Size Engineering and Crystallinity Control Enable High‐Capacity Aqueous Potassium‐Ion Storage of Prussian White Analogues

Abstract Aqueous potassium‐ion batteries have demonstrated huge potential in the field of energy storage, owing to their low cost, environmental friendliness, and high safety, yet with poor cycling stability and rate capability. Here, potassium Prussian white analogues (K‐PW) with different gradients in crystallinity and size have been synthesized by controlling the acidity of hydrothermal environment. The as‐synthesized K 1.93 Fe[Fe(CN) 6 ] 0.97 ⋅ 1.82H 2 O nanoparticles deliver considerable reversible capacities of 142 mAh g −1 at 75 mA g −1 , even 40 mAh g −1 at 9000 mA g −1 , and high capacity retention of 88 % after 300 cycles at 1500 mA g −1 in a KNO 3 aqueous electrolyte. High crystallinity and short ion‐diffusion length determine the fast ion‐intercalation kinetics and, thus, enable the superior rate capability. These results reveal the feasibility of using K‐PW for practical applications in aqueous potassium‐ion batteries.