Turning Berlin green frameworks into cubic crystals for cathodes with high-rate capability†

Prussian blue analogues (PBAs) have been considered as promising host frameworks for charge carriers because of their well-defined diffusion channel along the 〈100〉 direction. Among PBA families, Berlin green (BG) would be an ideal cathode platform because the empty carrier ion sites and two redox couples (Fe^3+/2+–CN–Fe^3+/2+) in the BG framework can deliver high specific capacity during battery operation. Nonetheless, in most solution-based precipitation processes, BG crystals are synthesized in irregular shapes rather than in well-defined cube shapes, thus limiting their capacities at high rate operations. In this work, given the aforementioned challenges, a simple two-step precipitation process to synthesize cubic BG without using any chelating agents and toxic acids was reported. Notably, an intermediate phase was identified as an important stage in converting irregularly shaped BG to cubic BG by releasing crystal water molecules from the framework. Utilizing well-aligned 〈100〉 channels in the cubic framework, cubic BG exhibits excellent electrochemical properties as a cathode for lithium-ion batteries, delivering a specific capacity of 107.2 mA h g⁻¹ at a high current density of 500 mA g⁻¹. A combined study of in situ X-ray diffraction and X-ray absorption fine structure analyses would provide a comprehensive structure–property relationship of BG cathodes.