Tilt Engineering in Prussian White Cathode Na1+xFe[Fe(CN)6] via Mg-doping for Enhanced Electrochemical Performance in Na-ion Batteries

Due to their robust open framework structure and facile synthesis techniques, Prussian blue analogs (PBAs) can be a suitable and cost-effective cathode for Na-ion batteries. Despite higher sodium content and low defects, the Prussian white (PW) still shows lower reversible capacity due to the limited participation of low-spin Fe and restricted Na-ion diffusion due to non-optimized octahedral tilts. Here, we report improving the electrochemical performance of PW by optimizing the octahedral tilts through Mg doping. The pristine Na_1.8Fe[Fe(CN)₆ ⋅ 2.75H₂O delivers the reversible capacity of 127 mAh g⁻¹ with a capacity retention of 69 % after 250 cycles. However, 10 % Mg-doped samples Na_1.81Mg_0.09Fe_0.81[Fe(CN)₆] ⋅ 2.58H₂O deliver an improved reversible capacity of 138.2 mAh g⁻¹ and capacity retention of 85 % after 250 cycles at C/10 rate. By replacing Fe, Mg incorporation into the Na_1+xFe[Fe(CN)₆] framework increases the electron density, which is propagated throughout the −Fe−CN−Fe−NC− chain and weakens the strong CN ligand field. This results in the participation of additional low-spin Fe in the electrochemical reaction. Mg incorporation also provides structural stability by reducing the distortion magnitude and enhancing diffusion kinetics by optimizing the octahedral tilt angle to improve the battery metrics of the cathode.