We study the ion-conductive and electrochemical properties of concentrated poly (ethylene carbonate) (PEC)-based electrolytes with Li salt concentrations greater than 100 mol%. An end-capped PEC having acetate groups (PEC-Ac) was synthesized as a novel polymer matrix for improving the thermal stability and oxidative stability. A concentrated PEC-Ac electrolyte with 120 mol% of LiFSI had a good conductivity of approximately 10−6 S/cm at 40 °C with an excellent Li transference number (t +) of 0.8. Linear sweep voltammetry measurement for the PEC-Ac electrolyte indicates that the oxidation tolerance is more than 5 V, and is greater than that of the original PEC-based electrolyte. A battery test for the LiFePO4 cell using the PEC-Ac electrolyte was conducted for the first time, finding good capacities ranging from 130 to 160 mAh/g and stable cyclability for more than 60 cycles.