Figure 3

Electrochemical characterizations and structural characterization. The initial five voltage composition curves of the amorphous FePO₄ cathode in (a) sodium and (b) potassium cells cycled between 3.7–1.2 and 3.5–1.5 V at current rates of 10 and 4 mAg⁻¹ rates, respectively. Low C-rate conditions were maintained because the ionic radii of Na⁺ (1.02 Å) and K⁺ (1.38 Å) ions are comparatively larger than that of the usual Li ion (0.76 Å). The active material loading for all of the cells was 2.5 mg cm⁻². The degree of impedance in the cells varies according to their respective ionic sizes in the increasing order of Na⁺<K⁺ ions. The cycle performances of the sodium and potassium test cells are plotted as insets in (a) and (b), respectively. The sodium test cell retains nearly 100% of its discharge capacity after 25 cycles, whereas a gradual capacity fade is observed in the case of the potassium cell.