Insights into Cationic Vacancies in a Prussian Blue Analogues Cathode for Enhanced Reversible Sodium Insertion

As promising cathode materials for sodium-ion batteries (SIBs), Prussian blue analogues (PBAs) have gained significant attention due to their facile synthesis and high theoretical capacity. However, the low practical capacity limits PBAs for battery applications. In this work, we propose a Fe vacancy-type PBA and systematically investigate its electrochemical activities, kinetics, and sodium storage mechanism. We demonstrate that although the capacity is enhanced as the vacancy content increases, an excessive concentration of vacancies leads to a deterioration of the cycling performance. Moreover, we show an enhancement of the Na⁺ diffusion kinetics due to Fe vacancies through multiscan rate cyclic voltammetry, electrochemical impedance spectroscopy, and molecular simulations. Furthermore, operando X-ray diffraction, ex situ solid-state nuclear magnetic resonance, and Raman reveal the sodium intercalation behavior, confirming the high structural reversibility of vacancy-type PBAs. This study not only proposes a novel vacancy-mediated strategy in PBAs but also highlights the significance of defect engineering in boosting the sodium storage for SIBs.