Ionic conductivity mechanisms in PEO-NaPF₆ electrolytes

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 (τ_c): D₊ ∼ τ_c^-0.87, D_- ∼ τ_c^-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 ∼ c^α 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.