Among the various synthesizing technologies of carbon nanofibers (CNFs), chemical vapor deposition (CVD) technology, which uses hydrocarbon gas or carbon monoxide as a carbon source gas and pyrolyzes it to grow CNFs on transition metal catalysts, such as Ni, Fe, and Co, has been regarded as the most inexpensive and convenient method to produce CNFs for industrial use. Experimental variables for CVD are source gas, catalyst layers, temperature, and reaction time. Since the particle size of metal catalysts has an influence on the diameter of CNFs, it is possible to control the diameter of CNFs by varying particle sizes of the metal. As such, it is possible to synthesize CNFs selectively through the selective deposition of catalyst metals. In this study, CNFs were grown by CVD on C-fiber textiles, which had catalysts deposited via electrophoretic deposition. The CNFs were coated with a silica layer via hydrolysis of TEOS (tetraethyl orthosilicate), and the CNFs were oxidized by nitric acid. Due to oxidation, a hydroxyl group was created on the CNFs, which was then able to be used as an activation site for the SiO2. CNFs and the CNFs/SiO2 composite can be used in various applications, such as a composite material, electromagnetic wave shielding material, ultrathin display devices, carbon semiconductors, and anode materials of Li secondary batteries. In particular, there is an increasing demand for lightweight, small-scale, and high-capacity batteries for portable electronic devices, such as laptop computers or smart phones, along with the escalating concern of fossil energy depletion. Accordingly, CNFs and CNFs/SiO2 composites are receiving attention for their use as anode materials of Li secondary batteries, which are eco-friendly, lightweight, and high capacity. Therefore, the physicochemical properties and electrochemical performance data of synthesized CNFs and CNFs/SiO2 composite are described in this chapter.
- carbon nanofibers
- chemical vapour deposition
- silicon composites
- anode materials
- Li secondary batteries
Carbon has various types of allotrope (graphite, diamond, carbon nanofibers (CNFs), carbon nanotubes, graphene, etc.) depending on the molecular bonding mode with hybridized bonding of sp, sp2, and sp3. Figure 1 shows a schematic of carbon fibers and tubes of various sizes.
CNFs among various allotropes of carbon are fibrous carbon materials with less than 1 μm thickness and over 90% carbon content and can take various forms, such as herringbone, platelet, and spiral. Such CNFs can be used in various applications, such as a composite material, electromagnetic wave shielding material, ultrathin display devices, carbon semiconductors, and anode materials of secondary batteries. In particular, there is an increasing demand for lightweight, small-scale, and high-capacity batteries for portable electronic devices, such as laptop computers or smart phones, along with the recent concern of fossil energy depletion. Accordingly, CNFs are receiving attention for their potential use as anode materials of Li secondary batteries, which are eco-friendly, lightweight, and high capacity. Secondary batteries can be charged and discharged several times for reuse. Lead and Ni-Cd batteries have been most commonly used prior to the introduction of Li secondary batteries, but they harbor the disadvantages of a memory effect and environmental pollution issues.
Recently, the development of Li secondary batteries received more attention, since they are environmentally safe, with lightweight, small-scale, and high-capacity features required for small IT devices, such as laptop computers or mobile phones [1–17].
1.1. Lithium secondary batteries
Li secondary batteries mainly consist of a cathode, anode, electrolyte, and separator. The anode reversibly intercalates and disintercalates the lithium ions coming from the cathode to allow electrons to flow through the internal circuit and generate electricity. Figure 2 shows a schematic diagram showing the principle behind the Li secondary battery.
Studies on the performance enhancement of lithium secondary batteries are mostly concerning the performance improvement of anode materials, including carbon materials and the development of new materials. Since the charging and discharging performance of Li secondary batteries is significantly influenced by the structure of the anode materials for intercalating lithium ions, this study focused on the performance improvement and development of carbon materials as anode materials to allow more Li ions to be inserted. For such anodes, graphite, first developed by Bell Labs in 1981, has been used in most cases. Graphite has the advantage of its crystal structure during charging and discharging, so that its volume does not significantly change; however, it limits the performance of the Li secondary batteries, due to its small maximum theoretical capacity of 372 mAh g−1.
Accordingly, studies are actively conducted to test new carbon-based materials that would allow high-capacity and performance improvement of Li secondary batteries. The CNFs of carbon-based anode materials do not change significantly in their crystal structure during the intercalation and disintercalation process of lithium ions, as in the case of graphite. They are accordingly receiving attention as a new anode material that would allow lithium secondary batteries to provide an excellent lifetime [18–20].
1.2. CNFs/silicon composites
Silicon is suitable as a high-capacity anode material because of its high maximum theoretical capacity of 4200 mAh g−1; however it is associated with a deteriorating electrical connection due to cracks forming as a result of the nearly 400% volume change during charging and discharging. It also has the problem of deteriorating charging and discharging characteristics by generating a large irreversible capacity. Various studies have been attempted to solve the problems of CNFs and silicon, and a method of synthesizing CNFs/Si composites was presented as one of the solutions .
Since CNFs provide a flexible space for the volume expansion of silicon in the process of synthesizing CNFs/Si composite, it has the potential to be a good candidate and as such has been selected as an anode material of Li secondary batteries in order to enhance its performance for use in this study. Figure 3 shows the grown CNFs on a Si surface which play a role as shrinked springs when the composites are charged.
CNFs were synthesized by using chemical vapor deposition (CVD), and the effects of synthesis conditions, such as synthesis temperature and the concentration ratio of catalysts on the growth of CNFs, were studied. CNFs/Si composites were synthesized based on these CNFs, and the physiochemical characteristics of CNFs/Si composites and the electrochemical characteristics as anode materials of Li secondary batteries were also investigated [9, 18–22].
2. Synthesis of CNFs on electroplated Fe/C-fiber textiles
2.1. Experimental process
C-fiber textiles (SGL Carbon Group) were put into 60% nitric acid and heated at 80°C for 30 min under reflux to form OH groups on the surface of C-fiber textiles. After the completion of oxidation, the oxidized C-fiber textiles were cleaned and stored in distilled water, without drying in an oven, in order to keep the OH groups attached to the surface [4, 7, 17].
2.1.2. Deposition of Fe catalyst
For the deposition of a Fe catalyst, Fe(NO3)3∙9H2O as a precursor was first dissolved in distilled water, and then the C-fiber textiles were immersed in the solution. The pH of the solution was lowered to 1.00 with nitric acid, and the solution was heated to 90°C. The ratio of C-fiber textiles and the amount of Fe in the solution was controlled as an experimental variable. Afterward, hydrolysis was carried out by slow dropping of 1 M urea solution.
It is known that Fe(OH)3 is formed by deposition-precipitation on the surface of oxidized C-fiber textiles, through the reaction of OH groups. For comparison, deposition of ferrous particles on the oxidized C-fiber textiles was carried out, by dipping the substrate into the solution, after the hydrolysis by urea was complete. The specimen was cleaned by distilled water, after completion of the deposition process, and dried for 24 hours at 80°C [4, 7, 17].
A reduction step was applied in order to convert iron hydroxides and oxides attached to the surface of C-fiber textiles into elemental iron using a tube furnace. Hydrogen balanced with nitrogen gas was used for the reduction process, and the flux of the reaction gas was controlled by an electronic mass flow controller (MFC). The reactor temperature was increased by 10°C/min up to 600°C in pure N2 atmosphere. Once the temperature reached 600°C, N2 gas balanced with 20% H2 gas was flowed into the reactor for 3 hours to carry out the reduction process [4, 7, 17].
2.1.4. Growth of CNFs
CNFs were grown on C-fiber textiles by CVD in a horizontal quartz tube reaction apparatus after completion of the reduction process. The flux of reaction gas was controlled by an electronic MFC, and ethylene gas (C2H4) was used to grow the CNFs as a carbon source. Hydrogen gas was used to eliminate the remaining hydroxyl group after the reduction process, while nitrogen was used to stabilize the reaction.
The prepared metal catalyst was evenly spread on a quartz boat, which was placed into the reactor under nitrogen atmosphere, and the reactor temperature was increased by 10°C/min. Once the temperature reached 700°C, it was maintained for 30 min. N2 gas balanced with 20% H2 gas was flowed into the reactor, and then 20% ethylene balanced with N2 gas was flowed into the reactor for 5 hours. After the completion of the reaction, the temperature was lowered to room temperature in N2 atmosphere [4, 7, 17].
2.2. Physicochemical properties of the grown CNFs
2.2.1. Scanning Electron Microscope (SEM)
Figures 4 and 5 show the shape of the CNFs grown on the C-fiber textiles by the CVD method, as taken by SEM, with 100,000× magnification. It was found that the diameters of CNFs increased with decreasing a Fe/C ratio. The diameters of CNFs grew up to 40–60 nm when the weight ratio of C-fiber textiles to Fe was 1:30, whereas CNFs grew up to 30–55 nm in diameter when the ratio was 1:70.
In contrast, it was observed that CNFs scarcely grew in the samples prepared by the dip-coating method if the ratio of C-fiber textiles to Fe reached 1:70; instead they started to cohere during the growth process. It is assumed that too many Fe catalyst particles were present on the surface of the C-fiber textiles, which interferes with the growth of CNFs. CNFs of 40–60 nm in diameter grew when the ratio was 1:10, while the thinnest and the most even CNFs, of 25–30 nm in diameter, grew when the ratio was 1:30 [4, 7, 17].
2.2.2. X-ray Diffraction (XRD)
XRD patterns were taken in order to analyze how the crystalline structure of the samples change according to the different processes, and the results are shown in Figure 6. After the deposition process, iron oxide phases of different stoichiometry were found. It is known that Fe(OH)3 present on the sample surface after precipitation-deposition transforms into iron oxide during the drying process, and then Fe particles of cubic crystalline structure remained after reduction.
The graphite peaks in the raw material mainly belong to the graphite flakes, which are present in the C-fiber textile. The peak broadening, which can be observed for the samples after CVD, can be attributed to the presence of CNFs [4, 7, 17].
2.2.3. X-ray Photoelectron Spectroscopy (XPS)
XPS analysis was carried out in order to analyze the binding energies of C, Fe, and oxygen. Measurements were performed for the sample where the CNFs (C:Fe 1:30, 30 min dip coating) were grown as uniformly as possible. Figure 7 shows the XPS analysis result after the growth of CNFs. The binding energy of the Fe catalyst particles deposited to CNFs, which were grown on the surface, did not appear on the XPS spectrum, while the binding energies of C and oxygen appeared on the spectra [4, 7, 17].
2.2.4. Thermogravimetry/Differential Thermal Analysis (TG/DTA)
Figure 8 shows TG/DTA thermograms for the CNF grown samples. After the Fe catalyst was deposited (a) by the deposition-precipitation method or (b) by the dip-coating method, with the same C:Fe ratio of 1:30, analysis was performed on the samples in which CNFs were grown. The TG/DTA curves showed similar tendencies when the experiment was conducted by both methods.
As can be seen in Figure 8, the decrease of TGA thermogram ranges from 490°C to 750°C, which resulted from the oxidation of CNFs and C-fiber textiles composing the sample. As for the DTA thermogram, exothermic peaks were found in both samples near 600°C, 690°C, and 850°C. The exothermic peak near 600°C appearing at the beginning of the DTA thermogram is deemed as oxidation reaction to carbon of CNFs; and meantime, exothermic peaks appearing in higher temperature of near 690°C and 850°C are deemed as peaks showing the oxidation reaction of C-fiber and graphite composing C-fiber textiles, which were used as substrate [4, 7, 17].
After CNFs had been synthesized in the CVD method, Raman analysis was performed in order to investigate the characteristics of C─ bonding, and the results are presented in Figure 9. After Fe catalysts were deposited on CNFs, they were grown on C-fiber textiles by the deposition-precipitation method or dip-coating method with a C:Fe ratio of 1:30. No differences were discovered in the patterns of Raman spectrum by the Fe catalyst deposition method; however the intensity value was relatively larger when the dip-coating method was used, when compared to the deposition-precipitation method.
As seen in Figure 9, a D band appeared around 1332 cm−1, a G band around 1582 cm−1, and a 2D band around 2700 cm−1; and the intensity and
3. Synthesis of CNFs and CNFs/SiO2 composite on electroplated Co-Ni/C-fiber textiles
3.1. Experimental process
3.1.1. Deposition of Co-Ni catalysts on C-fiber textiles
Using the electrophoretic method, Co-Ni catalysts were deposited on C-fiber textiles. Carbon electrodes and carbon sheets were used as the anode and cathode, respectively, while a mixture of cobalt nitrate and nickel nitrate was used as the electrolyte. To analyze the characteristics of the CNFs according to their cobalt and nickel contents, cobalt nitrate and nickel nitrate, with the weight ratio of 6:4 and 8:2, were used as the electrolyte for the deposition of Co-Ni catalysts by applying 0.04–0.05 A of current for 5 min [4, 7, 17, 18, 22].
3.1.2. Reduction of catalysts and synthesis of CNFs
After placing the C-fiber textiles with deposited catalysts into a furnace, the temperature was increased by 12°C/min up to 700°C in N2 atmosphere. While maintaining the temperature at 700°C, H2 gas was flowed into the furnace for 1 hour for the reduction of the catalysts.
Using CVD, CNFs were synthesized to electroplated Co-Ni/C-fiber textiles. Ethylene gas, as the C source, was flowed into the furnace for an hour at 700°C, during which reduction was completed. After the completion of the reaction, the temperature was lowered to room temperature in N2 atmosphere [4, 7, 17, 18, 22].
3.1.3. Synthesis of the CNFs/SiO2 composite
For the silica coating, a hydroxyl group was attached to the surface of the CNFs as an anchor group and oxidized for 30 min in 80°C nitric acid. Using the sol-gel process, SiO2 was applied as a coating through the hydrolysis of tetraethyl orthosilicate (TEOS) on the surface of the reduced CNFs. TEOS was hydrolyzed by dissolving hydrochloric acid and distilled water with ethanol and stirred for 6 hours at room temperature. After soaking the CNFs/C-fiber textiles in this solution and adding ammonia solution for gelation, the solution was stirred for 12 hours and SiO2 was coated on the surface of the CNFs [4, 7, 17, 18, 22].
3.1.4. Fabrication of coin cell
To investigate the electrochemical characteristics of the CNFs and the CNFs/SiO2 composite, a coin cell was fabricated. For the working electrode, the CNFs and CNFs/SiO2 deposited on C-fiber textiles were used without a binder, Li metal was used for the counter electrode, and polyethylene was used for the separator. For the electrolyte, LiPF6 was dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 vol.%) for a mixed solution. The coin cell was fabricated in a glove box filled with Ar gas [4, 7, 17, 18, 22].
3.2. Characterization of the CNFs/SiO2 composite
3.2.1. Scanning Electron Microscope (SEM)
Figure 10 shows the SEM images of CNFs synthesized according to the Co-Ni ratio. In both samples, CNFs in a curved form with a uniform diameter were synthesized. With a Co-Ni ratio of 8:2, 40 nm CNFs were synthesized, while 30 nm CNFs were synthesized with a ratio of 6:4 .
3.2.2. X-ray Photoelectron Spectroscopy (XPS)
Figure 11 shows the XPS spectra of CNFs that were synthesized using Co-Ni catalysts. With Co-Ni ratios of 8:2 and 6:4, a C═C (sp2) bond, C─C (sp3) bond, ─C═O bond, and ─COO bond were observed around 285, 286, 288, and 292 eV, respectively. It was found that the catalyst ratio did not affect the binding energy of CNFs as the same C-binding energy was observed with both catalyst ratios.
Figure 12 shows the XPS spectra of the as-synthesized CNFs/SiO2 composite. Around the binding energy of 103–105 eV for silicon, SiO2 and SiO were identified. Considering that the intensity of SiO2 bonding is higher than that of SiO bonding in larger areas, the coated layer on the surface of CNFs consisted mostly of SiO2 .
Figure 13 shows the Raman spectra of CNFs according to the Co-Ni ratios. Around 1590 cm−1, a G band indicating a C═C (sp2) bond was observed, while a D band indicating a C─C (sp3) bond was observed around 1350 cm−1. Considering that the intensity ratio of the D band and B band at both ratios was close to 1, it was determined that CNFs were synthesized with a 1:1 ratio of sp2 bonds and sp3 bonds. The intensity of CNFs synthesized at the Co-Ni ratio of 8:2 was higher than that of CNFs synthesized at a 6:4 ratio. Accordingly, this showed that CNFs synthesized at the Co-Ni ratio of 8:2 had higher crystallizability .
3.2.4. Transmission electron microscopy (TEM)
To observe the layer of silica coated on the surface of the CNFs, TEM images were examined and are shown in Figure 14. For the CNFs coated with silica, CNFs synthesized at the Co-Ni ratio of 8:2 with relatively high crystallizability were used. A layer of silica at a thickness of about 10 nm was coated evenly on the surface of the CNFs in a hollow form with about 8 nm center diameter .
3.2.5. Cycle performance
The discharge capacities of C-fiber textiles, CNFs, and CNFs/SiO2 composite were measured and are shown in Figure 15. The respective initial discharge capacities of the CNFs synthesized with the Co-Ni ratios of 8:2 and 6:4 were 258 and 234 mAh g−1, respectively, thereby showing higher results than that of C-fiber textiles with the initial discharge capacity of 190 mAh g−1 and a high retention rate of 95%. The CNFs/SiO2 composite showed the highest initial discharge capacity of 1468 mAh g−1 with the retention rate of 47% .
Based on the catalysts deposited using the electrophoretic method, CNFs were synthesized on the surface of C-fiber textiles through CVD. Upon hydrolysis of TEOS, silica was coated on the surface of CNFs to synthesize the CNFs/SiO2 composite. The characteristics of the as-synthesized CNFs and CNFs/SiO2 composite were analyzed through various spectroscopic methods. Upon fabricating a coin-type half-cell without a binder for the as-synthesized sample, its cycle performance when used as an anode material in Li secondary batteries was investigated. Based on the results, the following conclusions were deduced .
When the CNFs grew, after deposition of the Fe catalyst using the deposition-precipitation method, the diameters grew up to 40–60 nm and 30–55 nm, when weight ratios of C:Fe were 1:30 and 1:50, respectively.
When the CNFs grew, after deposition of Fe catalyst using the dip-coating method, the diameters grew up to 40–60 nm and 25–30 nm, when the weight ratios of C:Fe were 1:10 and 1:30, respectively [4, 7, 17].
With Co-Ni ratios of 8:2 and 6:4, CNFs with diameters of 40 and 30 nm were synthesized, respectively .
The Raman analysis results confirmed that CNFs with a 1:1 ratio of C sp2 bonds and sp3 bonds were synthesized when Co-Ni catalysts were used. When the Co-Ni ratio was 8:2, CNFs with higher crystallizability were synthesized compared to that synthesized with a ratio of 6:4 .
The discharge capacity of the CNFs synthesized on the surface of the C-fiber textiles was 230–260 mAh g−1, about 35% higher than that of pure C-fiber textiles, with a high retention rate of 95% .
CNFs/SiO2 composites showed the highest discharge capacity of 1468 mAh g−1 with a retention rate of 47% .
This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2015035858).