DMC

Electrochimica Acta 180 (2015) 778–787

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LiTDI as electrolyte salt for Li-ion batteries: transport properties in EC/ DMC Christopher L. Berhauta,c , Patrice Porionb , Laure Timpermana , Grégory Schmidtc , Daniel Lemordanta , Mériem Anoutia,* a b c

Laboratoire PCM2E (EA 6296), UFR Sciences et Techniques, Université de Tours, Parc de Grandmont, 37200 Tours, France ICMN CNRS - Université d'Orléans, UMR 7374, 1b rue de la Férollerie, 45071 Orléans Cedex 02, France ARKEMA, rue Henri Moissan, 69493 Pierre Bénite, France

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 July 2015 Received in revised form 29 August 2015 Accepted 31 August 2015 Available online 3 September 2015

The 4,5-dicyano-2-trifluoromethyl-imidazolide (TDI
) anion is a potential lithium salt anion for lithiumion batteries. In this paper the transport properties of LiTDI solutions in EC/DMC (50/50wt%) are compared to competing salts such as LiTFSI (lithium bis(trifluoromethylsulfonyl) imide), LiFAP (lithium tris(pentafluoroethane)-trifluorophosphate) and the most commonly used LiPF6 (lithium hexafluorophosphate). LiTDI in EC/DMC exhibits the lowest viscosity among the investigated electrolytes, but also the lowest conductivity: 6.84 mS cm
1 at 25  C (1 mol L
1). The self-diffusion coefficients (D(7Li) and D (19F)) determined by observing 7Li and 19F nuclei with the pulsed-gradient spin-echo (PGSE) NMR technique allowed access, through application of the Stokes-Einstein and Nernst-Einstein equations, to the lithium transference number (t+), the effective ionic radius and the ionic dissociation coefficient (aD) of LiTDI in EC/DMC. The ion-pair dissociation and t+ were also determined using conductivity and viscosity measurements and by following the Bruce-Vincent method respectively and were in good agreement with those obtained by PGSE-NMR. LiTDI displays in EC/DMC a cationic transference number t+ = 0.58, which is higher than that of LiPF6 in the same solvent mixture, meaning that the mobility of the PF6
anion is higher than the larger TDI
anion. Still, results from both methods indicate that, LiTDI, as many other lithium salts is only partially dissociated in EC/DMC. Only 31% of the ion pairs are dissociated at 25  C whereas for LiPF6, LiTFSI and LiFAP this number reaches at least 64% in 1 mol L
1 solutions. ã 2015 Published by Elsevier Ltd.

Keywords: LiTDI lithium-salt electrolyte ionic dissociation diffusion coefficients Li-ion battery

1. Introduction The lithium-ion battery (LiB) technology has become popular over the years for portable electronics [1]. Now in a world knowing a progressive electrification of transport systems LiBs are designed to fit the recommendations of large-scale applications such as electric vehicles but also aerospace and military devices. Therefore the continuing success and progress of the LiB technology depends on the careful choice of the battery components such as the electrode materials and the electrolyte [2–4]. It is needless to say that the electrolyte is a very important component in batteries as it allows ionic movement, and should be able to maintain its functions over a large life-span without presenting safety risks, especially if damaged. The most commonly used salt in electrolytes is lithium hexafluorophosphate (LiPF6), generally dissolved in mixtures of alkylcarbonates. LiPF6 is actually

* Corresponding author. Tel.: +33 247366951; Fax: +33 247367360. E-mail address: [email protected] (M. Anouti). http://dx.doi.org/10.1016/j.electacta.2015.08.165 0013-4686/ ã 2015 Published by Elsevier Ltd.

the reference Li-salt for LiBs due to its numerous convenient properties [3,5,6]. However its instability and moisture sensitivity, which induce high risks of releasing HF and/or PF5 via its thermal decomposition or hydrolysis in the presence of traces of water, have been highly criticized justifying the importance of finding a replacement. For these reasons in-depth studies have been conducted in order to select more stable and safer lithium salts such as lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) [6] and lithium tris(pentafluoroethane)-trifluorophosphate (LiFAP) [5,7,8]. In 1998, in looking for a truly covalent anion not prone to hydrolysis and HF generation, the concept of “Hückel anions” was introduced in the field of electrolytes for LiBs [9]. Owing to (i) its high thermal stability, (ii) stability in the presence of water and (iii) its electrochemical stability [10], the lithium 4,5-dicyano-2trifluoromethyl-imidazolide (LiTDI) has been proposed as a new lithium salt for use in LiBs. The electrolyte's most important function is charge transportation from one electrode to the other. The main requirements of the electrolyte are: electrical and chemical stability (no decomposition or gas generation at the

C.L. Berhaut et al. / Electrochimica Acta 180 (2015) 778–787

electrode interfaces), thermal stability (no phase change within the working temperature range), good wetting properties within the separator and the composite electrodes and the highest ionic conductivity in order not to hinder battery power at high currentrates. In this paper the transport properties (conductivity, viscosity, self-diffusion coefficients and transference number) and the ion dissociation coefficient of LiTDI dissolved in a mixture (50/50 wt%) of ethylene carbonate (EC) and dimethylcarbonate (DMC) have been determined and compared to those of LiPF6, LiTFSI and LIFAP in the same mixture with the aim of using LiTDI as an electrolyte salt in batteries. 2. Experimental section 2.1. Electrolyte, solvent mixture and cell preparation Highly pure (GC grade, molecular purity >99%) ethylene carbonate (EC) purchased from Fluka, and anhydrous dimethylcarbonate (DMC, molecular purity >99%) purchased from SigmaAldrich, were used as received. The alkyl-carbonate EC/DMC (50/ 50wt%) mixture used in this paper was prepared using a Sartorius 1602 MP balance with a 1 10
4 g accuracy in an argon filled MBraun glove box at 25  C with less than 5 ppm of moisture content. A GeneCust desiccant bag was placed in the mixture to reduce the water content to a minimal level. The battery grade lithium hexafluorophosphate (LiPF6) purchased from Fluorochem and extra dry (<20 ppm H2O) lithium bis (trifluoromethylsulfonyl) imide (LiTFSI) purchased from Solvionic were kept and used under dry atmosphere in the glove box. The main salt of this paper, lithium 4,5-dicyano-2-trifluoromethanoimidazolide (LiTDI) was supplied by Arkema. Before use LiTDI was dried under a vacuum at 120  C then placed under dry atmosphere in the glove box and kept there. The LiTDI, LiPF6 and LiTFSI in EC/DMC (50/50 wt%) electrolytes were prepared under dry atmosphere in the glove box at concentrations going from 1 to 1 10
4 mol.L
1 and remained there until analysis. The electrolyte containing 1 mol.L
1 of lithium tris(pentafluoroethane)-trifluorophosphate (LiFAP) in EC/DMC (50/50 wt%) was purchased from Merck. This commercial electrolyte is kept as received under dry atmosphere. Concentrations below 1 mol.L
1 were obtained by dilution. Prior to any measurement, the water content of each electrolyte was measured using an 831 Karl-Fisher Coulometer (Metrohm). The water content of each was lower than 20  1 ppm.

The coin-cells were prepared in the glove box with an atmospheric water and oxygen content below 5 ppm. 2.2. Experimental methods Conductivity measurements were performed using a Crison (GLP 31) digital multi-frequency (1000–5000 Hz) conductometer. The temperature (from 10 to 80  C) was controlled by a JULABO thermostated bath with an accuracy of 0.2  C. Before any measurements the conductometer was calibrated using standard KCl solutions at three different concentrations. Density and viscosity measurements were carried out from 10  C to 80  C using respectively an Anton Parr digital vibrating tube densitometer (model 60/602, Anton Parr, France) and an Anton Parr rolling-ball viscometer (Lovis 2000 M/ME, Anton Parr, France). In both cases the cell temperature was regulated within 0.02  C. Dynamic viscosity values reported in this paper were calculated by taking into account the effect of the sample density and the buoyancy of the ball in each sample as a function of temperature. The densitometer was firstly calibrated at all temperatures with degassed water and dehumidified air at atmospheric pressure as recommended by the constructor while ultra-pure water was used to calibrate the viscometer. The uncertainty of the density and viscosity measurements was inferior to 5  10
5 g.cm
3, and 1%, respectively. Lithium cation transference numbers (t+): also called transport number, were calculated using a VMP3 multichannel potentiostat and applying the method introduced by Bruce and Vincent. For a higher data consistency three coin-cells for each electrolyte of interest were prepared using lithium metal, separated by a Whatman glass microfiber filter, as Counter electrodes (CE) and working electrode (WE) in an argon filled glove box with less than 5 ppm of moisture content. Impedance measurements were performed with a 10 mV A.C. signal and a frequency range of 500 kHz to 50 mHz with 6 points per decade. Pulse-Gradient Spin-Echo NMR (PGSE-NMR) experiments were performed on a Brucker DSX100 NMR spectrometer with a 2.35T superconducting magnet, equipped with a 10 mm microimaging probe (Micro5 Brucker) without a lock system. The Larmor resonance frequencies are 100.00 MHz, 38.86 MHz and 94.08 MHz for 1H, 7Li and 19F nuclei, respectively. In order to determine the activation energy for the diffusion process of each species, the NMR runs were carried out over a temperature range from 20  C to 80  C within an accuracy of 1  C and a step DT = 5  C. The samples were prepared and placed in sealed glass tubes in a 20

20 18

0°C 25°C 50°C 80°C

a.

18 16

14

14

12

12

-1

σ (mS.cm )

-1

σ (mS.cm )

16

10 8

8 6

4

4

2

2

0,5

1,0

1,5 -1

C (mol.L )

2,0

b.

0°C 25°C 50°C 80°C

10

6

0 0,0

779

0 0,0

0,5

1,0

1,5 -1

C (mol.L )

Fig. 1. Conductivity at several temperatures in EC/DMC (50/50 wt%) vs. a) LiTDI concentration, b) LiTFSI concentration.

2,0

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Table 1 LiX (X = TDI, PF6 [11], FAP [12] and TFSI) maximum conductivities (s max) in EC/DMC (50/50 wt%) at 25  C and 80  C.

25  C 

80 C

smax/mS.cm
1 Cmax/mol.L
1 smax/mS.cm
1 Cmax/mol.L
1

LiTDI

LiPF6

LiTFSI

LiFAP

6.84 0.75 15.87 1.00

11.4 1.00 N/A

9.80 1.00 19.07 1.25

8.60 0.75 N/A

association and viscosity [13,14]. First the number of charge carriers per volume unit increases with the salt concentration causing the ionic conductivity to rise. At high concentration in salt, the viscosity increases drastically (refer to the Jones-Dole equation) and, at the same time, the ions associating in pairs contribute to decrease the number of charge carriers and hence the conductivity. The balance between the number of charge carriers and the viscosity is responsible for the flat maximum of conductivity which has also been reported for other systems [15]. Ionic mobility becomes a limiting factor sooner for LiTDI and LiFAP than it does for LiPF6 and LiTFSI. In the case of LiTDI it was reported that after 0.63 mol.kg
1 (about 0.75 mol.L
1), the percentage of free ions becomes lower than ion pairs and higher aggregates [16]. This fact could explain why LiTDI reaches its maximum of conductivity at a lower concentration than LiPF6 and LiTFSI.

glove box. For NMR measurements, samples were thermally equilibrated at each temperature for 30 min before any acquisition. Indeed, such experimental conditions allow a good homogeneity of the set point temperature across all the samples. Finally, all the chemical shift references were set to a particular arbitrary value for convenience. 3. Results and discussion

3.1.2. Ionicity and ion-pair dissociation The ionicity, empirical concept that helps understanding the nature of the chemical interactions in both molecules and crystals (salts), may be evaluated in ionic solutions using a Walden plot ln (L)=f[ln(h
1] as reported in Fig. 2. In this diagram the ionic mobility within each electrolyte, represented by the equivalent conductivity, L = s/Csalt (mS.cm2.mol
1), is plotted against the medium fluidity, h
1 (Poise
1), related to electrostatic interactions. The ideal line (dashed line) represents fully dissociated ionic solutions such as an aqueous KCl solution and the area above the ideal line is that of super-ionic solutions [17–19], i.e. solutions for which the ion transport mechanism is different (Grotthüs type). The variations of ln(L) as a function of ln(h
1) at various temperatures are displayed Fig. 2 for all four 1 mol.L
1 salt-based electrolytes in EC/DMC and also for the 0.75 mol.L
1 LiTDI electrolyte. When the ionicity of a solution increases, the representative line on the Walden diagram is closer to the ideal line. Hence, the ionicity of the EC/DMC solutions increases following the order: LiTDI < LiTFSI < LiFAP [20]
3.1. Effect of the salt concentration on conductivity 3.1.1. Concentrated solutions (C > mol.L
1) The effect of LiTDI concentration on ionic conductivity in EC/ DMC at several temperatures is presented in Fig. 1a. All curves behave similarly and reach a plateau around C = Cmax (ffi1 mol.L
1) before dropping at higher concentrations. The plateau concentration range depends on the temperature. At 25  C, the plateau onset takes place at 0.5 mol.L
1 and ends at 1 mol.L
1, whereas at 80  C it takes place at 0.75 mol.L
1 and ends at 1.4 mol.L
1. Since to our knowledge, the concentration and temperature influence on the conductivity of LiTFSI in EC/DMC has not yet been reported, it has been displayed also in Fig. 1b and share a similar pattern with a flat maximum which is shifted from 0.75 mol.L
1 at 0  C to 1.5 mol.L
1 at 80  C. As reported in Table 1, the maximum conductivity at 25  C in EC/ DMC is reached at 1 mol.L
1 for LiPF6 [11] and LiTFSI whereas it is reached at a concentration of 0.75 mol.L
1 in the case of LiFAP [12] and LiTDI. At 25  C the maximum conductivities reach 6.84 mS. cm
1 for LITDI, 9.8 mS.cm
1 for LiTFSI, 8.6 mS.cm
1 for LiFAP and 11.4 mS.cm
1 for LiPF6. These maxima are the result of a competition between the number of charge carriers, ionic

5,0 4,5 4,0

LiTDI 0.75M LiTDI 1M LiPF6 1M LiFAP 1M LiTFSI 1M

L KC s u eo u Aq

ln(Λ)

3,5

s on i t lu So

3,0

ty ici n o gi n i s rea c In

2,5 2,0 1,5 1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

ln(1/ η) Fig. 2. Walden plot showing the positioning of LiX (X = TDI, PF6[20], FAP[20] and TFSI) electrolytes in EC/DMC (50/50 wt%) over the temperature range 10  C to 80  C

C.L. Berhaut et al. / Electrochimica Acta 180 (2015) 778–787

781

Fig. 3. Walden product variation of the 1 M LiX (X = TDI, PF6 [20], FAP [20] and TFSI) electrolytes in EC/DMC (50/50 wt%) over a temperature range of 20  C to 70  C

Indeed the 0.75 mol.L
1 LiTDI solution has a higher molar conductivity than its 1 mol.L
1 counterpart since their conductivities over a temperature range of 10  C to 80  C stay close (Fig. 8). The Walden product W=sh represents the interdependence of conductivity and viscosity for an ionic solution at a given temperature. For each salt of interest in EC/DMC and at a concentration of 1 mol.L
1, W values are displayed in Fig. 3. W is almost independent of the temperature in the case of LiTDI, LiTFSI and LiFAP electrolytes. Only LiPF6 solutions exhibit a strong temperature dependence and W values are lower at higher temperatures. The Walden product W = sh has been extended to glass forming solutions (such as ionic liquids) under the form of the fractional Walden rule: W ¼ shn

ð1Þ

where n is a decoupling parameter, which increases with ionic size, and has a value between 0 and 1. The classical Walden rule is obtained when n equals 1 [18,21]. For LiTDI solutions, it is found that n = 0.91, a value which is close to the unity. This implies that for a fixed concentration the number of free ions does not vary with the temperature since any increase in sh also indicates an increase in salt dissociation [22]. This means also that in LiTDI solutions a strong coupling between conductivity and viscosity exists. As LiTDI electrolytes follow the Walden rule, an estimation of the ion-pair dissociation coefficient aD can be obtained from the following equation [18,23,24].

Lh W ¼  ¼ aD   W Lh

Fig. 4. Molar conductivity at high dilution as a function of the concentration for LiTDI in EC/DMC (50/50 wt%) at several temperatures.

ð2Þ

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Table 2 L, L , h, h and aD values at 25  C for the LiTDI, LiPF6, LiFAP and LiTFSI electrolytes in EC/DMC (50/50 wt%). [LiX]

C/mol.L
1

L/S.cm2.mol
1

L /S.cm2.mol
1

h/mPa.s

aD

LiTDI

0.75 1 1 1 1

9.4 6.75 11 7.6 9.9

47.52 47.52 49.41 44.64 40.65

2.03 2.91 4.31 4.35 3.4

0.35 0.31 0.85 0.64 0.73

LiPF6 LiFAP LiTFSI

where L, L , h and h (1.13 mPa.s) are the molar conductivity, the high dilution limiting molar conductivity, the medium viscosity and the pure solvent mixture viscosity. The limiting molar conductivity L values are obtained by extrapolating at high dilution the Debye-Hückel-Onsager equation (Eq. (3)) giving L as a function of C1/2 (Fig. 4). pffiffiffi L ¼ L
S 2 C ð3Þ where S is the slope of the line reported on the graph displayed in Fig. 4. According to the ion-pair dissociation coefficients listed in Table 2, it appears that lithium salts are not fully dissociated in the EC/DMC mixture and especially LiTDI. Moreover, Niedzicki et al. [16] showed that in a 1 mol.L
1 LiTDI-EC/2DMC mixture the freeion percentage was 25%. This result is consistent with the 31% calculated in Table 2, as the content in DMC, a low polarity solvent, is higher in EC/2DMC. 3.2. Viscosity and effective solute ion radius The relative viscosity hr, (hr = h/h0) is plotted at several temperatures as a function of LiTDI concentration in EC/DMC and fitted according to the Jones-Dole equation (Eq. (4)) as shown in Fig. 5. pffiffiffi hr ¼ 1 þ A C þ BC þ DC2 ð4Þ where A, B and D are constants representing respectively, the ionion long range interactions, the solute-solvent interaction (short distance) and D the ion-ion and solute-solvent structural interactions (short distances) [25]. Nevertheless, the physical meaning of the D coefficient still remain unclear [20]. It has been demonstrated that, for concentrations higher than 0.1 mol.L
1, the p A C term can be considered as negligible.For comparison

Table 3 Coefficients B and D of the Jones-Dole equation and effective solute radius (rs) for LiDTI in EC/DMC. T/ C

B/L.mol
1

D/L2.mol
2

rs/nm

20 30 40 60

0.69 0.68 0.66 0.59

0.90 0.76 0.65 0.54

0.48 0.48 0.47 0.45

purposes, the relative viscosity of LiTDI, LiPF6, LiTFSI and LiFAP solutions in EC/DMC have also been plotted against the molar salt concentration according to Eq. (4) at 40  C (see inset in Fig. 5). The fitting of the experimental data to Eq. (4) is excellent (R2 = p 0,99999 for each curve) justifying the fact that A C can be considered negligible here. From these plots, B and D values have been determined and reported in Table 3. Using the original Einstein relation for the viscosity of hard spheres in solution and its extension to spherical shaped suspensions [26], it is possible to have an estimation of the effective solute radius rs inasmuch as these relations can be applied to molecular dimensions (Eq. (5)). 4 B ¼ 2:5½ prs 3NA  3

rs ¼ 3

ð5Þ

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 Bj 10pNa

ð6Þ

where NA is the Avogadro constant (6.022  1023 mol
1) B, D and rs values for LiPF6, LiTFSI and LiFAP in EC/DMC have been reported elsewhere [27]. For these four salts the D coefficient, which takes into account the structural interactions, is strongly dependent on the temperature. This can be expected as any elevation of the temperature will increase the thermal agitation which will impact these interactions, whereas a lower temperature dependence is observed for B at least near room temperature (20 to 40  C) in the cases of LiPF6, LiFAP [27] and LiTDI. The effective solute radius, represents the radius of the hard sphere which has the same impact in viscosity as the salt molecule. LiTDI exhibits a nearly constant rs value between 20  C and 40  C as it has been already observed for LiPF6 and LiFAP [27]. The radius decreases at higher temperatures indicating some loss of solvating molecules. The lithium ion radius rLi = 0.076 nm and the EC molecule can be inscribed in an ellipse possessing a 0.33 nm major axis and a 5 -1

LiTDI (0.75 mol.L ) -1 LiTDI (1mol.L ) -1 LiPF6 (1mol.L )

4

Viscosity (mPa.s)

-1

LiFAP (1mol.L ) -1 LiTFSI (1mol.L ) 3

2

1 20 Fig. 5. Plot of (hr
1) as a function of LiTDI concentration in EC/DMC (50/50 wt%) at different temperatures. Inset: plot of (hr
1) as a function of salt concentration for LiX salts at 40  C.

40

T (°C)

60

80

Fig. 6. Viscosity of LiX (X = TDI, PF6 [20], FAP [20] and TFSI) at 1 mol.L
1 (and 0.75 mol.L
1 for LiTDI) in EC/DMC (50/50 wt%) as a function of the temperature

C.L. Berhaut et al. / Electrochimica Acta 180 (2015) 778–787

783

3.3. Temperature effect on the electrolyte viscosity, conductivity and transference number

Fig. 7. Conductivity of LiX (X = TDI, PF6, FAP and TFSI) at 1 mol.L
1 in EC/DMC (50/ 50 wt%) as a function of the temperature.

0.23 nm minor axis. By considering that each lithium cation is preferentially solvated by EC rather than DMC molecules as reported by Xu et al.[28] and by assuming that lithium is preferentially attracted by the double bonded EC oxygen, the calculated radius value of the solvated lithium rcal = 0.076 + 0.33 = 0.406 nm. By then considering that a paired TDI
anion is also present in the lithium solvation sphere as [Li+–EC–TDI
], the solute radius becomes rcal' = 0.406 + 0.43 = 0.449 nm which is close to the calculated rs. In this model the TDI
anion is considered as an oblate ellipsoid the dimensions of which are 0.43  0.64 nm and the most stable ion-pair configuration implies the smaller distance. It can also be noted that the rs value is also close to the radius of Li(EC)4+ species (0.37 nm) calculated by Matsuda et al.[29] suggesting that all lithium cations are solvated by an average of four EC molecules.

3.3.1. Viscosity The viscosities of LiTDI (0.75 and 1 mol.L
1) along with the other three lithium salts at 1 mol.L
1 in EC/DMC have been measured and plotted in Fig. 6 against the temperature. It should be noted, that LiTDI electrolytes have the lowest viscosity [20]. The reason for which the viscosity of LiTDI solutions is lower than the other lithium salts is due to different factors such as: (i) charge delocalization: electrostatic interactions between ions of opposite charge in LiTDI and LiTFSI should be less strong than in LiPF6 due to charge delocalization [10]. (ii) The size and shape of the anion: because of the flat shape of TDI
anions friction between solvent molecules linked to cations or not, is limited. On the contrary, the spherical PF6
or the unsymmetrical bulky FAP
anions may cause a high shear resistance leading to higher solution viscosities. (iii) The formation of ion-pairs: with a ion-pair dissociation coefficient of 0.31 (Table 2.), LiTDI remains mostly in the form of neutral species such as ion pairs or triple ions [Li (TDI)2
, Li2(TDI)+] [16] generating fewer interactions between charged species. Those three factors are cumulative but their respective contributions are unquantifiable. The Van der WaalsLondon forces between anions and solvent molecules will not differ too much owing to their similar molecular weight. 3.3.2. Conductivity In view of Li-ion batteries applications, the electrolyte conductivity is an important parameter to be taken into account. The conductivities of LiTDI and LiTFSI solutions in EC/DMC have been measured at different temperatures and reported alongside literature values for LiPF6 and LiFAP [20] in Fig. 7. According to the results displayed in Fig. 7, the LiTDI electrolyte offers the lowest conductivity (6.84 mS.cm
1), despite its low viscosity. At the opposite, the high viscosity LiPF6 electrolyte offers the highest conductivity (11.47 mS.cm
1). The substitution of three fluorine atoms by three –(CF2CF3) in PF6
to obtain the FAP
anion has a larger impact on conductivity than viscosity [20]. LiPF6, LiTFSI and LiFAP conductivities follow, as expected, the same trend as the ion-pair dissociation coefficients at 25  C reported in Table 2. The higher the dissociation coefficient, the higher the conductivity:

Fig. 8. VTF fitting of the 0.75 mol.L
1 LiTDI in EC/DMC electrolyte for conductivity (black squares) and viscosity (red dots) data.

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Table 4 VTF equation fitting parameters for the conductivity and viscosity of investigated LiX (X = TDI, PF6[20], FAP[20] and TFSI[6]) electrolytes in EC/DMC (50/50 wt%). Viscosity T0  5/K

Bh  0.05/kJ.mol
1

LiTDI 0,75 mol/L LiTDI 1 mol/L LiPF6 1 mol/L LiFAP 1mol/L LiTFSI 1mol/L

139 146 170 154 140

4.16 4.13 3,26 3,30 4.71

Conductivity LiX/(EC/DMC) LiTDI 0,75 mol/L LiTDI 1 mol/L LiPF6 1 mol/L LiFAP 1mol/L LiTFSI 1mol/L

T0  5/K 139 155 177 151 116

Bs  0.05/kJ.mol
1 3.80 3.59 2,42 2,97 4.43

h0 0,10 0.11 0,19 0,12 0.12

s0 121.0 140.0 127,9 209,6 181.8

R2 0.9999 0.9999 0.9996 0.9999 0.9989

R2 0.9999 0.9999 0.9999 0.9999 0.9988

with a dissociation coefficient in EC/DMC of 0.85, and 0.31 respectively, LiPF6 is the most conducting electrolyte and LiTDI the least. Viscosity and conductivity values of LiTDI solutions follow a non-Arrhenius behavior between 10  C and 80  C as do LiPF6, LiFAP and LiTFSI solutions, meaning a non-linear evolution of ln(h, s) vs. T
1. As a consequence, the Vogel-Tamman-Fulcher (VTF) equations (Eq. (7) and (8)) were used to represent the temperature dependency of these electrolytes: 1 Bh  R T
T0

ð7Þ

1
Bs  R T
T0

ð8Þ

h ¼ h0 exp ½

s ¼ s 0 exp ½

where Bh, Bs , h0, s 0 are fitting parameters and T0 is the ideal glass transition temperature which can be determined by DSC experiments at very low scanning rates. R is the perfect gas constant (8.314 J.mol
1.K
1). Viscosity and conductivity data are fitted using Eq. (7) and (8) as shown in Fig. 8 in the case of the 0.75 mol.L
1 LiTDI in EC/DMC electrolyte. Fitting parameters are reported in Table 4 together with those of LiTFSI, LiPF6 and LiFAP solutions [6,20]. The ideal glass transition temperature remains constant within 10  C whatever the transport property studied. Moreover Bh and Bs are similar, with a maximum difference of 0.84 kJ.mol
1 in the case of LiPF6. The LiPF6 electrolyte exhibits the lowest values meaning a steeper increase in conductivity and faster decrease in

viscosity when the temperature is raised than for all other studied electrolytes. The decrease in the Walden product observed previously for LiPF6 is linked to the difference between Bh and Bs values. Moreover, as presented by Angel et al. the viscosity and conductivity profiles follow an Arrhenius behavior at temperatures sufficiently higher than the ideal glass transition temperature. That is observed in the case of LiTDI between 25  C and 80  C (the correlation coefficient R2 = 0.998 for both profiles). The activation energies for the conductivity and viscosity processes were found to be Eas = 13.6 kJ.mol
1 and Eah = 14.1 kJ.mol
1. 3.4. Lithium cation transference number (t+) The lithium transference number, between 0 and 1, gives the fraction of the effective current crossing the cell which is the current carried by the lithium ions only. It has been calculated for each electrolyte of interest at 1 mol.L
1 in the EC/DMC solvent mixture by using the Bruce-Vincent original method [30]. First an impedance measurement was performed on the cell followed by a chronoamperometry measurement. A potential of 10 mV was applied to each sample until the current reached steady-state, i.e. when the variation in current is lower than 1% over 20 minutes. In our case steady-state was reached after 15 to 20 minutes (Fig. 9). The lithium transference number was calculated as tþ ¼ IIss0 , V ; Re and V are the steady-state current, the initial where Iss,I0 ¼ Re current, the electrolyte resistance and the D.C. applied voltage, respectively. Mean values are given in Table 5. The lithium transference number for LiTDI, measured by PGSENMR, is equal to 0.46 at 25  C [10]. t+ obtained using the BruceVincent method are different than those obtained by PGSE-NMR because in the first method only Li+ cations are taken into account for migration between the two lithium electrodes whereas in the second method, the diffusion of ion-pairs is also taken into account. The lithium transference number determined here for LiTDI by the Bruce-Vincent method is in agreement with the value given by Niedzicki et al. obtained in EC/2DMC (0.55) [16]. LiTDI has a higher average t+ than LiPF6 in the same solvent. This may be explained by a lower mobility of TDI
vs PF6
due to the larger anion size of the former and also to the lower dissociation coefficient of LiTDI. It must be noted that to be able to use the Bruce-Vincent method, the t+ + t
= 1 rule, expressing the fact that only free anions and cations migrate in the system, must be satisfied. As demonstrated previously this is not the case, the salts

Table 5 lithium transference number for the LiTDI, LiPF6, LiTFSI and LiFAP electrolytes in EC/ DMC (50/50 wt %) at 25  C.

Fig. 9. Chronoamperometry plot of the Li/1 M LiTDI in EC/DMC (50/50 wt%)/Li system used for estimating the lithium cation transference number.

LiTDI

Rel/ohm

I/mA

Iss/mA

t+ (0.05)

t+ (PGSE-NMR)

Cell 1 Cell 2 Cell 3

67 69 62

0.149 0.145 0.161

0.095 0.072 0.098

0.58

0.46 [10]

LiPF6 Cell 1 Cell 2 Cell 3

60 76 68

0.167 0.132 0.147

0.058 0.063 0.068

0.43

0.41 [10]

LiTFSI Cell 1 Cell 2 Cell 3

74 88 83

0.135 0.114 0.120

0.081 0.066 0.076

0.60

–

LiFAP Cell 1 Cell 2 Cell 3

63 80 65

0.159 0.125 0.154

0.082 0.067 0.087

0.54

0.48 [20]

C.L. Berhaut et al. / Electrochimica Acta 180 (2015) 778–787

785

in EC/DMC the salt is as can be noticed when comparing Tables 2 and Table 5. 3.5. Pulsed-gradient spin-echo NMR (PGSE-NMR) experiments 3.5.1. Self-diffusion coefficient According to the self-diffusion curves presented Fig. 10 species containing the 19F nuclei (such as the [Li+, TDI
] ion pair and the TDI
anion) diffuse faster than lithium species up to a concentration of 1.75 mol.L
1. At this concentration and higher, the ion-pair dissociation coefficient becomes so low that ion-pairs and higher aggregates are largely predominant and so D(7Li) D(19F). Li+ and TDI
self diffusion coefficients are presented in Table 6 up to a concentration of 1 mol.L
1. Using the D(7Li) and D(19F) values it is possible to calculate the cationic transference number at different salt concentrations following Eq. (9). tþ ¼ 7

19

Fig. 10. Li and F nuclei PGSE- NMR self-diffusion coefficients as a function of LiTDI concentration in EC/DMC.

Table 6 Concentration dependence of the self-diffusion coefficients D of the Li+ and TDI
ions in EC/DMC observed by PGSE-NMR at 26  1  C and of the cationic transference number. CLiTDI/mol.L
1

1010.D(7Li)/m2.s
1

1010.D(19F)/m2.s
1

t+(NMR)

1 0.75 0.5 0.25 0.1

2.084 2.681 3.189 3.924 4.334

2.417 3.426 4.117 5.373 6.253

0.46 0.44 0.44 0.42 0.41

not being fully dissociated in EC/DMC. This is another reason as to why the transference numbers measured using the Bruce-Vincent method are higher than those obtained by PGSE-NMR. The closer the values obtained from both methods are, the more dissociated

Dþ D þ D
þ

ð9Þ

The t+ values calculated are also reported in Table 6 which shows that t+ increases only slightly with the salt concentration. The selfdiffusion coefficient D(7Li) takes into account the mobility of free solvated Li+ ions and all entities paired with them such as ion pairs and hence the cationic transference numbers determined by NMR are affected by the ion-pair formation when their amount is not negligible. When CLiTDI increases, the t+ value increases also until it reaches a limiting value of t+ = 0.5 when all free ions are converted into pairs or aggregates and D(7Li) = D(19F). According to the results shown Fig. 10. this happens at a concentration over 1.75 mol.L
1. 3.5.2. The effective ionic radius Applying the Stockes-Einstein equation (Eq. (10)) to selfdiffusion coefficients can lead to an effective solute radius which takes into account all species involving lithium. The mean solute radius rs,Li = 0.37 nm and is almost concentration independent. D¼

kT 6phrLi;eff

ð10Þ

Fig. 11. LiTDI dissociation coefficient in EC/DMC as a function of the salt concentration. Black squares and red dots values are respectively obtained by applying the NernstEinstein equation and the Walden rule.

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where D0 (m2.s
1) is a pre-exponential factor and Ea the activation energy of the diffusion process. The fit values are reported in Table 7. The activation energies measured are consistent with those obtained when fitting the conductivity and viscosity against the temperature profiles and are higher than those measured in the case of LiPF6 in EC/DMC (12.991 kJ.mol
1 and 13.390 kJ.mol
1 for the 7Li and 19F nuclei respectively) [20] meaning a slower mobility increase for LiTDI in EC/DMC when the temperature rises than for LiPF6. 4. Conclusion

Fig. 12. Temperature dependence on 7Li, 19F and 1H nuclei PGSE-NMR self-diffusion coefficients in a 1 mol.L
1 LiTDI in EC/DMC electrolyte.

3.5.3. Ion-pair dissociation [31] The ion-pair dissociation, previously calculated by applying the Walden rule, can also be determined by using the present PGSENMR results as:

aD ¼

Lexp LNE

ð11Þ

where Lexp is the measured molar conductivity and LNE the molar conductivity obtained from the Nernst-Einstein equation when assuming a complete dissociation (aD = 1):

LNE ¼

F2 þ ðD þ D
ÞaD RT

ð12Þ
1

where F is the Faraday constant (96,500 C.mol ). Results obtained previously by applying the Walden rule and those obtained by using Eq. (11) and (12) are plotted together in Fig. 11 against the salt concentration. Values differ by only 0.05, and those obtained by the Walden rule are systematically lower than those obtained by the Nernst-Einstein relation. Nevertheless, this proves that LiTDI is poorly dissociated in EC/DMC. 3.5.4. Effect of temperature on the self-diffusion coefficients The evolution of the self-diffusion coefficients of the species containing 7Li, 19F and 1H (solvent molecules EC and DMC) nuclei with the temperature can be observed Fig. 12. The influence of temperature on the self-diffusion coefficients was quantified by Arrhenius plots and the experimental data fitted following Eq. (13). DðTÞ ¼ D0 expð

Ea Þ RT

ð13Þ

Table 7 Arrhenius fitting parameters for the self-diffusion coefficients of species containing 7 Li, 19F and 1H in a 1 mol.L-1 LiTDI in EC/DMC (50/50 wt %) electrolyte with R2 the correlation coefficient resulting from the fit procedure. NMR nucleus

108D0 (m2.s
1)

Ea (kJ.mol
1)

R2

7

7.008 15.16 19.11 19.06

14.079 15.638 14.529 14.044

0.9886 0.9981 0.9966 0.9957

Li F H (EC) 1 H (DMC) 19 1

In this work the transport and physical properties of LiTDI were compared to those of LiPF6, LiTFSI and LiFAP in the solvent mixture EC/DMC. LiTDI shows an interestingly lower viscosity due in part to smaller friction between the anion and solvent molecules and the lower electrostatic interactions generated by the delocalized charge on the TDI anion. However a weaker viscosity does not always mean a higher conductivity as, at 25  C, it is only half of that of LiPF6, a ratio which increases with the temperature. The LiTDI dissociation coefficient was investigated by means of two different methods. The first makes use of viscosity and conductivity data through the Walden rule. The second method makes use of selfdiffusion coefficients data and of the Nernst-Einstein equation. Both methods gave similar values: at 1 mol.L
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