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Synthesis and Electrochemical Performance of Transition Metal-Coated Carbon Nanofibers on Ni Foam as Anode Materials for Lithium Secondary Batteries
CHAPTER SEVEN
Synthesis and Electrochemical Performance of Transition Metal-Coated Carbon Nanofibers on Ni Foam as Anode Materials for Lithium Secondary Batteries Chang-Seop Lee1, Yura Hyun2 and Jin-Young Choi1 1
Department of Chemistry, Keimyung University, Daegu, South Korea Department of Pharmaceutical Engineering, International University of Korea, Jinju, South Korea
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Contents 7.1 Introduction 7.2 Synthesis and Electrochemical Performance of Ruthenium Oxide-Coated CNFs on Ni Foam 7.2.1 Synthesis of Carbon Nanofibers 7.2.2 Preparation of Ruthenium Oxide-Coated Carbon Nanofibers 7.2.3 Fabrication Process of Anode Materials for Lithium Secondary Batteries 7.3 Analyses 7.3.1 Scanning Electron Microscopy 7.3.2 Raman Spectroscopy 7.3.3 X-Ray Photoelectron Spectroscopy 7.3.4 Cyclic Voltammetry 7.3.5 Cycle Performances 7.4 Synthesis and Electrochemical Performance of Transition Metals Oxide-Coated Carbon Nanofibers on Ni Foam 7.4.1 Transition Metal-Coated Carbon Nanofibers 7.4.2 Fabrication Process of Anode Materials for Lithium Secondary Batteries 7.5 Analyses 7.5.1 Scanning Electron Microscopy 7.5.2 Raman Spectroscopy 7.5.3 X-Ray Photoelectron Spectroscopy 7.5.4 Cyclic Voltammetry 7.5.5 Cycle Performances 7.6 Conclusion References
Carbon-Based Nanofillers and their Rubber Nanocomposites. DOI: https://doi.org/10.1016/B978-0-12-813248-7.00007-9
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© 2019 Elsevier Inc. All rights reserved.
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7.1 INTRODUCTION As a new generation of green rechargeable batteries, the lithium ion secondary battery has many advantages over other candidates, such as better portability, higher energy density, longer cycle life, and nonmemory effect [1]. They are widely used not only in the electronic product, small electric tool, aerospace instrument, and related industries, but also as one of the backup power supplies for electric vehicles, which have attracted widespread interest worldwide [2]. The electrode material is an important component of a lithium ion battery, and its research has played a crucial role as one of the key subsystems for the development of the lithium-ion battery [3]. The lithium secondary battery has been highlighted as a promising power source, due to its high energy density and high output capacity, which enable it to offer high performance. As it is characterized by a long life cycle and low self-discharge, the lithium secondary battery is in high demand. As the electronics industry has rapidly developed, lightweight, small, and diverse electronic devices are required. Increasing interest is now being shown in the development of batteries with high capacity, high performance, and high density [4 10]. However, in lithium secondary batteries, in which lithium metal is used as the anode, crystals are likely to form on dendrite when charge/discharge is repeated. Due to the risk of short-circuit, the graphite-based anode active material is mainly used as the anode material of the lithium secondary battery [4 13]. The graphite-based anode active materials recently studied as anode materials for lithium secondary batteries include graphene, carbon nanotube, carbon nanofiber, and hollow or porous carbon. The carbon nanofibers have chemical stability, electrical conductivity, and high energy efficiency, and have a relatively larger specific surface area than general carbon materials. The carbon-based materials that are currently used as anode active material of the Li secondary battery have a maximum capacity of 372 mAh g 1. Because of their low theoretical capacity, they have capacitive limitations. Thus, there are many studies and suggestions on creating new anode materials with high capacity. One of them is to use silicon (Si). Si is known to be suitable as an anode material with high capacity, because it has high theoretical capacity (4200 mAh g 1) and low mass, and can interact with Li. However, Si has the problem of the electrode life being rapidly shortened due to the large volume expansion of up to 400% that occurs in the charge discharge process. To overcome the associated problems of both the carbon-based material and Si, many researchers are conducting studies on the composites of both materials. Carbon nanofiber/silicon composite can overcome the functional shortcomings of individual materials. Because carbon nanofiber provides flexible space to alleviate the
Synthesis and Electrochemical Performance of Transition
volume expansion of Si in the process of charging and discharging, it is expected to overcome the limitations and problems of the existing Li secondary batteries [14 18]. Thus, they can be employed in electrodes of fuel cells, absorbents, and energy storage. Since the physicochemical properties of carbon nanofibers, such as their diameter, presence of bonds, and number of layers, can be selected by changing the synthetic methods and conditions, carbon nanofiber might be a promising material that could replace the graphite-based anode active material with its structural limitation [11 13,19]. Since carbon-based anode electrode materials have problems, such as low charge/discharge capacity and low retention rate due to their high irreversible capacity, many studies are being actively conducted on transition metals as anode materials of lithium secondary batteries. The transition metals mitigate the degradation of the electrode and electrolyte, and improve the electron conductivity of the surface of the carbon nanofibers [11 13,16,19 24]. The aim of this study is to synthesize carbon nanofibers with high chemical stability and thermal conductivity for the use of anode materials of lithium secondary batteries, and coat carbon nanofibers with transition metals such as Ru, Fe, Co, Ni, and Cu to suppress the side reaction, enhance the retention rate, and extend the life of electrodes.
7.2 SYNTHESIS AND ELECTROCHEMICAL PERFORMANCE OF RUTHENIUM OXIDE-COATED CNFs ON NI FOAM [8] 7.2.1 Synthesis of Carbon Nanofibers Carbon nanofibers for use as carbon-based anode materials were synthesized using chemical vapor deposition (CVD) apparatus with a quartz tube. Fig. 7.1 shows a schematic of the experimental set-up for the synthesis of carbon nanofibers.
Figure 7.1 Schematic of the chemical vapor deposition apparatus for the preparation of carbon nanofibers [8].
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In this study, 20% ethylene (C2H4/N2, Korea standard gas) was used to synthesize carbon nanofibers as a carbon source, while 20% hydrogen (H2/N2, Korea standard gas) and high-purity nitrogen (N2, Korea standard gas) were used as a vapor reactionpromoting gas and a carrier gas, respectively. Carbon nanofibers were synthesized as follows: Ni foam which was employed as the current collector, was inserted into the chemical vapor deposition apparatus, and the temperature was then increased to 600 C at 10 C min 1 increment rate under nitrogen atmosphere. The temperature was then maintained at 600 C with flowing hydrogen gas for 30 min, followed by injection of ethylene/hydrogen mixed gas for 10 min. After the reaction ended, the apparatus was cooled down to room temperature by injecting nitrogen gas.
7.2.2 Preparation of Ruthenium Oxide-Coated Carbon Nanofibers RuCl3 solution was used to coat the carbon nanofibers with ruthenium oxide. Fig. 7.2 shows the dip-coating method that was used to coat the carbon nanofibers synthesized by chemical vapor deposition in 0.01 M RuCl3 solution. After Ni foam was dip-coated in the ruthenium chloride solution and it was air-dried for 5 min, it was then further dried for 12 h or more at 80 C.
7.2.3 Fabrication Process of Anode Materials for Lithium Secondary Batteries The three-electrode cell was prepared by applying RuO2/carbon nanofibers/Ni foam as anode active material of lithium secondary batteries. The three-electrode cell was assembled in a glove box filled with Ar gas. It was assembled as a half cell. Prepared active materials were used as a working electrode. Lithium was used as counter and
Figure 7.2 Preparation process of the ruthenium oxide-coated carbon nanofibers [8].
Synthesis and Electrochemical Performance of Transition
reference electrodes. A glass fiber separator was used as a separator membrane. The 1 M LiClO4 was employed as electrolyte, and dissolved in a mixture of ethylene carbonate (EC):propylene carbonate (PC) in a 1:1 volume ratio.
7.3 ANALYSES 7.3.1 Scanning Electron Microscopy Scanning electron microscopy images were measured to examine the morphologies of ruthenium oxide-coated Ni foam, current collector, and ruthenium oxide-coated carbon nanofibers/Ni foam. Fig. 7.3 shows the results of scanning electron microscopy image measurement. Fig. 7.3A shows that Ni foam was coated with ruthenium oxide, while it was cracked, while Fig. 7.3B shows that carbon nanofibers were grown on the Ni foam. Carbon nanofibers were synthesized by contact of thermally decomposed hydrocarbon with metal catalytic particle, such as Fe, Co, or Ni. This indicates that Ni foam, a current collector, played the role of a catalyst to grow carbon nanofibers. In addition, it shows that the grown carbon nanofibers were coated with ruthenium oxide.
7.3.2 Raman Spectroscopy Raman spectroscopy was performed to analyze the crystal quality of ruthenium oxide-coated Ni foam and ruthenium oxide-coated carbon nanofibers/Ni foam. Fig. 7.4 shows the results. Fig. 7.4B shows that in the case of synthesized carbon nanofibers, graphite-like band (G-band) and defect-like band (D-band) were observed
Figure 7.3 Scanning electron microscopy images of the synthesized (A) RuO2/Ni foam, and (B) RuO2/carbon nanofibers/Ni foam [8].
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Figure 7.4 Raman spectra of the synthesized (A) RuO2/Ni foam, and (B) RuO2/carbon nanofibers/Ni foam [8].
at (1340 and 1580) cm21, respectively. For D-band and G-band, sp3 (disordered graphite) and sp2 (ordered graphite) bonds were shown. The relative crystal quality of the material could be identified using the intensity ratio (D/G) of the G-band and Dband. In Fig. 7.4B, ID/IG was 1.57.
7.3.3 X-Ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy analysis was performed to examine the binding energy of ruthenium and carbon in ruthenium oxide-coated Ni foam and ruthenium oxide-coated carbon nanofibers/Ni foam. Fig. 7.5 shows the results. The binding energies of the atoms shown in the X-ray photoelectron spectroscopy results varied, depending on the differences in electronegativity. Fig. 7.5A and B shows that RuO2 bond with a binding energy of 280 eV was observed in Ru3d. Fig. 7.5B shows the data of the synthesized carbon nanofibers. CQC (sp2) bond with a binding energy of 284 eV, and C C (sp3) bond with a binding energy of 286 eV were observed in C1s. In addition, CQO bond with a binding energy of 288 eV was observed.
7.3.4 Cyclic Voltammetry Cyclic voltammetry was performed by using ruthenium oxide-coated Ni foam, RuO2-coated carbon nanofibers/Ni foam, carbon nanofibers grown on Ni foam, and graphite as control sample (Customcells, current collector: Cu foil, binder: PVDF), and as anode materials of Li secondary batteries. Fig. 7.6 shows the results. Cyclic voltammetry was performed in all samples by applying a current of 100 mA g 1 in the voltage range of 0.01 2.0 V (vs Li/Li1).
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Figure 7.5 X-ray photoelectron spectroscopy spectra of the synthesized (A) RuO2/Ni foam, and (B) RuO2/carbon nanofibers/Ni foam [8].
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Figure 7.6 Cyclic voltammograms of synthesized (A) RuO2/Ni foam, (B) RuO2/carbon nanofibers/Ni foam, (C) carbon nanofibers/Ni foam, (D) graphite [8].
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Specific sites with oxidation and reduction potential varied depending on the energy in lithium intercalation and deintercalation reactions of the electrodes. In the case of a fully reversible reaction, because there were small differences in the peaks of oxidation and reduction regardless of the voltage change rates, the patterns of cyclic voltammetry appeared symmetrically. Fig. 7.6A shows the results for RuO2-coated Ni foam used as the anode material of the Li-secondary battery. Upon first charge, reduction peaks were observed at 0.4 and 1.7 V. Upon discharge, oxidation peaks were observed at 0.6, 1.2, and 1.9 V. Upon second charge, reduction peaks were observed at 0.9 and 1.4 V. Upon discharge, oxidation peaks were similar to those in the first discharge. Fig. 7.6B shows the results for RuO2-coated carbon nanofibers/Ni foam. Upon first charge, reduction peaks were observed at 0.5 and 1.5 V. Upon discharge, oxidation peaks were observed at 0.3, 1.2, and 1.8 V. Upon second charge, reduction peaks were observed at 0.4 and 1.7 V. Upon discharge, the oxidation peaks were similar to those in the first discharge. Fig. 7.6C shows the results for carbon nanofibers grown on Ni foam. Upon first charge, reduction peaks were observed at 0.4 and 1.2 V. Upon second charge, no reduction peak was observed. Upon first and second discharge, an oxidation peak was observed at 0.4 V. Fig. 7.6D shows the results when purchased graphite was used. Upon first charge, reduction peaks were observed at 0.4 V. Upon second charge, no reduction peak was observed. Upon first and second discharge, no oxidation peak was observed. Reduction peaks that disappeared upon charge were associated with electrolyte decomposition and solid electrolyte interface (SEI) generation. In addition, areas of cyclic voltammograms were associated with capacity. RuO2-coated carbon nanofibers/Ni foam showed the largest cyclic voltammetry area. It showed the highest efficiency when compared with the capacity per cycle.
7.3.5 Cycle Performances In this study, RuO2-coated Ni foam and RuO2-coated carbon nanofibers/Ni foam were used. Carbon nanofibers directly grown on Ni foam and purchased graphite were used as control groups and a three-electrode cell was prepared by using them as anode active material. Charge and discharge characteristics were examined by applying the current of 100 mA g 1 in order to examine electrochemical characteristics such as capacity and cycle performance of the three-electrode cell. Fig. 7.7 shows discharge capacity and efficiency after 30 cycles. Fig. 7.7A showed reductions of initial capacity 1977 372 mAh g 1 after 30 cycles, and the retention rate of 18.8% when RuO2/Ni foam was used as the anode active
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After 30 cycles Discharge capacity Retention rate (%) (A) 1,977 372 18.8 (B) 494 234 47.4 (C) 276 129 46.7 (D) 203 21 10.3 Retention rate (%) = discharge capacity at 30 cycle/discharge capacity at maximum × 100
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Figure 7.7 Cycle performances of the synthesized (A) RuO2/Ni foam, (B) RuO2/carbon nanofibers/Ni foam, (C) carbon nanofibers/Ni foam, and (D) graphite [8].
material. Fig. 7.7B showed reductions of initial capacity 494 234 mAh g 1 after 30 cycles and the retention rate of 47.4% when RuO2/carbon nanofibers/Ni foam was used as the anode active material. Fig. 7.7C showed reductions of initial capacity 276 129 mAh g 1 after 30 cycles and the retention rate of 46.7% when carbon nanofibers/Ni foam (control group) was used as the anode active material. Fig. 7.7D showed reductions of initial capacity 203 21 mAh g 1 after 30 cycles and the retention rate of 10.3%. Thus, coating with transition metal oxide, such as ruthenium oxide (RuO2) with high capacity, could improve the low capacity of carbon nanofibers. Suppressing the side reactions of electrolytes also improved the retention rate.
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7.4 SYNTHESIS AND ELECTROCHEMICAL PERFORMANCE OF TRANSITION METALS OXIDE-COATED CARBON NANOFIBERS ON NI FOAM 7.4.1 Transition Metal-Coated Carbon Nanofibers The transition metal salts of Fe, Co, Ni, and Cu were dissolved in aqueous solution, and used to coat metals on the carbon fibers. Coating was achieved by dipping the carbon nanofibers synthesized via chemical vapor deposition method in 0.1 M transition metal salt solutions. Carbon nanofibers were grown on the Ni foam in chemical vapor deposition apparatus, and the foam was then dipped into the transition metal solution. After the foam was air-dried for 5 min, it was dried for more than 12 h at 80 C in an electric oven.
7.4.2 Fabrication Process of Anode Materials for Lithium Secondary Batteries A three-electrode cell was prepared by applying carbon nanofibers-transition metals as anode active materials of lithium secondary batteries. The three-electrode cell was assembled in a glove box filled with Ar gas. It was assembled as a half cell. Various prepared active materials were used as the working electrodes. Lithium was used as the counter and reference electrode, a glass fiber separator was used as a separator membrane, and 1 M LiClO4 was employed as the electrolyte, and was dissolved in a mixture of ethylene carbonate:propylene carbonate (EC:PC) at a 1:1 volume ratio.
7.5 ANALYSES 7.5.1 Scanning Electron Microscopy Fig. 7.8 shows the scanning electron microscopy images that were taken to examine the microscopic morphologies of the carbon nanofibers, after the grown carbon nanofibers were coated with the four transition metals of Fe, Co, Ni, and Cu. The synthesized carbon nanofibers were dipped in 0.1 M transitional metal solution, and the scanning electron microscopy images were then captured. Fig. 7.8A shows that the carbon nanofibers were evenly coated with Fe. On the other hand, Fig. 7.8B and D shows that large aggregations of Co and Cu were formed on the carbon nanofibers, and that some areas of the carbon nanofibers were not coated with the transition metals. Fig. 7.8C shows that the carbon
Synthesis and Electrochemical Performance of Transition
Figure 7.8 Scanning electron microscopy images of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers Fe, (B) carbon nanofibers Co, (C) carbon nanofibers Ni, and (D) carbon nanofibers Cu [25].
nanofibers were coated with Ni. While large aggregations were not formed, the carbon nanofibers Ni were less uniformly coated than the carbon nanofibers Fe. Among the four transition metals used to coat carbon nanofibers, the coating of Fe and Ni was relatively uniform, without large aggregations. This mitigated the degradation of carbon nanofibers in the Li insertion/extraction reactions.
7.5.2 Raman Spectroscopy Raman spectroscopy was performed to investigate the degree of crystal quality of the transition metal-coated carbon nanofibers. Fig. 7.9 shows the results, where all the Raman data show characteristic peaks of synthesized carbon nanofibers, such as a graphite-like band (G-band) and a defect-like band (D-band) observed at 1340 and 1580 cm21, respectively. Fig. 7.9A shows a peak of FeO at 680 cm21, while Fig. 7.9B shows a peak of CoO at 670 cm21; Fig. 7.9C shows a peak of NiO at 550 cm21, while Fig. 7.9D shows a peak of CuO at 260 cm21.
7.5.3 X-Ray Photoelectron Spectroscopy Fig. 7.10 shows the results of the X-ray photoelectron spectroscopy analysis that was performed to determine the binding energies of the transition metals coated on the carbon nanofibers. The binding energy varied depending on the electronegativity of transition metals. Transition metal with high electronegativity strongly attracted
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Figure 7.9 Raman spectra of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers Fe, (B) carbon nanofibers Co, (C) carbon nanofibers Ni, and (D) carbon nanofibers Cu [25].
electrons, so that it had a relatively low binding energy. Table 7.1 summarizes the binding energies of the four transition metals shown in Fig. 7.10 that were analyzed. The table shows that 2p1/2 and 2p3/2 peaks of the various chemical states of all transition metals appeared in the X-ray photoelectron spectroscopy spectra. Peaks that appear at 712 and 719 eV in Fig. 7.10A are assigned to FeO and Fe2O3, while peaks that appear at 781 and 787 eV in Fig. 7.10B are assigned to Co3O4 and CoO. Peaks at 857 and 862 eV in Fig. 7.10C and peaks at 934 and 945 eV in Fig. 7.10D are assigned to Ni(OH)2 and NiO, and Cu2O and CuO species, respectively.
7.5.4 Cyclic Voltammetry Fig. 7.11 shows the results of cyclic voltammetry measurements that were performed using transition metal coated carbon nanofibers as the anode materials of lithium secondary batteries. Carbon nanofibers and Si carbon nanofiber composites synthesized in this study have been applied as anode materials of Li secondary batteries, and assembled into
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Figure 7.10 X-ray photoelectron spectroscopy spectra of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers Fe, (B) carbon nanofibers Co, (C) carbon nanofibers Ni, and (D) carbon nanofibers Cu [25]. Table 7.1 X-ray Photoelectron Spectroscopy Results for the Transition Metal-Coated Carbon Nanofibers Composites: (A) Carbon Nanofibers Fe, (B) Carbon Nanofibers Co, (C) Carbon Nanofibers Ni, and (D) Carbon Nanofibers Cu [25] (A) (B) (C) (D) Combination Binding Energy (eV)
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Co3O4 CoO Co3O4 CoO
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Cu2O CuO Cu2O CuO
712 719 724 732
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three-electrode cells. Lithium was used as counter and reference electrodes. A glass fiber separator was used as a separator membrane. The 1 M LiClO4 was employed as an electrolyte, and dissolved in a mixture of ethylene carbonate:propylene carbonate (EC:PC) in a 1:1 volume ratio. Cyclic voltammetry measurements were performed by
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Figure 7.11 Cyclic voltammograms of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers, (B) carbon nanofibers Fe, (C) carbon nanofibers Co, (D) carbon nanofibers Ni, and (E) carbon nanofibers Cu [25].
applying 100 mA g 1 of current density in the 0.1 2.0 V (vs Li/Li1) voltage range, in order to investigate the electrochemical characteristics. In the lithium insertion/extraction reactions of an electrode, the particular position of oxidation/reduction potential varies, depending on the amount of energy. If it is a completely reversible reaction, the difference between the oxidation and reduction peaks is small, regardless of the rates of changing potential. This results in symmetric shape in the cyclic voltammogram.
Synthesis and Electrochemical Performance of Transition
Fig. 7.11A shows the results obtained using the synthesized carbon nanofibers as the anode material of a lithium secondary battery. Upon the first charge, reduction peaks were observed at 0.6 and 1.3 V; upon discharge, an oxidation peak was observed at 0.4 V; upon the second charge, a reduction peak was observed at 0.7 V; and upon discharge, an oxidation peak was observed at 0.4 V. Fig. 7.11B shows the results obtained using carbon nanofibers Fe. Upon the first charge, reduction peaks were observed at 0.5 and 1.7 V; upon discharge, oxidation peaks were observed at 1.2 and 1.9 V; upon the second charge, reduction peaks were observed at 0.6 and 1.3 V; and upon discharge, oxidation peaks were observed at 0.4 and 1.7 V. Fig. 7.11C shows the results obtained using carbon nanofibers Co. Upon the first charge, only one reduction peak was observed at 1.3 V. After that, no oxidation or reduction peaks were observed upon charge and discharge. Fig. 7.11D shows the results obtained using carbon nanofibers Ni. Upon the first charge, reduction peaks were observed at 0.6, 1, and 2.2 V; upon discharge, oxidation peaks were observed at 1.1, 1.8, and 2.2 V; upon the second charge, reduction peaks were observed at 0.8 and 2.2 V; and upon discharge, oxidation peaks were observed at 1.1, 1.7, and 2.2 V. Fig. 7.11E shows the results obtained using carbon nanofibers Cu. Upon the first charge, a reduction peak was observed at 1.4 V; upon discharge, an oxidation peak was observed at 2.1 V; upon the second charge, a reduction peak was observed at 0.8 V; and upon discharge, oxidation peaks were observed at 1.1 and 1.8 V. Oxidation and reduction peaks that disappear upon charge and discharge are believed to be related to the decomposition of the electrolyte, and the generation of a solid electrolyte interface (SEI). In addition, the cyclic voltammetry graph showing the area is related to the capacity of the carbon nanofibers. Carbon nanofibers Fe and carbon nanofibers Ni had the largest cyclic voltammetry area. When their capacities per cycle were compared, they showed the greatest efficiency.
7.5.5 Cycle Performances The electrochemical characteristics of transition metal-coated carbon nanofibers and carbon nanofibers/Ni foam (control) as anode active material of Li secondary batteries were investigated using a three-electrode cell. The characteristics of charge/discharge were examined to observe the electrochemical characteristics of the three-electrode cell by applying a current density of 100 mA g 1. Fig. 7.12 shows the results of discharge capacity and retention rate that were measured after 30 cycles. Fig. 7.12A shows the results of measuring the cell prepared using carbon nanofibers/Ni foam as the anode active material. The initial capacity was 310 mAh g 1. After 30 cycles, this was reduced to 154 mAh g 1. The retention rate was 49.7%.
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Figure 7.12 Cycle performances of the transition metal-coated carbon nanofibers composites (A) carbon nanofibers, (B) carbon nanofibers Fe, (C) carbon nanofibers Co, (D) carbon nanofibers Ni, and (E) carbon nanofibers Cu [25].
Fig. 7.12B E shows the results of measuring the cell prepared using transition metal-coated carbon nanofibers. After five cycles, the capacity of most transition metal-coated carbon nanofibers was reduced to about 360 mAh g 1. The capacity of the carbon nanofibers Co was reduced to 234 mAh g 1.
Synthesis and Electrochemical Performance of Transition
The initial capacity of carbon nanofibers Cu was the highest at 1028 mAh g 1. However, after 30 cycles, this reduced to 136 mAh g 1. The retention rate of carbon nanofibers Cu was the lowest at 13.2%. On the other hand, carbon nanofibers Fe had the lowest initial capacity (670 mAh g 1). After 30 cycles, this capacity reduced to 275 mAh g 1. The carbon nanofibers Fe showed the highest retention rate of 41%. The result of the cycle performances showed that the initial capacity of the transition metal-coated carbon nanofibers was high, compared to that of the carbon nanofibers without transition metal coating. However, after 30 cycles, the capacities of carbon nanofibers Co and carbon nanofibers Cu were similar to that of carbon nanofibers, because the transition metals were not uniformly coated. After 30 cycles, the capacity of carbon nanofibers Fe and carbon nanofibers Ni, in which transition metals were uniformly coated, was improved, compared to that of the carbon nanofibers. In particular, after 30 cycles, the capacity of carbon nanofibers Fe was higher than that of carbon nanofibers by 78%. This suggests that the low capacity of carbon nanofibers can be improved by coating carbon nanofibers with transition metals, and the retention rate can be improved by inhibiting the side reactions of the electrolyte.
7.6 CONCLUSION In this study, carbon nanofibers transition metal composites were prepared by dipping carbon nanofibers synthesized by chemical vapor deposition method in transition metal solutions. From the results of galvanostatic charge and discharge, when RuO2/carbon nanofibers/Ni foam was used as the anode active material, the initial capacity was reduced from 494 to 234 mAh g 1 after 30 cycles, and a retention rate of 47.4% was observed. According to the scanning electron microscopy images of transition metal-coated carbon nanofibers, it was found that carbon nanofibers were not uniformly coated with transition metals in the carbon nanofibers Co and carbon nanofibers Cu. The thickness of coating was not uniform in the carbon nanofibers Ni, but aggregated transition metals were not observed. Among the transition metal-coated carbon nanofibers, the carbon nanofibers Fe was the most uniformly coated. When the most uniformly coated carbon nanofibers Fe with high electronegativity was used as the anode material of lithium secondary battery, the oxidation and reduction peaks upon charge/discharge were symmetric, indicating reversible reaction, based on the results of cyclic voltammogram measurements. Among the four transition metals (Fe, Ni, Co, and Cu), the retention rate of carbon nanofibers Fe was the highest at 41%. In the case of carbon nanofibers Ni, Ni was relatively uniformly coated. The initial capacity and capacity after 30 cycles increased by 56%, compared to those of carbon nanofibers.
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The uniform RuO2-coating layer alleviates undesirable electrolyte and/or electrode decomposition. In addition, the RuO2-coating layer can also improve the surface electronic conductivity. As a result, the battery displays high performance, including high capacity, high rate, and long-life [26 28]. These results suggest that the uniformly coated transition metal on carbon nanofibers improved the low charge/discharge capacity of the lithium secondary battery by increasing the electric conductivity of the surface, and improved the retention rate by inhibiting the undesirable side reactions between the electrode and electrolyte.
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Synthesis and Electrochemical Performance of Transition Metal-Coated Carbon Nanofibers on Ni Foam as Anode Materials for Lithium Secondary Batteries Chang-Seop Lee1, Yura Hyun2 and Jin-Young Choi1 1
Department of Chemistry, Keimyung University, Daegu, South Korea Department of Pharmaceutical Engineering, International University of Korea, Jinju, South Korea
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Contents 7.1 Introduction 7.2 Synthesis and Electrochemical Performance of Ruthenium Oxide-Coated CNFs on Ni Foam 7.2.1 Synthesis of Carbon Nanofibers 7.2.2 Preparation of Ruthenium Oxide-Coated Carbon Nanofibers 7.2.3 Fabrication Process of Anode Materials for Lithium Secondary Batteries 7.3 Analyses 7.3.1 Scanning Electron Microscopy 7.3.2 Raman Spectroscopy 7.3.3 X-Ray Photoelectron Spectroscopy 7.3.4 Cyclic Voltammetry 7.3.5 Cycle Performances 7.4 Synthesis and Electrochemical Performance of Transition Metals Oxide-Coated Carbon Nanofibers on Ni Foam 7.4.1 Transition Metal-Coated Carbon Nanofibers 7.4.2 Fabrication Process of Anode Materials for Lithium Secondary Batteries 7.5 Analyses 7.5.1 Scanning Electron Microscopy 7.5.2 Raman Spectroscopy 7.5.3 X-Ray Photoelectron Spectroscopy 7.5.4 Cyclic Voltammetry 7.5.5 Cycle Performances 7.6 Conclusion References
Carbon-Based Nanofillers and their Rubber Nanocomposites. DOI: https://doi.org/10.1016/B978-0-12-813248-7.00007-9
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7.1 INTRODUCTION As a new generation of green rechargeable batteries, the lithium ion secondary battery has many advantages over other candidates, such as better portability, higher energy density, longer cycle life, and nonmemory effect [1]. They are widely used not only in the electronic product, small electric tool, aerospace instrument, and related industries, but also as one of the backup power supplies for electric vehicles, which have attracted widespread interest worldwide [2]. The electrode material is an important component of a lithium ion battery, and its research has played a crucial role as one of the key subsystems for the development of the lithium-ion battery [3]. The lithium secondary battery has been highlighted as a promising power source, due to its high energy density and high output capacity, which enable it to offer high performance. As it is characterized by a long life cycle and low self-discharge, the lithium secondary battery is in high demand. As the electronics industry has rapidly developed, lightweight, small, and diverse electronic devices are required. Increasing interest is now being shown in the development of batteries with high capacity, high performance, and high density [4 10]. However, in lithium secondary batteries, in which lithium metal is used as the anode, crystals are likely to form on dendrite when charge/discharge is repeated. Due to the risk of short-circuit, the graphite-based anode active material is mainly used as the anode material of the lithium secondary battery [4 13]. The graphite-based anode active materials recently studied as anode materials for lithium secondary batteries include graphene, carbon nanotube, carbon nanofiber, and hollow or porous carbon. The carbon nanofibers have chemical stability, electrical conductivity, and high energy efficiency, and have a relatively larger specific surface area than general carbon materials. The carbon-based materials that are currently used as anode active material of the Li secondary battery have a maximum capacity of 372 mAh g 1. Because of their low theoretical capacity, they have capacitive limitations. Thus, there are many studies and suggestions on creating new anode materials with high capacity. One of them is to use silicon (Si). Si is known to be suitable as an anode material with high capacity, because it has high theoretical capacity (4200 mAh g 1) and low mass, and can interact with Li. However, Si has the problem of the electrode life being rapidly shortened due to the large volume expansion of up to 400% that occurs in the charge discharge process. To overcome the associated problems of both the carbon-based material and Si, many researchers are conducting studies on the composites of both materials. Carbon nanofiber/silicon composite can overcome the functional shortcomings of individual materials. Because carbon nanofiber provides flexible space to alleviate the
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volume expansion of Si in the process of charging and discharging, it is expected to overcome the limitations and problems of the existing Li secondary batteries [14 18]. Thus, they can be employed in electrodes of fuel cells, absorbents, and energy storage. Since the physicochemical properties of carbon nanofibers, such as their diameter, presence of bonds, and number of layers, can be selected by changing the synthetic methods and conditions, carbon nanofiber might be a promising material that could replace the graphite-based anode active material with its structural limitation [11 13,19]. Since carbon-based anode electrode materials have problems, such as low charge/discharge capacity and low retention rate due to their high irreversible capacity, many studies are being actively conducted on transition metals as anode materials of lithium secondary batteries. The transition metals mitigate the degradation of the electrode and electrolyte, and improve the electron conductivity of the surface of the carbon nanofibers [11 13,16,19 24]. The aim of this study is to synthesize carbon nanofibers with high chemical stability and thermal conductivity for the use of anode materials of lithium secondary batteries, and coat carbon nanofibers with transition metals such as Ru, Fe, Co, Ni, and Cu to suppress the side reaction, enhance the retention rate, and extend the life of electrodes.
7.2 SYNTHESIS AND ELECTROCHEMICAL PERFORMANCE OF RUTHENIUM OXIDE-COATED CNFs ON NI FOAM [8] 7.2.1 Synthesis of Carbon Nanofibers Carbon nanofibers for use as carbon-based anode materials were synthesized using chemical vapor deposition (CVD) apparatus with a quartz tube. Fig. 7.1 shows a schematic of the experimental set-up for the synthesis of carbon nanofibers.
Figure 7.1 Schematic of the chemical vapor deposition apparatus for the preparation of carbon nanofibers [8].
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In this study, 20% ethylene (C2H4/N2, Korea standard gas) was used to synthesize carbon nanofibers as a carbon source, while 20% hydrogen (H2/N2, Korea standard gas) and high-purity nitrogen (N2, Korea standard gas) were used as a vapor reactionpromoting gas and a carrier gas, respectively. Carbon nanofibers were synthesized as follows: Ni foam which was employed as the current collector, was inserted into the chemical vapor deposition apparatus, and the temperature was then increased to 600 C at 10 C min 1 increment rate under nitrogen atmosphere. The temperature was then maintained at 600 C with flowing hydrogen gas for 30 min, followed by injection of ethylene/hydrogen mixed gas for 10 min. After the reaction ended, the apparatus was cooled down to room temperature by injecting nitrogen gas.
7.2.2 Preparation of Ruthenium Oxide-Coated Carbon Nanofibers RuCl3 solution was used to coat the carbon nanofibers with ruthenium oxide. Fig. 7.2 shows the dip-coating method that was used to coat the carbon nanofibers synthesized by chemical vapor deposition in 0.01 M RuCl3 solution. After Ni foam was dip-coated in the ruthenium chloride solution and it was air-dried for 5 min, it was then further dried for 12 h or more at 80 C.
7.2.3 Fabrication Process of Anode Materials for Lithium Secondary Batteries The three-electrode cell was prepared by applying RuO2/carbon nanofibers/Ni foam as anode active material of lithium secondary batteries. The three-electrode cell was assembled in a glove box filled with Ar gas. It was assembled as a half cell. Prepared active materials were used as a working electrode. Lithium was used as counter and
Figure 7.2 Preparation process of the ruthenium oxide-coated carbon nanofibers [8].
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reference electrodes. A glass fiber separator was used as a separator membrane. The 1 M LiClO4 was employed as electrolyte, and dissolved in a mixture of ethylene carbonate (EC):propylene carbonate (PC) in a 1:1 volume ratio.
7.3 ANALYSES 7.3.1 Scanning Electron Microscopy Scanning electron microscopy images were measured to examine the morphologies of ruthenium oxide-coated Ni foam, current collector, and ruthenium oxide-coated carbon nanofibers/Ni foam. Fig. 7.3 shows the results of scanning electron microscopy image measurement. Fig. 7.3A shows that Ni foam was coated with ruthenium oxide, while it was cracked, while Fig. 7.3B shows that carbon nanofibers were grown on the Ni foam. Carbon nanofibers were synthesized by contact of thermally decomposed hydrocarbon with metal catalytic particle, such as Fe, Co, or Ni. This indicates that Ni foam, a current collector, played the role of a catalyst to grow carbon nanofibers. In addition, it shows that the grown carbon nanofibers were coated with ruthenium oxide.
7.3.2 Raman Spectroscopy Raman spectroscopy was performed to analyze the crystal quality of ruthenium oxide-coated Ni foam and ruthenium oxide-coated carbon nanofibers/Ni foam. Fig. 7.4 shows the results. Fig. 7.4B shows that in the case of synthesized carbon nanofibers, graphite-like band (G-band) and defect-like band (D-band) were observed
Figure 7.3 Scanning electron microscopy images of the synthesized (A) RuO2/Ni foam, and (B) RuO2/carbon nanofibers/Ni foam [8].
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Figure 7.4 Raman spectra of the synthesized (A) RuO2/Ni foam, and (B) RuO2/carbon nanofibers/Ni foam [8].
at (1340 and 1580) cm21, respectively. For D-band and G-band, sp3 (disordered graphite) and sp2 (ordered graphite) bonds were shown. The relative crystal quality of the material could be identified using the intensity ratio (D/G) of the G-band and Dband. In Fig. 7.4B, ID/IG was 1.57.
7.3.3 X-Ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy analysis was performed to examine the binding energy of ruthenium and carbon in ruthenium oxide-coated Ni foam and ruthenium oxide-coated carbon nanofibers/Ni foam. Fig. 7.5 shows the results. The binding energies of the atoms shown in the X-ray photoelectron spectroscopy results varied, depending on the differences in electronegativity. Fig. 7.5A and B shows that RuO2 bond with a binding energy of 280 eV was observed in Ru3d. Fig. 7.5B shows the data of the synthesized carbon nanofibers. CQC (sp2) bond with a binding energy of 284 eV, and C C (sp3) bond with a binding energy of 286 eV were observed in C1s. In addition, CQO bond with a binding energy of 288 eV was observed.
7.3.4 Cyclic Voltammetry Cyclic voltammetry was performed by using ruthenium oxide-coated Ni foam, RuO2-coated carbon nanofibers/Ni foam, carbon nanofibers grown on Ni foam, and graphite as control sample (Customcells, current collector: Cu foil, binder: PVDF), and as anode materials of Li secondary batteries. Fig. 7.6 shows the results. Cyclic voltammetry was performed in all samples by applying a current of 100 mA g 1 in the voltage range of 0.01 2.0 V (vs Li/Li1).
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Figure 7.5 X-ray photoelectron spectroscopy spectra of the synthesized (A) RuO2/Ni foam, and (B) RuO2/carbon nanofibers/Ni foam [8].
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Specific sites with oxidation and reduction potential varied depending on the energy in lithium intercalation and deintercalation reactions of the electrodes. In the case of a fully reversible reaction, because there were small differences in the peaks of oxidation and reduction regardless of the voltage change rates, the patterns of cyclic voltammetry appeared symmetrically. Fig. 7.6A shows the results for RuO2-coated Ni foam used as the anode material of the Li-secondary battery. Upon first charge, reduction peaks were observed at 0.4 and 1.7 V. Upon discharge, oxidation peaks were observed at 0.6, 1.2, and 1.9 V. Upon second charge, reduction peaks were observed at 0.9 and 1.4 V. Upon discharge, oxidation peaks were similar to those in the first discharge. Fig. 7.6B shows the results for RuO2-coated carbon nanofibers/Ni foam. Upon first charge, reduction peaks were observed at 0.5 and 1.5 V. Upon discharge, oxidation peaks were observed at 0.3, 1.2, and 1.8 V. Upon second charge, reduction peaks were observed at 0.4 and 1.7 V. Upon discharge, the oxidation peaks were similar to those in the first discharge. Fig. 7.6C shows the results for carbon nanofibers grown on Ni foam. Upon first charge, reduction peaks were observed at 0.4 and 1.2 V. Upon second charge, no reduction peak was observed. Upon first and second discharge, an oxidation peak was observed at 0.4 V. Fig. 7.6D shows the results when purchased graphite was used. Upon first charge, reduction peaks were observed at 0.4 V. Upon second charge, no reduction peak was observed. Upon first and second discharge, no oxidation peak was observed. Reduction peaks that disappeared upon charge were associated with electrolyte decomposition and solid electrolyte interface (SEI) generation. In addition, areas of cyclic voltammograms were associated with capacity. RuO2-coated carbon nanofibers/Ni foam showed the largest cyclic voltammetry area. It showed the highest efficiency when compared with the capacity per cycle.
7.3.5 Cycle Performances In this study, RuO2-coated Ni foam and RuO2-coated carbon nanofibers/Ni foam were used. Carbon nanofibers directly grown on Ni foam and purchased graphite were used as control groups and a three-electrode cell was prepared by using them as anode active material. Charge and discharge characteristics were examined by applying the current of 100 mA g 1 in order to examine electrochemical characteristics such as capacity and cycle performance of the three-electrode cell. Fig. 7.7 shows discharge capacity and efficiency after 30 cycles. Fig. 7.7A showed reductions of initial capacity 1977 372 mAh g 1 after 30 cycles, and the retention rate of 18.8% when RuO2/Ni foam was used as the anode active
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Figure 7.7 Cycle performances of the synthesized (A) RuO2/Ni foam, (B) RuO2/carbon nanofibers/Ni foam, (C) carbon nanofibers/Ni foam, and (D) graphite [8].
material. Fig. 7.7B showed reductions of initial capacity 494 234 mAh g 1 after 30 cycles and the retention rate of 47.4% when RuO2/carbon nanofibers/Ni foam was used as the anode active material. Fig. 7.7C showed reductions of initial capacity 276 129 mAh g 1 after 30 cycles and the retention rate of 46.7% when carbon nanofibers/Ni foam (control group) was used as the anode active material. Fig. 7.7D showed reductions of initial capacity 203 21 mAh g 1 after 30 cycles and the retention rate of 10.3%. Thus, coating with transition metal oxide, such as ruthenium oxide (RuO2) with high capacity, could improve the low capacity of carbon nanofibers. Suppressing the side reactions of electrolytes also improved the retention rate.
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7.4 SYNTHESIS AND ELECTROCHEMICAL PERFORMANCE OF TRANSITION METALS OXIDE-COATED CARBON NANOFIBERS ON NI FOAM 7.4.1 Transition Metal-Coated Carbon Nanofibers The transition metal salts of Fe, Co, Ni, and Cu were dissolved in aqueous solution, and used to coat metals on the carbon fibers. Coating was achieved by dipping the carbon nanofibers synthesized via chemical vapor deposition method in 0.1 M transition metal salt solutions. Carbon nanofibers were grown on the Ni foam in chemical vapor deposition apparatus, and the foam was then dipped into the transition metal solution. After the foam was air-dried for 5 min, it was dried for more than 12 h at 80 C in an electric oven.
7.4.2 Fabrication Process of Anode Materials for Lithium Secondary Batteries A three-electrode cell was prepared by applying carbon nanofibers-transition metals as anode active materials of lithium secondary batteries. The three-electrode cell was assembled in a glove box filled with Ar gas. It was assembled as a half cell. Various prepared active materials were used as the working electrodes. Lithium was used as the counter and reference electrode, a glass fiber separator was used as a separator membrane, and 1 M LiClO4 was employed as the electrolyte, and was dissolved in a mixture of ethylene carbonate:propylene carbonate (EC:PC) at a 1:1 volume ratio.
7.5 ANALYSES 7.5.1 Scanning Electron Microscopy Fig. 7.8 shows the scanning electron microscopy images that were taken to examine the microscopic morphologies of the carbon nanofibers, after the grown carbon nanofibers were coated with the four transition metals of Fe, Co, Ni, and Cu. The synthesized carbon nanofibers were dipped in 0.1 M transitional metal solution, and the scanning electron microscopy images were then captured. Fig. 7.8A shows that the carbon nanofibers were evenly coated with Fe. On the other hand, Fig. 7.8B and D shows that large aggregations of Co and Cu were formed on the carbon nanofibers, and that some areas of the carbon nanofibers were not coated with the transition metals. Fig. 7.8C shows that the carbon
Synthesis and Electrochemical Performance of Transition
Figure 7.8 Scanning electron microscopy images of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers Fe, (B) carbon nanofibers Co, (C) carbon nanofibers Ni, and (D) carbon nanofibers Cu [25].
nanofibers were coated with Ni. While large aggregations were not formed, the carbon nanofibers Ni were less uniformly coated than the carbon nanofibers Fe. Among the four transition metals used to coat carbon nanofibers, the coating of Fe and Ni was relatively uniform, without large aggregations. This mitigated the degradation of carbon nanofibers in the Li insertion/extraction reactions.
7.5.2 Raman Spectroscopy Raman spectroscopy was performed to investigate the degree of crystal quality of the transition metal-coated carbon nanofibers. Fig. 7.9 shows the results, where all the Raman data show characteristic peaks of synthesized carbon nanofibers, such as a graphite-like band (G-band) and a defect-like band (D-band) observed at 1340 and 1580 cm21, respectively. Fig. 7.9A shows a peak of FeO at 680 cm21, while Fig. 7.9B shows a peak of CoO at 670 cm21; Fig. 7.9C shows a peak of NiO at 550 cm21, while Fig. 7.9D shows a peak of CuO at 260 cm21.
7.5.3 X-Ray Photoelectron Spectroscopy Fig. 7.10 shows the results of the X-ray photoelectron spectroscopy analysis that was performed to determine the binding energies of the transition metals coated on the carbon nanofibers. The binding energy varied depending on the electronegativity of transition metals. Transition metal with high electronegativity strongly attracted
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Figure 7.9 Raman spectra of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers Fe, (B) carbon nanofibers Co, (C) carbon nanofibers Ni, and (D) carbon nanofibers Cu [25].
electrons, so that it had a relatively low binding energy. Table 7.1 summarizes the binding energies of the four transition metals shown in Fig. 7.10 that were analyzed. The table shows that 2p1/2 and 2p3/2 peaks of the various chemical states of all transition metals appeared in the X-ray photoelectron spectroscopy spectra. Peaks that appear at 712 and 719 eV in Fig. 7.10A are assigned to FeO and Fe2O3, while peaks that appear at 781 and 787 eV in Fig. 7.10B are assigned to Co3O4 and CoO. Peaks at 857 and 862 eV in Fig. 7.10C and peaks at 934 and 945 eV in Fig. 7.10D are assigned to Ni(OH)2 and NiO, and Cu2O and CuO species, respectively.
7.5.4 Cyclic Voltammetry Fig. 7.11 shows the results of cyclic voltammetry measurements that were performed using transition metal coated carbon nanofibers as the anode materials of lithium secondary batteries. Carbon nanofibers and Si carbon nanofiber composites synthesized in this study have been applied as anode materials of Li secondary batteries, and assembled into
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Figure 7.10 X-ray photoelectron spectroscopy spectra of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers Fe, (B) carbon nanofibers Co, (C) carbon nanofibers Ni, and (D) carbon nanofibers Cu [25]. Table 7.1 X-ray Photoelectron Spectroscopy Results for the Transition Metal-Coated Carbon Nanofibers Composites: (A) Carbon Nanofibers Fe, (B) Carbon Nanofibers Co, (C) Carbon Nanofibers Ni, and (D) Carbon Nanofibers Cu [25] (A) (B) (C) (D) Combination Binding Energy (eV)
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three-electrode cells. Lithium was used as counter and reference electrodes. A glass fiber separator was used as a separator membrane. The 1 M LiClO4 was employed as an electrolyte, and dissolved in a mixture of ethylene carbonate:propylene carbonate (EC:PC) in a 1:1 volume ratio. Cyclic voltammetry measurements were performed by
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Figure 7.11 Cyclic voltammograms of the transition metal-coated carbon nanofibers composites: (A) carbon nanofibers, (B) carbon nanofibers Fe, (C) carbon nanofibers Co, (D) carbon nanofibers Ni, and (E) carbon nanofibers Cu [25].
applying 100 mA g 1 of current density in the 0.1 2.0 V (vs Li/Li1) voltage range, in order to investigate the electrochemical characteristics. In the lithium insertion/extraction reactions of an electrode, the particular position of oxidation/reduction potential varies, depending on the amount of energy. If it is a completely reversible reaction, the difference between the oxidation and reduction peaks is small, regardless of the rates of changing potential. This results in symmetric shape in the cyclic voltammogram.
Synthesis and Electrochemical Performance of Transition
Fig. 7.11A shows the results obtained using the synthesized carbon nanofibers as the anode material of a lithium secondary battery. Upon the first charge, reduction peaks were observed at 0.6 and 1.3 V; upon discharge, an oxidation peak was observed at 0.4 V; upon the second charge, a reduction peak was observed at 0.7 V; and upon discharge, an oxidation peak was observed at 0.4 V. Fig. 7.11B shows the results obtained using carbon nanofibers Fe. Upon the first charge, reduction peaks were observed at 0.5 and 1.7 V; upon discharge, oxidation peaks were observed at 1.2 and 1.9 V; upon the second charge, reduction peaks were observed at 0.6 and 1.3 V; and upon discharge, oxidation peaks were observed at 0.4 and 1.7 V. Fig. 7.11C shows the results obtained using carbon nanofibers Co. Upon the first charge, only one reduction peak was observed at 1.3 V. After that, no oxidation or reduction peaks were observed upon charge and discharge. Fig. 7.11D shows the results obtained using carbon nanofibers Ni. Upon the first charge, reduction peaks were observed at 0.6, 1, and 2.2 V; upon discharge, oxidation peaks were observed at 1.1, 1.8, and 2.2 V; upon the second charge, reduction peaks were observed at 0.8 and 2.2 V; and upon discharge, oxidation peaks were observed at 1.1, 1.7, and 2.2 V. Fig. 7.11E shows the results obtained using carbon nanofibers Cu. Upon the first charge, a reduction peak was observed at 1.4 V; upon discharge, an oxidation peak was observed at 2.1 V; upon the second charge, a reduction peak was observed at 0.8 V; and upon discharge, oxidation peaks were observed at 1.1 and 1.8 V. Oxidation and reduction peaks that disappear upon charge and discharge are believed to be related to the decomposition of the electrolyte, and the generation of a solid electrolyte interface (SEI). In addition, the cyclic voltammetry graph showing the area is related to the capacity of the carbon nanofibers. Carbon nanofibers Fe and carbon nanofibers Ni had the largest cyclic voltammetry area. When their capacities per cycle were compared, they showed the greatest efficiency.
7.5.5 Cycle Performances The electrochemical characteristics of transition metal-coated carbon nanofibers and carbon nanofibers/Ni foam (control) as anode active material of Li secondary batteries were investigated using a three-electrode cell. The characteristics of charge/discharge were examined to observe the electrochemical characteristics of the three-electrode cell by applying a current density of 100 mA g 1. Fig. 7.12 shows the results of discharge capacity and retention rate that were measured after 30 cycles. Fig. 7.12A shows the results of measuring the cell prepared using carbon nanofibers/Ni foam as the anode active material. The initial capacity was 310 mAh g 1. After 30 cycles, this was reduced to 154 mAh g 1. The retention rate was 49.7%.
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Figure 7.12 Cycle performances of the transition metal-coated carbon nanofibers composites (A) carbon nanofibers, (B) carbon nanofibers Fe, (C) carbon nanofibers Co, (D) carbon nanofibers Ni, and (E) carbon nanofibers Cu [25].
Fig. 7.12B E shows the results of measuring the cell prepared using transition metal-coated carbon nanofibers. After five cycles, the capacity of most transition metal-coated carbon nanofibers was reduced to about 360 mAh g 1. The capacity of the carbon nanofibers Co was reduced to 234 mAh g 1.
Synthesis and Electrochemical Performance of Transition
The initial capacity of carbon nanofibers Cu was the highest at 1028 mAh g 1. However, after 30 cycles, this reduced to 136 mAh g 1. The retention rate of carbon nanofibers Cu was the lowest at 13.2%. On the other hand, carbon nanofibers Fe had the lowest initial capacity (670 mAh g 1). After 30 cycles, this capacity reduced to 275 mAh g 1. The carbon nanofibers Fe showed the highest retention rate of 41%. The result of the cycle performances showed that the initial capacity of the transition metal-coated carbon nanofibers was high, compared to that of the carbon nanofibers without transition metal coating. However, after 30 cycles, the capacities of carbon nanofibers Co and carbon nanofibers Cu were similar to that of carbon nanofibers, because the transition metals were not uniformly coated. After 30 cycles, the capacity of carbon nanofibers Fe and carbon nanofibers Ni, in which transition metals were uniformly coated, was improved, compared to that of the carbon nanofibers. In particular, after 30 cycles, the capacity of carbon nanofibers Fe was higher than that of carbon nanofibers by 78%. This suggests that the low capacity of carbon nanofibers can be improved by coating carbon nanofibers with transition metals, and the retention rate can be improved by inhibiting the side reactions of the electrolyte.
7.6 CONCLUSION In this study, carbon nanofibers transition metal composites were prepared by dipping carbon nanofibers synthesized by chemical vapor deposition method in transition metal solutions. From the results of galvanostatic charge and discharge, when RuO2/carbon nanofibers/Ni foam was used as the anode active material, the initial capacity was reduced from 494 to 234 mAh g 1 after 30 cycles, and a retention rate of 47.4% was observed. According to the scanning electron microscopy images of transition metal-coated carbon nanofibers, it was found that carbon nanofibers were not uniformly coated with transition metals in the carbon nanofibers Co and carbon nanofibers Cu. The thickness of coating was not uniform in the carbon nanofibers Ni, but aggregated transition metals were not observed. Among the transition metal-coated carbon nanofibers, the carbon nanofibers Fe was the most uniformly coated. When the most uniformly coated carbon nanofibers Fe with high electronegativity was used as the anode material of lithium secondary battery, the oxidation and reduction peaks upon charge/discharge were symmetric, indicating reversible reaction, based on the results of cyclic voltammogram measurements. Among the four transition metals (Fe, Ni, Co, and Cu), the retention rate of carbon nanofibers Fe was the highest at 41%. In the case of carbon nanofibers Ni, Ni was relatively uniformly coated. The initial capacity and capacity after 30 cycles increased by 56%, compared to those of carbon nanofibers.
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The uniform RuO2-coating layer alleviates undesirable electrolyte and/or electrode decomposition. In addition, the RuO2-coating layer can also improve the surface electronic conductivity. As a result, the battery displays high performance, including high capacity, high rate, and long-life [26 28]. These results suggest that the uniformly coated transition metal on carbon nanofibers improved the low charge/discharge capacity of the lithium secondary battery by increasing the electric conductivity of the surface, and improved the retention rate by inhibiting the undesirable side reactions between the electrode and electrolyte.
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