Breaking Ion Clusters: Size Asymmetry for Fast Ion Transport in Polymer Electrolytes

Solid polymer electrolytes (SPEs)—mixtures of polymers and inorganic salts—have emerged as promising candidates for next-generation solid-state batteries, as they offer an attractive combination of mechanical robustness, thermal stability, and safety. However, their widespread application remains limited by inherently low ionic conductivity and, consequently, reduced energy efficiency. As the salt concentration in an SPE increases, the system often undergoes microphase segregation due to ion clustering and unfavorable polymer–ion interactions, leading to a substantial drop in conductivity. In this work, we perform large-scale coarse-grained molecular dynamics (CGMD) simulations to establish the microscopic correlation between ion distribution, morphology, and ionic transport in SPEs. Our results reveal that introducing asymmetry in ion size effectively suppresses microphase separation by disrupting ion–ion correlations, thereby enhancing overall ionic conductivity. Furthermore, in the limit of low dipole moment, we identify the emergence of a novel phase wherein ions self-organize into percolating pathways throughout the polymer matrix, enabling a faster ion transport mechanism. These findings provide new molecular-level insights into the design principles of ion-conducting polymers and demonstrate that tuning the ion size ratio represents a viable strategy to engineer ultrahigh-ion-conductive SPEs for advanced energy storage applications.