Random copolymers of ethylene carbonate (EC) and ethylene oxide (EO) having various EC/EO ratios were synthesized using double metal cyanide catalyst, and their LiFSI electrolytes were prepared. All of the copolymer electrolytes had greater conductivity and lower T g than the EC homopolymer system, and the copolymer with 53% EO content (LiFSI 120 mol%) exhibited the highest conductivity of 2.9 × 10−4 S cm−1 at 60 °C, with an excellent Li-ion transference number of nearly 0.7. FT-IR measurement showed that the carbonate groups of the copolymer interact preferentially with Li ions without the formation of any stable coordination structure, while free FSI anions decrease and the aggregated ions increase with increasing salt concentration. A Li/LiFePO4 coin cell with copolymer electrolyte having LiFSI 120 mol% had a discharge capacity of approximately 160 mAh g−1 for the first few cycles at 40 °C. Electrochemical impedance spectroscopy measurement on the cell after discharge found two semicircles in the Nyquist diagrams based on the passivation layer and charge-transfer resistance between the electrode and electrolyte gradually increase in size with increasing charge-discharge cycling.