(323d) Electronic Charge Transport Properties of Electrochemically Oxidized Block Copolymers – Lithium Battery Application

Block copolymers that can simultaneously conduct electronic and ionic charges on the nanometers length scale can serve as innovate conductive binder material for solid-state battery electrodes.  The purpose of this work is study the electronic charge transport of poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-PEO) copolymers electrochemically oxidized with lithium bis(trifluoromethanesulfonyl) imide  (LiTFSI) salt.  We use a solid-state three-terminal electrochemical cell that enables simultaneous conductivity measurements and control over electrochemical doping of P3HT.  At low oxidation levels, the electronic conductivity increases from 10^-8 S/cm to 10^-4 S/cm.  At high oxidation levels, electronic conductivity_­­­ approaches 10^-2 S/cm.  When P3HT-PEO is used as a conductive binder in a positive electrode with LiFePO­₄ active material, P3HT is electrochemically active within the voltage window of a charge/discharge cycle.  The electronic conductivity of the P3HT-PEO binder is in the 10^-4 to 10^-2 S/cm range over most of the potential window of the charge/discharge cycle.  This allows for efficient electronic conduction and observed charge/discharge capacities approach the theoretical limit of LiFePO₄.  However, at the end of the discharge cycle, the electronic conductivity decreases sharply to 10^-7 S/cm, which means the “conductive” binder is now electronically insulating.  The ability of our conductive binder to switch between electronically conducting and insulating states in the positive electrode provides an unprecedented route for automatic overdischarge protection within the battery.  This is in stark contrast to traditional lithium ion batteries where external electronics is used to provide overdischarge protection.