Chemical reduction of Prussian blue nanocubes to obtain alkali ion containing cathodes and their battery applications

Abstract

A facile process for the preparation of alkali ion containing Prussian blue cathodes by chemical reduction and their battery applications are reported. Two cathodes, Li_xFe[Fe(CN)₆] and Na_xFe[Fe(CN)₆], have been prepared through chemical lithiation/sodiation of Fe[Fe(CN₆)] through a rapid (<10 min) and low temperature process (<100 °C). Structural analysis confirmed that the as-prepared Fe[Fe(CN)₆], lithiated and sodiated compounds maintained a cubic structure. Nano-sized cuboidal morphology was confirmed by electron microscopy and it was unaffected by the chemical lithiation/sodiation process. Based on the first cycle electrochemical capacity, it is realized that chemical sodiation into Fe[Fe(CN₆)] is more efficient than lithiation. The sodiated Fe[Fe(CN)₆] showed a capacity retention of about 70% after 500 cycles with 78 mA h g⁻¹ discharge capacity at 100 mA g⁻¹ for sodium ion battery applications. On the other hand, lithiated Fe[Fe(CN)₆] showed a better capacity retention of 92% after 300 cycles with 111 mA h g⁻¹ discharge capacity at 100 mA g⁻¹ for lithium ion battery applications. Ex situ X-ray diffraction analysis of the electrodes at the end of the first charge, at the end of the first cycle and at the end of 5 cycles was carried out and the results were compared with those of the as-sodiated Na_xFe[Fe(CN)₆] electrode. This investigation confirmed that for the chemically sodiated electrode a cubic phase at the end of the first charge transformed to a monoclinic structure at the end of discharge (first cycle and at the end of the 5^th cycle). The cubic to monoclinic structural phase transition is identified to be happening at the threshold sodium concentration around x = 1.3 in Na_xFe[Fe(CN)₆]. These results are correlated with ex situ surface chemical analysis. Such a cost-effective chemical reduction process to achieve a sodium-rich phase has not been reported so far to the best of our knowledge.