Effect of Polymer Chain Length and Fluoroethylene Carbonate Plasticizer on Lithium-Ion Transport in PAN–LiFSI Solid Polymer Electrolytes: A Molecular Dynamics Study

Abstract The development of solid polymer electrolytes for lithium batteries requires a molecular-level understanding of how host architecture and additives jointly control ion transport. Here, classical molecular dynamics simulations, complemented by density-functional theory, are used to investigate the independent effects of polyacrylonitrile (PAN) chain length and fluoroethylene carbonate (FEC) plasticizer content on lithium-ion transport and solvation in the PAN–LiFSI–FEC system. Increasing the chain length is found to suppress lithium diffusion, as longer chains progressively cage the cation within the nitrile coordination environment. Incorporation of FEC enhances the ionic conductivity by more than an order of magnitude, saturating over a broad 30–40 wt % range, despite a concurrent increase in bulk density that rules out a simple free-volume mechanism. Radial-distribution and residence-time analyses reveal that FEC does not enter the lithium first coordination shell but instead acts as a second-sphere plasticizer that renders the lithium–polymer coordination dynamically labile; the diffusion coefficient correlates strongly and inversely with the coordination residence time. Density-functional calculations show that FEC binds lithium more strongly than the polymer nitrile, demonstrating that its exclusion from the first shell is entropic and steric rather than energetic. These results establish a coordination-dynamics mechanism for plasticizer-enhanced transport and identify a plasticized intermediate-length PAN host as a favorable electrolyte design.