Diffusion of Ions in Diblock Copolymers: Understanding the Molecular Weight Effect Through Coarse-Grained Modeling

Diblock copolymers in which one microphase is mechanically robust while the other solvates and allows conduction of ions are of interest as solid battery electrolytes. The transport of ions through the conducting microphase is generally slower than through the analogous homopolymer, and is thought to depend on the distribution of ions within the conducting phase, among other factors. To understand these effects, we use coarse-grained molecular dynamics simulations and consider a wide range of systems with various ion-polymer and ion-ion interaction strengths. Our model reproduces the experimental trend of increasing ion transport with copolymer molecular weight, and this trend is more dramatic as ions are solvated in one polymer block more strongly or as the ion-ion interactions get stronger. The degree to which ions are locally concentrated, quantified by their average number of nearest ion neighbors out to a distance of approximately three diameters, is a good predictor of the ion diffusion constant. Specifically, systems whose ions are more locally aggregated (have more ion neighbors) have a reduced diffusion constant.