K + extraction induced phase evolution of KFeO 2 .

Orthorhombic KFeO 2 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, KFeO 2 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 KFeO 2 and KFeO 2 after exposure to ambient air and after immersion in water, respectively. We found that the phase evolution of KFeO 2 was composed of K-redistribution and phase transition, both of which originated from K + extraction. We observed that K + cations were extracted after exposing KFeO 2 to ambient air, resulting in the formation of K 2 CO 3 ·1.5 H 2 O outside KFeO 2 and lattice expansion inside KFeO 2 . 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 KFeO 2 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.