By potentiodynamic polarization in saturated aqueous NaNO3 solution, the composite LiFePO4/C was transformed completely to NaFePO4/C composite. The voltammograms of sodiated olivine were successfully recorded at the rate for two order of magnitude higher than reported for the same material in organic electrolyte. A high storage capacity amounting to 118 mAh g−1 at a scan rate of 10 mV s−1 was measured. The diffusion coefficients have been calculated via cyclic voltammetry and faster diffusion of sodium in comparison to lithium was found. This finding was confirmed also with the impedance measurements. The presence of dissolved oxygen in the electrolyte did not disturb the electrochemical behaviour of this composite material.