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

Abstract 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 K 1.66 Fe 0.25 Co 0.75 [Fe(CN) 6 ]·0.83H 2 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.