# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes  
A simple SPE consists of a polymer and a salt where both are dissolved in a solvent before the solvent is allowed to evaporate, while the ions continue to be free and mobile inside the polymer matrix [3]. The salt can be either inorganic or organic salt but when used in DSSCs application, the SPE must contain additional materials, i.e., iodine (I\({}_{2}\)) crystals to act as redox mediator, while the salt must be iodide-based salts, since iodide/triiodide (I\({}^{-}\)/I\({}_{3}^{-}\)) redox couple is commonly used [17]. Likewise, for application in QDSCs, the basic SPE should have sulfide/polysulfide (S\({}^{2-}\)/S\({}_{2}^{-}\)) redox mediator with sulfide-based salts. Similarly, the salt should be lithium-based, sodium-based, magnesium-based and proton-based if it is going to be employed in cells of Li-ion, Na-ion and Mg-ion and proton batteries as well as PEMFCs. Besides this, the salt must meet the criterion of having low lattice energy for easy solvation in polymer matrix [3]. Tables 1 and 2 list some of the most popular polymers and salts used in polymer electrolytes, along with their special traits that enable them to be favored among others by the researchers worldwide. The \(T_{g}\) of polymer will affect the crystallinity of the electrolytes, and usually low \(T_{g}\) is preferred. Meanwhile, the lattice energy of salt will determine how easy it can dissociate in the polymer matrix. The lattice energies of the salts in Table 2 are calculated in this work, using the Kapustinskii equation [18]. All materials have both their own benefits and drawbacks. For instance, PEO, being the first polymer host used in polymer electrolytes and still remaining one of the most popular polymers until now, is favored for its talent to solvate many types of salts and exhibit good mechanical strength and thermal stability. It is a semicrystalline macromolecule, and thus, its crystalline part restricts the increment of conductivity. As for the salts, lithium perchlorate (LiClO4), as an example, shows good ionic conductivity, but it is a strong oxidizer, hence making it explosive, and so it must be handled with caution. As for the solvent choice, certain features, including dielectric constant, melting point, boiling, point and viscosity, have to be taken into consideration for salt dissociation and solvation of polymer. Some frequently employed solvents are summarized in Table 3, along with their physical attributes. \begin{table}
\begin{tabular}{c c c c} \hline
**Solver** & **D Donor** & **Dielectric Constant,** & **Viscosity (cP)** & **Melting Point,** & **Boiling** \\  & **Number, DN** & \(\epsilon\) **at 25 \({}^{\circ}\)C** & **at 25 \({}^{\circ}\)C** & \(T_{m}\) (\({}^{\circ}\)C)** & **Point (\({}^{\circ}\)C)** \\ \hline Acetone & 17.0 & 20.6 & 0.30 & \(-94.7\) & 56.1 \\ Acetonitrile (ACN) & 14.1 & 35.9 & 0.37 & \(-43.8\) & 81.6 \\ N-methyl-2-pyrrolidone (NMP) & 27.3 & 32.2 & 1.67 & \(-24.4\) & 202 \\ Tetrahydrofuran (THF) & & 7.58 & 0.46 & \(-108.4\) & 60 \\ N,N-Dimethylacetamide (DMAc) & 27.8 & 37.8 & 0.93 & \(-20\) & 166.1 \\ Dimethyl sulfoxide (DMSO) & 29.8 & 46.5 & 1.99 & 18.5 & 189 \\ Gamma-butylrolactone (gBL) & 18.0 & 39.0 \({}^{\ddagger}\) & 1.70 & \(-44\) & 204 \\ N,N-Dimethylformamide (DMF) & 26.6 & 36.1 & 0.80 & \(-61\) & 153 \\ Ethylene carbonate (EC) & 16.4 & 89 \({}^{\ast}\) & 1.90 \({}^{\ast}\) & 36 & 238 \\ Propylene carbonate (PC) & 15.1 & 64 & 2.50 & \(-49\) & 241 \\ Ethanol & 19.2 & 25 & 1.08 & \(-114\) & 78 \\ Water & 18.0 & 78 & 0.89 & 0 & 100 \\ \hline \end{tabular}  
* At 40 \({}^{\circ}\)C + At 20 \({}^{\circ}\)C.  
\end{table} 
Table 1: Typical polymers used in polymer electrolytes, along with their characteristics.