Journal of Power Sources 200 (2012) 108 112

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Journal of Power Sources

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Short communication

Feasibi ercelectro ors

Hongyu a State Key Lab ese Acb Advanced Res

a r t i c l

Article history:Received 7 SepReceived in reAccepted 24 OAvailable onlin

Keywords:Quaternary alkGraphitePropylene carbElectrochemicIntercalation

ite/ad. Theand thely. Tentsg theum d

contsion ropo

1. Introduction

In the big family of electric energy storage systems, electro-chemical capacitors are becoming more and more attractive inmany applitors (EDLCsto meet thties, hybrid drawing modeveloped aelectrode, gsay, TEMAPpropylene citors demoas compareEDLCs). If wpoint, we cconsisting electrode. Cgraphite nealkyl ammoAC positivequestion w

CorresponE-mail add

(H. Wang).

ammonium-based organic electrolytes can be used in practicalapplications aroused our research interest.

Of course, the attempt of graphitic carbon/AC capacitors usingLi+-based electrolytes proved promising [6]. Here the charge stor-

0378-7753/$ doi:10.1016/j.cation elds. Traditional electric double-layer capaci-) still prevail in most of the application scope. However,e needs of both high power and high energy densi-capacitors with an asymmetric conguration have beenre and more research attention in recent years. We havectivated carbon (AC)/graphite capacitors (AC negativeraphite positive electrode) using organic electrolytes,F6 (TEMA, triethylmethyl ammonium) dissolved inarbonate (PC) [15]. This kind of asymmetrical capac-nstrate more promise for high energy density utilityd with the symmetrical AC/AC capacitors (commone look at the asymmetric capacitors from another view-an get a new kind of capacitors. That is a capacitorof a graphite negative electrode and an AC positiveonsequently, the charge storage mechanism at thegative electrode involves the insertion of quaternarynium (like TEMA+) cation into graphite, while that at the

electrode relates to the adsorption of PF6 anion. Thehether graphite/AC capacitors using quaternary alkyl

ding author. Tel.: +86 431 85262287; fax: +86 431 85262287.resses: hongyuwang@ciac.jl.cn, wanghongyu@hotmail.com

age mechanism at the graphitic carbon negative electrode involvesthe Li+ intercalation while at the AC positive electrode relates toanion adsorption. This capacitor system has the operating volt-age over 4 V. Moreover, the graphite negative electrode deliversa high capacity of charge storage. However, there are two big prob-lems with this capacitor system using Li+-based electrolytes. Onone hand, the risk of Li metal deposition on the negative car-bon electrode cannot be avoided, which can cause the explosionof capacitor. On the other hand, the incompatibility between thegraphitic carbon and the Li+PC electrolytes is also a severe problem[7,8].

Quaternary alkyl ammonium-based electrolytes obviouslyexclude these two shortcomings of Li+-based electrolytes. At rst,no metal deposition can take place during charge process becausethe electrolytes are free of metal ions in the capacitors. Moreover,since the quaternary alkyl ammonium cations are not so tightlybonded with solvent molecules in the electrolytes as Li+ [5], theco-intercalation of solvent molecules (like PC) together with thecations into graphite is unlikely to happen. Therefore the dras-tic decomposition of the solvent molecules in the interlayer spacebetween the graphene layers and the great exfoliation of graphitecan hardly occur at low potentials of graphite electrode.

In fact, some previous investigations have been performed onthe electrochemical intercalation of quaternary alkyl ammonium

see front matter 2011 Elsevier B.V. All rights reserved.jpowsour.2011.10.086lity of quaternary alkyl ammonium-intde materials in electrochemical capacit

Wanga,, Masaki Yoshiob

oratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinearch Center, Saga University, 1341 Yoga-machi, Saga 840-0087, Japan

e i n f o

tember 2011vised form 22 October 2011ctober 2011e 29 October 2011

yl ammonium

onateal capacitors

a b s t r a c t

The electrochemical capacitor of graphorganic electrolytes has been proposeinto pores of the AC positive electrode graphite negative electrode, respectivinvestigated by in situ XRD measuremhas been intensively addressed. Amonthe biggest cation tetrabutyl ammonigraphite, but the best cycle-ability. Byinto graphite gives the smallest expanof capacitor. A mechanism has been p/ locate / jpowsour

alated graphite as negative

ademy of Sciences, 5625 Renmin Street, Changchun 130022, China

ctivated carbon (AC) using quaternary alkyl ammonium-based charge storage mechanisms involve the adsorption of anionse intercalation of ammonium cations into crystal lattice of thehe intercalation processes of ammonium cations have been

on graphite negative electrodes and the effect of cation size four quaternary alkyl ammonium cations, the intercalation ofemonstrates the largest degree of crystal lattice expansion ofrast, the insertion of second big cation tetraethyl ammoniumdegree of graphite crystal lattice, but poor cycle performancesed to explain the above phenomenon.

2011 Elsevier B.V. All rights reserved.

H. Wang, M. Yoshio / Journal of Power Sources 200 (2012) 108 112 109

cations into graphite [915]. The consensus appeared that sincequaternary alkyl ammonium is large enough to expand the inter-layer spaces between graphene layers signicantly, the drasticvolume expansion may cause big stress in graphite particles andthus deteribelieve thiseffect of quasibility of gelectrochemsurements.electrolytes

2. Experim

PW15Mfrom Timcaelectrode mproperties the past regenerally kAC (PW15MThe electro1.5 M X-BFcell fabricawere the sagalvanostatperformed cut-off voltof the totacapacity (mto the followtime for chweight of A

Computage was pmolecular orbital) eneThe geometoptimized wtheory (DFTtional and basis set wa

3. Results

The cyccapacitors uFig. 1. The drops downof AC/graphthat the gratry. This shgraphite elmatter of fait is expecteelectrode cthan that of

In situ XRthe charge Fig. 2 showelectrode inTBABF4PCno change ing the charpeak appeavoltage, thi

ycle performance of the graphite/AC and AC/graphite capacitors using thete of 1 M TEMABF4PC.

tion angles. At the same time, the intensity of (0 0 2) peak ofte becomes weaker and two new diffraction peaks appear at

diffraction angles. One is close to the (0 0 2) peak of graphite.r weak peak locates at diffraction angles higher than 30.new diffraction peaks provide a profound evidence of theformation of the cation-graphite intercalation compound

charge process. The electrolytes of 1 M DEDMABF4 andF4PC give series of in situ XRD patterns of graphite negativede similar to Fig. 2. Here DEDMA represents diethyldimethylnium.contrast, the in situ XRD result of graphite negative elec-using the electrolyte of 1 M TEABF4PC has big difference

n situ XRD patterns of the graphite negative electrode in the initial chargeof a graphite/AC capacitor using the electrolytes of 1 M TBABF4PC. Ther voltages rise in the following order: OCV (open circuit-voltage), 1.9 V, 2.3 V,

V, 3.1 V, 3.3 V, 3.4 V, 3.5 V.orate the cycle performance of graphite electrode. We anticipation lacks the systematic investigations on theternary alkyl ammonium cations. In this paper, the fea-raphite/AC capacitors has been investigated by typicalical methods in connection with the in situ XRD mea-

The inuence of quaternary alkyl ammonium-based has been addressed in detail.

ental

13130 (AC from Kureha Co. Ltd.) and KS6 (graphitel Co. Ltd.) were chosen as the positive and negativeaterials for capacitors, respectively. Some physical

of PW15M13130 and KS6 have been described inports [2,3]. The weight ratio of graphite to AC wasept at 1. From the pore size distribution result of the13130), most of the pores have the size near 1 nm.

lytes employed in this study were denoted as 1 M or4 dissolved in PC (X stands for the cation type). Thetion, glove box conditions and in situ XRD proceduresme as those described in the past reports [25]. Theic chargedischarge tests of the coin cell were generallyat the constant current density of 0.4 mA cm2. Theages were set as 0 and 3.5 V. Charge storage abilityl capacitor (coin cell) was expressed in the terms ofAh g1). The capacity values were calculated accordinging formula: Q = IT/w+ (I, constant current (mA); T, the

arge or discharge between cut-off voltages (h); w+, theC positive electrode (g)).ational calculation with the Gaussian09 program pack-erformed to evaluate the HOMO (highest occupiedorbital) and LUMO (lowest unoccupied molecularrgy values of these quaternary alkyl ammonium cations.ries of quaternary alkyl ammonium cations were fullyithout symmetry constraints by the density functional) method, with hybrid Becks three-parameter func-LeeYangParr functional (B3LYP). The 6-311G (d,p)s used for all the atoms.

and discussion

le performance of the graphite/AC and AC/graphitesing the electrolyte of 1 M TEMABF4PC is shown in

discharge capacity delivered by graphite/AC capacitor more remarkably with cycles as compared with thatite capacitor. The poor cycle-ability seems to provephite/AC capacitor is not applicable in practical indus-ortcoming probably comes from the deterioration ofectrode as suggested in the previous study [15]. As act, the ionic size of TEMA+ is bigger than that of BF4. Sod that the intercalation of TEMA+ into the of graphite

ause more severe volume expansion in crystal lattice BF4.D measurement is a powerful technique to investigatestorage mechanism of ions in the graphite electrode.s the in situ XRD patterns of the graphite negative

the graphite/AC capacitor using the electrolyte of 1 M (TBA stands for tetrabutyl ammonium). There is almostin the (0 0 2) peak of graphite (26.5) until 2.7 V dur-ge process. As the cell voltage rises up to 2.9 V, a smallrs at a lower diffraction angle. With the increase in cells peak grows up in the intensity and shifts to lower

Fig. 1. Celectroly

diffracgraphihigherAnotheThese stage duringTEMABelectroammo

By trode

Fig. 2. Iprocess capacito2.7 V, 2.9

110 H. Wang, M. Yoshio / Journal of Power Sources 200 (2012) 108 112

Fig. 3. In situ process of a gcapacitor volta3.3 V, 3.5 V, 3.7

with those celectrolytesFig. 3 showtrode in thTEABF4PCchange in tdecrease inage rises upthe previoutwo broad dof graphiteThese two respectivelylittle strongthe (0 0 2) pdiffraction ppatterns is sWe deliber3.9 V to mak

peak of graphite at 26.5 does not exist in the XRD pattern. Instead,two broad peaks can be clearly observed at about 24.2 and 30.4,respectively.

In the case of 1.5 M LiBF4PC electrolyte, the appearance of newpeaks can be hardly seen during the charge process although wehave raised the cell voltage to 4.1 V, only the irreversible decreasein the intensity of graphite (0 0 2) peak takes place in the in situ XRDmeasurements.

Table 1 summarizes the in situ XRD results of the graphitenegative electrode in the graphite/AC capacitor using these elec-trolytes. To correlate with the cation effect, the ionic radius valuespicked up from previous reports have also be included in this table[16,17]. Here, we have coined a term of stage initiation voltageto stand for the cell voltage at which the new diffraction peaksother than graphite (0 0 2) peak begin to appear (stage formation

on-graphite intercalation compound). The stage initiations co

A+ catioge iage red wn comf Li+co-ind inge foy dese Stoated

Li+) alsoe el

tor u

Table 1In situ XRD res

Electrolyte: X-BF4 dissol

Ionic radius Stage initiatDiffraction p

Underline valuXRD patterns of the graphite negative electrode in the initial chargeraphite/AC capacitor using the electrolytes of 1 M TEABF4PC. Theges rise in the following order: OCV, 1.9 V, 2.3 V, 2.7 V, 2.9 V, 3.1 V,

V, 3.9 V.

of cativoltageDEDMbiggerthe stagish stcompacalatiocase oof the insertethe stagas mathat thPC solv(naked

Wenegativcapaciorresponding to other quaternary alkyl ammoniumPC. Here TEA is the abbreviation of tetraethyl ammonium.s the in situ XRD patterns of the graphite negative elec-e graphite/AC capacitor using the electrolyte of 1 M. During the initial charge process of the capacitor, thehe XRD patterns at rst is reected by the signicant

the intensity of graphite (0 0 2) peak when the cell volt- to over 2.7 V. This trend is the same as that reported ins study [15]. As the cell voltage gets higher than 3.1 V,iffraction peaks appear at both sides of the (0 0 2) peak

, which phenomenon has not been found before [15].peaks deviate apart from the (0 0 2) peak of graphite

and the intensities of these two new peaks become aer along with the rise in cell voltage. At the same time,eak of graphite becomes weaker. The presence of neweaks besides (0 0 2) peak of graphite in the in situ XRDomehow different from the result in the previous study.ately charged the capacitor to high cell voltages untile sure of the new diffraction peaks. At 3.9 V, the (0 0 2)

tion anglesparameter graphite siinterlayer das peathe interlaydiffraction tions corresTBA+ < TEMof the biggecrystal lattiDEDMA+ orthe least vo

It is comlattice durinfor the cracble for the long cyclesthe in situ Xpredict the

ults of the graphite negative electrode in the graphite/AC capacitor using different electr

1 M 1 M 1 M ved in PC DEDMA TEMA TEA

of cation X [nm] 0.313 0.327 0.343 ion voltage [V] 2.3 2.3 3.1 eaks at 3.5 V [] 23.01 () 22.68 () 24.73

26.56 26.61 26.5 27.62 27.55 (Grap

29.64

e is residual (002) peak of the graphite phase.rresponding to the cations rank in the following order:TEMA+ < TBA+ < TEA+ < Li+PC. Generally speaking, then gets more difcult to intercalate into graphite, thus

nitiation voltage becomes higher. However, the slug-initiation voltage for TEA+graphite is very unique asith other quaternary alkyl ammoniumgraphite inter-pounds. The absence of new diffraction peaks in the

PC electrolyte can be easily understood in the termstercalation of PC molecules with Li+ into graphite. PCto graphite crystal lattice may be decomposed beforermation of intercalation compounds and the resultingtroy the crystal structure of graphite. It should be notedcks ionic radius value of 0.408 nm is chosen to stand for

Li+ instead of the crystallographic value of 0.076 nm.

listed in Table 1 the diffraction peaks of graphiteectrode at the cell voltage of 3.5 V of the graphite/ACsing these electrolytes. The peak at lower diffrac-

(

H. Wang, M. Yoshio / Journal of Power Sources 200 (2012) 108 112 111

Fig. 4. Compaelectrolytes ofTBA+).

quaternaryof TBA+ < TE

Fig. 4 coin the electrof Li+, DEDMthe discharas the cyclin dischargThe cycle-acycles) of tTBA+ > TEMdemonstratsince PC-sotrode and dcharge procthe (0 0 2) psurement. capacitor usis just contin situ XRDthe interlaygraphite, coof the capathe interlayrior cycle pcations.

Besides intercalatiofactors maytrode. One oammoniumet al. has shsmaller catwith our coLUMO enerrank in theaccordance

Table 2HOMO and LU

LUMO [eV] HOMO [eV]

ompang the

stronaphit

thaing tic st

of aycle elecA+ aions.

for .m th4PCraphdy. A/graplyte tor isson he elcatioroughrage

ente the phiterison of the cycle performance of () graphite/AC (+) capacitor in the X-BF4 dissolved in PC (X stands for Li+, DEDMA+, TEMA+, TEA+, and

alkyl ammonium-based electrolytes ranks in the orderMA+ < DEDMA+ < TEA+.mpares the cycle performance of graphite/AC capacitorolytes of X-BF4 dissolved in PC (X stands for the cationsA+, TEMA+, TEA+, and TBA+). For all these electrolytes,

ge capacity of the capacitor generally becomes smallere number increases. However, the fading tendenciese capacity using these electrolytes are quite different.bility (retention extent in the discharge capacity withhe capacitor ranks in the following order of cations:A+ > DEDMA+ > TEA+ > Li+. The worst cycle performanceed in the case of LiBF4PC can be well understoodlvated Li+ intercalates into the graphite negative elec-ecompose to break the crystal lattice of graphite in theess. This can be veried by the irreversible decrease ineak density of graphite during the in situ XRD mea-

To our surprise, the order in the cycle-ability of theing the quaternary alkyl ammonium-based electrolytesrary to the prediction from the peak positions in the

results. The biggest cation (TBA+), which can expander distance most signicant during intercalation intontributes to the most satisfactory cycle performancecitor. On the other hand, TEA+, which can expander distance to the lowest extent, provides the infe-

erformance among all the quaternary alkyl ammonium

Fig. 5. Citors usi

be a the grbelieveincludcathodmancepoor cbased DEDMreductpoundresults

FroTBABFof the gthis stuthe ACelectrocapacithe reausing tTBA+ TBA+ (the avecationsduringAC/grathe expansion degree of crystal lattice during a cationn into a graphite negative electrode, there are other

also inuence the cycle-ability of the graphite elec-f them is likely to be the tolerance of quaternary alkyl

cation towards cathodic reduction. The work of Ueown that TBA+ is more cathodically stable than otherions (Table IV in Ref. [18]). This conclusion coincidesmputational calculation results as listed in Table 2. Thegy values of these quaternary alkyl ammonium cations

following order: TBA+ > TEA+ > TEMA+ > DEDMA+, in with their abilities against cathodic reduction. It may

MO energy values of some quaternary alkyl ammonium cations.

DEDMA+ TEMA+ TEA+ TBA+

3.528 3.377 3.145 2.74214.096 13.823 13.643 12.170

4. Conclus

In concluammoniumenergy stornary alkyl involves thtice, whichgraphite nequaternaryin the elecfour quaterand TBA+), applicationof the capaintercalatiobetween grcations.rison of cycle performance of the AC/graphite and graphite/AC capac- electrolyte of 1 M TBABF4PC.

g proof for the extraordinarily stable cycle life ofe/AC capacitor using the TBA+-based electrolyte. Wet the delicate compromise between different factorshe volume change of graphite crystal lattice and theability of a cation, etc., determines the cycle perfor-

graphite/AC capacitor at last. However, the ratherperformance of the graphite/AC capacitor using TEA+-trolyte is an open question. TEA+ is not inferior tond TEMA+ in terms of tolerance towards cathodic

Moreover, the stage formation of intercalation com-TEA+graphite is not uent as shown the in situ XRD

e viewpoint of cycle performance, the electrolyte of appears the most valuable for the practical applicationite/AC capacitor systems among the electrolytes used inctually, the electrolyte of TBABF4PC is not suitable forhite capacitor at all. As shown in Fig. 5, given the sameof 1 M TBABF4PC, the cycle performance of AC/graphite

much poorer than that of graphite/AC capacitor. As tofor the poor cycle-ability of the AC/graphite capacitorectrolyte of TBABF4PC, we ascribe it to the irreversiblen trapping in the micro-pores of AC. Since the size ofly estimated to be 0.431 2 = 0.862 nm) is very close to

pore size of AC (near 1 nm), it is likely that some TBA+

ring inside the smaller pores of AC cannot be driven outde-sorption process (corresponding to the discharge of

capacitor), similar to the case in a previous report [19].

ionsion, the graphite/AC capacitors using quaternary alkylPC electrolytes show some promise as an electricage device. The charge storage mechanism of quater-ammonium cations at the graphite negative electrodee intercalation of these cations into graphite crystal lat-

can be observed from the in situ XRD patterns of thegative electrode during charge process. The size of the

alkyl ammonium cations plays a very important roletrochemical performance of the capacitor. Within thenary alkyl ammonium cations (DEDMA+, TEMA+, TEA+,the biggest cation of TBA+ appears most suitable for the

in the capacitor because of the good cycle performancecitor using the electrolyte of TBABF4PC, although then of TBA+ into graphite expands the interlayer distanceaphene layers to the biggest extent among these four

112 H. Wang, M. Yoshio / Journal of Power Sources 200 (2012) 108 112

Acknowledgements

This work was nancially supported by National Natural ScienceFoundation of China (21173206) and Hundred Talents Program ofChinese Academy of Sciences.

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Feasibility of quaternary alkyl ammonium-intercalated graphite as negative electrode materials in electrochemical capacitors1 Introduction2 Experimental3 Results and discussion4 ConclusionAcknowledgementsReferences