# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Strategies to Improve Ionic Conductivity of SPEs
#### 3.2.5 Plasticized Solid Polymer Electrolytes

Table 6: _Cont._
It is interesting to highlight that ionic liquids can play three roles, viz as plasticizer (to enhance the flexibility of polymer backbone), conducting salt (provides cation and anion) and solvent. They are non-volatile, safe (due to non-flammability), chemically and thermally stable, and are not prone to evaporation and crystallization [130]. In addition, they exhibit low melting temperature and outstanding ionic conductivity at room temperature condition, and they also have superior solvating ability. In the work of [PERSON] et al. [114], increment in conductivity has been observed after incorporating ionic liquid, S\({}_{2}\)TFSI into  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Electrolyte System**} & **Preparation Method** & **Conductivity, \(\sigma\) (S cm\({}^{-1}\))** & **Important Findings** & **Reference** \\ \hline \multirow{6}{*}{\ \begin{tabular}{} \end{tabular}  } & & & N719/TiO\({}_{2}\)/SPE/Pt DSSC (light intensity of 100 mW cm\({}^{-2}\)): \\  & & & OCV = 0.53 V; \(J_{cc}\) = 2.62 mA cm\({}^{-2}\); \\  & & & \(FF\) = 0.52; \(\eta\) = 0.739\% & [126] \\  & & & Without EMImSCN: \\  & & & \(\sigma\) = 1.21 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) (25 \({}^{\circ}\)C) \\  & & & OCV = 0.35 V; \(J_{cc}\) = 1.05 mA cm\({}^{-2}\); \\  & & & \(FF\) = 0.34; \(\eta\) = 0.13\% & \\ \hline \multirow{6}{*}{ \begin{tabular}{} \end{tabular}  } & & & \(T_{m}\) = 152.8 \({}^{\circ}\)C; \%\%\%\%\%\%\%\%\%\%\%\%  
PEO-LiTFSI SPE without affecting Li\({}^{+}\) transference number. From Table 6, it is to be noted that N1222 FSI is not an ionic liquid but an ionic plastic crystal that shows similar features to that of ionic liquids [115]. In contrast to conventional plasticizers, the mechanical integrity can still be retained upon addition of N1222 FSI into the electrolyte besides reducing the polymer crystallinity and dissociating the salt. Moreover, the interfacial contact between electrolyte and electrode can be improved [115]. Another organic plastic crystal, i.e., succinonitrile (SN) is favored for its outstanding solvating capability in dissociating many salts including Li-based salts in battery application. As a result, conductivity is enhanced as can be observed in refs [105; 112] from Table 6. Unfortunately, the plastic nature of SN deteriorates the mechanical strength of the electrolyte. Nonetheless, this can be easily avoided by preparing the electrolyte membrane in a different way. [PERSON] et al. [131] prepared free-standing film containing PEGDA (M\({}_{w}\) 6000 g mol\({}^{-1}\)), LiTFSI and SN via photopolymerization procedure using bis(2,4,6-trimethylbenzoylyl)-phenylphosphine oxide (also known as Irgacure 819) as photo-initiator but without solvent. Such electrolyte which was highly amorphous with crosslinking network exhibited good ambient conductivity of 1.10 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\), electrochemical stability window of 4.80 V with improved mechanical strength (0.24 MPa tensile strength, 84% elongation at break). Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\)/Li cell employing this electrolyte gave promising performance (The first discharge capacity of 140 mAh g\({}^{-1}\) at 0.2 C) with 93% capacity retention for 25 cycles [131]. On the other hand, it is evident from Table 6 that PEG of low molecular weight acted as plasticizer rather than secondary polymer in PEO-KI electrolyte which showed better conductivity [113] than PEG-free SPE even though PEG might have crosslinked with PEO. Instead, conductivity improvement is attributed to extra passageways for ion migration through PEG segmental movement.