Nickel Hexacyanoferrate with Dual-Redox Centers for High-Performance Aluminum-ion Batteries

The development of high-performance cathode materials for aluminum-ion batteries (AIBs) is hindered by the strong electrostatic interactions of Al³⁺ ions with host frameworks, leading to sluggish kinetics and structural degradation. Herein, nickel hexacyanoferrate (NiHCF) nanomaterials with rhombohedral (r-NiHCF) and cubic (c-NiHCF) phases are synthesized using a chelator-regulated solvothermal approach at 80 and 150 °C, respectively. The r-NiHCF exhibits a more ordered lattice with higher Na⁺ content, fewer [Fe(CN)₆] vacancies, and reduced water content, resulting in a larger specific surface area (58.34 vs 13.13 m² g^-1) and smaller particle size (37 vs 224 nm). Electrochemical tests reveal that r-NiHCF delivers twice the initial discharge capacity of c-NiHCF (128.8 vs 67.1 mAh g^-1 at 0.4 A g^-1) and maintains 77.1% capacity retention after 500 cycles. It also demonstrates superior rate performance (38.5 mAh g^-1 at 2.0 A g^-1) and a higher Al³⁺ diffusion coefficient (1.04 × 10^-11 vs 2.99 × 10^-13 cm² s^-1). Mechanistic studies confirm a highly reversible dual-redox behavior (Ni²⁺/Ni³⁺ and Fe²⁺/Fe³⁺) in r-NiHCF, facilitated by a phase transition that stabilizes the framework during cycling. This work highlights crystal phase engineering as a strategic approach to optimize Prussian blue analogue cathodes for high-performance multivalent-ion batteries.