# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Strategies to Improve Ionic Conductivity of SPEs
#### 3.2.6 Composite Solid Polymer Electrolytes  
Although addition of plasticizers in SPEs can easily improve the ionic conductivity, the mechanical strength of the SPEs is compromised. On the contrary, incorporating fillers in SPEs is known to improve the performance of SPEs not merely the ionic conductivity but also its mechanical strength since fillers can reduce the crystallinity of SPE films. SPE incorporated with an inorganic filler are termed as composite solid polymer electrolytes (CSPEs). In most CSPEs systems, the polymer is usually regarded as Lewis base if it donates its lone pair of electrons, while the filler is a Lewis acid in accepting the electrons. Fillers can be categorized into two types, i.e., active and inactive. Some examples of inactive fillers are titanium dioxide (TiO\({}_{2}\)), silicon dioxide (SiO\({}_{2}\)), alumina (Al\({}_{2}\)O\({}_{3}\)) and zirconia (ZrO\({}_{2}\)). The effects of fillers are varied depending on their particles size, surface nature and amount or concentration in the CSPEs. [PERSON] et al. [132] investigated the effect of SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) on the PMMA-based SPEs containing LiCr\({}_{3}\)SO\({}_{3}\) salt. Addition of Al\({}_{2}\)O\({}_{3}\) has improved the ionic conductivity by one order of magnitude from 1.36 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) to 2.05 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\). However, this effect cannot be seen for CSPE containing SiO\({}_{2}\)[132]. The reason behind Al\({}_{2}\)O\({}_{3}\) showed significant improvement in conductivity may be because of the OH surface nature in alumina that provided additional coordination sites to oxygen atom in PMMA for ionic conduction. Moreover, the presence of H\({}^{+}\) on the alumina surface also help in salt dissociation thus increasing the number density of ions leading to ionic conductivity improvement of the CSPE [132]. In the same work, they also reported that the amount of Al\({}_{2}\)O\({}_{3}\) influenced the conductivity of CSPE [132]. The presence of high amount of alumina grains in CSPE may cause the polymer chain to be less mobile thus affecting the transport of ions that resulted in conductivity decrement. Nonetheless, it has been reported that the mechanical strength of CSPE was enhanced at high concentrations of filler [133]. Too much or too low content of fillers would not bring positive effect on the ionic conductivity [134]. Only at optimized concentration can lead to its improvement in the CSPE [134]. The findings on acidic surface nature of alumina that enhanced the conductivity of SPE [132] are also supported by [PERSON] et al. [135]. Following Reference [135], it can be understood that extra pathway for ion transport was formed via hydrogen bonding when the surface of Al\({}_{2}\)O\({}_{3}\) nanofiller (particle size 5.8 nm) was acidic. The SPE system that comprised PEO, LiCF\({}_{3}\)SO\({}_{3}\) and acidic Al\({}_{2}\)O\({}_{3}\) exhibited ionic conductivity of 2.10 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) at 30 \({}^{\circ}\)C. Slight reduction in conductivity (1.20 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) at 30 \({}^{\circ}\)C) was observed for Al\({}_{2}\)O\({}_{3}\) neutral surface which may be due to weaker interactions as compared to that in acidic form [135]. On the contrary, there was no interaction between filler, anion of salt and polymer in the case of basic Al\({}_{2}\)O\({}_{3}\) surface thus subsequently causing detrimental outcome to conductivity (7.30 \(\times\) 10\({}^{-6}\) S cm\({}^{-1}\) at 30 \({}^{\circ}\)C) [135]. In another work, it was also observed that the addition of SiO\({}_{2}\) successfully boosted the ionic conductivity for PVA-PVP-PEG-NaI electrolyte [136]. An improvement in amorphous region of the electrolyte film via the interaction of functional groups of the polymer with the Si-O-Si of the filler may be the reason for the conductivity increase.