Nitrogen-Doped Carbon Nanoparticles by Flame Synthesis as Anode Material for Rechargeable Lithium-Ion Batteries

Nitrogen-Doped Carbon Nanoparticles by Flame Synthesis as AnodeMaterial for Rechargeable Lithium-Ion BatteriesDhrubajyoti 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) areprepared using fast single-step flame synthesis by directly burning acetonitrilein air atmosphere and investigated as an anode material for lithium-ionbatteries. The as-prepared N-doped carbon NPs show excellent Li-ion stoarageproperties with initial discharge capacity of 596 mA h g−1, which is 17% morethan that shown by the corresponding undoped carbon NPs synthesized byidentical process with acetone as carbon precursor and also much higher thanthat of commercial graphite anode. Further analysis shows that the charge−discharge process of N-doped carbon is highly stable and reversible not only athigh current density but also over 100 cycles, retaining 71% of initial dischargecapacity. Electrochemical impedance spectroscopy also shows that N-dopedcarbon has better conductivity for charge and ions than that of undopedcarbon. The high specific capacity and very stable cyclic performance areattributed to large number of turbostratic defects and N and associated increased O content in the flame-synthesized N-dopedcarbon. To the best of our knowledge, this is the first report which demonstrates single-step, direct flame synthesis of N-dopedturbostratic 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 futurepower source for portable electronic devices and electrical/hybrid vehicles.1−5 After commercialization of Li-ion battery,carbon-based rechargeable batteries have extensively gainedmuch attention. In particular, graphite has been used as themost popular anode material for current Li-ion batteries due toits high Coulombic efficiency, stability, and safety. However, ithas suffered from limitation in meeting the increasing highenergy demands for electricity storage stations and electricvehicles due to its low specific capacity and lithium intercalationpotential close to that of lithium plating.6 Intensive efforts havebeen made to develop alternatives of the graphite for next-generation new LIBs with more attractive features, includinglow cost, high energy density, and good cycling and rateperformance.7−9 Therefore, various forms of carbon materialssuch as micro- to macroporous carbon materials,9−14 carbonnanotubes,15,16 nanofibers,17 and graphene18−21 have beenstudied as alternative anode materials to increase the energydensity of the LIBs. Overall, the disordered carbon rather thangraphitic 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 havebeen prepared and applied as alternative anode materials toenhance the Li storage capability of carbon materials.28,29 Thefirst-principles calculations for the lithium adsorption propertiesof N- and B-doped materials were reported in the literature,which showed that the B-doping decreases the Li adsorption

energies, while the N-doping increases the Li adsorptionenergies.27,30,31 The N-doping has been reported to yieldseveral different N-carbon species depending on the sitelocation in carbon framework including the pyridinic carbon,which is considered most suitable for Li storage with a highstorage capacity.23

In recent years, soot consisting of carbon particles has beenintroduced using facile flame synthesis method, and severalapplications of the candle soot have been demonstrated.32−34

Inamdar et al. reported a simple flame pyrolysis method toprepare uncapped maghemite (γ-Fe2O3) and carbon-coatedmaghemite nanoparticles (NPs).35,36 Herein, for the collectiveeffect of N content and energy-efficient simple synthesis, wereport the flame synthesis of N-doped carbon NPs by directburning of acetonitrile as a nitrogen and carbon source anddemonstrate the N-containing turbostratic carbon as apromising alternative anode material with high Li-ion storagecapacity, stable cyclic, and rate performances. The effect of N-doping on change of charge density in carbon frameworkneighboring to N is discussed, and its most possible effect on Listorage is proposed.

Received: August 31, 2013Revised: November 29, 2013Published: December 17, 2013

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2. EXPERIMENTAL SECTION2.1. Preparation of N-Doped Turbostratic Carbon Nano-

particle Soot. The earlier reported flame pyrolysis method was usedwith some modifications to prepare the carbon NP in the form ofsoot.36,37 In brief, 30 mL of flammable acetonitrile was burned in asmall glass vessel under air. The soot was deposited over a cold surfaceof conical water containing flask for cooling. The product in the formof black soot was collected and used without any purification orprocessing. For comparison, carbon NPs without N-doping was alsosynthesized by identical process using acetone as carbon precursor.The entire synthesis process was carried out in well-ventilated fumehood chamber to avoid the toxic CN likely to be released fromacetonitrile during burning.2.2. Characterization of Carbon Soot Nanoparticles. For

characterization of N-doped carbon NPs, high-resolution scanningelectron microscopy (HR-SEM), transmission electron microscopy(HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectros-copy (XPS), and Raman spectroscopy were used. The TEM imageswere obtained by using a field emission transmission electronmicroscope (JEM 2200-FS) operated at 200 kV, and HR-SEM imageswere obtained by using a Hitachi S-5500 ultrahigh-resolution scanningelectron microscope operated at 30 kV. Powder XRD was measuredwith 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 outin an AXIS-NOVA (Kratos) X-ray photoelectron spectrometer usingmonochromatic 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 thecarbon was degassed at 250 °C to 20 mTorr for 12 h. The specificsurface areas were determined from nitrogen adsorption using theBrunauer−Emmett−Teller (BET) equation. The Raman spectrum wasrecorded by a Nanofinder-30 with a He−Ne laser (1.017 mW, 631.81nm) to understand the molecular structure of the carbon material.Thermogravimetric analysis (TGA) measurement was carried out on aBruker TG-DTA 2000SA from room temperature to 1000 °C at aheating 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-dopedcarbon as anode materials was studied with half-cell confuration inCR2032 coin-type cells (Hohsen Corp., Japan). For comparison andunderstanding of Li storage capacity, carbon NPs without N andgraphite were also tested. The cell preparation steps were performed inan argon glovebox with the oxygen and the humidity level of 1 ppm orless, respectively. A pure Li metal foil (purity 99.9% and 150 mmthick) was used as a reference electrode and counter electrode. Thecarbon NPs was mixed well with acetylene black (as a conductivityenhancer) and poly(vinyl diflouride) (PVdF as a binder) at a weightratio of 8:1:1 in N-methyl-2-pyrrolidone solvent to form homogeneousslurry. The slurry was uniformly pasted with 30 mm thickness on Cufoil. The as-prepared working electrodes were dried at 120 °C in avacuum oven and pressed under a pressure around 4000 psi. For allthe coin cells, 1.0 M LiPF6 in ethylene carbonate (EC)−dimethylcarbonate (DMC) (1:1 by volume) was used as an electrolyte, and atypical polypropylene−polyethylene material (Celgard 2400) was usedas a separator. The charge−discharge behaviors of the coin cells werecharacterized in a BaSyTec multichannel battery testing system atconstant room temperature. The instrument was programmed to readin each 10 s step. The cells were cycled in the voltage range 3.0−0.02V at a rate of 37.2 mA g−1 during an initial formation process and atdifferent rates in the following cycles. Electrochemical impedancespectroscopy (EIS) measurements were carried out in the frequencyrange of 10 kHz to 100 mHz with a zero-bias potential and 10 mV ofamplitude. Impedance spectra were analyzed by fitting the spectra tothe proposed equivalent circuit using Z-view software.

3. RESULTS AND DISCUSSIONA flame contains several temperature zones depending onavailability of oxygen, among which the dark and luminous

zones have very low supply of oxygen and have temperatureranging from 800 to 1200 °C.38 The pyrolysis process starts inthis zone and is maintained by highly exothermic reaction ofcombustible organic material, resulting in carbon soots. Ourflame synthesis exploits this process for synthesis of carbonNPs. By using organic volatile material with heteroatom presentin molecular structure, carbon NPs doped with the heteroatomcan be synthesized.37 Therefore, flame synthesis can be hailedas crude but cheap synthesis method for turbostratic carbonNPs. Since the flame pyrolysis method is performed at strictconditions for reagent concentration, temperature, and timespan, the process, although simple, leads to repetitive formationof heteroatom-doped carbon materials having similar micro-structure and physical properties, and as a result theperformances of the materials in various applications are highlyreproducible.XRD analysis was performed for evaluation of crystalline

phase of the carbon NPs prepared by flame pyrolysis ofacetonitrile and acetone. As shown in Figure 1a, the XRD

patterns of as-prepared N-doped carbon and undoped carbonNPs are characteristic of disordered turbostratic carbon phase,which consists of one sharp peak at 25.4° and another smallbroad peak at around 42.3°.6,17 There is no visible difference inthe XRD patterns of N-doped carbon and undoped carbonNPs, and the increase in interlayer distance for the (002) planefrom usual 0.33 nm for pristine graphite to 0.35 nm can beattributed to the turbostratic disorder.39−41 Figure 1b displaysN2 adsorption−desorption isotherms for N-doped carbon andundoped carbon NPs, which reveal a type II isotherm for both,which is characteristic of monolayer−multilayer adsorption byfinely divided nonporous NPs.14 The specific surface areas ofN-doped carbon and undoped carbon NPs were 89.6 and 87.5m2 g−1, respectively, as calculated from the N2 adsorption data.Figures 1c and 1d show the HR-SEM images of synthesized N-doped carbon and undoped carbon. The surface morphologiesreveal 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 2bshows the particle size distribution plot derived from Figure 2a,which indicates a wide distribution of size from 10 to 50 nm forN-doped carbon NPs. Formation of such small nanoparticles bysimple flame synthesis technique without need for any kind of

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

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stabilizing agent is really astonishing and may have great meritfor commercial production. Figure 2c shows high-resolutionimage of a single NP, which reveals the presence of disorderedarrays of graphitic lattice planes in accordance with XRDresults.17 The stability of N-doped carbon is an important issuefor long-term operation in various applications, and TGA canprovide a rapid method for determination of carbon contentand thermal stability.42 Figure 2d shows a TGA curve recordedfor as-prepared N-doped carbon NPs in air. The oxidation ofN-doped carbon NPs is started at 470 °C, lower than that ofideal graphite (600 °C). With increase in temperature, thecarbon decomposes rapidly in air with initial decompositiontemperature of 530 °C and exhausts at 660 °C as the finaldecomposition temperature. The average of the initial and finaldecomposition temperatures is usually referred as the thermal

stability of a material. The prepared N-doped carbon NPexhibits high thermal stability temperature of 595 °C (indicatedby arrow in Figure 2d), which is higher than those reported forN- and B-doped graphene sheets (531 and 588 °C,respectively) and that of undoped graphene (502 °C).23 Noash or residue was observed from the carbon NP aftercombustion at 1000 °C, indicating high purity material withoutany 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 carboncaused by structural disorders and defects due to the presenceof in-plane substitution heteroatoms, vacancies, grain bounda-ries, or other defects and by finite size effects, all of which lowerthe crystalline symmetry of the quasi-infinite lattice.19 On theothe hand, G band corresponds to the first-order scattering ofthe stretching vibration mode (E2g) observed for sp2 carbondomains.24 The ratio of intensity for D to G bands is directlyproportional to amount of turbostratic disorder of carbon. TheD/G ratio is found to be 1.12, which is close to the ratioreported for synthetic reduced graphene oxide materials.23,24

XPS is one of the most reliable characterization tool fordetermining elemental composition of a material and theiroxidation state.23−25,43 As depicted in Figure 3b, the XPSsurvey spectrum of the N-doped carbon NPs possesses threemajor peaks at 284.5, 398.5, and 533.4 eV, which are associatedwith C 1s, N 1s, and O 1s, respectively, as expected. Therelative atomic percentage of C 1s, N 1s, and O 1s is calculatedfrom XPS as 90.0, 2.6, and 7.4%, respectively, whereas undopedcarbon NPs shows presence of only C 1s and O 1s with relativeatomic % of 94.6 and 5.4, respectively (Figure 3b). Tounderstand the bonding environment of the elements, high-resolution N 1s, O 1s, and C 1s spectra are analyzed with

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

Figure 3. (a) Raman spectrum of N-doped carbon. (b) XPS survey plots for N-doped carbon and undoped carbon. High-resolution XPS spectrawith 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.

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Gaussian fitting program, and the results are shown in Figures3c, 3d, and 3e. The XPS spectrum of the C 1s is found to becombination of three peaks, among which the peak at 284.5 eVis related to the sp2 carbon and the peak at 285.2 eV is relatedto combination of sp3 carbon and C−N bonding.15,23−26 It canbe observed from comparison of Figures 3e and 3f that C 1sspectra of N-doped carbon has an additional peak at 288.8 eVassignanable to N−CO bond, which is absent in C 1s spectraof undoped carbon. This observation further confirms thepresence of N bonded to carbon framework and its potentialcontribution to Li storage performance.15 Figure 3c shows N 1sXPS spectrum and contains four different bonding environ-ments. The peaks at 398.2, 399.2, and 400.7 eV are assigned topyridinic, pyrrolic, and graphitic nitrogen, and the peak at 404eV is credited to terminal N−O bonding. The high-resolutionO 1s spectra in Figure 3d also show evidence of N−CObonding which is in accordance with C 1s spectra. Oxygen alsoas doped heteroatom may play a positive role for Li storage byincreasing defects, disorder, or local electron density around Oatoms. Interestingly, the N-doped carbon has more oxygencontent as well compared to the corresponding undopedcarbon 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-cellconfiguration at a low current rate of 37.2 mA g−1, whichcorresponds to 0.1 C rate (1.0 C = 372 mA g−1 for graphite) inthe voltage range of 0.02−3.0 V (vs Li+/Li). The representationof charge−discharge used for in this study is widely acceptedfor the carbon anodes, i.e., charge for Li insertion process in theanode (lithiation) and discharge for the reverse process(delithiation). The initial ten charge−discharge voltage cyclesof cells are shown in Figure 4a. The first charge profile shows

two visible plateaus at 1.7 and 0.9 V. The first plateau is formedmay be due to irreversible reduction of dioxygen molecules oroxygenated functional groups present in the synthesizedcarbon.47 The second plateau at 0.9 V is common in allcarbon-based anode materials. It is primarily due to the reactionof lithium with the electrolyte, which causes decomposition ofthe electrolyte and formation of solid electrolyte interphase

(SEI) passivation layer on the surface of the carbon electrode. Itcan be seen that N-doped carbon NPs exhibit first charge anddischarge capacity of 1190 and 596 mA h g−1, which accountsfor irreversible capacity loss of 594 mA h g−1, i.e. 50%Coulombic efficiency. Low initial Coulombic efficiency is acommon phenomenon for porous turbostratic carbon anodematerials due to to follwing reasons. The first reason isirreversible reduction of dioxygen molecules or oxygneatedfunctional groups present in turbostratic carbon.47 The secondreason is decomposition of electrolyte and the formation of SEIfilms at the electrode/electrolyte interface.25,47−49 All of thesecause irreversible consumption of Li ion and contribute to thelarge irreversible charge capacity. The charge−discharge profilesbecome stable and Coulombic efficiency increases substantiallyto 85% in the second cycle. This enhancement suggests that theSEI layer becomes steady for subsequent lithiation anddelithiation, indicating a good cycling performance for the N-doped carbon electrode.Figure 4b shows comparative of charge−discharge profile

from second cycle for N-doped carbon and undoped carbonNPs along with commercial graphite. It can be seen that N-doped carbon NPs shows 17% more discharge capacity (596mA h g−1) than undoped carbon (508 mA h g−1) and almost 2times more than that shown by commercial graphite (270 mA hg−1). Li storage in graphite occurs through intercalation−deintercalation of Li ion between two adjacent graphene planesand within a plane. Each lithium is associated with six carbonatom forming LiC6 composition. As a result, graphite hastheoretical capacity of 372 mA h g−1. But turbostratic carbonpossesses large number of topological defects as evident fromXRD and Raman spectroscopic analysis. These defects form adisordered carbon structure and creates large number of surfaceactive sites or cavities, which can accommodate extra Li-ionspecies.8 Also, there is an increase interlayer distance for the(002) plane from usual 0.33 nm for pristine graphite to 0.35nm due to turbostratic disorder, which may also help in more Liintercalation.18 Since the undoped carbon NPs have similarsurface area, the capacity enhancement in the N-doped carbonNPs clearly demonstrates the positive effect of N doping andassociated increased O heteroatom on Li adsorption. This ismay 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 arealso reported by Wu et al. for N- and B-doped graphenes and Liet al. for mesoporous N-doped carbon anode materials.23,50

Evaluation of LIB performance depends not only on specificcapacity but also on its reversibility for long-term operation andat high current density. These parameters are strongly affectedby the physical and chemical properties of the active electrodematerials.13 Retention of capacity at high rate is one of themandatory electrochemical features of Li-ion batteries to powerthe high-energy applications such as electric vehicles.13 The rateperformances of N-doped carbon, undoped carbon, andgraphite are shown in Figure 4c as a plot of discharge capacitywith increase in current density from 0.1 to 1 C and then revertback to 0.1 C. Similar fading behavior of discharge capacity canbe observed in N-doped carbon and undoped carbon withincrease in current density. But in the case of graphite, thecapacity fading is lower than both of them. Faster capacity dropis a common phenomenon in the case of turbostratic carbon.This occurs due to less in-plane conductivity of turbostraticcarbon than graphite, induced by presence of defects anddisorder and also by the high amount of SEI layer. Despite of

Figure 4. Charge−discharge profiles of (a) N-doped carbon in thevoltage 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, undopedcarbon, and graphite.

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this, the N-doped carbon reveals much better capacity thanthose of undoped carbon and graphite at all current densities.Even at high current density of 1.0 C, the N-doped carbonshows a capacity of ∼300 mA h g−1, which exhibits about 2times higher capacity than commercial graphite. After 50 cycles,when the current rate is reduced from 1.0 to 0.1 C, the initialcapacity is recovered, indicating excellent rate performance ofthe N-doped carbon NP as anode material.Figure 4d shows a comparative plot of discharge capacities

for N-doped carbon, undoped carbon, and graphite withrespect to charge−discharge cycle numbers. The dischargecapacities of N-doped carbon and undoped carbon are found todrop relatively faster than that of graphite in initial 10 cyclesand then stabilize. Overall, N-doped carbon shows capacityretention of 72% after 100 charge−discharge cycle, which isquite impressive. Figure 4d also shows the change inCoulombic efficiency of N-doped carbon NPs with cyclenumbers. For the first few cycles of charge−discharge, theCoulombic efficiency of the N-doped carbon is low, but itincreases sharply after every subsequent cycles and increasesquickly up to 95% at 14th cycle and reached to ca. 97% after 20cycles. This observation is also in good agreement with thosereported for other porous carbon materials.13,46

All of the above results suggest that N-doping can enhancethe Li storage capacity to some extent. To better understandphysical properties and related LIB performance of N-dopedcarbon, electrochemical impedance spectroscopy (EIS) meas-urement was performed at open circuit potential from 10 kHzto 100 mHz region with 10 mV sinusoidal amplitude. Thecomparative Nyquist plots of N-doped carbon and undopedcarbon are shown in Figure 5. Both of the plots show a well-

defined semicircle in high to medium frequency followed bylinear part toward lower frequency. The Nyquist plots areinterpreted with a probable electric equivalent circuit showninset of Figure 5.23 The parameters include electrolyteresistance (RS), charge transfer resistance (RCT), constantphase element (CPE), Warburg impedance (ZW), and doublelayer capacitance (C).23 By fitting of the impedance spectrumto the proposed equivalent circuit using the ZView program,several kinetic parameters are derived. The results shows thatthe RCT and ZW values in the N-doped carbon are 25.6 and 10.7Ω, which are lower than 27.8 and 11.5 Ω for the correspondingundoped carbon. The double layer capacitance of the N-dopedcarbon is 7.5 mF, which is also higher than 5.9 mF for theundoped carbon. These results matches well with the

performance achieved in charge−discharge measurementstudies and further support the positive effect of N-dopinginto carbon framework on Li storage.

4. CONCLUSIONSIn this study, we demonstrated for the first time an easy and fastsingle-step synthesis of nitrogen-doped carbon nanoparticlesusing the flame pyrolysis method. Our approach involvesincomplete combustion of acetonitrile in air, resulting in carbonsoots doped with N. The synthesized N-doped carbon NPswere tested as anode material for Li-ion battery and exhibitedexcellent 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 thatshown by commercial graphite anode. Further analysis with rateperformance, cycle efficiency and EIS specrscopy also showsthat N-doped carbon has better activity than that of undopedcarbon, successfully demonstrating the effect of N-doping onreversible Li-ion capacity for long-term operation and at highcurrent density. A large number of surface defects induced fromN-doping and higher electronegativity of N and associated Oatoms than carbon are proposed as main factor for enhancedLIB performance. The demonstrated flame synthesis methodfor N-doped carbon is extremely simple, economic, energyefficient, and can be easily scaled up for large scale production.Thus, we believe that the flame synthesized N-doped carbon isabsolutely unprecedented and entirely worthy material formaking a stroke in Li-ion storage field.

■ AUTHOR INFORMATIONCorresponding Author*E-mail [email protected]; Fax +82 44-860-1331; Tel +8244-860-1494 (J.-S.Y.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by NRF Grant (NRF-2010-0029245)and Global Frontier R&D Program on Center for MultiscaleEnergy System (NRF 2011-0031571) funded by the Ministryof Education, Science and Technology through the NationalResearch Foundation of Korea. The authors also thank theKorean Basic Science Institutes at Jeonju, Chuncheon, andDaejeon for SEM, TEM, and XRD measurements.

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