# 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
[PERSON] and [PERSON] [137] studied the effect of SiO\({}_{2}\) particle sizes (4 \(\upmu\)m and 7 nm) in hydroxide conducting poly(acrylamide) (PAM) based SPE for alkaline batteries. The hydroxide conductivity of the SPE was amplified with the incorporation of 7 nm SiO\({}_{2}\) but not for SPE containing 4 \(\upmu\)m SiO\({}_{2}\). Based on the micrographs and Raman studies, smaller size of SiO\({}_{2}\) significantly helped to delay polymer crystallization [137]. Similar observation has been reported for Al\({}_{2}\)O\({}_{3}\) where the smaller particle size prevented the formation of dendrites [138]. Other than size, the morphology of filler will also give different outcome to the CSPE. Lithium dendrites formation was minimized in the case of electrolyte containing PEO, lithium (bis trifluoromethyl)sulfate, SCN and SiO\({}_{2}\) nanofibers as compared to similar electrolyte but with SiO\({}_{2}\) nanoparticles [139]. It was observed that SiO\({}_{2}\) aggregates was found in the latter, while the SiO\({}_{2}\) nanofibers were homogeneously distributed in the electrospun CSPE as evidenced from SEM images. The CSPE of SiO\({}_{2}\) nanofibers that had a 3D structure demonstrated better mechanical strength and flexibility with improved performance in lithium battery [139]. Incorporation of filler in electrolyte also yielded better interfacial contact with lithium metal electrode [140]. At 30 \({}^{\circ}\)C, SiO\({}_{2}\) nanofibers CSPE exhibited the conductivity of 9.32 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) which was highest when comparing to CSPE with SiO\({}_{2}\) nanoparticles (5.28 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\)) and SPE without SiO\({}_{2}\) (4.72 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\)) [139]. Occasionally, an increment in cationic transference number was observed upon the addition of fillers in SPEs [3]. As discussed earlier, the contribution of inactive or passive fillers in ionic conductivity of SPEs is clear, i.e., improving the mobility of charge carriers either through the mobility of polymer chain (amorphousness) or by providing the additional transportation sites. Active fillers, such as Li\({}_{1+\times}\)Al\({}_{x}\)Ge\({}_{2-\times}\)(PO\({}_{4}\))\({}_{3}\)[141], Li\({}_{1+\times}\)Al\({}_{x}\)Ti\({}_{2-\times}\)(PO\({}_{4}\))\({}_{3}\)[142], Li\({}_{2}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\)[143], Li\({}_{0.33}\)La\({}_{0.557}\)TiO\({}_{3}\)[144] and Li\({}_{6.4}\)La\({}_{3}\)Zr\({}_{1.4}\)Ta\({}_{0.6}\)O\({}_{12}\)[145] on the other hand, contribute directly to the lithium-ion conductivity apart from impeding crystallization and improving the mechanical integrity of the SPEs. These materials by their own serve as solid electrolytes [146; 147; 148; 149; 150]. Nonetheless, the fabrication processes of these ceramics are usually complicated and costly [139]. In addition, the thickness of these solid electrolytes have to be large (around 100 \(\upmu\)m in pellet form), since they are fragile and brittle [139] thus resulting in high interfacial resistance. These drawbacks lead to introducing them in SPEs as fillers instead. Filler-added SPEs are flexible, more stable and have better interfacial compatibility with Li metal electrodes [141; 151]. Furthermore, the SPEs possess better processability and higher ionic conductivity than the solid electrolytes [151; 152]. Similar to inactive fillers, the size, concentration and morphology of active fillers impose different impact on the CSPEs. Among the inorganic active fillers, Li\({}_{2}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\) (LLZO) having garnet structure is favored due to its chemical stability against lithium metal and large electrochemical window (>5 V) [143]. LLZO nanofibers prepared via the electrospinning method have been introduced to an electrolyte comprising PVDF-HFP as a polymer host, LiTFSI salt and ionic liquid plasticizer [153]. The CSPE delivered high ambient conductivity (6.50 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\)), reduced crystallinity and wide electrochemical window (5.3 V) than plasticized SPE without LLZO as filler [153].