

Kim, H.; Seo, M.; Park, M. H.; Cho, J., A Critical Size of Silicon Nano‐Anodes for

Lithium Rechargeable Batteries. Angewandte Chemie International Edition 2010, 49 (12), 21462149. 35.

Chen, Y.; Hu, Y.; Shao, J.; Shen, Z.; Chen, R.; Zhang, X.; He, X.; Song, Y.; Xing, X.,

Pyrolytic Carbon-Coated Silicon/Carbon Nanofiber Composite Anodes for High-Performance Lithium-Ion Batteries. J Power Sources 2015, 298, 130-137. 36.

Seo, J. H.; Lee, Y. S.; Jeon, M. S.; Song, J. K.; Jeong, C. W.; Han, D. B.; Rha, S. K.,

Study of the Post-Annealing Effect for the Mosi2 Compound. J Ceram Process Res 2009, 10 (3), 335-339. 37.

Manukyan, K.; Aydinyan, S.; Aghajanyan, A.; Grigoryan, Y.; Niazyan, O.; Kharatyan, S.,

Reaction Pathway in the Moo3 + Mg + C Reactive Mixtures. Int J Refract Met H 2012, 31, 2832. 38.

Luo, W.; Wang, X. F.; Meyers, C.; Wannenmacher, N.; Sirisaksoontorn, W.; Lerner, M.

M.; Ji, X. L., Efficient Fabrication of Nanoporous Si and Si/Ge Enabled by a Heat Scavenger in Magnesiothermic Reactions. Sci Rep-Uk 2013, 3, 2222. 39.

Nie, M.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L., Silicon Solid Electrolyte

Interphase (Sei) of Lithium Ion Battery Characterized by Microscopy and Spectroscopy. The Journal of Physical Chemistry C 2013, 117 (26), 13403-13412. 40.

Han, X.; Chen, H.; Liu, J.; Liu, H.; Wang, P.; Huang, K.; Li, C.; Chen, S.; Yang, Y., A

Peanut Shell Inspired Scalable Synthesis of Three-Dimensional Carbon Coated Porous Silicon Particles as an Anode for Lithium-Ion Batteries. Electrochim Acta 2015, 156, 11-19. 41.

Son, S. B.; Trevey, J. E.; Roh, H.; Kim, S. H.; Kim, K. B.; Cho, J. S.; Moon, J. T.;

DeLuca, C. M.; Maute, K. K.; Dunn, M. L., Microstructure Study of Electrochemically Driven

ACS Paragon Plus Environment

25

ACS Applied Materials & Interfaces

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

Lixsi. Adv Energy Mater 2011, 1 (6), 1199-1204. 42.

Han, X.; Chen, H.; Li, X.; Wang, J.; Li, C.; Chen, S.; Yang, Y., Interfacial Nitrogen

Stabilizes Carbon-Coated Mesoporous Silicon Particle Anodes. J Mater Chem A 2016, 4 (2), 434-442. 43.

Han, X.; Chen, H.; Li, X.; Lai, S.; Xu, Y.; Li, C.; Chen, S.; Yang, Y., Nisix/a-Si

Nanowires with Interfacial a-Ge as Anodes for High-Rate Lithium-Ion Batteries. Acs Appl Mater Inter 2016, 8 (1), 673-679. 44.

Netz, A.; Huggins, R. A.; Weppner, W., The Formation and Properties of Amorphous

Silicon as Negative Electrode Reactant in Lithium Systems. J Power Sources 2003, 119–121, 95100. 45.

Usui, H.; Nomura, M.; Nishino, H.; Kusatsu, M.; Murota, T.; Sakaguchi, H., Gadolinium

Silicide/Silicon Composite with Excellent High-Rate Performance as Lithium-Ion Battery Anode. Materials Letters 2014, 130, 61-64.

ACS Paragon Plus Environment

26

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Table of Contents

ACS Paragon Plus Environment

27