Effects of Homopolymer Additives on Conductivity of Salt-Doped Block Copolymers from Molecular Dynamics Simulations

As promising solid electrolyte materials for batteries, salt-doped block copolymers (BCP) simultaneously have ionic conductivity and mechanical robustness, due to the combination of two distinct polymer types. Recent experiments revealed that homopolymer additives, at high enough molecular weight (MW), can increase ionic conductivity in such systems. To understand this effect and guide further study, we employ coarse-grained molecular dynamics (MD) simulations. We analyze the microphase distribution of added homopolymers in salt-doped BCPs and relate this to the overall dynamics and ion conductivity. In particular, we find that longer homopolymers form an interlayer in the middle of the conducting microphase, while shorter chains are more fully integrated across the conducting microphase. This leads to a greater degree of overlap of the ion and homopolymer density profiles with increasing homopolymer MW, which may explain the increased conduction with MW. Ion concentration, segregation strength, and ion solvation energy also affect the ion distribution and can potentially be adjusted to enhance ionic conductivity.