## 3 Varieties of Epoxide-Based Polymer Electrolyte
### Silicone-Based Reagents
At 25 \({}^{\circ}\)C, the ionic conductivity was 2.66 \(\times\) 10\({}^{-4}\) S/cm and the lithium ion transference number was 0.39 [25]. Despite having a different mechanism of crosslinking, this polymer electrolyte was stable up to 4.9 V [25]. As this is similar to the other silane-containing polymer electrolytes that are discussed in this work, it is anticipated the epoxides (which are terminal groups here) are electrochemically stable. PDMS has also been used as a component of solid polymer electrolytes. Like silanes, siloxanes such as PDMS confer high thermal and mechanical stability to the resultant elastomers and can confer these properties to polymer electrolytes when they are crosslinked [26]. In addition, the high chain flexibility, large free volume and lower affinity for lithium ions of PDMS can result in high ion transport capabilities [26]. [PERSON] et al. prepared solid polymer electrolytes using PDMS, DGEPEG, LiTFSI and pentaerythhtol tetrakis (3-mercaptoethanolpropionate) (PEMP) as a linker [26]. The resultant electrolytes were thermally stable up to 200 \({}^{\circ}\)C and had Tg values between \(-\)33.2 and 36.5 \({}^{\circ}\)C depending on the LITFSI loading with higher loading resulting in lower thermal stability and higher Tg [26]. The ionic conductivity of this electrolyte is lower than those of the silane-containing materials that are presented above as it is on the order of 10\({}^{-6}\) S/cm at 30 \({}^{\circ}\)C [26]. The transference number is similar to that of PEO (\(-\)0.20) and the activation energy for lithium transport is 0.1 eV [26]. Despite limited gains in ionic conductivity relative to polyether-based electrolytes, the high stability of PDMS does have a significant impact on the electrochemical stability of the electrolyte material which is stable up to 5.3 V with respect to Li\({}^{+}\)/Li [26]. [PERSON] et al. prepared solid polymer electrolytes using a PDMS-like polymer: polyhedral oligomeric silsesquioxane (POSS) that had been combined with a PEO-polyppropylene oxide co-polymer P(EO-co-PO) via the ring opening polymerization reaction using LiTFSI as an activator [27]. The resultant solid polymer electrolytes were thermally stable, up to 350 \({}^{\circ}\)C and had a T\({}_{\text{g}}\) value of \(-\)43 \({}^{\circ}\)C [27]. The POSS-based polymer had a higher room temperature conductivity than the polymer that was based on PDMS (1.1 \(\times\) 10\({}^{-4}\) S/cm), a higher transference number (0.62) and a lower activation energy for ion transport as calculated by the Vogel-Tamman-Fulcher method (0.037 eV) [27]. The superior ion transport properties of the POSS-based electrolyte were attributed to the relatively rigid POSS framework keeping polymer chains separated from one another preventing crosslinking and/or tangling between PEO-co-PPO chains which were responsible for ion conduction (Figure 4) [27]. Using POSS instead of PDMS also resulted in improvements in electrochemical stability as the POSS-based electrolyte was stable up to 5.4 V relative to Li\({}^{+}\)/Li using an LiFePO4 (LFP) electrode for up to 500 h [27]. The reported improvements in electrochemical stability stem ming from the inclusion of POSS in this solid polymer electrolyte translated to improved electrochemical performance relative to other solid polymer electrolyte-based batteries that employ PEO-based systems [7; 81]. The POSS-based electrolyte was cycled at room temperature in a Li-LFP cell between 2.5 and 4.0 V [27]. Relatively high capacities were maintained at cycling rates ranging between 0.1 and 1 C (Figure 5) [27]. Additionally, 95% capacity was maintained for 50 cycles at 0.1 C and 92% capacity was maintained for 50 cycles at 0.2 C (Figure 5) [27]. The properties of the above electrolytes show that the inclusion of silanes or siloxanes in epoxide-based solid polymer electrolytes can improve ionic conductivity and electrochemical stability via the creation of a highly miscible organic-inorganic interface, reduced affinity for the polymer by lithium ions and improved crosslinking.