## 3 Varieties of Epoxide-Based Polymer Electrolyte
### Plasticizers and Gel Electrodes  
In addition to introducing various functional groups into epoxide-based solid polymer electrolytes to improve ionic conductivity, plasticizers can be added to these polymer frameworks for further enhancement of the ionic conductivity [36; 38]. Plasticizers improve ionic conductivity in electrolytes as a result of solvating ions, decreasing their affinity for the  
Figure 6: Ambient temperature ionic conductivity of polymer electrolytes comprised of DGEBA, PEG, LiTf and nano-silica using either Jeffamine D400 or metaxllenediamine (MXDA) as a curing agent. The lithium to ether oxygen ratio was 0.125 in all cases. Materials Science and Engineering B, 2017.  
polymer phase and therefore increasing ion mobility [88]. Plasticizers also impact the solid polymer phase as they tend to increase the free volume between polymer chains thereby increasing chain mobility, preventing tangling and increasing lithium ion transport [44; 89]. However, as plasticizers are typically liquids, the crosslinked epoxy-based polymer serves as a scaffold allowing the use of plasticizers in all solid-state systems [89]. The impact of plasticizers on the ionic conductivity of solid electrolytes was demonstrated by [PERSON] et al. who added propylene carbonate (PC) to a crosslinked epoxide-based polymer that was prepared using DGEPEG, DGEBA, Jeffamine D400 and LiClO4 that was prepared in the absence of solvents [35]. The solid electrolyte had a room temperature ionic conductivity on the order of 10\({}^{-9}\) S/cm in the absence of the PC plasticizer [35]. However, when 22 wt% PC was added to the system, the room temperature conductivity was increased to 3.1 \(\times\) 10\({}^{-5}\) S/cm [35]. The increased ionic conductivity was attributed to increased polymer segment mobility as the addition of 22% PC decreased the T\({}_{\text{g}}\) from \(-4\)\({}^{\circ}\)C to \(-40\)\({}^{\circ}\)C when the DGEBA loading was 10% [35]. It is for this reason that polymer electrolytes containing plasticizers are generally more conductive than their completely solid counterparts. [PERSON] et al. prepared polymer electrolytes comprised of various ratios of polyvinylidene fluoride co hexafluoropropylene (PVDF-HFP) and DGEPEG cross linked with poly(ethylenimine) (DIEPEG-PEI) [36]. These electrolytes were soaked in a mixture of ethylene carbonate, dimethyl carbonate and LiPF\({}_{6}\)[36]. The sample with a PVDF-HFP to DIEPEG-PEI ratio of 60:40, had the greatest uptake of the lithium salt-plasticizer mixture [36]. Absorption of the plasticizer mixture resulted in this film having an ionic conductivity of 2.3 \(\times\) 10\({}^{-3}\) S/cm at room temperature and an activation energy of 0.88 eV [36]. Figure 7: **(A)** Specific capacity of an NPEG-TMPEG polymer electrolyte-LFP cell at 60 \({}^{\circ}\)C cycled at various rate between 2.8 and 4.0 V. (**B**) Capacity of the NPEG-TMPEG polymer electrolyte-LFP cell after 100 cycles at 0.2 compared with a PEO-LFP cell. Journal of Power Sources, 2020. This amine-containing material is at least an order of magnitude more conductive than the amine-containing epoxy polymers that were discussed in the previous section. The increase in ionic conductivity was attributed to the affinity of the lithium cations for the plasticizer resulting in increased mobility. All the PVDF-HFP-DIEPEG-PEI based electrolytes prepared by [PERSON] et al. were thermally stable up to about 200 \({}^{\circ}\)C and were electrochemically stable up to about 4.8 V relative to Li\({}^{+}\)/Li [36]. The effect of absorbed plasticizers in Jeffamine-containing epoxyide-based solid polymer electrolytes is detailed in the work of [PERSON] and [PERSON] who prepared solid polymer electrolytes consisting of DGEPEG, PEGB and 3-glycidyloxypropanesulfonate (LiGPS) that were swollen with a mixture of PC and LiTFSI [37]. These electrolytes were thermally stable up to 322 \({}^{\circ}\)C and had a T\({}_{\text{g}}\) value of \(-\)55 \({}^{\circ}\)C when LiGPS was included in the mixture [37].