K⁺ extraction induced phase evolution of KFeO₂

Orthorhombic KFeO₂ has a unique structure where K⁺ cations can migrate inside the Fe-O skeleton, thus making it a promising material for heterogeneous catalysis and electrochemical energy storage devices. However, KFeO₂ is sensitive to conditions such as moisture and carbon dioxide, which would trigger severe phase evolution and consequently deteriorate the performance. In this work, we investigated the phase evolution using freshly prepared KFeO₂ and KFeO₂ after exposure to ambient air and after immersion in water, respectively. We found that the phase evolution of KFeO₂ was composed of K-redistribution and phase transition, both of which originated from K⁺ extraction. We observed that K⁺ cations were extracted after exposing KFeO₂ to ambient air, resulting in the formation of K₂CO₃·1.5 H₂O outside KFeO₂ and lattice expansion inside KFeO₂. We also observed that water molecules were crucial to K⁺ extraction when calculating the function between potassium and the adjacent oxygen atoms via ab initio molecular dynamics simulations. Moreover, we successfully reinserted K⁺ cations into lattice expanded KFeO₂ by high-temperature calcination at 900 °C; such a reversible extraction-insertion process would have great potential for application in catalyst reactivation and rechargeable high-temperature batteries.