Prussian Blue Analogues as Positive Electrodes for Mg Batteries: Insights into Mg²⁺ Intercalation

Potassium manganese hexacianoferrate has been prepared by co-precipitation from manganese (II) chloride and potassium citrate, with chemical analysis yielding the formula K_1.72 Mn[Fe(CN)₆ ]_0.92 □_0.08  ⋅ 1.1H₂ O (KMnHCF). Its X-ray diffraction pattern is consistent with a monoclinic structure (space group P 2₁ /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 Mg²⁺ 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 Mg²⁺ intercalation is at 32f sites (considering a pseudo cubic F m-3m phase), which are located between 8c and Mn sites.