Impact of Side Chain Chemistry on Lithium Transport in Mixed Ion-Electron Conducting Polymers

Mixed ion-electron conducting polymers (MIECs) have recently been reported to significantly improve rate capability when used as binders in Lithium-ion battery cathodes. While it is widely known that the transport of Li+ is of utmost importance for battery functionality, this property has yet to be well characterized in MIECs. Here, we show that two thiophene derivates functionalized with chemically distinct ion conducting side chains, Poly{3-[6’-(N-methylimidazolium) hexyl] thiophene} (P3HT-Im+) and Poly[3-(methoxyethoxyethoxymethyl) thiophene] (P3MEEMT), show contrasting lithium solvation and transport properties. P3HT-Im+ can solvate and conduct ions up to a molar concertation of r=1.0 ([moles of salt]/[moles of monomer]), achieving an ionic conductivity of  ~10^–3 S/cm at 80°C, and a lithium transference number of 0.36. On the other hand, P3MEEMT shows a peak conductivity of ~10^–5 S/cm at r=0.05 and 80°C, with 0 lithium transport. This work shows that multiple high dielectric moieties can be used to impart ion conduction in semiconducting polymers, but diffuse, cationic side chains such as imidazolium are ideal for lithium conduction.