(310e) Computational Study of Lithium-Ion Transport in Mixed Ionic/Electronic Conducting Materials

Mixed ionic/electronic conductors are of broad interest for robotic materials and for energy storage and production. In these dual conducting materials, a single molecule contains both electron conducting hydrophobic blocks, which usually form solid-like nanodomains, and ion conducting hydrophilic blocks which form liquid-like nanodomains. Rational material design requires the understanding of the connection between the morphology of these nanodomains and the corresponding ion/electron transport. Extensive efforts have been devoted to understanding the ion conduction in PEO-based system and several ionic transport mechanisms have been identified, including both intra- and inter-segmental hopping events as well as the codiffusion of cations and segments. However, ion transport in dual conducting materials has been much less studied. In this talk, we focus on the mechanism of lithium-ion transport in mixed ionic/electronic conducting materials, targeting oligomers containing poly(ethylene oxide) (PEOn) segments for ion conduction and oligothiophene segments for electron conduction. We use Molecular Dynamics simulations to study the self-assemble behavior of various molecular designs having different shapes (linear and T-shape) and segmental fractions. We study in detail lithium-ion solvation and mobility in selected systems that were found to form promising percolating structures. For some species, both ordered and disordered phases were studied for comparison. We characterized the lithium-ion coordination environment to help us identify the ion solvation shell and track hopping events. We extracted the time scale of the codiffusion mechanism by analyzing the correlation between ion mobility and the corresponding solvation shell mobility. Our analysis shows that different ion binding motifs occur depending on the length of the PEO segments. The binding motif is defined according to the number of coordinating oxygen atoms and the number of coordinating molecules. Our simulation shows that different binding motifs tend to favor different ion hopping mechanism, which provide insights on the connection between molecular design and the observed ionic conductivities. For molecules with relatively short PEO segments, two molecules are usually required to form the ion solvation shell. When a new PEO segment attacks the solvation shell, it will only replace half of the coordinating oxygen atoms. For molecules with relative long PEO segments, only one molecule is required to form the solvation shell; hence, when a new PEO segment attacks the solvation shell, it will tend to replace all the coordinating oxygen atoms. We also find that the oligothiphene segments generate varying degrees of steric hindrance to the lithium-ion hopping events, depending on their spatial arrangement and the type of prevailing ion binding motif.