Effects of Anion Structure

The oxidative stability of anions mainly determines the anodic limit of the ILs. Fluorinated anions such as BF4 anti PF()t and amide-type anions, such as [TFSA]~ and [FSA]', or imide-type anions, such as |TFS11_, are frequently used in the electrolytes owing to their good electrochemical stability. The fluorinated anions interact weakly with organic cations in the liquids because they are Lewis bases. When a lithium salt is dissolved in solvents (e.g., propylene carbonate), the Li+ ion is stabilized

TABLE 10.3

Properties of Quaternary Ammonium Imide Ionic Liquids [6]

ILs

Cation Structure

Ts

°c

Tm

°C

T„

°c

Density

g_1mL'/20°C

Conductivity (a) 10J S-'cm’ 20°C

Viscosity (rj) mPa-'s-1 20°C (50°C)

N1114TFS1

(CH,),C4H,N+

-78

8/17

400

1.41

17

148 (30)

NIII4ITFSI

(CH,)3CF3(CH2)3N*

90

408

4 (70°C)

N|,3,TFSI

CH,C,H5(C3H7)2N*

15/29

405

1.32

8

155

n1123tfsi

(CH,)2C2H5(C,H;)N*

-95

-9

402

1.39

14

82 (40)

nI224tfsi

CH,(C2H5)2(C4H,,)N*

-89

11

398

1.34

10

161(41)

nI224Itfsi

CH3(C2H5)2(CF3C3H,)N*

-70

36

420

1.46

2

774(119)

NII24ITFSI

(CH,)2C2H5(CF3C,H6)№

51

410

4

N„I6TFS1

(CH,)3C6HI3N*

-76

32

395

1.32

8

205 (52)

NmJFSl

(CH3)3C„H17N+

-77

7

380

1.26

5

257 (65)

Tg = glass transition temperature; Tm = melting point; Td = degradation temperature.

TABLE 10.4

Theoretical and real viscosity of ionic liquids mixed with different percentages of ethylene carbonate [2]

% EC

N,raTFSI

n„24tfsi

EMITFSI

Theory

Reality

Theory

Reality

Theory

Reality

10

19.8

15.8

23.3

22.1

11.3

-

15

13.8

11.1

15.9

15.6

8.6

-

20

10.4

9.3

11.7

12.3

6.9

14.1

25

8.1

7.0

9.0

10.1

5.8

_

Reproduced with permission. Copyright 2010, ACS

by solvation and the salt dissociates into a solvated cation [Li(solvent)J+ and counter anion. In the case of a binary mixture of lithium salt and IL, the Li+ is solvated by anions and forms complexes [Li - (X)J(x - 1)" (X: anion).

For example, P. Le et al. reported that Li+ and [TFSI]- may form complexes of [Li(TFSI)J- in certain binary mixtures [2]. The hydrodynamic radius of Li+ is increased due to the complex formation, and the viscosity of the mixture is increased with increasing lithium salt concentration, resulting in an ionic conductivity decrease. In comparison with the conventional electrolyte (lithium salt and organic solvent), the quaternary ammonium cation-based ILs (QAILs) showed a good oxidation stability with a potential of more than 6.0 V versus Li/Li+. The oxidation potential of QAILs is slightly higher than that of LiPF(,/EC:DMC (2:1), an electrolyte for high- performance lithium-ion batteries. The imidazolium ionic liquid exhibits a lower oxidation stability than others. The potential limitation of 1.5 V is found to be comparable for all the ILs with TFSI anion. The reduction wall is associated with the imide anion reduction and the lack of passive layer formation [29-32]. It is the fact that the addition of lithium salt increases the reduction stability with the formation of a passive layer derived from an insoluble salt.

In the binary mixture of Li salt/IL, complexes between anion and Li+ make Li+ relatively low, resulting in low limiting current density in the Li-ion cells. Matsumoto et al. reported that the viscosity of ILs is significantly affected by the anion structure; thus, the design of anion structure is critical.

The Li+ mobility in the electrolyte was enhanced by low viscosity. Li salts were dissolved in various ILs. The advantages of [TFSI]-based ILs as electrolytes for lithium batteries have been reported by many researchers [2,3,7,33]. The electrode/electrolyte interfacial charge transfer process is also greatly affected by the anion structure of the ILs. At the interface, the decomplexation (desolvation) of [Li(X)J(x - 1)_ occurs and anions are liberated. Therefore, the interaction between Li+ and anions has a significant effect on the interfacial charge transfer process [34]. Graphite is a representative, commonly employed as negative electrode material for Li-ion batteries. During the charging of a Li-ion cell, the reduction of graphite occurs and Li+ intercalates into the layered graphite structure to maintain electrical neutrality. In the case of lithium salt/IL electrolytes (except for [FSA]-based ILs), it is known that the organic cation is inserted into the graphite instead of Li+, and destruction of the layered structure of graphite takes place. However, in the [FSA]-based ILs, the intercalation of organic cation into graphite is suppressed and reversible Li+ intercalation occurs [5].

Figure 10.5 shows first charge and cycling performance of hard carbon electrode in binary mixtures of Li[TFSI]/ILs. In the case of Li[TFSI]/[C2mim][TFSI], the [C2mim]+ cation is intercalated into the graphite at an electrode potential of ca. 1 V versus Li/Li+ during the first cathodic scan, and this intercalation is irreversible. On the other hand, in the case of [FSA]-based ILs, the Li+ intercalation reaction takes place reversibly in the potential range of 0.2-0 V. In addition, in the case [FSA]-based ILs, the reductive decomposition of [C2mim]+ cation at the negative electrode is also prohibited. At present, it is not clear why the intercalation

(a) 1st discharge capacity of half-cell (-) Na I EC-PC (1:1) + x %wt. EMI-

FIGURE 10.5 (a) 1st discharge capacity of half-cell (-) Na I EC-PC (1:1) + x %wt. EMI-

TFSI +1M NaTFSI I Hard carbon, (b) Cycling discharge capacity as a function cycle number.

of organic cation and decomposition of [C2mim]+ cation are prevented in the [FSA]-based ILs. A small amount of [FSA]~ anion decomposition during the initial charging stabilizes passivation layer on the electrode surface. To support this hypothesis, another mechanism was also proposed recently [35]. As well as lithium salt/organic IL binary systems, the sodium salt/organic ILs can also be used as the electrolytes for Na and Na-ion batteries [5].

Effect of Organic Solvent Added to ILs

The effects of the incorporation of ethylene carbonate (EC) or dimethyl carbonate (DMC) on the physicochemical and electrochemical properties of ionic liquids (ILs) based on aliphatic quaternary ammonium and imide anion were studied [4]. The evolution of the melting point, glass transition, ionic conductivity, diffusion coefficient, and electrochemical stability were evaluated. The addition of a low amount of solvent, that is, 20wt% resulted in significantly improve the conductivity values, reaching 12 mS cm-1 at 40°C. The incorporation of a polar solvent, EC, has no positive effect on the IL dissociation. Moreover, the incorporation of EC in ILs improves the electrochemical stability toward reduction, whereas the high anodic stability is maintained. The addition of LiTFSI in IL + solvent mixed electrolytes reduces the ionic conductivity, but still higher than pure ILs, showing the beneficial effect of the additive solvent [4].

Phung Le and co-workers [5] conducted a calculation based on VTF fit of ionic conductivity in the range of ~25°C—60°C for supercooled liquids and glasses rather than a straight-line Arrhenius behavior to explain the conduction mechanism of the complex electrolyte:

where T„ is referred to as the Vogel temperature, equal to the glass transition in ideal glasses, the effects of charge carrier concentration, often related to the refactor, A, and segmental motion, related to the activation energy, E( on overall conductivity, a, at a given temperature T. Fits were performed by linearizing the data according to the form of the equation. The solver tool within Microsoft Excel and verified by the fminbnd function of MATLAB® R2016b was used with manually varying the value of T„ confirming the local maximum in R-squared to maximize the linearity of the resulting data.

Table 10.5 showed that the activation energy of EMI-TFSI has a decreasing tendency with an increase in the percentage of EC. This is due to the fact that dilution of solutions partially holds up ionic bond slightly, which is easy to them extract and become non-electrical charged particles. On the contrary, an opposite tendency for electrolytes with an IL used as a co-solvent was observed with an increase in the IL addition amount. This result is due to the increase in viscosity as well as the presence of significant ionic interactions between anion-cation and ion-dipole molecules. However, the degree of activation energy values deduced are gently lower than those of an IL used as the main solvent (Table 10.5).

TABLE 10.5

Activation Energy of the Mixed Electrolytes: EMI-TFSI - xwt% EC and EC-PC (1:1) + xwt% EMI-TFSI [5]

Electrolytes (EMI-TFSI as Main Solvent)

E, (| mol"’)

Electrolytes (EMI-TFSI as Co-solvent)

f, () mol-’)

Pure IL

2714

EC-PC + 1 M NaTFSI

1812

IL + 0.5 M NaTFSI

3267

EC-PC + 10% IL + 1M NaTFSI

1927

IL + 5% EC + 0.5 M NaTFSI

3009

EC-PC + 20% IL + 1M NaTFSI

2097

IL+ 10% EC+ 0.5 M NaTFSI

2868

EC-PC + 25% IL + 1M NaTFSI

2147

IL+ 15% EC + 0.5 M NaTFSI

2726

EC-PC + 30% IL + 1M NaTFSI

2219

IL + 20% EC + 0.5 M NaTFSI

-

EC-PC + 20% IL + 2% FEC + I M NaTFSI

1832

IL + 25% EC + 0.5 M NaTFSI

2477

EC-PC + 25% IL + 2% FEC + I M NaTFSI

1928

IL + 30% EC + 0.5 M NaTFSI

2564

EC-PC + 30% IL + 2% FEC + I M NaTFSI

2011

EC-PC + 50% IL + 1M NaTFSI

2320

Reproduced with permission. Copyright 2010, ACS.

Solvent-in-Salt Electrolytes

We can also prepare electrolytes using various solvents to enhance the solvation of lithium salt rather than ILs [2-4,7]. In these electrolytes mixed with alkaline salt, the Li+ and solvent form a solvate [Li(solvent)J+ cation. The coordination number of Li+ in liquids is typically 4-5. Therefore, in extremely dilute electrolytes with the amount of solvent higher than that of salt, all the solvent molecules can be assumed to be in the first solvation shell of Li+, and free solvents thus scarcely exist in the solution. However, the extremely concentrated electrolytes possess unique properties, some of them either very similar to those of solvate ILs or different from those of common electrolyte solutions with salt concentrations less than 2M [36]. Regarding the organic solvent-in-salt electrolytes for Li batteries in a review article by Yamada et al., the advantages of the electrolytes, such as the wide electrochemical windows, suppression of volatility, suppression of Al corrosion, and highly reversible reactions of Li metal and graphite electrodes, were well highlighted [36]. Recently, the solvent- in-salt concept was extended to the aqueous electrolytes [37-41] for Li-ion cells. The anodic limit expansion can be attributed to the decrease in solvent in the solution. The cathodic limit expansion is attributed to the formation of a LiF-based passivation layer, which is a decomposition product of the [TFSI ]~ anion. The layer passivates the electrode surface and suppresses the further reductive decomposition of electrolyte.

Li+-Conducting Polymer Electrolytes Containing Ionic Liquids

ILs can be mixed with polymers [42], resulting in gels, hereafter termed “ion-gels,” which can be used as electrolytes for batteries. In ion-gels (polymer electrolyte), ILs are confined in the polymer matrix, and this is advantageous to avoid the leakage of

Ionic conductivity as a function of temperature for NTFSI + solvent

FIGURE 10.6 Ionic conductivity as a function of temperature for Nm4TFSI + solvent: (■) Nnl4TFSI, (0) Nm4TFSI + 20wt % ЕС. (Д) Nm4TFSI + 30 wt % EC. (O) Nm4TFSI + 50 wt % EC, (A) NII14TFSI + 20wt % DMC. (□) Nm4TFSI + 30wt % DMC. (A) Nnl4TFSI + 50wt % DMC [4], Reproduced with permission. Copyright 2010, ACS.

electrolytes and resolve the safety issue of batteries. Ion-gels, however, are clearly different from conventional polymer electrolytes. In the case of Li+~-conducting conventional polymer electrolytes, Li salts are dissolved in polymers such as polyethylene oxide) (PEO), and the ionic conduction is coupled with the segmental motion of the polymer chains [43]. Therefore, the ionic conductivities of conventional polymer electrolytes are as low as 10~6-10~4 S cm-1 at room temperature. On the other hand, in the case of ion-gels, the liquid-state salts such as Li salt/organic ILs are mixed with polymers and the ILs behave as both charge carriers and plasticizers in the gels. By using ILs, we can prepare “polymer-in-salt” [44]-type electrolytes having relatively high ionic conductivity of ca. 10-4—10-3 S cm-1 at room temperature. Polyethylene oxide (PEO) [45], polydnethyl methacrylate) (PMMA) [46], and copolymer of poly(vinylidene fluoride- co-hexafluoropropylene) (PVDF-HFP) [47] have been reported to be compatible with ILs. It is known that the PVDF-HFP has relatively good mechanical strength due to the partially crystalline nature even if some plasticizer is included in the matrix [42]. The ionic conductivity is enhanced when increasing the content of IL in an ion-gel; however, the gel also becomes mechanically weaker. To achieve both high ionic conductivity and sufficient mechanical strength, the polymer cross-linking is effective [46].

 
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