A small amount of thiophene or ethylene dioxythiophene (EDOT) is introduced into the liquid electrolyte of lithium-ion cells as an additive. These organic additives are electrochemically oxidized to form a thin conductive polymer film on the surface of the cathode at high potential. With the liquid electrolyte containing different additives, the lithium-ion cells composed of carbon anode and LiNi1/3Co1/3Mn1/3O2 cathode are assembled, and their cycling performances are evaluated. Adding small amounts of thiophene or EDOT to the liquid electrolyte is found to reduce the interfacial resistance in the cells and thus the cells containing an organic additive exhibit less capacity fading and better high-rate performance. Differential scanning calorimetric studies show that the thermal stability of the charged Li1−xNi1/3Co1/3Mn1/3O2 cathode is also enhanced in the presence of an organic additive.