Effects of sodium nitrate and heat treatment atmosphere on the synthesis of α–NaFeO2 layered oxide

• Published in: Materials chemistry and physics Vol. 249; p. 122948
• Main Authors: Kugai, Junichiro, Mine, Hirotaka, Seino, Satoshi, Nakagawa, Takashi, Yamamoto, Takao A., Yamada, Hirohisa
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 15-07-2020; Elsevier BV
• Subjects: CO2 absorption; Crystal lattices; Crystal structure; Decomposition reactions; Electrode materials; Ferric ions; Heat treatment; Inert atmospheres; Intercalation; Iron oxides; Nitrates; Oxidation; Oxygen; Phase transitions; Purity; Sodium; Sodium ferrite; Sodium ion batteries; Storage batteries; Topotactic reaction; Valence; α-NaFeO2; α-NaFeO2; Sodium ferrite; Topotactic reaction; Sodium ion batteries; CO2 absorption
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2020.122948

α-NaFeO2 type layered oxides shows reversible intercalation/de-intercalation of sodium ions between the oxide layers, which is suitable as regenerable CO2 absorber and cathode material for secondary batteries. This paper reports a critical effect of heating atmosphere to synthesize pure α-NaFeO2 from equimolar sodium nitrate and iron oxide. Reaction of NaNO3 with γ-Fe2O3 was much faster in nitrogen than oxygen atmosphere and high-purity α-NaFeO2 was yielded in a few hours of heat treatment in nitrogen while in oxygen atmosphere the reaction was sluggish, resulting in partially sodiated γ-Fe2O3. Inert atmosphere thermodynamically enhances nitrate decomposition, which controls the reaction rate of NaNO3 with γ-Fe2O3, and increases the solubility of γ-Fe2O3 in molten NaNO3 to promote topotactic phase transformation to α-NaFeO2. Molten NaNO3 works as a solvent for ferric ions and a redox buffer for maintaining the lattice oxygen structure and oxidation state of γ-Fe2O3. Larger iron oxide crystal was found to be suitable for production of high-purity α-NaFeO2 in a wide temperature range.
•High-purity α-NaFeO2 was quickly formed by heating NaNO3 with γ-Fe2O3 in an inert gas.•Heating in an inert gas thermodynamically promotes the reaction of NaNO3 with γ-Fe2O3.•Heating in an inert gas promotes topotactic phase transformation to α-NaFeO2.•NaNO3 functions as a redox buffer for maintaining the structure of iron oxide.•Larger crystal of iron oxide is advantageous for selective production of α-NaFeO2.