Low Potential Prussian Blue Analogs: Manganese Hexacyanochromate

Abstract

Prussian blue analogues (PBAs) have recently shown outstanding electrochemical properties ascribable to their unique open-framework crystal structure that allows the reversible insertion of alkali ions with negligible perturbation to the framework itself. Many hexacyanoferrate materials have shown excellent properties and are some of the most promising sodium- and potassium-ion cathode materials in both aqueous and organic electrolytes. However, there is a distinct lack of candidate PBA materials that operate at low potentials as their characteristic crystalline framework shows instability. In this article we characterise the structure and electrochemical behavior of manganese hexacyanochromate which exhibits reversible sodium insertion at - 0.86 V vs. SHE (1.84 V vs. Na<sup>+</sup>/Na), whilst maintaining the characteristic PBA cubic structure. This is the lowest redox potential of reported PBA materials and shows fast kinetics in a high voltage water-in-salt electrolyte. Further reduction in potential in an organic electrolyte shows decomposition of the crystalline structure.

Origin of the high specific capacity in sodium manganese hexacyanomanganate

“…Second, the low-potential behavior of a closely related composition, sodium manganese hexacyanochromate (Na x Mn[Cr(CN) 6 ]), has previously been explored experimentally. 16 The lattice was found to be stable below the minimum potential (about 1.21 V versus Na + /Na) that these calculations predict is necessary to insert the third sodium ion. Third, closely related PBA compositions have been found to be thermally stable up to 200 • C or higher.…”