Simultaneous Electronic and Ionic Conduction in a Block Copolymer: Application in Lithium Battery Electrodes†

Conjugated polymers such as poly(3-hexylthiophene) (P3HT) have been used extensively as the active material in various electronic devices such as solar cells, field-effect transistors, and light-emitting diodes. Coupling these conjugated polymers to ionic conductors allows use of the polymers in electrochemical energy storage devices, which require the transport of both ions and electronic charge. The need for developing new materials for electrochemical energy storage in the emerging energy landscape, which includes electric cars and renewable stationary energy, is now widely recognized. We report herein the synthesis and characterization of poly(3hexylthiophene)-b-poly(ethylene oxide) (P3HT-PEO). Lithium batteries composed of a cathode with LiFePO4 particles dispersed in a P3HT-PEO matrix, a solid polymer electrolyte, and lithium metal anodes were assembled. In contrast to current lithium battery electrodes, which require an inert polymer to bind the cathode particles, carbon additives for electronic conduction and a liquid electrolyte for lithium ion conduction, our system uses one material that serves as the binder and transporter of electronic charge and lithium ions. Scheme 1 shows the synthesis of P3HT-PEO. Grignard metathesis (GRIM) polymerization was used to produce an ethynyl-terminated, regioregular P3HT (P3HT-1). Regioregular P3HT forms well-defined, nanostructured polymers, while the ethynyl functional group is required for the next step. The 1,3-dipolar cycloaddition “click” reaction was used to couple P3HT-1 to a commercially available, azide-terminated PEO, and was performed in the presence of CuI and diisopropylethylamine (DIPEA) in THF for 72 h at 40 8C to result in P3HT-PEO (detailed experimental procedures and polymer characterization (Figures S1 and S2) are shown in the Supporting Information). Tapping-mode AFM phase images of P3HT-1 homopolymer and the P3HT-PEO copolymer thin films are shown in Figure 1a and Figure 1b, respectively. Both images show the nanofibrillar morphology that is usually reported for P3HT and its derivatives, which results from the p–p stacking of the well-defined P3HT chains. Both samples have nanofibril widths of approximately 20 nm, which are consistent with reported values. The P3HT-1 nanofibrils appear to have a very compact arrangement with relatively little phase variaScheme 1. Synthesis of P3HT-PEO block copolymer. dppp=1,3-bis(diphenylphosphino)propane.