# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Preparation Methods of SPEs  
SPEs can be prepared via several methods, including solution casting [31], phase inversion [32], photopolymerization [33] and electrospinning [31] techniques, to name a few. Solution cast or solvent casting method is the most conventional and easy way to prepare electrolyte film. In this procedure, the polymer and salt are dissolved in an appropriate solvent before casting onto suitable substrates (e.g., glass plates). The solvent is then evaporated after being left to dry either at ambient condition or elevated temperature inside oven. Solution casting route can be performed at room temperature atmosphere or inside glove box, depending on its needs. After complete drying, the SPE is a free standing membrane with thickness usually in millimeter range. [PERSON] and co-authors [31] have compared PVA-based SPE containing potassium iodide (KI), potassium chloride (KCl) and sodium chloride (NaCl) salts prepared by using electrospinning and solution-casting techniques. Regardless of which salt was used, the electrolyte fabricated via the former method demonstrated superior ionic conductivity (5.95 \(\times\) 10\({}^{-6}\) S cm\({}^{-1}\) for NaCl-containing SPE) than that prepared by the latter (1.87 \(\times\) 10\({}^{-6}\) S cm\({}^{-1}\) for SPE having NaCl) [31]. The electrospinning approach can produce film of fibers morphology with a large surface-area-to-volume ratio. Moreover, the electrospun membranes were more thermally stable than the solution-cast prepared films. Among the three salts, electrolytes comprising NaCl exhibited the highest conductivity irrespective of the preparation method [31]. Nonetheless, there are many parameters that need to be controlled and optimized in electrospinningness process such as polymer molecular weight and solution viscosity, conductivity and surface tension. Furthermore, the electrospinningning setup conditions, e.g., flow rate, applied voltage and distance from tip of syringe containing solution to collector, are tedious and time-consuming. In contrast to electrospinningning, the solution-casting route is a more straightforward and convenient approach for membrane preparation. The phase inversion technique can produce porous structure which is beneficial for ionic transport. Using this method, SPE membrane consisting of PDVF-HFP, LiClO\({}_{4}\) and cerium oxide (CeO\({}_{2}\)) exhibited sponge-like morphology with tiny pores and ionic conductivity of 2.50 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\) at 25 \({}^{\circ}\)C [32]. In this procedure, a solution containing PVDF-HFP, CeO\({}_{2}\) and NMP solvent was stirred for 24 h before being coated onto a glass plate via the doctor-blade method. This was followed by immersion in deionized water (about 3 to 5 h) for solvent extraction and phase inversion, which took place before drying under vacuum-pressure condition. The last step was to soak the membrane in LiClO\({}_{4}\) liquid electrolyte for 6 h [32]. [PERSON] et al. [34] have combined phase-inversion and chemical-reaction routes to obtain PVDF-PAN-SiO\({}_{2}\)-LiPF\({}_{6}\) SPE membrane that gave ambient conductivity of 3.32 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\). For comparison, the same composition membrane has been prepared by phase-inversion method, and it was found that the ambient conductivity was 2.83 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\)[34]. The higher conductivity obtained by the membrane prepared by the combined method was attributed to larger porosity and better homogeneous pores distribution as compared to that developed from the phase-inversion technique only [34]. The former also showed better properties in terms of heat resistance, mechanical strength and electrochemical stability than the latter [34]. An unconventional way to prepare SPE films is through dry-mixing without the use of solvent. Dry-mixing is combined with hot-pressing. [PERSON] et al.