Dual Redox-Active Copper Hexacyanoferrate Nanosheets as Cathode Materials for Advanced Sodium-Ion Batteries

Abstract Prussian Blue Analogs (PBAs) such as copper hexacyanoferrate (CuHCF) are traditional intercalation cathodes for rechargeable Na-ion batteries. However, the electrochemical performance of these PBAs suffers from insufficient activation and sharp performance deterioration. Here, the insufficient electrochemical activity and frame deformation issues in the CuHCF cathodes were investigated to enhance their specific capacity and improve their cycling stability. The high-crystallinity CuHCF nanosheets (CuHCF-P) with low-water-content were synthesized by a pyrophosphate-assistant co-precipitation method. It has a highly reversible 1.5-Na insertion/desertion capability with a specific capacity of ∼120 mAh g−1 at 0.1 C, which is the highest among all the CuHCF cathodes reported. First-principle study and XPS detection demonstrate that both Cu and Fe are redox-active centers in CuHCF-P cathode, which facilitates a high Na+ storage capability. And the decrease of water content in CuHCF framework increases Fe/Cu 3d-orbital occupy-sites, which activates both of the transition-metals. Furthermore, the full cells fabricated with as-prepared CuHCF-P cathode and commercial hard carbon anode exhibit excellent performances with a reversible capacity of 109 mAh g−1 at 0.1 C over 200 cycles.