Diversification of battery technologies is crucial both to reduce dependence of specific raw materials and, most important, to adapt to specific use requirements including not only performance figures of merit (energy density, power, lifetime etc.,) but also economic and environmental considerations.  Among the different possible rechargeable battery concepts, those based on multivalent charge carrier ions, such as Mg²⁺, Ca²⁺, Zn²⁺ or Al³⁺, have attracted great attention, especially coupled to the use of the corresponding metal as negative electrode. Proof-of-concept for Mg batteries was achieved already in 2000 while exploration of the analogous systems is much more recent. None of such concepts has yet reached the market, as significant hurdles remain, affecting not only electrolyte and negative electrode (efficiency of plating/stripping process) but also the positive electrode. In this regard, migration of multivalent charge carrier ions in inorganic hosts has been proved to be sluggish due to strong coulombic interactions which can be diminished by enhancing the covalency of the bonding and moving from oxides to sulfides, which penalizes the operation potential. 

Open framework structures have also recently attracted attention, amongst which Prussian Blue analogues (PBAs) A_xM[M’(CN)₆]_y · zH₂O (A = alkaline metal (mostly K⁺ or Na⁺); M and M’ = transition metals; 0 ≤ x ≤ 2; y ≤ 1) represent an interesting alternative, which has proved excellent performances in Na and K based batteries. Their structure oconsists of a double perovskite framework with (C≡N)⁻ anions bridging MN₆ and M’C₆ octahedra.  A⁺ and H₂O occupy the cubes defined by the transition metal framework. These have shown electrochemical activity in aqueous electrolytes containing multivalent cations but controversies remain as to whether the redox response observed being related to the intercalation of protons rather than divalent ions.

The study of these compounds in non-aqueous electrolytes is appealing, especially if both M and M’ are redox active (e.g. Fe, Mn), and they have shown excellent performance in K⁺ and Na⁺ batteries with very good kinetics and cycle life. The presentation will deal with analogous studies carried out in Mg²⁺ and Ca²⁺ containing electrolytes.  Electrochemical performance, possible side reactions, and operando diffraction studies aimed at unravelling the redox mechanism will be discussed.