C composite nanofibers as free-standing anode materials for Li-ion batteries

Journal Pre-proof Synthesis and characterization of SiO2/C composite nanofibers as free-standing anode materials for Li-ion batteries Ayaulym Belgibayeva, Izumi Taniguchi PII:

S0013-4686(19)31972-3

DOI:

https://doi.org/10.1016/j.electacta.2019.135101

Reference:

EA 135101

To appear in:

Electrochimica Acta

Received Date: 23 July 2019 Revised Date:

4 October 2019

Accepted Date: 15 October 2019

Please cite this article as: A. Belgibayeva, I. Taniguchi, Synthesis and characterization of SiO2/C composite nanofibers as free-standing anode materials for Li-ion batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135101. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Synthesis and characterization of SiO2/C composite nanofibers as freestanding anode materials for Li-ion batteries Ayaulym Belgibayeva and Izumi Taniguchi∗

Department of Chemical Science and Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan Abstract One dimensional SiO2/C composite nanofibers were successfully synthesized

from

precursor solutions with polyvinylpyrrolidone (PVP) as a carbon source by electrospinning with two-step heat treatment, comprising preoxidation at 280°C in the air followed by annealing at 700°C for 1 h in a reduced atmosphere. The effects of the preoxidation treatment on the morphology, chemical structure, and electrochemical properties of the synthesized materials were studied by scanning electron microscopy, X-ray diffraction analysis, Fourier transform infrared spectroscopy, and solid-state nuclear magnetic resonance spectroscopy. The two-step heat treatment had strong positive impacts not only on the morphology but also on the structure and electrochemical performance of the materials. Furthermore, free-standing SiO2/C composite nanofiber mats (FS-SiO2/C-CNFMs) were prepared from precursor solutions with different PVP concentrations by electrospinning with the two-step heat treatment. The FS-SiO2/C-CNFM obtained from the precursor solution with 5 wt.% PVP had a geometric mean diameter of 79 nm and a specific surface area of 642 m2 g–1. It exhibited high initial discharge and charge capacities of 1800 and 984 mAh (SiO2-g)–1, respectively, and finally retained a charge capacity of 754 mAh (SiO2-g)–1 after 200 cycles with 100% Coulombic efficiency.

Keywords: Li-ion batteries, anode, SiO2/C composite, nanofibers, electrospinning

∗

Corresponding author. Tel. & Fax: +81-3-5734-2155

E-mail: [email protected] 1

1. Introduction

The rapid development of different technologies to meet the needs of today’s information-rich society requires a consecutive improvement of Li-ion batteries (LIBs) as key components of portable, entertainment, and telecommunication equipment [1]. LIBs have been the energy storage system of choice for various applications owing to their high energy and power density, light weight, and compactness [2]. On the other hand, the current demand for energy lies far beyond the capacity of commercially available LIBs, which is limited by their electrode materials [1–4]. Research over the past several decades has been devoted to the development of new electrode materials with special attention to cathodes [5–7]. As a result of extensive studies, novel high-energy technologies with the potential to be implemented in the near future have been proposed [8]. Nevertheless, when coupled with high-capacity cathode materials, the commercially available anode materials still have a limited capacity (372 mAh g−1) [4]. Since the discovery of the electrochemical activity of SiO2 towards Li+ ions by Gao et al. [9], SiO2 has been attracting more attention as a promising alternative anode material owing to its high theoretical capacity (1965 mAh g−1) [10]. Furthermore, the natural abundance, low cost, and environmental friendliness of

SiO2 make it a commercially viable electrode

material for LIBs [11]. However, there are some limitations hindering the practical application of this material. First, the high capacity of SiO2 cannot be fully utilized owing to its poor electronic conductivity. The actual reversible capacity of pure commercial SiO2 nanoparticles (7 nm diameter) has been reported to be ca. 400 mAh g−1 [9]. Another problem is capacity fading caused by volume expansion during cycling. To overcome these issues, carbon coating and nanostructuring are believed to be effective approaches [10–18]. 2

Among the different techniques for the synthesis of carbon composite nanostructures, electrospinning is recognized as a simple and cost-effective synthesis technique. It can produce one dimensional (1D) carbon composite nanofibers of a large number of materials, including organics and inorganics [19–22]. The 1D fibers are considered to be an excellent conductive substrate for host nanomaterials owing to the very short paths for Li+ on the crosssection of the fibers and the large interior surface area [23]. Polymer composite nanofibers of SiO2 can be easily synthesized via electrospinning as an intermediate product of porous carbon (C), silicon (Si) or Si/C nanofiber synthesis [24–28]. Recently, an electron-conductive SiO2@C nanofiber has been developed and applied as a microporous layer in a protonexchange membrane fuel cell [29]. However, there are only a few studies on the electrochemical performance of SiO2/C composite nanofibers as an anode material for LIBs [30–33]. Wu et al. [30] developed nanostructured SiO2/C composites by mixing SiO2 nanoparticles with polyacrylonitrile (PAN) as a carbon precursor, which was subjected to electrospinning and subsequent heat treatments. Prepared composite fibers containing 15 wt.% SiO2 showed stable cycling performance with a high reversible capacity of 658 mAh g−1 after 100 cycles at a current density of 50 mA g−1. Another research group [31] further enhanced the cyclability of SiO2/C nanofibers up to 1000 cycles by fabricating flexible and robust N-doped freestanding nanofiber films from a mixture of PAN and tetraethyl orthosilicate (TEOS) as the SiO2 source. On the other hand, the low utilization of SiO2 (23%), which is caused by the precipitation of PAN upon mixing with the SiO2 precursor solution [32], still limits the overall capacity of the composite. Furthermore, PAN has relatively low solubility in many solvents, and the commonly used solvent dimethylformamide (DMF) is hazardous. The introduction of another polymer source, along with an increase in the overall SiO2 content in the electrode, should be considered. 3

Wang et al. [34] developed mesoporous carbon nanofibers from thermoplastic polyvinylpyrrolidone (PVP), which is more soluble than PAN in many solvents, including green water and ethanol, making it much easier to develop a wide range of carbon-based composite nanofibers. Ren’s group [33] then mixed SiO2 nanoparticles with PVP and obtained SiO2/C composite fibers by electrospinning with heat treatments. The composite containing 44 wt.% SiO2 showed a reversible capacity of 465 mAh g−1 at a current density of 50 mA g−1 up to 50 cycles. On the other hand, the uniform dispersion of nanoparticles in the polymer solution cannot be ensured, and inhomogeneous mixing of the components may negatively affect the cyclability of the composite [31]. In this work, we have attempted to develop free-standing SiO2/C composite nanofibers with a high SiO2 content from the homogeneous precursor solutions containing PVP and TEOS via electrospinning with two-step heat treatment

comprising preoxidation at 280°C

in the air followed by annealing at 700°C for 1 h in a reduced atmosphere. The excellent mechanical flexibility and strength of the free-standing nanofibers ensure the stability of the electrode during cycling. Furthermore, elimination of the additive of conductive carbon black and electrochemically inactive binder, and metal current collector, to some extent, decrease the weight of cell and improve the energy density [31, 35]. We have investigated the effects of the process parameters on the structure and morphology of the prepared nanofibers and the correlation between the physical and electrochemical properties of SiO2/C nanofibers as the anode material for LIBs. To the best of our knowledge, this is the first report on the effects of the preoxidation process and fiber size on the electrochemical performance of SiO2/C nanofibers.

2. Experimental

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2.1 Precursor solutions

The precursor solution used for the electrospinning was prepared by dissolving 0.24 g of TEOS (99%) in a mixture of 0.48 g of PVP (Mw=1,300,000) with 8 mL of ethanol, which was followed by stirring for 2 h. The concentration of PVP in the ethanol was varied from 4 to 7 wt.%, while the weight ratio of PVP to TEOS was fixed at 2:1. Nitric acid (HNO3) was added to the solutions as a catalyst with a TEOS: HNO3 molar ratio of 20:1.

2.2 Experimental setup and procedure

Fig. 1 shows a schematic diagram of the electrospinning apparatus, which has been adopted from an electrostatic spray deposition (ESD) setup [36–39]. It consists of two main parts: a liquid-precursor feed unit and an electrostatic spin unit. The liquid-precursor feed unit comprises a micro-feeder (1) (Furue Science, JP-V) with a glass syringe (2). The electrostatic spin unit consists of a flexible stainless steel needle (3) (19G), a high-DC-voltage power supply (4) (Matsusada Precision, HAR-30P2), and a grounded collector (7) covered with aluminum (Al) foil. The precursor solution is fed to the stainless steel needle at a feed rate of 1 mL h−1. A high positive voltage of 15 kV is applied to the tip of the stainless steel needle, causing the formation and ejection of a jet. The charged jet gradually elongates to the grounded collector and forms a randomly oriented nanofiber with the evaporation of the solvent (6), which is captured on the collector located at a distance of 10 cm from the stainless steel needle. A light source (9) inside the box allows the formation of fibers (6) from the tip of the needle to be observed by a CCD camera connected to a monitor (10). The humidity inside the box was controlled using silica gel (11).

5

The as-spun fibers were dried at 150°C for 12 h to remove the remaining solvent. The dried fibers were detached from the Al foil, then directly annealed in a horizontal tubular furnace at 700°C for 1 h in an N2+H2 (97/3 v.v%) atmosphere at a heating rate of 5°C min−1. Sample of these fibers is denoted as SiO2/C–700°C. To stabilize the fibrous morphology, asspun samples were also preoxidized at 280°C for 1 h in an air oven (samples denoted as PO– 280°C). The preoxidized samples were further annealed at 700°C for 1 h in an N2+H2 (97/3 v.v%) atmosphere at a heating rate of 5°C min−1 (samples denoted as SiO2/C–280°C–700°C). The synthesis route of the SiO2/C nanofibers by electrospinning with heat treatments is illustrated in Fig. 2.

2.3 Physical characterization

X-ray diffraction (XRD) analysis was performed using a Rigaku Ultima IV diffractometer with a D/teX Ultra X-ray detector and Cu-Kα radiation to identify the crystalline phases of the samples. The molecular structure of the samples was studied from their Fourier transform infrared (FTIR) absorption spectra recorded by a Shimadzu IRAffinity-1 Miracle-10 FTIRATR spectrophotometer at a resolution of 4 cm−1 with a minimum of 80 scans averaged per spectrum. Solid-state nuclear magnetic resonance (NMR) spectra were recorded on a Bruker BioSpin 400 MHz AVANCE III 54-mm‐bore NMR system. The structure of carbon was analyzed by Raman spectroscopy (NRS-2100, JASCO Co.). The morphology of the prepared samples was observed by scanning electron microscopy (SEM, KEYENCE, VE-9800SP) or field-emission SEM (FE-SEM, Hitachi, S4500). The fiber diameter distribution, geometric mean diameter dg, and geometric standard deviation σg, were calculated by randomly selecting about 500 fibers in the SEM images. The equations used for the calculations are given elsewhere [40]. The microstructure of the samples was 6

characterized by transmission electron microscopy (TEM, JEOL Ltd., JEM2010F). The elemental distributions in the samples were observed by FE-SEM (Hitachi, SU9000) with energy dispersive spectroscopy (EDS, Ametech, Genesis-APEX) at 30 kV. The nitrogen adsorption–desorption isotherms were examined using Micromeritics TriStar-II or 3Flex instruments at 77 K and the specific surface area was calculated using the Brunauer–Emmett–Teller (BET) equation. The contents of carbon, hydrogen, and nitrogen of the final samples were determined using an element analyzer (CHNS, Elementar, Vario Micro Cube).

2.4 Electrochemical characterization

The electrochemical performance of the prepared samples was evaluated in CR2032 coin-type cells assembled in high-purity argon gas (99.9995% purity). To prepare the positive electrode, mixtures containing SiO2/C composite nanofibers, polyvinylidene fluoride (PVDF) as a binder, and acetylene black (AB) as a conductive agent (with a weight ratio of 8:1:1) were dispersed in N-methyl-2-pyrrolidone (NMP) and coated on a Cu foil current collector using the doctor blade technique. The coated electrodes were dried in a vacuum oven at 110oC for 4 h and then pressed to obtain strong adherence between the electrode material and the current collector. The electrodes were punched into circular discs with a diameter of 1.55 cm and scraped to standardize the area of each electrode to 1 cm2. A 1 mol L−1 solution of LiPF6 in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 volume ratio was used as the electrolyte, and Li foil was used as the negative electrode. Galvanostatic discharge–charge tests were performed on multi-channel battery testers (Hokuto Denko, HJ101mSM8A) in the voltage range between 0.01 and 3.0 V vs. Li+/Li at a 50 mA (SiO2/C+AB)-g −1 discharge–charge rate. 7

As-prepared free-standing SiO2/C composite nanofibers were cut into 1 cm2 pieces and directly used as positive electrodes without any coating procedure. The mass of prepared electrodes was in the range of 2–4 mg. The rate performance of these was measured at different current densities from 100 to 1000 mA g−1 in the voltage range between 0.01 and 3.0 V vs. Li+/Li. Cyclic voltammetry (CV) was conducted over the potential range from 0 to 3.1 V at a scan rate of 0.1 mV s−1, and electrochemical impedance spectroscopy (EIS) measurements were performed at 5 mV AC amplitude in the frequency range from 100 kHz to 0.01 or 0.001 Hz using a Solartron SI 1287 electrochemical interface. All the electrochemical measurements were carried out at room temperature.

3. Results and discussion

3.1 Synthesis of SiO2/C composite fibers

Fig. 3 shows the SEM images of the samples

prepared by electrospinning from the

precursor solution with 7 wt.% PVP before and after the heat treatments under different conditions. The electrospun sample clearly shows a fibrous morphology with continuous long smooth fibers (Fig. 3a). On the other hand, the morphology was drastically changed after the direct heat treatment at 700°C by the interconnection of the fibers with each other (Fig. 3b). To prevent the drastic change in morphology during the heat treatment, additional heat treatment as a preoxidation was introduced. The electrospun samples were treated at 280°C in air for 1 h and then annealed at 700°C for 1 h in an N2+H2 (97/3 v.v%) atmosphere. Figs. 3c and 3d show the SEM images of the samples after the preoxidation and then the following heat treatment, respectively. The SEM images of the samples without and with peroxidation show that the fibrous morphology is retained when peroxidation is introduced. The successful 8

preoxidation and carbonization were confirmed from the change in sample color after each heat treatment, as shown in the digital photographs in Fig. 3e. Fig. 4(a) shows the XRD patterns of the samples prepared by electrospinning from the precursor solution with 7 wt.% PVP after the heat treatments under different conditions. The broad peak near 23o is the characteristic peak of amorphous SiO2, and the peaks near 25o (002 peak of carbon) and 43o (100 peak of carbon) imply the presence of amorphous carbon [41]. All the XRD patterns exhibit weak broadened peaks in the range between 20 and 30o, which may be due to the overlap of the SiO2 peak and the 002 peak of carbon. On the other hand, the peak near 43o can be barely detected in the XRD pattern of the sample obtained by electrospinning with the preoxidation and carbonization processes. Thus, the carbon and silicon structures of the samples were studied in detail by FTIR analysis. Fig. 4(b) presents the FTIR spectra of the samples whose XRD patterns are shown in Fig. 4a. The FTIR spectrum of electrospun PVP fibers is also given for reference. The strong broad bands at 1130−1000 cm−1 and the smaller bands at 820 cm−1 in the FTIR spectra are assigned to Si–O–Si asymmetric and symmetric stretching vibrations, respectively [12]. The additional bands at 962 cm−1 correspond to Si−OH stretching vibrations [30]. After the direct heat treatment at 700°C (SiO2/C–700°C), all the peaks of PVP disappear, and new weak peaks assigned to aromatic –C=C− (carbon−carbon double bond) appear, confirming the decomposition and carbonization of PVP. Furthermore, the peak position of the Si–O–Si band is shifted to a lower wavenumber (1053 cm−1), which may be due to the decrease in the Si–O–Si angle caused by the chemical bonding of SiO2 with carbon, forming cyclic SiOC, where some oxygen atoms are substituted by carbon [42]. The FTIR spectra of the samples after the preoxidation at 280°C (PO–280°C sample) and the following heat treatment at 700°C (SiO2/C–280°C–700°C sample) differ from that of the sample subjected to direct heat treatment (SiO2/C–700°C sample ). As compared with the 9

spectrum of the PVP sample, the C−N and C=O bands in the spectrum of the PO–280°C sample shift to a higher wavenumber, indicating a change in the C−N−C angle, probably caused by the opening of the heterocycle and the formation of carboxyl −COOH groups, respectively. Furthermore, the stretching vibration band of the −C=C− double bond of alkene groups at 1690−1630 cm−1 appears after the preoxidation and becomes the −C=C− double bond of aromatic groups at 1600−1475 cm−1 after the further heat treatment at 700°C [32]. The peak position of the Si–O–Si band is located at 1095 cm−1 for the PO–280°C sample. However, it shifts from 1095 to 1066 cm−1 after further heat treatment at 700°C. The difference in the peak position of the Si–O–Si band in the FTIR spectra of the SiO2/C–700°C and SiO2/C–280°C–700°C samples may be due to the difference in their chemical structure. In order to further understand the structural changes of the sample during heat treatments under different conditions and to estimate the structure of the final samples, solid-state NMR analysis was conducted. Figs. 4(c) and 4(d) respectively show 13C and 29Si NMR spectra of the samples after heat treatments. All peaks in the 13C spectra are consistent with the carbon structures observed in the FTIR spectra. A large peak centered around 130 ppm, corresponding to aromatic carbon, is observed in the 13C spectra of all samples. In the spectra of the PO–280°C sample, additional peaks of uncyclized alkene groups and oxidized carbon structures are detected at 110 and 165 ppm, respectively. An additional peak of C–N is clearly observed at approximately 55 ppm for the SiO2/C–280°C–700°C sample. On the other hand, a difference in the structure of the Si tetrahedra can be clearly seen in the 29Si spectra. The main peak of the SiO4 group at approximately −100 ppm is present in the spectra of all samples [43]. Additional significant peaks of silicon from carbon-substituted oxides (SiO3C, SiO2C2, SiOC3) appears in the spectra of the SiO2/C–700°C sample [44, 45]. The formation of SiO3C, SiO2C2, and SiOC3 after the direct heat treatment at 700°C may have led to the

10

different peak positions of the Si–O–Si band in the FTIR spectra of the SiO2/C–700°C and SiO2/C–280°C–700°C samples. Table 1 summarizes the elemental composition of the as-spun sample before and after the heat treatment under different conditions determined by CHNS analysis. After the heat treatment, the weight percentage of the residual carbon hardly changed in the SiO2/C–280°C– 700°C sample, whereas it decreased from 50 to 28 wt.% in the SiO2/C–700°C sample. It was confirmed by the morphology observation and structural characterization that the preoxidation has significant effects on the morphology and structure of the final products. Furthermore, in the structural characterization of the as-spun sample by FTIR analysis, the characteristic bands of both PVP and SiO2 were observed, whereas the band at 820 cm−1, corresponding to the Si–O–Si symmetric stretching vibration, as well as some O−H and C−H vibrations of PVP were absent or had low intensities (Fig. S1). The structures of the as-spun, SiO2/C–700°C, PO–280°C, and SiO2/C–280°C–700°C samples could be approximately estimated from the characteristic bands in the FTIR spectra and the peaks in the NMR spectra. Schematic illustrations of possible structural changes in the present synthesis process for the SiO2/C composite fibers are shown in Fig. 5. The morphological change during the direct heat treatment at 700°C can be mainly attributed to the general PVP pyrolysis process and the subsequent structural degradation in an inert atmosphere (process (1a) in Fig. 5). The melting of PVP and its decomposition by the breakdown of the C−N bonds connecting the polyvinyl skeleton with the heterocycle take place in the intermediate stage of the pyrolysis between 365 and 460°C, forming unsaturated vinyl radicals (Fig. 5(b)). Elimination of the heterocycle is confirmed from the reduced nitrogen content (Table 1) and the FTIR and NMR spectra of the SiO2/C–700°C sample (Figs. 4(b) and 4(c)). Furthermore, the morphological change during the direct heat treatment at 700°C can also be

related to the carbonization process. When the oxide content is 11

sufficiently high, the oxides bond with the polymer and restrict the free movement of unsaturated vinyl radicals. As a result, the carbonization process is localized within the oxide carcass and does not cause any damage to the fibrous structure [46]. On the other hand, the lower content of the oxide in the form of SiO2 allows the free movement of the radicals; thus, they can easily connect with the radicals of the neighboring fibers and form interconnected structures. Another possible reason is the formation of intermediate pyrolysis byproducts, which is also accompanied by the melting of the medium and leads to the connection of fibers. As confirmed from the FTIR and NMR spectra of SiO2/C–700°C and illustrated in Fig. 5 (c), SiO2 bonded with carbon via the substitution of oxygen atoms. This indicates the strong reducing capability of the intermediate pyrolysis product caused by oxygen deficiency. The oxygen deficiency in fibers restricts the smooth elimination of the pyrolysis byproducts and the morphology changes.

In this case, the elimination of organic byproducts by the

preoxidation treatment below the melting point of PVP (~300°C) could help to improve the morphology of the heat-treated composites by providing additional oxygen molecules. As shown in Fig. 5 (d), during the preoxidation at 280°C (process (1b) in Fig. 5) instead of C−N bond-breaking the carbonyl group of the ketone oxidizes to dicarboxylic acid with the opening of the heterocycle. This preoxidation process helps to maintain the fibrous morphology by preventing the melting of the medium. Furthermore, a SiO2/C composite without carbon-substituted SiO4-xCx species could be obtained after further high-temperature heat treatment, as shown in Fig. 5 (e). Both the morphology and structure of the material are expected to have a strong effect on its electrochemical performance.

3.2 Effect of preoxidation treatment on the electrochemical properties of SiO2/C composite fibers

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Fig. 6(a) shows the two initial discharge and charge curves of the SiO2/C–700°C and SiO2/C–280°C–700°C samples at a current density of 50 mA g−1. Both samples delivered similar initial discharge and charge capacities of about 901 and 505 mAh g−1, respectively, with an initial Coulombic efficiency of 56%. For the SiO2/C–700°C sample, the initial discharge curve consists of a barely visible plateau at about 0.6 V and a smooth long plateau below 0.3 V. This type of discharge curve is common for the SiOC materials, as reported by Wilson et al. [41]. From the above structure analysis of the SiO2/C–700°C sample and the electrochemical reaction of amorphous nano-SiO2 with Li+ ions reported by Guo et al. [10], we predict that the lithiation into the SiO2/C–700°C sample occurs by the following electrochemical reactions:

(x+1)SiO4-xCx + 8Li+ + 8e-→ xSiO3-xCx+1+4Li2O+ Si SiO2 + 4Li + + 4e- → Si + 2Li2O 2SiO2 + 4Li+ + 4e-→ Si + Li4SiO4 Si + xLi+ + xe- ↔ LixSi

(1a) (1b) (1c) (2)

In the above reactions, Eq. (1a) represents the lithiation into the SiOC materials, and Eqs. (1b) and (1c) express the irreversible reaction of SiO2 with Li+ ions to form pure Si and electrochemically inactive LiO2. Eq. (2) indicates that the Si further reversibly reacts with Li+ ions at a voltage of less than 0.3 V. The large irreversible capacity in the first cycle is caused by the formation of inactive byproducts and a solid electrolyte interphase (SEI) layer. The difference between the discharge curves of the first and second cycles may be caused by the irreversible reactions and the SEI formation. This can also be seen in the discharge curves of the SiO2/C–280°C–700°C sample; the initial discharge curve of the SiO2/C–280°C–700°C

13

sample shows plateaus at approximately 0.8, 0.4, and 0.25 V and is similar to that of a SiO2/C composite fiber electrode [33]. To further investigate the reaction mechanism during the electrochemical process, CV measurements of the SiO2/C–700°C and SiO2/C–280°C–700°C samples were conducted at a scan rate of 0.1 mV s−1. Fig. 6 (b) shows the cyclic voltammograms of both samples for the initial two cycles. In the initial discharge process, an irreversible peak corresponding to SEI formation begins to appear at about 0.7 V on the voltammograms of both samples. On the other hand, the SiO2/C–700°C sample exhibits only one additional cathodic peak starting at 0.4 V, whereas the SiO2/C–280°C–700°C sample shows two cathodic peaks at 0.28 V and below 0.25 V. The positions of the reduction peaks in the initial CV curves are in good agreement with the positions of the discharge plateaus in the discharge–charge profiles in Fig. 6 (a), confirming the validity of the proposed lithiation mechanism. Fig. 7 shows the cyclability of the SiO2/C–700°C and SiO2/C–280°C–700°C samples up to 50 cycles. Despite the similar initial capacities, the cyclability of both samples is different. The capacity of the SiO2/C–700°C sample gradually decreases to 330 mAh g−1 after 50 cycles, which may be attributed to the formation of electrochemically inactive species such as SiC4 finally obtained from SiO4-xCx [47]. Furthermore, the dense structure of the electrode can not accommodate the volume expansion during continuous cycling. On the other hand, the capacity of the SiO2/C–280°C–700°C sample increases after a small decrease in the first 10 cycles and reaches 470 mAh g−1 after 50 cycles. The improved electrochemical performance is attributed to the fibrous morphology and chemical structure of the sample obtained with the preoxidation treatment. Elimination of the binder and current collector via the formation of a free-standing electrode may markedly improve the energy density of batteries. Furthermore, we can omit several complicated processes in the manufacture of the electrode. Thus, as described in the 14

next sections, we attempted to develop free-standing SiO2/C composite nanofibers and studied their physical and electrochemical properties.

3.3 Synthesis of free-standing SiO2/C composite fibers

The concentration of PVP in the precursor solution may be a key parameter affecting the morphology of the prepared samples. Pure PVP nanofibers were obtained from solutions with PVP concentrations ranging from 6 to 10 wt.%. Since the addition of TEOS to the PVP solution and the hydrolysis of TEOS lead to increased viscosity of the precursor solution, the PVP concentration was initially chosen as 7 wt.% and then decreased to 4 wt.% while keeping the PVP:TEOS ratio constant (2:1). The effect of the PVP concentration on the morphology of the final samples was observed by SEM. Figs. 8 (a)-(d) show the SEM images of the SiO2/C–280°C–700°C samples prepared from the precursor solutions with PVP concentrations ranging from 4 to 7 wt.%. As can be seen from the SEM images, the SiO2/C– 280°C–700°C samples have a uniform fibrous morphology consisting of long and smooth nanofibers. The geometric mean diameter of the fibers has a linear relationship with the PVP concentration and decreases from 177 to 54 nm with decreasing PVP concentration from 7 to 4 wt.%. The SiO2/C composite nanofibers prepared from the solutions with 6 and 5 wt.% PVP have narrower diameter distributions with geometric standard deviations of 1.5 and 1.6, respectively. The digital images in the insets of Fig. 8 show the maximum folding angle of the fiber mats allowed by their flexibility. The sample obtained from 7 wt.% PVP solution is not foldable owing to its brittleness, whereas the sample obtained from 4 wt.% PVP solution can be easily folded without any breakage. The flexibility of the SiO2/C composite nanofibers increases with decreasing geometric mean diameter.

Finally, we concluded that the free-

standing SiO2/C composite nanofiber mats (FS-SiO2/C-CNFMs) were successfully 15

synthesized from the precursor solutions with PVP concentrations ranging from 4 to 6 wt.% by the SiO2/C–280°C–700°C synthesis route. The element composition of the FS-SiO2/CCNFMs is given in Table 2. Fig. 9 shows the N2 adsorption isotherms of the FS-SiO2/C-CNFMs prepared with different PVP concentrations. The isotherms are steep at very low relative pressures, which occupy the main part of the adsorbed volume and correspond to micropores. All the isotherms are a combination of type I and type II or IV isotherms [47], indicating the presence of micro-, meso-, and macropores. From the N2 adsorption isotherms, the BET, micropore, and external specific surface areas were evaluated by the t-plot method. Furthermore, the micropore and the mesopore-to-macropore volumes were also calculated using the t-plot method and the Barrett–Joyner–Halenda (BJH) model, respectively. The median pore width was calculated using the Horvath–Kawazoe method. Table 3 summarizes these values for the FS-SiO2/C-CNFM samples. Among the FS-SiO2/C-CNFM samples with almost the same elemental composition, the FS-SiO2/C-CNFM sample obtained from the precursor solution with 5 wt. % PVP shows the highest pore volume and specific surface area. Fig. 10 illustrates the detailed morphology and elemental mappings of the FS-SiO2/CCNFM prepared from the precursor solution with 5 wt.% PVP. It can be clearly seen from the SEM and TEM images (Figs. 10(a) and 10(b)) that the surface morphology of the nanofibers at the site of interest is smooth without any defects. The elemental mappings of Si, O, and C in Fig. 10 (c) exhibit the uniform distribution of elements throughout a single fiber. These findings again confirm that the SiO2 and carbon composite can be prepared at the nanoscale by the present synthesis route. To investigate the graphitization degree of carbon in the FS-SiO2/C-CNFM sample prepared from the precursor solution with 5 wt.% PVP, Raman spectroscopy was employed. As shown in Fig. 11, the FS-SiO2/C-CNFM sample exhibits two clear bands, which are 16

assigned to the typical D band (1360 cm–1 for defect-induced carbon) and G band ( 1590 cm– 1

for graphite crystal planes). The IG/ID intensity ratio is 1.08, revealing a moderate

graphitization degree of the carbon framework and indicating sufficient electronic conductivity to facilitate the electrochemical lithiation and delithiation of SiO2.

3.4 Electrochemical properties of free-standing SiO2/C composite nanofiber mats

Fig. 12 (a) shows the first and second discharge and charge curves of the FS-SiO2/CCNFMs prepared from the precursor solutions with different PVP concentrations, cycled between 0.01 and 3.0 V at 100 mA g–1 current density. As confirmed in our preliminary testing of carbon nanofibers prepared from pure PVP (Fig. S2), the carbon in the FS-SiO2/CCNFMs did not contribute to the electrochemical lithiation and delithiation, and the current densities and specific capacities were calculated on the basis of the mass of SiO2. As can be seen from the discharge and charge curves, the profiles of different samples overlap, indicating coinciding discharge–charge behavior and high initial discharge and charge capacities of 1800 and 984 mAh (SiO2-g)–1, respectively. Despite the absence of additional conductive carbons, such as AB or ketjen black, in the electrode (~50 wt.% SiO2, Table 2), these capacities are much higher than those of the SiO2/C composite nanofiber electrodes prepared by coating onto the current collector with the addition of conductive AB. This is attributed to the shortened Li+-ion pathway with the decreased mean diameter of the prepared nanofibers. To clarify the effects of the morphological characteristics, such as the fiber diameter, specific surface area, and the pore volume, on the electrochemical properties of the FSSiO2/C-CNFMs, rate capability testing was conducted at current densities ranging from 100 to 1000 mA g–1. Fig. 12 (b) shows the rate capability of the FS-SiO2/C-CNFMs up to 1000 17

mA g–1. At a low current density of 100 mA g–1, the FS-SiO2/C-CNFMs obtained from precursor solutions with different PVP concentrations exhibit similar charge capacities. On the other hand, a significant difference in the charge capacity of the FS-SiO2/C-CNFMs can be seen with increasing current density. The FS-SiO2/C-CNFMs obtained from the precursor solution with 5 wt.% PVP has the highest charge capacities of 576, 506, and 422 mAh g–1 at 200, 400, and 1000 mA g–1, respectively, owing to its largest pore volume and specific surface area. When the current density is returned to 100 mA g–1, all cells exhibited charge capacities of approximately 710 mAh g–1 at the 50th cycle, which indicates the good reversibility of the cells. To obtain further insight into the effects of the morphological characteristics, the cell kinetics was studied by EIS. Nyquist plots and the equivalent circuit models of the cells containing the FS-SiO2/C-CNFMs before cycling and after 20 cycles at 200 mA g–1 are shown in Figs. 12 (c) and 12 (d), respectively. The intersection with the real axis represents the equivalent series resistance (Rs) of the electrolyte, separator, and other components of the cell fabrication. All the impedance spectra of the cells before cycling consist of a depressed semicircle at medium and high frequencies and an oblique straight line at low frequencies. The depressed semicircle represents the interfacial resistance (Rct) related to the charge transfer at the solid–liquid interface, whereas the oblique straight line corresponds to the Warburg impedance (W) related to the solid–state diffusion of Li+ in the electrode. The additional semicircle in the impedance spectra of the cells after 20 cycles in Fig. 12(d) is attributed to the impedance (Rf) formed by the SEI on the surfaces of the electrodes [48]. The EIS spectral data were fitted using the equivalent circuit model and the fitting results are summarized in Table 4. When the mean fiber diameter decreased from 130 to 79 nm, the BET specific surface area, micropore volume, and meso-to-macropore volume of the FS-SiO2/C-CNFMs increased 18

(Table 3). This led to a decrease in Rct and the total resistance (Rf + Rct) in the cell, increasing the rate capability. However, further decreasing the mean fiber diameter from 79 to 54 nm reduced the rate capability. This may be due to the decrease in the BET specific surface area and pore volume of the FS-SiO2/C-CNFMs. On the other hand, the rate capability of the FSSiO2/C-CNFMs obtained from the precursor solution with 6 wt.% PVP is similar to that obtained from the precursor solution with 4 wt.% PVP. However, its BET specific surface area and total pore volume are larger than those of the FS-SiO2/C-CNFMs obtained from the precursor solution with 4 wt. % PVP. These samples have different micropore surface areas and micropore volumes, whereas their external surface area, median pore width, and mesopore-to-macropore volume are quite similar. From these results, we conclude that the specific surface area and pore volume derived from the meso- and macropores of the FSSiO2/C-CNFMs are key factors for improving the electrochemical properties of the FSSiO2/C-CNFMs. Fig. 13 illustrates the cyclability of the FS-SiO2/C-CNFMs obtained from the precursor solution with 5 wt. % PVP up to 200 cycles at 100 mA g–1. After the rapid decay in the initial 10 cycles, the capacity gradually increased and a capacity of 754 mAh (SiO2-g)–1 was retained after 200 cycles with 100% Coulombic efficiency. This is attributed to the fibrous morphology and flexibility of the electrode, which can accommodate the volume expansion during continuous cycling. To better understand the reason for the capacity fading in the initial cycles, the CV of the FS-SiO2/C-CNFMs obtained from the precursor solution with 5 wt. % PVP was conducted at 0.1 mV s–1 up to 20 cycles. Fig. S3 shows the cyclic voltammograms of the sample. The initial cycle contains only one anodic peak starting from 0.2 V. On the other hand, the intensity of this peak decreases cycle by cycle with the appearance of additional peaks at 0.13 and 1.2 V. The capacity decay in the initial cycles could be attributed to the structural 19

changes and formation of byproducts. The peak at 1.2 V is consistent with the reversible turning of Li2Si2O5 to SiO2 [14]. Increase in the capacity cycle by cycle could be related to the activation of this reaction. Fig. S4 exhibits Nyquist plots of FS-SiO2/C-CNFMs obtained from the precursor solution with 5 wt. % PVP before cycling and after 100 discharge-charge cycles at 200 mA g– 1

. Plots of the first cycle contain only one semicircle. As shown in Fig. 12 (d), additional

semicircle appears after several cycles, indicating the formation of the SEI layer, which could be another reason for the capacity decay in the initial cycles. On the other hand, this semicircle disappears after 100 discharge-charge cycles. Furthermore, the diameter of the semicircle decreases after continuous cycling in comparison with that of the fresh cell, indicating reduced impedance from the charge transfer.

4. Conclusions

SiO2/C composite nanofibers were successfully synthesized by electrospinning with twostep heat treatment, comprising the preoxidation at 280°C in the air followed by annealing at 700°C for 1 h in a reduced atmosphere. The effects of the preoxidation treatment on the morphology, chemical structure, and electrochemical properties of the synthesized materials were studied by SEM-EDS, TEM, XRD analysis, FTIR spectroscopy, NMR spectroscopy, and electrochemical testing. FS-SiO2/C-CNFMs with different fiber diameters were prepared from precursor solutions with PVP concentrations ranging from 4 to 6 wt.% by electrospinning with the two-step heat treatment. From the physical and electrochemical characterizations, it was found that the specific surface area and pore volume derived from the meso- and macropores of the FS-SiO2/C-CNFMs are key factors in enhancing its rate capability. The FS-SiO2/C-CNFMs obtained from the precursor solution with 5 wt.% PVP 20

had a geometric mean diameter of 79 nm and a specific surface area of 642 m2 g–1. They exhibited high initial discharge and charge capacities of 1800 and 984 mAh (SiO2-g)–1, respectively, and after the rapid decay in the initial 10 cycles, finally retaining the capacity of 754 mAh (SiO2-g)–1 after 200 cycles with 100% Coulombic efficiency.

Acknowledgment This research was supported by Tokyo Institute of Technology. The authors are also grateful to Mr. J. Koki and Mr. Y Sei, staff members of the Center for Advanced Materials Analysis (Tokyo Institute of Technology, Japan) for the analysis of samples. Moreover, the authors gratefully acknowledge Mr. A. Michishita (Shimadzu Co.) for help in obtaining the pore size distributions of the samples.

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Table 1 Elemental content of the sample before and after the heat treatment at different conditions. Sample

Mass of sample, mg

C, mg

H, mg

N, mg

572 106 252 160

286 (50 wt.%) 30 (28 wt.%) 108 (43 wt.%) 78 (49 wt.%)

40(7 wt.%) 2 (2 wt.%) 8 (3 wt.%) 3 (2 wt.%)

57(10 wt.%) 2 (2 wt.%) 20 (8 wt.%) 6 (4 wt.%)

As-spun SiO2/C–700°C PO–280°C SiO2/C–280°C−700°C

Table 2 Elemental composition of the FS-SiO2/C-CNFMs prepared from the solutions with different PVP concentrations. PVP concentration C H N wt.% wt.% wt.% wt.% 6 48 2 4 5 46 2 4 4 47 2 4

Table 3 Specific surface area, pore volume and median pore width of the FS-SiO2/C-CNFMs prepared from the solutions with different PVP concentrations. PVP concentration wt.% 6 5 4

Micropore surface area m2 g−1 385 603 260

External surfaces area m2 g−1 20 39 19

BET surface area m2 g−1 405 642 279

Median Pore width nm 0.47 0.49 0.46

Micropore volume by t-plot cm3 g–1 0.15 0.24 0.10

Meso- to Macropore volume by BJH cm3 g–1 0.06 0.11 0.05

Table 4 Fitting results of the Nyquist plots of the FS-SiO2/C-CNFMs prepared from the solutions with different PVP concentrations. PVP Before cycling After 20 cycles concentration Rs / Ohm Rct / Ohm Rs / Ohm Rf / Ohm Rct / Ohm 6 wt.% 5 wt.% 4 wt.%

2.369 2.220 2.135

659.1 217.1 342.0

2.708 2.840 3.002

27

939.6 474.0 710.7

3541 1257 5946

Figure caption Fig.1 Schematic illustration of electrospinning apparatus. Fig. 2 Synthesis flowchart of SiO2/C composite nanofibers. Fig. 3 SEM images of as-spun samples before and after the heat treatment at different conditions and corresponding digital photographs. Fig. 4 XRD patterns of the samples after the heat treatment (a) and FTIR spectra of as-spun PVP and the sample before and after the heat treatments at different conditions (b),

13

C (c)

and 29Si (d) NMR spectra. Fig. 5 Possible structural changes of the as-spun sample during the heat treatments at different conditions. Fig. 6 Charge−discharge curves (a) and cyclic voltammetry (b) of the SiO2/C composite fibers, prepared without and with preoxidation. Fig. 7 Cycle performance of the SiO2/C composite fibers, prepared without and with preoxidation, cycled between 0.01−3 V (vs. Li+/Li) at a current density of 50 mA g−1. Fig. 8 SEM images and corresponding fiber diameter distributions of SiO2/C composite fibers prepared from the precursor solutions with different PVP concentrations. Insets: digital images. Fig. 9 N2 adsorption isotherms of the FS-SiO2/C-CNFMs prepared from the precursor solutions with different PVP concentrations. Fig. 10 High magnification SEM (a), TEM (b) images and elemental mapping (c) of the FSSiO2/C-CNFM prepared from the precursor solution with a PVP concentration of 5 wt.%. Fig. 11 Raman shifts of the FS-SiO2/C-CNFM prepared from the precursor solution with a PVP concentration of 5 wt.%.

28

Fig. 12 Voltage profiles (a), rate-capability (b) and Nyquist plots before cycling (c) and after 20 discharge−charge cycles at 200 mA g–1 (d) of the FS-SiO2/C-CNFMs prepared from the precursor solutions with different PVP concentrations. Fig. 13 Cycle performance of the FS-SiO2/C-CNFM prepared from the precursor solution with a PVP concentration of 5 wt.%.

29

Fig. 1 Schematic illustration of the electrospinning apparatus.

30

Fig. 2 Synthesis flowchart of SiO2/C composite nanofibers.

31

Fig. 3 SEM images of as-spun samples before and after the heat treatment at different conditions and corresponding digital photographs.

32

Fig. 4 XRD patterns of the samples after the heat treatment (a) and FTIR spectra of as-spun PVP and the sample before and after the heat treatments at different conditions (b), and 29Si (d) NMR spectra.

33

13

C (c)

Fig. 5 Possible structural changes of the as-spun sample during the heat treatments at different conditions.

34

Fig. 6 Charge−discharge curves (a) and cyclic voltammetry (b) of the SiO2/C composite fibers, prepared without and with preoxidation.

35

Fig. 7 Cycle performance of the SiO2/C composite fibers, prepared without and with preoxidation, cycled between 0.01−3 V (vs. Li+/Li) at a current density of 50 mA g−1.

36

Fig. 8 SEM images and corresponding fiber diameter distributions of SiO2/C composite fibers prepared from the precursor solutions with different PVP concentrations. Insets: digital images.

37

Quantity adsorbed / cm3 g-1

200 150 100 PVP 6 wt.% 5 wt.% 4 wt.%

50 0

0

0.2 0.4 0.6 0.8 Relative pressure / p/p°

1

Fig. 9 N2 adsorption isotherms of the FS-SiO2/C-CNFMs prepared from the precursor solutions with different PVP concentrations.

38

Fig. 10 High magnification SEM (a), TEM (b) images and elemental mapping (c) of the FSSiO2/C-CNFM prepared from the precursor solution with a PVP concentration of 5 wt.%.

39

Fig. 11 Raman shifts of the FS-SiO2/C-CNFM prepared from the precursor solution with a PVP concentration of 5 wt.%.

40

Fig. 12 Voltage profiles (a), rate-capability (b) and Nyquist plots before cycling (c) and after 20 discharge−charge cycles at 200 mA g–1 (d) of the FS-SiO2/C-CNFMs prepared from the precursor solutions with different PVP concentrations.

41

100

800

80

600

60 PVP concentration: 5 wt.%

400 200 0 0

Current density: 100 mA g –1

40

80 120 160 Cycle number

40 20

Coulombic efficiency / %

–1

Charge capacity / mAh (SiO2-g)

1000

0 200

Fig. 13 Cycle performance of the FS-SiO2/C-CNFM prepared from the precursor solution with a PVP concentration of 5 wt.%.

42

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: