Chain Topological Effects on Collective Ion Transport in Lamellar Block Copolymer Electrolytes

Microphase-separated block copolymer electrolytes (BCPEs) show promising potential for battery applications. In BCPEs, both ionic conductivity and mechanical stability can be improved with increasing polymer chain molecular weight (MW). The domain interfaces modulate ion transport properties through a complex interplay between various factors, characterized by a transient, non-monotonic correlation between MW and conductivity at low MW. In this work, we model lithium salt-doped poly(styrene-b-ethylene oxide) (PS-b-PEO) BCPEs by molecular dynamics (MD) simulations in microsecond time scale and united atom resolution. We capture the non-monotonic trend of conductivity with respect to polymer chain MW, which is attributed to a cross-over between decreasing vehicular contribution and increasing collective cation conduction as polymer chain MW increases. The collective cation movement is associated with collective polymer chain segmental motions in directions parallel to the interface, imposed by microphase separation. The apparent interfacial segmental mixing is largely driven by capillary effects, limiting the actual impact of local ion retardation. Our modeling which embody the chemically specific polymer-ion associations shed new light on the importance of polymer chain dynamics on the ionic conductivity of BCPEs, where the topological constraints play key roles for the anomalies.