(3ch) Simultaneous Electronic and Ionic Conducting Block Copolymers for Lithium Battery Applications

The primary use of polymeric materials for lithium batteries has focused on the electrolyte.  Due to the flammability and reactivity of organic electrolytes used in traditional lithium-ion batteries, researchers have utilized solid polymer electrolytes, which are sufficiently ion conducting and stable during cycling.  In addition, researchers have used nanostructured block copolymer electrolytes where one of the blocks allows for mechanical strength. However, researchers have not considered the use of nanostructured block copolymers for the cathode (positive) electrode of lithium batteries where both electronic and ionic charges are transported.  For that reason, the use nanostructured block copolymer for lithium battery electrodes has been the primary focus of my PhD research.  For reference, the traditional porous lithium battery cathode consists of a redox-active material, carbon black for electronic conduction, and non-conductive binder that holds the particles in place.  In addition, the pores are backfilled filled with organic electrolyte for ionic conduction.  Due to the poor electronic and ionic conduction, the active materials are often are used in the nanoparticle form, thus requiring the transport of charges to occur on the nanometer length scales.  Therefore, the use of block copolymers that simultaneously conducts electronic/ionic charges and that self-assemble on the nanometer length scales is the ideal material.  The modified cathode thus only consists of the redox-active material and the nanostructured block copolymers. 

The primary focus of my PhD work involved the synthesis and characterization of the poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) as a nanostructured electrode binder where the P3HT-domains conduct electronic charges and PEO-domains conducts ionic charges.  The results of ac impedance spectroscopy and dc measurements provided the first evidence of the simultaneous conduction of electronic and ionic charges in a block copolymer.  In addition, my work was the first attempt to study the relationship between morphology and simultaneous electronic and ionic transport.  Furthermore, the application of this material in a lithium battery with LiFePO₄ showed specific capacity reaching the theoretical limit and with minimal capacity fade, thus demonstrating the practical application of P3HT-b-PEO as conductive binder material.  I believe the results of my work provides a foundation for substantial future work on nanostructured electronic/ionic conducting block copolymers for battery and other energy related devices.  The scope of my PhD research fell in the fields of both polymer physics and electrochemistry.  Knowledge in both fields will help develop my future academic research projects that both provide fundamental scientific knowledge and have a broad real world impact on energy applications.