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

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_1.40Fe_0.90Ni_0.10[Fe(CN)₆]_0.85 ⋅ 1.3H₂O, 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 Fe²⁺C₆/Fe³⁺C₆ 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.