Salt-Dependent Ion Transport Behavior in Liquid and Polymer Electrolytes: A Molecular Dynamics Study

Highly concentrated electrolytes are promising candidates for improving the stability of

rechargeable batteries. In this work, molecular dynamics simulations were performed to investigate

the effects of salt concentration on ion transport and ionic conductivity in both liquid and polymer

electrolyte systems. Two representative systems were considered: EC–LiTFSI (ethylene

carbonate–lithium bis(trifluoromethane sulfonyl)imide) and PEO–NaPF₆ (polyethylene oxide–

sodium hexafluorophosphate). The results reveal that ionic conductivity initially increases with salt

concentration, reaches an optimum value, and then decreases at higher concentrations for both types

of electrolytes. Interestingly, the mechanisms underlying this behavior differ between liquid and

polymer systems. Our analysis proposes a unified equation (𝜎(𝑐)~𝑐^𝛼 𝑒^-c/c₀ (𝛼 > 0) linking ionic

conductivity (𝜎) to salt concentration (𝑐), where 𝑐^𝛼 term represents uncorrelated ionic motion and

𝑒^-c/c₀ term describes viscosity-driven effects induced by salt addition. Structural correlations were

further analyzed using radial distribution functions and coordination numbers to elucidate ion-pair

formation and local organization.