Electrochimica Acta 171 (2015) 156–164

N-doped graphene/graphite composite as a conductive agent-freeanode material for lithium ion batteries with greatly enhancedelectrochemical performance

Wu Guanghui a, Li Ruiyi a, Li Zaijun a,b,*, Liu Junkang a, Gu Zhiguo a, Wang Guangli a

a School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Chinab The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, Wuxi 214122, China

A R T I C L E I N F O

Article history:Received 15 January 2015Received in revised form 22 April 2015Accepted 4 May 2015Available online 5 May 2015

Keywords:graphitegraphenecompositelithium ion batteryelectrochemical performance

A B S T R A C T

Present graphite anode cannot meet the increasing requirement of electronic devices and electricvehicles due to its low specific capacity, poor cycle stability and low rate capability. The study reported apromising N-doped graphene/graphite composite as a conductive agent-free anode material for lithiumion batteries. Herein, graphite oxide and urea were dispersed in ultrapure water and partly reduced byascorbic acid. Followed by mixing with graphite and hydrothermal treatment to produce graphene oxide/graphite hydrogel. The hydrogel was dried and finally annealed in Ar/H2 to obtain N-doped graphene/graphite composite. The result shows that all of graphite particles was dispersed in three-dimensionalgraphene framework with a rich of open pores. The open pore accelerates the electrolyte transport. Thegraphene framework works as a conductive agent and graphite particle connector and improves theelectron transfer. Electrical conductivity of the composite reaches 5912 S m�1, which is much better thanthat of the pristine graphite (4018 S m�1). The graphene framework also acts as an expansion absorber inthe anodes of lithium ion battery to relieve the large strains developed at high discharge rates. As a result,the N-doped G/C electrode provides an excellent electrochemical performance for lithium ion battery,including high specific capacity (781 mA h g�1), outstanding rate capability (351 mA h g�1 at 10 C) andintriguing cycling stability (98.1% capacity retention at 10 C after 1000 cycles).

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1. Introduction

Lithium ion battery has been widely used since its firstcommercialization by Sony Corporation in 1990s. It is alsoconsidered as an attractive power source for electric vehicle[1–3] and for stationary energy storage of solar and wind power[4,5]. These applications have more demands for lithium ionbatteries in all aspect such as specific capacity, performance underhigh charge/discharge rate, cycling stability, safety and cost. Atpresent, graphite plays a dominating role in field of the anodematerial in commercial lithium ion batteries owing to desirablecharge potential profile, good safety feature and low cost. However,present graphite anodes can not provide high enough specificcapacity, cycle stability and rate capability. These greatly limit itsfurther development [6].

Two strategies have been used for the fabrication of graphiteanode materials to improve the electrochemical performance. One

* Corresponding author. Tel.: +86 13912371144.E-mail address: zaijunli@263.net (L. Zaijun).

http://dx.doi.org/10.1016/j.electacta.2015.05.0160013-4686/ã 2015 Elsevier Ltd. All rights reserved.

is to reduce the particle size of graphite, especially nanoscale [7].The nanostructured carbon is effective for improving lithium ioninsertion/extraction by providing large surface area, which isbeneficial to the accommodation of additional lithium ions, and ashorter path for lithium ion diffusion [8,9]. However, it also sufferfrom a large irreversible capacity loss during the initial cycling, inaddition to the mass scalability of carbon material beingchallenging. Another is to modify the surface of graphite particles[10,11]. To date, amorphous carbon [12], carbon nanotube [13] andgraphite oxide [14] were used for the above aim. Amorphouscarbon has an isotropic feature different from graphite, whichenables lithium ions to transport inside randomly and thusexhibits a good rate capability. Besides, amorphous carbon alsooffers a minor volume change during the charge/discharge processand good cyclic performance [15]. Many investigations havedemonstrated that the surface modification by coating amorphouscarbon is an effective approach to improve the electrochemicalperformance. However, a proper coating layer is essential tosurface structure and electrochemical properties of the graphite.An incomplete coating layer leads to the exposure of part of thegraphite surface, while excessive coating can cause a thick layer.

W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164 157

This leads to increase the diffusion distance of lithium ions andaffect the rate capability. Wang et al. studied the effect of coatingamount of glucose on the surface structure and the electrochemi-cal properties [16]. The result shows that the sample coated with5 wt.% glucose exhibits the highest reversible specific capacity,excellent rate capability, and good cyclic performance. Since thediscovery of carbon nanotube, its applications to energy storagedevices have been highly issued. Compared with amorphouscarbon, carbon nanotube provides a shorter diffusion path for theelectron transfer, larger surface area and faster electronic/thermalconductions. Moreover, carbon nanotube can also entangle withparticles to form an interconnected network, which is easier forelectrons to distribute and transport between particles [17]. Leeet al. used carbon nanotube as a functional additive on the C-ratecapabilities of composite anodes [18]. The additive was observed toact as a particle connector or an expansion absorber in the anodesof lithium-ion batteries. By controlling the dispersion process,carbon nanotube bundles were divided and dispersed betweenhost particles. The rounded graphite with 2.0 wt.% of carbonnanotube exhibited a high rate capacity even at high electrodedensity. In addition, the well-dispersed carbon nanotube bundlesenabled to relieve the large strains developed at high dischargerates and to keep the electrical contact between the host particlesduring repeated intercalation/deintercalation. However, the sur-face modification of graphite with amorphous carbon or carbonnanotube does not substantially increase the specific capacity ofgraphite anode materials. Graphene is a single atomic layer ofcarbon connected by sp2-hybridized bonds and attracted strongscientific interest since its recent discovery [19]. It has beenrecognized as the most promising electrode material for lithiumion battery due to superior electrical conductivity, high surfacearea, and wide electrochemical window [20]. The researches haveproved that the specific capacity of graphene anode for lithium ionbattery can exceed 1000 mA h g�1, which is about 3-fold that of thegraphite electrode [21]. However, the capacity will rapidly decaywith the increase of cycle number due to its graphitization. Theproblems can be effectively solved by the introduction of otherparticles in the graphene layers. Gu et al. developed a graphite/graphene oxide composite as the anode material of lithium ionbattery [22]. The composite has more than 690 mA h g�1 reversiblecapacity at the rate of 0.5C, simultaneously with excellent cycleperformance and rate capability. However, the results of previousresearches has proved that graphene oxide is not very suitable tobe used as electrode materials for lithium batteries. On the onehand, graphene oxide is an insulating material. Its poor electricalconductivity brings a low high-rate capability. Its use need to add alarge amounts of conductive agent to improve the electricalconductivity. This will increase the mass and volume of theelectrode material and thus limits its many applications in highpowder batteries. On the other hand, graphene oxide contains richof active groups such as hydroxyl, carboxyl and epoxy groups. Thecombination of active groups with lithium ions will produce anirreversible capacitance and thus leads to large reversiblecapacitance loss. Moreover, the decomposition of active groupsin graphene oxide inevitable forms some free water during therepeated charge/discharge process. This will bring a decline inelectrochemical performance [23,24].

In the study, we reported a new strategy for the synthesis ofN-doped graphene/graphite composite (termed as N-doped G/C).The result shows that all of graphite particles was dispersed inthree-dimensional graphene framework with rich of open pores.The unique structure creates ultrafast electron transfer andelectrolyte transport, and stable structure of the anode material.The as-prepared N-doped G/C as conductive agent-free anodematerial exhibits a greatly enhanced electrochemical performancefor lithium ion batteries.

2. Experimental

2.1. Synthesis of N-doped G/C

Graphite oxide (GO) (1.0 g) and urea (2.0 g) were dispersed inultra pure water (200 mL) by the ultrasonication. Followed by theadding ascorbic acid (2.0 g) and the heating (90 �C) for 1 h to form auniform and viscous solution. The solution was mixed withgraphite (6.7 g) by the mechanical stirring for 1 h and subsequentlytreated by the hydrothermal reaction for 10 h under 160 �C toproduce a graphene oxide/graphite gel. The gel was dried at 105 �Cfor 2 h and finally annealed at 400 �C for 6 h in Ar/H2 atmosphere(Ar:H2 = 95:5) to obtain the N-doped G/C composite. To determinethe amounts of graphene in the composite, two control samples,including single graphite and single graphite oxide, were treatedby using the same procedure. According to the mass changes ofgraphite and graphene oxide before and after treated, we cancalculate the content of graphene in the composite. The resultshows that the content of graphene in the as-prepared N-dopedG/C is 10.7%. To study on the effect of nitrogen doping and thermalannealing on the electrochemical performance, the graphene/graphite (G/C) and the N-doped graphene oxide/graphite (N-dopedGO/C) were also fabricated by using above method unless noaddition of urea or no use of the thermal annealing.

2.2. General characterization

Scanning electron microscope (SEM) was performed usingHITACHI S4800 with a X-ray Energy Dispersive Spectrometer (EDS)(INCA Energy, Oxford Instruments). X-ray diffraction (XRD) wasmeasured on the D8 Advance with a Cu Ka radiation. Ramanmeasurements were carried out using InVia laser micro-Ramanspectrometer. X-ray photoelectron spectroscopy (XPS) was per-formed by PHI 5700 using Al KR radiation. N2 adsorption anddesorption isotherms were measured at 77 K on a QuantachromeNova 2000 and the CO2 isotherms were recorded at 273 K with aMicromeritics TriStar 3000. Prior to the gas sorption measure-ments, all samples were outgassed in a vacuum at 120 �C for 24 h.The specific surface area and the pore size distribution werecalculated using the Braunauer–Emmett–Teller (BET) method, andthe relative pressure range of p/p0 from 0.1 to 0.3 was used formultipoint BET calculations. Non-local density functional theory(NLDFT) assuming the pores are slit/cylinder shaped was used todetermine the pore size distribution and mesopore volume.Electrical conductivity measurement was carried out in aST-2258C multifunction digital four-probe tester, which equippedwith a SZT-D semiconductor powder resistivity tester (SuzhouJingge Electronic Co., Ltd. China). The powder sample was directlyplaced in a round hole with the diameter of 5 mm on the SZT-Dsemiconductor powder resistivity tester. Then, the powder wasslowly compressed to 100 atmospheres by the piston on thesystem. At the same time, the electrical conductivity values underdifferent pressures were recorded.

2.3. Electrochemical measurement

Electrochemical properties of N-doped G/C, G/C and graphiteelectrodes were evaluated using 2016 coin cells. The activematerial was mixed with polyvinylidene fluoride (PVDF, Sigma–Aldrich) at weight ratio of 9:1 in N-methylpyrrolidone (NMP,Sigma–Aldrich) solvent to form uniform slurries, which were thencoated on copper foils. The loading density of active materials wasabout 3.0 mg cm�2. Subsequently dried in a vacuum oven at 120 �Covernight and rolled by using a rolling machine. The copper foilwas placed on the specialized slicing machine to prepare theworking electrode. These working electrodes were incorporated

Fig. 2. Optical photographs of GO dispersion (a), GO dispersion partly reduced GOby ascorbic acid (b), mixture of graphite and partly reduced GO (c) and GO/graphitehydrogel (d).

158 W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164

into 2016 coin cells, in which Li foils were serviced as the counterand reference electrode, Celgard 2400 as the separator, and amixed solvent of ethylene carbonate, dimethyl carbonate anddiethylene carbonate (1:1:1) containing of 1 M LiPF6 as theelectrolyte. The assembly process was conducted in an argon-filled glove box having O2 and H2O contents below 0.1 ppm.

Discharge-charge tests were performed at a potential range of0.01�2.0 V (vs. Li/Li+) on a CT2001A LAND Battery Test System. Alldischarge-charge rates were denoted using the C-rate where370 mA g�1 was assigned to be current density of 1C based ontheoretical capacity of graphite (370 mA h g�1). Cyclic voltammo-grams (CV) were performed in a CHI 660D ElectrochemicalWorkstation over the potential range of 0.01�2.0 V at a scanningrate of 1 mV s�1. Electrochemical impedance spectroscopy (EIS)measurements were carried out on the coin cells using a CHI 660DElectrochemical Workstation. A potential amplitude of �5 mV anda frequency range of 0.01 to 105 Hz were adopted.

Electro-active area of the working electrode was estimated bycyclic voltammograms at various scan rates. For a reversibleprocess, the Randles-Sevcik formula was used [25].

Ipa ¼ 260 � 105n3=2AC0D1=2R v1=2

Here, Ipa, n, A, DR, C0 and v present the anodic peak current, thenumber of electron transferred, the electro-active area (cm2) ofworking electrode, the diffusion coefficient of Li+ (cm2 s�1) in theelectrolyte solution, the bulk concentration of Li+ (mol cm�3) in theelectrolyte and the scan rate of potential perturbation (V s�1),respectively. According to the equation, the electro-active areas ofN-doped G/C electrode and graphite electrodes were found to be13.51 cm2 and 11.82 cm2, respectively.

3. Results and discussion

3.1. N-doped G/C synthesis

The synthesis of N-doped G/C includes three assembleprocesses, i.e. the partly reduction of GO, mixing of GO withgraphite, and nitrogen doping of graphene (shown in Fig. 1). First,graphite oxide and urea were dispersed in ultra pure water andpartly reduced by ascorbic acid to form a viscous GO dispersion.Owing to the existence of big difference in the density between thegraphite and the GO dispersion, a simple mixing of graphite withGO dispersion is difficult to form a stable dispersion by ordinarymechanical stirring. Thus, one step for increasing GO viscosity isstrongly required prior to the mixing. An appropriate viscosity ofGO dispersion become a key factor to obtain good dispersion in thefinal product. Often, GO can be slowly reduced into graphene byheating. The partly reduced GO sheets will self-assembly andfinally form GO hydrogel, which offers a relatively high viscosity. Inthe step, we used ascorbic acid as a moderate reducing agent forreducing GO. The result shows that the addition of ascorbic acidremarkably speeds up the reaction rate, the color of GO dispersionrapidly changes into black from yellow (shown in Fig. 2). Because ofthe loss of some hydrophilic groups (hydroxyl, carboxyl and epoxygroups), the partly reduced GO sheets have a lower water-solubility compared with pristine GO and results in an obvious

Fig. 1. The procedure for the prepara

increase in the viscosity. Moreover, urea can cross-link with GOsheets by the chemical bonds and serves to increase the viscosity.However, we also noted that graphite particles will be moredifficult to enter into the internal of the GO hydrogel structureduring the mixing process with the increase of the gelation degree.When the heating time increases to 2.5 hours, the GO dispersionwill be completely changed into the GO hydrogel (see Fig. S1). Inthis case, the addition of graphite into the system may not get agood composite. The resulting product contains a relatively bigpieces and small powders (see Fig. S2). In order to determine thetypes of the piece and the powder, the collected piece sample andpowder sample from the product were investigated by the SEM-EDS analysis, respectively. The results in Fig. s3 exhibit that thepiece sample has rich of folds and porous structure, indicatingtypical morphology features of graphene materials. The powdersample is comprised of relatively big particles with smooth surface,indicating typical morphology feature of graphite materials.Further, the EDS analysis reveals that the piece sample containscarbon (C), nitrogen (N) and oxygen (O) elements, which areconsistent with the composition of N-doped graphene. The powdersample only contains carbon element, which is consistent with thecomposition of graphite (shown in Fig. S4). The above resultsdemonstrate that an excessive gelation leads to produce a mixtureof N-doped graphene aerogel and graphite, in which the N-dopedgraphene aerogel and graphite particles exist alone and are notwell mixed to form a good composite. This is because an excessivegelation leads to form a large number of the solidified porousstructures. When the pore size is less than the particle size ofgraphite, the graphite particle is difficult to enter into internal ofthe hydrogel structure. In addition, the GO hydrogel will be brokeninto a large number of small pieces of N-doped graphene aerogelunder agitating. For the reason, a strict control of the reactiontemperature and time was strongly required to obtain an optimalgelation level.

The viscous GO solution was mixed with the amounts ofgraphite under vigorous agitation. Followed by the hydrothermal

tion of N-doped G/C composite.

W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164 159

reaction to form a GO/graphite hydrogel. By mechanical stirring,the high viscosity of GO dispersion make it firmly adhere to thesurface of graphite particles. As a result, the final N-doped G/Ccomposite offers a relatively good dispersion. During thehydrothermal reaction, these GO sheets will be further reducedand result a higher gelation degree of GO. Because the most ofhydrophilic groups in the GO sheets were exfoliated, the GO/graphite gel become insoluble in water and finally precipitatedfrom the system. Moreover, the volume of gel is largely compressedunder the high pressure and high temperature. The treatmentachieves to firmly block graphite particles in porous structure ofthe gel, which reduces the distance between graphite and N-dopedgraphene sheets. This is very important to improve the electricalconductivity and tap density of the final product.

The GO/graphite hydrogel was dried in air and finally annealedin Ar/H2 to obtain a N-doped G/C composite. In this step, we needto consider two key issues. One is how to keep three-dimensionalstructure of graphene framework. Often, three-dimensionalstructure of GO hydrogel will be completely destroyed duringthe drying due to rapidly evaporation of water. For the reason,many researchers have to adopt freeze-drying for the preparationof GO aerogel. However, the process is time-consuming andenergy-consuming and difficult to be used in the industrialproduction. In the study, the most of space in the hydrogel has beenoccupied by graphite particles. The feature in structure allow us touse a simple method to remove water from the hydrogel. The resultconfirms that its high mechanical strength and three-dimensionalstructure can be well maintained during the drying. Another is howto achieve the best match of fast electron transfer and rapidelectrolyte transport for the composite electrode material. Bythermal annealing, the GO/graphite hydrogel was fully reducedand formed a graphene/graphite aerogel. To evaluate the effect ofgraphene on the electrical conductivity, a four-probe method wasemployed for the electrical conductivity measurement of thecomposite and graphite powder samples. Often, the electricalconductivity of powder sample will increase with increase of thepressure. The electrical conductivity lends to the constant whenexceeded a certain pressure. Our investigation shows that theelectrical conductivity can remain almost unchanged when the

Fig. 3. The SEM images of graphite (a) and N-doped G/C (b) and enlarged SEM o

pressure is more than 80 atmospheres for the composite andgraphite. Thus, the pressure of 100 atmospheres was selected forthe electrical conductivity measurement. The electrical conduc-tivity of N-doped G/C composite was found to be 5912 S m�1, whichis more than 1.4-fold that of pristine graphite (4018 S m�1). This isbecause graphene framework as a particle connector of graphitegreatly improves the electron transfer. In addition, the thermalannealing also contributes to produce a very small gap betweengraphite particles and graphene sheets owing to their difference inthermal expansion coefficient [26]. This will further reduce theblocking of graphene sheets on the electrolyte transport.

3.2. Structure characterization

The morphology and structure of N-doped G/C and graphitewere characterized by SEM-EDS. As shown in Fig. 3, pristinegraphite particles exhibit a smooth surface and clear edges andcorners, indicating a high crystallinity. There only is one carbonpeak on the EDS pattern of graphite, verifying that the graphitesample has a high purity. The N-doped graphene framework ispresent in the N-doped G/C composite, in which graphite particleswere dispersed in the framework. The selected region EDS analysisproves that the framework are composed of carbon (C), nitrogen(N) and oxygen (O) elements. The chemical composition isdifferent from that of pristine graphite and is consistent withthe composition of N-doped graphene, verifying that theframework is composed of N-doped graphene. Owing to uniqueelectrical, mechanical and mechanical properties, such a N-dopedgraphene framework can act as a particle connector to enhance theelectrical conductivity, and an expansion absorber in the anodes oflithium ion battery to relieve the large strains developed at highdischarge C rates. The enlarged SEM images further reveals that thesurface of graphite particles was coated by the wrinkled graphenesheets. The self-assembly of graphene sheets leads to form a three-dimensional graphene framework with a rich of open porousstructure, which will improve the electrolyte transport. In order towell understand the structure of graphene framework, a controlsample (termed as graphene aerogel) was prepared by the sameprocedure unless no addition of graphite. Fig. S5 indicates that all

f N-doped G/C (c) and EDS patterns of graphite and N-doped graphene(d).

160 W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164

of graphene sheets was intertwined each other to form a grapheneaerogel. The aerogel shows a three-dimensional structure withwell-defined porous distribution. The macropores provides suffi-cient space for the entry of graphite particles for the hybridizationof graphene oxide with graphite. The meso- and micropores will beserved as the channel for fast electrolyte transport. The result alsoconfirms that a 3D graphene gel can be formed during thesynthesis of N-doped G/C composite.

Fig. 4 presents typical nitrogen adsorption/desorption isother-mal curves and pore size distribution of the N-doped G/C. The typeIV nitrogen adsorption/desorption isothermal curves offer ahysteresis loop at high relative pressure. The fact demonstratesthe existence of plentiful mesopores in the composite. The poresize distribution exhibits one sharp peak at 2.8 nm and one broadpeak at 12 nm. The broad pore size distribution, spanning fromseveral to 100 nm, implies that the composite is rich in thehierarchical pores. The macropores could originate from theinterconnected hollow space, the micro- and mesopores could begenerated by the wrinkled morphology of graphene sheets.Moreover, we also noted that BET surface area of the as-preparedN-doped G/C reaches 25.6 m2g�1. The value is more than that of thepristine graphite (4.2 m2g�1), indicating that the introduction ofgraphene aerogel increases the specific area. This is because thegraphene aerogel provides a bigger BET surface area whencompared with the pristine [27].

XPS technology was applied to measure the element type andchemical state of N-doped G/C. Fig. 5 shows that total XPSspectrum offers four peaks at 284.58, 531.64, 399.71 and 169.34 eV,respectively. The result shows that the samples contains carbon(C), oxygen (O), nitrogen (N) and sulfur (S) elements. The C1s XPSspectrum indicates four peaks at 284.6, 285, 286.6 and 288.1 eV,respectively. The peaks at 284.4, 286.6 and 288.1 eV are assigned toC��C, C��O and C¼O species respectively [28]. The XPS peak at285 eV is assigned to sp2 C bonded to N. The N1s XPS spectrum hasthree peaks at 398.4, 399.8 and 401 eV, respectively. The peak at398.4 eV can be assigned to N2, corresponding to pyridinic N. Thepeak at 399.8 eV is assigned to amide, amine or pyrrolic N. The peakat 401 eV can be assigned to N4, corresponding to graphitic N. Thisreveals that nitrogen replaced carbon in graphene sheets and wasincorporated into the carbon network [29]. Moreover, we alsonoted that the peak intensity of amide nitrogen was obviouslystronger than that of pyridinic nitrogen and graphitic nitrogen,implying that amide nitrogen was dominant in the as-preparedmaterials. Because pyridinic nitrogen and pyrrolic nitrogen arelocated at the edge of the carbon materials, which will induce moredisorders of the final carbon materials and leads to produce a broad

Fig. 4. Nitrogen adsorption/desorption isotherm (A) and por

D band in the Raman spectrum. These results demonstrates thatwe have successfully achieved nitrogen doping by the thermalannealing.

The pristine graphite and N-doped G/C samples were subjectedto XRD and Raman measurement to evaluate the effect ofgraphene. Fig. 6A shows that two samples exhibit a very similarXRD patterns. The main peaks of 2u at 26.42�, 44.52�, and 54.52�

can be indexed to a typical hexagonal graphite structure (JCPDF no.41-1487). This demonstrates that the hybridization process ofgraphite with graphene did not destroy the crystal structure ofgraphite. However, we also noted that the N-doped G/C gives ahigher diffraction intensity than the pristine graphite. The increasein crystallinity might imply that a small change on the surfacemorphology of graphite takes place, i.e., the graphene surface layeris formed. Fig. 6B presents typical Raman spectra of graphite,graphene and N-doped G/C. Often, Raman spectrum of graphene ischaracterized by two main features: G band arising from the firstorder scattering of E1g phonon of sp2 carbon atoms and D bandarising from a breathing mode of point photons of A1g symmetry.The most of carbon in the graphite exists as sp2 carbon, thus itsRaman spectrum offers the highest G band and the smallest D bandamong three samples. However, carbon atoms in the edge ofgraphene sheets are changed into sp3 carbon during the oxidationprocess. This will result in an obvious decrease of G band and theincrease of D band. From Fig. 6B, we can calculate the R value (theintensity ratio of D band and G band) of the each of carbonmaterials. The result shows that the R value of N-doped G/C (0.54)is greater than that of pristine graphite (0.052) while less than thatof graphene (0.83). The result again demonstrates that theas-prepared N-doped G/C is the mixture of graphite and graphene.

3.3. Electrochemical property

Fig. 7 shows the cyclic voltammograms (CVs) of N-doped G/Celectrode and graphite electrode from 0 V to 2.0 V. Two kinds ofelectrodes offer a very similar shape of CV curve. The each CV curvecan be analyzed into two sections. One section from 0.5 V to 2.0 V iscorresponding to the formation of SEI film on the interferebetween the electrode and the electrolyte [30]. Because theformation of SEI film leads to some fading of active sites, the peakarea of CV from 0.5 V to 0.0 V in the second cycle is obviously lessthan that in the first one. Moreover, we also find that the reducingpeak potential of N-doped G/C electrode can remain almostunchanged in two cycles. This is because the graphite surfaces inthe N-doped G/C are well coated by graphene sheets. This willeffectively inhibit the formation of the SEI film and improves the

e-size distribution curve for N2 (B) of the N-doped G/C.

0

20000

40000

60000

80000

100000

120000

0 200 400 600 800 1000

Cou

nts

(s)

Binding energy (eV)

C1s

N1s O1s

Total

S2p

0

10000

20000

30000

40000

50000

60000

70000

280 283 286 289 292 295

Cou

nts

(a.u

)

Binding energy (eV)

C1s

C-C

C-N

C-O

C=O

0

1000

2000

3000

4000

5000

6000

7000

395 398 401 404 407 410

Cou

nts

(s)

Binding energy (eV)

N1s

0

1000

2000

3000

4000

5000

6000

525 528 531 534 537 540

Cou

nts

(s)

Binding energy (eV)

O1sO-C

O=C

Fig. 5. Total, C1s, N1s and O1s XPS spectra of the N-doped G/C.

0

300

600

900

1200

1500

10 20 30 40 50 60 70

Inte

nsit

y (a

.u)

2θ (degree)

graphite

N-doped G/C

A

0

4000

8000

12000

16000

20000

500 1000 1500 2000 2500 3000

Inte

nsit

y (a

.u)

Raman shift (nm)

graphit e

graph ene

N-doped G/C

B

Fig. 6. XRD pattern (A) and Raman spectra (B).

W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164 161

-15

-10

-5

0

5

10

15

0 0.4 0.8 1.2 1.6 2

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rent

(mA

cm

-2)

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first/second-cylce

-15

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-5

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5

10

15

0 0.4 0.8 1.2 1.6 2

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rent

(mA

cm

- 2)

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B

first/second-cycle

Fig. 7. CV curves of N-doped G/C electrode (A) and graphite electrode (B) at 0.1 mV/s.

Table 1Impedance parameters of the graphite and N-doped G/C electrodes after 50 cyclesat 0.2C in 25 �C

Anode material Rs (V) Rf (V) Rct (V) D (cm2 s�1)

Graphite 133.1 1644.2 4234.5 4.24 �10�16

N-doped G/C 26.5 28.0 134.4 6.01 �10�12

162 W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164

electrochemical performance. However, the reducing peak poten-tial of graphite electrode increases from 0.07 V to 0.1 V, indicating arelatively poor electrical conductivity. Another section from 0.0Cand 0.5 V is corresponding to Li+ intercalation/deintercalationprocess. Fig. 7 shows that the CV curve of each electrode gives apair reversible oxidation/reduction peak in the second section ofCV each curve. The peak potential of N-doped G/C is similar withcorresponding graphite electrode, verifying that the introductionof graphene does not change the electrochemical process ofgraphite material. However, the N-doped G/C electrode shows alower peak potential and higher peak current when compared withgraphite electrode, indicating a faster Li+ intercalation/deinterca-lation process. These improved CV performances are attributedthat the introduction of graphene enhances the electricalconductivity and the formed SEI film.

Fig. 8 shows typical impedance spectra of N-doped G/C andgraphite electrodes in the fully discharged state after the first,second and fiftieth cycles at 0.2C rate. For the each of electrodes,the overall shape of Nyquist plot is composed of a depressedsemicircle at high frequency region and a straight line at lowfrequency region. The intercept of the depressed semicircle athigh-frequency region on the real axis can be assigned to the totalinterfacial resistance, which consists of the resistance of charge

Fig. 8. AC impedance spectra of N-doped G/C electrode (A) and graphite elec

transfer at the solid film interface and the resistance of Li+

migration through SEI film [31]. The straight line is attributed tothe diffusion of the Li+ into the bulk of the electrode material [32].Fig. 8 shows that total interfacial resistance of the electrode in thesecond cycle are smaller than that in the first cycle, verifying thatthe formed SEI film improves the electrical conductivity. However,total interfacial resistance in the fiftieth cycle is slightly smallerthat in the second cycle, indicating high stability of the SEI film. Anequivalent circuit of the impedance spectra is presented in theinset in Fig. 8, which Rs, Rf and Rct, are denote the solutionresistance, the contact resistance and the charge-transfer resis-tance, respectively [33]. The parameters and the diffusioncoefficient of Li+ (D) in the fiftieth cycle are calculated by usinga reported method [34]. Table 1 indicates that the N-doped G/Coffers a lower Rct value and a larger Li+ diffusion coefficientcompared with he graphite. This demonstrates that the presence of

trode (B) with the equivalent circuit from the EIS measurements (inset).

Fig. 10. Cyclic performance of N-doped G/C electrode at 10 C.

W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164 163

graphene can significantly suppress the rise of both of the surfacefilm resistance and the charge transfer resistance, which willimprove the Li+ diffusion in the SEI film and the bulk electrodematerials.

3.4. Cell performance

Fig. 9 shows the charge/discharge curves of N-doped G/C, G/Cand graphite electrodes at a potential range of 0.0–2.0 V withdifferent rate from 0.1 C to 10 C. It can be seen that the N-doped G/Celectrode exhibits the highest specific capacity at the each of ratesamong four electrodes. Its discharge capacity reaches 781 mA h g�1

at 0.1 C, which is more than 2-fold that of theoretical capacity ofgraphite electrode (372 mA h g�1). The enhanced capacity isattributed to the introduction of N-doped graphene, because thespecific capacity of graphene can excess 1500 mA h g�1 for lithiumion batteries [34]. The capacity will reduces to 737 mA h g�1 after afive cycles. The reduced capacity mainly originates from somefading of active sites on the electrode due to the formation of SEIfilm. The reversible capacity can remain 351 mA h g�1 at 10 C,indicating an excellent rate capability. This is because theintroduction of N-doped graphene enhances the electricalconductivity of electrode material and SEI film, which enhancesthe rate capability. In the study, N-doped GO/C was used as acontrol sample to examine the effect of thermal annealing. Fig. 9shows that the N-doped GO/C electrode can only provide a slightlybigger specific capacity at all charge/discharge rate whencompared with the graphite electrode, which is largely smallerthan that of the N-doped G/C and G/C electrodes. The resultdemonstrates that the thermal annealing is very important toimprove the electrochemical performance. On the one hand, arelatively large number of active groups in the N-doped GO/C cancombine with Li+ to form stable compound during the chargeprocess. This will bring a high irreversible capacity and thedecrease of reversible capacity. On the other hand, its poorelectrical conductivity will lead to a poor rate capacity. Moreover,the graphene oxide sheets in N-doped GO/C tightly wrapped on thesurface of graphite particles. This partly blocks the electrolytetransport and results in the decrease of specific capacity, especiallyspecific capacity at high rate. In the study, the G/C sample was usedas a control to evaluate the effect of nitrogen doping. Fig. 9 showsthat the G/C electrode provides a lower specific capacity at the each

Fig. 9. Charge/discharge curves of N-doped G/C, G/C, N-doped GO/C and graphiteelectrodes at various discharge rate (from under to down).

of all rate when compared with the N-doped G/C electrode. Theresult confirms that the nitrogen doping plays an important role inthe improvement of cell performance. The introduction of nitrogenatom in the graphene sheet increases the polarity of graphenematerial due to its big electronegativity, and destroy some planarstructure of graphene sheets owing to its sp3 nitrogen atom. Theseare beneficial to improve mass transport of the electrolyte andresults in an enhanced electrochemical performance.

The cyclic performance is very important to practical applica-tion of electrode materials for lithium ion batteries. Fig.10 presentscontinual galvanostatic charge/discharge 1000 tests for theN-doped G/C electrode with potential window of 0.0–2.0 V at10 C. The discharge capacity was 365 mA h g�1 in the first cycle.The capacity can remain 359 mA h g�1 after 1000 charge/dischargecycles. The capacity loss is about 2%, indicating an excellentcycle stability. This is attributed that the graphene framework inthe composite acts as an expansion absorber in the anodes oflithium ion battery to relieve the large strains developed at highdischarge C rates. This is beneficial to keep electrical contactbetween the host particles during the repeated intercalation/deintercalation.

4. Conclusions

We have demonstrated the fabrication of N-doped graphene/graphite anode material for lithium ion batteries. Since three-dimensional N-doped graphene framework greatly improveselectrical conductivity of the electrode material, mass transportof the electrolyte and structure stability of the electrode, the N-doped graphene/graphite electrode exhibits an enhanced electro-chemical performance for lithium ion batteries. The study alsoprovides an attractive approach for building on the graphite-basedanode materials for high power storage devices.

Acknowledgements

The authors acknowledge the financial support from Prospec-tive Joint Research Project: Cooperative Innovation Fund (No.BY2014023-01), the country “12th Five-Year Plan” to supportscience and technology project (No. 2012BAK08B01), NationalNatural Science Foundation of China (No. 21176101), FundamentalResearch Funds for Central Universities (No. JUSRP51314B) andMOE & SAFEA for the 111 Project (B13025).

164 W. Guanghui et al. / Electrochimica Acta 171 (2015) 156–164

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.electacta.2015.05.016.

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