Sodiation vs. Lithiation of FePO4: A comparative kinetic study

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.