Selective edge etching to improve the rate capability of Prussian blue analogues for sodium ion batteries

Prussian blue analogues (PBA) possess a high theoretical specific capacity for sodium ion batteries. However, cycling PBA to a high current density causes severe capacity fading. Here, we develop a selective edge-etching approach to tackle this long-standing issue of poor rate capability. Well-crystallized PBA particles were produced by hydrothermal treatment of a sodium hexacyanoferrate precursor dissolved in muriatic acid solution, which were then eroded in hydrochloric acid solution to promote selective etching along the edges of the PBA crystals. The defect concentration ([Fe(CN) 6 ] 4− ) on the edge is denser than that at the face or corner, which stimulates the preferred etching of edges via the defect-induced heterogeneous mechanism. Due to the increasing exposed surface area and active sites, the etched PBA display much improved electrochemical performance with a capacity of 167 mA h g −1 at a current density of 5 mA g −1 and a capacity retention of 82.7% when the current density was increased to 40 mA g −1 , demonstrating fast sodium ion transfer and high rate capability. Prussian blue analogues prefer to be etched along the edge in HCl solution, resulting in much enhanced ionic diffusions and thus rate capability.