Abstract
Mixed ion-electron conducting polymers have garnered significant attention in recent years for applications in batteries, electrochemical transistors and many such electrochemical devices. Due to contradictory design rules for electron and ion conduction, the library of such mixed conducting polymers is small. While conjugated polymers with Li+-ion conducting oligoether side chains have previously shown mixed conducting ability, their ionic conductivity (∼10-7 S/cm) lags far behind their electronic counterpart ((∼10-1 S/cm). In this regard, our group has previously shown that cationic conjugated polyelectrolytes such as polythiophenes with alkyl side chains attached to imidazolium pendant units and bulky TFSI- counterions can lead to long-range polymer ordering and diffuse ion interactions resulting in high Li+ transport (∼10–4 S/cm at 80 °C) and electronic conductivity (∼ 10-4 S/cm). In this work, we have further engineered the polymer architecture to incorporate alkoxy side chains with imidazolium pendants to increase stability in the doped state, thereby further enhancing electronic conductivity and electrochemical stability. We observed that even with Br- counterions, before ion exchange with bulky TFSI-, bulk ionic conductivity is of the order of 10-3 S/cm at 80°C, while electronic conductivity was increased by several orders of magnitude, up to 10-2 S/cm upon vapor doping using HTFSI. This demonstrates considerable enhancement of mixed ion-electron conduction on incorporation of alkoxy side chains compared to alkyl side chains. Our current efforts are focused on measuring Li+ ion diffusion using PFG NMR, morphological investigation using GIWAXS and finally application as battery binders upon electrostatic compatibilization with an oppositely charged polymeric ionic liquid (PIL) which has also been previously established by our group.
