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 (~3.7V) and reductive low-V (~1.5V) plateaus. The observed capacities for both the plateaus are 60–80% of the ideal value (Q ideal =162.45mAh/g) calculated based on the monomer unit. Significantly, the material shows very rapid discharge process up 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.