Boosting the sodium storage performance of Prussian blue analogues via effective etching

Prussian blue analogues (PBAs) have gained significant popularity as cathode materials for sodium-ion batteries (SIBs) due to their remarkable features such as high capacity and convenient synthesis. However, PBAs usually suffer from kinetic problems during the electrochemical reactions due to sluggish Na+ diffusion in the large crystals, resulting in low-capacity utilization and inferior rate capability. In this study, we present a facile etching method aiming at activating the sodium storage sites and accelerating the Na+ transport of Na2NiFe(CN)(6) (denoted as NaNiHCF) by precisely controlling its morphologies. A progressive corner passivation phenomenon occurred in NaNiHCF during the etching process, which led to a substantial augmentation of the specific surface area as the morphology transitioned from a standard cube to a dice shape. Notably, by controlling the etching time, the obtained NaNiHCF-3 electrode exhibited boosted electrochemical performance with high reversible capacity of 83.5 mAh g(-1) (98.2% of its theoretical capacity), superior rate capability (71.2 mAh g(-1) at 10 C), and stable cycling life-span at different temperatures. Both experimental and computational methods reveal the remarkably reversible structural evolution process and improved Na+ diffusion coefficient. We believe that this work can serve as an indispensable reference to tailor the structure of PBAs to obtain improved electrochemical performance.