Iron-oxalato framework with one-dimensional open channels for electrochemical sodium-ion intercalation

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]₃·2H₂O 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.