**6. Application of IL based GPEs in Lithium batteries**

Because of having several required properties, IL based polymer electrolytes are frequently used in many application as in supercapacitors, batteries and fuel cell etc. Particularly, in Li batteries, they are gaining much attention due to their high energy density, flexibility and safety. In recent years, almost all the electronic equipment are being run by polymer batteries as laptop, mobile phones, power banks, portable media players etc. Many reports include the application of GPEs in Li batteries. As one of the main advantage of GPE is that it forms stable solid electrolyte interface (SEI) passive layer between the electrode-electrolyte and provides higher cyclic stability to Li battery. Battery performance of PEO based GPEs have already been studied in literature. Gupta et al. [51] synthesized the GPE, (PEO + 20 wt% LiTFSI + 30 wt% 1-butyl-3-methyl pyridinium TFSI), and reported its performance in Li battery in (Li/LiFePO4) configuration. They obtained maximum discharge capacity ~160 mAh/g and 99% Coulombic efficiency upto 35 cycles at 40°C (**Figure 10(a)** and **(b)**). In another report, they used the pyrrolidinium based IL in polymer system, PEO + 20% LiFSI + 10% PYR13FSI, with graphene oxide coated LiFePO4 cathode and obtained maximum discharge capacity ~163 mAh/g at C/10 rate at room temperature (RT) (**Figure 10(c)**) [49]. It was the result of high conductivity of IL, PYR13FSI and LiFSI salt as well as high electronic conductivity and large surface area of graphene oxide (GO) which enhanced the electron transfer rate and hence capacity of Li battery. This Li battery provided almost constant capacity and Coulombic efficiency upto 100 cycles (**Figure 10(d)**). Balo et al. [36] reported the same system (Li/LiFePO4) using imidazolium IL based polymer electrolyte PEO + 20 wt.% LiFSI + 7.5wt.%EMIMFSI at RT.

They found maximum discharge capacity ~143 mAh/g at C/20 rate which decreased upto 130 mAh/g at C/10 and further reduced upto 20 mAh/g at 2C discharge rate. This reduction of discharge capacity was reported due to the increase of electrolyte ohmic drop and limited Li<sup>+</sup> ion diffusion in composite cathode. The above polymer systems have been also tested with high voltage and capacity cathode materials such as LiNixMnyCozO2 (NMC) and LiNixCoyAlzO2 (NCA). These electrolytes are electrochemically stable even at high voltage which deliver high capacity and cyclic stability to the Li battery. Gupta et al. [52] used the phosphonium based IL (trihexyltetradecylphosphonium bis TFSI) in PEO-LiTFSI polymer system.

They fabricated the Li cell (Li/LiNi0.6Mn0.2Co0.2O2) and obtained maximum discharge capacity ~148 mAh/g with 95% Coulombic efficiency upto 150th cycle at C/10 rate in the voltage range of 2.4–4.2 V (**Figure 11(a, b)**). The impedance of the Li cell was also evaluated with cycling (inset of **Figure 11(b)**). It showed the slight increment in the interfacial resistance value and hence, resulted very small capacity fading of Li cell (**Figure 11(b)**). Balo et al. [50] used imidazolium IL (EMIMFSI)

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

**Figure 10.**

*Electrochemical performance of (a, b) 1-butyl-3-methyl pyridinium TFSI and (c, d) N-propyl-Nmethylpyrrolidinium-FSI IL based polymer electrolytes with Li/LiFePO4 and graphene oxide coated LiFePO4 cathode respectively.*

based GPE (PEO + 20 wt% LiTFSI + 10 wt% EMIMFSI) in Li/NCA cell. They observed the discharge capacity ~175 mAh/g at C/10 current rate which remained almost stable throughout cycling (**Figure 11(c, d)**) and only 0.05% of total capacity was lost during 200 cycles (inset of **Figure 11(d)**). The use of Li metal electrode in Li batteries are in demand due to its higher energy density and capacity (3862 mAh/g), but it could not be frequently used in application purpose because of the formation of Li dendrites. This Li dendrite is formed due to the deposition of Li+ ions on the Li metal surface during cycling which starts to grow and causes short circuiting and results low cyclic stability. Therefore, in order to obtain high capacity and safer Li battery, suppression of dendrite growth is important. It was reported that the dendrite growth becomes faster with liquid solvents. But the use of GPEs in Li battery is able to suppress its growth because of having mechanical stability. Therefore, the use of GPEs provides safety and cyclic stability to Li battery. The electrochemical stability of GPEs with Li electrode is reported in literature. Wang et al. [53] reported the combination of the use of LAGP-PEO (LiTFSI) composite solid electrolyte and the modification of Li anode with PEO500000 (LiTFSI) in Li/LiMn0.8Fe0.2PO4 battery. They obtained that the use of both can effectively prevent the Li dendrite growth. Kim et al. used three different ILs (BMITFSI, BMIBF4 and BMICF3SO3) in polymer system (PEO-LiTFSI) and reported that the Polymer electrolyte with BMITFSI IL resulted low and stable interfacial resistance or dendrite growth on lithium metal. Balo et al. [50] examined the performance of EMIMFSI IL in PEO-LiTFSI system. They found the stable and uniform formation of Li dendrite between lithium and GPE during cycling (**Figure 12(a)**). They also evaluated the interfacial resistance of this passive layer and observed that except the initial few cycles almost stable interfacial resistance 380 Ω/cm2 was obtained throughout the cycling (**Figure 12(b)**).

#### **Figure 11.**

*Discharge capacity, efficiency (at C/10 rate) and capacity fading of the Li cell (a, b) Li/ (PEO + 20wt%LiTFSI + 20wt% trihexyltetradecylphosphonium TFSI)/LiNi0.6Mn0.2Co0.2O2 and (c, d) Li/ (PEO + 20 wt% LiTFSI + 10 wt% EMIMFSI)/NCA.*

#### **Figure 12.**

*(a) Voltage vs. time profile of lithium deposition and (b) evolution of interfacial resistances during cycling using GPE (PEO + 20wt% LiTFSI + 10wt% EMIMFSI).*

Other reports on the electrochemical performance of Li batteries using PEO based GPEs are also tabulated in **Table 3**. All these analysis shows that the use of GPEs in Li battery maintains the cyclability and electrochemical stability of the Li battery much more compared to liquid solvents.

Therefore, from the above discussions it can be concluded that the IL based GPEs not only provide good ionic conductivity, flexibility and mechanical stability but also act as a potential candidate in order to enhance the capacity, cyclicity and safety to Li battery.

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


#### **Table 3.**

*Electrochemical performance of Li batteries using IL based GPEs.*
