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

Typical design strategies for mixed ion–electron conduction in polymers have focused on overall ionic conductivity, without specificity for anion vs cation conduction. Here, we demonstrate that side chain chemistry can be used to control Li+ conductivity in semiconducting polymers. This design principle is significant for applications that require Li+-specific transport, such as Li-ion batteries. We show that a polythiophene functionalized with an ionic liquid side chain demonstrates higher conductivity and lithium transference than a more commonly studied ether-functionalized P3AT derivative. Poly(3-(6′-(N-methylimidazolium) hexyl)thiophene TFSI–) (P3HT-Im+TFSI–) can solvate and conduct ions up to salt concentrations of r = 1.0 (where r = [moles of salt]/[moles of monomer]) while achieving an ionic conductivity of ∼10–3 S/cm at 80 °C and a lithium transference number of 0.36. On the other hand, poly(3-(methoxyethoxyethoxymethyl)thiophene) (P3MEEMT) shows a peak conductivity of ∼10–5 S/cm at r = 0.05 and 80 °C, with near-zero lithium transport. This work shows that multiple high dielectric moieties can be used to drive ion conduction in semiconducting polymers, but diffuse, cationic side chains such as imidazolium are preferred for Li-ion conduction.