Layered metal-organic framework based on tetracyanonickelate as a cathode material for Li-ion storage.

Prussian blue analogs (PBAs) formed with hexacyanide linkers have been studied for decades. The framework crystal structure of PBAs mainly benefits from the six-fold coordinated cyano functional groups. In this study, in-plane tetracyanonickelate was utilized to engineer an organic linker and design a family of four-fold coordinated PBAs (FF-PBAs; FeNi(CN), MnNi(CN), FeNi(CN), CuNi(CN), CoNi(CN), ZnNi(CN), and NiNi(CN)), which showed an interesting two-dimensional (2D) crystal structure. It was found that these FF-PBAs could be utilized as cathode materials of Li-ion batteries, and the Ni/Fe system exhibited superior electrochemical properties compared to the others with a capacity of 137.9 mA h g at a current density of 100 mA g. Furthermore, after a 5000-cycle long-term repeated charge/discharge measurement, the Ni/Fe system displayed a capacity of 60.3 mA h g with a coulombic efficiency of 98.8% at a current density of 1000 mA g. In addition, the capacity of 86.1% was preserved at 1000 mA g as compared with that at 100 mA g, implying a good rate capability. These potential capacities can be ascribed to an reduction of Li in the interlayer of Ni/Fe instead of the formation of other compounds with the host material according to XRD characterization. These specially designed FF-PBAs are expected to inspire new concepts in electrochemistry and other applications requiring 2D materials.