Some Concepts of IL-Based Electrolytes for Li–Ion/Na–Ion Batteries

Low-Melting Alkaline Salts

Low-Melting lithium Salts

Many conventional lithium salts were combined with other ingredients to use as electrolytes for Li-ion batteries with the melting point typically above 200°C, for example, 236°C for LiCl04, 296.5°C for LiBF4, and 200°C for LiPF6. This is mainly because of the strong electrostatic attraction between the Li+ ion and the anion, ligand structure, or asymmetric structure of the salts. The charge density of Li+ ion is high, because of its small ionic radius, resulting in a strong electrostatic attraction between the Li+ ion and the anion. Sometimes, the factors described above make us difficult to predict the melting temperatures of lithium salts because interactions between Li+ and anions are relatively weak due to Lewis acidity of Li+; thus, it is difficult to lower the melting point of a lithium salt down to room temperature. However, even the lithium salt of a large aluminate anion which is weak Lewis base has a melting point much higher than room temperature [13,14]. Fujinami et al. reported that the introduction of ether groups into the aluminate structure gave liquid-like Li salts [15,16].

The ether groups act as Li+ coordinating ligands, dissociating the Li+ cations from the central atom of the anion’s centers. Watanabe and co-workers reported ILs consisting of Li+ cation and borates anion having electron-withdrawing groups, to reduce the basicity, as well as Li+ coordinating ether ligands, to dissociate the lithium cations from the central atom of the anions [17-19].

Mixtures of Alkaline Imide Salts

The melting point of A[TFSI] (A: alkali metal; TFSI: bis(trifluoromethanesulfonyl) imide = [(C'F2S02)2N~) is relatively high, about 236°C. The melting points of the imide salt as well as the ionic conductivity of the molten salts are low at room temperature because of their very high viscosity ion pairs. The strong Lewis-acidic nature of the Li+ ion favors ion pairing even in the molten state. However, to increase the ionic conductivity, the temperature should be elevated.

Alkaline Salts Dissolved in Organic Ionic Liquids

There are numerous aprotic ILs composed of organic cations and counter anions (Table Ю.2), having melting points lower than room temperature without containing Lf7Na+ ions; therefore, lithium or sodium salts should be mixed with the organic ILs to form mixtures that are used as electrolytes for Li-ion/Na-ion batteries, and the ILs are used as electrolyte solvents for the lithium or sodium salts. The physicochemical properties of lithium salt/IL binary mixtures are significantly affected by the structures of cations and anions of the organic IL. Herein, we highlight the characteristics of lithium salt/IL binary mixtures [2,5].

TABLE 10.2

Abbreviations of Typical Cations and Anions Comprising Aprotic ILs

Cation

[C„mim]

[Cnmpyr]

[Cnmpip]

[DEME]

[Nabcd]

[Pabcd]

[Cndmim]

[dema]

[DBU]

(Continued)

TABLE 10.2 (Continued)

Abbreviations of Typical Cations and Anions Comprising Aprotic ILs

Anion

[TFSA]

[FSA]

[FTA]

[BETA]

[TSAC]

[FAP1

[TfO]

[MS]

[DFOB]

[DCA]

Effects of Cation Structure

The oxidation stability of the organic ILs is strongly affected by the chemical nature of the cations. It is widely known that aliphatic quaternary ammonium (AQA) cations and aliphatic quaternary phosphonium (AQP) cations have excellent oxidation stability [20,211- Hence, many researchers have investigated AQA- and AQP-based ILs as electrolyte solvents for Li-ion/Na-ion batteries [7].

Besides, within the same cation structure, the changes in alkyl chain length affect the oxidation stability of ionic liquids [6] (Figure 10.3).

The electrochemical reactions of negative electrode materials are affected by not only the cation species but also the anion species. Therefore, the combination of cation and anion of an IL and the lithium salt or sodium salt should be selected carefully for battery applications. The organic cations, especially ionic radius, the anion size, and the flexibility of cation and anion as well as functional groups contained in the cation/anion structures also affected many properties of lithium salt/IL binary mixtures such as transport property of Li+, viscosity, and conductivity. For example, organic cations containing ether groups tend to result in lower viscosities for the Li salt/IL binary mixtures owing to the interaction between the Li+ and the ether group (Figure 10.4) [22-28] (Table 10.3).

For some mixtures of ILs in different cation types, P. Le et al. [2] reported the comparison between the theoretical and practical viscosity of IL mixtures to demonstrate the effect of cation type on viscosity. The work shows that if the presence of solvent EC in electrolyte mixtures created an ideal solution (i.e., there is no interaction between molecules), theoretical viscosity will be calculated by the formula:

CV curves (a) different pure ionic liquids compared with conventional electrolyte I M LiPF/EC-DMC

FIGURE 10.3 CV curves (a) different pure ionic liquids compared with conventional electrolyte I M LiPF6/EC-DMC (1:1) [2]; (b) Variation of alkyl chains length of quaternary ammonium cation [6]. (Platinum working and counter electrodes. Scan rate 0.1 mV/s, room temperature).

Ionic conductivity and viscosity as a function of temperature for several ionic liquids [6], Reproduced with permission. Copyright 2010, ACS

FIGURE 10.4 Ionic conductivity and viscosity as a function of temperature for several ionic liquids [6], Reproduced with permission. Copyright 2010, ACS.

Ln 77mix = X| In /71 + X2 In 7]2 [2], where X, and X2 are mole fractions of solution 1 (ionic liquid) and solution 2 (polar solvent), respectively. The difference between the theoretical value and the real value is relatively high, up to 51% (Table 10.4). It can be demonstrated that these solutions are not ideal and some interactions between solvent with ionic liquid exist. The larger the cations are, the stronger the interaction between the solvent and the IL is shown.

 
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