Nitrogen-Doped Carbon Nanoparticles by Flame Synthesis as Anode

Dec 17, 2013 - Facile Synthesis of Nitrogen-Doped Double-Shelled Hollow Mesoporous Carbon Nanospheres as High-Performance Anode Materials for Lithium ...
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Nitrogen-Doped Carbon Nanoparticles by Flame Synthesis as Anode Material for Rechargeable Lithium-Ion Batteries Dhrubajyoti Bhattacharjya, Hyean-Yeol Park, Min-Sik Kim, Hyuck-Soo Choi, Shaukatali N. Inamdar, and Jong-Sung Yu* Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 339-700, Republic of Korea ABSTRACT: Nitrogen-doped turbostratic carbon nanoparticles (NPs) are prepared using fast single-step flame synthesis by directly burning acetonitrile in air atmosphere and investigated as an anode material for lithium-ion batteries. The as-prepared N-doped carbon NPs show excellent Li-ion stoarage properties with initial discharge capacity of 596 mA h g−1, which is 17% more than that shown by the corresponding undoped carbon NPs synthesized by identical process with acetone as carbon precursor and also much higher than that of commercial graphite anode. Further analysis shows that the charge− discharge process of N-doped carbon is highly stable and reversible not only at high current density but also over 100 cycles, retaining 71% of initial discharge capacity. Electrochemical impedance spectroscopy also shows that N-doped carbon has better conductivity for charge and ions than that of undoped carbon. The high specific capacity and very stable cyclic performance are attributed to large number of turbostratic defects and N and associated increased O content in the flame-synthesized N-doped carbon. To the best of our knowledge, this is the first report which demonstrates single-step, direct flame synthesis of N-doped turbostratic carbon NPs and their application as a potential anode material with high capacity and superior battery performance. The method is extremely simple, low cost, energy efficient, very effective, and can be easily scaled up for large scale production.

1. INTRODUCTION Lithium-ion batteries (LIBs) are currently highlighted as future power source for portable electronic devices and electrical/ hybrid vehicles.1−5 After commercialization of Li-ion battery, carbon-based rechargeable batteries have extensively gained much attention. In particular, graphite has been used as the most popular anode material for current Li-ion batteries due to its high Coulombic efficiency, stability, and safety. However, it has suffered from limitation in meeting the increasing high energy demands for electricity storage stations and electric vehicles due to its low specific capacity and lithium intercalation potential close to that of lithium plating.6 Intensive efforts have been made to develop alternatives of the graphite for nextgeneration new LIBs with more attractive features, including low cost, high energy density, and good cycling and rate performance.7−9 Therefore, various forms of carbon materials such as micro- to macroporous carbon materials,9−14 carbon nanotubes,15,16 nanofibers,17 and graphene18−21 have been studied as alternative anode materials to increase the energy density of the LIBs. Overall, the disordered carbon rather than graphitic carbon provides a better Li-ion storage capacity.5 Recently, heteroatom-doped carbons such as nitrogen (N)22−25 or boron (B)26−28 doping to carbon materials have been prepared and applied as alternative anode materials to enhance the Li storage capability of carbon materials.28,29 The first-principles calculations for the lithium adsorption properties of N- and B-doped materials were reported in the literature, which showed that the B-doping decreases the Li adsorption © 2013 American Chemical Society

energies, while the N-doping increases the Li adsorption energies.27,30,31 The N-doping has been reported to yield several different N-carbon species depending on the site location in carbon framework including the pyridinic carbon, which is considered most suitable for Li storage with a high storage capacity.23 In recent years, soot consisting of carbon particles has been introduced using facile flame synthesis method, and several applications of the candle soot have been demonstrated.32−34 Inamdar et al. reported a simple flame pyrolysis method to prepare uncapped maghemite (γ-Fe2O3) and carbon-coated maghemite nanoparticles (NPs).35,36 Herein, for the collective effect of N content and energy-efficient simple synthesis, we report the flame synthesis of N-doped carbon NPs by direct burning of acetonitrile as a nitrogen and carbon source and demonstrate the N-containing turbostratic carbon as a promising alternative anode material with high Li-ion storage capacity, stable cyclic, and rate performances. The effect of Ndoping on change of charge density in carbon framework neighboring to N is discussed, and its most possible effect on Li storage is proposed. Received: August 31, 2013 Revised: November 29, 2013 Published: December 17, 2013 318

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2. EXPERIMENTAL SECTION

zones have very low supply of oxygen and have temperature ranging from 800 to 1200 °C.38 The pyrolysis process starts in this zone and is maintained by highly exothermic reaction of combustible organic material, resulting in carbon soots. Our flame synthesis exploits this process for synthesis of carbon NPs. By using organic volatile material with heteroatom present in molecular structure, carbon NPs doped with the heteroatom can be synthesized.37 Therefore, flame synthesis can be hailed as crude but cheap synthesis method for turbostratic carbon NPs. Since the flame pyrolysis method is performed at strict conditions for reagent concentration, temperature, and time span, the process, although simple, leads to repetitive formation of heteroatom-doped carbon materials having similar microstructure and physical properties, and as a result the performances of the materials in various applications are highly reproducible. XRD analysis was performed for evaluation of crystalline phase of the carbon NPs prepared by flame pyrolysis of acetonitrile and acetone. As shown in Figure 1a, the XRD

2.1. Preparation of N-Doped Turbostratic Carbon Nanoparticle Soot. The earlier reported flame pyrolysis method was used with some modifications to prepare the carbon NP in the form of soot.36,37 In brief, 30 mL of flammable acetonitrile was burned in a small glass vessel under air. The soot was deposited over a cold surface of conical water containing flask for cooling. The product in the form of black soot was collected and used without any purification or processing. For comparison, carbon NPs without N-doping was also synthesized by identical process using acetone as carbon precursor. The entire synthesis process was carried out in well-ventilated fume hood chamber to avoid the toxic CN likely to be released from acetonitrile during burning. 2.2. Characterization of Carbon Soot Nanoparticles. For characterization of N-doped carbon NPs, high-resolution scanning electron microscopy (HR-SEM), transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used. The TEM images were obtained by using a field emission transmission electron microscope (JEM 2200-FS) operated at 200 kV, and HR-SEM images were obtained by using a Hitachi S-5500 ultrahigh-resolution scanning electron microscope operated at 30 kV. Powder XRD was measured with a Bruker D-8 diffractometer equipped with a Cu Kα radiation (1.5406 Å) operated at 40 kV and 30 mA. XPS analysis was carried out in an AXIS-NOVA (Kratos) X-ray photoelectron spectrometer using monochromatic Al Kα (150 W) source under base pressure of 2.6 × 10−9 Torr. N2 adsorption−desorption isotherms were measured at −196 °C on Micromeritics ASAP 2020 surface area and porosity analyzer after the carbon was degassed at 250 °C to 20 mTorr for 12 h. The specific surface areas were determined from nitrogen adsorption using the Brunauer−Emmett−Teller (BET) equation. The Raman spectrum was recorded by a Nanofinder-30 with a He−Ne laser (1.017 mW, 631.81 nm) to understand the molecular structure of the carbon material. Thermogravimetric analysis (TGA) measurement was carried out on a Bruker TG-DTA 2000SA from room temperature to 1000 °C at a heating rate of 10 °C min−1 in air. 2.3. Cell Construction and Electrochemical Characterization. Electrochemical behavior of Li-ion battery based on the N-doped carbon as anode materials was studied with half-cell confuration in CR2032 coin-type cells (Hohsen Corp., Japan). For comparison and understanding of Li storage capacity, carbon NPs without N and graphite were also tested. The cell preparation steps were performed in an argon glovebox with the oxygen and the humidity level of 1 ppm or less, respectively. A pure Li metal foil (purity 99.9% and 150 mm thick) was used as a reference electrode and counter electrode. The carbon NPs was mixed well with acetylene black (as a conductivity enhancer) and poly(vinyl diflouride) (PVdF as a binder) at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone solvent to form homogeneous slurry. The slurry was uniformly pasted with 30 mm thickness on Cu foil. The as-prepared working electrodes were dried at 120 °C in a vacuum oven and pressed under a pressure around 4000 psi. For all the coin cells, 1.0 M LiPF6 in ethylene carbonate (EC)−dimethyl carbonate (DMC) (1:1 by volume) was used as an electrolyte, and a typical polypropylene−polyethylene material (Celgard 2400) was used as a separator. The charge−discharge behaviors of the coin cells were characterized in a BaSyTec multichannel battery testing system at constant room temperature. The instrument was programmed to read in each 10 s step. The cells were cycled in the voltage range 3.0−0.02 V at a rate of 37.2 mA g−1 during an initial formation process and at different rates in the following cycles. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 10 kHz to 100 mHz with a zero-bias potential and 10 mV of amplitude. Impedance spectra were analyzed by fitting the spectra to the proposed equivalent circuit using Z-view software.

Figure 1. (a) XRD patterns and (b) nitrogen adsorption−desorption isotherms at 77 K of N-doped carbon and undoped carbon NPs. HRSEM images of (c) N-doped carbon and (d) undoped carbon NPs.

patterns of as-prepared N-doped carbon and undoped carbon NPs are characteristic of disordered turbostratic carbon phase, which consists of one sharp peak at 25.4° and another small broad peak at around 42.3°.6,17 There is no visible difference in the XRD patterns of N-doped carbon and undoped carbon NPs, and the increase in interlayer distance for the (002) plane from usual 0.33 nm for pristine graphite to 0.35 nm can be attributed to the turbostratic disorder.39−41 Figure 1b displays N2 adsorption−desorption isotherms for N-doped carbon and undoped carbon NPs, which reveal a type II isotherm for both, which is characteristic of monolayer−multilayer adsorption by finely divided nonporous NPs.14 The specific surface areas of N-doped carbon and undoped carbon NPs were 89.6 and 87.5 m2 g−1, respectively, as calculated from the N2 adsorption data. Figures 1c and 1d show the HR-SEM images of synthesized Ndoped carbon and undoped carbon. The surface morphologies reveal small NPs of irregular shape for both of the samples. Figure 2a shows the HR-TEM image of N-doped carbon NPs showing aggregated particle-type structure. Figure 2b shows the particle size distribution plot derived from Figure 2a, which indicates a wide distribution of size from 10 to 50 nm for N-doped carbon NPs. Formation of such small nanoparticles by simple flame synthesis technique without need for any kind of

3. RESULTS AND DISCUSSION A flame contains several temperature zones depending on availability of oxygen, among which the dark and luminous 319

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stability of a material. The prepared N-doped carbon NP exhibits high thermal stability temperature of 595 °C (indicated by arrow in Figure 2d), which is higher than those reported for N- and B-doped graphene sheets (531 and 588 °C, respectively) and that of undoped graphene (502 °C).23 No ash or residue was observed from the carbon NP after combustion at 1000 °C, indicating high purity material without any residue. Figure 3a shows Raman spectrum of the N-doped carbon NPs, which displays two broad bands at 1350 and 1585 cm−1 assignable to the disordered (D) band and graphitic (G) band. D band is related to the breaking of symmetry in sp2 carbon caused by structural disorders and defects due to the presence of in-plane substitution heteroatoms, vacancies, grain boundaries, or other defects and by finite size effects, all of which lower the crystalline symmetry of the quasi-infinite lattice.19 On the othe hand, G band corresponds to the first-order scattering of the stretching vibration mode (E2g) observed for sp2 carbon domains.24 The ratio of intensity for D to G bands is directly proportional to amount of turbostratic disorder of carbon. The D/G ratio is found to be 1.12, which is close to the ratio reported for synthetic reduced graphene oxide materials.23,24 XPS is one of the most reliable characterization tool for determining elemental composition of a material and their oxidation state.23−25,43 As depicted in Figure 3b, the XPS survey spectrum of the N-doped carbon NPs possesses three major peaks at 284.5, 398.5, and 533.4 eV, which are associated with C 1s, N 1s, and O 1s, respectively, as expected. The relative atomic percentage of C 1s, N 1s, and O 1s is calculated from XPS as 90.0, 2.6, and 7.4%, respectively, whereas undoped carbon NPs shows presence of only C 1s and O 1s with relative atomic % of 94.6 and 5.4, respectively (Figure 3b). To understand the bonding environment of the elements, highresolution N 1s, O 1s, and C 1s spectra are analyzed with

Figure 2. (a) TEM image, (b) particle size distribution, (c) HR-TEM image, and (d) TGA plot for the N-doped carbon NPs.

stabilizing agent is really astonishing and may have great merit for commercial production. Figure 2c shows high-resolution image of a single NP, which reveals the presence of disordered arrays of graphitic lattice planes in accordance with XRD results.17 The stability of N-doped carbon is an important issue for long-term operation in various applications, and TGA can provide a rapid method for determination of carbon content and thermal stability.42 Figure 2d shows a TGA curve recorded for as-prepared N-doped carbon NPs in air. The oxidation of N-doped carbon NPs is started at 470 °C, lower than that of ideal graphite (600 °C). With increase in temperature, the carbon decomposes rapidly in air with initial decomposition temperature of 530 °C and exhausts at 660 °C as the final decomposition temperature. The average of the initial and final decomposition temperatures is usually referred as the thermal

Figure 3. (a) Raman spectrum of N-doped carbon. (b) XPS survey plots for N-doped carbon and undoped carbon. High-resolution XPS spectra with Gaussian fitting for (c) N 1s peak, (d) O 1s peak, and (e) C 1s peak of N-doped carbon and (f) C 1s peak of undoped carbon. 320

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Gaussian fitting program, and the results are shown in Figures 3c, 3d, and 3e. The XPS spectrum of the C 1s is found to be combination of three peaks, among which the peak at 284.5 eV is related to the sp2 carbon and the peak at 285.2 eV is related to combination of sp3 carbon and C−N bonding.15,23−26 It can be observed from comparison of Figures 3e and 3f that C 1s spectra of N-doped carbon has an additional peak at 288.8 eV assignanable to N−CO bond, which is absent in C 1s spectra of undoped carbon. This observation further confirms the presence of N bonded to carbon framework and its potential contribution to Li storage performance.15 Figure 3c shows N 1s XPS spectrum and contains four different bonding environments. The peaks at 398.2, 399.2, and 400.7 eV are assigned to pyridinic, pyrrolic, and graphitic nitrogen, and the peak at 404 eV is credited to terminal N−O bonding. The high-resolution O 1s spectra in Figure 3d also show evidence of N−CO bonding which is in accordance with C 1s spectra. Oxygen also as doped heteroatom may play a positive role for Li storage by increasing defects, disorder, or local electron density around O atoms. Interestingly, the N-doped carbon has more oxygen content as well compared to the corresponding undoped carbon as seen in Figure 3b, suggesting additional benefit. The electrochemical performances of the as-prepared carbon NPs as LIB anode materials were investigated in a half-cell configuration at a low current rate of 37.2 mA g−1, which corresponds to 0.1 C rate (1.0 C = 372 mA g−1 for graphite) in the voltage range of 0.02−3.0 V (vs Li+/Li). The representation of charge−discharge used for in this study is widely accepted for the carbon anodes, i.e., charge for Li insertion process in the anode (lithiation) and discharge for the reverse process (delithiation). The initial ten charge−discharge voltage cycles of cells are shown in Figure 4a. The first charge profile shows

(SEI) passivation layer on the surface of the carbon electrode. It can be seen that N-doped carbon NPs exhibit first charge and discharge capacity of 1190 and 596 mA h g−1, which accounts for irreversible capacity loss of 594 mA h g−1, i.e. 50% Coulombic efficiency. Low initial Coulombic efficiency is a common phenomenon for porous turbostratic carbon anode materials due to to follwing reasons. The first reason is irreversible reduction of dioxygen molecules or oxygneated functional groups present in turbostratic carbon.47 The second reason is decomposition of electrolyte and the formation of SEI films at the electrode/electrolyte interface.25,47−49 All of these cause irreversible consumption of Li ion and contribute to the large irreversible charge capacity. The charge−discharge profiles become stable and Coulombic efficiency increases substantially to 85% in the second cycle. This enhancement suggests that the SEI layer becomes steady for subsequent lithiation and delithiation, indicating a good cycling performance for the Ndoped carbon electrode. Figure 4b shows comparative of charge−discharge profile from second cycle for N-doped carbon and undoped carbon NPs along with commercial graphite. It can be seen that Ndoped carbon NPs shows 17% more discharge capacity (596 mA h g−1) than undoped carbon (508 mA h g−1) and almost 2 times more than that shown by commercial graphite (270 mA h g−1). Li storage in graphite occurs through intercalation− deintercalation of Li ion between two adjacent graphene planes and within a plane. Each lithium is associated with six carbon atom forming LiC6 composition. As a result, graphite has theoretical capacity of 372 mA h g−1. But turbostratic carbon possesses large number of topological defects as evident from XRD and Raman spectroscopic analysis. These defects form a disordered carbon structure and creates large number of surface active sites or cavities, which can accommodate extra Li-ion species.8 Also, there is an increase interlayer distance for the (002) plane from usual 0.33 nm for pristine graphite to 0.35 nm due to turbostratic disorder, which may also help in more Li intercalation.18 Since the undoped carbon NPs have similar surface area, the capacity enhancement in the N-doped carbon NPs clearly demonstrates the positive effect of N doping and associated increased O heteroatom on Li adsorption. This is may be due to higher electronegativity of N and O than C, which can enhance the electron density around N and O atoms, and this will help to hold even more Li ion. Similar results are also reported by Wu et al. for N- and B-doped graphenes and Li et al. for mesoporous N-doped carbon anode materials.23,50 Evaluation of LIB performance depends not only on specific capacity but also on its reversibility for long-term operation and at high current density. These parameters are strongly affected by the physical and chemical properties of the active electrode materials.13 Retention of capacity at high rate is one of the mandatory electrochemical features of Li-ion batteries to power the high-energy applications such as electric vehicles.13 The rate performances of N-doped carbon, undoped carbon, and graphite are shown in Figure 4c as a plot of discharge capacity with increase in current density from 0.1 to 1 C and then revert back to 0.1 C. Similar fading behavior of discharge capacity can be observed in N-doped carbon and undoped carbon with increase in current density. But in the case of graphite, the capacity fading is lower than both of them. Faster capacity drop is a common phenomenon in the case of turbostratic carbon. This occurs due to less in-plane conductivity of turbostratic carbon than graphite, induced by presence of defects and disorder and also by the high amount of SEI layer. Despite of

Figure 4. Charge−discharge profiles of (a) N-doped carbon in the voltage range of 0.02−3.0 V (vs Li+/Li) at 0.1 C current rate. Comparison of (b) charge−discharge profiles at 0.1 C rate, (c) discharge capacity at different currant rate ranging from 0.1 to 1 C, and (d) discharge capacity for 100 cycles for N-doped carbon, undoped carbon, and graphite.

two visible plateaus at 1.7 and 0.9 V. The first plateau is formed may be due to irreversible reduction of dioxygen molecules or oxygenated functional groups present in the synthesized carbon.47 The second plateau at 0.9 V is common in all carbon-based anode materials. It is primarily due to the reaction of lithium with the electrolyte, which causes decomposition of the electrolyte and formation of solid electrolyte interphase 321

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performance achieved in charge−discharge measurement studies and further support the positive effect of N-doping into carbon framework on Li storage.

this, the N-doped carbon reveals much better capacity than those of undoped carbon and graphite at all current densities. Even at high current density of 1.0 C, the N-doped carbon shows a capacity of ∼300 mA h g−1, which exhibits about 2 times higher capacity than commercial graphite. After 50 cycles, when the current rate is reduced from 1.0 to 0.1 C, the initial capacity is recovered, indicating excellent rate performance of the N-doped carbon NP as anode material. Figure 4d shows a comparative plot of discharge capacities for N-doped carbon, undoped carbon, and graphite with respect to charge−discharge cycle numbers. The discharge capacities of N-doped carbon and undoped carbon are found to drop relatively faster than that of graphite in initial 10 cycles and then stabilize. Overall, N-doped carbon shows capacity retention of 72% after 100 charge−discharge cycle, which is quite impressive. Figure 4d also shows the change in Coulombic efficiency of N-doped carbon NPs with cycle numbers. For the first few cycles of charge−discharge, the Coulombic efficiency of the N-doped carbon is low, but it increases sharply after every subsequent cycles and increases quickly up to 95% at 14th cycle and reached to ca. 97% after 20 cycles. This observation is also in good agreement with those reported for other porous carbon materials.13,46 All of the above results suggest that N-doping can enhance the Li storage capacity to some extent. To better understand physical properties and related LIB performance of N-doped carbon, electrochemical impedance spectroscopy (EIS) measurement was performed at open circuit potential from 10 kHz to 100 mHz region with 10 mV sinusoidal amplitude. The comparative Nyquist plots of N-doped carbon and undoped carbon are shown in Figure 5. Both of the plots show a well-

4. CONCLUSIONS In this study, we demonstrated for the first time an easy and fast single-step synthesis of nitrogen-doped carbon nanoparticles using the flame pyrolysis method. Our approach involves incomplete combustion of acetonitrile in air, resulting in carbon soots doped with N. The synthesized N-doped carbon NPs were tested as anode material for Li-ion battery and exhibited excellent discharge capacity of 596 mA h g−1, which is 17% higher than that of corresponding undoped carbon NPs. Moreover, the discharge capacity was much higher than that shown by commercial graphite anode. Further analysis with rate performance, cycle efficiency and EIS specrscopy also shows that N-doped carbon has better activity than that of undoped carbon, successfully demonstrating the effect of N-doping on reversible Li-ion capacity for long-term operation and at high current density. A large number of surface defects induced from N-doping and higher electronegativity of N and associated O atoms than carbon are proposed as main factor for enhanced LIB performance. The demonstrated flame synthesis method for N-doped carbon is extremely simple, economic, energy efficient, and can be easily scaled up for large scale production. Thus, we believe that the flame synthesized N-doped carbon is absolutely unprecedented and entirely worthy material for making a stroke in Li-ion storage field.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +82 44-860-1331; Tel +82 44-860-1494 (J.-S.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NRF Grant (NRF-2010-0029245) and Global Frontier R&D Program on Center for Multiscale Energy System (NRF 2011-0031571) funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea. The authors also thank the Korean Basic Science Institutes at Jeonju, Chuncheon, and Daejeon for SEM, TEM, and XRD measurements.

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Figure 5. Nyquist plot from EIS analysis of fresh cells made from Ndoped carbon and undoped carbon. Inset shows the equivalent electronic circuit used for fitting of the Nyquist plot.

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