Colloidal to micrometer-sized iron oxides and oxyhydroxides as anode materials for batteries and pseudocapacitors: Electrochemical properties

Electrode materials of high-performance are the key for lithium-ion and aqueous batteries, and supercapacitors. Although there are many studies about iron oxides as electrode materials, few or no studies focused on the dependence of electrochemical properties on both their structure and crystal size. Here, we focused on electrochemical properties of iron oxides of different structures such as hematite (α-Fe₂O₃), akaganeite (β-FeOOH) and ferrihydrite with unique structure (Fe₂O₃·2FeOOH·2.6H₂O) and of different crystal sizes and morphologies. When used as anode materials for LIBs, hematites with nanosized crystals showed higher capacity than the hematites with micrometer crystals. Among the different crystal structures, both hematite and akaganeite outperformed ferrihydrite even though the latter has the smallest crystals. Both hematite and akaganeite electrode materials showed similar charge and discharge behaviors for aqueous batteries, while ferrihydrite showed pseudocapacitive behavior. Thus, both crystal size and structure of iron oxides are important in tailoring them for electrode materials.