Zero-strain reductive intercalation in a molecular framework

Dual Redox Reactions of Silver Hexacyanoferrate Prussian Blue Analogue Enable Superior Electrochemical Performance for Zinc-ion Storage

Prussian blue analogues (PBAs) have been employed as host materials of aqueous zinc-ion batteries (ZIBs), however, they suffer from low capacity and poor cycling stability due to limited electron transfer and the presence of interstitial water in PBAs. Herein, a vacancy and water-free silver hexacyanoferrate K0.95Ag3.05Fe(CN)6 (AgHCF-3) was synthesized by adjusting the ratio of K and Ag in the framework. It offers nearly four electrons involving two sequential redox reactions, namely, Fe3+/Fe2+ and Ag+/Ag0, to deliver a large capacity of 179.6 mAh g-1 at 20 mA g-1 with Coulomb efficiency of ~100 %. The Zn//AgHCF-3 cell delivers an estimated energy density of 200 Wh kg-1, surpassing reported PBAs-based ZIBs. The formation of Ag0 in the cycling process merits favorable rate performance of AgHCF-3 (156.4 mAh g-1 at 200 mA g-1 with 80.3 % capacity retention after 100 cycles). This investigation paves new pathways for high-capacity PBA cathodes.

Effect of Extra-Framework Cations on Negative Linear Compressibility and High-Pressure Phase Transitions: A Study of KCd[Ag(CN)2]3

The negative thermal expansion material potassium cadmium dicyanoargentate, KCd[Ag(CN)₂]₃, is studied at high pressure using a combination of X-ray single-crystal diffraction, X-ray powder diffraction, infrared and Raman spectroscopy, and density functional theory calculations. In common with the isostructural manganese analogue, KMn[Ag(CN)₂]₃, this material is shown to exhibit very strong negative linear compressibility (NLC) in the crystallographic c direction due to structure hinging. We find increased structural flexibility results in enhanced NLC and NTE properties, but this also leads to two pressure-induced phase transitions-to very large unit cells involving octahedral tilting and shearing of the structure-below 2 GPa. The presence of potassium cations has an important effect on the mechanical and thermodynamic properties of this family, while the chemical versatility demonstrated here is of considerable interest to tune unusual mechanical properties for application.

Dicyanometallates as Model Extended Frameworks

We report the structures of eight new dicyanometallate frameworks containing molecular extra-framework cations. These systems include a number of hybrid inorganic–organic analogues of conventional ceramics, such as Ruddlesden–Popper phases and perovskites. The structure types adopted are rationalized in the broader context of all known dicyanometallate framework structures. We show that the structural diversity of this family can be understood in terms of (i) the charge and coordination preferences of the particular metal cation acting as framework node, and (ii) the size, shape, and extent of incorporation of extra-framework cations. In this way, we suggest that dicyanometallates form a particularly attractive model family of extended frameworks in which to explore the interplay between molecular degrees of freedom, framework topology, and supramolecular interactions.