This dissertation work examines the electrochemical properties of various solid polymer electrolytes (SPEs) through the lens of composition-function relationships. The analyses presented offer unique design perspectives for improving the performance of SPEs for use in lithium-ion batteries (LIBs). Specifically, three distinct strategies are explored to enhance the lithium ion (Li+) conductivity and reduce the electrode/electrolyte interfacial resistance, two of the major challenges of adopting SPEs as alternatives to common organic liquid electrolytes. The basis for improving ionic conductivity, in all three strategies, is the inclusion of additives in the polymer matrix to plasticize the SPE and improve ionic transport. In one strategy, an ionic liquid (IL) is used as a plasticizer to fabricate free-standing ILSPEs membranes based on a poly(ethylene oxide) (PEO) matrix with an appropriate lithium salt. Optimized ILSPE compositions were able to achieve room temperature ionic conductivity of 0.96 mS/cm, a value suitable for commercial applications, as well as long cycle life in a lithium-metal battery with a capacity of 150—175 mAh/g and >99% coulombic efficiency. In a second strategy, the IL was swapped with water as the plasticizer to fabricate PEO-based aqueous SPEs (ASPEs). The ASPEs exhibited excellent transport properties, with room temperature conductivity values of 0.68—1.75 mS/cm. Molecular dynamics simulations revealed the origin of the exceptional transport properties as the presence of highly interconnected Li+(H2O)n domains. In a final strategy, the concepts of the ILSPE and ASPE were combined through the incorporation of both IL and water into a polymer matrix. For this strategy, the polymer matrix was also changed from PEO to polyacrylonitrile (PAN) to limit the effects of crystallinity and oxidation. These “hybrid aqueous/ionic liquid” SPEs (HAILSPEs) demonstrated the exceptional transport properties of the ASPE system with the improved stability and passivation of the ILSPE system. An analysis of the composition-function relationships correlated the dramatic rise in ionic conductivity to the nearly complete decoupling of ion transport from polymer chain mobility while the unique passivating properties were shown to derive from the choice of ionic liquid, with solid electrolyte interphases comprised of LiF, Li2CO3, Li2S, and Li3N.