# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Strategies to Improve Ionic Conductivity of SPEs
#### 3.2.2 Modification on Polymer Host
[70] employed an in situ polymerization procedure to prepare SPE based on crosslinking poly(ethylene glycol) diglycidyl ether (PEG) and bisphenol A diglycidyl ether (BADGE) with 1-ethyl-3-methylimidazol bis(trifluoromethylsulfonyl)imide (EMIMTFSI), using PEI as crosslinker without solvent. The SPE was prepared directly on activated carbon electrode for supercapacitor application before the curing process. Such an SPE film showed a conductivity of 2.41 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\) at 25 \({}^{\circ}\)C and capacitance value of 19.8 F g\({}^{-1}\) at 2 mV s\({}^{-1}\) scan rate with decent cyclability upon 10,000 cycles [70]. Chitosan in 1% acetic acid solution was subjected to a crosslinking process with methanesulfonic acid (MSA) and dodecylbenzene sulfonic acid sodium salt (DBSA-Na), using sulfuric acid as an agent to form chitosan-MSA and chitosan-DBSA membranes, respectively [71]. Such an action improved the proton conductivity and the thermal and mechanical stabilities of the chitosan-based membranes, which can be applied as solid electrolyte membranes in PEMFCs. At the elevated temperature of 90 \({}^{\circ}\)C, the proton conductivity was 3.09 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\) for chitosan-DBSA film, whereas 2.18 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\) was the conductivity exhibited by the chitosan-MSA membrane [71]. The non-crosslinked chitosan membrane was also prepared for comparison and showed lowest conductivity of 1.04 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\) at same temperature [71]. The grafted copolymers consisting of natural rubber and methyl methacrylate, namely 49% PMMA-grafted NR (MG49) [48; 72; 73] and 30% PMMA-grafted NR (MG30) [74], have been used as polymer hosts to produce polymer electrolytes. [PERSON] et al. [72] reported a conductivity of 1.00 \(\times\) 10\({}^{-12}\) S cm\({}^{-1}\) for pure MG49 at 30 \({}^{\circ}\)C, and this value increased to 2.30 \(\times\) 10\({}^{-7}\) S cm\({}^{-1}\) when MG49 was incorporated with 20 wt.% LiBF\({}_{4}\) salt. Such an electrolyte of MG49-LiBF\({}_{4}\) was then added to another salt (LiI and LiCF\({}_{3}\)SO\({}_{3}\)) by the same group of researchers for the mixed-salt effect [48]. It was found that the ambient conductivity increased with different salts and combination of salts as follows: LiCF\({}_{3}\)SO\({}_{3}\) (2.15 \(\times\) 10\({}^{-11}\) S cm\({}^{-1}\)) < LiI (4.73 \(\times\) 10\({}^{-9}\) S cm\({}^{-1}\)) < LiCF\({}_{3}\)SO\({}_{3}\)+LiBF\({}_{4}\) (7.71 \(\times\) 10\({}^{-9}\) S cm\({}^{-1}\)) < LiBF\({}_{4}\) (1.42 \(\times\) 10\({}^{-8}\) S cm\({}^{-1}\)) < LiBF\({}_{4}\)+LiI (1.89 \(\times\) 10\({}^{-6}\) S cm\({}^{-1}\)) [48]. This clearly demonstrates that the ionic conductivity was enhanced significantly for the MG49-based SPE having double salts of LiBF\({}_{4}\) and LiI at a ratio of 30:70, while the addition of LiCF\({}_{3}\)SO\({}_{3}\) into the MG49-LiBF\({}_{4}\) electrolyte had an adverse effect on the conductivity. The transference number of ions for the five samples was observed to increase in the same order of conductivity [48]. It can be understood that the degree of crystallinity increased in the reverse order [48]. Modified natural rubber has superior elastic features for better interfacial contact between electrolyte and electrode when applied in electrochemical devices. Another strategy to improve the properties of electrolyte is to bind the anion directly to the polymer chain to form a single-ion-conducting polymer electrolyte. Such an electrolyte consisting of poly[lithium 1-[3-(methacryloyloxy) proppulsulfonyl]-1-(trifluoromethylsulfonyl)imide] (PLIMTTSI) attached to PEG-crosslinked poly(2-hydroxyethylacrylate) (PHEA) showed improved mechanical strength as compared to an electrolyte of crosslinked PEG-PHEA doped with LiTFSI, even though its conductivity is lower than the latter [75]. Nonetheless, the single-ion-conducting polymer electrolyte could minimize lithium dendrite formation and gave better results in Li-ion cells.