Prussian Blue Analogues as Positive Electrodes for Mg Batteries: Insights into Mg Intercalation.

Potassium manganese hexacianoferrate has been prepared by co-precipitation from manganese (II) chloride and potassium citrate, with chemical analysis yielding the formula K Mn[Fe(CN) ] □  ⋅ 1.1H O (KMnHCF). Its X-ray diffraction pattern is consistent with a monoclinic structure (space group P 2 /n, no. 14) with cell parameters a=10.1202(6)Å, b=7.2890(5)Å, c=7.0193(4)Å, and β=89.90(1)°. Its redox behavior has been studied in magnesium containing electrolytes. Both K ions deintercalated from the structure upon oxidation and contamination with Na ions coming from the separator were found to interfere in the electrochemical response. In the absence of alkaline ions, pre-oxidized manganese hexacianoferrate showed reversible magnesium intercalation, and the process has been studied by operando synchrotron X-ray diffraction. The location of Mg ions in the crystal structure was not possible with the available experimental data. Still, density functional theory simulations indicated that the most favorable position for Mg intercalation is at 32f sites (considering a pseudo cubic F m-3m phase), which are located between 8c and Mn sites.