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. 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. I propose to use poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) as a nanostructured binder that conducts both electronic and ionic charges. P3HT-b-PEO self-assembles on the nanometer length scale with P3HT-domains that conduct electronic charges and PEO-domains that conducts ionic charges. The modified cathode thus only consists of the redox-active material and P3HT-b-PEO.
I report on the relationship between morphology and electronic/ionic charge transport of P3HT-b-PEO and lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI) mixtures. Using ac impedance spectroscopy, I show that P3HT-b-PEO/LiTFSI mixtures can conduct electronic and ionic charges simultaneously. At 90 °C, the electronic conductivity of P3HT-b-PEO/LiTFSI mixtures ranged from 10-8 to 10-5 S/cm depending on the volume fraction of P3HT, which is below the decoupled ionic conductivity of ~10-4 S/cm. For the material to work effectively, the electronic conductivity needs to match or exceed the ionic conductivity. Nevertheless, the application of P3HT-b-PEO in a lithium battery with LiFePO4 active material showed specific capacity approaching the theoretical capacity and with minimal capacity fade. An ac impedance measurement indicated that the total resistance of the battery decreases upon charging and increases upon discharge. This suggested the electronic resistance of P3HT-b-PEO was changing within the potential range of the battery through electrochemical doping of P3HT with LiTFSI. We designed an all solid-state electrochemical cell with three terminals to measure the electronic conductivity of P3HT-b-PEO under applied potentials. The addition of a third terminal within the P3HT-b-PEO layer allows for the in-situ measurement of the electronic conductivity as a function of the P3HT electrochemical doping level. The results of the in-situ electronic conductivity measurements as a function of electrochemical doping level and block copolymer composition will be presented.