## 1 Introduction
In the case of epoxide-based polymer electrolytes, the ring opening polymerization reaction, which occurs as a result of an attack on the carbon-oxygen bond forming an alkoxide ion, tends to occur via the activated monomer mechanism where a Lewis acid bonds to the oxygen (Scheme 1) [8,14]. Interaction with a Lewis acid accelerates the rate of the ring opening polymerization reaction as it reduces the electronic density in the epoxide ring [8]. Under acidic conditions, the epoxide functional group is converted into a hydroxyl group which can go on to react with hydroxyl functional groups or other epoxides yielding C-O-C linkages (Scheme 1) [16,17]. As the presence of multiple epoxide or hydroxyl functional groups opens up the possibility for various polymerization pathways, control of the final product is obtained by selecting the molar ratio of each reagent [16]. The primary advantage of Lewis acid activation is that the polymerization of less reactive functional groups becomes more likely which is advantageous for polymer tunability. Additionally, activation allows the reaction to occur under mild conditions which decreases the total energy required to produce the electrolyte. Lithium salts are Lewis acids that can be used to activate the ring opening polymerization reaction [6,8]. During the activation process, the lithium cation coordinates to the oxygen on the epoxide which accelerates the ring opening process by creating a good leaving group (Scheme 1) [6]. Significant advantages of using lithium salts to catalyze the ring opening polymerization reaction include: (i) excluding potentially inactive and poisoning amines from the system, (ii) allowing the reaction to be performed as a \"one-pot\" synthesis as lithium salts tend to be soluble in liquid epoxide precursors and (iii) lithium salts do not react with the epoxide monomer to form hydroxyl groups during the curing process which can be harmful to lithium electrodes during battery cycling [14]. As a wide variety of solid-state polymer electrolytes can be produced via the ring opening polymerization of epoxide-containing reagents, this review will discuss the ionic conductivities, thermal properties and electrochemical stability of various types of epoxide-based polymers (Table 1). These properties are discussed in more detail in Section 2 of this review. In order to compare epoxide-based solid polymer electrolytes to PEO-based solid polymer electrolytes more directly, the first part of Section 3 of this review will discuss the characteristics of epoxide-based polymer electrolytes that are based on a polyether backbone as polymerized ethylene oxide is the main source of ionic conductivity in these polymers. Then, polymer electrolytes containing silicones (silane and siloxane) are discussed to demonstrate the impact of crosslinking organic and inorganic moieties on the thermal and electrochemical properties of the resultant electrolytes. The next part discusses polymer electrolytes that were polymerized by a combination of lithium salt activation and amine-based curing agents which have been classically used in epoxy curing reactions. The last part of the Section 3 investigates the impact of the incorporation of plasticizers on the ionic conductivity and crystallinity of crosslinked epoxide-based polymer electrolytes. Plasticizers tend to improve ionic conductivity by solubilizing lithium cations and polymer chains.