3-Hexylthiophene was incorporated into battery electrolyte as a life-extending additive. Upon oxidation, this monomer forms a conductive, conjugated polymer, resulting in significant increase in the cycle lifetimes of lithium half-cells containing high capacity Li1.2Ni0.15Co0.1Mn0.55O2 and high voltage LiNi0.5Mn1.5O4 cathodes. This additive significantly reduces the interfacial impedance growth observed in the cell without additive, which indicates an effective protection of the interface when using 3-hexythiophene. We also observed an improvement in safety when 3-hexylthiophene was incorporated. Electron microscopy images show the new polymer on the cathode surface, specifically poly(3-hexylthiophene) as supported by characterization of the cathode surface using vibrational spectroscopy (Raman and infrared) and mass spectrometry (MALDI). (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.039302jes] All rights reserved.