**Table 1.**

*Properties of ionic liquid.*

#### **Figure 5.**

*Flexibility and mechanical stability of IL based gel polymer electrolyte.*

They also provide good adhesive nature with the electrode surfaces. The transportation of ions in GPEs occurs by hopping or diffusion process. IL enhances the performance of polymer electrolyte in two ways (i) it acts as the plasticizer therefore reduces the crystalline phase or enhances the amorphous region, (ii) Supplier of free charge carriers, hence helps to improve the ionic conductivity of polymer electrolytes [19]. The ionic conductivity of different IL based GPEs are shown in **Table 2**.

*Ionic Liquid-Based Gel Polymer Electrolytes for Application in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.93397*


#### **Table 2.**

*Ionic conductivity of ILs based GPEs.*

### **5. Results and discussion**

#### **5.1 Surface morphology and crystallinity of IL based GPEs**

The effect on surface morphology and degree of crystallinity of PEO based SPE with the addition of IL is reported in literature. Gupta et al. [47] studied that, by using the phosphonium based IL (Trihexyltetradecylphosphonium TFSI) in polymer electrolyte (PEO + 20 wt% LiTFSI), increment in amorphous region was obtained (**Figure 6**). The surface morphology of PEO based GPE is given in **Figure 6**. **Figure 6(a)** shows the crystalline region of polymer PEO, when IL (up to 20 wt%) was incorporated into PEO-LiTFSI system, smoother morphology was observed which resulted the amorphous nature of GPE. Singh et al. [48] also reported that the suppression in rough surface nature of polymer electrolyte (PEO + 20 wt% LiTFSI) was observed when BMIMTFSI IL was added into it. The smoother surface morphology of polymer electrolyte was observed due to the plasticization effect of ionic liquid which reduced the interaction between the polymer chain and made it more flexible.

From the XRD analysis, crystallinity of GPEs is also reported in many studies. Gupta et al. [47] reported the variation in crystallinity of SPE with the addition of phosphonium based IL They showed the semi-crystalline nature of polymer PEO. When LiTFSI salt and IL were added into the PEO, broadening of halo region and FDHM of polymer electrolytes were noticed which resulted the decrement in crystalline region or enhancement in amorphous region of polymer electrolyte (**Figure 7(A)**).

Singh et al. [48] reported the effect of BMIMTFSI IL on the crystallinity of SPE (PEO + 20 wt% LiTFSI). They showed that with the addition of IL into SPE, halo region was increased substantially and relative intensities of the crystalline peaks reduced. This proved the enhancement in amorphous phase in polymer electrolyte (**Figure 7(B)**). Therefore, presence of IL in polymer electrolyte improves its amorphous region which is desirable for the conduction of Li+ ions, since, conduction in polymer electrolytes occurs only in this region.

**Figure 7.**

*XRD pattern of (A) phosphonium IL based polymer electrolytes, PEO + 20wt.%LiTFSI + x% IL (x = 0, 10, 20, 40); and (B) polymer electrolytes (a) PEO with PEO + 20wt.%LiTFSI + xwt.% BMIMTFSI (b) x = 0, (c) x = 5, (d) x = 10, (e) x = 15 and (f) x = 20.*

#### **5.2 Ionic and Li<sup>+</sup> ions conductivity of IL based GPEs**

In batteries, electrolyte plays the major role to transport the ions between the two electrodes. So, conductivity of electrolyte is very important parameter for Li battery. It is well known that the IL acts as a plasticizer and its presence in polymer electrolyte enhances the ionic conductivity. IL also provides free charge carriers and therefore helps to promote the ionic conductivity of polymer electrolyte.

*Ionic Liquid-Based Gel Polymer Electrolytes for Application in Rechargeable Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.93397*

The increment in the conductivity of polymer electrolyte with IL concentration is also reported by Gupta et al. [49] and Balo et al. [36] which are depicted in **Figure 8(a)** and **(b)** respectively. Gupta et al. [49] showed that ionic conductivity of polymer electrolyte (PEO + 20 wt% LiFSI) increases with IL (PYR13FSI) concentration.

Whereas, Balo et al. [36] synthesized the PEO based polymer electrolyte (PE O + 20wt.%LiFSI + xwt.%EMIMFSI; for 0 ≤ x ≤ 15) and mentioned the use of optimized concentration of IL in polymer electrolyte. They found similar increasing trend of conductivity of polymer electrolyte with IL, but after certain concentration, it showed decreasing trend (**Figure 8(b)**). It happened because with the addition of IL into polymer electrolyte, large number of FSI− anions was available which started to interact with Li<sup>+</sup> ions present in the polymer electrolyte. Therefore, they formed ion pairs instead of participating in interaction with ether oxygen of PEO, due to which conductivity of electrolyte was reduced.

#### **5.3 Electrochemical stability of IL based GPEs**

Electrochemical stability of electrolyte is an important parameter as it decides the performance of the battery in working voltage range. Electrochemical stability of GPEs should be high so that it can be used in high voltage Li batteries. Many studies have been carried out on the electrochemical performance of PEO and IL

#### **Figure 8.**

*Variation in (a) ionic conductivity as well as (b) Li+ ion conductivity and degree of crystallinity of polymer electrolyte with IL concentration.*

**Figure 9.** *Electrochemical stability of (a) imidazolium (EMIMFSI) and (b) pyrrolidinium (PYR13FSI) IL based GPEs.*

based GPEs. Singh et al. [48] synthesized the GPE, PEO + 20 wt% LITFSI + xwt% BMIMTFSI (x = 5, 10, 15, 20) for Li battery and the electrochemical stability of 20 wt% IL containing GPE was reported ~4 V vs. Li/Li<sup>+</sup> .

Balo et al. used the same polymer system with EMIMFSI and EMIMTFSI ILs and found that the GPE, PEO + 20 wt% LiTFSI + 10 wt% EMIMFSI was electrochemically stable up to ~6.4 V vs. Li/Li+ (**Figure 9(a)**) [50]. In another study they showed that the polymer electrolyte, PEO + 20 wt% LiTFSI + 10 wt% EMIMTFSI, was stable upto ~5.1 V vs. Li/Li+ [42]. Also, Gupta et al. [49] reported the electrochemical stability of pyrrolidinium IL based GPE (PEO + 20% LiFSI + 10% PYR13FSI) which was ~4.8 V vs. Li/Li+ (**Figure 9(b)**). Therefore, it can be concluded that IL based GPEs show enough electrochemical stability to be used in high voltage Li batteries application.
