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Electrochimica Acta 114 (2013) 95– 104

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

omparative study on transport properties for LiFAP and LiPF6 inlkyl-carbonates as electrolytes through conductivity, viscosity andMR self-diffusion measurements

atrice Poriona, Yvon Rodrigue Dougassab, Cécile Tessierc, Loubna El Ouatanic,ohan Jacquemind, Mérièm Anoutib,∗

Centre de Recherche sur la Matière Divisée, CNRS-Université d’Orléans, FRE3520, 1b rue de la Férollerie, 45071 Orléans Cedex 02, FranceUniversité Franc ois Rabelais, Laboratoire PCMB (EA 4244), équipe (CIME), Parc de Grandmont, 37200 Tours, FranceDirection de la Recherche – SAFT, SAFT, 33000 Bordeaux, FranceCenTACat, The School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, United Kingdom

r t i c l e i n f o

rticle history:eceived 18 August 2013eceived in revised form7 September 2013ccepted 1 October 2013vailable online 19 October 2013

eywords:ithium hexafluorophosphateithiumris(pentafluoroethane)-trifluorophosphateulsed-gradient spin-echo NMRelf-diffusion coefficientstokes–Einstein

a b s t r a c t

We present in this work a comparative study on density and transport properties, such as the conductivity(�), viscosity (�) and self-diffusion coefficients (D), for electrolytes based on the lithium hexafluorophos-phate, LiPF6; or on the lithium tris(pentafluoroethane)-trifluorophosphate, LiFAP dissolved in a binarymixture of ethylene carbonate (EC) and dimethylcarbonate (DMC) (50:50 wt%). For each electrolyte, thetemperature dependence on transport properties over a temperature range from 10 to 80 ◦C and 20to 70 ◦C for viscosity and conductivity, respectively, exhibits a non-Arrhenius behavior. However, thisdependence is correctly correlated by using the Vogel-Tamman-Fulcher (VTF) type fitting equation. Ineach case, the best-fit parameters, such as the pseudo activation energy and ideal glass transition tem-perature were then extracted. The self-diffusion coefficients (D) of the Li+ cation and PF6

− or FAP− anionsspecies, in each studied electrolyte, were then independently determined by observing 3Li, 19F and 31Pnuclei with the pulsed-gradient spin-echo (PGSE) NMR technique over the same temperature range from20 to 80 ◦C. Results show that even if the diffusion of the lithium cation is quite similar in both electrolytes,the anions diffusion differs notably. In the case of the LiPF6-based electrolyte, for example at T ≈ 75 ◦C(high temperature), the self-diffusion coefficients of Li+ cations in solution (D (Li+) ≈ 5 × 10−10 m2 s−1) is1.6 times smaller than that of PF6

− anions (D (PF6−) = 8.5 × 10−10 m2 s−1), whereas in the case of the LiFAP-

based electrolyte, FAP− anions diffuse at same rate as the Li+ cations (D (FAP−) = 5 × 10−10 m2 s−1). Based on

these experimental results, the transport mobility of ions were then investigated through Stokes–Einsteinand Nernst–Einstein equations to determine the transport number of lithium tLi

+, effective radius of sol-vated Li+ and of PF6

− and FAP− anions, and the degree of dissociation of these lithium salts in the selectedEC/DMC (50:50 wt%) mixture over a the temperature range from 20 to 80 ◦C. This study demonstratesthe conflicting nature of the requirements and the advantage of the well-balanced properties as ionicmobility and dissociation constant of the selected electrolytes.

. Introduction

Electrolytes used in commercial lithium-ion batteries areainly formulated by the dissolution of the lithium hexafluo-

ophosphate, LiPF6 in cyclic carbonates like ethylene carbonateEC) or propylene carbonate (PC) and linear carbonates such as

imethyl carbonate (DMC), ethyl methyl carbonate (EMC) andiethyl carbonate (DEC), which are, to date, the most sustainablelectrolytes, dealing with their cost and properties, for lithium-ion

∗ Corresponding author. Tel.: +33 247366951; fax: +33 247367360.E-mail address: meriem.anouti@univ-tours.fr (M. Anouti).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.10.015

© 2013 Elsevier Ltd. All rights reserved.

battery applications, LiBs [1]. However, it is well described intothe literature that LiPF6 is thermally unstable and decomposes inLiF and PF5, as well as, that PF5 can be hydrolyzed in presenceof residual water to form HF and PF3O [2–4]. Furthermore, thebattery lifetime is often linked to the quantity of these decom-position products, as they are highly reactive to both negativeand positive electrodes in LiBs [5,6]. Despite these technolog-ical issues, LiPF6 is still used as the reference salt in LiBs tillmore than a decade because of its unique properties in Li-ion

devices providing a good ionic conductivity of the electrolyte, aswell as, based-on its ability to passivate an aluminum currentcollector, and to participate to the passivation layer on the neg-ative electrode [7–9]. Therefore, the selection of another safer

9 imica

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2

2

cAi±sf

paLu

6 P. Porion et al. / Electroch

ithium salt to be dissolved in alkyl-carbonates operating over aide temperature range has received increasing attentions [10,11].mong this and according to the structure similarity between

he hexafluorophosphate, LiPF6 and tris(pentafluoroethane)-rifluorophosphate, LiPF3(CF2CF3)3 (denoted herein LiFAP) anions,lectrolytes-based on the LiFAP, which are now commercially avail-ble from many suppliers as the Merck company [12], are currentlynvestigated by several groups [13,14], because the substitutionf three fluorine atoms by three (CF2CF3) groups seems tomprove both the hydrolytic and thermal stabilities of this lithiumalt series. Additionally, several works demonstrated that LiFAP-ased electrolytes containing common alkyl-carbonates such asC, DMC or DEC have better LiBs properties than those reportedor LiPF6, lithium bis[(trifluoromethyl)sulfonyl]imide, LiTFSI orithium bis[(pentafluoroethyl)sulfonyl]imide, LiBeti solutions [8].or examples, both LiBs anodes and cathodes perform better iniFAP solutions as higher capacity and lower capacity fading uponycling than those using other lithium salt-based electrolytes areescribed in the literature [8,13–15]. Furthermore, it was alsobserved that both electrodes are stabilized faster upon repeatedithiation–delithiation cycling in LiFAP than in LiPF6 solutions [8].nanaraj et al. [8] suggest that the absence of HF and the rela-

ively higher stability and lower reactivity of the FAP− anion inomparison with the PF6

− prevent detrimental solution-electrodenteractions and allow the development of surface films originat-ng from solvent reactions that well protect the electrode’s active

ass. Furthermore, even if the conductivity of LiFAP-based elec-rolytes is generally 30% lower than that of LiPF6 ones, better LiBserformances are reported [8,13–15]. Therefore, it is necessary toompare the transport properties of both lithium salts dissolvedn a common alkyl-carbonate-based lithium electrolytes solvent or

ixture to understand the key parameters which explain observedifferences on their cycleability, properties and LiBs performances.

n the present study, we first compared the physical properties:ensity, ionic conductivity and viscosity of two lithium ion bat-eries based electrolytes containing 1 mol L−1 of lithium salt, LiPF6r LiFAP, dissolved in the alkyl-carbonates EC/DMC (50:50 wt%)ixture as a function of temperature from 20 to 80 ◦C. Secondly,

ulsed-gradient spin-echo NMR technique has been applied toeasure the self-diffusion coefficients of lithium, fluorine, and

hosphorus nuclei in order to detect the differences on the ionsobility between the two selected electrolytes. Finally, by using

he Stokes–Einstein and Nernst–Einstein equations, the temper-ture dependence on the self-diffusion coefficients and on theiscosity has been investigated to determine the effective radius ofithium cations, PF6

− and FAP− anions in each electrolyte, as wells the degree of dissociation of each salt in the selected EC/DMC50:50 wt%) mixture.

. Experimental

.1. Materials and mixtures preparations

Highly pure (GC grade, molecular purity > 99.99%) ethylenearbonate (EC), and dimethylcarbonate (DMC), purchased fromldrich, were used as received. The alkyl-carbonate mixture stud-

ed into this work has been prepared by mass with an accuracy of1 × 10−4 g using a Sartorius 1602 MP balance under a dry atmo-

phere in a glove box at 25 ◦C and is denoted in mass fraction asollow: EC/DMC (50:50 wt%).

The highly pure (99.99%) lithium hexafluorophosphate (LiPF6)

urchased from Sigma–Aldrich was kept and used under a drytmosphere in a glove box. The electrolyte based on 1 mol L−1 ofiPF6 salt dissolved in EC/DMC (50:50 wt%) was prepared by masssing a Sartorius 1602 MP balance under a dry atmosphere in a

Acta 114 (2013) 95– 104

glove box, and kept inside the glove box before further analyses.Firstly, the mixture of EC/DMC (50:50 wt%) was prepared by massand its density measured as a function of temperature. Secondly,a precise quantity of EC/DMC mixture has been placed in a precisevolume vessel at constant temperature (e.g. 25 ◦C) and an appropri-ate quantity (in mass) of LiPF6 was dissolved in it to reach a LiPF6 saltcomposition of 1 mol L−1 of solution. Finally, the density of this elec-trolyte was measured as a function of temperature. The electrolytecontaining the 1 mol L−1 of LiFAP salt in EC/DMC (50:50 wt%) waspurchased, as is, from Merck within a molecular purity higher than99.99%. This commercial electrolyte was kept and used as receivedfrom the manufacturer under a dry atmosphere in a glove box. Addi-tionally, prior to any measurement, electrolytes were analyzed forwater content using coulometric Karl-Fischer (Coulometer 831 –Metrohm) titration. The water content of selected electrolytes islower than (20 ± 1) ppm.

2.2. Experimental and theoretical methods

The optimized 3D structure of each investigated ion and solventwas carried out by using DFT calculations within Gaussian software(Gaussian version 03-D1, DFT-B3LYP-DGTZVP) [16], to generatethen each COSMO file (Turbomole version 5.7, BP-DFT-Ahlrichs-TZVP) [17] by using the same methodology as already presentedpreviously by our group [18–20] Additionally, COSMO volume andsigma profile of investigated species were obtained by using theCOSMOthermX software (version 2.1, release 01.08) [21]. Briefly,as the COSMO-RS theory (from Conductor-like Screening Modelfor Real Solvents) uses the concept of an ideally screened moleculeenergy and charge distribution on surface of molecule, the moleculebeing embedded in a virtual conductor with an infinite permit-tivity = ∞ [21]. For COSMO-RS calculations, knowledge of COSMOparameters describing the 3D chemical structure of the moleculesis required. The interaction energy is then expressed by local sur-face descriptors calculated from quantum chemical methods. Theprincipal descriptor and a key parameter for molecular interactionsin COSMO-RS methodology is the screening charge density � at thesurface of each molecule (e.g. the sigma profile of each molecule)”,based on which affinity between species can be highlighted anddepicted [18–21].

Conductivity measurements were performed by using a Crison(GLP 31) digital multi-frequencies conductometer between 1000and 5000 Hz. The temperature control (from 10 ◦C to 80 ◦C) wascarried out within an accuracy of 0.2 ◦C by means of a JULABO ther-mostated bath. Prior any measurement, the conductometer wasfirst calibrated with a standard solution of known conductivity suchas 0.1 and 0.01 mol L−1 KCl aqueous solutions, as well as, using a1 mol L−1 of LiPF6 salt dissolved in EC/PC/3DMC (20:20:60 wt%) asthe electrolyte standard (� = 12 mS cm−1 at 25 ◦C) [7,22]. From thisstudy the repeatability and the uncertainty of conductivity mea-surements did not exceed ±0.5% and ±1%, respectively.

Density and viscosity measurements were conducted from 20to 70 ◦C using an Anton Parr digital vibrating tube densitometer(model 60/602, Anton Parr, France) and an Anton Parr rolling-ballviscometer (model Lovis 2000M/ME, Anton Parr, France), respec-tively. In both cases, the temperature in the cell was regulatedwithin ±0.02 ◦C. Dynamic viscosity values reported herein werecalculated by taking into account the effect of the sample den-sity and the buoyancy of the ball in each sample as a function oftemperature. Additionally, as specify by the constructor, densitydata is required to calculate a dynamic and kinematic viscosityby using the Lovis M/ME micro-viscosimeter. For that we decided

to perform simultaneously density and viscosity measurementsusing the sample, as both instruments are connected with hosesand simultaneously filled with the same sample. Furthermore, thethermo-balance technology was used to obtain precise viscosity

imica Acta 114 (2013) 95– 104 97

maapwut5

pspn7

vrwteliwbots

3

3

iaatacohvut

mctrfstaLwocoamscwita

Fig. 1. Density measurements, �, of the selected electrolytes containing 1 mol L−1 ofLiX salt (X = �, PF6

− or �, FAP−) in EC/DMC (50:50 wt%) as a function of the temper-

P. Porion et al. / Electroch

easurements over the whole temperature range using air/waterdjustments. The densitometer was firstly calibrated at all temper-tures with degassed water and dehumidified air at atmosphericressure as recommended by the constructor. Eleven readingsere taken for each density measurement reported therein. While,ltra-pure water was used to calibrate the viscometer. The uncer-ainty of the density and viscosity measurements were better than

× 10−5 g cm−3, and 1%, respectively.Pulse-Gradient Spin-Echo NMR (PGSE-NMR) experiments were

erformed on a Bruker DSX100 NMR spectrometer with a 2.35 Tuperconducting magnet, equipped with a 10 mm microimagingrobe (Micro5 Bruker) without a lock system. The Larmor reso-ance frequencies are 39.91 MHz, 94.20 MHz, and 40.53 MHz forLi, 19F and 31P nuclei, respectively. In order to determine the acti-ation energy for the diffusion process of each species, the NMRuns were carried out over a temperature range from 20 ◦C to 80 ◦Cithin an accuracy of ±1 ◦C and a step �T = 5 ◦C. In this study,

o achieve better experimental measurements for these diffusionxperiments, the inner diameter of the NMR tube was 6 mm and theiquid height was 10 mm. The samples were prepared and placedn sealed tubes in a glove box. For RMN measurements, samples

ere thermally equilibrated at each temperature during 30 minefore any acquisition. Indeed, such experimental conditions allowbtaining a good homogeneity of the set point temperature acrosshe whole samples. Finally, all the chemical shift references wereet to a particular arbitrary value for convenience.

. Results and discussion

.1. Structure and sigma profile of selected solvents and anions

The structure, name, abbreviation, minimum and maximumntramolecular distances, sigma profile and COSMO volume evalu-ted by using the COSMOthermX interface of studied solvents andnions in lithium salt LiX are presented in Table 1 and in Fig. S1 ofhe supporting information. As reported in Table 1 and in Fig. S1, forll the selected species, FAP− anion has a larger volume than thosealculated for either the PF6

− anion, or selected solvents. In the casef the selected solvents, as expected from their 3D structure DMCas a larger volume (108.34 A3) than that of the EC (94.17 A3). Aolume ratio close to 1.15 is calculated between DMC and EC vol-mes. From this investigation it appears that volume increases inhe following order: Li+ � EC < PF6

− < DMC < FAP−.According to the surface charge distributions, minimum and

aximum intramolecular distances, as well as to the sigma profilesalculated for all studied anions (Table 1 and Fig. S1), it appearshat the charge in the symmetrical PF6

− anion is much localizedesulting in strong Li+-anion Coulombic interactions and ion-pairormation in solution. Contrarily, the charge in the FAP− anion istrongly affected by the substitution of three fluorine atoms byhree (CF2CF3) groups, which increases the steric effect of thenion and the van der Waals forces, resulting in relatively weakeri+-anion Coulombic interactions than those with the PF6

− anion,hich reduce the likelihood of ion association in solution, than that

bserved with the PF6−. These differences between these anions

an explain differences on LiBs performances of electrolytes basedn these salts described above. Additionally, oxygen atoms onlkyl-carbonates solvents (shown by the yellow-orange color in theapped surface charge distributions in Table 1) participate to the

olvation of lithium cations in solution. Better solvation of lithiumations with these solvents can be expected from a salt having

eak cation–anion interaction in solution like LiFAP. However, the

ncrease of the van der Waals force resulting to the substitution ofhree fluorine atoms by three (CF2CF3) groups from PF6

− to FAP−

nions, can induce in fact an increase of the viscosity and a decrease

ature from 20 to 70 ◦C at 0.1 MPa. The lines represent linear-type fittings with theparameters indicated in Eq. (1) within a standard deviation lower than 10−5 g cm−3

in both cases.

of the conductivity of the electrolyte, especially at low temperaturedue to the increase of the steric effect of the anion. Both positive andnegative effects have in fact an impact on the salts solvation proper-ties in molecular solvents, which can affect the transport properties,as well as the electrochemical performances of the electrolytes.

3.2. Volumetric and transport properties of selected electrolytes

3.2.1. DensityThe density of selected electrolytes was measured as a function

of the temperature from 20 to 70 ◦C and results are presented inFig. 1 with data reported in Table S1 of the supporting information.From which and as expected, it can be seen that the density for bothelectrolytes decreases linearly with the temperature (see Eq. (1)).Additionally, it appears that the electrolyte containing 1 mol L−1ofLiFAP salt dissolved in EC/DMC (50:50 wt%) is denser over the wholetemperature range studied than the LiPF6 one. For example, thedensity values are close to 1.402 and 1.308 g cm−3 at 20 ◦C and 1.340and 1.251 g cm−3 at 70 ◦C, for LiFAP and LiPF6 based electrolytes,respectively.

�LiPF6= 1.33150 − 1.1519 × 10−3(T/◦C)

�LiFAP = 1.42708 − 1.2499 × 10−3(T/◦C)(1)

where �LiPF6and �LiFAP are the density of the electrolyte based

on EC/DMC (50:50 wt%) mixture containing 1 mol L−1 of LiPF6 andLiFAP at the temperature T from 20 to 70 ◦C, respectively.

The density changes on an electrolyte series can be affected bytwo main parameters.

The first one can be linked to the molecular weight of eachion and selected solvent in solution, which affects the volumetricproperties of electrolytes. Molar volumes, Vm, as a function of thetemperature were calculated from density data reported in TableS1 of the supporting information and the molecular weight of eachelectrolyte, determined by the knowledge of the composition (massand number of moles) of each species in solution (e.g. 93.57 g mol−1

and 120.34 g mol−1 for the selected electrolyte containing the LiPF6and LiFAP salt, respectively). As expected, the molar volume of

electrolytes based on 1 mol L−1 of LiX salt in EC/DMC (50:50 wt%)increases with the temperature and is higher for the LiFAP-basedelectrolyte than for LiPF6 solution. For example, the molar vol-umes are close to 71.5 and 85.8 cm3 mol−1 at 20 ◦C and 74.8 and

98 P. Porion et al. / Electrochimica Acta 114 (2013) 95– 104

Table 1Structure, name, abbreviation, minimum and maximum intramolecular distances (Min/max), sigma profile and COSMO volume evaluated by using the COSMOthermXinterface of studied anions (X−) in lithium salt LiX and solvents.

Solvent/ion Structure Min/max (nm) COSMO volume (Å3)

0.41/0.87 108.34

0.49/0.67 94.17

0.25/0.25 [23] 103.71

0.25/0. 38 [23] 321.37

8r

isaat(aetfwtSsib

9.8 cm3 mol−1 at 70 ◦C, for LiPF6 and LiFAP based electrolytes,espectively.

The second parameter than can affect the electrolyte densitys related to interactions in solution (e.g. ion–ion, ion–solvent,olvent–solvent interactions), as these interactions can reflect howtoms, ions or molecules are packed together in a solution. Here,ccording to Table 1 and Fig. S1 of the supporting information,he main differences between the two electrolytes are the sizeCOSMO volumes are close to 321 A3 and 104 A3 for FAP− and PF6

−

nions, respectively), the charge distribution on the surface, andspecially the molecular weight of the anions. Based on which,he substitution of three fluorine atoms by three (CF2CF3) groupsrom PF6

− to FAP− anions induces an increase of the moleculareight of the anion and a steric effect. Furthermore, this substi-

ution affects also the charge distribution (sigma profile see Fig.

1 of the supporting information), as well as the interaction inolutions. Nevertheless, according to the limited salt compositionn the EC/DMC (50:50 wt%) mixture (1 mol L−1 of LiX salt) it maye assumed that no major reorganization of the solution can be

expected by substituting LiPF6 by LiFAP. In other words, we assumeherein that the molecular weight and steric effect of the anion arethe predominant factor explaining the observed density increasesby substituting LiPF6 by LiFAP in selected electrolytes.

3.2.2. ViscosityThe viscosity data of the selected electrolytes containing

1 mol L−1 of LiX salt (X = PF6− or FAP−) in EC/DMC (50:50 wt%) as

a function of the temperature from 20 to 70 ◦C are shown in Fig. 2and tabulated at two selected temperatures in Table 2. It can beobserved that despite the fact that LiPF6-based electrolytes is moreconductive than LiFAP in EC/DMC (50:50 wt%) (see Table 2), theviscosity of the two electrolytes is similar, for example, the vis-cosity values are close to 4.21 and 4.30 mPa s at 25 ◦C and 1.89and 1.84 mPa s at 70 ◦C, for LiFAP and LiPF6 based electrolytes,

respectively. This experimental observation reinforce the assump-tion that the substitution of three fluorine atoms by three (CF2CF3)groups from PF6

− to FAP− anions induces an increase of the vander Waals force, but decreases also Coulombic interaction with

P. Porion et al. / Electrochimica Acta 114 (2013) 95– 104 99

Fig. 2. Viscosity measurements, �, of electrolytes based on 1 mol L−1 of LiX salt(X = �, PF − or �, FAP−) in EC/DMC (50:50 wt%) as a function of the temperaturefL

tpooti2tottvbmmbpss

td1rg

�

wumt

TDo

Table 3VTF equation parameters for the conductivity and the viscosity (T0, �0, �0, B, and B′)of selected electrolytes based on 1 mol L−1 of LiX salt (X = PF6

− or FAP−) in EC/DMC(50:50 wt%) at 0.1 MPa.

Conductivity T0 (K) �0 (mS cm−1) B� · R (kJ mol−1) R2 a

1 mol L−1 LiPF6 inEC/DMC (50:50 wt%)

176.9 127.9 2.419 0.9999

1 mol L−1 LiFAP inEC/DMC (50:50 wt%)

151.2 209.6 3.966 0.9999

Viscosity T0 (K) �0 (mS cm−1) B� · R (kJ mol−1) R2 a

1 mol L−1 LiPF6 inEC/DMC (50:50 wt%)

169.9 0.1969 3.259 0.9996

1 mol L−1 LiFAP in 153.5 0.1205 4.300 0.9999

thermore, the increase of the van der Waals forces and of the stericeffect of the structure caused by the substitution of three fluorineatoms by three (CF2CF3) groups from PF6

− to FAP− anions affectsmore strongly the conductivity of the electrolyte than its viscosity

6

rom 20 to 70 ◦C at 0.1 MPa. The insert figure represents the VTF type fitting of theiFAP-based electrolyte experimental data with parameters reported in Table 3.

he Li+ cations in the solution (see Table 1 and Fig. S1 of the sup-orting information). In fact, both effects seem to act oppositelyn the cohesive energy of the solution, and have a huge impactn the electrolytes viscosity. As expected, and shown in Fig. 2,he viscosity of both electrolytes decreases as the temperaturencreases from 4.7 to 1.8 mPa s when temperature decreases from0 to 70 ◦C as reported in Table S1 in the supporting informa-ion and shown in Fig. 2. This effect is due to the higher mobilityf ions when the solvent organization is reduced (by increasinghe global entropy of the solution). Although all type of interac-ions: ion–ion, ion–solvent and solvent–solvent contribute to theiscosity, it is generally admitted that the viscosity is dominatedy intermolecular bonding between solvent, here alkyl-carbonateolecules together, and between solvent molecules and Li+ cationsore solvated than anions, All of these interactions can be affected

y a temperature increase. For example, an increasing of the tem-erature reduces then the micro-domains created by the strongolvation of lithium cation, which leds more mobility of species inolution [24].

Because, each variation of viscosity values as a function ofemperature exhibits a non-Arrhenius behavior, i.e. a distinctownward curvature at in the Arrhenius plot behavior (ln(�) versus/T), Vogel-Tamman-Fulcher (VTF) equation (Eq. (2)) was used toepresent the temperature dependence on the viscosity of investi-ated electrolytes.

= �0 exp[

B�

T − T0

](2)

here B�, �0 and T0 are fitting parameters. B� and T0 constants

sually referred to as pseudo activation energy (B� · R; with R is theolar gas constant) and ideal glass transition temperature, respec-

ively.

able 2ensity, ionic conductivity and viscosity of selected electrolytes containing 1 mol L−1

f LiX salt (X = PF6− or FAP−) in EC/DMC (50:50 wt%) at selected temperatures.

Electrolyte EC/DMC(50:50 wt%)

� (g cm3) � (mS cm−1) � (mPa s)

25 ◦C 70 ◦C 25 ◦C 70 ◦C 25 ◦C 70 ◦C

1 mol L−1 LiPF6 1.3027 1.2508 11.66 22.44 4.212 1.8881 mol L−1 LiFAP 1.3959 1.3396 8.12 17.54 4.301 1.840

EC/DMC (50:50 wt%)

a Correlation coefficient.

Experimental viscosity data of each electrolyte as a function oftemperature from 20 to 70 ◦C are then fitted by using Eq. (2) asshow in insert in Fig. 2 in the case of the LiFAP-based electrolytes,for example. Fitting parameters determined during this study arepresented herein in Table 3.

3.2.3. ConductivityConductivity data of the selected electrolytes containing

1 mol L−1 of LiX salt (X = PF6− or FAP−) in EC/DMC (50:50 wt%) as

a function of the temperature from 10 to 80 ◦C are shown in Fig. 3and tabulated at two selected temperatures in Table 2. As shownin Fig. 3, conductivity data were measured during a heating and acooling loads (see Table S2 in the supporting information).

As expected, the conductivity of these electrolytes increaseswith the temperature from 10.48 and 7.26 mS cm−1 at 20 ◦C andreaches 24.6 and 19.8 mS cm−1 at 80 ◦C for selected electrolytesbased on the LiPF6 and LiFAP salt, respectively. At 20 ◦C, the con-ductivity of the 1 mol L−1 LiPF6 in EC/DMC (50:50 wt%) electrolyteis close to that reported by Aravindan et al. [23] (e.g. 10.5 mS cm−1),showing that in controlled atmosphere, the decomposition of thePF6

− anion is prevented and not observed for temperature up to80 ◦C, as showed by the absence of hysteresis in the variation ofthe conductivity as function of the temperature (see Fig. 3). Fur-

Fig. 3. Influence of temperature on the conductivity of electrolytes based on1 mol L−1 of LiX salt (X = �, PF6

− or �, FAP−) in EC/DMC (50:50 wt%) at 0.1 MPa. Filledand open symbols correspond to the conductivity data obtained during heating andcooling loads, respectively. The solid line serves as a guide to the eye.

100 P. Porion et al. / Electrochimica Acta 114 (2013) 95– 104

F he ioni tting.

aam

((c

�

�

wpo

pp

ttwvftsacophaweeoh

3

iw(mF

3.3.1. Self-diffusion coefficient measurementsThe PGSE-NMR method [29–31] is used to measure the self-

diffusion of a variety of ionic species [32]. For each electrolyte inthe present study, the temperature dependence on the cationic

ig. 4. Arrhenius (a) and VTF (b) plots describing the temperature dependences on tn EC/DMC (50:50 wt%) at 0.1 MPa. The solid lines represent the Arrhenius or VTF fi

s discussed above (see Table 2 and Figs. 2 and 3). For example,t 25 ◦C the LiPF6-based electrolyte (11.62 mS cm−1) is 1.43 timesore conductive than LiFAP one (8.12 mS cm−1).As shown in Fig. 4a, both systems present a non-Arrhenius (Eq.

3)) behavior, therefore the Vogel-Tamman-Fulcher (VTF) equationEq. (4)) was used to represent the temperature dependence on theonductivity of investigated electrolytes, as shown in Fig. 4b.

= �0 exp[−Ea

RT

](3)

= �0 exp[ −B�

T − T0

](4)

here �0 (mS cm−1), B� (K), and T0 (K) are fitting parameters. Theroduct B� · R (where R is the molar gas constant) has the dimensionf the activation energy (kJ mol−1).

Fig. 4b represents the VTF plot. According to Eq. (4), fittingarameters can be determined for the studied electrolytes and areresented herein in Table 3.

As shown in Table 3, for each selected electrolyte, it appearshat the T0 VTF parameter, which can be assimilated as the glassransition temperature of the electrolyte, seems to be constanthatever the transport properties VTF-type fittings. A higher T0

alue is obtained in the case of the LiPF6-based electrolyte thanor the LiFAP one. This is consistent with the fact that the glassransition decreases with increasing asymmetry of molecules inolution [25]. Interestingly, as presented also in Table 3, the pseudo-ctivation energy, e.g. the product Bi · R; (with i = � or �, for the ioniconductivity and viscosity, respectively) are both higher in the casef the LiFAP-based electrolyte. For example, it appears that theseudo-activation energy for the ionic conductivity is 1.63 timesigher for LiFAP-based system than for the LiPF6 one. This indicates

larger increase in conductivity and a larger decrease in viscosityhen temperature increases for the LiFAP-based electrolytes. This

xperimental observation can be linked to the fact that LiFAP-basedlectrolytes containing common alkyl-carbonates such as EC, DMCr DEC have better LiBs properties than those reported for LiPF6 atigh temperature [9].

.2.4. IonicityClassical Walden rule diagram was used to determine the

onicity of studied electrolytes. From that the ionic mobilities

ere represented through the equivalent conductivity, � = �/Csalt

S cm2 mol−1), as function of fluidity (Poise−1), � = 1/�, of theedium, which is related to the ions association in solution [26,27].

ig. 5 shows the variation of log(�) versus log(1/�) at various

ic conductivity of electrolytes based on 1 mol L−1 of LiX salt (X = �, PF6− or �, FAP−)

temperatures from 20 to 70 ◦C for selected EC/DMC (50:50 wt%)solution containing the 1 mol L−1 of LiX salt (X = PF6

− or FAP−).According to the Walden rule, the ideal line represents fully disso-ciated ionic solutions having ions of equal mobility such as aqueousKCl solutions at high dilution [26–28]. Fig. 5 shows, that both stud-ied electrolytes can be classified as “good ionic liquid” accordingto the Walden classification. Furthermore, according to this clas-sification, electrolyte based on LiFF6 salt is more “ionic” than thatbased on the LiFAP. As the ionicity represents the balance betweenconductivity and fluidity, this observation is quite logical since bothelectrolytes have a same viscosity, but the electrolyte-based on theLiFAP has a lower conductivity than LiPF6. In other words, the bet-ter LiBs performances reported in the literature [9] of LiFAP thanLiPF6 electrolytes, cannot be explained by using the Walden classi-fication, but could be associated to a difference on the self-diffusioncoefficients of ions and on their transport numbers in solution, asdescribed below.

3.3. Pulsed-gradient spin-echo NMR (PGSE-NMR)

Fig. 5. Walden plot showing the classification of investigated electrolytes based on1 mol L−1 of LiX salt (X = �, PF6

− or �, FAP−) in EC/DMC (50:50 wt%) over a temper-ature range from 20 to 70 ◦C at 0.1 MPa. The line corresponds to the Walden Plotideal line corresponding to aqueous KCl solution [26].

P. Porion et al. / Electrochimica Acta 114 (2013) 95– 104 101

F ith bipolar gradient pulses used in this study to suppress effects of convection. � is thee ddy-current delay (see text for values), g is the intensity of the pulse gradient and ı itsl e cycling (see Ref. [33] for more detail about this sequence).

stpkptwJeFtt

cscfi

E

wsg�dasaromaeu1Tpeid

ntcaealmLprc

Table 4Temperature dependences of the self-diffusion coefficients D of the different ionsin the 1 mol L−1 LiPF6 in EC/DMC (50:50 wt%) system as a function of the nucleusobserved by PGSE-NMR experiments.

1 mol L−1 LiPF6 in EC/DMC (50:50 wt%)

7Li PGSE-NMR 19F PGSE-NMR 31P PGSE-NMR

T (◦C) D (m2 s−1) T (◦C) D (m2 s−1) T (◦C) D (m2 s−1)

79.98 5.539 × 10−10 80.36 8.950 × 10−10 80.36 7.765 × 10−10

74.95 5.199 × 10−10 75.07 8.326 × 10−10 75.07 7.645 × 10−10

69.94 4.940 × 10−10 70.29 7.857 × 10−10 70.29 6.865 × 10−10

64.95 4.595 × 10−10 64.95 7.240 × 10−10 64.95 6.877 × 10−10

60.00 4.260 × 10−10 60.22 6.822 × 10−10 60.22 6.903 × 10−10

54.98 4.007 × 10−10 54.87 6.262 × 10−10 54.87 6.076 × 10−10

50.00 3.764 × 10−10 50.21 5.857 × 10−10 50.21 5.913 × 10−10

44.97 3.430 × 10−10 45.47 5.453 × 10−10 45.47 5.068 × 10−10

39.99 3.165 × 10−10 40.09 4.988 × 10−10 40.09 4.806 × 10−10

34.98 2.912 × 10−10 35.36 4.620 × 10−10 35.36 4.611 × 10−10

30.03 2.657 × 10−10 30.03 4.200 × 10−10 30.03 4.096 × 10−10

27.03 2.522 × 10−10 25.33 3.767 × 10−10 25.33 3.754 × 10−10

24.97 2.431 × 10−10 20.00 3.548 × 10−10 20.60 3.584 × 10−10

During discharge the LiB, the Li ions migrate and diffusethrough the electrolyte to the cathode where they recombine withthe electrons, whereas anions migrate in the opposite directiontoward the anode. However, only the part of the current that is

Table 5Temperature dependences of the self-diffusion coefficients D of the different ions in1 mol L−1 LiFAP in EC/DMC (50:50 wt%) system as a function of the nucleus observedby PGSE-NMR experiments.

1 mol L−1 LiFAP in EC/DMC (50:50 wt%)

7Li PGSE-NMR 19F PGSE-NMR 31P PGSE-NMR

T (◦C) D (m2 s−1) T (◦C) D (m2 s−1) T (◦C) D (m2 s−1)

85.04 5.703 × 10−10 80.36 5.633 × 10−10 80.36 5.102 × 10−10

79.98 5.286 × 10−10 75.07 5.138 × 10−10 75.07 4.658 × 10−10

74.95 4.906 × 10−10 70.29 4.914 × 10−10 70.29 4.634 × 10−10

69.94 4.191 × 10−10 64.95 4.513 × 10−10 64.95 3.897 × 10−10

64.95 4.276 × 10−10 60.22 4.180 × 10−10 60.22 3.931 × 10−10

60.00 3.770 × 10−10 54.87 3.771 × 10−10 54.87 3.415 × 10−10

54.98 3.595 × 10−10 50.21 3.493 × 10−10 50.21 3.247 × 10−10

50.00 3.079 × 10−10 45.47 3.156 × 10−10 45.47 3.137 × 10−10

44.97 2.953 × 10−10 40.09 2.882 × 10−10 40.09 2.537 × 10−10

39.99 2.482 × 10−10 35.36 2.544 × 10−10 35.36 2.495 × 10−10

34.98 2.414 × 10−10 30.03 2.321 × 10−10 30.03 2.155 × 10−10

30.03 1.967 × 10−10 25.33 1.995 × 10−10 25.33 2.089 × 10−10

ig. 6. (a) Schematic view of the double stimulated spin-echo (DSTE) experiment wffective diffusion time, is gradient recovery delay and TLED is the longitudinal eength. (b) The selected coherence transfer pathways selected by an adequate phas

elf-diffusion coefficients are evaluated by lithium (7Li) NMR, whilehe anionic ones are extracted both from fluorine (19F), and phos-horus (31P) NMR experiments. At high temperature, it is wellnown that the convection currents induced in non-viscous sam-les by the temperature gradients along the Oz-axis affects stronglyhe self-diffusion measurements [33,34]. To avoid such artifacts,e have used a specific NMR sequence, originally proposed by

erschow and Muller [33], based on a Double-Stimulated Echoxperiment, DSTE, with bipolar gradient pulses as schematized inig. 6. In this pulse sequence, a delay TLED is also added just beforehe acquisition of the FID to reduce the distortion of the spectra dueo the eddy-currents.

By using the PGSE-NMR method, self-diffusion coefficients werealculated by measuring the decrease of the NMR echo signal inten-ity through increasing magnetic field gradients (g). Self-diffusionoefficients, D, were then obtained by a simple linear least-squaretting of the echo attenuation E(q, �) according to Eq. (5):

(g, �) = I(g, �)I(0, �)

= exp

[−2g2ı2D

(� + 4ı

3+ 3

2

)](5)

here I(g, �) and I(0, �) are the echo intensities, respectively, mea-ured with and without the field gradient g (varying between 0 andmax), ı is its duration, is the gyromagnetic ratio of the nuclei,

is the diffusion time (fixed to 20 ms), is the gradient recoveryelay (fixed to 300 �s) and D is the self-diffusion coefficient. Forll PGSE-NMR runs, the magnetic field gradient duration (ı) waset to 4 ms, the longitudinal eddy-current delay (TLED) was 2 msnd the maximum value of the magnetic field gradient (gmax) wasanged from 0.7 to 1.8 T m−1 depending on the gyromagnetic ratiof the NMR nuclei and the set point temperature of the measure-ents. The self-diffusion coefficient D was then extracted from

series of measurements involving different g values. As suchxperiments are very time-consuming; only 8 gradient values aresually taken for this study. The recovery delay was varied between0 s and 30 s in the relation with the longitudinal relaxation time1 value measured by the inversion recovery (180◦-tau-90◦-Acq)ulse sequence. In addition, the 90◦ pulse length was calibrated forach sample and varied from 11.4 �s to 15.25 �s. For both stud-ed electrolytes, the self-diffusion temperature dependences of theifferent ion species are summarized in Tables 4 and 5.

Hayamizu et al. [35] have evaluated the cationic transportumber tLi

+ calculated from the self-diffusion coefficient asLi

+ = DLi/(DLi + DAnion) in alkyl-carbonates. Their results show thatyclic carbonates like EC or PC have cation transport numbersround 0.40 because the solvated lithium ion diffuses slower thanach isolated anion. In acyclic esters or alkyl-carbonates like ECnd DMC, the cation transport number is larger (tLi

+ ∼ 0.52). Hence,ithium ions diffuse almost at the same speed as the anion in these

edia. Herein, calculated tLi+ = 0.38 ± 0.01 and tLi

+ = 0.48 ± 0.01 for

iPF6 and LiFAP electrolytes, respectively. It is notable that trans-ort number of lithium in LiPF6-based electrolyte is close to thateported in the case of Li+ transport number tLi

+ in EC or PC, whichan be linked also to the fact that each solvated lithium ion diffuses

20.00 2.201 × 10−10

16.02 2.155 × 10−10

slower than the isolated anion [36]. While, in the case of LiFAP,the cation transport number is larger, hence, lithium ions diffusealmost at the same speed than the anion in these media. By com-parison tLi

+ calculated for LiTFSI is close to 0.40 and 0.52 in EC andDMC, respectively [35].

+

24.97 1.960 × 10−10 21.20 1.860 × 10−10 20.60 1.876 × 10−10

20.00 1.586 × 10−10 20.60 1.960 × 10−10

17.02 1.605 × 10−10

15.02 1.567 × 10−10

102 P. Porion et al. / Electrochimica Acta 114 (2013) 95– 104

10-10

10-9

2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5

Li+ 7

Li NMR self-diffusion

PF6

- 19F NMR self-diffusion

PF6

- 31P NMR self-diffusion

Se

lf-d

iffu

sio

n c

oe

ffic

ien

t D

(

m2/s

)

1000 / T (K-1

)

LiPF6 1M / (EC/DMC)

(a)

10-10

10-9

2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5

Li+ 7

Li NMR self-diffusion

FAP- 19

F NMR self-diffusion

FAP- 31

P NMR self-diffusion

Se

lf-d

iffu

sio

n c

oe

ffic

ien

t D

(

m2/s

)

1000 / T (K-1

)

LiFAP 1M / (EC/DMC)

(b)

F coeffii w wit

caoti(balticittsndmaiTpo

3

dtbabsti

TAE

ig. 7. Temperature dependences on 7Li, 19F and 31P nuclei PGSE-NMR self-diffusionn EC/DMC (50:50 wt%). The solid lines represent the best fits using the Arrhenius la

arried by the Li+ cations perform useful work (i.e. the electronsssociated with those Li+ ions in the outer circuit). The proportionf the current that is carried by a particular ion is relied to theransport number for that ion. As transport number for Li+ in a typ-cal Li-ion cell using a LiPF6 electrolyte salt is between 0.35 and 0.40here 0.38). This means that only 40% of the total current within theattery is from the lithium ions and the other 60% from the PF6

−

nions. These considerations applied to transport properties mayead to a simplistic approach, the lower the transport number Li+,he greater the adverse concentration gradient that occurs, caus-ng concentration polarization in the same way as a concentrationell which then absorbs part of the battery’s energy and reducests performances. However, Dalinov and Notten show by modelinghat in the very early stages of current application when no concen-ration profile has been developed yet, the transport of Li+ ions areolely carried by the migration process [37,38]. Subsequently, sig-ificant concentration profiles develop in time, contributing to theiffusion. Consequently, for fully dissociated salts, the diffusion andigration contribution to the total over-potential were exactly bal-

nced. Their results show clearly that the contribution of migrationncreases strongly when the degree of salt dissociation declines.hese results clearly show that migration is of major importance inoorly dissociated electrolytes, the transference number of Li-ionsnly determines the instantaneous voltage drop.

.3.2. Effect of temperature on self-diffusion coefficientFirstly, according to Fig. 7, it clearly appears that the self-

iffusion coefficients of lithium cations Li+ are quite similar withinhe precision of the PGSE-NMR method (i.e. few per cent) foroth electrolytes at higher temperature. On the contrary, thenions self-diffusion behaviors are notably different: for LiPF6-

ased electrolyte, across the entire temperature range studied, theelf-diffusion coefficients of PF6

− anions are always 1.6 greater thanhat of Li+ cations (Fig. 7a); whereas for LiFAP based electrolyte, allons species diffuse at the same rate as shown in Fig. 7b.

able 6rrhenius equation parameters (D0 and Ea) for the self-diffusion coefficients of the differC/DMC (50:50 wt%) as a function of the nucleus observed by PGSE-NMR experiments. W

Sample Ion, NMR nucleus

1 mol L−1 LiPF6 inEC/DMC (50:50 wt%)

Li+, 7Li NMR

PF6− , 19F NMR

PF6− , 31P NMR

1 mol L−1 LiFAP inEC/DMC (50:50 wt%)

Li+, 7Li NMR

FAP− , 19F NMR

FAP− , 31P NMR

cients in selected electrolytes based on 1 mol L−1 of LiX salt (a, X = PF6−; b, X = FAP−)

h parameters reported in Table 6.

Secondly, the influence of the temperature on the self-diffusioncoefficients was then quantified by Arrhenius plots and the exper-imental data was fitted according to:

D(T) = D0 exp(

− Ea

RT

)(6)

where D0 (m2 s−1) is a pre-exponential factor, Ea (kJ mol−1) is theactivation energy for the diffusion process and R is the molar gasconstant. Best-fit values are reported in Table 6.

According to Table 6, it appears that the activation energy Ea forall ionic species involved in the LiFAP electrolyte are 30% higherthan those in the LiPF6 solution. This can be explained by thehigh molar mass of the FAP− anion due to CF2CF3 groups, whichimpacts the energy of activation of both Fluorine and Phosphorusnucleus, and indirectly as a result of anion–cation interactions ofthose Li+. Finally, we may link also this difference on these activa-tion energies by a difference on the resistance to viscous flow dueto an increase of the van der Waals forces associated to the substi-tution of F by CF2CF3 groups by substitution PF6

− anion by FAP−

in solution.

3.3.3. The effective ionic radiusAccording to the Stokes–Einstein principle, the self-diffusion

coefficient of a spherical particle in solution of viscosity � can berepresented as:

Di = kT

6��ri,eff(7)

where Di is the self-diffusion coefficient, k is the Boltzmann con-stant, T is the absolute temperature, � is the viscosity of thesolution, and ri,eff is the hydrodynamic radius of molecule species,

i. The effective radius of each investigated species was thenevaluated herein (see Fig. 8) using the self-diffusion coefficientand viscosity measurements (Figs. 2 and 7). Herein, the calcu-lated reff of Li+ in both electrolytes, rLi,eff

+ = (0.34 ± 0.02) nm, is

ent ions in selected electrolytes based on 1 mol L−1 of LiX salt (X = PF6− or FAP−) in

here, R2 is the correlation coefficient resulting from the fit procedure.

D0 (m2 s−1) Ea (kJ mol−1) R2

4.662 × 10−8 12.991 0.99898.535 × 10−8 13.390 0.99963.890 × 10−8 11.394 0.9727

1.353 × 10−7 16.285 0.99281.236 × 10−7 15.809 0.99757.430 × 10−8 14.621 0.9871

P. Porion et al. / Electrochimica Acta 114 (2013) 95– 104 103

Fig. 8. Self-diffusion coefficients of: Li+ cation in investigated electrolytes basedon 1 mol L−1 of LiX salt (X = �, PF6

− or , FAP−) in EC/DMC (50:50 wt%); and anionspecies: �, PF6

−; �, FAP− , where filled and open symbols correspond to the self-diffusion coefficients obtained using, respectively 19F and 31P nuclei PGSE-NMR data,atE

a[t0cEmtbirSItiitMttPltmtv

3

da

�

wb(cd

a

s a function of (T/�) ratio according to Stokes–Einstein principle used to estimatehe effective radius of ions in solutions. The solid lines represent the best fits usingq. (7).

lmost four times larger than the radius of naked Li+ (0.073 nm)39]. Knowing that the one-half intramolecular average dis-ances of the EC and DMC molecules are close to 0.290 and.320 nm, respectively (see Table 1), it is then possible to cal-ulate the radius, rcal.,i of a Li+ cation solvated by a ring ofC molecules (rcal.,EC = 0.073 nm + 0.290 nm = 0.363 nm) and DMColecules (rcal.,DMC = 0.073 nm + 0.320 nm = 0.393 nm). According

o the mole fraction of EC and DMC in the solution, it is then possi-le to calculate an average of radius rcal.sol of a Li+ cation solvated

n EC/DMC (50:50 wt%) solution, which is close to 0.378 nm. Thiscal.,sol value is close (e.g. �r/reff < 10%) to that calculated using thetokes–Einstein principle, e.g. rLi,eff

+ = 0.34 ± 0.02 nm using Eq. (7).f consider that each Li+ cation is preferentially solvated by EChan by DMC molecules, even under high concentration of DMCn the electrolyte, as reported by Xu et al. [40], the retained values rcal.,sol = 0.363 nm. Interestingly, the rLi,eff

+ radius for both elec-rolytes is close to the radius of Li(EC)4

+ species calculated byatsuda et al. as close to 0.370 nm [41]. On the other hand, by using

he same methodology, the reff of PF6− anion is estimated as close

o 0.23 ± 0.03 nm, which is quite similar to the radius of the nakedF6

− anion (e.g. 0.265 nm, see Table 1), as well as that reported in theiterature (0.255 nm) [23], for example. In the case of FAP− anion,he reff was estimated as 0.34 ± 0.02 nm. This value is in range of

aximum (e.g. P-CF2-CF3) and minimum (e.g. P-F) dimensions ofhe naked FAP− anion (0.38 nm, see Table 1), in agreement withalue reported in the literature (0.377 nm) [23].

.3.4. Dissociation degree of LiX salt (X = FAP and PF6)From the Nernst–Einstein equation (Eq. (7)), the specific con-

uctivity �NMR can be evaluated using the diffusion rates of cationsnd anions.

NMR = NAe2(D+ − D−)kT

(8)

here �NMR, specific conductivity (S m−1 mol); NA, Avogadro num-er; e, elementary charge; D+, cation self-diffusion coefficientm2 s−1); D−, anion self-diffusion coefficient (m2 s−1); k, Boltzmann

onstant; T, absolute temperature (K); �cond/�NMR, dissociationegree.

The self-diffusion coefficients obtained by PGSE-NMR pointed at translational motion (self-diffusion) of the NMR-sensitive nuclei.

Fig. 9. Molar conductivity ratios (�cond/�NMR) for electrolytes based on 1 mol L−1 ofLiX salt (X = �, PF6

− or �, FAP−) in EC/DMC (50:50 wt%) as a function of temperatureat 0.1 MPa.

Thus, the �NMR values were derived from the assumption that allof the diffusing species detected during the PGSE-NMR measure-ment contributed to the molar conductivity. On the other hand, the�cond values (i.e. the direct current: d.c., conductivity) were basedon the migration of the charged species under an electric field.Hence, the Nernst–Einstein equation holds only if species involvedin diffusion were also responsible for conduction. In the presenceof neutral ion pairs, the �NMR values were expected to differ fromthose determined from conductivity (�cond).

The strong ion–ion interactions occurring in electrolyte shouldaffect the D and � values by separate mechanisms. The calculatedionic dissociation degree can then be evaluated using the measuredresults of molar conductivity as ratios (�cond/�NMR) for selectedelectrolytes. Fig. 9 summarizes the dissociation degree of LiPF6 andLiFAP in the selected EC/DMC (50:50 wt%) mixture. Fig. 9 shows,that the ionic dissociation degree of the LiFAP salt this mixture,which is close to 0.52, is higher than that calculated in the caseof LiPF6 salt (e.g. 0.47). This higher ionic dissociation degree of theLiFAP than of LiPF6 in the selected EC/DMC (50:50 wt%) mixture,can be then explained by the higher delocalization of the negativecharge in FAP− anion (see Table 1) and steric hindrance, resulting inrelatively weaker Li+-anion Coulombic interactions which reducethe likelihood of ion association in solution, than that observedwith the PF6

−. However, Fig. 9 shows also that the dissociationof LiFAP in the solution is less affected by a temperature varia-tion than the LiPF6, as its ionic dissociation increases strongly withtemperature.

4. Conclusions

Herein, we have investigated the volumetric and transport prop-erties, such as the conductivity (�), viscosity (�) and self-diffusioncoefficients (D), of two electrolytes based on 1 mol L−1 of LiPF6 orLiFAP salt dissolved in a binary mixture of EC/DMC (50:50 wt%),as a function of temperature from 20 to 70 ◦C at 0.1 MPa. Firstly,we showed that the viscosity of the two electrolytes is basicallythe same, while the density and the conductivity data of the elec-trolyte based on the LiPF6 are lower and higher that those reportedfor the LiFAP solution, respectively. These properties differencescan be associated by the increase of the van der Waals forcesand of the steric effect of the structure caused by the substi-

tution of three fluorine atoms by three (CF2CF3) groups fromPF6

− to FAP− anions. Secondly, we demonstrated by using thePGSE-NMR, that the self-diffusion of the lithium cation is quitesimilar in both electrolytes and close to that of FAP− anions, but

1 imica

1poltrcnseaoLtiLawLacoiatwoa

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[

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[

[

[

[

[

[

[

[

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[

[

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[

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[40] K. Xu, A. von Cresce, Li+-solvation structure directs interphasial processes on

04 P. Porion et al. / Electroch

.6 times smaller than that of PF6− anions. Additionally, the trans-

ort number of lithium tLi+, effective radius of solvated Li+ and

f PF6− and FAP− anions, and the degree of dissociation of these

ithium salts in EC/DMC (50:50 wt%) solution were then inves-igated through Stokes–Einstein and Nernst–Einstein equations,espectively. Based on which, we have demonstrated that the cal-ulated effective radius of solvated Li+ is higher than the radius ofaked Li+, demonstrating again that the Li+ is solvated by a ring ofolvent molecules in solution. While both anions have a calculatedffective radius close to those reported in the case of the nakednions. Furthermore, by using the Nernst–Einstein equation, webserved a higher ionic dissociation degree of the LiFAP than ofiPF6 in the selected EC/DMC (50:50 wt%) mixture, which can behen explained by the higher delocalization of the negative chargen FAP− anion and steric hindrance, resulting in relatively weakeri+-anion Coulombic interactions which reduce the likelihood of ionssociation in solution, than that observed with the PF6

−. In otherords, we showed herein that the differences between LiFAP than

iPF6 electrolytes, cannot be explained by using solely the viscositynd conductivity measurements or the Walden classification, butould be associated to a difference on the self-diffusion coefficientsf ions, on their transport numbers, and on the salt dissociationn solution. We can therefore conclude that the transport numbersnd the dissociation degree of ions in solutions clearly demonstratehe conflicting nature of the requirements and the advantage of theell-balanced transport properties (�, �, D) of electrolyte based

n lithium salt to enhance its performance as electrolyte for LiBpplications.

cknowledgments

Thanks to SAFT and Région Centre, France for their financialupport through the Y.R.D. PhD thesis. The DSX100 Bruker spec-rometer used for the PGSE-NMR study was purchased thanks tohe funding allocated by the Région Centre, France.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.electacta.013.10.015.

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