New α-NaFeO₂ synthesis route for green sodium-ion batteries

New technologies have been investigated to replace the use of lithium and cobalt ions, raw materials of the cathode active material of lithium-ion batteries. Among the emerging technologies stands out one that uses sodium (Na+) and iron ions. Sodium iron oxide (NaFeO2) has polymorphism, with only the α phase being active for the reversible deintercalation of sodium ions, so this phase has potential application as an electroactive material in green sodium-ion batteries. The novel synthesis of α-sodium iron oxide through the sol–gel route, which provides a material with small particles and high crystallinity, is described in this work. Through X-ray diffraction and Rietveld refinement, it was found that the initial chelating agent/metals ratio affects the concentration of the α and β phases at the end of the synthetic route. The α-sodium iron oxide, obtained with an appropriate chelating agent/metals ratio, showed high purity and crystallinity. A discharge capacity of approximately 110 mAh/g was achieved when the α-sodium iron oxide electrode, obtained through the sol–gel route, was cycled from 1.00 to 4.00 V against sodium ions/sodium (Na), corresponding to the intercalation of approximately 0.5 sodium ions of the Na1−x FeO2 formula. The success of the synthesis of the α-sodium iron oxide phase can lower the cost and ensure the economic viability of green sodium-ion batteries.