## 3 Varieties of Epoxide-Based Polymer Electrolyte
### Polyether-Based
Increasing lithium salt content was also found to increase the activation energy (0.12 eV at 10 wt% LiTFSI and 5 wt% LiBF\({}_{4}\)) for lithium motion which was attributed to increased ion pairing decreasing ion mobility [10]. The lithium transference number, determined based on ionic conductivity, was 0.23 which is not that much higher than those that are commonly reported for PEO-based polymers. This could be the result of the increased incidence of ion pairs at higher salt loadings. The polymer electrolytes proved to have high electrochemical stability as the sample with 10 wt% LiTFSI and 5 wt% LiBF\({}_{4}\) was stable up to 5.5 V vs. Li\({}^{+}\)/Li [10]. It was found that LiTFSI was less electrochemically stable than LiBF\({}_{4}\) as a sample that contained only LiBF\({}_{4}\) was stable up to 7.0 V (Figure 2). Electrochemical stability windows were instead more closely related to LiTFSI content. The sample containing the most LiTFSI (15 wt%) was simultaneously the least conductive and the least stable relative to Li\({}^{+}\)/Li. These results suggest that the choice of lithium salt can have a significant impact on the electrochemical properties of the resultant electrolyte. In addition to investigating electrochemical stability relative to Li\({}^{+}\)/Li, [PERSON] et al. also constructed full lithium metal-LFP and lithium metal-NMC batteries using their DGEPEG-based electrolyte [10]. Both cathodes were comprised of 90 wt% active material, 5 wt%  
Figure 2: Electrochemical stability of DGEPEG-based polymer electrolytes with various lithium salt contents relative to Li\({}^{+}\)/Li at 0.1 C. P95 contains 5 wt% LiBF\({}_{4}\), P90A contains 5 wt% LiBF\({}_{4}\) and 5 wt% LiTFSI, P85A contains 5 wt% LiBF\({}_{4}\) and 10 wt% LiTFSI and P80A contains 5 wt% LiBF\({}_{4}\) and 15 wt% LiTFSI. Chemistry of Materials, 2019.  
polyvinylidene fluoride and 5 wt% carbon black [10]. These batteries were assembled in the coin cell format and were cycled at various rates to determine the lower (2.5 to 4.0 V) and higher (3.0 to 4.4 V) voltage performance of the epoxide-based electrolyte material. Plots of voltage as a function of specific capacity, various cycling rates at 60 \({}^{\circ}\)C, show that the epoxide-based electrolyte maintains significant Coulombic efficiency up to 1 C when cycled against LFP and up to 0.2 C when cycled against NMC (Figure 3) [10]. The ability of this electrolyte to withstand cycling with NMC above 4.0 V suggests that the presence of the epoxide provides some improvement over more traditional PEO-based electrolytes which have been widely reported to become unstable over 4.0 V [72]. Like PEO, DGEPEG can be combined with numerous reagents to yield polymer electrolytes via the ring opening polymerization reaction [16; 20]. An example of this is the polymer electrolyte that was prepared by [PERSON] et al. who used LiTFSI to activate the polymerization of DGEPEG and poly(ethylene glycol) diacrylate (PEGDA) [20]. The resultant films had similar ionic conductivities to the polyether-based epoxide-containing solid polymer electrolytes that are discussed above, 5.3 \(\times\) 10\({}^{-5}\) S/cm, despite being prepared from a mix of more conductive materials [20]. The resultant electrolyte was thermally stable up to 200 \({}^{\circ}\)C and had a Tg of \(-\)36.9 \({}^{\circ}\)C [20]. However, the material was shown to be relatively electrochemically robust as it was stable up to 4.7 V versus Li\({}^{+}\)/Li and maintained a 99% Coulombic efficiency for up to 150 cycles at 0.2 C and 55 \({}^{\circ}\)C [20]. In addition to being used to produce electrolytes for lithium batteries, epoxide-based polymer electrolytes have also been proposed for use in batteries that function based on other types of ions. This demonstrates the versatility of this synthesis technique. [PERSON] et al. prepared a solid polymer electrolyte that conducted calcium ions for use in calcium ion batteries [16].