High Capacity and Fast Kinetics Enabled by Metal‐Doping in Prussian Blue Analogue Cathodes for Sodium‐Ion Batteries

Abstract Prussian blue analogues (PBAs) have gained tremendous attention as promising low‐cost electrochemically‐tunable electrode materials, which can accommodate large Na + ions with attractive specific capacity and charge‐discharge kinetics. However, poor cycling stability caused by lattice strain and volume change remains to be improved. Herein, metal‐doping strategy has been demonstrated in FeNiHCF, Na 1.40 Fe 0.90 Ni 0.10 [Fe(CN) 6 ] 0.85 ⋅ 1.3H 2 O, delivering a capacity as high as 148 mAh g −1 at 10 mA g −1 . At an exceptionally high rate of 25.6 A g −1 , a reversible capacity of ~55 mAh g −1 still can be obtained with a very small capacity decay rate of 0.02 % per cycle for 1000 cycles, considered one of the best among all metal‐doped PBAs. This exhibits the stabilizing effect of Ni doping which enhances structural stability and long‐term cyclability. In situ synchrotron X‐ray diffraction reveals an extremely small (~1 %) change in unit cell parameters. The Ni substitution is found to increase the electronic conductivity and redox activity, especially at the low‐spin (LS) Fe center due to inductive effect. This larger capacity contribution from LS Fe 2+ C 6 /Fe 3+ C 6 redox couple is responsible for stable high‐rate capability of FeNiHCF. The insight gained in this work may pave the way for the design of other high‐performance electrode materials for sustainable sodium‐ion batteries.