Synthesis and Characterization of Prussian Blue Analogue Materials for Li-ion and post-Li Batteries

The global challenge responding to the need of efficient electrical energy storage is answered by rechargeable batteries, which are based on high-rate intercalation reaction of lithium ions into nano- and microstructured porous materials. A class of insetion-type materials is represented by Prussian blue analogues (PBAs), characterized by porous open 3D-frameworks which allow for a facile insertion/ extraction of ions with negligible lattice strain. In the present work we focused on the synthesis and characterization of PBAs in Li-ion and post-Li battery systems, their redox activity, electronic and structural reversibility while cycling. All synthesized materials exhibit good structural stability and negligible lattice strain during (de)insertion of ions and redox processes ascribable to one or more redox species. For instance, copper hexacyanoferrate features two redox sites, copper and iron, contrarily to what reported in the literature, while copper nitroprusside has been demonstrated to possess three redox centres, including the two metals, as well as the non innocent nitrosyl ligand as third site. Electrosynthesized copper hexacyanoferrate results extremely versatile towards a wide selection of ions in aqueous solution, ranging from monovalent to multivalent ions, while titanium hexacyanoferrate may reach a capacity equal to 55 mAh/g in potassium nitrate aqueous solution. This led to the conclusion that a H2O-based system would be feasible for the studied materials, and more in general for this class of compounds. Although they do not feature high specific capacities, they are characterized by good cycling ability and efficiency, as well as ion-versatility which can be favorable to a post-lithium strategy. The investigation of their reaction mechanism has led to the deep understanding of limiting steps, whereby it is possible to tailor new promising materials that could result competitive in the next future.