Electrodeposited Silicon Nanowires from Silica ... - ACS Publications

Subscriber access provided by Columbia University Libraries

Article

Electrodeposited Silicon Nanowires from Silica Dissolved in Molten Salts as a Binder-Free Anode for Li-Ion Batteries Wei Weng, and Wei Xiao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01870 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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 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 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.

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 50 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 Energy Materials

Electrodeposited Silicon Nanowires from Silica Dissolved in Molten Salts as a Binder-Free Anode for Li-Ion Batteries Wei Weng and Wei Xiao * School of Resource and Environmental Sciences, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan 430072, PR China. * Corresponding author: [email protected]

Abstract: Rational manipulation of size, surface chemistry and electrode configuration is of prime importance to enhance lithium storage capability of siliconbased anode for Li-ion batteries. Herein, Si nanowires with small sizes (20 - 30 nm in diameter) and ultrathin surface oxide layer (~1 nm in thickness) are directly electrodeposited on carbon cloth from soluble SiO2 in molten chloride salts. Benefiting from the small size, ultrathin oxide layer and binder-free electrode configuration, the electrodeposited Si nanowires on carbon cloth therefore has a high initial Coulombic efficiency and a reversible capacity of 711 mAh g−1 after 200 galvanostatic charge/discharge cycles at 1000 mA g−1. The present study highlights that the molten salt electrolysis of soluble silica can be a customized preparation of Si for advanced Liion batteries.

Keywords: Si nanowires; oxide layer; electrodeposition; Li-ion batteries; binder-free electrode

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 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

1. Introduction

Rechargeable Li-ion batteries (LIBs) are one of the most important energy storage devices for portable electronics and electric vehicles. However, with the rapid development of electric vehicles, LIBs with higher specific energy are in urgent need. This requirement can be realized by replacing the currently used graphite anode with electrode materials that possess a higher specific capacity.1-3 Silicon (Si) is a promising candidate for next-generation anode materials due to its high reversible specific capacity of 3580 mAh g-1 (about tem times that of graphite).4-7 Nonetheless, dramatic volume change (up to 400%) associated with the lithium alloying and de-alloying reactions leads to pulverization of Si anode materials and thus rapid loss of capacity with prolonged cycling.8-9 Compared with macro/micro-sized Si materials, nanostructured Si anode shows higher resistance to fracture and pulverization during volume variations.1, 9-12 Among various Si nanostructures, Si nanowires can facilitate axial charge transfer and shorten radial Li ion diffusion distance due to the one-dimensional character, thus being an interesting Si morphology as anode material for LIBs.10 Specially, Si nanowires with smaller diameters possess higher electrical conductivity, larger specific areas and enhanced structural robustness upon violent volume variations, yielding a higher specific capacity, better cyclability and high-rate performance.10, 13-15 In addition to the size of Si nanowires, the surface oxide layer is another important factor affecting the performance of Si-based anodes for LIBs. Generally, the surface of

2


Page 2 of 50

Page 3 of 50 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


nanostructured Si materials has a native amorphous SiO2 layer with different thickness. 10, 13-14

When the diameter of Si nanowires is less than 50 nm, the surface native oxide

layer can induce compressive hydrostatic stress during lithiation, efficiently preventing the volume expansion and thus improving the cycling performance.10 However, excessively thick oxide layer can decrease the mass of active silicon in Si nanowires and limit the extent of lithiation, causing a decline in capacity.13-14 The excessively thick surface oxide layer is also detrimental for formation of stable Solid Electrolyte Interphase (SEI), tending to deteriorate the cycle performance. It is recently reported that the optimal thickness of oxide layer is less than 5 nm.14 Constructing Si nanowires with small diameter and ultrathin amorphous oxide layer is an effective way to increase both the capacity and cycling performance. Si-based anodes also suffer from inadequate high-rate performance due to the intrinsically poor electrical conductivity of silicon powder. A large amount of conductive agents and binders is essential for fabrication of Si-based electrode from Si powder.16-18 Array electrode configurations consisting of active materials directly deposited on conducting substrate without any conducting agents and binders are beneficial to better electron transfer and enhanced high-rate performance.15,

19-21

Currently, the binder-free Si nanowires are mostly prepared by the chemical vapor deposition (CVD) method from SiH4/SiCl4, with the aid of catalysts. 20, 21 However, the difficulties in the massive production of Si nanowires and the usage of toxic gaseous raw materials restrict the large-scale deployment of the CVD method. 20 In recent years, preparation of Si nanowires has been realized by molten salt 3



electrolysis of silica.22-26 Compared with the CVD method,

Page 4 of 50

9-10, 21

the molten salt

electrolysis approach for Si nanowires preparation has the following merits. Firstly, SiO2, which is earth-abundant, no-toxic and easily accessible, can be chosen as the raw materials;15, 23 Secondly, massive preparation of Si nanowires can be realized in low temperatures (generally 650 - 900 oC).23, 27-28 In fact, this approach is verified to be upscalable.27 Thirdly, the morphology of Si nanomaterials prepared by molten salt electrolysis is easily tunable by simply regulating the electrolysis conditions. 15, 27-28 If small-sized Si nanowires with an ultrathin surface oxide layer for high-performance anode materials in LIBs can be prepared by molten salt electrolysis, the advantage of this approach can be further highlighted. The morphology of electrolysis products in molten salts is highly related to the reduction mechanism.29-30 Generally, two reduction mechanisms, which means electrodeoxidation (electrolysis of solid SiO2) or electrodeposition (electrolysis of soluble SiO2, which exists in the form of SiO32- in molten salt with a solubility of 1.56 wt%),27,

29

are verified for Si nanowires formation in molten salts. Although the

electrolysis conditions (such as cell voltages, temperature, molten salt composition et al.) for Si nanowires formation were optimized,27-30 the size difference and surface oxide layer dissimilarity between the Si nanowires prepared by these two different mechanisms are not well understood.

4


Page 5 of 50 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


Figure 1. Illustration of the carbon cloth cathode before and after deposition of Si nanowires.

In this paper, the size and surface oxide layer of Si nanowires prepared by molten salt electrolysis from solid SiO2 (Electrolyzed Si powder) and soluble SiO2 (Electrodeposited Si on carbon cloth) are compared. It is found that, by using a carbon cloth cathode, the Electrodeposited Si on carbon cloth can tightly be adhered to the carbon substrate, as shown in Figure 1. Specially, the Electrodeposited Si on carbon cloth not only have much smaller diameters, but also possess a thinner surface oxide layer. When evaluated as anodes for LIBs, the Electrodeposited Si on carbon cloth shows a higher initial Coulombic efficiency, much higher reversible specific capacity and better high-rate performance than that of Electrolyzed Si powder. The results highlight the importance of surface, size and electrode configuration controlling on lithium storage capability of electro-prepared Si from silica in molten salts.

2.

Experiments and materials

Molten salt electrolysis: All the chemical reagents are of analytical purity. An alumina crucible (inner diameter: 70 mm; height: 150 mm) containing 500 g NaCl5



CaCl2 (molar ratio of NaCl and CaCl2 is 1:1.) electrolyte at 800 oC were sealed into a steel tube in a high-purity Ar atmosphere at a flow rate of 150 mL min-1. Pre-electrolysis of the molten salt was conducted at 2.6 V between a nickle sheet and a graphite anode for 10 h to remove the residual moisture and impurities in the melts. A graphite rod (20 mm in diameter) was used as the anode in all experiments. For the electro-deoxidation of solid SiO2, SiO2 pellets (20 mm in diameter, 2.5 mm in thickness, about 1.8 g in mass) were prepared by die-pressing SiO2 spheric powders at 7 MPa and subsequently sintering at 700 oC for 4 h. The obtained pellets were then sandwiched between two molybdenum meshes to form the assembled cathode, which were potentiostatically electro-reduced for 12 h by using the graphite anode and a quartz-sealed high temperature Ag/AgCl reference electrode.27 Correspondingly, electrolysis of soluble SiO2 was conducted in the same molten salt electrolyte. 0.1 M nano-SiO2 (15 - 30 nm) spheres were added into the molten salt and then SiO2-saturated electrolyte was obtained after keeping the molten salt still for more than 12 h before electrolysis. Si was electro-deposited on a carbon cloth (WOS1002, purchased from Cetech Co., Ltd.) cathode after electrolysis of 1-2 h by using the same graphite anode and Ag/AgCl reference electrode. After electrolysis, the cathodes were uplifted out of the molten salt and cooled in argon, and then repeatedly washed in diluted hydrochloric solution (pH=2~3) and distilled water for several times. Finally, the products (Electrolyzed Si powder and Electrodeposited Si on carbon cloth) were obtained after dried at 60 oC overnight. Materials characterization: The obtained samples were characterized by X-ray 6


Page 6 of 50

Page 7 of 50 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


diffraction spectroscopy (XRD, Shimadzu X-ray 6000 with Cu Kα1 radiation at λ =1.5405Å), field emission scanning electron microscopy (FESEM, FEI Sirion field emission gun), transmission electron microscopy (TEM, JEM2010-HT), highresolution TEM (JEM 2010-FEF) and X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd., UK). The binding energy of the obtained XPS spectra was calibrated by placing the principal C1s peak at 284.8 eV. Cyclic voltammograms: Cyclic voltammograms (CVs) of solid SiO2 were recorded using a SiO2-loaded metallic molybdenum (Mo) cavity working electrode in the eutectic NaCl-CaCl2 molten salt by an electrochemical workstation (CHI1140A, Shanghai Chenhua, China).29 Correspondingly, CVs of soluble SiO2 were conducted on a Mo rod working electrode (diameter in 2 mm, and immersion depth of 20 mm in molten salt) in the SiO2-saturated NaCl-CaCl2 molten salt. The immersed area (1.26 cm2) of the Mo rod working electrode in molten salt is same with that of Mo cavity electrode. For all CV tests, a graphite rod and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. LIBs performances: To prepare the working electrode from powder, a slurry was obtained by thoroughly mixing the Electrolyzed Si powder (obtained from solid SiO2 electrolysis), sodium carboxymethyl cellulose (CMC), polyacrylic acid (PAA) and acetylene black in de-ionized water. The weight ratio of Si powder, CMC, PAA and acetylene black is 70:5:5:20. Then, the electrodes containing around 1.0 mg cm-2 electrolytic Si powder were fabricated by blade-coating the above slurry onto the copper foil and vacuum-dried at 120 oC overnight. For the Si obtained from soluble 7



SiO2, the Electrodeposited Si on carbon cloth containing ~1.0 mg cm-2 of Si was directly used as the working electrode, without addition of any binders and conductive additives. The CR2016-type coin cells were fabricated in an Ar-filled glove box with moisture and oxygen below 1 ppm, with Li foils and Celgard 2400 membrane as the counter electrode and separator respectively. A mixture of ethylene carbonate and diethyl carbonate (mass ratio of 1:1) containing 1 M LiPF6 was used as the electrolyte. Five consecutive CV tests were conducted at a scanning rate of 0.2 mV s-1 on a CHI1140A workstation. The galvanostatic charge/discharge profiles were obtained on a Neware battery tester within the voltage range of 0.01 - 1.5 V (vs. Li/Li+). Electrochemical impedance spectra (EIS) of the electrodes were tested at open-circuit potentials ranging from a frequency of 105–0.1 Hz with an AC amplitude of 5 mV by using a Solartron workstation.

3 Results and discussion

3.1 Size comparison Two mechanisms, solid-to-solid transformation and dissolution-electrodeposition, 27, 29

exist for Si preparation during the electrolysis of SiO2 in molten chloride salts. Si

was generated by the dissolution-electrodeposition mechanism for electrolysis of soluble SiO2 (denoted as Electrodeposited Si on carbon cloth in Figure 2a),27 while the solid-to-solid transformation mechanism was dominant for electrolysis of solid SiO2 pellets (denoted as Electrolyzed Si powder in Figure 2b).29 After electrolysis of soluble SiO2, the black surface of carbon cloth becomes yellowish (Figure S1), implying the 8


Page 8 of 50

Page 9 of 50 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


deposition of nanostructured Si materials.8 According to the XRD patterns in Figure 2a, in addition to the peaks of carbon cloth, new diffraction peaks appear for the sample of Electrodeposited Si on carbon cloth, which are well indexed to Si (Si, JCPDS No. 271402, cubic, Fd3m, a=5.430 A). From the insets in Figure 2a, the smooth surface of carbon fibers becomes branched, further verifying the deposition of Si on the surface of carbon cloth. The XRD pattern of the Electrolyzed Si powder in Figure 2b are also well indexed to Si, confirming that solid SiO2 pellets are successfully electro-reduced to Si.27

Figure 2. XRD patterns of (a) carbon cloth and Electrodeposited Si on carbon cloth, and (b) Electrolyzed Si powder. Electrolysis potential: 0.3 V vs. Ca/Ca2+. Electrolysis time: 1.5 h.

9



Different mechanism contributes to diverse morphologies. Figure 3 shows the morphologies of the Electrodeposited Si on carbon cloth (Figure 3b and c) and Electrolytic Si powder (Figure 3d and e). The bare carbon cloth consists of highly tangled carbon fibers with a diameter of around 10 μm (inset in Figure 3a), showing a very smooth surface (Figure 3a). Interestingly, the smooth surface of carbon fibers becomes rough (inset in Figure 3b) after electrodeposition, completely covered by electrodeposited hairy-like Si (Figure 3b). From the magnified image in Figure 3c, the surface consists of uniform Si nanowires with diameters between 20 - 30 nm. The morphology of the Electrolyzed Si powder is also analyzed, as shown in Figure 3d and e. Similarly, the Electrolyzed Si powder also consists of homogeneous tangled Si nanowires (Figure 3d), with a much larger size (with diameters in 100-200 nm). Figure 3e clearly presents a Si nanowire with a diameter of about 200 nm in the Electrolyzed Si powder.

Figure 3. SEM images of bare carbon cloth (a), the Electrodeposited Si on carbon cloth (b, c) and the Electrolytic Si powder (d); TEM image of the Electrolytic Si powder (e). All the electrolysis is conducted at 0.3 V vs. Ca/Ca2+; Electrolysis time is 1.5 h for soluble SiO2 and 12 h for solid SiO2 pellets. 10


Page 10 of 50

Page 11 of 50 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


Obviously, the diameter of the Electrodeposited Si on carbon cloth from soluble SiO2 is five times smaller than that from solid SiO2 pellets (the Electrolyzed Si powder), which implies that the former might have better performances as anode materials for LIBs.10, 13, 31-32 In addition, the Electrodeposited Si on carbon cloth from soluble SiO2 are tightly attached to the cathode substrate (Figure 3b). Even after repeated wash by de-ionized water and dilute hydrochloric acid to remove the entrained chloride salts, detachment of the electrodeposited Si nanowires from carbon cloth was not observed. The strong attachment of electrodeposited Si nanowires on carbon cloth is likely due to the thermodynamically spontaneous formation of SiC (Si + C= SiC, ΔGθ= -63.21 kJ mol-1, 800 oC) on the interface between carbon cloth and Si.33 Moreover, resulting from a sluggish reaction kinetics for SiC formation,34, 35 the surface of carbon cloth can be quickly covered by Si nanowires, avoiding the excessive contact between Si nanowires and carbon cloth. Therefore, massive formation of SiC is not observed. The above results imply that the Si nanowires electrodeposited on carbon cloth can directly be used as an anode for LIBs, free of any binders and conductive additives.15 This is beneficial to decrease the mass of LIBs, contributing to high specific energy densities. The electrolysis conditions for Si nanowires electrodeposition on carbon cloth from soluble SiO2 are optimized (Figure S2). The proper potential range for Si nanowires deposition is 0.15 – 0.35 V vs. Ca/Ca2+ (Figure S2a). The Si nanowires electrodeposited at 0.30 V vs. Ca/Ca2+ shows a highest crystallinity degree (Figure S2b). The electrolysis time can affect the mass of electrodeposited Si nanowires on carbon 11



cloth. After electrolysis of 1 h, the carbon cloth is only partly covered by Si nanowires (Figure S3a and d). By increasing the electrolysis time to 1.5 h, carbon cloth with fully covered Si nanowires is observed (Figure S3b and e). Further prolonging the electrolysis time to 2 h results in a thicker Si deposition layer (Figure S3c and f). The results indicate that the mass and crystallinity of electrodeposited Si nanowires from soluble SiO2 are tunable, which is beneficial to the one-pot fabrication of Si/carbon cloth anode for LIBs with controllable mass loading of Si nanowires.

Figure 4. CV curves of solid (black curve) and soluble (red curve) SiO2. Scan rate: 20 mV s-1.

The CVs of solid SiO2 and soluble SiO2 in the molten salts are shown in Figure 4. The reduction peaks (R1) at 0.85 V vs. Ca/Ca2+ for soluble SiO2 (red line) and at 0.75 V vs. Ca/Ca2+ for solid SiO2 (black line) are attributed to the electroreduction of SiO2 to Si.27 Obviously, both the onset potential and the peak potential of R1 for soluble SiO2 reduction are slightly more positive than those for solid SiO2 reduction. This fact indicates that the electro-reduction of soluble SiO2 is easier. 29 In addition, the reduction current of R1 for soluble SiO2 finishes at about 0.76 V vs. Ca/Ca2+ while that for solid SiO2 finishes at a more negative potential of about 0.4 V vs. Ca/Ca2+, implying that 12


Page 12 of 50

Page 13 of 50 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


electrochemical reduction of solid SiO2 to Si is more sluggish. Compared with solid SiO2, electrochemical reduction of soluble SiO2 can proceed easier and quicker. The reduction peaks appear at potentials more negative than 0.15 V vs. Ca/Ca2+ are attributed to the formation of Si-Ca alloys and deposition of alkaline or alkaline earth metals.27, 29 Upon reversing the scan, three oxidation peaks (marked as O1, O2 and O3 in Figure 4) for both solid SiO2 and soluble SiO2 appears, revealing that at least three types of silicides form, which may be Ca2Si, CaSi and CaSi2 according to the Si-Ca binary phase diagram. 27 Both solid SiO2 pellets and soluble SiO2 can electrochemically be converted to Si nanowires in molten chlorides, however, the size of the Si nanowires prepared by these two ways shows a huge difference. For the electrolysis of solid SiO2 pellets, the reduction reaction mainly propagates through the Si/SiO2/melt three-phase interlines (3PIs).28 Therefore, both the new Si nucleus formation and initial Si nanowires growing-up proceed at the limited 3PIs areas, leading to the intimate contact between newly formed Si nucleus and Si nanowires. Resultantly, growing-up of Si nanowires by coalescence of the surrounding Si nucleus is easier to happen, leading to formation of large-sized Si nanowires by continually nucleation growth in the Electrolyzed Si powder. For the Si electrodeposition from soluble SiO2, all the surfaces of carbon cloth cathode are the active sites for Si nucleus formation. In fact, the carbon cloth with a microstructure of tangled carbon wires provides a large specific surface area. In addition, as discussed in Figure 4, the electrochemical reduction of soluble SiO2 proceeds easier and quicker, also contributing to a quicker nucleation process.29, 13


36


Therefore, explosive nucleation growth can take place during the electrodeposition of Si from soluble SiO2, facilitating the formation of small-sized Si nanowires in the Electrodeposited Si on carbon cloth. 3.2 Surface chemistry

Figure 5. (a) TEM images of the electrolyzed Si nanowire from solid SiO2; (b) magnification of the area in the dashed square of Figure 5a; (c) TEM image of the electrodeposited Si nanowire; (d) magnification of the area in the dashed square of Figure 5c (Electrolysis potential: 0.3 V vs. Ca/Ca2+; electrolysis time: 1.5 h).

In addition to the size, the surface composition is also different between Si nanowires obtained by these two mechanisms. The typical TEM images of the Si nanowires are shown in Figure. 5. Obviously, the crystalline Si nanowires are covered by an amorphous SiO2 shell (Figure 5b and 5d). For the Electrolyzed Si powder, the thickness of the surface SiO2 shell is around 12 nm (Fig. 5a and b). The thickness of amorphous oxides on the surfaces of the Electrodeposited Si on carbon cloth from soluble SiO2 is only ~1 nm (Fig. 5c and d), nearly 11 times thinner than that of Si 14


Page 14 of 50

Page 15 of 50 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


nanowires from solid SiO2 pellets. The lattice labelled by the parallel white lines in both Figure 5b and d is measured to be 0.31 nm, agreeing well with (111) d-spacing of cubic silicon. This observation indicates that both the Electrodeposited Si on carbon cloth and Electrolytic Si powder consist of crystalline Si in the inner core of nanowires.

Figure 6. XPS results of the Electrodeposited Si on carbon cloth and Electrolyzed Si powder: (a) survey spectra; (b) Si 2p spectra.

The surface compositions of the Si nanowires are further investigated by XPS, as shown in Figure 6. The survey (Figure 6a) XPS shows the existence of Si and O on the surfaces of the both samples. Specially, the intensity of O 1s for the Electrolyzed Si powder from solid SiO2 pellets is obviously stronger than that of the Electrodeposited 15



Si on carbon cloth from soluble SiO2. The atomic ratios of Si/O on the surfaces of the Electrolyzed Si powder and the Electrodeposited Si on carbon cloth are 0.88 and 1.44, respectively. The results further indicate a thicker oxide layer exists on the surface of the Electrolyzed Si powder from solid SiO2 pellets. Figure 6b presents the Si 2P XPS spectra. In the Electrolyzed Si powder from solid SiO2 pellets, peaks ascribed to Si (Si0) and SiOx (Si-O) both appear. The peak intensities of Si0 and Si-O are comparable, indicating severe oxidation of the Si nanowire surfaces. The atomic ratio of Si0 and Si-O on surface is 53:47. In the Electrodeposited Si on carbon cloth from soluble SiO2, Si0 and Si-O are also found, with an atomic ratio (Si0/Si-O) of 66: 34. Obviously, the content of surface Si-O bond is much lower in the Electrodeposited Si on carbon cloth, meaning a less oxidation content compared with the Electrolyzed Si powder. In addition to Si0 and Si-O, Si-C bond located at 101.71 eV is also detected in the Electrodeposited Si on carbon cloth,33, 37-38 as shown in Figure 6b. The formation of Si-C bond indicates that a strong interaction exists between the electrodeposited Si nanowires and the carbon cloth cathode, which is likely the reason for the tight attachment between Si nanowires and carbon cloth. Figure S4 shows the high-resolution TEM image of the carbon cloth after removing the electrodeposited Si nanowires by ultrasonication for more than 12 h in dilute hydrochloric acid solution. The spacing of crystalline lattices on the surface of the carbon fiber is measured to be 0.25 nm (Figure S4), corresponding to the (111) planes of SiC phase (JCPDS No. 29-1129).33 The results are in accordance with that in Figure 6b and further verify that SiC is formed on the contact sites between carbon 16


Page 16 of 50

Page 17 of 50 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


fibers and electrodeposited Si nanowires, contributing to a strong adherence of electrodeposited Si nanowires onto the surface of carbon cloth.

Figure 7. (a) Voltages between graphite anode and carbon cloth cathode monitored during the potentiostatic electrolysis of soluble SiO2 at 0.35 V and 0.25 V vs. Ca/Ca2+; (b) the theoretical decomposition voltages of SiO2 for formation of Si and SiC (Calculated by the HSC 5.1 software).

The voltages between graphite anode and carbon cloth cathode is monitored during the potentiostatic electrolysis process of soluble SiO2 (Figure 7a). For the potentiostatic electrolysis at both 0.35 and 0.25 V vs. Ca/Ca2+, a transition voltage plateau (denoted as Plateau 1 in Figure 7a) at the initial stage is observed, and then a stable higher voltage plateau (denoted as Plateau 2 in Figure 7a) is formed. As shown in Figure 7b, the theoretical formation voltage of SiC is lower than that of Si during electrolysis of SiO2, 17



Page 18 of 50

indicating that the lower voltage Plateau 1 at the initial stage is attributed to formation of SiC while the higher voltage Plateau 2 is for electrodeposition of Si nanowires. It should be mentioned that Plateau 1 only lasts for several minutes. However, the Plateau 2 keeps stable until the end of electrolysis. This observation indicates that, although thermodynamically favorable, formation of SiC suffers from unfavorable reaction kinetics. Therefore, massive formation of Si nanowires is observed, and SiC is only generated at the contact sites between carbon fibers and Si nanowires. Generally, smaller-sized Si nanowires with larger specific surface area are easier to be oxidized when exposed to oxygen-containing atmospheres.10,

13

However, the

Electrodeposited Si on carbon cloth from soluble SiO2 not only have smaller sizes, but also shows a thinner oxide layer. This is likely due to the following reasons. Firstly, formation of minor SiC with high oxidation resistance (as shown in Figure 6b and Figure S4) on the surface of Si nanowires retards Si atoms from oxidation.33 Secondly, the Electrodeposited Si on carbon cloth from soluble SiO2 are tightly attached to the carbon cloth cathode (Figure 3b), leading to a decreased area in contact with oxygencontaining atmosphere and retarded circulation of air among the Si nanowires clusters. Thirdly, the tight and abundant contact between Si nanowires and carbon cloth provides enhanced electron transfer pathways,39 helpful to deep deoxidation of the products on cathode. Si nanowires with a small diameter (20 - 30 nm) and an ultrathin surface oxide layer (~1 nm) are tightly electrodeposited on carbon cloth cathode from soluble SiO2. Compared with solid SiO2 pellets, reduction of soluble SiO2 can proceed at more 18


Page 19 of 50 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


positive potentials. Moreover, carbon cloths cathode can provide much more reduction sites for soluble SiO2 reduction than that of the 3PIs can provide for solid SiO2 reduction. Resultantly, the rate of Si nucleation process is quicker than that of Si nanowire growing-up process, contributing to the formation of smaller-sized Si nanowires. A thinner oxide layer is attributed to the formation of Si-C bonds and the tightly contact between Si nanowires and carbon cloths, which decrease the tendency of Si oxidation. Smaller-sized Si nanowires can facilitate the electrons transfer and lithium ions diffusion. In addition, the ultrathin oxide layer on the surface of Si nanowires can accommodate fast lithium penetration and mitigate the volume expansion.8,

10, 40-41

Therefore, the Si nanowires electrodeposited from soluble SiO2 is likely to show a good performance as anode materials in LIBs. 3.3 Performance as anodes in LIBs The as-obtained Si nanowires were evaluated as anodes for LIBs to understand the influence of the size, surface oxide layer and electrode configuration on lithium-storage performances. Figure 8a shows the CV curves of the Electrodeposited Si on carbon cloth. In the first cycle, a sloped initial reduction current from 1.5 to 0.3 V vs. Li/Li+ is attributed to the irreversible reduction of electrolyte and the formation of a solid electrolyte interphase (SEI) film, which gets disappeared in the following cycles.40-41 The cathodic peaks at 0.15 and 0.1 V vs. Li/Li+ are attributed to the formation of Li-Si alloys. In the anodic scans, peaks located at 0.35 and 0.55 V are related to the delithiation of Li-Si alloys to amorphous Si.40 The redox current increases with increasing scan number, indicating that the irreversible swelling effect of Si nanowires 19



during cycling leads to the increase of the surface area.

Figure 8. The CV curves(a), discharge-charge curves (b) of the Electrodeposited Si on carbon cloth from soluble SiO2; (c) cycling performance of the Electrodeposited Si on carbon cloth and the Electrolyzed Si powder as well as the Coulombic efficiency of Electrodeposited Si on carbon cloth at 500 mA g-1; (d) rate capabilities of the Electrodeposited Si on carbon cloth and the Electrolyzed Si powder.

Figure 8b shows the discharge-charge curves of the Electrodeposited Si on carbon cloth at 200 mA g-1. The first alloying and de-alloying capacity are 2000 and 1750 mAh g-1 respectively, meaning an initial Coulombic efficiency as high as 87.5%. The irreversible capacity loss in the first cycle is likely attributed to the formation of SEI film and the irreversible reduction of surface oxide layer to Si. The Coulombic efficiency increases to 91.5% in the second cycle, and further increases to approximately 100% in the following cycles. Correspondingly, the first alloying and de-alloying capacity of the Electrolyzed Si powder are 2090 and 1194 mAh g-1 respectively (as shown in Figure S5), however, with an initial Coulombic efficiency of only 57.1 %. Obviously, the initial Coulombic efficiency for the Electrodeposited Si on carbon cloth is much higher than that of the Electrolyzed Si powder. The thin surface 20


Page 20 of 50

Page 21 of 50 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


oxide layer is beneficial for the swift formation of the stable SEI film. In addition, the side reactions can largely be eliminated by directly using the binder-free Electrodeposited Si on carbon cloth as the electrode. Furthermore, a thinner surface oxide layer means a lower capacity loss from the irreversible reduction of SiO2 to Si. The capacity loss caused by the irreversible reduction of surface SiO2 layer to Si is shown in Figure 9a (details for calculation of capacity loss in Figure 9a are present in Figure S6 and equation S5). Obviously, the irreversible capacity loss caused by the reduction of SiO2 to Si increases quickly with surface oxide thickness. For the Electrodeposited Si on carbon cloth (with a thin surface oxide layer of ~1 nm and a diameter of 20 – 30 nm), the capacity loss is only 3- 4%. However, the capacity loss increases to 6 – 12% for the Electrolyzed Si powder (with a thick surface oxide layer of ~12 nm and diameter of 100 – 200 nm).

21



Figure 9. Capacity losses of Si nanowires with different diameters and surface oxide thickness (a) and the electrochemical impedance spectra of the Electrodeposited Si on carbon cloth and the Electrolytic Si powder measured at open circuit potential before cycling (b).

The electrochemical impedance spectra (EIS) of the Electrodeposited Si on carbon cloth and the Electrolytic Si powder before cycling test are further measured to understand the resistance difference, as shown in Figure 9b. The typical EIS curve consists of a semicircle in the high-frequency region that is attributed to the charge transfer resistance (Rct) at the electrode/electrolyte interface and a straight line in the low-frequency region belonging to the diffusion of the electrolyte ions into the active electrode.

21

As shown in Figure 9b, the semicircle is significantly smaller in the

spectrum recorded for the Electrodeposited Si on carbon cloth than that for the Electrolytic Si powder, pointing out that the Rct is lower in the former sample. The 22


Page 22 of 50

Page 23 of 50 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


results indicate that the charge transfer across the electrode/electrolyte interface for the Electrodeposited Si on carbon cloth is faster than that for the Electrolytic Si powder because of the thinner surface oxide layer and smaller diameter in the former sample. The cycling performance of the as-prepared Electrolyzed Si powder and the Electrodeposited Si on carbon cloth are compared based on galvanostatic chargedischarge cycling at a constant current density of 500 mA g-1 between 0.01 V and 1.5 V. As shown in Figure 8c, a reversible capacity as high as of 1265 mAh g-1 still remains for the Electrodeposited Si on carbon cloth after cycling at 500 mA g-1 for 200 cycles, 76.8% of the first capacity (1647 mAh g-1). The corresponding Coulombic efficiency increases quickly in the initial few cycles and then kept constant at approximately 100% afterwards. In comparison, the specific capacity of the Electrolyzed Si powder decays rapidly with prolonged cycling numbers, remaining at a low value of 79 mAh g-1 after cycling at 500 mA g-1 for 200 cycles, only about 4.6% of the first capacity (1730 mAh g-1). Cycle performance of the Electrodeposited Si on carbon cloth is also evaluated at a large current density of 1000 mA g-1. As shown in Figure 8c, even at such a large current density, the reversible capacity still shows negligible capacity decay in the former 100 cycles. With further prolonging the cycling number, the capacity begins to decay slowly to 711 mAh g-1 after 200 cycles, but still eight times higher than that of the Electrolyzed Si powder. The initial alloying capacity (IAC), initial Coulombic efficiency (ICE), alloying capacity at the 200th cycle (AC 200th) and the capacity retention (CR) of the Electrodeposited Si on carbon cloth and the Electrolyzed Si powder are summarized in Table 1. It should be mentioned that the loading of active Si 23



Page 24 of 50

nanowires can be easily tuned by varying the electrolysis time. For example, when the electrolysis time is 1.5 h, the loading of Si nanowires on carbon cloth is ~1.0 mg cm-2. By increasing the electrolysis time to 2 h, a higher loading of ~1.4 mg cm-2 can be obtained. The capacity of the Electrodeposited Si on carbon cloth with a loading of ~1.4 mg cm-2 remains a capacity of 1280 mAh g-1 after 100 cycles at 500 mA g-1.

Table 1. Comparison of lithium storage capacity between the Electrodeposited Si on carbon cloth and the Electrolytic Si powder.

Electrodeposited Si on carbon cloth at 500 mA g-1 Electrodeposited Si on carbon cloth at 1000 mA g-1 Electrolyzed Si powder at 500 mA g-1

IAC

ICE

AC 200th

CR

(mAh g-1)

(%)

(mAh g-1)

(%)

1647

91.6

1265

76.8

1328

89.4

711

53.5

1730

59.1

79

4.6

The rate capabilities of the Electrodeposited Si on carbon cloth and the Electrolyzed Si powder are also evaluated and compared. As shown in Figure 8d, for the Electrodeposited Si on carbon cloth, the alloying capacities of 1580, 1316 and 1218 mAh g-1 can be delivered at 200, 500 and 1000 mA g-1, respectively. However, the capacities for the Electrolyzed Si powder are only 596, 284 and 147 mAh g-1 at the corresponding current densities. When the current density is again reversed from 1000 mA g-1 to 200 mA g-1, the capacity of the Electrodeposited Si on carbon cloth returns to 1586 mAh g-1, with negligible capacity decay in the following extended cycles. However, the capacity of the Electrolyzed Si powder only returns to 363.7 mAh g-1, with the capacity rapidly decreasing to 142 mAh g-1 at the 60th cycles. 24


Page 25 of 50 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


The above electrochemical tests suggest that the Electrodeposited Si on carbon cloth have higher initial Coulombic efficiency, much enhanced cycle stability and better high-rate performance. Such an enhanced lithium storage capability of the Electrodeposited Si on carbon cloth to the Electrolyzed Si powder can be rationalized to the smaller size, much thinner surface oxide layer and unique electrode configuration of the former. The smaller diameter of the Electrodeposited Si nanowires shortens the distance for lithium diffusion and provides more electrochemically active area. The present surface oxide layer with a thickness as less as ~1 nm is beneficial to fast formation of stable SEI and uncompromised Li+ penetration. The unique arrayed electrode configuration with Si nanowires directly electrodeposited on highly conducting carbon cloth ensures a prompt electron transfer in the whole electrode,15, 4243

in which the absence of any binder and conducting agents is advantageous to avoid

side reactions. In addition to the carbon cloth cathode, the electrodeposition of Si nanowires with a small diameter of 20 - 30 nm on a nickle foam cathode were also verified, as shown in Figure 10. The results imply that this method is universal for small-sized Si nanowires deposition on highly conductive substrate as binder-free nanostructured Si anode for LIBs.

25



Page 26 of 50

Figure 10. SEM images of the Si nanowires electrodeposited on nickel foam cathode with different magnification times: 100 X (a); 20 K X (b); 60 K X (c); 100 K X (d). Electrolysis potential: 0.3 V vs. Ca/Ca2+; Electrolysis time: 1 h.

It is well known that serious pulverization of Si anode materials due to volume variation during the alloying/de-alloying process causes inferior capacity retention. To mitigate the volume expansion, coating a carbon layer on the surface of Si materials is widely adopted and proven to be an effective way.

21, 44-47

As shown in Table 2, the

performances of Si anode materials (Si nanoparticles and Si nanowires) are greatly enhanced after coating a carbon layer or wrapped by a graphene layer. However, extra steps are needed to coat a carbon layer on the surface of nanostructured Si materials, leading to a more complex preparation process. 21 To simplify the fabrication procedure, no carbon layer is coated on the surface of Si nanowires in this paper. Therefore, to reasonably evaluate the electrochemical performance of Si nanowires herein, the capacity of Si nanowires should be compared with those of Si nanowires without a carbon layer. 26


Page 27 of 50 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


Table 2 the capacities of Si nanowires with/without a carbon layer Coating a carbon layer or not

Capacity (mAh g-1)

Cycling number

Charge/discharge rate (mA g-1)

Morphology of Si

Ref.

Without coating Coating a carbon layer

~1100 ~1600

70th

300

Si nanoparticles

[44]


155 603

120th

200

3D porous Si derived from bamboo charcoal

[45]


1251 1932

30th

400

Si nanoparticles in carbon foam

[46]


~0 2000

100th

400

Si nanowires

[50]

Without coating With a graphene layer

250 1335

80th

200

Si nanowires

[47]

Coating a carbon layer

3362

120th

100

Si nanowires

[21]

Without coating

~1000 ~900

120th 200th

400

Si nanowires

[39]

Without coating

1600

50th

400

Si nanowires

[51]

Without coating

250

80th

200

Si nanowires

[47]

Without coating

~375 ~1000

120th

400

Si nanowires Si nanotube

[52]

Without coating

1430 1386 1265

50th 120th 200th

500

Si nanowires

This work

As shown in Table 2, when compared with the capacities of Si nanowires without a carbon layer, the Si nanowires prepared from SiO2 by molten salt electrolysis in this work shows an obvious superiority. It is prudently anticipated that the capacity of Si nanowires in this work can be further improved by coating a carbon layer. However, the effect of carbon layer on the capacity of Si nanowires is not the aim of this work.

27



Therefore, extra steps to coat a carbon layer on the surface of Si nanowires are not adopted. Silicon-nanowires deposited on carbon-cloth were previously reported by the chemical vapor deposition (CVD) method. 21,48,49 Although being highly effective for synthesis of Si nanowires with high purity, the CVD method suffers from some intrinsic drawbacks. For example, toxic SiH4/SiCl4 are generally used as the silicon precursors, 21

which are prepared after complex reaction, separation and purification steps. In

addition, the CVD method is not appropriate for large-scale synthesis of Si nanowires. 20

Moreover, catalysts are indispensable for the preparation of Si nanowires by CVD

method. 21, 48, 49 To obtain Si nanowires on carbon cloth, deposition of catalysts on the carbon cloth substrate are needed prior to the growth of Si nanowires, 21 adding extra step to this method. In this work, the Si nanowires are deposited on carbon cloth by molten salt electrolysis of soluble SiO2. Compared with the CVD method, the molten salt electrolysis method has the following merits. Firstly, instead of toxic SiH4/SiCl4, the safe, nontoxic and easily accessible SiO2 is used as the raw material. Secondly, no catalysts are needed, which could reduce the cost and simplify the preparation process. Thirdly, molten salt electrolysis method is suitable for the massive preparation of Si nanowires. For example, the molten salt electrolysis method has been used for the massive production of metal aluminum with the annual yields as high as several hundred million tons. The preparation of Si nanowires by this method has also been

28


Page 28 of 50

Page 29 of 50 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


proven to be up-scalable. 27 Therefore, the preparation of Si nanowires on carbon cloth in this work is of merits. In summary, the present work provides an effective and affordable preparation of Si nanowires electrodes with rationally designed size, surface chemistry and electrode configuration. And the Si nanowires electrodeposited on carbon cloth in this work shows a superiority in the electrochemical performance as anode materials in lithium ion batteries.

4 Conclusions

Electrolysis of silica feedstock in molten salts for preparation of Si nanowires were herein compared regarding the two different electrochemical mechanisms, viz. electrodeposition of Si on carbon cloth from dissolved silica and direct electrodeoxidation of solid silica to solid Si powder. The corresponding Electrodeposited Si nanowires on carbon cloth from soluble silica and Electrolyzed Si nanowires from solid silica show different microstructures. For the Electrolyzed Si nanowires from solid SiO2, the diameters are 100 - 200 nm while the thickness of surface oxide layer is about 12 nm. Diameters of the Electrodeposited Si nanowires from soluble SiO2 are only 20 - 30 nm, with a surface oxide layer of ~1 nm. Different from the continually nucleation growth in the solid SiO2 electrolysis, the explosive nucleation process during the electrolysis of soluble SiO2 results in the formation of Si nanowires with a much smaller size. In addition, the strong interaction between the carbon cloth cathode and Si nanowires retards the oxidation of electrodeposited Si 29



nanowires, contributing to the formation of an ultrathin oxide layer on surfaces of Si nanowires. Smaller-sized Si nanowires can facilitate charge transfer and Li-ions diffusions, and a thin oxide layer can mitigate the volume expansion during lithiation/delithiation process without compromise in Li+ penetration. When evaluated as anodes for LIBs, the binder-free electrodes consisting of the Electrodeposited Si nanowires on carbon cloth shows a higher initial Coulombic efficiency, much higher capacity, better cyclability and better high-rate performance than that of the Electrolyzed Si powder electrodes. The Electrodeposited Si nanowires on carbon cloth therefore has a reversible capacity of 711 mAh g−1 after 200 galvanostatic charge/discharge cycles at 1000 mA g−1.The results highlight the importance of surface chemistry, size and electrode configurations on addressing the challenge of Si-based anodes in capacity retention and high-rate performance. The herein reported protocol based on molten salt electrolysis of soluble silica can also be a promising method for large-scale preparation of silicon with high values.

Acknowledgment

The authors acknowledge the funding support from the National Natural Science Foundation of China (51722404, 51674177 and 51804221).

Associated Content 30


Page 30 of 50

Page 31 of 50 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


Supporting Information Available: Digital images, XRD patterns, SEM images and High-resolution TEM image of Carbon cloth cathodes before and/or after electrolysis are available. The discharge- charge curves of the Electrolyzed Si powder electrode and calculation of capacity loss caused by reduction of SiO2 to Si are also available.

Reference

1. Cui, L. F.; Ruffo, R.; Chan, C. K.; Peng, H.; Cui, Y. Crystalline-amorphous Coreshell Silicon Nanowires for High Capacity and High Current Battery Electrodes. Nano Lett. 2009, 9, 491-495. 2. Yu, Y.; Gu, L.; Zhu, C.; Tsukimoto, S.; Aken, P. A. V.; Maier, J. Reversible Storage of Lithium in Silver-coated Three-dimensional Macroporous Silicon. Adv. Mater. 2010, 22, 2247-2250. 3. Zhou, Y.; Guo, H.; Yan, G.; Wang, Z.; Li, X.; Yang, Z.; Zheng, A.; Wang, J. Fluidized Bed Reaction Towards Crystalline Embedded Amorphous Si Anode with Much Enhanced Cycling Stability. Chem. Commun. 2018, 54, 3755-3758. 4. Song, T.; Cheng, H.; Town, K.; Park, H.; Black, R. W.; Lee, S.; Park, W. I.; Huang, Y.; Rogers, J. A.; Nazar, L. F.; Paik, U. Electrochemical Properties of Si-Ge Heterostructures as an Anode Material for Lithium Ion Batteries. Adv. Funct. Mater. 2014, 24, 1458-1464. 5. Xia, F.; Kim, S. B.; Cheng, H.; Lee, J. M.; Song, T.; Huang, Y.; Rogers, J. A.; Paik,

31



U.; Park, W. I. Facile Synthesis of Free-standing Silicon Membranes with Threedimensional Nanoarchitecture for Anodes of Lithium Ion Batteries. Nano Lett. 2013, 13, 3340-3346. 6. Sun, X. W.; Zhang, Y. X.; Losic, D. Diatom Silica, an Emerging Biomaterial for Energy Conversion and Storage. J. Mater. Chem. A 2017, 5, 8847-8859. 7. Le, Q. J.; Wang, T.; Tran, D. N. H; Dong, F.; Zhang, Y. X.; Losic, D. Morphologycontrolled MnO2 modified silicon diatoms for high-performance asymmetric supercapacitors. J. Mater. Chem. A 2017, 5, 10856–10865. 8. Yuang Y.; Xiao, W.; Wang, Z.; Fray, D. J.; Jin, X. Highly Efficient Nanostructuring of Silicon by Electrochemical Alloying/Dealloying in Molten Salts for Superb Lithium Storage. Angew. Chem. Int. Ed. 2018, https://doi.org/10.1002/anie.201809646 9. Liu, N.; Wu, H.; McDowell, M. T.; Yao, Y.; Wang, C.; Cui, Y. A Yolk-shell Design for Stabilized and Scalable Li-ion Battery Alloy Anodes. Nano Lett., 2012, 12, 33153321. 10. McDowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y. Novel Size and Surface Oxide Effects in Silicon Nanowires as Lithium Battery Anodes. Nano Lett., 2011, 11, 4018-4025. 11. Li, D.; Wang, H.; Zhou, T.; Zhang, W.; Liu, H. K.; Guo, Z. Unique Structural Design and Strategies for Germanium-Based Anode Materials Toward Enhanced Lithium Storage. Adv. Energy Mater. 2017, 7, 1700488. 12. Ng, S. H.; Wang, J.; Wexler, D.; Konstantinov, K.; Guo, Z. P.; Liu, H. K. Highly 32


Page 32 of 50

Page 33 of 50 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


Reversible Lithium Storage in Spheroidal Carbon-coated Silicon Nanocomposites as Anodes for Lithium-ion Batteries. Angew. Chem. Int. Ed. 2006, 45, 6896-6899. 13. Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-dependent Fracture of Silicon Nanoparticles during Lithiation. ACS Nano 2012, 6, 1522-1531. 14. Zheng, G.; Xiang, Y.; Xu, L.; Luo, H.; Wang, B.; Liu, Y.; Han, X.; Zhao, W.; Chen, S.; Chen, H.; Zhang, Q.; Zhu, T.; Yang, Y. Controlling Surface Oxides in Si/C Nanocomposite Anodes for High-Performance Li-Ion Batteries. Adv. Energy Mater. 2018, 1801718. 15. Xie, H.; Zhao, H.; Liao J.; Yin, H.; Bard, A.J. Electrochemically Controllable Coating of a Functional Silicon Film on Carbon Materials. Electrochim. Acta 2018, 269, 610-616. 16. Dong, Y.; Slade, T.; Stolt, M. J.; Li, L.; Girard, S. N.; Mai, L; Jin, S. Lowtemperature Molten-salt Production of Silicon Nanowires by the Electrochemical Reduction of CaSiO3. Angew. Chem. Int. Ed. 2017, 56, 14453-14457. 17. Seng, K. H.; Park, M. H.; Guo, Z. P.; Liu, H. K.; Cho, J. Self-assembled Germanium/Carbon Nanostructures as High-power Anode Material for the Lithium-ion Battery. Angew. Chem. Int. Ed. 2012, 51, 5657-5661. 18. 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, 6749-6755. 19. Song, T.; Cheng, H. Y.; Choi, H.; Lee, J. H.; Han, H.; Lee, D. H.; Yoo, D. S.; Kwon, 33



M. S.; Choi, J. M.; Doo, S. G.; Chang, H.; Xiao, J. L.; Huang, Y. G.; Park, W. I.; Chung, Y. C.; Kim, H.; Rogers, J. A.; Paik, U. Si/Ge Double-Layered Nanotube Array as a Lithium Ion Battery Anode. ACS Nano. 2012, 6, 303-309. 20. Usui, H.; Kiri, Y.; Sakaguchi, H. Effect of Carrier Gas on Anode Performance of Si Thick-Film Electrodes Prepared by Gas-Deposition Method. Thin Solid Films 2012, 520, 7006-7010. 21. Wang, X. L.; Li, G.; Seo, M. H.; Lui, G.; Hassan, F. M.; Feng, K.; Xiao, X. C.; Chen, Z. W. Carbon-Coated Silicon Nanowires on Carbon Fabric as Self-Supported Electrodes for Flexible Lithium-Ion Batteries. ACS Appl. Mater. Interfaces, 2017, 9, 9551-9558. 22. 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, 6749-6755. 23. Dong, Y.; Slade, T.; Stolt, M. J.; Li, L.; Girard, S. N.; Mai, L.; Jin, S. Lowtemperature Molten-salt Production of Silicon Nanowires by the Electrochemical Reduction of CaSiO3. Angew. Chem. Int. Ed. 2017, 56, 14453-14457. 24. Cho, S. K.; Fan, F. R.; Bard, A. J. Electrodeposition of Crystalline and Photoactive Silicon Directly from Silicon Dioxide Nanoparticles in Molten CaCl2. Angew. Chem. Int. Ed. 2012, 51, 12740-12744. 25. Cho, S. K.; Fan, F. R. F.; Bard, A. J. Formation of a Silicon Layer by Electroreduction of SiO2 Nanoparticles in CaCl2 Molten Salt. Electrochim. Acta. 2012, 65, 57-63. 34


Page 34 of 50

Page 35 of 50 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


26. Zou, X.; Ji, L.; Yang, X.; Lim, T.; Yu, E. T.; Bard, A. J. Electrochemical Formation of a p-n Junction on Thin Film Silicon Deposited in Molten Salt. J. Am. Chem. Soc. 2017, 139, 16060-16063. 27. Xiao, W.; Jin, X.; Chen, G. Z. Up-scalable and Controllable Electrolytic Production of Photo-responsive Nanostructured Silicon. J. Mater. Chem. A 2013, 1, 10243-10250. 28. Xiao, W.; Jin, X.; Deng, Y.; Wang, D.; Hu, X.; Chen, G. Z. Electrochemically Driven Three-phase Interlines into Insulator Compounds: Electroreduction of Solid SiO2 in Molten CaCl2. Chemphyschem. 2006, 7, 1750-1758. 29. Xiao, W.; Wang, X.; Yin, H.; Zhu, H.; Mao, X.; Wang, D.; Verification and Implications of the Dissolution–electrodeposition Process during the Electro-reduction of Solid Silica in Molten CaCl2. RSC Adv. 2012, 2, 7588-7593. 30. Xiao, W.; Wang, D.; The Electrochemical Reduction Processes of Solid Compounds in High Temperature Molten Salts. Chem. Soc. Rev. 2014, 43, 3215-3228. 31. Tang, C.; Liu, Y.; Xu, C.; Zhu, J.; Wei, X.; Zhou, L.; He, L.; Yang, W.; Mai, L.; Ultrafine Nickel-Nanoparticle-Enabled SiO2 Hierarchical Hollow Spheres for High‐ Performance Lithium Storage. Adv. Funct. Mater. 2018, 28, 1704561 32. Chen, H.; Xu, J.; Chen, P.; Fang, X.; Qiu, J.; Fu, Y.; Zhou, C. Bulk Synthesis of Crystalline Core/Amorphous Shell Silicon Nanowires and Their Application for Energy Storage. ACS Nano. 2011, 5, 8383-8390. 33. Zou, X.; Ji, L.; Lu, X.; Zhou, Z. Facile Electrosynthesis of Silicon Carbide Nanowires from Silica/Carbon Precursors in Molten Salt. Sci. Rep. 2017, 7, 9978.

35



34. Juzeliūnas, E.; Fray, D. J.; Kalinauskas, P.; Valsiūnas, I.; Niaura, G.; Selskis, A.; Jasulaitienė, V. Electrochemical Synthesis of Photoactive Carbon-carbide Structure on Silicon in Molten Salt. Electrochem. Commun. 2018, 80, 6-10. 35. Xu, R.; Wang, G.; Zhou, T.; Zhang, Q.; Cong, H. P.; Sen, X.; Rao, J.; Zhang, C.; Liu, Y.; Guo, Z.; Yu, S. H. Rational Design of Si@Carbon with Robust Hierarchically Porous Custard-pple-like Structure to Boost Lithium Storage. Nano Energy. 2017, 39, 253-261. 36. Yang, X.; Ji, L.; Zou, X. L.; Lim, T.; Zhao, J.; Yu, E. T. Bard, A. J. Toward Costeffective Manufacturing of Silicon Solar Cells: Electrodeposition of High-quality Si Films in a CaCl2-based Molten Salt. Angew. Chem. Int. Ed. 2017, 56, 15078-15082. 37. Zou, X.; Zheng, K.; Lu, X.; Xu, Q.; Zhou, Z.; Solid Oxide Membrane-assisted Controllable Electrolytic Fabrication of Metal Carbides in Molten Salt. Faraday Discuss. 2016, 190, 53-69. 38. Zou, X.; Chen, C.; Lu, X.; Li, S.; Xu, Q.; Zhou, Z.; Ding, W. Solid Oxide Membrane (SOM) Process for Facile Electrosynthesis of Metal Carbides and Composites. Metall. Mater. Trans. B. 2016, 48, 664-677. 39. Xia, F.; Kwon, S.; Lee, W. W.; Liu, Z.; Kim, S.; Song, T.; Choi, K. J.; Paik, U.; Park, W. I. Graphene as an Interfacial Layer for Improving Cycling Performance of Si Nanowires in Lithium-Ion Batteries. Nano Lett. 2015, 15, 6658-6664. 40. Liu, Z.; Guan, D.; Yu, Q.; Xu, L.; Zhuang, Z.; Zhu, T.; Zhao, D.; Zhou, L.; Mai, L. Monodisperse and Homogeneous SiO/C Microspheres: A Promising High-capacity and Durable Anode Material for Lithium-ion Batteries. Energy Storage Mater. 2018, 13, 36


Page 36 of 50

Page 37 of 50 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


112-118. 41. Yu, Q.; Ge, P.; Liu, Z.; Xu, M.; Yang, W.; Zhou, L.; Zhao, D.; Mai, L. Ultrafine SiOx/C Nanospheres and their Pomegranate-Like Assemblies for High-Performance Lithium Storage. J. Mater. Chem. A 2018, 6, 14903-14909. 42. Song, T.; Xia, J.; Lee, J. H.; Lee, D. H.; Kwon, M. S.; Choi, J. M.; Wu, J.; Doo, S. K.; Chang, H.; Park, W. I.; Zang, D. S.; Kim, H.; Huang, Y.; Hwang, K. C.; Rogers, J. A.; Paik, U. Arrays of Sealed Silicon Nanotubes as Anodes for Lithium Ion Batteries. Nano Lett. 2010, 10, 1710-1716. 43. Song, T.; Jeon, Y.; Samal, M.; Han, H.; Park, H.; Ha, J.; Yi, D. K.; Choi, J. M.; Chang, H.; Choi, Y. M.; Paik, U. A Ge Inverse Opal with Porous Walls as an Anode for Lithium Ion Batteries. Energ. Environ. Sci. 2012, 5, 9028. 44. Yu, J. L.; Yang, J.; Feng, X. J.; Jia, H.; Wang, J. L.; Lu, W. Uniform Carbon Coating on Silicon Nanoparticles by Dynamic CVD Process for Electrochemical Lithium Storage. Ind. Eng. Chem. Res. 2014, 53, 12697-12704. 45. Zhang, C. C.; Cai, X.; Chen, W. Y.; Yang, S. Y.; Xu, D. H.; Fang, Y. P.; Yu, X. Y. 3D Porous Silicon/N-Doped Carbon Composite Derived from Bamboo Charcoal as High-Performance Anode Material for Lithium-Ion Batteries. ACS Sustainable Chem. Eng. 2018, 6, 9930-9939. 46. Botas, C.; Carriazo, D.; Zhang, W.; Rojo, T.; Singh, G. Silicon-Reduced Graphene Oxide Self-Standing Composites Suitable as Binder-Free Anodes for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 28800-28808.

37



47. Zhu, Y.; Liu, W.; Zhang, X. Y.; He, J. C.; Chen, J. T.; Wang, Y. P.; Cao, T. B. Directing Silicon-Graphene Self-Assembly as a Core/Shell Anode for HighPerformance Lithium-Ion Batteries. Langmuir 2013, 29, 744-749. 48. Zeng, B. Q.; Xiong, G. Y.; Chen, S.; Wang, W. Z.; Wang, D. Z.; Ren, Z. F. Field Emission of Silicon Nanowires Grown on Carbon Cloth. Appl. Phys. Lett. 2007, 90, 033112. 49 Chockla, A. M.; Klavetter, K. C.; Mullins, C. B.; Korgel, B. A. Tin-Seeded Silicon Nanowires for High Capacity Li-Ion Batteries. Chem. Mater. 2012, 24, 3738-3745. 50 Bogart, T. D.; Oka, D.; Lu, X.; Gu, M.; Wang, C.; Korgel, B. A. Lithium Ion Battery Performance of Silicon Nanowires with Carbon Skin. ACS Nano 2014, 8, 915-922. 51 Kang, K.; Lee, H. S.; Han, D. W.; Kim, G. S.; Lee, D.; Lee, G.; Kang, Y. M.; Jo, M. H. Maximum Li Storage in Si Nanowires for the High Capacity Three-Dimensional LiIon Battery. Appl. Phys. Lett. 2010, 96, 053110. 52 Wu, H.; Chan, G.; Choi, J.W.; Ryu, I.; Yao, Y.; MacDowell, M.T.; Lee, S.W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y.; Stable Cycling of Double-Walled Silicon Nanotube Battery Anodes through Solid–Electrolyte Interphase Control. Nat. Nanotechnol. 2012, 7, 310-315.

38


Page 38 of 50

Page 39 of 50 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


For Table of Contents Only

39



TOC figure 84x52mm (300 x 300 DPI)


Page 40 of 50

Page 41 of 50 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


Figure 1. Illustration of the carbon cloth cathode before and after deposition of Si nanowires. 84x41mm (300 x 300 DPI)



Figure 2. XRD patterns of (a) carbon cloth and Electrodeposited Si on carbon cloth, and (b) Electrolyzed Si powder. Electrolysis potential: 0.3 V vs. Ca/Ca2+. Electrolysis time: 1.5 h. 84x133mm (300 x 300 DPI)


Page 42 of 50

Page 43 of 50 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


Figure 3. SEM images of bare carbon cloth (a), the Electrodeposited Si on carbon cloth (b, c) and the Electrolytic Si powder (d); TEM image of the Electrolytic Si powder (e). All the electrolysis is conducted at 0.3 V vs. Ca/Ca2+; Electrolysis time is 1.5 h for soluble SiO2 and 12 h for solid SiO2 pellets. 84x51mm (300 x 300 DPI)



Figure 4. CV curves of solid (black curve) and soluble (red curve) SiO2. Scan rate: 20 mV s-1.


Page 44 of 50

Page 45 of 50 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


Figure 5. (a) TEM images of the electrolyzed Si nanowire from solid SiO2; (b) magnification of the area in the dashed square of Figure 5a; (c) TEM image of the electrodeposited Si nanowire; (d) magnification of the area in the dashed square of Figure 5c (Electrolysis potential: 0.3 V vs. Ca/Ca2+; electrolysis time: 1.5 h). 84x76mm (300 x 300 DPI)



Figure 6. XPS results of the Electrodeposited Si on carbon cloth and Electrolyzed Si powder: (a) survey spectra; (b) Si 2p spectra. 84x137mm (300 x 300 DPI)


Page 46 of 50

Page 47 of 50 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


Figure 7. (a) Voltages between graphite anode and carbon cloth cathode monitored during the potentiostatic electrolysis of soluble SiO2 at 0.35 V and 0.25 V vs. Ca/Ca2+; (b) the theoretical decomposition voltages of SiO2 for formation of Si and SiC (Calculated by the HSC 5.1 software). 84x125mm (300 x 300 DPI)



Figure 8. The CV curves(a), discharge-charge curves (b) of the Electrodeposited Si on carbon cloth from soluble SiO2; (c) cycling performance of the Electrodeposited Si on carbon cloth and the Electrolyzed Si powder as well as the Coulombic efficiency of Electrodeposited Si on carbon cloth at 500 mA g-1; (d) rate capabilities of the Electrodeposited Si on carbon cloth and the Electrolyzed Si powder. 84x57mm (300 x 300 DPI)


Page 48 of 50

Page 49 of 50 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


Figure 9. Capacity losses of Si nanowires with different diameters and surface oxide thickness (a) and the electrochemical impedance spectra of the Electrodeposited Si on carbon cloth and the Electrolytic Si powder measured at open circuit potential before cycling (b). 84x123mm (300 x 300 DPI)



Figure 10. SEM images of the Si nanowires electrodeposited on nickel foam cathode with different magnification times: 100 X (a); 20 K X (b); 60 K X (c); 100 K X (d). Electrolysis potential: 0.3 V vs. Ca/Ca2+; Electrolysis time: 1 h. 84x57mm (300 x 300 DPI)


Page 50 of 50

Electrodeposited Silicon Nanowires from Silica ... - ACS Publications

Recommend Documents