# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Strategies to Improve Ionic Conductivity of SPEs
#### 3.2.1 Incorporation of More Than One Salt  
One viable approach to boost the ionic conductivity of SPEs is to add another salt. The combination of two or more salts in the right proportions can give synergistic outcomes. In most cases, the sum concentration of salts is the same as the highest conducting electrolyte that contained only on one type of salt. This means that the total weight of salts remained unchanged, but an appropriate amount for salt one has been substituted by salt two. Since the amount of each salt in a mixed salt electrolytes is lower than the concentration of salt in single-salt electrolyte systems, the probability of ion recombination to form ion pairs will be reduced in former case, and thus, there are more free ions for conduction [48,49]. In addition, decrement in \(T_{g}\) and degree of crystallinity (%\(\chi_{\text{C}}\)) of the electrolyte are observed in mixed salt systems [48]. [PERSON] and [PERSON] [50] reported that highest conductivity was achieved at 5 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) (30 \({}^{\circ}\)C) in PEO-based SPE containing lithium borate and lithium aluminate salts as compared to PEO electrolyte with one salt only regardless which one. Only minimal capacity fading was observed in LiNi\({}_{0.8}\)Co\({}_{0.2}\)O\({}_{2}\)/Li cell with double salt-containing SPE after 30 th cycle, but no results were given for that of single salt [50]. The type of cation and anion determine the electrochemical performance of devices. For Li-ion, Na-ion and Mg-ion cells, it is obvious that dissimilar anions influence the performance of the devices. For DSSCs, the variation of cations since the anion is fixed following the redox mediator used. Studies of mixed salt systems containing different cations [51,52] or dissimilar anions [49,53,54] have shown superior ionic conductivity than single salt systems. For instance, higher conductivity was obtained for an electrolyte based on PEO-CaBr\({}_{2}\)-CaI\({}_{2}\) (30:1:1) than for a PEO-based electrolyte with only one salt, i.e., either calcium iodide (CaI\({}_{2}\)) or calcium bromide (CaBr\({}_{2}\)) [52]. Similarly, enhanced conductivity has been detected for PEO electrolyte having dual salts of zinc bromide (ZnBr\({}_{2}\)) and lithium bromide (LiBr) [51] and PEO-based electrolyte comprising the same compositions of binary salts of copper trifluoromethanesulfonate (Cu(CF\({}_{3}\)SO\({}_{3}\))\({}_{2}\)) and zinc trifluoromethanesulfonate (Zn(CF\({}_{3}\)SO\({}_{3}\))\({}_{2}\)) [55]. The incorporation of double salts does not always warrant higher conductivity than single salt but can still result in better performance of electrochemical devices, since conductivity is not the sole influencing factor. This has been showcased in the work of [PERSON] et al. [56], who have prepared PEO-based SPE with KI and TPAI salts via the solution-cast method. SPE containing sole salt of KI gave higher conductivity than electrolyte having double salts. Conductivity was lowest for electrolyte with lone TPAI salt since its cation is larger than K\({}^{+}\) and thus less mobile. Moreover, the degree of crystallinity for PEO-TPAI electrolyte was higher than that of PEO-TPAI-KI SPE [56]. DSSC employing hybrid salts displayed the best performance, with an efficiency (\(\eta\)) of 4.22% and short circuit current density (\(J_{sc}\)) of 8.00 mA cm\({}^{-2}\), followed by cells having single TPAI (\(J_{sc}\) = 6.38 mA cm\({}^{-2}\); \(\eta\) = 3.54%) and KI (\(J_{sc}\) = 6.52 mA cm\({}^{-2}\); \(\eta\) = 3.09%) under illumination of 100 mW cm\({}^{-2}\). Their open-circuit voltage (OCV) values were 0.69, 0.77 and 0.75 V for KI, TPAI and KI+TPAI electrolytes, respectively [56]. A large-sized cation boosted OCV value, while it reduced \(J_{sc}\), as can be seen when comparing the two single salt-containing electrolytes. The addition of a second salt decreased the cation conduction and enhanced \(J_{sc}\), leading to a higher \(\eta\)[56].