Unveiling the Effect of Al Doping on the Sodium Iron Hexacyanoferrate Cathode for the Development of High‐Performance Na‐Ion Battery

Sodium iron hexacyanoferrates (SFHCF) are the potential positive electrodes for Na ⁺ ‐ion batteries due to their large interstitial sites and three‐dimensional porous framework. However, poor cycling stability caused by lattice strain and volume change remained a challenge. SFHCF, with a reduced particle size, could decrease the lattice strain by accommodating large structural defects and stresses. In the present work, aluminum‐doped sodium iron hexacyanoferrates (Al‐SFHCF) are demonstrated to accommodate the structural defects and strains. The strain‐relaxed Gibbs–Thomson (SRGT) model calculations predict that Al ³⁺ (∼53.5 pm) partially replaces Fe ³⁺ (∼64.5 pm) ions in SFHCF during the growth process, causing an increase in free energy through an elastic contribution and elevating the chemical potential, which results in slower growth rates. Al doping influences the growth kinetics, leading to the formation of uniform and smaller particle sizes in the nanoscale (∼20–50 nm) and improves electrochemically active surface area. Furthermore, Al doping leads to the formation of Fe vacancies in SFHCF, which contributes to the improvement in structural stability. The Na‐ion cell performs reversibly even at higher C‐rates. The rational design strategy of metal doping to alter the growth of the Prussian blue cathode may pave the way for designing other electrode materials for high‐performance Na‐ion batteries.