RESUMEN
Discovery of a new class of ion intercalation compounds is highly desirable due to its relevance to various electrochemical devices, such as batteries. Herein, we present a new iron-oxalato open framework, which showed reversible Na(+) intercalation/extraction. The hydrothermally synthesized K4Na2[Fe(C2O4)2]3â 2 H2O possesses one-dimensional open channels in the oxalato-bridged network, providing ion accessibility up to two Na(+) per the formula unit. The detailed studies on the structural and electronic states revealed that the framework exhibited a solid solution state almost entirely during Na(+) intercalation/extraction associated with the reversible redox of Fe. The present work demonstrates possibilities of the oxalato frameworks as tunable and robust ion intercalation electrode materials for various device applications.
RESUMEN
Phase separation and transformation induced by electrochemical ion insertion are key processes in achieving efficient energy storage. Exploration of novel insertion electrode materials/reactions is particularly important to unravel the atomic/molecular-level mechanism and improve the electrochemical properties. Here, we report the unconventional phase separation of a cyanide-bridged coordination polymer, Eu[Fe(CN)6]·4H2O, under electrochemical Na-ion insertion. Detailed structural analyses performed during the electrochemical reaction revealed that, in contrast to conventional electrochemical phase separation induced by the elastic interaction between nearest neighbors, the phase separation of NaxEu[Fe(CN)6]·4H2O is due to a long-range interaction, namely, cooperative rotation ordering of hexacyanoferrates. Kolmogorov-Johnson-Mehl-Avrami analysis showed that the activation energy for the phase boundary migration in NaxEu[Fe(CN)6]·4H2O is lower than that in other conventional electrode materials such as Li(1-x)FePO4.