Modification of the Electron Density in Prussian Blue Iron Sites to Improve the Charge Extraction at High Voltages in Sodium Ion Batteries

Abstract

An interesting material for the cathode of Na-Ion battery is the metal-organic open framework called Prussian Blue AFe[Fe(CN)]6 (A= alkali metals), in which alkali ions occupy half of the empty cubic cavities created by a cubic network of alternating high spin Fe and low spin Fe in the vertices and -C≡N- ligands on the edges. These materials exhibit interesting properties such as low cost, easiness of synthesis and good stability. During the charge process this material exhibit two redox processes, the first associated to high spin Fe (coordinated to nitrogen) and the second to low spin Fe (coordinated to carbon); however, the extracted charge is mainly obtained during the discharge process at lower potentials (lower energy). Thus, in this work, in order to enhance the energy density, the high spin Fe in Prussian blue is partially replaced by Mn2+ and Co2+, i.e. samples NaxMnFe[Fe(CN)]6 and NaxCoFe[Fe(CN)]6. All materials were synthetized by precipitation method, the XRD revealed that materials crystallized in cubic cell, whereas the IR and Raman spectroscopy suggested that high spin Fe is in oxidation state 3+ and iron low spin is found in 2+. Mössbauer spectroscopy was used to confirm the oxidation state of the samples; moreover, the characterization indicates that the electronic density of the iron center is modified by the presence of Mn or Co in the structure. The electrochemical performance indicated that the charge extracted during the discharge process is similar in NaxCoFe[Fe(CN)]6 and NaxFe[Fe(CN)6] whereas in the NaxMnFe[Fe(CN)]6 the higher amount of extracted charge occurs in the second process at higher voltage. Thus, the incorporation of a second metal in the structure of the Prussian Blue cathode is capable to modify the electronic structure of the sites and lead to the possibility to control the extraction of charge at high voltages.