Higher voltage plateau cubic Prussian White for Na-ion batteries

Cubic sodium Prussian White, Na2-xFe2(CN)6·yH2O, and potassium Prussian White, K2-xFe2(CN)6·yH2O, are prepared following a mild synthetic methodology. While cubic symmetry is confirmed by XRD and TEM, IR and XPS show characteristic features different from Prussian Blue compositions. When investigated as cathode materials in sodium ion batteries, both compounds exhibit reversible capacities above 140 mAh g−1 at 1C (ca. 80 mA g−1). While sodium Prussian White shows better high rate capability (10C/0.1C = 0.64), potassium Prussian White exhibits longer cycle stability, with up to 80% of capacity retention after 500 cycles. Interestingly, the potassium Prussian White phase also provides an increase of 0.35 V in the high voltage redox peak compared to the sodium Prussian White analogue ascribed to the preferential insertion of K+ ions instead of Na+, resulting in an increment of the gravimetric energy density. On the other hand, the insertion of Na+ seems to occur at the lower voltage plateau. This hybrid Na+ and K+ insertion in the framework of potassium Prussian White is most likely the responsible of the long cycle stability as a consequence of synergistic effects.