Rechargeable lithium-ion batteries are widely used in portable devices and electric vehicles. The electrolyte, composed of organic liquid and lithium salt, is a critical part of the battery that conducts ions during charging and discharging. Because of the flammability of the organic liquid, conventional electrolytes cause safety concerns, especially under thermal runaway conditions. Moreover, liquid electrolytes are not fully compatible with lithium metal, a promising high capacity anode material for future lithium batteries, because of the uneven electrodeposition of Li (Li dendrites). Solid polymer electrolytes (SPEs) have been reported as promising candidates to replace liquid electrolytes with advantages on processability, mechanical strength, and non-flammability. However, the low ionic conductivity of SPEs at room temperature preclude their practical application. This dissertation describes studies intended to improve the ionic conductivity and Li dendrite suppression of a series of poly(ethylene oxide) (PEO) and polyether based polymer electrolytes. We first did a systematic property-structure relationship study on a family of cross-linked hydrocarbon/PEO electrolytes by tuning the crystallinity of the hydrocarbon backbone. We were able to develop a hydrogenated polynorbornene backbone that increased the ionic conductivity to ~10-3 S/cm at room temperature without significant sacrifice on its Li dendrite resistance. With the help of theoretical chemists, we then synthesized an alternating copolymer of ethylene oxide (EO) and trimethylene oxide (TMO), which was predicted to have higher ionic conductivity than that of PEO at room temperature. The cyclic ether monomer of this alternating polymer was prepared from a novel ester-to-ether reduction. A series of EO/TMO random copolymers were also synthesized for comparison. One of the EO/TMO random copolymers showed an ionic conductivity of 10-4 S/cm at room temperature, among the highest ionic conductivities reported for SPEs. Finally, we explored further the application of the ester-to-ether reduction on some polyesters and successfully synthesized polyethers that have never been accessed before. We anticipate that the polymer electrolytes described in this work will provide useful insights for future design of SPEs and the concept of direct synthesis of polyethers from polyesters may help prepare new polyethers for SPE and other applications.