## 3 Varieties of Epoxide-Based Polymer Electrolyte  
### Polyether-Based  
Although solid Epoxide-based electrolytes that contain only polyether groups tend to be heavily based on PEO as lithium conduction occurs primarily through ether oxygen groups in these films. In fact, initial attempts at polyether-based epoxide electrolytes involved the incorporation of PEO in the crosslinked material. [PERSON] et al. prepared a solid electrolyte using crosslinked PEO and epoxy resin [19]. The addition of LiClO\({}_{4}\) initiated the formation of a film via the cationic ring opening polymerization reaction [19]. The resultant solid polymer electrolyte had a room temperature ionic conductivity on the order of 10\({}^{-6}\) S/cm which is one to two orders of magnitude higher than that of solid polymer electrolytes that are comprised of PEO alone (about 10\({}^{-7}\) to 10\({}^{-8}\) S/cm) [19]. The electrolyte material that was developed by [PERSON] et al. was cycled in a Li-Li symmetric cell at temperatures ranging between 50 and 80 \({}^{\circ}\)C and achieved cycling performances that were comparable to those of PEO-based electrolytes between 100 and 120 \({}^{\circ}\)C [19]. However, it was noted by the authors that this system could be improved as decreases in battery capacity (after 3 cycles at 50 \({}^{\circ}\)C and after 20 cycles at 80 \({}^{\circ}\)C) were linked to decreases in the ionic conductivity of the polymer electrolyte membrane over time [19]. In order to get around the limitations in room temperature conductivity that are imposed by the direct incorporation of PEO in the electrolyte, [PERSON] et al. prepared an epoxide-based solid polymer electrolyte using LiClO\({}_{4}\) to polymerize DGEPEG and triglycerides ether of glycerol (TGEG) [14]. Like PEO, DGEPEG has ethylene oxide units and can be used to conduct lithium cations [77]. Additionally, the material has a low T\({}_{\text{g}}\) and can be used to form films via the lithium salt-activated ring opening polymerization reaction [77]. The biggest advantage in using DGEPEG as opposed to PEO is that the resultant films are much less crystalline at room temperature [77]. The room temperature ionic conductivity of this electrolyte was around 10\({}^{-5}\) S/cm which is greater than that of both PEO and epoxide-based polymers that are prepared directly from PEO [14]. Incorporating less crystalline epoxides into polymer electrolytes tends to cause the resultant polymer to be more mobile, increasing ionic conductivity. [PERSON] et al. also prepared epoxide-based polymer electrolytes using DGEPEG but lithium difluoro(oxalate)borate (LiDFOB) was used to catalyze the ring opening polymerization reaction [7]. The resultant polymer electrolyte was a bit more conductive at room temperature with a conductivity of 8.9 \(\times\) 10\({}^{-5}\) S/cm [7]. This may be a result of LiDFOB having a higher electrochemical stability than LiTFSI. This is because increased electrochemical stability increases salt concentration which, along with charge carrier mobility, tends to result in increased conductivity [78] The electrochemical stability was evaluated using a LiFePO\({}_{4}\)-Li cell at a current density of 0.1 C [7]. The electrolyte was cycled for 100 cycles with 74.2% efficiency and was stable up to 4.5 V, this is greater than the 3.8 V that is often cited for PEO, relative to Li\({}^{+}\)/Li [7]. Further improvements in the electrochemical properties of a DGEPEG-based polymer electrolyte were obtained by [PERSON] et al. who catalyzed the ring opening polymerization of DGEPEG using a mixture of LiTFSI and LiBF\({}_{4}\)[10]. The resultant polymer had an ionic conductivity of 1.1 \(\times\) 10\({}^{-4}\) S/cm under ambient conditions at 10 wt% and 5 wt% of LiTFSI and LiBF\({}_{4}\) respectively [10]. Increased lithium salt content was found to result in higher ionic conductivity and higher T\({}_{\text{g}}\) values [10]. This was attributed to having an increased number of charge carriers with an increase in salt-polymer interactions.