Anode performance of boron-doped graphites prepared from shot and sponge cokes

Anode performance of boron-doped graphites prepared from shot and sponge cokes
Anode performance of boron-doped graphites prepared from shot and sponge cokes

Journal of Power Sources 195 (2010) 1714–1719

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Journal of Power

Sources

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j p o w s o u

r

Anode performance of boron-doped graphites prepared from shot and sponge cokes

Tao Liu a ,Ruiying Luo a ,?,Seong-Ho Yoon b ,??,Isao Mochida b

a School of Science,Beihang University,Beijing 100083,China

b

Institute for Materials Chemistry and Engineering,Kyushu University,Kasuga,Fukuoka 816-8580,Japan

a r t i c l e i n f o Article history:

Received 4August 2009

Received in revised form 29August 2009Accepted 31August 2009

Available online 26 September 2009Keywords:Shot coke Sponge coke

Boron-doped carbons Graphitization

Electrochemical properties

a b s t r a c t

The structures and anode performances of graphitized pristine and boron-doped shot and sponge cokes have been comparatively studied by means of scanning electron microscope (SEM),X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS)and galvanostatic measurement.The results show that high degree of graphitization can be obtained by the substituted boron atom in the carbon lattice,and boron in the resultant boron-doped graphites mainly exist in the form of boron carbide and boron substi-tuted in the carbon lattice.Both of boron-doped graphites from shot and sponge cokes obtain discharge capacity of 350mAh g ?1and coulombic ef?ciency above 90%.Apart from commonly observed discharge plateau for graphite,boron-doped samples in this study also show a small plateau at ca.0.06V.This phenomenon can be explained that Li ion stores in the site to be void-like spaces that are produced by “molecular bridging”between the edge sites of graphene layer stack with a release of boron atoms substituted at the edge of graphene layer.The effect of the amount of boron dopant and graphitization temperature on the anode performance of boron-doped graphite are also investigated in this paper.

? 2009 Elsevier B.V. All rights reserved.

1.Introduction

Synthetic graphite has been well applied to the anodic mate-rial of commercial Li ion battery due to its large reversible capacity and long plateau in the low voltage [1–6].It is usually prepared from graphitizable carbon such as needle coke,meso-carbon microbeads (MCMB)and mesophase pitch-based carbon ?bers (MPCF)through graphitization above 2800?C.Such high temperature heat-treatment incurs high cost in mass produc-tion.

It has been well established that,boron,as a graphitization cata-lyst,can enhance the graphitization of carbons.Thus,Boron doping can help graphitizable carbon obtain the high degree of graphi-tization under a lower graphitization temperature.Furthermore,boron doping can modify the electronic properties of the host car-bon materials due to boron atom substituted for carbon atom in the carbon lattice [7–18].Theoretically,the presence of boron should be bene?cial for lithium intercalation/de-intercalation behavior of carbon material,because the substituted boron atom acts as an electron acceptor.Many studies on the boron doping into MCMB [10,11],MPCF [12–14]and pitch coke [15–17]have approved

?Corresponding author.Tel.:+861082338267;fax:+861082338267.??Corresponding author.Tel.:+81925837801;fax:+81925837798.

E-mail addresses:ryluo@https://www.360docs.net/doc/d36535131.html, (R.Luo),yoon@cm.kyushu-u.ac.jp (S.-H.Yoon).

that boron doping can improve the discharge capacity and initial coulombic ef?ciency.

Shot and sponge cokes are byproducts of delayed coking oper-ations that use petroleum feedstocks to produce valuable needle coke [19,20].Both of them are anisotropic in nature.Shot coke is an undesirable product in the form of small spheres and no commer-cial application has been identi?ed.Sponge coke can be used for fuel or to manufacture carbon anodes for electrolytic aluminum pro-duction.Therefore,compared with other currently used precursor materials,shot and sponge cokes are attractive in their low cost.

In this study,graphitized shot and sponge cokes and boron-doped graphites were characterized by X-ray diffraction (XRD)and X-ray photoelectron spectroscopy (XPS).Their anode performances were investigated by galvanostatic measurement.Furthermore,the effects of the amount of boron dopant and graphitization temper-ature on the anode performances of boron-doped graphites were also studied.

2.Experimental 2.1.Materials

Shot coke and sponge coke were used in this study,and their compositions of as-received forms are summarized in Table 1.

After grinding into the size of less than 75?m,shot and sponge cokes were calcined at 1300?C (abbreviated as SHC13

0378-7753/$–see front matter ? 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jpowsour.2009.08.104

T.Liu et al./Journal of Power Sources 195 (2010) 1714–1719

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Table 1

Industrial analysis and elemental analysis of as-received shot and sponge cokes (wt.%).Cokes Fixed carbon Volatile Ash Water C H N Diff.SHC 88.7411.080.18 1.2688.54 3.93 3.06 4.47SPC

89.26

10.21

0.53

5.40

90.09

3.66

2.31

3.94

and SPC13,respectively),and then were graphitized at 2400and 2800?C,respectively,under Ar atmosphere (abbreviated as SHC24,SHC28,SPC24and SPC28).Boric acid powder (H 3BO 3)was mixed with SHC13and SPC13by 20wt.%,respectively,and then were heat-treated at 800?C for 1h under Ar atmosphere,followed by graphitization at 2800?C under Ar atmosphere (abbre-viated as SHCB28-2and SPCB28-2,respectively).To investigate the effect of the amount of boron dopant on the anode perfor-mances of the resultant boron-doped graphites,H 3BO 3was also mixed with SHC13by 10and 30wt.%followed by the same treat-ment as SHCB28-2,and the resultant boron-doped graphites were abbreviated as SHCB28-1and SHCB28-3.SHCB24-2/SPCB24-2was prepared by graphitizing the mixture of SHC13/SPC13and 20wt.%H 3BO 3at 2400?C to compare with SHCB28-2/SPCB28-2.2.2.Characterization of samples

The microtexture of the prepared samples was studied by a scanning electron microscope (SEM;JSM-6700F JEOL,Japan).The crystallographic data were collected using an X-ray diffractometer (Ultima III Rigaku,Japan)with Cu K ?( =0.15406nm)radiation,and the crystallographic parameters were calculated according to the revised Gakushin method [21].The standard silicon powder (200Mesh,99.99%,Soekawa Chemical Co.,Japan)was mixed with each sample to be measured by 10wt.%as an internal standard.The chemical state of boron and carbon in the boron-doped samples was analyzed using an X-ray photoelectron spectroscopy (JPS-9010MC JEOL,Japan)with Mg K ?radiation (1253.6eV).The C 1s peak of car-bon was used as a reference for the chemical shift determination,assuming that its binding energy is 284.2eV.2.3.Measurement of anode performance

Test electrode was fabricated by spreading a slurry mixture of active material (90wt.%)and PVdF (polyvinylidene ?uoride,

10wt.%)dissolved in NMP (1-methyl-2-pyrrolidone)solution onto the copper foil,and rolling it after drying under vacuum for 12h at 120?C.Two-electrode test cell was assembled by using lithium foil as the counter electrode,1M LiPF 6in EC/DEC (1:1,v/v)as the elec-trolyte and polyethylene ?lm as the separator.The cell assembly was carried out in an argon-?lled glove box.

Electrochemical evaluation was performed using TOSCAT-3100battery testing unit (TOYO SYSTEM CO.LTD.,Japan).The carbon electrodes were ?rst charged from open circuit voltage to 0V at 30mA g ?1,and then constant voltage (CV)charging was applied at 0V until current decreased to 3mA g ?1.Constant current (CC)discharging was performed at 30mA g ?1until 2V.

3.Results

3.1.Characterization of as-prepared samples

Fig.1shows SEM images of as-received shot and sponge cokes,SHCB28-2and SPCB28-2.Microtexture of cokes evidently changes from bulky form to a highly developed plate-like texture with par-allel alignment of the https://www.360docs.net/doc/d36535131.html,pared with boron-doped graphite from sponge coke,more byproducts,which are boron carbide according to XRD and XPS results later,are observed on the surface of boron-doped graphite from shot coke.

Fig.2shows XRD patterns of graphitized shot and sponge cokes and boron-doped graphites.As graphitization temperature increases,the (002),(004)and (006)peaks shift to a higher diffraction angle,in corresponding to the decrease of interlayer spacing,d 002.The (101),(103)and (112)peaks become clearer,which indicate a more developed three-dimensional stacking layer structure of https://www.360docs.net/doc/d36535131.html,pared with undoped samples,boron-doped samples also show a similar tendency in spite of the same graphitization temperature.It implies improvement of graphite crystallinity with boron doping.In addition,the peaks attributed to B 4C are observed in the range of 30–40?for SHCB28-2,while they do not appear in XRD pattern of SPCB28-2,which is in consistence with their SEM images.

The calculated lattice sizes of as-prepared samples are shown in Table 2.Boron-doped sample shows a lower d 002and a growth in crystallite thickness (L c(002))and lattice constant (a 0).The

decrease of d 002is thought to be related to the depleted p-electrons between graphite layers [13].The increase of a 0is because B–C bond caused

Fig.1.Low and high magni?cation SEM images of as-received shot coke (a and b),as-received sponge coke (c and d),boron-doped shot coke (e and f)and boron-doped sponge coke (g and h)graphitized at 2800?C.

1716T.Liu et al./Journal of Power Sources

195 (2010) 1714–1719

Fig.2.XRD patterns of graphitized and boron-doped shot (a)and sponge (b)cokes.

by boron atom substituted in the carbon lattice had a longer bond length than C–C bond.It supports the result calculated through molecular simulation method by Endo et al.[8].Furthermore,a 0monotonously increases with an increasing amount of boron-containing compound,which indicates more substituted boron atoms.On the other hand,for shot coke,the crystallite width (L a(110))of boron-doped sample is lower than that of undoped sam-ple,while for sponge coke,L a(110)of the boron-doped sample is higher.

Fig.3shows XPS C 1s and B 1s spectra of graphitized shot and sponge cokes and boron-doped graphites.For both types of cokes,C 1s peaks of boron-doped samples are located at a slightly lower binding energy compared with those of boron-undoped samples.The reduction of binding energy of C 1s peak for boron-doped sam-ple might be caused by the reduction of the Fermi level due to the redistribution of ?-electrons within the graphite layers [22,23].As expected,the B 1s peaks appear in boron-doped samples,but their forms and positions are different and determined by the sample,the amount of boron-containing compound added and graphitization temperature.SHCB28-1,similar to SPCB28-2,only has one B 1s peak near 185.7eV,attributed to boron in boron car-bide.While for SHCB28-2and SHCB28-3,apart from B–C peak,the other B 1s peak is also observed at 187.3eV,which is assigned to boron substituted in carbon lattice according to the results reported in the previous literature [24].In addition,when the graphitization temperature increases from 2400to 2800?C,B 1s peak attributed to boron substituted in carbon lattice is reduced largely or disap-Table 2

The calculated lattice size of as-prepared samples (nm).Sample d (002)L c(002)L a(110)a 0

SHC240.337891650.24613SHC280.33671401160.24613SHCB28-10.3361158990.24636SHCB28-20.3355214830.24651SHCB28-30.3356218850.24654SPC240.338881720.24608SPC280.3365183910.24615SPCB24-20.33602061040.24649SPCB28-2

0.3356

225

108

0.24643

pears.It indicates that the substituted boron atom begins to leave its substitutional site above 2400?C.

3.2.Anode performances of as-prepared samples

Fig.4and Table 3show the ?rst charge/discharge pro?les,spe-ci?c capacities and initial coulombic ef?ciencies of as-prepared samples.First of all,both cokes calcined at 1300?C show the charge/discharge characteristic of soft carbon,i.e.,small charge capacities in the CV stage and no discharge plateau at around 0.1V.After graphitization at 2400?C,de?nite plateaus in the volt-age range 0–0.3V,like graphite,are observed,although discharge capacities are almost the same as those of calcined cokes.With the increase of graphitization temperature,discharge capacities of cokes are improved.Especially for sponge coke,discharge capacity of SPC28increases over 70mAh g ?1compared with that of SPC24.For the boron-doped graphites,both of them obtain discharge capacity of about 350mAh g ?1and initial coulombic ef?ciency above 90%.The increase of discharge capacities is because of a well developed graphite crystallinity caused by boron doping.

To better understand the effect of boron doping in carbon mate-rials,differential capacitance (d Q /d V )plots for the ?rst and second charge/discharge cycles are shown in Fig.5.For SHC13and SPC13,only peak A corresponding to the discharge plateau at ca.0.05V,as shown in Fig.4,can be observed,while for cokes graphitized over 2400?C,four peaks can be obviously observed.Their charge and discharge d Q /d V plots are basically corresponding to each other,and the corresponding peak is labeled with the corresponding cap-ital and lowercases.In charge,the peaks in the d Q /d V plots for the second cycle shift to a higher potential compared with the peaks in the d Q /d V plots for the ?rst cycle,and peak D is more evident.In dis-charge,the peaks in the d Q /d V plots for the ?rst and second cycles are basically superposed.Peaks B,C and E can be observed with cokes graphitized at 2800?C,and they correspond to the discharge plateaus at ca.0.1,0.14and 0.22V in Fig.4,respectively.These three peaks are commonly observed with graphite,and they are assigned based on the literature to a coexistence of stages 1and 2,a coex-istence of stages 2and 2L (liquid-like stage 2)and a coexistence of stages 4and 1 (dilute stage 1),respectively.Peak D,accounted as

T.Liu et al./Journal of Power Sources195 (2010) 1714–1719

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Fig.3.XPS B1s and C1s spectra of graphitized and boron-doped shot(a and b)and sponge(c and d)

cokes.

Fig.4.The?rst charge/discharge pro?les of calcined,graphitized and boron-doped shot(a)and sponge(b)cokes.intercalation into turbostratic disordered layers[4],is observed in the discharge pro?le of graphitized cokes.Furthermore,intensity of peak D reduces with the increase of graphitization tempera-ture,and disappears in boron-doped graphites.It indicates that the degree of graphitization increases in the sequence of cokes graphi-tized at2400?C,cokes graphitized at2800?C and boron-doped cokes graphitized at2800?C,which is in consistence with the result of XRD.On the other hand,the peaks of boron-doped graphites are located at a higher potential compared with the corresponding peaks of graphitized cokes except peak A.

It is interesting to note that a well distinct peak A appears at ca.0.06V in the d Q/d V plots of cokes graphitized at2400?C and boron-doped graphites.This peak commonly corresponds to Li ion stored in the site to be void-like spaces.This kind of site is supposed to be produced by“molecular bridging”between the edge sites of graphene layer stack with a release of boron atoms substituted at the edge of graphene layer.Furthermore,with the increase of the heat-treatment temperature,such molecular bridg-ing is expected to further proceed to ultimately produce isolated voids or interlayer spaces[25].Therefore,peak A observed in the d Q/d V plots of cokes graphitized at2400?C cannot be observed in the d Q/d V plots of cokes graphitized at2800?C.However,for the boron-doped graphites,when the graphitization temperature is higher than2400?C,as mentioned above,the substituted boron atom begins to leave its substitutional site,especially for the sub-stituted boron atom existing at the edge of graphene layer,thus forms the Li ion storage site corresponding to peak A.

Fig.6shows the?rst charge/discharge pro?les of graphitized shot cokes mixed with different amount of boron-containing compound.In spite of different mixing ratio,the resultant boron-

Table3

Speci?c capacities and initial coulombic ef?ciencies of as-prepared samples.

Sample Speci?c capacities(mAh g?1)Coulombic ef?ciency(%)Sample Speci?c capacities(mAh g?1)Coulombic ef?ciency(%) Charge Discharge Charge Discharge

SHC13355.5271.676.4SPC13322.2263.881.9

SHC24285.8270.394.6SPC24261.3247.994.9

SHC28396.4313.779.1SPC28352.6324.992.1

SHCB28-2387.0350.390.5SPCB28-2382.3350.091.6

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195 (2010) 1714–1719

Fig.5.Differential capacitance (d Q /d V )plots for the ?rst and second charge/discharge cycles of calcined,graphitized and boron-doped shot (a)and sponge (b)cokes.

doped graphites have the same discharge capacities in the range of 0–0.4V.Nevertheless,with the increasing amount of boron-containing compound,charge/discharge plateaus shift toward a higher potential,because the presence of substituted boron strengthens the chemical bond between the intercalated Li and the boron–carbon host compared to the pure carbon host [1].In addition,the initial coulombic ef?ciency is reduced with the increase of mixing ratio,due to the increasing amount of B 4C formed,which is inactive for Li ion

intercalation.

Fig.6.The ?rst charge/discharge pro?les of boron-doped shot cokes with different amount of boron dopant graphitized at 2800?

C.

Fig.7.The relationship between discharge capacity and graphitization temperature of boron-doped cokes.

Fig.7shows the relationship between discharge capacity and graphitization temperature of boron-doped cokes.Discharge capacity of SHCB24-2and SPCB24-2are only smaller for about 5mAh g ?1than that of SHCB28-2and SPCB28-2,but larger than that of SHC28and SPC28.It means that boron doping makes cokes even graphitized at 2400?C obtain a perfect graphite crystal struc-ture.Therefore,graphitization temperature of synthetic graphite can be decreased from over 2800to 2400?C by boron doping.The reduction of electric power consumption caused by this is favorable to reduce the preparation cost of anode material.4.Conclusion

Boron-doped graphites have been prepared from shot and sponge cokes by mixing with boric acid in this study,and they show obvious plate-like structure.High degree of graphitization was obtained by the substituted boron atom in the carbon lattice.Boron in the resultant boron-doped graphites mainly exist in the

T.Liu et al./Journal of Power Sources195 (2010) 1714–17191719

form of boron carbide and boron substituted in the carbon lattice according to the result of XPS measurement.

Both of boron-doped graphites from shot and sponge cokes obtained discharge capacity of350mAh g?1and initial coulombic ef?ciency above90%.Apart from commonly observed discharge plateau for graphite,boron-doped samples in this study also showed a small plateau at ca.0.06V,which is correspond to Li ion storage site to be void-like spaces that are produced by“molec-ular bridging”between the edge sites of graphene layer stack with a release of boron atoms substituted at the edge of graphene layer.With the increasing amount of boron-containing compound, charge/discharge curves shift toward the higher potential,and ini-tial coulombic ef?ciency is reduced due to the increasing amount of B4C.On the premise of near discharge capacity,graphitization tem-perature of synthetic graphite can be decreased from over2800to 2400?C by boron doping.

In conclusion,boron-doped graphite from shot and sponge cokes is attractive for its application to anode materials for Li ion battery due to its low cost.

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