Electrochimica Acta 55 (2010) 11131119

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Electrochimica Acta

journa l homepage: www.e lsev ier .com

Electro romtheir eu

Keigo Ku GoGraduate Scho

a r t i c l

Article history:Received 15 JuReceived in reAccepted 6 SepAvailable onlin

Keywords:Bis(triuoromMolten saltIonic conductiViscosityElectrochemic

ctivitTFSAyrevt alkasitio

1. Introdu

We have recently reported binary and ternary phase diagramsfor MTFSAs (M=Li, Na, K, Rb, Cs) [1,2]. Table 1 shows the eutec-tic compositions and temperatures of binary MTFSA salt mixtures[1]. Although the melting points of single MTFSA salts are rela-tively highpossessing373473Kreported thing as an elto its high cdow, 5.0V wgood cycle

In this(densities,propertiesoSAs and disproperties o

2. Experim

Reagentreported innewproced

CorresponE-mail add

uritymeasurement. HTFSA (Morita Chemical Industries, purity >99.0%)and M2CO3 (M=Na, K, Rb, Cs) were starting chemicals. MTFSAsexcept for LiTFSAwere synthesized by the reaction betweenHTFSAand M2CO3.

Densities, viscosities and ionic conductivities were measured

0013-4686/$ doi:10.1016/j.(typically above 400K), it is possible to obtain mixturesmelting points in the intermediate temperature range[1,2]. Among the ternary systems, we have alreadyat the LiTFSAKTFSACsTFSA system is highly promis-ectrolyte for rechargeable lithium metal battery owingonductivity, 14.2mScm1, wide electrochemical win-ith a cathode limit of lithium metal deposition with a

performance at 423K [3].study, we examined physicochemical properties

viscosities, ionic conductivities) and electrochemicalf single saltmelts and thebinaryeutecticmelts forMTF-cussed on the effect of mixing on the physicochemicalf MTFSAs.

ental

s and synthetic procedures are basically the same asthe previous study [2]. Several important points andures aredescribedhere. LiTFSA (MoritaChemical Indus-

ding author. Fax: +81 75 753 5906.ress: hagiwara@energy.kyoto-u.ac.jp (R. Hagiwara).

from the temperatures higher than the melting points by about20K up to around 573K. Densities weremeasured by Archimedeanmethod. Viscosities were measured by a viscometer (DV-II+ PRO,Brookeld Engineering Laboratories). Ionic conductivities weremeasured with an AC impedancemethod (IVIUMSTAT, Ivium tech-nologies, frequency: 1MHz). The cell constant was determined bythe measurement of a KCl standard solution at 291K.

Electrochemical windows of the eutectic LiTFSAMTFSA(M=Na, K, Rb, Cs), and NaTFSACsTFSA melts were measured bymeans of cyclic voltammetry. The limiting potentials are dened asthe potentials at a current density of 0.1mAcm2. Electrochemicalmeasurements were performed by the three-electrode method inan argon-lled glove box. A nickel disk (6mm in diameter, 0.1mmin thickness) or a glassy carbon rod (3mm in diameter) was usedfor the working electrode. A glassy carbon rod (3mm in diameter)was also used for the counter electrode. A lithium foil was used forthe reference electrode in the eutectic LiTFSAMTFSA (M=K, Rb,Cs) melts. In the cases of Li67Na33 (the abbreviation of the eutec-tic LiTFSANaTFSA with xLiTFSA =0.33. All the binary salts describedhere are eutectic mixtures.) and Na07Cs93 melts, sodium metalwas used for the reference electrode material. The scan rate for thecyclic voltammetry was 10mVs1. Galvanostatic electrolysis wasperformed to obtain the deposit for the identication of the reac-

see front matter 2009 Elsevier Ltd. All rights reserved.electacta.2009.09.024chemical properties of alkali bis(triuotectic mixtures

bota, Kenichiro Tamaki, Toshiyuki Nohira, Takuyaol of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan

e i n f o

ly 2009vised form 3 September 2009tember 2009e 15 September 2009

ethylsulfonyl)amide

vity

al window

a b s t r a c t

Densities, viscosities and ionic condubis(triuoromethylsulfonyl)amides, Mrangeof413573K.Cyclic voltammetrwindowsof5.06.0V. Itwas found thalimit potentials. The order of the depo

ction tries, p/ locate /e lec tac ta

ethylsulfonyl)amides and

to, Rika Hagiwara

ies of single salts and the binary eutectic mixtures of alkalis (M=Li, Na, K, Rb, Cs), were measured in the temperatureealed that thebinaryeutecticmeltshavewideelectrochemicallimetals reversiblydeposit onanickel electrodeat thecathoden potential was determined to be Na> Li > (K, Rb, Cs).

2009 Elsevier Ltd. All rights reserved.

>99.0%) is purchased and dried under vacuum before

1114 K. Kubota et al. / Electrochimica Acta 55 (2010) 11131119

Table 1The eutectic temperatures and compositions of binary systems.

System xeu System Teu (K)

LiTFSANaTFSA xLiTFSA = 0.67 Li67Na33 459LiTFSAKTFSA xLiTFSA = 0.43 Li43K57 429

LiTFSARbTFSA xLiTFSA = 0.25 Li25Rb75 421xLiTFSA = 0.60 Li60Rb40 426

LiTFSACsTFSA xLiTFSA = 0.07 Li07Cs93 391xLiTFSA = 0.70 Li70Cs30 434

NaTFSAKTFSA xNaTFSA = 0.25 Na25K75 456NaTFSARbTFSA xNaTFSA = 0.25 Na25Rb75 431NaTFSACsTFSA xNaTFSA = 0.07 Na07Cs93 392

tions at the cathode limits. The deposited substances on a nickelelectrode after the electrolysis were analyzed by means of dif-ferential scanning calorimetry (DSC) and X-ray diffraction (XRD)analysis.

3. Results

3.1. Viscosi

Fig. 1 sh(b) eutectictic NaTFSAcurvaturesthe followinapplied [4]:

(T) = A

where A, BT0 is calledof the MTFequation w

Among tof the LiTFSwith the otThe viscosiof the const

The visLiTFSAMTLiTFSAandhcosityof Li6tohigh concfor the eute

Table 2VTF paramete

System

Li100Na100K100Rb100Cs100Li67Na33Li43K57Li25Rb75Li07Cs93Na25K75Na25Rb75Na07Cs93and discussion

ties of single salts and binary eutectic mixtures

ows Arrhenius plots of viscosities for (a) single salts,LiTFSAMTFSA (M=Na, K, Rb, Cs) and (c) the eutec-MTFSA (M=K, Rb, Cs). Distinct upward trends in theare observed at lower temperatures. In such a case,g VogelTammanFulcher (VTF) equation is frequently

T exp

(B

T T0

)(3.1)

and T0 are constants to be determined empirically.the ideal glass transition temperature. The viscositiesSA melts were tted very well (R2 >0.999) to the VTFith the tting parameters shown in Table 2.he single salts, the viscosity of Li100 (the abbreviationA single salt) is extremely high, the viscosity of the saltsher cations becoming lower with increasing their radii.ties of binary mixtures are somewhere between thoseituent single salts.cosity of Li07Cs93 is the lowest in the eutecticFSA (M=Na, K, Rb, Cs) due to the low concentration ofighconcentrationofCsTFSA.On theotherhand, thevis-7Na33 is thehighest in theeutectic LiTFSAMTFSAdueentrationof LiTFSA. The same tendency is also observedctic NaTFSAMTFSA (M=K, Rb, Cs) melts.

rs for viscosities of MTFSA melts.

3 1/210 A (cPK ) B (K) T0 (K)66.4 569 4041.37 2260 1734.08 1414 215

10.5 949 25213.8 850 2500.875 2330 2246.61 1280 2650.749 2204 2174.95 1260 2206.68 1150 2535.96 1100 2594.34 1310 216

Fig. 1. Arrhenius plots of viscosities for (a) single MTFSA salts, (b) eutecticLiTFSAMTFSA (M=Na, K, Rb, Cs) melts, and (c) eutectic NaTFSAMTFSA (M=K, Rb,Cs) melts.

K. Kubota et al. / Electrochimica Acta 55 (2010) 11131119 1115

Table 3VTF parameters for ionic conductivities of MTFSA melts.

System 104 A (mS cm1 K1/2) B (K) T0 (K)Li100Na100K100Rb100Cs100Li67Na33Li43K57Li25Rb75Li07Cs93Na25K75Na25Rb75Na07Cs93

3.2. Ionic comixtures

Fig. 2 shosalts, (b) euNaTFSAMTobserved atsuch a tempequation [5

(T) = AT

where A , BT0 is calledductivities oto this VTFThose of somof data poin

The ionicLiTFSAMTtion of LiTFhand, the iotic LiTFSAThe same te(M=K, Rb, Cing temperamelts are noliquids. Howthe applicamelts contatemperaturtrolytes of las LiTFSA, Lties of RTIL8.8mScm

tion of 0.3Li43K57 isto EMImTFSdirectly as t

3.3. Relatio

The relatis given by

= Md

where isweight or a

Fig. 3 shosingle MTFS0.254 567 391 5.47 1290 1902.72 902 2271.59 1159 2942.44 1070 2581.67 784 2722.68 881 2364.24 1160 2272.98 987 2402.51 840 242

nductivities of MTFSA single salts and binary eutectic

ws Arrhenius plots of ionic conductivities for (a) singletectic LiTFSAMTFSA (M=Na, K, Rb, Cs) and (c) eutecticFSA (M=K, Rb, Cs). Slightly downward curvatures arelower temperatures. Similarly to the case of viscosity,erature dependence is described by the following VTF]:

exp(

BT T0

)(3.2)

and T0 are constants to be determined empirically.the ideal glass transition temperature. The ionic con-f most MTFSA melts were tted quite well (R2 >0.999)equation by the tting parameters shown in Table 3.e melts could not be tted simply due to the shortage

ts.conductivity of Li07Cs93 is the highest in the eutectic

FSA (M=Na, K, Rb, Cs) melts due to low concentra-SA and high concentration of CsTFSA. On the othernic conductivity of Li67Na33 is highest in the eutec-MTFSA melts due to the high concentration of LiTFSA.ndency is also observed for the eutecticNaTFSAMTFSAs)melts. If the comparison ismade just above themelt-tures, the ionic conductivities of these binary eutectict so high compared to those of room temperature ionicever, they are considered to be reasonably high for

tion of battery electrolytes taking account that thesein highly concentrated LiTFSA or NaTFSA at workinges from the beginning. In order to apply RTILs for elec-ithium batteries it is necessary to add lithium salts suchiBF4 and LiPF6 which signicantly increases the viscosi-s. In fact, the ionic conductivity of neat EMImTFSA is1 at 298K, but it decreases to 5.1mScm1 by the addi-mol kg1 of LiTFSA [6]. On the other hand, LiTFSA inhighly concentrated and have ionic conductivity equalA. Thus thesemolten salts arehighly possible to beusedhe electrolytes for lithium and sodium batteries.nship between molar conductivity and viscosity

ion between ionic conductivity andmolar conductivitythe following equation:

(3.3)

molar conductivity, d is density and M is molecularverage molecular weight of the melt.ws the temperature dependence of the densities for (a)A salts and (b) binary eutectic mixtures. For all MTFSA

Fig. 2. Arrhenius plots of ionic conductivities for (a) single MTFSA salts, (b) eutecticLiTFSAMTFSA (M=Na, K, Rb, Cs) melts, and (c) eutectic NaTFSAMTFSA (M=K, Rb,Cs) melts.

1116 K. Kubota et al. / Electrochimica Acta 55 (2010) 11131119

Fig. 3. Tempereutectic MTFS

salts, densitbe expresse

d(T) = A Bwhere A anTable 4. Fomolecular wof density w

Table 4The tting par

System

Li100Na100K100Rb100Cs100Li67Na33Li43K57Li25Rb75Li07Cs93Na25K75Na25Rb75Na07Cs93aturedependenceofdensities for (a) singleMTFSAsalts and (b)binaryA melts.

ies linearly decrease with the temperature elevation tod by the following equation:

T (3.4)

d B are constants. The tting parameters are given inr single salts, densities increase with the increase ofeight (from Li to Cs) whose effect exceeds the decreaseith increase of molecular volume (from Li to Cs). In

ameters of the densities.

A (g cm3) 104 B (K1)2.27 10.02.15 7.032.12 6.052.38 6.842.77 10.52.25 9.712.27 9.392.43 9.202.74 11.12.28 8.932.45 9.192.75 11.2

Fig. 4. Plots of logarithmic molar conductivity against the logarithmic reciprocalviscosity for (a) single MTFSA salts, (b) eutectic LiTFSAMTFSA (M=Na, K, Rb, Cs)melts, and (c) eutectic NaTFSAMTFSA (M=K, Rb, Cs) melts.

K. Kubota et al. / Electrochimica Acta 55 (2010) 11131119 1117

Table 5The constants of fractional Waldens rule ofMTFSA melts.

System

the case ofstituent sin

Fig. 4 shagainst thethe eutecticeutectic Naconductivitmolar cond

= const.This equ

uid is goverto =1. Alttheir gradiecalled decoviscosity [8tures, the rshould be dthe followin

= conswhere is aeach systemsalts inwhiby that alkacan move emelts, the cincrease wia tendencyandanion, tMTFSA melthat thedecsome of binnot fall in thlocated abotivity of Li0(b)) althougof Cs100 (sable for theby reducing

3.4. Electro

Fig. 5 shA pair of cLi/Li+ on aare interprelution, resp10mAcmtion at the

combined cyclic voltammogram at a Ni electrode (negative potential0.32 to 3.0V) and a glassy carbon electrode (positive potential region,) in eutectic Li43K57 melt at 443K (scan rate: 10mVs1).

metallic luster was obtained. When the deposit was ana-yDSC, endothermicpeakswereobservedonly at themeltingf lithium metal, 454K as well as the melting points of theic salt, 423K. Thus, it was conrmed that only lithium metalted odicectroidatiote thK57tenticm

6 shtaine57. ILi100 0.845Na100 0.844K100 0.861Rb100 0.908Cs100 0.900Li67Na33 0.804Li43K57 0.813Li25Rb75 0.743Li07Cs93 0.834Na25K75 0.833Na25Rb75 0.789Na07Cs93 0.821

binary mixtures, densities fall in between those of con-gle salts.ows the plots of the logarithmic molar conductivitylogarithmic reciprocal viscosity of (a) single salts, (b)LiTFSAMTFSA (M=Na, K, Rb, Cs) melts and (c) the

TFSAMTFSA (M=K, Rb, Cs) melts. If the viscosity andy of the electrolyte obeys Waldens rule, the product ofuctivity and viscosity is constant [7]:

(3.5)

ation is interpreted as that ionic conductivity of the liq-nedby their viscosity. Adashed line inFig. 4 correspondshough all the plots ofMTFSAmelts are on straight lines,nts are slightly different from unity. This tendency isupling in behavior of ionic conductivity from that of,9]. Thus, in the cases of MTFSA melts and their mix-elationship between molar conductivity and viscosityescribed by the fractional Waldens rule presented byg equation [10]:

t. (3.6)

positive constant smaller than one. Table 5 shows for. Such small of less than one is observed for molten

ch small singly charged ions exist [11]. This is explainedlimetal cationswhicharemuchsmaller thanTFSAanionasily among TFSA anions. In the case of single MTFSAonstants fall in the range of 0.840.90 and they tend toth increasing the radius of alkali metal cations. There isthat the larger the difference of radius between cationhegreater thedegreeof decoupling. In the caseof binaryts, is smaller than those of single melts. This suggestsoupling is enhancedbymixing twodifferent cations. ForaryMTFSA salts such as Li07Cs93, theWalden plots doemiddle of those for the constituent single salts but areve them. This is attributed to the fact that ionic conduc-7Cs93 is very close to that of Cs100 (see Fig. 2(a) andh the viscosity of eutectic Li07Cs93 is higher than thatee Fig. 1(a) and (b)). Such an effect of mixing is favor-

Fig. 5. Aregion,3.85.2V

partiallyzedbpoint oeutectdeposiAn anobon elthe oxelucidaof Li43ing po0.1mA

Fig.The obLi43Kimprovement of the ionic conductivity of electrolytesthe inuence of viscosity.

chemical properties of the binary eutectic melts

ows cyclic voltammograms in Li43K57 melt at 443K.athodic and anodic currents is observed at 0V vs.nickel electrode. These cathodic and anodic currentsted as the deposition of lithium metal and its disso-ectively. Galvanostatic electrolysis was performed at2 on a nickel electrode in order to identify the reac-cathode limit. As a result, a dendritic deposit with

Fig. 6. A comregion, 0.263.25.2V) in en the nickel electrode at the cathode limit of Li43K57.current is observed at 5.0V vs. Li/Li+ on a glassy car-de. This anodic current is considered to correspond ton of TFSA anion, though more studies are required toe detail of this reaction. The electrochemical windowmelt is determined as 5.0V at 443K, when the limit-

als are dened as the potential at a current density of2.ows cyclic voltammograms in Li25Rb75 melt at 443K.d cyclic voltammograms are quite similar to that forn the samemanner as described above, itwas concludedbined cyclic voltammogram at a Ni electrode (negative potentialto 3.0V) and a glassy carbon electrode (positive potential region,utectic Li25Rb75 melt at 443K (scan rate: 10mVs1).

1118 K. Kubota et al. / Electrochimica Acta 55 (2010) 11131119

Fig. 7. A combined cyclic voltammogram at a Ni electrode (negative potentialregion, 0.20 to 2.2V) and a glassy carbon electrode (positive potential region,3.25.3V) in eutectic Li07Cs93 at 403K (scan rate: 10mVs1).

that thecathode limit is lithiumdeposition, and theelectrochemicalwindow is 5.0V at 443K.

Fig. 7 shows a cyclic voltammogram in Li07Cs93melt at 403K.A voltammetric feature is similar to the cases described above,except theis a slightlyLi25Rb75anion is conrmedbyDelectrode at

Fig. 8 shmelt at 403a nickel elecsition/dissoon a glassy

Fig. 8. A comregion, 0.3 t3.35.2V) in e

Fig. 9. A comregion, 1.03.04.8V) in e

is 5.0Vat 40on the nickmeasureme

compCs93hatcorreit istion/ssoluined9 shcaseThe centiat pead Nae limanodic current is observed at 5.2V vs. Li/Li+. This valuehigher than that of the anode limit for Li43K57 andat 443K. Thus, the decomposition potential of TFSAsidered to have temperature dependence. It was con-SCmeasurement that lithiummetal deposits on anickelthe cathode limit at 403K.ows cyclic voltammograms in the eutectic Na07Cs93K. A pair of cathodic and anodic currents is observed ontrode at 0V vs. Na/Na+, which corresponds to the depo-lution of sodium metal. An anodic current is observedcarbon electrode at 5.0V. The electrochemical window

InNa07than ttemsanion,deposition/dideterm

Fig.In thistrode.thepotcurrenof Li ancathodbined cyclic voltammogram at a Ni electrode (negative potentialo 2.1V) and a glassy carbon electrode (positive potential region,utectic Na07Cs93 at 403K (scan rate: 10mVs1).

450K. TheLiNa alloymetal. Theeutectic salare close tolithiummealong withanodic dissrately. Betwnegative polithium meing to the oand lithiumbly due to ois observedclusion, forwindow is

It is revewindows alimit. Thus,or sodiumsodiumsubined cyclic voltammogram at a Mo electrode (negative potentialto 2.0V) and a glassy carbon electrode (positive potential region,utectic Li67Na33 melt at 493K (scan rate: 10mVs1).

3 K. It is also conrmed that only sodiummetal depositsel electrode at the cathode limit from the result of DSCnt for the deposit.arison of cyclic voltammograms in Li07Cs93 and, the electrochemical window of Li07Cs93 is widerof Na07Cs93. Since anodic currents of those sys-spond to the same oxidation reaction of TFSAreasonably concluded that the potential of sodium

dissolution is higher than that of lithium deposi-tion. Thus, the order of deposition potential wasto be Na> Li > (K, Rb, Cs).

ows cyclic voltammograms in Li67Na33melt at 493K., a sodium liquid metal was used as the reference elec-athodic current is observed at 0.2V vs. Na+/Na. Whenl sweep is reversed to thepositive direction, twoanodicks are observed, which suggests that the deposit is bothmetal. In the DSC measurement for the deposit at theit, endothermic peaks were observed at 360, 369 and

peak at 360K corresponds to the eutectic melting of. The peak at 369K corresponds to the melting of Napeak at 450K possibly corresponds to the melting ofts and lithiummetal because the melting temperatureseach other, 459K for the eutectic salt, and 454K, for

tal. Thus, the deposition of the lithiummetal could occurthe deposition of the sodium metal. On the other hand,olutions of lithium and sodium seem to proceed sepa-een two peaks of oxidation currents, the peak at moretential is suggested to correspond to the dissolution oftal and the other peak to that of sodium metal accord-rder of deposition potential. The deposition of sodiumoccurs simultaneously on the nickel electrode proba-verpotential for sodium deposition. An anodic currentat 4.5V vs. Na/Na+ at a glassy carbon electrode. In con-the eutectic LiTFSANaTFSA melt, the electrochemical4.7V at 493K.aled that those binarymelts havewide electrochemicalnd lithium and sodium metals deposit at the cathodethosemelts areexpected touseaselectrolytesof lithiummetal batteries such as lithium secondary battery andlfur battery.

K. Kubota et al. / Electrochimica Acta 55 (2010) 11131119 1119

4. Conclusions

Densities, viscosities, ionic conductivities and electrochemicalwindows of the binary eutectic MTFSA salts have been studied.The densities are measured in order to calculate the molar con-ductivities. The viscosities of the melts are higher and the ionicconductivities become lower when the concentration of the saltcontaining a small cation such as lithium ion is higher. However,the binary eutectic MTFSA melts have higher ionic conductivitiesat intermediate temperatures than room temperature ionic con-ductivities of room temperature ionic liquids containing lithiumsalts even with high concentration of lithium salts or sodium salts.The viscosities and the ionic conductivities are described well byVTF equations. Logarithmic molar conductivity is proportional tologarithmic reciprocal viscosity and obeys a fractional Waldensrule. The binary eutectic MTFSA salts have large electrochemicalwindows of about 5.0V at intermediate temperatures. The reac-tions at the cathode limit of Li43K57, Li25Rb75 and Li07Cs93melts are identied to be the lithium metal deposition. In the caseof Na07Cs93 melt, the reaction at the cathode limit is sodiummetal deposition. In the case of Li67Na33melt, the reaction at thecathode limit is simultaneous deposition of sodium and lithium

probably due to overpotential for sodium deposition. It has beenfound that alkali metal deposition is possible in these melts. Theanode limit of the binary eutectic melts is considered to be the oxi-dation of TFSA anion. The binary eutecticMTFSAmelts are expectedto use as electrolytes of lithium or sodium metal batteries.

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Electrochemical properties of alkali bis(trifluoromethylsulfonyl)amides and their eutectic mixturesIntroductionExperimentalResults and discussionViscosities of single salts and binary eutectic mixturesIonic conductivities of MTFSA single salts and binary eutectic mixturesRelationship between molar conductivity and viscosityElectrochemical properties of the binary eutectic melts

ConclusionsReferences