Plasticizers play a crucial role in enhancing cation transport in single-ion conducting polymer electrolytes. However, a fundamental understanding of how different plasticizers facilitate ionic conductivity remains incomplete. To elucidate the molecular mechanisms by which plasticizers promote lithium-ion transport, equilibrium molecular dynamics simulations were performed to investigate lithium-ion transport behavior in modified polyethylene terephthalate (mPET) electrolytes plasticized with varying concentrations of fluoroethylene carbonate (FEC) and propylene carbonate (PC). Our simulation results show that both systems exhibit comparable lithium-ion diffusion coefficients and ionic conductivities at plasticizer concentrations below 40 wt %. In contrast, at concentrations above 40 wt %, the PC-plasticized system displays higher lithium-ion diffusion coefficients and ionic conductivities than the FEC-plasticized system. These observations can be attributed to the synergistic effects of plasticizer electrostatic properties and polymer chain flexibility. Specifically, quantitative comparisons of electrostatic surface potentials among FEC and PC indicate that the electron distribution of plasticizers governs their ability to compete with the polymer matrix for lithium-ion coordination, thereby determining the local coordination environment. In addition, radius of gyration analysis reveals that mPET chains in PC-plasticized systems exhibit greater flexibility, providing more continuous pathways for lithium-ion hopping between polymer chains. This enhanced flexibility is consistent with the reduced mean lifetimes of Li-O coordination pairs observed at plasticizer concentrations above 40 wt %. Overall, this work provides molecular-level insights that can guide the rational design of plasticizers to improve ionic conductivity in single-ion conducting polymer electrolytes.