Electrochemical properties of cast films of a prototypical polythiophene, regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT), were investigated in the lithium-ion battery (LIB) cell. The discharge curve shows both the oxidative high-V (similar to 3.7 V) and reductive low-V (similar to 1.5 V) plateaus. The observed capacities for both the plateaus are 60-80% of the ideal value (Q(ideal) = 162.45 mAh/g) calculated based on the monomer unit. Significantly, the material shows very rapid discharge process up to similar to 1500C. The rapid discharge process was ascribed to the formation of electric double layer (EDL) like state, in which charged polymers are electrically neutralized by the approach of the Li+ (ClO4-) at the surface region of the polymer aggregates.