Applications of Ionic Liquids in Batteries

## **Chapter 1**

## Ionic Liquids: Applications in Rechargeable Lithium Batteries

*Dipika Meghnani and Rajendra Kumar Singh*

## **Abstract**

World is passing through the energy crises due to the rapid depletion of fossil fuels. To address this crisis and to fulfill the energy demands worldwide, development of energy storage devices have increased rapidly. Also, renewable energy resources are intermittent, and therefore nevertheless, this energy resources are not always available. In that context, rechargeable lithium batteries are most promising energy storage devices owing to high energy and power density. Although, the development of the component of rechargeable battery such as anode, cathode and electrolyte are in progress as they play major role in enhancing the electrochemical performance of lithium-ion battery. Among them, electrolyte plays crucial role as it provides the path for diffusion of Li<sup>+</sup> ions between the electrodes. In that context, ionic liquid-based electrolytes are widely used as it acts as plasticizer and thus increases the conductivity of electrolyte considerably. In this chapter, we have discussed basics of ionic liquids and its application in electrolyte system. Also, in this chapter, we have discussed various properties of ionic liquid-based electrolytes and their application in rechargeable lithium battery.

**Keywords:** ionic liquid, rechargeable battery, IL-based polymer electrolyte, ionic conductivity, lithium-ion conductivity

## **1. Introduction**

During the past decades, lithium-ion batteries (LIBs) have drawn more attention of researchers in the area of energy storage due to high energy density (250 Wh/kg) and high-power density, good cycle life etc. Also, LIBs are considered as the most efficient energy storage devices that can store the vast amount of energy from the renewable energy sources (such as wind, solar etc.) and thus help us in making the "Fossil free world". Furthermore, in order to maximize the energy and power density, several industries and R&D groups have continuous worked hard on the improvement of electrochemical performance of electrode materials as well as electrolytes for LIBs [1–5]. Due to high power and energy density, good cycle life and bring light weight, lithium-ion batteries are widely used and industrialized in transportation devices as well as hybrid and electric vehicles [6] as shown in **Figure 1**. Commercial rechargeable LIBs have three components mainly anode, cathode and electrolyte. Each component of battery has significant role in enhancing the performance of LIBs. Among them,

#### **Figure 1.**

*Schematic presentation of various application of Lithium-ion battery.*

electrolyte provides the path for the diffusion of ions between the electrodes and also act as separator. Initially, organic based conventional electrolytes are commonly used in LIBs due to their high ionic conductivity (10�3to 10�<sup>2</sup> s*=*cm) [7]. Organic liquid electrolytes are usually lithium salts that are dissolved in organic solvents such as ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC) etc. However, they are highly flammable in nature, highly reactive towards the lithium electrode which causes the unwanted lithium metal electrode growth commonly knowns as lithium dendrite growth [8–10] as shown in the **Figure 2**. This inevitable lithium dendrite growth is the major problem of short circuiting of battery. Therefore, from the safety concern point, development of organic liquid-based electrolyte is restricted on large-scale applications. To address the above problem, solid polymer electrolytes are promising candidate as they have good mechanical strength, better thermal and chemical stability, good electrochemical stability window. Also, growth of lithium dendrite can be suppressed by such type of quasi-solid electrolytes.

However, they suffer from low room temperature ionic conductivity (<10�6s*=*cm<sup>Þ</sup> [11, 12]. In order to enhance the ionic conductivity of solid polymer electrolyte, several strategies [13, 14] are adopted such as:

<sup>1.</sup>Use of ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC) as conventional plasticizer.

*Ionic Liquids: Applications in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.107941*

#### **Figure 2.** *Showing the lithium dendrite growth in rechargeable lithium batteries.*


By adopting this approach, considerable increase in the ionic conductivity of solid polymer electrolyte has been achieved but it needs to be increased further for Li-ion battery applications. For that, a new class of material known as the ionic liquid has been introduced in the solid polymer electrolyte that has flexible nature, good ionic conductivity, better mechanical stability and wide electrochemical window and ease of processibility as well as portability and is free from the corrosion and leakage problem [15–18]. Ionic liquids have enormous effect on the research field due to its wide range of properties that have a good impact on the development of energy technologies. Ionic liquids are generally molten salts having large organic cation and organic/inorganic anion and therefore the ionic forces between them are weaker and thus low melting temperature. ILs have generally melting temperature less than 100°C and some ionic liquids are liquid at or below the room temperature usually known as room temperature ionic liquids (RTILs) [19, 20]. Also, RTILs have gained increasing attention in the electrochemical device due to their excellent properties such as nonvolatility & nonflammability and thermal stability. In addition, ILs have low vapor pressure, display wide electrochemical stability window (ESW), have high ionic conductivity and display high thermal and chemical stabilities. Few common anions and cations used in the formation of ILs [21, 22] are shown in the **Figure 3**. The choice of anions and cations plays a crucial role on the physical properties of ILs because these cations and anions can combine in millions of possible ways to give the IL having the specific properties for a particular interest. Due to these properties, ionic liquids are also known as *"Designer Solvents"* [23]. Therefore, due to their uniqueness in properties ionic liquids are widely used as plasticizer in solid polymer electrolytes. Ionic liquid based solid electrolytes are usually composed of polymer host matrix, lithium salts and ionic liquid. Ionic liquid-based polymer electrolytes not only have

#### **Figure 3.**

*Some of cations and anions used in the formation of ionic liquids.*

high ionic conductivity but have good electrode-electrolyte contact like liquid electrolyte, good mechanical strength, flexibility, wide electrochemical window and good thermal stability and chemical stability also. Therefore, in electrochemical devices especially, in lithium-ion batteries, ionic liquid-based electrolytes are widely used.

In this chapter, we discussed the physical, thermal, structural and electrochemical properties of IL-based polymer electrolytes and its application as electrolyte in lithium polymer batteries.

## **2. Different role of ionic liquids in lithium batteries**

#### **2.1 Pure Ionic liquids as electrolytes**

Nowadays, room temperature ionic liquids are replaced by conventional organic carbonate-based electrolytes due to their unique features, such as (a) wide ECW, (b) non-flammability, (c) low vapor pressure (d) higher conductivity (e) wide operation range of temperature (f) non-toxicity (g) non-flammability. Surprisingly, some electrode materials show good performance with pure IL-based electrolytes, compromising of lithium salt and IL which are not working with the conventional organic carbonate-based electrolyte [24]. However, practical applications of ionic liquid electrolyte are limited due to their high viscosity. As the conductivity is affected by viscosity, higher the viscosity, it will be more difficult for Li<sup>+</sup> ions to migrate. Among different types of ILs, 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) IL has highest conductivity (<sup>10</sup><sup>2</sup> S/cm at room temperature) due to smallest viscosity [25]. Ishikawa et al. [26] reported pure ionic liquid electrolytes such as the 0.8 M LiTFSI/EMI-TFSI, 0.8 M LiTFSI/EMI-FSI, or 0.8 M LiTFSI/P13-FSI and studied their electrochemical behavior using natural graphite as a negative electrode and Li as a counter electrode. They found that LiTFSI/EMI-TFSI shows outstanding performance than that of LiTFSI/EMI-FSI or LiTFSI/P13-FSI

electrolyte and reversible capacity of a graphite negative electrode with LiTFSI/EMI-FSI as the electrolyte is 360 mAh g<sup>1</sup> at C/5 rate remains stable during 30 cycles which favorably is comparable with that of the EC + DEC solvent.

Also, the Li-salt concentration greatly affects the conductivity of ionic liquid system. Shiro Seki et al. [27] have reported effect of lithium salt concentration in 1,2 dimethyl-3- propyl imidazole bis(trifluoromethylsulfonyl) imide ([DMP][ImTFSI]) based binary electrolyte on the electrolyte/electrode interfacial resistance, chargedischarge performance, and ionic conductivity. They found that ionic conductivity of DMPIMTFSI-LiTFSI mixed electrolyte is decreased with on decreasing LiTFSI concentration and also with optimized mixed binary electrolyte with LiCoO2 cathode, Li-cell shows high reversibility during charge-discharge performance for more than 100 cycles.

## **2.2 Conventional carbonate-based electrolyte with added ILs**

Due to strong ion-ion interaction, ionic liquids have high viscosity and low ionic conductivity which limits their application in batteries. In order to solve these mentioned problems, organic solvents such as ethylene carbonate (EC), diethylene carbonate (DEC), dimethyl carbonate (DMC) are added to ILs which significantly reduce the viscosity and enhance the ionic conductivity of system than that of pure ionic liquid electrolyte system [28]. Marco Agostini et al. [29] added ethylene carbonate (EC): dimethyl carbonate (DMC) in N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide, lithium bis(trifluoromethanesulfonyl)imide (Py1,4 TFSI– LiTFSI) ionic liquid-based system and found that ionic conductivity as well as lithiumion transference number increases from 10<sup>3</sup> to 10<sup>2</sup> S/cm and 0.25 to 0.38 respectively. This increase in lithium-ion transference number and ionic conductivity is due to the solvation of Li<sup>+</sup> -ions and dissociation of Li-salts due to more conductive system.

### **2.3 Quasi solid-state electrolytes containing ionic liquid**

Quasi solid-state electrolyte containing ionic liquids are also known as ionic liquidbased electrolyte which have gained significantly more attention due to their high mechanical and chemical stability, good thermal and wide electrochemical stability, highly flexible in nature, non-flammability in nature and high ionic conductivity. In such system, ionic liquid is added into the polymer-based electrolyte as plasticizer which helps to improve the ionic conductivity, electrochemical window as well as Li-transference number. Singh et al. [30] added the 1-butyl-3- methylimidazolium bis (trifluoromethylsulfonyl)imide ([BMIM][TFSI]) ionic liquid into PEO + 20 wt.% of LiTFSI polymer electrolyte system and found the ionic conductivity of optimized ionic liquid-based polymer electrolyte (PEO + 20 wt.% LiTFSI) + 20 wt.% BMIMTFSI around 1.5 <sup>10</sup><sup>4</sup> S/cm at 30°C. Furthermore, addition of ILs in to polymer-based electrolyte, increases the amorphicity as well as ionic conductivity of polymer electrolyte system due to the plasticizing effect of ILs. Simonetti et al. [31] have reported the PEO-based polymer electrolyte containing N-methyl-N-propylpyrrolidinium bis (fluorosulfonyl)imide ionic liquid and ionic conductivity values 3.46<sup>10</sup><sup>4</sup> and 2.43<sup>10</sup><sup>3</sup> S/cm at 20 and 20°C, respectively, with electrochemical stability window 4.5 V vs. Li/Li<sup>+</sup> . Furthermore, Kumar et al. [32] have also reported the effect of 1-ethyl 3-methyl imidazolium trifluoromethanesulfonate (EMITf) IL on the PEO and lithium trifluoromethanesulfonate (LiCF3SO3 or LiTf) (in the ratio 25) polymer electrolyte system. They found that PEO25.LiTf +40 wt.% (EMITf) electrolyte system


#### **Table 1.**

*Properties of Ionic liquid-based polymer electrolyte system.*

shows ionic conductivity of <sup>3</sup> <sup>10</sup><sup>4</sup> S/cm at RT with wide electrochemical stability window (4.9 V vs. Li/Li<sup>+</sup> ) and excellent thermal stability which proves the suitability of electrolyte in various energy storage/conversion devices. Also, such IL-based polymer electrolytes have freestanding and flexible nature along with excellent thermal and mechanical stabilities which proves the suitability of such electrolytes in Liion battery. Some of the IL-based polymer electrolyte systems are listed in **Table 1** along with some properties such as ionic conductivity, Li-ion transference number and electrochemical stability window (ESW). From the **Table 1**, it can be seen that IL incorporation in the polymer and salt electrolyte system has increased the ionic conductivity as well ESW which proves the suitability of IL-based polymer electrolytes for Li-battery. In this chapter, we mainly discuss the physical, thermal, structural and electrochemical properties of IL-based polymer electrolyte system and its application in lithium metal battery (LMB).

## **3. Structural, thermal and ionic transport properties of ionic liquid-based polymer electrolytes**

#### **3.1 Structural properties of ionic liquid-based polymer electrolytes**

It is reported in the literature that on addition of IL in the PEO-based polymer electrolyte system, its amorphicity is increased and hence mobility of free ions which in turn increases the ionic conductivity of IL-based polymer electrolyte system. Such

*Ionic Liquids: Applications in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.107941*

increase in the amorphicity is observed by the X-ray Diffraction study (XRD) of ILbased polymer electrolyte system. Meghnani et al. [8] have reported that when in the PEO + 20 wt.% LiFSI system, 1-butyl-3-methlypyridinium bis(trifluoromethylsulfonyl) imide (BMPyTFSI) IL is added, the amorphous nature of polymer electrolyte system increases which in turn increases the mobility of charge carrier due to free volume. **Figure 4** shows the XRD pattern of pristine PEO and polymer electrolytes PEO + 20 wt.% LiFSI + X wt.% IL (b) X = 0, (c) X = 10, (d) X = 15, and (e) X = 20 at room temperature and it can be seen that on addition of ionic liquid in polymer electrolyte system, the halo region as well as FWHM increases which in turn decreases the crystallinity of the system. Gupta and Singh et al. [30, 33] observed the similar behavior, they reported the effect of addition of phosphonium based IL and BMIMTFSI IL in the PEO + 20 wt.% LiTFSI, and observed that the halo region of peak increases which makes the polymer system amorphous. This amorphous nature of polymer electrolyte system favors the Li ion diffusion and hence enhances the Li-ion conductivity.

Besides that, addition of ionic liquid does not only affect the XRD pattern of polymer electrolyte system, but also affects the surface morphology of it. It is reported in the literature that on addition of IL, surface of polymer electrolyte becomes smooth than that of without ionic liquid. We [40] have reported that addition of BMPyTFSI IL into PEO-based electrolyte system makes the surface of polymer electrolyte system smoother as shown in the **Figure 5**. Same behavior is also observed by Gupta et al. [33] They have reported the smoother surface morphology of IL based polymer electrolyte

#### **Figure 4.**

*X-ray diffraction patterns of (a) PEO and PEO + 20 wt.% LiFSI + X wt.% IL (b) X = 0, (c) X = 10, (d) X = 15, and (e) X = 20 polymer electrolytes at RT.*

**Figure 5.** *SEM images of (a) Pristine PEO (b) PEO +20 wt.% LiFSI +20 wt.% IL.*

and found that addition of IL into polymer electrolyte system increases the amorphicity of the system and thus makes it surface smoother.

## **3.2 Effect of IL on the thermal behavior of polymer electrolyte system**

The effect of IL on the thermal stability of polymer electrolyte system is examined by Thermogravimetric and Differential scanning calorimetry analysis using Mettler Toledo DSC/TGA system (TGA and DSC study). For practical application of IL-based polymer electrolyte system in lithium batteries at high temperature, it is necessary to know the thermal stability as well as melting temperature of IL based polymer electrolyte system. Meghnani et al. [41] synthesized the IL-based polymer electrolytes PEO + 20 wt.% LiFSI + X wt.% N-methyl-N-propyl piperidinium bis(fluorosulfonyl) imide (PP13FSI) (X = 10, 20, 30 and 40) by solution cast technique [8] and found the thermal stability �210°C as shown in the **Figure 6(a)**. The authors have also reported the melting temperature as well as degree of crystallinity of IL-based polymer electrolytes by DSC technique. They found that on increasing the IL concentration in PEO + 20 wt.% LiFSI system, melting temperature (*Tm*) is shifted towards the lower temperature (�47.6°C) and also area under the endothermic curve deceases due to decrease in the crystallinity of the system. This decrease in the crystallinity of the system is calculated by the formula [42]:

$$\text{Degree of }\text{Crystallinity} = \frac{\Delta H\_m}{\Delta H\_m^0} \times 100\% \tag{1}$$

Where Δ*Hm* and Δ*H*<sup>0</sup> *<sup>m</sup>* are the enthalpy value of electrolyte and pristine PEO respectively. The value of Δ*Hm* is the area under the endothermic peak related to melting curve whereas the value of Δ*H*<sup>0</sup> *<sup>m</sup>* is 213.7 Jg�<sup>1</sup> for 100% crystalline polymer PEO. It is found that on addition of IL, the melting temperature as well as degree of crystallinity decrease and the lowest value is found to be for 40 wt.% IL containing polymer electrolyte (see **Figure 6(b)**). This is due to the enhancement of the amorphicity of the polymer electrolyte system which result in the increase of the segmental motion of polymeric chain.

#### **Figure 6.**

*(a) Thermal stability, (b) DSC thermograms of polymer electrolyte PEO + 20 wt.% LiFSI + X wt.% PP13FSI (X = 0, 10, 20, 30, 40) and (c) TGA thermogram and (d) DSC thermograms of pristine PEO, polymer electrolytes films PEO + 20 wt.% LiFSI + X wt.% IL (X = 0, 10, 15, and 20).*

Meghnani et al. [8] also reported the effect of BMPyTFSI IL on the thermal stability of PEO-based electrolyte system. They have found that that IL based polymer electrolyte PEO + 20 wt.% LiTFSI + X wt.% (X = 10, 15, 20) shows three step decomposition and thermal stability is found in the range 200–220°C as shown in **Figure 6(c)**. Also, on increasing the IL concentration, melting temperature shifted towards lower temperature side (see **Figure 6(d)**). Therefore, IL based polymer electrolyte is more suitable electrolyte for Li-battery applications at high temperature due to its good thermal stability.

### **3.3 Effect of IL on the ionic conductivity of polymer electrolyte system**

One of the crucial effects of IL on the polymer electrolyte system is ionic conductivity which is usually examined by the complex impedance spectroscopy technique. It has been reported in literature that on addition of IL into the polymer electrolyte system, the conductivity of parent system increases because IL acts as plasticizer which enhances flexibility of polymer electrolyte system and thus ionic conductivity of the system. The ionic conductivity of IL-based polymer electrolyte system is calculated by the equation:

$$
\sigma = \frac{1}{R\_b} \frac{L}{A} \tag{2}
$$

Where L and A are the thickness and area of polymer electrolyte respectively and Rb is the bulk resistance of electrolyte.

Meghnani et al. [41] have reported that on increasing the IL concentration the bulk resistance of polymer electrolyte PEO + 20 wt.% LiFSI + X wt.% PP13FSI (X = 0, 10, 20, 30, 40) decreases and the lowest bulk resistance or highest ionic conductivity is found to be for 40 wt.% IL containing polymer electrolyte as shown in **Figure 7(a,b)**.

This decreasing trend may be due to the plasticizing nature of IL which enhances the flexibility of polymer chain and thus ionic conductivity of polymer electrolyte. They have also studied the temperature dependent conductivity and found that on increasing the temperature, ionic conductivity of the system increases and show Arrhenius type thermally activated behavior:

$$
\sigma = \sigma\_0 e^{-\frac{E\_t}{kT}} \tag{3}
$$

Where Ea, K and T are the activation energy, Boltzmann constant and temperature respectively and *σ*<sup>0</sup> is pre-exponential factor. It is found that activation energy of electrolyte system decreases with increasing the IL concentration as seen from **Figure 7(c)**. In another study Meghnani et al. [8] have reported the conductivity of PEO + 20 wt.% LiFSI + X wt.% BMPyTFSI (X = 10, 15, 20) system and found that on increasing IL content, conductivity of system increases and follows Arrhenius type thermally activated behavior as shown in the **Figure 7(d)**. Also, they have observed that conductivity and activation energy both are inverse in nature (see **Figure 7(e)**). Balo et al. [43] also observed almost similar behavior in PEO + 20 wt.% LiTFSI + X wt. % EMIMFSI (x = 0, 2.5, 5, 7.5, 10, 12.5, and 15) polymer electrolyte system. They found that on increasing the IL concentration upto 10 wt.%, ionic conductivity of system increases and thereafter it is decreases. It is because beyond this IL concertation, formation of ion pairs starts which reduces the ionic conductivity of the system.

## **3.4 Effect of IL on the Li<sup>+</sup> diffusion coefficient** *DLi* ð Þ <sup>þ</sup> **and Li-ion ionic conductivity of polymer electrolyte system**

For application of IL-based polymer electrolyte system in Li-battery, it is important to know the Li<sup>+</sup> diffusion coefficient *DLi* ð Þ <sup>þ</sup> and Li-ion ionic conductivity. Li<sup>+</sup> diffusion coefficient is usually estimated by restricted diffusion method [44]. In this method, polymer electrolyte is sandwich between two non-blocking electrodes and constant potential is applied for a fixed period of time to set up constant current. Thereafter, potential is interrupted and drop of potential is recorded with respect to time. By determining the slope of this curve, diffusion coefficient of electrolyte is calculated using the formula [44]:

$$Slope = \frac{-\pi^2 D}{L^2} \tag{4}$$

Where L is the thickness of the electrolyte. Meghnani et al. [41] have estimated the Li<sup>+</sup> diffusion coefficient *DLi* ð Þ <sup>þ</sup> for PEO + 20 wt.% LiFSI + X wt.% (0, 10, 40) PP13FSI system (see **Figure 8(a,b)**). It is found that *DLi*<sup>þ</sup> value for PEO + 20 wt.% LiFSI system is �3*:*<sup>51</sup> � <sup>10</sup>�<sup>9</sup> cm2s�<sup>1</sup> at room temperature (RT) while when IL is added in

*Ionic Liquids: Applications in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.107941*

**Figure 7.**

*(a) Nyquist plot, (b) variation of conductivity with IL concentration at 30°C and 40°C and (c) Arrhenius plot of the cell SS/PEO + 20 wt.% LiFSI + X wt.% PP13FSI/SS (X = 0, 10, 20, 30, 40) (d) temperature-dependent conductivity of polymer electrolytes PEO + 20 wt.% LiFSI + X wt.% IL (X = 10, 15, and 20) and (e) variation of conductivity (σ), and activation energy (Ea) with IL concentration.*

the PEO + 20 wt.% LiFSI system, *DLi*<sup>þ</sup> increases progressively and its maximum value is observed �1*:*<sup>32</sup> � <sup>10</sup>�8cm2 <sup>s</sup>�<sup>1</sup> for 40 wt.% IL containing polymer electrolyte as shown in the **Figure 8(c)**. This increase in *DLi*<sup>þ</sup> value confirms the higher lithium-ion

#### **Figure 8.**

*(a-b) Li+ ion Diffusion coefficient measurement using RDM of the polarized cell Li/PEO + 20 wt.% LiFSI + X wt. % (0, 10, 40) PP13FSI/Li shown by (e) Variation of Li+ ion diffusion coefficient with respect to IL concentration.*

conductivity which makes IL-based electrolyte system more suitable for Li-battery applications.

Besides that, IL also affects the lithium-ion transference number of polymer electrolyte system. Meghnani et al. [8] have determined the Li-ion transference no by dc polarization technique. In this technique, IL-based polymer electrolyte is sandwiched between two Li-metal foils (Li/IL-based-PE/Li) and a small constant potential (0.1 V) is applied across this cell configuration for a fixed period of time and corresponding current is recorded with respect to time, as shown in **Figure 9(a)**. Also, the impedance plot is recorded before and after polarization (see **Figure 9(b)**). Authors have calculated the transference number of Li + ion using the Bruce and Vincent' formula [45]:

$$t\_{Li^{+}} = \frac{I\_{\text{sr}}(\Delta V - I\_{0}R\_{0})}{I\_{0}(\Delta V - I\_{\text{SS}}R\_{\text{SS}})} \tag{5}$$

Where *ISS* and *RSS* are the steady- state current and passive layer resistance after polarization and *I*0&*R*<sup>0</sup> are the initial current and passive layer resistance before polarization respectively.

#### **Figure 9.**

*(a) dc polarization curve of cell (Li/20 wt.% IL containing PEs/Li) at the voltage 0.05 V, (b) Nyquist plot of the cell (Li/20 wt.% IL containing PEs/Li) before and after polarization and (c) Variation of Li+ transference number and Li<sup>+</sup> ion conductivity with IL concentration.*

From these studies they found that *tLi*<sup>þ</sup> as well Li-ion conductivity both are increased with IL concentration and the maximum *tLi*<sup>þ</sup> value is �0.37 for 20 wt.% IL-containing polymer electrolyte as seen from **Figure 9(c)**.

Besides that, Balo et al. [34] also reported the same result for PEO + 20 wt.% LiTFSI + x wt.% EMIMTFSI (x = 0, 2.5, 5, 7.5, 10 and 12.5) IL-based polymer electrolyte system. They found that for PEO + 20 wt.% LiTFSI, *tLi*<sup>þ</sup> is around 0.11 while on addition of IL it is reached to 0.16 for PEO + 20 wt.% LiTFSI +2.5 wt.% EMIMTFSI and increases gradually on increasing the IL concentration and maximum value is found to be �0.39 for PEO + 20 wt.% LiTFSI +12.5 wt.% EMIMTFSI.

#### **3.5 Effect of IL on the electrochemical stability window of polymer electrolyte**

For application of IL-based polymer electrolyte in Li-battery, it is important to know the electrochemical stability window (ESW) of electrolyte. For this motive, Linear sweep voltammetry (LSV) technique is used. In this technique electrolyte is sandwiched between stainless steel electrode which work as reference electrode and Li-electrode which act as working electrode and current is record with respect to voltage. Meghnani et al. [41] have observed the effect of IL on the ESW (**Figure 10**)

#### **Figure 10.**

*Linear sweep voltammetry measurement of Li/PEO + 20 wt.% LiFSI + X wt.% (0, 40) PP13FSI/SS at scan rate* � *0.05 mV/s.*

and found that for polymer system PEO + 20 wt.% LiFSI without IL, the ESW is �3.67 V vs. Li/Li<sup>+</sup> [while for 40 wt.% IL containing polymer electrolyte, electrochemically stable window �4.72 V vs. Li/Li<sup>+</sup> . From the above discussion, it can be seen that the presence of IL improves the electrochemical stability window of polymer system significantly which is good for high voltage Li-battery application.

## **4. Application of IL-based polymer electrolyte in Li-metal battery**

In Li-battery, charging and discharging take place by means of electrochemical redox reaction. For Li-ion insertion and extraction during charging-discharging process, electrolyte is required. Electrolyte plays the crucial role in determining the Libattery performance. Among the different type of electrolytes, IL-based polymer electrolytes have gained more attention due to its some unique properties such as high flexibility, wide ECW, good thermal and mechanical stability as well as high ionic conductivity.

IL-based polymer electrolyte which is most commonly known as gel-polymer electrolyte is widely used in Li-battery carries the hybrid properties of solid electrolyte with an embedded liquid electrolyte that is liquid electrolyte is embedded in the polymer matrix. Due to immobilization of electrolyte, enhanced ionic conductivity as well as wide ESW is achieved when compared to solid and liquid electrolytes. Also, it reduces the probability of short-circuiting due to stable solid-interphase formation (SEI) which reduces in turn the lithium-dendrite growth, there is ease of portability, safety issue of battery enhances due to no electrolyte leakage and absence of volatile reaction. Therefore, IL-based polymer electrolyte is attractive choice for Li-batteries. Among the alternative clear energy resources, Li-metal battery have gained more attention. They are widely used in the market such as portable electronic devices, electric vehicles (EVs), hybrid electric vehicles (HEVs), grid energy storage system and so on [46–48].

Furthermore, from safety point of view specially in lithium-metal battery (LMB), IL -based polymer electrolyte is widely used but still there needs to improve some


*Ionic Liquids: Applications in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.107941*

> **Table 2.**

*Electrochemical performance of lithium metal batteries (LMBs) with IL-based polymer electrolytes.* parameters like internal resistance of the cell should be low and also cell must deliver maximum capacity with 100% coulombic efficiency and better capacity retention. For that purpose, IL-based polymer electrolytes having high ionic conductivity must be used in LMBs so that cell resistance as well as SEI layer resistance would be low. The performance of some of LMBs with IL-based polymer electrolytes are listed in **Table 2**.

Meghnani et al. [41] have reported the performance of lithium metal polymer battery (LMPB) using the synthesized IL-based polymer electrolyte, PEO + 20 wt.% LiFSI +40 wt.% PP13FSI in the cell Li/PEO + 20 wt.% LiFSI +40 wt.% PP13FSI/ Pristine NCA and Li/PEO + 20 wt.% LiFSI +40 wt.% PP13FSI/BiPO4@NCA configuration. In the cell configuration Li/pristine NCA, cell delivers a discharging capacity 150 mAh. <sup>g</sup><sup>1</sup> at C/10 rate while Li/BiPO4@NCA cell delivers the discharge capacity 164 mAh. g<sup>1</sup> at C/10 rate as shown in **Figure 11(a,b)**. Also, the coulombic efficiency of the cell Li/BiPO4@NCA is found to be 99.96% after 150 cycles and for the cell Li/pristine NCA, it is around 91.68% as shown in **Figure 11(c,d)**.

Balo et al.[43] have studied the electrochemical performance of Li-battery using the IL based polymer electrolyte, PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI in the cell Li/ PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI /GO-LiFePO4 and Li/PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI/NCA configuration. They found that the cell Li/GO-

#### **Figure 11.**

*(a,b) charge-discharge curve and (c,d) cyclic stability of cell Li/PEO + 20 wt.% LiFSI +40 wt.% PP13FSI/Pristine NCA and Li/PEO + 20 wt.% LiFSI +40 wt.% PP13FSI/BiPO4@NCA respectively.*

## *Ionic Liquids: Applications in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.107941*

LiFePO4 delivers the maximum discharge capacity 145, 83, 38, 20 mAh/g at C/10, C/5 and 1 C and 2 C rate respectively (see **Figure 12(a)**). While another cell configuration Li/NCA delivers maximum discharge capacity 175, 168, 157, 150 mAh/g at C/10, C/5, 1 C and 2 C rate respectively (**Figure 12(c)**). When these cells are cycled at C/10 upto 200 cycles it is found that for cell Li/GO-LiFePO4 in the initial few cycles discharge capacity is lower and it reaches to around �145 mAh/g within the 10th cycle (see **Figure 12(b)**). However, 142 mAh/g capacity remains after 200th cycle. They have also calculated capacity fading per cycle (inset of **Figure 12(b)**). It is seen that it shows linear behavior and there is only 0.01% capacity loss per cycle. From the cyclic performance of the cell Li/NCA it was inferred that the discharge capacity is low (�126 mAh/g) in initial 6–7 cycles thereafter it value increases gradually and attains the maximum capacity �175 mAh/g (see **Figure 12(d)**). However, after that, the capacity remains constant during the cycling process. The maximum discharge capacity and better cyclic stability with good coulombic efficiency of these two cells may be consistent with the higher lithium-ion conductivity of ionic liquid-based polymer electrolyte and lower interfacial resistance due to better contact with the electrode.

From the above discussion, it can be concluded that IL-based polymer electrolytes are most promising candidate for Li-metal polymer batteries.

**Figure 12.**

*(a, c) Charge-discharge curve and (b, d) cyclic performance of the cell Li/PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI/GO-LiFePO4 and Li/PEO + 20 wt.% LiTFSI+10 wt.% EMIMFSI/NCA.*

## **5. Conclusion**

As we can see that electrolyte plays the crucial role in batteries performance. It not only acts as a separator between electrodes but also provides the path for ion transportation. Therefore, electrolyte should possess high ionic conductivity, wide electrochemical window, good thermal and mechanical stability. All these properties are embedded in ionic liquid-based gel polymer electrolyte. IL-based polymer electrolytes not only have high ionic conductivity but also are free from leakage problem, portability issue and short-circuiting problem which is usually faced in case of liquid electrolytes. The unique properties of IL-based polymer electrolytes proves its suitability for battery applications. For this reason, IL-based polymer electrolytes are widely used in Li-battery and it is found that IL-based polymer electrolyte with Limetal shows better cyclic stability as well good coulombic efficiency. Thus, IL-based polymer electrolytes are most attractive choice for lithium metal polymer battery.

## **Author details**

Dipika Meghnani and Rajendra Kumar Singh\* Ionic Liquid and Solid-State Ionics Lab, Department of Physics, Institute of Science, Banaras Hindu University, Varanasi, India

\*Address all correspondence to: rksingh\_17@rediffmail.com; rajendrasingh.bhu@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Ionic Liquids: Applications in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.107941*

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## **Chapter 2**

## High Ionic Conductivities of Ionic Materials as Potential Electrolytes

*Pradip K. Bhowmik, Si L. Chen, Haesook Han, Khairul Anwar Ishak, Thamil Selvi Velayutham, Umama Bendaoud and Alfonso Martinez-Felipe*

## **Abstract**

Ionic liquids (ILs) are salts consisting of organic cations and inorganic/organic anions having melting transitions lower than 100°C. They hold promise as engineered materials in a variety of modern fields. They are used as green solvents or catalysts for chemical reactions, biocatalysts, biopolymers processing, active pharmaceutical ingradients in medicine, even as electrolytes for batteries. For batteries applications, ionic liquids must have high ionic conductivity, but most of the ionic liquids (monocationic) have low conductivities. To address this limitation, we describe in this chapter dicationic ionic liquids based on extended viologens. The colossal conductivities, σdc ~ 10−1.5·S cm1 of new diatonic ionic liquids in the same range of benchmark materials/electrolytes applied in fuel cells and batteries is reported. The relatively new class of ionic liquids consist of extended viologen bistriflimides containing oligoethyleneoxy groups were prepared via Zincke reaction under mild conditions and are excellent candidates as components in devices for energy conversion and storage applications. The synthesis and ionic conductivities of other ionic liquids and dicationic organic salts will be contrasted with dicationic ionic liquids in this chapter.

**Keywords:** extended viologens, ionic liquids, Zincke salt, dicationic ionic liquid crystals, ionic conductivity, dielectric impedance spectroscopy

## **1. Introduction**

Ionic liquids (ILs) are salts consisting of organic cations and inorganic/organic anions having melting transitions (*T*m) lower than 100°C, and even below ambient temperatures, and with cryogenic glass transition temperatures (*T*g). ILs can be engineered for a variety of modern applications, including green solvents or catalysts [1, 2], biocatalysts [3], biopolymers [4–7], pharmaceutical ingredients [8], and electrolytes for batteries [9–11]. Despite they were discovered over 30 years ago, interest by researchers and industrialists in ILs continues to grow due to their versatility.

The introduction of multiple charges in low-molar ionic liquids and poly(ionic liquid)s widens the range of physical properties, leading to improvements in density, surface tension and viscosity, facilitated by their higher molecular weights [12, 13]. The large charge densities and electrostatic interactions normally increase the ILs

thermal stabilities [14, 15] and electrical capacities [16, 17], and results in better performance as antimicrobial agents [18] and stationary phases for gas chromatography [19], among others [20, 21].

Multi-charged ILs are particularly attractive as electrolytes used in energy storage and conversion materials and devices, due to their combination of low viscosity (like traditional ILs) and high ionic conductivity (like poly(ionic liquid)s). The physical properties of multi-charged ILs can be fine-tuned by combining different cations and anions, with well-defined chemical structures that avoid polydispersity issues. Current multi-charged ILs include ammonium, phosphonium, imidazolium, pyridinium, pyrrolidinium, piperidinium, triazolium and 4,4′-bipyridinium (viologen) cations, but the perspectives for new and tailored materials are almost unlimited.

In this chapter, we showcase the potential of three series of different ionic liquids as electrolytes in energy applications, by correlating their ionic conductivities to structural effects.

## **2. Conductivity measurements**

The conductivity of the ionic liquids was studied by impedance spectroscopy [22]. Small amounts (few mg) of molten samples were inserted into commercial indium tin oxide (ITO) cells, *via* capillary method (Instec Inc.). The parallel capacitance *C*p and the dielectric loss tangent ( ( ) ε δ ε ′ <sup>=</sup> ′ ′ tan , where ε ′′ is the dielectric loss factor, and ε ′ the dielectric modulus) were measured in frequency sweeps ranging between *f* = 0.1 Hz and 106 Hz, and at different temperatures. Two instruments were used and combined for these impedance measurements: a laboratory-made dielectric spectrometer (10−1–104 Hz) and a commercial impedance analyser (Agilent 4294A) (102 –106 Hz). The results were analysed in terms of the complex permittivity, εω ε ε ( ) <sup>∗</sup> = ′ − *j* ′′, and conductivity, σω σ σ ( ) <sup>∗</sup> = ′ − *j* ′′ , where σ ωε ε*<sup>O</sup>* ′ = ′′ and σ ωε ε = *<sup>O</sup>* ′′ ′ with ω= 2π*f* is the angular frequency (rad·s−1).

## **3. Results: materials preparation and conductivity**

We have prepared and assessed three series of dicationic salts as novel ionic liquid electrolytes: dicationic stilbazolium salts, dicationic asymmetric viologens, and dicationic ionic liquids. Our general strategy for their synthesis is based on quaternization by SN 2 aka Menshutkin reactions, followed by metathesis of anions [12–21, 23].

### **3.1 Dicationic stilbazolium salts and their ionic conductivities**

The dicationic stilbazolium salts (**I-1 – I-4**) were prepared by the reaction of *trans-*4-octyloxy-4-stilbazole with α,ω-methylene ditosylates by SN 2 , followed by metathesis reaction of the corresponding ditosylates salts with lithium triflimide salt to yield the dicationic stilbazolium bistriflimide salts (**II-1- II-4**), as shown in **Figure 1**. The detailed synthetic procedures were described elsewhere [24]. Their chemical structures were established by using <sup>1</sup> H, 13C and 19F nuclear magnetic resonance (NMR) spectra and elemental analysis [24].

*High Ionic Conductivities of Ionic Materials as Potential Electrolytes DOI: http://dx.doi.org/10.5772/intechopen.107949*

#### **Figure 1.**

*Synthesis of dicationic stilbazolium dicationic salts containing bistosylate and bistriflimide counterions.*

Despite the presence of mesogenic units, these salts do not display mesomorphic properties, and instead solely exhibit crystalline polymorphism, confirmed by the presence of several peaks in the differential scanning calorimetric thermograms (DSC), **Figure 2**.

We found that the salts containing tosylate ions (**I-***n*) display stronger dielectric response than the triflimide analogues (**II-***n***,** see **Figure 3(a)**), and show more complex profiles, associated to the additional aromatic groups. All these dielectric relaxations promote short-range conductivity in the salts, see **Figure 3(b)**, associated to local displacements of the charges. Despite the strong *ε* "response, the tosylate salts did not develop signs of direct conductivity (σdc) in the range of temperatures and frequencies under study. We hypothesise that the presence of bulky aromatic groups may hinder long-range transport of ionic charges in these compounds. The triflimide salts, alternatively, show DC values that reach the 10–4.5 S·cm−1 range in the isotropic

#### **Figure 2.**

*Differential scanning calorimetry thermograms (DSC) obtained on heating at 10°C·min−1 for the bistosylate (a) and bistriflimide (b) salts.*

#### **Figure 3.**

*Dielectric results obtained from the impedance frequency sweeps of dicationic stilbazolium salts: (a) dielectric loss factor,* ε′′ *and (b) real component of the complex conductivity,* σ′ *of* **II-3;** *(c) Arrhenius plots for the direct current conductivity (* σ*dc ) obtained for* **II-4***.*

phase, see **Figure 3c**), but then fall rapidly following a Vogel- Fulcher-Tammann, VFT, profile [24–26].

These correlations between conductivity and structure highlight that the transport of ionic charges in these salts may require an amorphous environment, even if they involve short molecular range. The strong temperature dependence of σdc also confirms that the conductivity response is strongly coupled to segmental viscous-like motions.

### **3.2 Dicationic asymmetric viologens, 6BP***n***(s)**

The synthesis of asymmetric viologens with hexyl terminal groups and different alkyl chain lengths, and their synthetic routes are shown in **Figure 4**. The detailed synthetic procedures were described elsewhere, and their chemical structures were determined by using <sup>1</sup> H, 13C and 19F NMR spectra and elemental analysis [27].

The **6BPn(s)** exhibit Smectic T phases in a broad range of temperatures, including room temperature, confirmed by polarised optical microscopy and X-ray diffraction, see **Figure 5**.

The **6BP***n***(s)** undergo one main dielectric relaxation, **Figure 6(a)**, associated to the rotation of the molecular core, and high frequency conductivity, **Figure 6(b)**, related to short-range triflate ion-hoping. The materials reach direct current conductivities (σdc) in the 10−5 to 10−3 S cm−1 range, **Figure 6(c)**, which are very promising for organic media. These dielectric processes depict a clear Vogel-Fulcher-Tammann (VFT) behaviour, indicating that the dielectric and conductivity response of these LCIs is strongly coupled to segmental-type motions [27].

#### **3.3 Dicationic extended viologens and their ionic conductivities**

The dicatonic extended viologens **1–3** were prepared according to the **Figure 7** (vide infra). The 4-oligoethyleneoxypheylanilines were prepared according to

*High Ionic Conductivities of Ionic Materials as Potential Electrolytes DOI: http://dx.doi.org/10.5772/intechopen.107949*

#### **Figure 4.**

*Synthetic routes for the asymmetric viologen bistriflimide salts of a 4,4***′***-bipyridinium core (BP) alkylated with hexyl group with n = 5, 7, 10, 11, 12, 14, 16, 18 and 20.*

#### **Figure 5.**

*(a) Photomicrograph of* **6BP16** *obtained under the polarised optical microscope, showing the SmT phase; (b) phase diagram of the* **6BP***n***(s)** *as a function of terminal chain, n, showing a sketch of a proposed SmT structure.*

modified literature procedures step 1 and step 2 [28, 29]. The synthesis of bis-(4 oligoethyleneoxyphenyl)-4,4′-bipyridinium dichlorides (**P1-P3**) with different ethyleneoxy groups is also summarised in **Figure 7**. The synthetic routes involved: (i) the aromatic nucleophilic substitution between the 1-chloro-2,4-dinitrobenzene

#### **Figure 6.**

*Dielectric results obtained from the impedance frequency sweeps of* **6BP14:** *(a) dielectric loss factor,* ε′′ *; (b) real component of the complex conductivity,* σ′ *; (c) Arrhenius plots for the frequency dependence of* ε′′ *and* σ′ *maxima, and direct current conductivities (* σ*dc ).*

and 4,4′-bipyridine in acetonitrile on heating to reflux, to yield the so-called Zincke salts [30, 31] (step 3); and (ii) subsequent anionic ring opening and ring closing reactions (ANROC) with the corresponding 4-oligoethyleneoxypheylanilines, in N,N-dimethylacetamide (DMAc) at room temperature (step 4). Lastly**, P1-P3** were converted to **1–3** by metathesis with lithium triflimides in methanol [32] (step 5). Detailed synthetic procedures and analyses are also given elsewhere [33]. The chemical structures of the intermediates and final products were confirmed by their Fourier-transform infrared (FT-IR) spectra, <sup>1</sup> H, 13C, and 19F spectra in CD3OD and their purity was determined by elemental analysis [33]. To our knowledge, these are the first examples of ionic liquids prepared *via* Zincke reactions.

Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and polarised optical microscopy (POM) were used to determine the thermal properties and phase behaviour of these salts. The three salts' degradation temperatures, *T*d, range from ~311 to 334°C, with less than 5% weight loss up to 300°C in nitrogen [33]. Although it was anticipated that bistriflimide ions would impart high thermal stabilities, the high *T*d values demonstrate that the presence of flexible oxyethylene groups has no destabilising effect on these salts.

The DSC thermograms of salts **1** and **2** exhibit first-order endotherms associated with crystal-to-crystal (**2**) and melting (**1** and **2**) processes, whereas salt **3** only exhibits a low-temperature glass transition (*T*g = −6°C) [1–11, 34, 35]. Both **1** and **2** melt upon heating as expected where the increase in the oxyethylene termination length reduces the melting point. Due to inhibition of crystallisation at sufficiently long ethyleneoxy chains, n = 3, the absence of first-order transitions in the corresponding thermogram indicates that **3** behaves like an amorphous salt. In subsequent heating and cooling scans, there are no additional thermal events were visible for **1** and **2**, suggesting that crystallisation of these samples must be a slow process. The absence of liquid crystal behaviour in these salts, contrasts with the recent report of smectic phases by analogous alkoxy-terminated (n ≥ 6) viologens (see **Figure 8**) [32] and others [36–38]. Even though comparable lengths of terminal chains would have been expected to promote microphase separation and smectic behaviour in **1–3**, the formation of stronger interactions by the ethyleneoxy

*High Ionic Conductivities of Ionic Materials as Potential Electrolytes DOI: http://dx.doi.org/10.5772/intechopen.107949*

#### **Figure 7.**

*Multiple steps for the synthesis of the ionic liquids and salts* **1–3.**

#### **Figure 8.**

*General chemical structures of alkoxy-terminated viologens [32].*

groups may limit the local mobility required to yield liquid crystallinity. The effect of terminal chain lengths on nanosegregation between polar chains and aromatic cores in similar viologens is being studied further.

Viologens and their countless derivatives have already been proposed as potential redox active functional materials for use in electrochromic devices, diodes and transistors, memory devices, molecular machines, and dye-sensitised solar cells [39–41]. The incorporation of the oxyethylene(s) terminations is warranted for not one but two distinct reasons. On the one hand, one of our goals is to reduce (at least partially) the rigidity of the four-ring phenyl core (which could increase viscosity). On the other hand, the presence of polar chains can help delocalize the triflimide anions and avoid complexation, both of which would inhibit ion mobility [42]. This is because polar chains have a higher dipole moment.

**Figure 9** shows the dielectric and conductivity response of the extended viologens **1–3** as a function frequency measured at room temperature. The complex permittivity and conductivity values of these salts have been found to be extraordinarily high due to the highly polar nature of ionic liquids and salts [33]. The ε" plot demonstrates a linear increase (with slopes of −1) at sufficiently lower frequencies representing the increase in direct current (DC) conductivity [43]. This DC component dominates any potential dielectric relaxation, despite the presence of peaks in the salts **1** in **Figure 9(a)**. The third salt has the highest conductivity values of the three salts. The formation of plateaus in the double logarithmic σ' versus f plots in **Figure 9(b)** [22] confirms the existence of DC conductivity. The exemplary temperature plot of complex permittivity (ε\*) and conductivity (σ\*) of salt **2** is shown

*Complex permittivity (a) and conductivity (b) obtained for compounds* **1***,* **2** *and* **3** *measured at room temperature.*

*High Ionic Conductivities of Ionic Materials as Potential Electrolytes DOI: http://dx.doi.org/10.5772/intechopen.107949*

#### **Figure 10.**

*Complex permittivity (a) and conductivity (b) obtained for compounds* **2** *measured on heating from 30°C (see the arrow).*

in **Figure 10**. The DC conductivity values, σdc, are estimated by extrapolating the constant σ' ranges to *f*→0 at various temperature. The resulting Arrhenius plots (not shown here) were used to evaluate the activation energy *E*a of the conductivity process using the equation; σdc = σ0exp(Ea/RT), where *R* is the gas constant, 8.31 J. mol−1.K−1, *T* is the absolute temperature, and *σ0* is a pre-exponential term. Salts **1** and **2** have activation energies of 95.9 kJ/mol and 84.5 kJ/mol, respectively. These values are significantly higher for locally activated processes and are consistent with the occurrence of so-called β-relaxations, which involve the rotation of rod-like molecules (extended viologen moieties) within the crystal lattice around their long axis [33, 44]. When the -(CH2CH2O)- terminal chains are short, it appears that the motions around the bulky four-phenyl core dominate (and partially hinder) the conductivity process [43, 45–47]. On the other hand, salt **3** shows exceptional σdc values (~10-1.5 S/cm) comparable to the bench electrolytes used in fuel cells [48] and batteries [49]. Longer ethyleneoxy terminal chains enhance conductivity in salts by a plasticizing effect, thereby promoting ionic motions in the material. Moreover, salt **3** is one of the few examples of an organic salt with such large conductivities under anhydrous conditions and at temperatures close to room temperature [9, 50–54]. The formation of a rubbery phase above its low glass transition *T*<sup>g</sup> − 6°C (from DSC measurement) with large free volumes that facilitate ionic motion can explain the higher σdc values observed for **3** within the series [55, 56]. In this molten-like state, conductivity has very weak temperature dependence [57–59]. The ionic liquids **1** and **2**, on the other hand, remain in a glassy state throughout the temperature range under study, and the σdc values are lower.

## **4. Conclusions**

We have prepared new viologens by using Zincke reactions, which led to the formation of ionic liquids and salts with strong dielectric responses. Our results confirm that a fine balance between local/charge interactions and mobility is needed to optimise phase behaviour and conductivity of the ionic liquids. In general terms, the formation of amorphous, liquid crystalline, or crystal phases, can be tuned by the aspect ratio of the rigid core and the flexible terminations.

Whilst compounds having alkyl terminations still fall some orders of magnitude below those exhibited by reference phosphonium or imidazolium-based ionic liquid electrolytes (10−2 S·cm−1) [60, 61], we found that the presence of oxyethylene groups promotes high conductivities in the 10−1.5 S·cm−1 range, comparable to those required in commercial batteries or fuel cells [48, 49]. The presence of additional polar sites in these terminations may be the key to facilitate long-range transport in these materials. Interestingly, the presence of tosylate ions with a strong dielectric response does not promote long-range conductivity, and triflimide ions appear to be more suitable for ionic transport. Delocalisation of the charges can therefore be a key enabler for high σdc values.

These results highlight the potential use of these and other ionic liquids in energy devices such as fuel cells, batteries, supercapacitors, or solar cells. By extending the central rigid core, exchanging different cations, or modifying the composition and length of the terminations, this work opens new avenues for the design of ionic liquids with tuned electrostatic interactions and nanostructures.

## **Acknowledgements**

We sincerely acknowledge Dr. Kousaalya Bakthavatchalam for critically reading and making insightful suggestions for the improvement of the article. TSV acknowledges the Ministry of Higher Education under the Fundamental Research Grant Scheme [FRGS/1/2018/STG07/UM/02/6] for the financial support. AMF would like to thank the Carnegie Trust for the Universities of Scotland, for the Research Incentive Grant RIG008586, the Royal Society and Specac Ltd. for the Research Grant RGS\ R1\201397, the Royal Society of Edinburgh and the Scottish Government for one Sapphire project, and the Royal Society of Chemistry for the award of a mobility grant (M19-0000). UB thanks the School of Engineering (University of Aberdeen) for the award of one Summer Scholarship.

## **Conflict of interest**

The authors declare no conflict of interest.

*High Ionic Conductivities of Ionic Materials as Potential Electrolytes DOI: http://dx.doi.org/10.5772/intechopen.107949*

## **Author details**

Pradip K. Bhowmik1 \*, Si L. Chen1 , Haesook Han1 , Khairul Anwar Ishak<sup>2</sup> , Thamil Selvi Velayutham3 , Umama Bendaoud4 and Alfonso Martinez-Felipe4

1 Department of Chemistry and Biochemistry, University of Nevada Las Vegas, Las Vegas, NV, USA

2 Faculty of Science, Institute of Biological Sciences, Universiti Malaya, Kuala Lumpur, Malaysia

3 Faculty of Science, Department of Physics, Low Dimensional Materials Research Centre, Universiti Malaya, Kuala Lumpur, Malaysia

4 Chemical Processes and Materials Research Group, School of Engineering, Centre for Energy Transition, King's College, University of Aberdeen, Old Aberdeen, UK

\*Address all correspondence to: pradip.bhowmik@unlv.edu

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 3**
