Structural regulated nickel hexacyanoferrate with superior sodium storage performance by K-doping

Prussian blue analogs with unique open frame structure have aroused great concern because of their low cost, environmental friendliness and relatively easy synthesis. Especially, nickel hexacyanoferrate (NiHCF) attracts great attention due to its outstanding cycling stability. Herein, we regulate the structure of NiHCF by K-doping at Na-site and investigate the sodium storage performance by combining experiments with density functional theory calculations. Our results reveal that K-doping induces the transformation of NiHCF from cubic to monoclinic and provides more sodium storage sites. In addition, K+ ions can play a role of pillar in stabling the structure. More importantly, K-doping can improve the transport ability of sodium ions and electrons. As a result, the K-doped NiHCF electrode delivers an extremely high initial discharge capacity of 87.1 mAh g−1 at 10 mA g−1 with a very low capacity fading ratio of 0.016% per cycle at 800 mA g−1 over 1000 cycles. Such K-doped NiHCF composite shows its potential application prospects in the field of sodium-ion batteries.