Sodiation vs. Lithiation of FePO₄: A comparative kinetic study

Highlights: • Comparison of intercalation kinetics: sodiation vs. lithiation of FePO₄. • Rate constant of sodiation is significantly lower compared to lithiation. • Solid state diffusion of sodium in FePO₄ is slower compared to lithium. • Larger activation energies of sodiation compared to lithiation of FePO₄. - Abstract: A comparative study of sodiation and lithiation of FePO₄ host material for secondary sodium- and lithium-ion batteries is performed. To ensure the best possible comparability, NaFePO₄ based electrodes are prepared by an electrochemical displacement technique using LiFePO₄ as the starting material. The electrodes are identical in view of particle size distribution, active material loading, binder, conductive additives, and carbon coating, etc. All differences in the kinetics are exclusively determined by the substitution of the alkali metal cation. Electrochemical impedance spectroscopy is carried out at different temperatures. Kinetic parameters as the exchange current density and the diffusion coefficients of sodium and lithium in the FePO₄ host material are obtained by fitting the data to an appropriate equivalent circuit. The study reveals substantially slower intercalation kinetics and larger activation energies for sodiation in comparison to lithiation of FePO₄. This is most likely attributed to the different ionic radii and molar masses of sodium and lithium.