Potassium nickel hexacyanoferrate as a high-voltage cathode material for nonaqueous magnesium-ion batteries

The magnesium insertion capability of Prussian blue (PB) analogue, potassium nickel hexacyanoferrate K0.86Ni[Fe(CN)(6)](0.954)(H2O)(0.766) (KNF-086), is demonstrated as a cathode material for rechargeable magnesium-ion batteries using a conventional organic electrolyte. K1.51Ni[Fe(CN)(6)](0.954)(H2O)(0.766) is synthesized first, and potassium ions are electrochemically extracted to prepare the KNF-086 cathode. The electrochemical test cell is composed of KNF-086 as the working electrode, an activated carbon as the counter and reference electrode, and 0.5 M Mg(ClO4)(2) in acetonitrile as the electrolyte. The cell shows a reversible magnesium insertion/extraction reaction with a discharge capacity of 48.3 mAh g(-1) at a 0.2 degrees C rate, and an average discharge voltage at 2.99 V (vs. Mg/Mg2+) that is the highest among the cathode materials ever reported for magnesium-ion batteries. Elemental analysis and Fourier electron-density map analysis from powder X-ray diffraction data confirm that the magnesium-inserted phase is Mg0.27K0.86Ni[Fe(CN)(6)](0.954)(H2O)(0.766) (MKNF-086), and the magnesium ions in MKNF-086 are positioned at the center of the large interstitial cavities of cubic PB. Compared to KNF-086, MKNF-086 exhibits a decreased unit cell parameter (0.8%) and volume (2.4%). These results demonstrate that a PB analogue, potassium nickel hexacyanoferrate, could be utilized as a potential cathode material for conventional organic electrolyte-based magnesium-ion batteries. (C) 2017 Elsevier B.V. All rights reserved.