In Situ Fe‐Substituted Hexacyanoferrate for High‐Performance Aqueous Potassium Ion Batteries

The eco-friendliness, safety, and affordability of aqueous potassium batteries (AKIBs) have made them popular for large-scale energy storage devices. However, the cycling and rate performance of research materials, particularly cobalt hexacyanoferrate, have yet to meet satisfactory standards. Herein, a room-temperature drafted K1.66 Fe0.25 Co0.75 [Fe(CN)6 ]·0.83H2 O (KFCHCF) sample is reported using an in situ substitution strategy. A higher concentration of ferrocyanide ions decreases the water content and increases the potassium content, while citric acid works as a chelating agent and is responsible for Fe-substitution in the KFCHCF sample. The resultant KFCHCF sample exhibits good rate performance, and about 97% and 90.6% of discharge capacity are conserved after 400 and 1000 cycles at 100 and 200 mA g-1 , respectively. The full cell using the KFCHCF cathode and 1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide (PNTCDA) anode maintains ≈74.93% and 74.35% of discharge capacity at 200 mA g-1 and 1000 mA g-1 for 1000 and >10,000 cycles, respectively. Furthermore, ex situ characterizations demonstrate the high reversibility of K-ions and structural stability during the charge-discharge process. Such high performance is attributed to the fast K-ion migration and crystal structure stabilization caused by in situ Fe-substitution in the KFCHCF sample. Other hexacyanoferrates can be synthesized using this method and used in grid-scale storage systems.