Layered metal–organic framework based on tetracyanonickelate as a cathode material for <i>in situ</i> 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; Fe²⁺Ni(CN)₄, MnNi(CN)₄, Fe³⁺Ni(CN)₄, CuNi(CN)₄, CoNi(CN)₄, ZnNi(CN)₄, and NiNi(CN)₄), 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/Fe²⁺ system exhibited superior electrochemical properties compared to the others with a capacity of 137.9 mA h g^-1 at a current density of 100 mA g^-1. Furthermore, after a 5000-cycle long-term repeated charge/discharge measurement, the Ni/Fe²⁺ system displayed a capacity of 60.3 mA h g^-1 with a coulombic efficiency of 98.8% at a current density of 1000 mA g^-1. In addition, the capacity of 86.1% was preserved at 1000 mA g^-1 as compared with that at 100 mA g^-1, implying a good rate capability. These potential capacities can be ascribed to an <i>in situ</i> reduction of Li⁺ in the interlayer of Ni/Fe²⁺ instead of the formation of other compounds with the host material according to <i>ex situ</i> XRD characterization. These specially designed FF-PBAs are expected to inspire new concepts in electrochemistry and other applications requiring 2D materials.