Antimony into Porous Electrospun Carbon

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Encapsulating Silica/Antimony into Porous Electrospun Carbon Nanofibers with Robust Structure Stability for High-Efficiency Lithium Storage Hongkang Wang, Xuming Yang, Qizhen Wu, Qiaobao Zhang, Huixin Chen, Hongmei Jing, Jinkai Wang, Shao-Bo Mi, Andrey L. Rogach, and Chunming Niu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b09092 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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Encapsulating Silica/Antimony into Porous Electrospun Carbon Nanofibers with Robust Structure Stability for High-Efficiency Lithium Storage Hongkang Wang,*,a Xuming Yang,b Qizhen Wu,a Qiaobao Zhang,*,c Huixin Chen,d Hongmei Jing,e Jinkai Wang,a,f Shao-Bo Mi,e Andrey L. Rogach,*,b Chunming Niua a

Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical

Insulation and Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, P. R. China b

Department of Materials Science and Engineering, and Centre for Functional

Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, P. R. China c

Department of Materials Science and Engineering, College of Materials, Xiamen

University, Xiamen, Fujian 361005, China. d

Xiamen Institute of Rare Earth Materials, Haixi institute, Chinese Academy of

Sciences, Xiamen 361005, P. R. China e

State Key Laboratory for Mechanical Behavior of Materials & School of

Microelectronics, Xi'an Jiaotong University, Xi'an 710049, P. R. China f

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 320027, P.

R. China

ABSTRACT: To address the volume change-induced pulverization problems of electrode materials, we propose a “silica reinforcement” concept, following which silica-reinforced carbon nanofibers with encapsulated Sb nanoparticles (denoted as SiO2/Sb@CNFs) are fabricated via an electrospinning method. In this composite structure, insulating silica fillers not only reinforce the overall structure but also contribute to additional lithium storage capacity; encapsulation of Sb nanoparticles into the carbon-silica matrices efficiently buffers the volume changes during Li-Sb alloying-dealloying processes upon cycling and alleviates the mechanical stress; porous carbon nanofiber framework allows for the fast charge transfer and electrolyte diffusion. These advantageous characteristics synergistically contribute to the superior 1

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lithium storage performance of SiO2/Sb@CNF electrodes, which demonstrate excellent cycling stability and rate capability, delivering reversible discharge capacities of 700 mA h/g at 200 mA/g, 572 mA h/g at 500 mA/g and 468 mA h/g at 1000 mA/g each after 400 cycles. Ex situ as well as in situ TEM measurements confirm that the structural integrity of silica-reinforced Sb@CNF electrodes can efficiently withstand the mechanical stress induced by the volume changes. Notably, the SiO2/Sb@CNF//LiCoO2 full cell delivers high reversible capacities of ~400 mA h/g after 800 cycles at 500 mA/g and ~336 mA h/g after 500 cycles at 1000 mA/g.

KEYWORDS: silica reinforcement; Sb nanoparticles; porous carbon nanofibers; robust structure stability; lithium storage

Lithium ion batteries (LIBs) have been commercially employed in a wide range of portable electronics, such as cell phones and laptops.1-8 With the increasing requirements on large-scale energy storage in the emerging electric vehicles and smart grids, LIBs with higher capacity, higher rate capability and higher energy density are required. However, the graphite anodes in present commercial LIBs show relatively low theoretical gravimetric and volumetric capacities of ~372 mA h/g and ~840 mA h/cm3, respectively, and also suffer from safety problems owing to the low Li+ insertion/deposition

potential.

As

a

promising

alternative

anode

material,

alloying-type antimony (Sb) possesses higher theoretical gravimetric and volumetric capacities of 660 mA h/g and 1750 mA h/cm3, respectively, and operates on the reversible alloying/dealloying reaction Sb + 3Li+ + 3e– ↔ Li3Sb.9-15 However, alloying-type anodes suffer from the large volume expansion as a result of the lithiation, so that the transformation of Sb to Li3Sb is accompanied by ~150% volume increase.15 Such large volume change often results in the electrode pulverization and contact losses with current collector, finally causing the rapid capacity fading.16-20 Moreover, the volume changes cause the repeated construction-destruction of solid electrolyte interphase (SEI) coating on such anode materials, while fresh SEI formation would result in continuous lithium consumption, which is detrimental for 2


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the long-term cycle stability. Many studies have thus been devoted to finding efficient strategies allowing for buffering the volume changes and keeping the structural integrity of the alloying-type electrode materials.21, 22 One widely adopted approach is to combine Sb with other electrochemically active or inactive components, such as carbonaceous materials like carbon nanotubes, graphene and carbon nanofibers (CNFs).15,

23-30

These carbonaceous materials not

only serve as a buffer matrix to alleviate the large volume changes of Li-Sb alloying/dealloying, but also bring additional lithium storage capacity and improve the electrode conductivity. On the other hand, silica (SiO2) has been generally regarded as an electrochemically inactive component in such composites, till findings of Gao et al. who reported that SiO2 nanoparticles with size of 7 nm could react with Li in the voltage range of 0~1.0 V (vs. Li/Li+) and deliver a reversible capacity of 400 mA h/g.31 Silica has thus been also considered as an alternative anode material for LIBs because of its potentially high (albeit controversial) specific capacity of 1965 mA h/g32 and low discharge potentials.31-39 Guo et al. reported a composite made of nano-SiO2 (24 wt.%) and hard carbon made via hydrothermal method and subsequent carbonization at 1000 oC, which delivered a reversible capacity of 630 mA h/g (namely, 1675 mA h/g for SiO2). Yao et al. reported the synthesis of carbon-coated SiO2 (50 wt.%) by carbonizing the nano-SiO2-sucrose at 900 oC, which exhibited a discharge capacity of 500 mA h/g after the 50 cycles at 50 mA/g.38 Sasidharan et al. synthesized hollow silica nanospheres (size: ~30 nm), which exhibited a reversible discharge capacity of 336 mA h/g after the 500 cycles at 1C current density.40 Yan et al. prepared hollow porous SiO2 nanocubes which exhibited a reversible capacity of 919 mA h/g over 30 cycles at 100 mA/g.32 Favors et al. reported the template synthesis of SiO2 nanotubes which delivered a stable reversible capacity of 1266 mA h/g after 100 cycles at 50 mA/g.39 Therefore, silica holds a great potential in the capacity improvement, which relies on the delicate structure design with the aim to alleviate the volume expansion effect and enhance the conductivity. Besides, silica is an important industrial material with low cost and rich abundance on the Earth, which has been widely used as filler in composites, owing to its reinforcing effect on the 3


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dynamic-mechanical and physical properties of the composites.41 Relying on the potential reinforcement effect of silica on the mechanical strength in silica-based composites, as well as different working potentials of silica (0-1.0 V vs. Li/Li+) and Sb (0.5-1.0 V vs. Li/Li+), we herein designed a composite SiO2/Sb@CNF structure consisting of silica and Sb nanoparticles, which were integrated into the electrospun porous CNFs via a facile electrospinning method and a subsequent annealing under inert atmosphere. The silica with a low density (similar to that of graphite) possesses a high compatibility with CNFs and greatly reinforces the fiber strength. When used as anode materials for LIBs, the SiO2/Sb@CNF electrodes show excellent lithium storage performance including high specific capacity, superior cycling stability and rate capability, while their structural characteristics are demonstrated to greatly enhance the stability and efficiently resist the volume change induced mechanical strain. RESULTS AND DISCUSSION The fabrication process of the SiO2/Sb@CNF composite is schematically shown in Figure S1. Precursor solution prepared by dissolving polyvinylpyrrolidone (PVP), SbCl3 and tetrathylorthosilicate (TEOS) in N,N-Dimenthyformaide (DMF) was used as the fluid for electrospinning (see Experimental Section in the Supporting Information for details). The electrospun PVP/SbCl3/TEOS fibers (SEM and EDS data presented in Figure S2) were first stablized at 300oC in air (the phase evolution is shown by XRD patterns given in Figure S3) and then annealed at 700oC in Ar. During the annealing process, a number of processes is supposed to happen. PVP serving as a carbon/nitrogen source undergoes carbonization with simultaneous N-doping; ionic Sb species are reduced to metallic Sb nanoparticles by the carbonized CNFs, which at the same time serve as encapsulating matrix; TEOS decomposes into silica nanoparticles which also become embedded within the CNFs. We note that both the electrospun SbCl3/PVP and TEOS/PVP nanofibers are not able to maintain the fibrous morphology after annealing under the same conditions (SEM data given in Figure S4), indicating that TEOS-derived silica nanoparticles as well as SbCl3-derived Sb 4


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nanoparticles synergistically enhance the structural stability and maintain the structural integrity of the resulting SiO2/Sb@CNF composite. SiO2/Sb@CNF composite has been investigated by scanning electron microscopy (SEM). Figure 1a shows the low-magnification SEM image of the SiO2/Sb@CNFs, illustrating multiple fibers with the length reaching hundreds of micrometers. As shown in Figure 1b, the fiber diameters are mostly in the range of 500-700 nm, while thinner ones with diameters of ~100 nm could also be observed. Different from the smooth surface of the precursor fibers (Figure S2), the surfaces of SiO2/Sb@CNFs are wrinkled after annealing, and the fractured sections are rough, indicating the presence of cavities within the fibers (Figure 1c).

Figure 1. Structural characterization of the SiO2/Sb@CNF composite. (a-c) SEM images at different magnification; (d) TEM image of a single SiO2/Sb@CNF and (e, f) HRTEM images of two representative Sb nanoparticles which are embedded within 5


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the fiber. (g) HAADF-STEM image of a single fiber with corresponding EDS maps of (h) Sb, (i) C, (j) Si, (k) O and (l) N elements. (m) SEM and (n) TEM images of the macroporous CNFs obtained by etching SiO2/Sb@CNF composites with aqueous HF solution.

Transmission electron microscopy (TEM) and high-resolution (HR) TEM analyses were further performed to reveal the microstructures of the SiO2/Sb@CNFs. Figure 1d displays the TEM image of a single SiO2/Sb@CNF fiber, revealing that Sb nanoparticles with size ranging from several nanometers to several tens of nanometers are fully embedded inside. HRTEM images taken from two representative particles show well resolved lattice fringes with the same spacing of 0.31 nm (Figure 1e-f), which can be assigned to the (012) plane of the metallic Sb. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) study further confirms the porous structure of SiO2/Sb@CNFs (Figure 1g); white dots on that image correspond to Sb nanoparticles which is well consistent with the TEM analysis. The energy-dispersive X-ray spectroscopy (EDS) elemental maps reveal the presence of Sb, C, Si, O and N elements (Figure 1h-l, respectively), which are equally distributed within the SiO2/Sb@CNF. We note that both Sb and silica can be easily removed by etching with HF aqueous solution, resulting in macroporous CNFs (denoted as mp-CNFs). As shown in Figure 1m, the mp-CNFs shrink due to the removal of both SiO2 and Sb. Their TEM image displays highly porous structures with pore size up to hundreds of nanometers (Figure 1n). These large pores are mostly located within the inner parts of the fibers, which further confirms full encapsulation of Sb and silica nanoparticles within CNFs. Brunauer-Emmett-Teller (BET) measurements (Figure S5) reveal that etched CNFs exhibit much larger surface area of ~488 m2/g than that of SiO2/Sb@CNFs (~91 m2/g).

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Figure 2. (a) XRD pattern, (b) EDS spectrum, and (c) survey XPS spectrum of the SiO2/Sb@CNF composite, with insets in (c) showing the high-resolution XPS spectra of N 1s and Si 2p. (d) Raman spectrum of the SiO2/Sb@CNF composite, with the inset showing the enlarged part in the range of 400-2000 cm-1.

The crystallographic structure of the SiO2/Sb@CNF composite was examined by X-ray diffraction (XRD) analysis (Figure 2a); all the prominent diffraction peaks can be indexed to the rhombohedral phase of crystalline Sb (JCPDS No. 35-0732). No peaks belonging to silica are observed, but EDS spectrum clearly reveals the presence of Si element (Figure 2b), indicating the amorphous nature of the silica which is embedded within the electrospun CNFs. X-ray photoelectron spectroscopy (XPS) was performed to investigate the composition and the chemical states of the elements constituting SiO2/Sb@CNFs. Figure 2c shows the survey XPS spectrum with insets presenting the high resolution XPS spectra of N 1s and Si 2p. The survey XPS 7


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spectrum reveals the presence of Sb, Si, C, N and O elements. The peaks in N 1s XPS spectrum can be deconvoluted into three components centered at 401.0, 400.0 and 398.3 eV, which are related to the graphitic (N-Q), pyrrolic (N-5) and pyridinic (N-6) nitrogen, respectively, indicating N-doping of the CNFs.42 The Si 2p peak centered at 103.5 eV can be assigned to the silica (SiO2) with the oxidization state of Si4+. The relative contents of silica, Sb and carbon were roughly estimated by the thermogravimetric analysis (TGA, Figure S6) and EDS (Table S1), which are approximately 42, 18 and 40 wt.% in the resulting SiO2/Sb@CNF composites. Figure 2d shows the Raman spectrum of the SiO2/Sb@CNFs, in which four peaks are observed. The one located at around 150 cm-1 can be indexed to the A1g band from the Sb phase,43 while the one at around 260 cm-1 can be related to the SiO2 phase.44 Notably, two typical bands located at 1360 cm-1 and 1589 cm-1 can be attributed to the D and G bands of graphite, respectively. The D band is the A1g ring-breathing mode which relates to the defects or lattice disorders in the graphene structure but is an indicator of graphitic ring structure, while the G band is the E2g vibration mode corresponding to the relative motion of sp2 carbon atoms.45 The relative intensity ID/IG is widely used to reflect the disorder degree in graphite,24 and the ID/IG ratio in the SiO2/Sb@CNFs is ~0.89. Besides, both D and G bands are relatively broad, indicating the disordered graphitic structure.24 The Raman data thus suggest the presence of short-range graphitic structure in the CNFs, even though the long-range graphitic order is absent in the SiO2/Sb@CNFs as no visible graphitic peaks are observed in the XRD pattern (Figure 2a).

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Figure 3. Electrochemical properties of SiO2/Sb@CNF electrodes. (a) CV curves at 0.2 mV/s, with an inset showing the subsequent CV curve at 0.1 mV/s; (b) Galvanostatic discharge/charge profiles, with an inset showing the differential capacity (dQ/dV) profiles at 100th cycle; (c) Cycle performance at different current densities; (d) Comparison of rate performances between SiO2/Sb@CNF and Sb@C electrodes; (e) Nyquist plots of the SiO2/Sb@CNF electrodes before and after cycling. (f) CV curves of the SiO2/Sb@CNF electrode at different scan rates and (g) plots of the evolutions of the anodic/cathodic peak currents along with scan rates. (h) Comparison of the specific capacities of various Sb-based anode materials at different current densities.

Electrochemical properties of SiO2/Sb@CNF electrodes as anode materials for LIBs were first investigated in half-cells using lithium foil as reference and counter electrode. Figure 3a shows the cyclic voltammetry (CV) curves of the SiO2/Sb@CNF electrode at different cycles. There is an apparent reduction peak at around 0.53 V in 9


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the first cathodic scan, which disappears in the following CV cycles, indicating an irreversible reaction, that is the electrolyte decomposition and formation of SEI film.46 In the first anodic scan, a prominent peak appears at around 1.1 V but with a little shift to lower potential in the following cycles, which can be attributed to the structural evolution after the first CV cycle, related to dealloying of Li3Sb. Correspondingly, cathodic peaks appear at around 0.82 V in the following CV cycles, which are assigned to the Li-Sb alloying reaction. In addition, sharp cathodic peaks and broad anodic peaks below 0.5 V can be attributed to the reversible lithiation/delithiation of the carbon and silica anode materials.6, 32, 39 The CV curve measured at a scan rate of 0.1 mV/s shows two cathodic peaks at around 0.32 and 0.49 V with corresponding anodic peaks at 0.01 and 0.25 V (inset of Figure 3a), which are related to the reversible deliathiation-lithiation in the silica anode materials according to the reactions SiO2 + Li+ + e- ↔ Li2Si2O5 + Si and Si + xLi+ + xe- ↔

LixSi, consistent with the literature reports.32-34, 37 Figure 3b displays galvanostatic discharge/charge profiles of the SiO2/Sb@CNF electrode at 200 mA/g. The initial discharge and charge capacities are as high as 1390 and 923 mA h/g, respectively, with an initial Coulombic efficiency of 66.4%, while the large capacity loss can be due to the irreversible formation of SEI film and an irreversible electrochemical reaction between silica and Li+ ions (such as, formation of lithium silicates).35 In addition, it is observed that the discharge/charge capacities steadily decrease with increasing cycle number, but become almost constant after 50 cycles. The differential capacity (dQ/dV) profiles that are derived from the 100th discharge-charge profiles are displayed in the inset of Figure 3b, from which the lithiation mechanism for the SiO2/Sb@CNF electrode can be derived. A pair of pronounced lithiation/delithiation peaks are present at ~0.88/1.04 V, which correspond to the Li-Sb alloying and dealloying processes. Figure 3c presents the cycling performance of the SiO2/Sb@CNF electrode at different current densities. It shows excellent cycle stability, delivering reversible discharge capacities of 700 mA h/g at 200 mA/g, 572 mA h/g at 500 mA/g and 468 mA h/g at 1000 mA/g each after 400 cycles. Figure 3d compares the rate 10


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performances of the SiO2/Sb@CNF and Sb@C electrodes at various current densities ranging from 100 to 2000 mA/g. The discharge capacities for SiO2/Sb@CNF electrode are 867, 775, 688, 614 and 520 mA h/g each after 10 cycles at 100, 200, 500, 1000 and 2000 mA/g, respectively. When recovering the current density to 100 mA/g, a discharge capacity of 796 mA h/g is maintained after another 10 cycles, indicating the high rate capability. In contrast, the capacities of the reference Sb@C electrode with no SiO2 introduced are much lower than that of the SiO2/Sb@CNF counterpart (Figure 3d), indicating that silica formed as a result of decomposition of TEOS greatly enhances the lithium storage performance. As silica is an insulating oxide, electrochemical impedance spectroscopy (EIS) was performed to reveal its effect on the electrode charge transfer resistance and the kinetics of the electrochemical processes. Figure 3e shows Nyquist plots of the fresh SiO2/Sb@CNF electrode, and the counterpart electrode after 400 cycles, where the semicircle diameter represents the charge transfer resistance (Rct). The fresh SiO2/Sb@CNF electrode exhibits a low Rct of around 80 Ω, which even decreases to around 40 Ω after 400 cycles, indicating that introduction of silica does not result in the decrease of the conductivity. In addition, the intercept at real part (Z’) represents the resistance of the electrolyte (Rs), and the Rs values are measured to be 2.0 and 9.9 Ω for the SiO2/Sb@CNF electrodes before cycling and after 400 cycles, respectively. The increased resistance of the electrolyte may be due to its decomposition, and formation of the SEI layer during the long-term cycling. The steep inclined lines at the low frequency indicate the fast lithium ion diffusion processes within the electrode. For better understanding of the electrochemical kinetics of SiO2/Sb@CNF electrode, CV measurements were performed at various scan rates ranging from 0.1 to 2.0 mV/s. On the basis of the CV curves (Figure 3f), two fitting straight lines can be obtained with the anodic/cathodic peak currents as y-axis and / as x-axis, where v is the scan rate (V/s). The calculated line slopes are approximately 0.65 and 0.80 for the anodic and cathodic peaks (Figure 3g), which are higher than that of the 11


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recently reported Sb@C yolk-shell spheres (0.54).15 According to the Randles-Sevcik equation 2.69 10 / / / (where A stands for the anode area (cm2), C represents the shuttle concentration (mol/cm3), and n is the involved electron number),15 the lithium diffusion coefficient D (cm2/s) is positively proportional to the fitted line slope ( / / ) for a certain electrode (assuming A is a constant). Thus the high slope of our SiO2/Sb@CNFs (BET surface area = 91 m2/g) may indicate a high lithium diffusion coefficient as compared to the Sb@C yolk-shell spheres (D = 1.2 10-9 cm2 s-1; BET surface area = 204.9 m2/g).15 To the best of our knowledge, our SiO2/Sb@CNFs have demonstrated the highest reversible lithium storage capacity as compared with various previously reported Sb-based electrode materials (Figure 3h and Table S2).

Figure 4. (a, b) TEM and (c) HRTEM images of SiO2/Sb@CNF electrodes which were initially discharged to 0.01 V. (d) Ex situ XRD patterns of the discharged SiO2/Sb@CNFs electrode in the first cycle. (e) TEM and (f) HRTEM images of SiO2/Sb@CNF electrodes which were initially charged to 3.0 V. (g) TEM image of the charged SiO2/Sb@CNF electrode after 400 cycles at 500 mA/g.

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The electrochemical behaviors of SiO2/Sb@CNF electrodes were further investigated via ex situ TEM, XRD and XPS analyses, in order to reveal the synergistic interaction of each component and their effect on the lithium storage. Ex situ TEM measurements were performed to uncover the structural evolution upon discharging and charging in the first cycle (Figure 4). As shown in Figure 4a, a discharged SiO2/Sb@CNF electrode at 0.01 V in the first cycle shows no apparent changes in the main body as compared with the unreacted counterpart (Figure 1d), while larger particles with size of 50-70 nm emerge at the fractured cross section (Figure 4b), which can be attributed to the formation of Li3Sb alloys. This was further proved by ex situ XRD (Figure 4d), which reveals the presence of Li3Sb (JCPDS No. 65-3011). However, the diffraction peaks of Li3Sb alloy disappear soon after exposure in air owing to the oxidation, thus the lattice fringes with spacing of 0.21 nm (Figure 4c) may be attributed to certain Li3Sb-derived compound. In contrast, when further charging to 3.0 V in the first cycle, many particles are still fully encapsulated within the well-preserved nanofiber (Figure 4e). The HRTEM image shows lattice spacings of 0.21 and 0.38 nm, which can be indexed to the (012) and (003) planes of metallic Sb (Figure 4f). In addition, in situ XRD analysis was further performed and the formation of Li3Sb alloy was clearly observed in the first discharging process (Figure S7). In order to reveal the structural stability upon long-term cycling, TEM analysis was performed on the SiO2/Sb@CNF electrode after 400 cycles at 500 mA/g. Figure 4g shows the TEM images of a single charged fiber, where the 1D fibrous structure with diameter of ~600 nm is still well preserved, and appears as almost the same as the unreacted SiO2/Sb@CNFs. Moreover, small particles are well distributed within the inner parts of the fibers, indicating the robust structural stability of the SiO2/Sb@CNFs upon long-term lithiation-delithiation processes. The fibrous structure can still be well preserved even after long-term high rate cycling at 1 A/g after 400 cycles (Figure S8a), in which Sb nanoparticles without coarsening were efficiently confined within the carbon-silica fibers (Figure S8b). EDS elemental maps of Sb, C, Si, O and N elements show that they were uniformly distributed within the 13


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CNFs, which is consistent with the pristine SiO2/Sb@CNFs (Figure S8c-h vs. Figure 1g-l). Ex situ XPS analysis was performed in order to investigate the evolution of the chemical states of Si element. Figure S9 compares the Si 2p XPS spectra of the SiO2/Sb@CNF electrodes at different discharge/charge states. When first discharged to 0.01 V, the binding energy shifts to 102.7 eV, indicating the reduction of SiO2. The broad Si 2p peak indicates the co-existence of several chemical states rather than a single one. Owing to the coverage of SEI layer on the electrode and the limited detection depth of XPS technique, it is still hard to reveal the exact electrochemical reaction pathways between lithium and silica, which have been partially discussed in the previous studies but still remain controversial.32, 34, 35, 37, 39

Figure 5. (a) Schematic illustration of the experimental setup used for in situ TEM analysis of the SiO2/Sb@CNF electrodes and (b) TEM image of a real pair of SiO2/Sb@CNF//Li/Li2O electrodes used in in situ TEM analysis. (c) Schematic illustration

of

the

relative

dimensions

of

Sb

and

Li3Sb

spheres

upon

alloying/dealloying, with 150% volume expansion for Li3Sb. A sequence of TEM images collected at the different times depicts the lithiation (d) and delithiation (e) processes of a SiO2/Sb@CNF electrode.

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In situ TEM analysis was carried out to follow the structural and phase evolution of SiO2/Sb@CNF electrodes during lithiation/delithiation processes. The schematic and the real architectures of the SiO2/Sb@CNF//Li/Li2O electrodes used in in situ TEM analysis are shown in Figure 5a and b, respectively. SiO2/Sb@CNF electrode was loaded on an Au tip, and connected to Li/Li2O on a W tip. The as-formed Li2O on Li served as a solid electrolyte. When applying a negative bias at the Au end, the SiO2/Sb@CNF is lithiated, while the subsequent delithiation is realized when applying a positive bias. As schematically shown in Figure 5c, fully lithiated Li3Sb spheres with 150% volume expansion only display slight diameter increase in the in-plane view. Structural changes of the SiO2/Sb@CNF electrode during the lithiation and delithiation processes are illustrated by a sequence of TEM images in Figure 5d and e, respectively, captured from the in situ collected videos (Supplementary Videos S1 and S2). As seen from Figure 5d, upon lithiation, only Sb nanoparticles expand as evidenced by slight volume expansion after a few seconds. It is noted that the SiO2/Sb@CNF electrode maintains its intrinsic structure well during this fast lithiation process without any other structural change. Reversely, the lithiated Sb nanoparticles shrank during the delithiation process as shown in Figure 5e. As seen from Figure 5d-e, the carbon-silica matrix efficiently accommodates the volume changes of Li-Sb alloying-dealloying, and the fiber itself undergoes little change after full lithiation and delithiation. Selected area electron diffraction (SAED) patterns were recorded to study the phase evolution (Figure S10), in which the phase transition between Sb and Li3Sb was further confirmed. This is consistent with the in/ex situ XRD results. To further explore the dynamic changes of the SiO2/Sb@CNF electrode, videos S3 and S4 were recorded under the in situ TEM mode for the second lithiation and delithiation cycles. During the second lithiation process (video S3), the tiny Sb nanoparticles become coarsened and slightly expand but still remain fully embedded within the carbon-silica fiber. In the second delithiation process, the inner Sb nanoparticles slightly decrease in size but the fibrous structure remains similar to that in the pristine stage (video S4). Except for the slight changes of Sb nanoparticles, 15


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there is no cracking or fracturing happening in the SiO2/Sb@CNF electrode during the whole alloying-dealloying process. Such robust structure integrity is greatly beneficial both for the electrical contact and for the stable cycling performance of the anode materials. In order to demonstrate the feasibility in practical LIB devices, SiO2/Sb@CNF anodes were further examined in full cells using commercial LiCoO2 as cathode material. Figure 6a shows the discharge/charge profiles of the full cell measured at a current density of 500 mA/g in the voltage range of 1.0-3.9 V. The initial charge/discharge capacities are 600/625 mA h/g with an initial Coulombic efficiency of 96%, which further increase to 99.51% and 99.73% for the 100th and 200th cycles, respectively. Figure 6b shows the cycle performance of the full cells at 500 mA/g, delivering a reversible capacity of ~450 mA h/g after 200 cycles, with a capacity retention of 75%. The rate performance of the full cell was also investigated and is shown in Figure 6c. The reversible capacities of the SiO2/Sb@CNFs at 100, 200, 500, 1000 and 2000 mA/g could be preserved at 869.2, 741.5, 630.2, 510.7 and 361.7 mA h/g each after 10 cycles, respectively. When steadily increasing the current densities, comparable capacities can be recovered. Notably, a capacity of ~400 mA h/g can be still maintained after additional 700 cycles at 500 mA/g. Even at a higher current density of 1000 mA/g, the full cell can still deliver a high capacity of ~336 mA h/g after 500 cycles (Figure 6d). These results further reveal the superior cycle stability and rate capability of the SiO2/Sb@CNFs.

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Figure 6. (a) Discharge/charge curves and (b) cycle performance of the full cell that paired the SiO2/Sb@CNF anode with a commercial LiCoO2 cathode at a current density of 500 mA/g. (c) Rate performance of the full cell measured at different current densities. (d) Cycle performance of the full cell at a high rate of 1000 mA/g. Plots of Coulombic efficiencies along with cycle numbers are also shown in Figures (b-d).

CONCLUSIONS In summary, a robust composite structure of silica-Sb-carbon nanofibers (SiO2/Sb@CNFs) is produced via electrospinning and subsequent annealing, where both Sb and silica nanoparticles are encapsulated into CNFs. The in situ generated silica nanoparticles, formed during thermal decomposition of TEOS, greatly enhance the structural stability of the resultant SiO2/Sb@CNF composite. Ex situ and in situ 17


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TEM measurements indicate that the SiO2/Sb@CNF framework can efficiently maintain the integrity of electrodes, where both aggregation of Sb nanoparticles as well as the volume changes are largely alleviated during the repeated alloying-dealloying processes, which consequently contribute to the excellent lithium storage performance in both half-cell and full-cell tests. The structural advantages of SiO2/Sb@CNF composite can be summarized as follows. First, the electrochemically active materials of Sb and silica are both encapsulated into the fibrous carbon matrices, which efficiently buffer the volumetric changes owing to the confinement effect of CNFs on the lithiation/deliathiation reactions between Sb/silica and lithium ions. Besides, the different work potentials of Sb, silica and carbon allow for the stepwise volume changes of each component in the hybrid structure, so that the unreacted components can accommodate the strain yielded by the reacted phase. Second,

the

porous

CNFs

enhance

the

conductivity

and

improve

the

charge/electrolyte transfer. Therefore, an excellent lithium storage performance including superior cycle stability and high rate capability have been achieved for SiO2/Sb@CNF electrodes. When used as anode materials for lithium ion batteries, SiO2/Sb@CNF electrodes in half-cells demonstrate excellent cycling stability and rate capability, delivering reversible discharge capacities of 700 mA h/g at 200 mA/g, 572 mA h/g at 500 mA/g and 468 mA h/g at 1000 mA/g each after 400 cycles. Moreover, the SiO2/Sb@CNF//LiCoO2 full cell can maintain a high reversible capacity of ~336 mA h/g after 500 cycles at 1000 mA/g. Most importantly, silica reinforcement concept introduced here can be applied to design other hybrid electrode materials able to withstand severe volume changes, which has demonstrated great potential in electrochemical energy storage application. EXPERIMENTAL SECTION Fabrication. Silica-reinforced Sb@carbon nanofibers (denoted as SiO2/Sb@CNFs) were prepared via electrospinning method followed by annealing. In a typical synthesis, 0.75 g polyvinylpyrrolidone (PVP, (C6H9NO)n, MW=1300000, J&K) was dissolved in 4 mL N, N-Dimenthyformaide (DMF, C3H7NO, J&K, Super-Dry) under 18


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magnetic stirring until the complete dissolution of PVP. Afterwards, 1 mL tetrathylorthosilicate (TEOS, Si(OC2H5)4) and 0.64 g antimony trichloride (SbCl3, Amethyst) were added under vigorous stirring at 90 oC. The resulting transparent solution was loaded into a plastic syringe for electrospinning. The distance between the needle and the fiber collector was ~15 cm, and a high voltage of 16 kV was applied to initiate the electrospinning. A flow rate of 0.6 mL/h was employed, and an aluminum foil was used to collect the fibers. The collected fibers were first dried at 80 o

C and then stabilized at 300 oC for 1 h with a heating rate of 5 oC/min, which were

further annealed under Ar atmosphere at 700 oC for 2h with a heating rate of 5 oC/min, yielding the final SiO2/Sb@CNF product. Besides, in order to reveal the effect of TEOS, a precursor solution containing the same amount of SbCl3, PVP and DMF but without addition of TEOS was also prepared for electrospinning under the same conditions. The resulting product underwent the same annealing treatments and was denoted as Sb@C; its fibrous morphology has disappeared after annealing. Structural Characterization. Phase structures of the products were examined by powder X-ray diffraction (XRD) on a Bruker D2 PHASER X-ray diffractometer using Cu Kα radiation (λ=1.5418 Å) with voltage at 30 kV and current at 10 mA. The morphologies of the products were characterized using a scanning electron microscope (SEM, FEI Quanta 250F) and transmission electron microscope (TEM, JEOL-2100 TEM and JEM-F200 Field Emission TEM). High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) was performed on an aberration-corrected JEOL ARM200F instrument with energy-dispersive X-ray spectroscopy (EDS) for elemental analysis. The compositions and chemical states of the products were recorded by X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi). Thermogravimetric analysis (TGA) was conducted in air with a heating rate of 5oC/min on a METTLER TOLEDO TGA/DSC thermal analyzer. Raman spectroscopy was carried out using a Renishaw Raman RE01 scope with Ar excitation laser at 514 nm. The surface area and pore structures were examined using a Quantachrome Autosob IQ analyzer. Electrochemical Measurements. The electrochemical properties of the samples were 19


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evaluated using 2025 coin-type cells using lithium foils as counter and reference electrodes, which were assembled in an Ar-filled glove box with both H2O and O2 contents below 1.0 ppm. The working electrode slurry was firstly prepared by mixing active materials, namely conductive acetylene black (Super P) and binder (polyacrylic acid, PAA, Mw=100000, Sigma-Aldrich) in a weight ratio of 8:1:1 using water as solvent. The slurry was then coated onto a copper foil by doctor blade method, followed by vacuum-drying at 120oC overnight in order to completely evaporate the absorbed water. The areal loading amount was in the range of 0.8-1.0 mg cm-2. A Celgard 2400 microporous membrane was used as separator. 1 M LiPF6 in the dimethyl carbonate / ethylene carbonate (1:1 by volume) was used as an electrolyte. Galvanostatic discharge-charge tests were conducted on a NEWARE battery test system (Neware Technology Co., Ltd., China) in the range of 0.01-3 V (vs. Li+/Li) in an incubator with temperature constant at 25oC. Cyclic voltammetry (CV) measurements were performed on an electrochemical workstation (Autolab PGSTAT 302N) at a scan rate of 0.2 mV/s in 0.01-3 V. Electrochemical impedance spectroscopy (EIS) was carried out on a CHI 660E electrochemical workstation (Chenhua, Shanghai) with a voltage amplitude of 10 mV in the frequency range of 10 MHz to 0.01 Hz. For the full cell test, the cathode electrode was fabricated by mixing 80 wt.% commercialized LiCoO2 (LCO) cathode with 10 wt.% carbon black and 10 wt.% polyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone (NMP) to form a slurry, which was then spread onto an Al foil current collector and dried under vacuum at 80 °C for 24 hours. After prelithiation, the half-cells were disassembled and the SiO2/Sb@CNF and LCO (Figure S11) electrodes were combined in full cells (CR-2025 coin-type cells) in a Ar-filled glovebox. The electrolytes and separator in the full cell were the same as those in the half-cells described above. Electrochemical analysis of the full cell was carried out in the voltage window between 1.0 V and 3.9 V. The full cell was designed with a N/P reversible capacity ratio of 1:1, and the capacity of the full cell was calculated based on the weight of active materials in the anode.

In Situ TEM Measurements. SiO2/Sb@CNFs were attached to a gold wire by 20


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scratching the SiO2/Sb@CNF powder. The gold wire was then transferred and fixed by inserting it into a tiny-diameter pipe welded to the sample holder frame. Fresh Li was attached to an electrochemically etched tungsten tip that was inserted into the three-dimensional movable part of the piezo-driven holder inside a glove box. The sample holder was then transferred to a TEM within a few seconds, with a thin layer of Li2O formed due to the exposure to air. The tungsten tip contacted a single SiO2/Sb@CNF with the Li2O layer and the construction of the nanobattery was thus completed. Once the contact between a SiO2/Sb@CNF and Li2O layer was established, a constant bias of -6V/6V was applied to the SiO2/Sb@CNF against Li metal to initiate the fast lithiation and delithiation processes. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx. Scheme for the synthetic approach; Additional XRD patterns, EDS, SEM, TEM images of the intermediate products; Si 2p XPS spectra of SiO2/Sb@CNFs electrodes; TGA curve of the SiO2/Sb@CNFs; N2 sorption isotherms of the SiO2/Sb@CNFs and the macroporous CNFs; TEM images and EDS elemental maps of the SiO2/Sb@CNFs after cycling; Tables summarising the EDS data, and the electrochemical performance of various Sb-based electrodes. (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID Hongkang Wang: 0000-0003-4893-5190

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant No. 51402232, 21703185, 51521065 and 61471307), and by the Fundamental Research Funds for the Central Universities (Xiamen University: 20720170042). H.W. appreciates the support of the Tang Scholar Program from the Cyrus Tang Foundation, and the support by State Key Laboratory of Electrical Insulation and Power Equipment (Grant No. EIPE17308). We thank Dr. Jiao Li from the Instrument Analysis Center of Xi’an Jiaotong University for her assistance with TEM/EDS analyses. We also appreciate the support of State Key Laboratory of Silicon Materials of Zhejiang University on the in situ TEM study. REFERENCES (1) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359–367. (2) Chan, C. K.; Peng, H. L.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. High-Performance Lithium Battery Anodes Using Silicon Nanowires. Nat. Nanotechnol. 2008, 3, 31– 35. (3) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652–657. (4) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4, 366–377. (5) Jiang, H.; Ren, D. Y.; Wang, H. F.; Hu, Y. J.; Guo, S. J.; Yuan, H. Y.; Hu, P. J.; Zhang, L.; Li, C. Z. 2D Monolayer MoS2-Carbon Interoverlapped Superstructure: Engineering Ideal Atomic Interface for Lithium Ion Storage. Adv. Mater. 2015, 27, 3687–3695. (6) Wang, H.; Wang, J.; Cao, D.; Gu, H.; Li, B.; Lu, X.; Han, X.; Rogach, A. L.; Niu, C. Honeycomb-Like Carbon Nanoflakes As A Host for SnO2 Nanoparticles Allowing Enhanced Lithium Storage Performance. J. Mater. Chem. A 2017, 5, 6817–6824. (7) Wang, H.; Xi, L.; Tucek, J.; Ma, C.; Yang, G.; Leung, M. K. H.; Zboril, R.; Niu, C.; Rogach, A. L. Synthesis and Characterization of Tin Titanate Nanotubes: Precursors for Nanoparticulate Sn-Doped TiO2 Anodes with Synergistically Improved Electrochemical Performance. ChemElectroChem 2014, 1, 1563–1569. (8) Chen, S.; Shen, L.; van Aken, P. A.; Maier, J.; Yu, Y. Dual-Functionalized Double Carbon Shells Coated Silicon Nanoparticles for High Performance Lithium-Ion Batteries. Adv. Mater. 2017, 29, 1605650. (9) Park, C. M.; Kim, J. H.; Kim, H.; Sohn, H. J. Li-Alloy Based Anode Materials for Li Secondary Batteries. Chem. Soc. Rev. 2010, 39, 3115–3141. (10) He, M.; Kraychyk, K.; Walter, M.; Kovalenko, M. V. Monodisperse Antimony Nanocrystals for High-Rate Li-Ion and Na-Ion Battery Anodes: Nano versus Bulk. Nano Lett. 2014, 14, 1255–1262. (11) Yang, Y. C.; Yang, X. M.; Zhang, Y.; Hou, H. S.; Jing, M. J.; Zhu, Y. R.; Fang, L. B.; Chen, Q. Y.; 22


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Direct Laser Writing of Graphene Features into Graphene Oxide Films Involves Photoreduction and Thermally Assisted Structural Rearrangement. Carbon 2016, 99, 423–431. (46) Shi, L.; Pang, C.; Chen, S.; Wang, M.; Wang, K.; Tan, Z.; Gao, P.; Ren, J.; Huang, Y.; Peng, H.; et al. Vertical Graphene Growth on SiO Microparticles for Stable Lithium Ion Battery Anodes. Nano Lett. 2017, 6, 3681–3687.

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Silica-reinforced Sb-encapsulated porous N-doped carbon nanofibers demonstrate superior lithium storage performance and robust structure stability upon repeated lithiation/delithiation. 361x284mm (72 x 72 DPI)


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Antimony into Porous Electrospun Carbon

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