Ionic conductivity mechanisms in PEO–NaPF6 electrolytes
Abstract
Understanding ion transport mechanisms in sodium ion-based polymer electrolytes is critical, considering the emergence of sodium ion electrolyte technologies as sustainable alternatives to lithium-based systems. In this paper, we employ all-atom molecular dynamics simulations to investigate the salt concentration (c) effects on ionic conductivity (σ) mechanisms in sodium hexafluorophosphate (NaPF6) in polyethylene oxide (PEO) electrolytes. Sodium ions exhibit ion solvation shell characteristics comparable to those of lithium-based polymer electrolytes, with similar anion coordination but more populated oxygen coordination in the polymer matrix. We find that the diffusion coefficient of Na+ and PF6− follows the Stokes–Einstein behavior with viscosity (η) and ion-pair relaxation timescales (τc): D+ ∼ τc−0.87, D− ∼ τc−0.93, D+ ∼ η−1.08, and D− ∼ η−1.09, emphasizing the role of ion–polymer coordination and relaxation behavior in governing ion transport. Further analysis reveals an intriguing nonmonotonic trend in the Nernst–Einstein and true ionic conductivity as a function of c, peaking near c = 1 M. We model this behavior as σ ∼ cα exp(−c/c0), where the nonlinear term (α = 1.6) reflects efficient ion transport due to the absence of ion–ion correlations at low c, and the exponential decay quantifies viscosity-driven losses in ionic conductivity at high c. Our work establishes molecular guidelines to optimize conductivity in sodium-conducting polymer electrolytes, advancing next-generation sodium ion electrolyte technologies.