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Article Journal of Nanoscience and Nanotechnology

Copyright © 2015 American Scientific Publishers All rights reserved Printed in the United States of America

Vol. 15, 8951–8960, 2015 www.aspbs.com/jnn

Characteristics and Electrochemical Performance of Si-Carbon Nanofibers Composite as Anode Material for Binder-Free Lithium Secondary Batteries Yura Hyun1 , Heai-Ku Park2 , Ho-Seon Park3 , and Chang-Seop Lee1 ∗ 2

1 Department of Chemistry, Keimyung University, Daegu, 704-701, South Korea Department of Chemical System Engineering, Keimyung University, Daegu, 704-701, South Korea 3 Chair of Materials Processing, University of Bayreuth, D-95440 Bayreuth, Germany

The carbon nanofibers (CNFs) and Si-CNFs composite were synthesized using a chemical vapor deposition (CVD) method with an iron-copper catalyst and silicon-covered Ni foam. Acetylene as a carbon source was flowed into the quartz reactor of a tubular furnace heated to 600 C. This temperature was maintained for 10 min to synthesize the CNFs. The morphologies, compositions, and crystal quality of the prepared CNFs were characterized by Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), X-ray Diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical characteristics of the Si-CNFs composite as an anode of the Li secondary batteries were investigated using a three-electrode cell. The asdeposited Si-CNF composite on the Ni foam was directly employed as an working electrode without any binder, and lithium foil was used as the counter and reference electrode. A glass fiber separator was used as the separator membrane. Two kinds of electrolytes were employed; 1) 1 M LiPF6 was dissolved in a mixture of EC (ethylene carbonate): PC (propylene carbonate): EMC (Ethyl methyl carbonate) in a 1:1:1 volume ratio and 2) 1 M LiClO4 was dissolved in a mixture of propylene carbonate (PC): ethylene carbonate (EC) in a 1:1 volume ratio. The galvanostatic charge–discharge cycling and cyclic voltammetry measurements were carried out at room temperature by using a battery tester. The resulting Si-CNFs composite achieved the large discharge capacity of 613 mAh/g and much improved cycle-ability with the retention rate of 87% after 20 cycles.

Keywords: Carbon Nanofiber, Fe–Cu Catalyst, Silicon, Chemical Vapor Deposition, Li Secondary Battery.

1. INTRODUCTION A lithium secondary battery consists of a cathode, an anode, a separator, and electrolyte. The cathode for the early stage lithium secondary cell is often made of lithium to take advantage of its high potential and energy density for lightness. With lithium having the lowest electrode potential at 3.045 V among metals, lithium batteries benefit from its extremely low electronegativity, which helps to generate cations and donate electrons. Despite its advanced maximum capacity, however, lithium grows in the form of dendrites over repetitive charging–discharging procedures. Eventually, these procedures result in the dissolution or depositing of lithium by ionization and eventual ∗

Author to whom correspondence should be addressed.

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internal short-circuiting that affects the safety of a cell. Furthermore, lithium demands particular attention when handled because it shows an intense exothermic reaction when exposed to moisture. The most common alternative anode material employed to overcome such problems is carbon-based materials such as graphite. With lithium ions placed inside of a carbonbased anode material under stable conditions, the use of a carbon-based anode material offers a decent solution to secure the safety of a cell. Furthermore, the constant and repetitive oxidation-reduction reactions take place due to the lithium-like electrochemical reaction potential of the carbon-series materials, and the crystal structure barely varies during the course of the intercalation– deintercalation of lithium ions.6–9 18 22

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Characteristics and Electrochemical Performance of Si-CNFs Composite as Anode Material

Carbon materials feature varied structures and, thus, have a wide scope of lithium storage capacities and storage mechanisms; due to the development of the most optimal carbon material, use of these carbon materials as an anode material for a lithium secondary battery is underway.1 2 4 5 However, there should much room to develop the theoretical capacity because the carbon material anode features a comparatively lower theoretical capacity of 372 mAh/g than those of other newly developed anode materials. For this reason, the development of non-carbon anode active material is on the rise to meet the demands from the full-scale secondary cells market for higher capacity and output for use in, without limitation, electronic cars by taking advantage of silicon or tin that features a much improved theoretical capacity than the carbon-series materials.3 10–13 19 Among others, silicon (Si) is known to feature the highest capacity (4,200 mAh/g), which makes itself to be a very attractive anode material for the Si–Li alloy. Silicon is a very attractive anode material despite the rapid decrease in capacity and short lifecycle due to the large variation in volume (up to 300%) during the course of the charging and discharging of lithium. A number of studies are underway to solve this problem. These studies include the micro-granulation of silicon particles, the use of multiphase alloy that reacts with lithium, and different methods to generate an active/inactive silicon composite and a silicon/carbon composite.14–17 20 21 Among the materials consisting of electrode, Binder, electronic conducting additives and current collector are also regarded as very important composition materials in the process of secondary batteries, since the overall quality of the cell depends upon the performances of these materials. Among the rest, binder performs the best role in the cell, because it maintains the physical and chemical stability of the electrode.25 At present, in the case of commercialized graphite electrode, about 10% of volume change is observed for the electrode, during insertion and de-intercalation of Li ion in the electrode and this phenomena gets worse in the high capacitance electrode. When this volume change is continuously repeated, the bonding between active materials is weakened and the contact resistance with conductive additives is increased.26 27 These kinds of mechanical problems can be solved by employing binder so that the role of binder becomes more important. In this study, we synthesized Si-Carbon nanofibers composites on the Ni-foam based on Fe–Cu bimetallic catalysts via Chemical Vapor Deposition, and aimed to solve the theoretical limit of charge capacity of carbon material and improve the problem of volume expansion of Si simultaneously. We also try to investigate the role of binder in the performance of the electrode active materials in the Li secondary batteries by measuring electrochemical characteristics of synthesized anode materials. 8952

2. EXPERIMENTAL DETAILS 2.1. Materials for Experiments Gases and other materials used in this study are summarized in Tables I and II, respectively. The transition metals such as iron nitrate (Fe(NO3 3 · 9H2 O) and copper nitrate (Cu(NO3 2 · 3H2 O) were used as the catalyst for synthesis with aluminum nitrate (Al(NO3 3 · 9H2 O), ammonium molybdate ((NH4 6 Mo7 O24 · 4H2 O), and ammonium carbonate ((NH4 2 CO3 further used as a transition metal supporter, an anti-cohesive agent during high-temperature reaction of transition metal particles, and a precipitator, respectively. For the synthesis ofthe Si-CNFs composites, the pure silicon with a particle size of 1∼5 m was used. The carbon source of C2 H4 /N2 (20/80 vol%) was employed for the synthesis of the carbon nanofibers with H2 /N2 (20/80 vol%) as a promoting gas, and N2 further was used as a carrier gas. 2.2. Preparation of the Catalysts In this study, a Fe–Cu catalyst was prepared out of the coprecipitation method and used in the synthesis of carbon nanofibers. The preparation process of catalysts is shown in Figure 1. Fe(NO3 3 · 9H2 O and Cu(NO3 2 · 3H2 Owere used as Fe–Cu catalysts to prepare a catalyst with the weight ratio of 7:3(Fe:Cu). Sample Solution A is composed of aluminum nitrate, which helps to generate alumina (Al2 O3 that serves as a supporter of the transition metal catalysts in the transition metal nitrate, dissolved in distilled water. With the foregoing supporter working to capture the nano-metal catalyst, the coagulation phenomenon occurs when the temperature is increased up to the temperature for the synthesis of carbon nanofibers without supporter, because of the unstable nano-metal particles. The usage of supporter helps carbon nanofibers grow without a clustered catalyst and thereby serves as a matrix that prevents catalyst coagulation. Table I. Reagents used in the synthesis of CNFs and Si-CNFs composite. Name of reagents

Specification (%)

Manufacturer

Iron(III) Nitrate Nonahydrate (Fe(NO3 3 · 9H2 O)

98

DAEJUNG CHEMICALS & METALS CO.

Copper(II) Nitrate Trihydrate (Cu(NO3 2 · 3H2 O)

99


Aluminum Nitrate Nonahydrate (Al(NO3 3 · 9H2 O) Ammonium MolybdateTetrahydrate ((NH4 6 Mo7 O24 · 4H2 O)

98

DAEJUNG CHEMICALS & METALS CO. DAEJUNG CHEMICALS & METALS CO.

Ammonium Carbonate ((NH4 2 CO3

30


99.9

Alfa Aesar

Silicon powder (Si)

98

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Table II. Gases used in the synthesis of CNFs and Si-CNFs composite. Gas N2 gas H2 /N2 gas C2 H4 /N2 gas

Specification (%)

Manufacturer

99 20 20

Korea standard gas Korea standard gas Korea standard gas

Meanwhile, it is preferred to mix passive metals such as Mn, Cr, Mo, W, Zr, and Tito control the inter-particle coagulation of transition metals such as Fe, Co, and Ni, which all have catalytic activity against the reaction gas during high-temperature reaction. This study employed a mixture of the foregoing Solution A and another Solution B, which is composed of ammonium molybdate and distilled water. The Solution C is made of ammonium carbonate, which acts as a precipitator that works to precipitate the transition metals and the aluminum included in the foregoing Solution A. Precipitation was induced by the gradual blending of the mixture composed of the Solutions A and B and the mixture of the Solution C. This step was followed by agitation for the stability of precipitation. Next, filtering and oven-drying at 80 C were done for at least 24 hours for the catalyst powder, which is then added to the ethanol solvent and mixed in excess of twelve hours by using a ball mill. 2.3. Preparation of Silicon The Si powder solution was prepared by adding silicon powder, an Alfa Aesar product with a size of 1∼5 m to the ethanol solvent and mixed in excess of twelve hours by using a ball mill. 2.4. Catalyst and Silicon Deposition on Ni Foam Without Binder The catalyst and silicon deposition on Ni foam without binder is shown in Figure 2. The Ni foam was used as a collector that helps to supply the electrons demanded

Figure 1.

Preparation process of catalysts.


Figure 2.

Catalysts and Silicon deposition on Ni foam.

for the electrochemical reactions or to collect the electrons generated by the electrochemical reaction of the active electrode materials. The employed Ni foam was then added to the foregoing catalyst solution or the catalystsilicon composite, followed by dip-coating, and vacuumdrying at 120 C for 12 hours in an oven. 2.5. Synthesis of CNFs and Si-CNFs Composites The CNFs and Si-CNFs composites were synthesized in a quartz tube reactor by chemical vapor deposition. The experimental apparatus is shown in Figure 3. Ethylene (C2 H4 , hydrogen, and nitrogen gases were used as a carbon source, as well as a promoting gas and a carrier gas for the synthesis of the CNFs and Si-CNFs composite. In a reactor, the Fe–Cu catalyst or Ni foam dip-coated with Si/Fe–Cu was added, followed by a temperature increase up to 10 C/min while maintaining a nitrogen atmosphere. After reaching 600 C, the nitrogen gas and 20% hydrogen gas (N2 balance) were added once more while maintaining the temperature for half an hour. This step was followed by the 10-minute addition of hydrogen (N2 balance) gas and 20% ethylene (N2 balance) gas. The addition of gases lasted until the end of reaction. This step was followed by flowing nitrogen into the reactor for inactivity and cooling-down until room temperature was reached and the BF (binder free) –CNFs/Ni foam and BFSi-CNFs composites were synthesized.

Figure 3. Schematic diagram of experimental apparatus for preparation of BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam.

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employed was the mixture of the 1:1 solution of EC (ethylene carbonate):PC (propylene carbonate) and 1 M LiClO4 . 2.8. Analysis 2.8.1. Scanning Electron Microscope (SEM) The fiber shape and growth of the BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam were analyzed by using a Scanning Electron Microscope (SEM, Hitachi, S-4800). 2.8.2. Energy Dispersive Spectroscopy (EDS) An energy dispersive spectroscope installed with a Scanning Electron Microscope (SEM, Hitachi, S-4800) was used for the qualitative and quantitative analyses of the synthesized CNFs and Si-CNFs composites at a specific portion of the SEM images.

Figure 4.

Preparation process of CNFs/Ni foam and Si/Ni foam.

2.6. Catalyst and Silicon Deposition on Ni Foam with Binder Carbon nanofibers with Silicon grown based on Fe–Cu catalysts was mixed with PTFE (Polytetrafluoroethylene) as binder (8:2 wt%) in IPA (Isopropyl Alcohol) solution. This slurry was stirred with homogenizer with 2000 rpm to make dipping solution and this dipping solution was used to coat the prepared electrode materials to the Ni foam which is employed as a current collector. CNFs/Ni foam and Si/Ni foam with binder were fabricated by drying in atmosphere at room temperature followed by drying at 80 C during 12 hrs in an oven. 2.7. Fabrication Process of Anode Material for Lithium Secondary Battery By using the CNFs and Si-CNF composites as an anode active material for a lithium secondary battery, the threeelectrode cells were fabricated in a glove box filled with Ar gas. The most common means to improve the binding of anode active material and collector is use of a binder. In this study, however, the CVD method was used instead for the synthesis of the CNFs and the Si-CNFs composites directly on the Ni foam for improved binding of the collector and the anode active materials. The three-electrode cells were fabricated into half cells with the foregoing active materials, lithium metal, and the glass fiber separator immersed in electrolyte used as a working electrode, a counter and reference electrode, and a separator, respectively.23 24 Two different electrolytes were used with EC (ethylene carbonate):PC (propylene carbonate):EMC (Ethyl methyl carbonate) mixed at the ratio of 1:1:3 for a solution to which 1 M LiPF6 was later added. Another solution 8954

2.8.3. X-ray Diffraction (XRD) An X-ray Diffractometer (XRD, PANalytical, X’pert PROMPD) was employed for the analysis of the crystal structure and the microstructure of the synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam. For XRD analysis, the target was Cu; a Bragg angle of 10∼97 and a scanning speed of 1 per minute were also used. 2.8.4. Raman Spectroscopy (Raman) A Raman spectroscope (Horiba Jobin-Yvon, LabRam HR) was employed for the crystal structure analysis of the synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam. 2.8.5. X-ray Photoelectron Spectroscopy (XPS) An X-ray photoelectron spectroscope (Thermo Fisher Scientific, Multilab-2000) was employed for measurement of and comparison among the binding energies for the analysis of the binding energies of carbon, iron and, copper in the synthesized BF-CNFs/Ni foam and binding energy of silicon in BF-Si-CNFs/Ni foam. 2.8.6. Electrochemical Performance In order to investigate the electrochemical performance (Solartron 1287 electrochemical interface), synthesized CNFs and Si-CNFs composites were used as anode active materials. After the three-electrode cells were assembled, the reversibility and charge–discharge capacity of the cells were measured when 0.1∼2 V of voltage and 100 mA/g of current were applied.

3. RESULTS AND DISCUSSION 3.1. Synthesis of BF-CNFs/Ni Foam and BF-Si-CNFs/Ni Foam The BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam were synthesized as per the CVD method for two different Ni foam specimens dip-coated with either the Fe–Cu catalyst or Si/Fe–Cu. Ethylene gas was employed as the carbon source and was transported by non-reactive H2 /N2 and N2 . J. Nanosci. Nanotechnol. 15, 8951–8960, 2015

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shown in Figure 6. According to the foregoing images, the growth of BF-CNFs/Ni foam was verified with an average diameter of 130 nm. Also, the growth of BF-SiCNFs/Ni foam was verified with an average diameter of 10 nm. This diameter represents the diameter of the BF-SiCNFs/Ni foam when using silicon-introduced Fe–Cu catalyst reduces, for 1/10 of that of BF-CNFs/Ni foam when using Fe–Cu catalyst only. Figure 5. Schematic diagram of three-electrode cell for lithium secondary batteries.

When using the Fe–Cu catalyst, the synthesis took place at 600 C for the BF-CNFs/Ni foam, and, when using Ni foam dip-coated with Si/Fe–Cu for the BF-Si-CNFs/Ni foam the synthesis also took place at 600 C. 3.1.1. SEM The carbon nanofibers were synthesized when pyrolyzed hydrocarbon contacts metal catalysts such as Fe, Co, and Ni at high temperature and dissolveds in such catalysts, thereby generating itself a growth core after equilibrium solubility and growing itself carbon nanofibers under a constant supply of carbon source by way of surface diffusion. This surface diffusion occured with the dissolved carbon or metal particle in the metal particle itself. In this study, the SEM images were taken to analyzethe shape of the fully grown carbon nanofibers and their ability to grow. The SEM images of the BF-CNFs/Ni foam and BF-SiCNFs/Ni foam, synthesized by way of the CVD method with Ni foam dip-coated by Fe–Cu and Si/Fe–Cu, are

Figure 6.

3.1.2. EDS EDS was measured for the qualitative and quantitative analyses of the synthesized BF-CNFs/Ni foam and BF-SiCNFs/Ni foam, and the results are shown in Figure 7. The average values of the carbon elements of the synthesized BF-CNFs/Ni foam in the Ni foam dip-coated with Fe–Cu catalyst and the synthesized BF-Si-CNFs/Ni foam in Ni foam dip-coated with Si–Fe–Cu catalyst were 92.75% and 90.75%, respectively. 3.1.3. XRD The XRD experiments for the analysis of the crystal structure of the synthesized BF-CNFs/Ni foam and BF-SiCNFs/Ni foam were carried out, and the results are shown in Figure 8. As shown in Figure 8, the XRD patterns of the synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam show the crystalline carbon peak on the surfaces C(002) and C(331) and the crystalline copper peak on the surfaces Cu(111) and Cu(200). Also, the characteristic peaks of the crystalline silicon of the surfaces Si(002), Si(220), Si(311), and Si(422) were observed. Since the C(002) peaks of BF-CNFs/Ni foam showed narrower width and

SEM images of of BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam synthesized from ethylene at 600 C.


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Figure 7.

EDS images of of BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam.synthesized from ethylene at 600 C.

higher intensity than that of BF-Si-CNFs/Ni foam, it indicated that the carbon nanofibers with better crystallinity was synthesized in the case of BF-CNFs/Ni foam compared to the case of BF-Si-CNFs/Ni foam. 3.1.4. Raman In most cases, the carbon nano-materials are made out of the pure carbon called carbon allotrope such as Diamond,

Figure 8. XRD patterns of synthesized of BF-CNFs/Ni foam and BFSi-CNFs/Ni foam.

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Graphite, Fullerene (C60), Graphene, CNT (Carbon nanotube), and CNF (Carbon nanofiber) each of which is C–C bonded and varies in an orientation that can be observed by the Raman spectroscopy. The Raman analysis was employed for the comparison of the crystallinity between the BF-CNFs/Ni foam and BFSi-CNFs/Ni foam, and the results are shown in Figure 9. As shown in Figure 9, the G-band (Graphite-like band)nd the D-band (Defect-like band) appeared at 1,340 cm−1 and 1,580 cm−1 , respectively, with the G-band representing graphitic carbon nanofibers and the D-band representing carbon impurity or structural defect in graphite. The carbons in the D-band and the G-band are identical to those of sp3 (disordered graphite) and sp2 (ordered graphite) structures with the intensity ratio (D/G) between two different bands representing relative crystallinity.

Figure 9. Raman spectra of synthesized BF-CNFs/Ni foam and BF-SiCNFs/Ni foam.


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Carbon nanofibers were comparatively more defective than the carbon nanotube, and the amorphocity and D/G band intensity ratio are proportionate to each other. With the respective D/G ratios of the BF-CNFs/Ni foam and BFSi-CNFs/Ni foam being 0.93 and 0.86, respectively, for this study, it was observed that the greater the disordered graphite structure (sp3 of the BF-CNFs/Ni foam became, the higher the amorphocity got. 3.1.5. XPS The XPS analysis was performed to analyze the binding energies of the carbon and silicon in the synthesized proportionate, and the results are shown in Figure 10. As shown in Figure 10, the binding energies for carbon were observed at when nearing 284∼285 eV, 286 eV, 287 eV, 288 eV, and 290 eV, representing C–C(sp2 , C–C(sp3 , COOH, C O, and CO2 bonds, respectively. The binding energies for the silicon were observed at when nearing 100 eV and 104 eV, representing SiC and SiO2 bonds, respectively.

Figure 10. XPS spectra of synthesized BF-CNFs/Ni foam and BF-SiCNFs/Ni foam.


Figure 11. Cyclic voltammograms of BF-CNFs/Ni foam and BF-SiCNFs/Ni foam at a 100 mV/s sweep rate.

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Table III. Cycle performances of synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam after 20th cycles in 1 M LiPF6 (EC:PC:EMC = 1:1:3). After 20 cycles Samples

Max. discharge capacity (mAh/g)

Discharge capacity

Retention rate (%)

356 802

104 510

29 64

BF-CNFs/Ni foam BF-Si-CNFs/Ni foam

Note: Retention rate (%) = discharge capacity at 20 cycle/discharge capacity at maximum × 100.

Figure 12. Discharge–charge curves of BF-CNFs/Ni foam in 1 M LiClO4 (PC:EC = 1:1).

3.2. Electrochemical Performance The capacity and cycle performance of the as-prepared BFCNFs/Ni foam and BF-Si-CNFs/Ni foam were analyzed using cyclic voltammetry (CV), Charge–discharge curves and galvanostatic charge–discharge measurements varying by type of electrolyte. 3.3. Cyclic Voltammetry The cyclic voltammetry was performed at a scan rate of 5 mV/s in the potential scope of 0.1–2.0 V (vs. Li/Li+ for the proper analysis of the electrochemical properties of the synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam by type of electrolyte. The CVs of the BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam were measured for two different electrolytes as shown in Figure 11. For the case of BF-CNFs/Ni foam, when 1 M LiPF6 in EC/PC/EMC was employed as an electrolyte, the reduction peak and the broad oxidation peak were observed at below 0.4 V and 1.4 V, respectively. For BF-Si-CNFs/Ni foam, the reduction peak and the oxidation peak were observed at when

nearing 0.5 V, 1.2 V and 1.0 V, 1.8 V, respectively. It seems that these values are associated with the insertion and deinsertion of Li into the disordered carbon contained in the composites. Furthermore, the formation of solid electrolyte interface (SEI) was combined with Li intercalation at below 0.5 V during the cathodic sweep and deintercalation of Li during reverse scan nearing 0.6 V. On the other hand, for the case of BF-CNFs/Ni foam, when LiClO4 was employed as an electrolyte, the reduction peak was observed during charging at when nearing 0.5 V, which is due to the SEI layer, where as the oxidation peak was not apparent at during discharging. As for the BF-Si-CNFs/Ni foam, the reduction peak was not apparent compared to that of the BF-CNFs/Ni foam, thus indicating little formation of the SEI layer. It was found that the formation of the SEI layer was a little prevented using electrolyte LiClO4 than using electrolyte LiPF6 . 3.3.1. Discharge–Charge Curves Figure 12 shows the charge–discharge curve of BFCNFs/Ni foam obtained by applying 10 mA/g and 100 mA/g of current. By comparing two charge–discharge curves, we know that the potential of the cell maintains with flat level of slope as the current density applied to the electrode is lowered, especially the largest discharge 1000

BF-CNFs / Ni foam BF-Si-CNFs / Ni foam 800

600

400

200

BF-CNFs / Ni foam BF-Si-CNFs / Ni foam 800

600

400

200

0

0 0

5

10

15

20

Cycle Number Figure 13. Cycle performances of BF-CNFs/Ni foam and BF-SiCNFs/Ni foam up to 20th cycles in 1 M LiPF6 (EC:PC:EMC = 1:1:3).

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Discharge capacity (mAh/g)


1000

0

5

10

15

20

Cycle Number Figure 14. Cycle performances of BF-CNFs/Ni foam and BF-SiCNFs/Ni foam up to 20th cycles in 1 M LiClO4 (PC:EC = 1:1).


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Table IV. Cycle performances of synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam after 20th cycles 1 M LiClO4 (PC:EC = 1:1).

Table V. Cycle performances of synthesized CNFs/Ni foam and SiCNFs/Ni foam after 20th cycles 1 M LiClO4 (PC:EC = 1:1).

After 20 cycles Samples BF-CNFs/Ni foam BF-Si-CNFs/Ni foam


Discharge capacity

Retention rate (%)

578 706

303 613

52 87

After 20 cycles Samples


Discharge capacity

Retention rate (%)

477 5082

258 423

54 8

CNFs/Ni foam Si-CNFs/Ni foam

Note: Retention rate (%) = discharge capacity at 20 cycle/discharge capacity at maximum × 100.

Note. Retention rate (%) = discharge capacity at 20 cycle/discharge capacity at maximum × 100.

capacity of 520 mAh/g was shown in the 10 mA/g of applied current. This can be explained by the generally observed phenomena in the electrochemical reaction, that is, a total reaction is governed by mass transfer reaction in a high applied current and charge transfer reaction in a low applied current, which is called rate characteristics in the cell. On the other hand, the case of BF-Si-CNFs/Ni foam did not show any significant experimental results.

in discharge capacity is apparently due to the higher theoretical capacity of the Si particles that gave rise to the higher discharge capacity of the overall BF-Si-CNFs/Ni foam. (b) 1 M LiClO4 in PC:EC The discharging capacity of the PC (propylene carbonate):EC (ethylene carbonate) with use of LiClO4 as an electrolyte is shown in Figure 14, and the results are summarized in Table IV. As shown in Figure 14, the earlystage capacity of 578 mAh/g was decreased to 303 mAh/g after twenty cycles with 52% retention rate for the BFCNFs/Ni foam anode. Whereas, for the BF-Si-CNFs/Ni foam anode, the early-stage capacity of 706 mAh/g was decreased to 613 mAh/g with 87% retention rate after twenty cycles. The foregoing results demonstrate that the anode active material in the LiClO4 electrolyte features more enhanced discharging capacity and retention rate than that of the LiPF6 electrolyte. Furthermore, the higher theoretical capacity of the Si particles gave rise to the higher discharging capacity compared to that of the overall BF-Si-CNFs/Ni foam.

3.3.2. Galvanostatic Charge–Discharge The charging and discharging properties employing two different electrolytes were analyzed by applying a 100 mA/g current density in recognition of such electrochemical characteristics as capacity and cyclic ability of the synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam. (a) 1 M LiPF6 in EC:PC:EMC The cycle performance of the BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam with use of LiPF6 as an electrolyte is shown in Figure 13, and the results are summarized in Table III. Using the BF-CNFs/Ni foam as the anode active material for lithium secondary batteries, the earlystage capacity of 356 mAh/g was decreased to 104 mAh/g after twenty cycles with 29% retention rate. Whereas, using the BF-Si-CNFs/Ni foam, the early-stage capacity of 802 mAh/g was decreased to 510 mAh/g after twenty cycles with 64% retention rate. The foregoing increment 5500 Si / Ni foam CNFs / Ni foam


5000 4500

Figure 15 shows charge–discharge capacities and retention rate of Si and CNFs with binder as anode materials of Li secondary batteries after 20 cycles respectively, and the results are summarized in Table V. In the case of pure Si, it is observed that the initial capacity of 5,082 mAh/g, was decreased to 423 mAh/g after 20th cycles with the retention rate of 8%, which shows a serious volumetric expansion of Silicon. Whereas, the case of CNFs showed 477 mAh/g of initial discharge capacity, decreased to 258 mAh/g with 54% of retention rate after 20th cylces.

4000 3500

4. CONCLUSIONS

3000 2500 2000 1500 1000 500 0 0

5

10

15

20

Cycle Number Figure 15. Cycle performances of Si/Ni foam and CNFs/Ni foam up to 20th cycles in 1 M LiClO4 (PC:EC = 1:1).


In this study, we synthesized the BF-CNFs/Ni composite by using a Fe–Cu catalysts via chemical vapor deposition and fabricated the BF-Si-CNFs/Ni foam by way of mixing Si particles and Fe–Cu catalysts. We investigated the physicochemical and electrochemical properties of such materials when used as anode materials of the lithium secondary batteries. Based on the results, we obtained the following conclusions: (1) SEM images for the synthesized BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam via CVD method on the Ni foam dip-coated with Fe–Cu catalyst and Si, showed the average 8959


diameters of the BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam to be 130 nm and 10 nm, respectively. (2) EDS results showed that the composition of carbon element for the BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam were observed at 92.75% and 90.75%, respectively. (3) XRD analysis based on the intensity of crystalline peaks of C(002) revealed that the BF-CNFs/Ni foam showed higher intensity than the BF-Si-CNFs/Ni foam, which demonstrates the higher contents of the pure carbon nanofibers. (4) In the Raman Analysis, D-band and G-band were observed at 1,340 cm−1 and 1,580 cm−1 , respectively, with the D/G ratios of BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam at 0.93 and 0.86, respectively. This means that the BF-Si-CNFs/Ni foam has the higher contents of SP2 carbon structure than BF-CNFs/Ni foam. (5) In the XPS analysis, the carbon-binding energies were observed at 284∼285 eV, 286 eV, 287 eV, 288 eV, and 290 eV, representing C–C(sp2 , C–C(sp3 , COOH, C O, and CO2 bonds, respectively. The binding energies for silicon were observed at 100 eV and 104 eV, representing SiC and SiO2 bonds, respectively. (6) CV analysis showed that the LiClO4 as an electrolyte represents more reversible reactions than LiPF6 for both BF-CNFs/Ni foam and BF-Si-CNFs/Ni foam. (7) When LiPF6 was used as an electrolyte, the charge and discharge capacity of BF-CNFs/Ni foam as an anode material of Li secondary batteries at the 20th cycle were decreased from 356 mAh/g to 104 mAh/g with the retention rate of 29%. The charge and discharge capacity of BF-Si-CNFs/Ni foam as an anode material at the 20th cycle were decreased from 802 mAh/g to 510 mAh/g with the retention rate of 64%. (8) When LiClO4 was used as an electrolyte, the charge and discharge capacity of BF-CNFs/Ni foam as an anode material of Li secondary batteries at the 20th cycle were decreased from 578 mAh/g to 303 mAh/g with retention rate of 52%. The charge and discharge capacity of BF-Si-CNFs/Ni foam were decreased from 706 mAh/g to 613 mAh/g at the 20th cycle with retention rate of 87%, which is relatively high value. (9) Si-CNFs composite showed the better charge capacity and retention rate than Si or CNFs as anode materials of Li secondary batteries, regardless of the existence of binder condition. (10) Binder free condition on Ni foam enhanced the compatibility of anode active materials to the current collector and resulted in a physical stability and the better performance of the electrode.

Hyun et al.

Acknowledgments: This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2012026209).

References and Notes 1. K. H. Kwon, Master Thesis, Myongji University (2002). 2. J. S. Lim, Master Thesis, Myongji University (2004). 3. S. S. Kim, Master Thesis, Graduate School, Soonchunhyang University (2013). 4. Y. R. Hyun, H. S. Kim, and C. S. Lee, Bull. Korean Chem. Soc. 33, 3177 (2012). 5. E. S. Park, J. W. Kim, and C. S. Lee, Bull. Korean Chem. Soc. 35, 1687 (2014). 6. S. M. Jang, J. Miyawaki, M. Tsuji, I. Mochida, and S. H. Yoon, Carbon 47, 3383 (2009). 7. H. P. Liu, W. M. Qiao, L. Zhan, and L. C. Ling, New Carbon Mater. 24, 124 (2009). 8. Y. Liu, K. Huang, Y. Fan, Q. Zhang, F. Sun, T. Gao, Z. Wang, and J. Zhong, Electrochim. Acta 102, 246 (2013). 9. S. Y. Kim, B. H. Kim, and K. S. Yang, J. Electroanal. Chem. 705, 52 (2013). 10. D. Ohms, M. Kohlhase, G. Benczur-Urmossy, and G. Schadlich, J. Power Sources 105, 127 (2002). 11. M. Endo, C. Kim, K. Nishimura, T. Fujino, and K. Miyasita, Carbon 28, 183 (2000). 12. Y. Asami, K. Tsuchoya, H. Nose, S. Suzuki, and K. Mizushina, J. Power Sources. 54, 146 (1995). 13. H. Wang and M. Yoshio, J. Power Sources. 93, 123 (2001). 14. H. Wang, T. Ikeda, K. Fukuda, and M. Yoshio, J. Power Sources 83, 141 (1999). 15. J. K. Lee and B. W. Cho, Carbon Sciences 3, 155 (2002). 16. S. Flandriois and B. Simon, Carbon 37, 165 (1999). 17. L. E. Murphy, J. Winnick, and P. A. Kohl, J. Power Sources. 102, 294 (2001). 18. I. Martin-Gullon, J. Vera, J. A. Conesa, J. L. Gonzalez, and C. Merino, Carbon 44, 1572 (2006). 19. J. Zhang, Z. Xie, W. Li, S. Dong, and M. Qu, Carbon 74, 153 (2014). 20. N. Ding, J. Xua, Y. Yao, G. Wegner, I. Lieberwirth, and C. Chen, J. Power Sources. 192, 644 (2009). 21. J. Brumbarov and J. Kunze-Liebhäuser, J. Power Sources. 258, 129 (2014). 22. Y. Du, M. Hou, D. Zhou, Y. Wang, C. Wang, and Y. Xia, J. Energy. Chem. 23, 315 (2014). 23. H. K. Park, O. Y. Jung, and M. H. Lee, J. Ind. Eng. Chem. 16, 491 (2005). 24. S. J. Park, M. H. Lee, and H. K. Park, J. Ind. Eng. Chem. 21, 211 (2010). 25. C. C. Chang, L. J. Her, T. K. Chen, L. C. Chen, and J. L. Hong, J. New Mat. Electr. Sys. 11, 42 (2008). 26. Z. Chen, L. Christensen, and J. R. Dahn, J. Appl. Polym. Sci. 90, 1891 (2003). 27. H. W. Lee and Y. M. Lee, J. Korean Electrochem. Soc. 15, 115 (2012).

Received: 18 July 2014. Accepted: 16 December 2014.

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