Intercalation of Mg ²⁺ into electrodeposited Prussian Blue Analogue thin films from aqueous electrolytes

So-called Prussian Blue Analogues (PBAs) attract significant research interest for a large variety of electrochemical applications. Recently, they have been recognized as promising electrode materials to implement in aqueous Na-ion batteries for grid-scale energy storage systems. Further types of aqueous batteries using, for instance, alkali metal cations such as Li ⁺ and K ⁺ or multivalent ions like Mg ²⁺ , Zn ²⁺ and Al ³⁺ as insertion species for PBA-based electrodes are being explored. In this study we systematically investigate intercalation of Mg ²⁺ into four electrodeposited PBA thin films: Na ₂ Ni[Fe(CN) ₆ ], Na ₂ Co[Fe(CN) ₆ ], Na ₂ VO _x [Fe(CN) ₆ ], and NaIn[Fe(CN) ₆ ]. All four electrodes are capable of hosting Mg ²⁺ , while NaIn[Fe(CN) ₆ ] shows better stability upon repeated (dis)charging compared to the other three systems. Surprisingly, even though the hydration energies of Na ⁺ and Mg ²⁺ differ significantly, the mean half-charge potentials of (de)intercalation are comparable for both cations. A correlation between the empirical radii of transition metals present in the structure of PBAs and the mean half-charge potential of (de)intercalation, previously identified for Na ⁺ , is also found for Mg ²⁺ .