Iron Containing Layered Oxides for Sodium Ion Battery Positive Electrodes
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NaxFeyM1-yO2 phases with M = Mn, Ti, Sn, Co, Co0.5Ni0.5, Ni, and others including a P2-Na2/3Mn1/3Fe1/3Co1/3O2 phase are synthesized and applied in Na-ion half cells for use as positive electrode materials. The potential use and application of Fe in Na-ion layered cathodes is explored. The feasibility of Fe for potential use in a commercial layered cathode material is assessed. On the basis of capacity retention, it is determined that the use of Fe in layered oxides may be prohibitive for commercial purposes unless coupled with Co in order to utilize high gravimetric and volumetric energy densities comparable to commonly used Li-ion systems. Using 57Fe Mössbauer spectroscopy and a new model which aids in detection of Na-vacancies, it is found that Na vacancies in the Na layer have a measureable affect the quadrupole splitting of edge sharing octahedral Fe3+ sites. Using this model, it is hypothesized that the position of Na-vacancies during insertion and extraction play a key to stabilization of Fe in the transition metal layer when large amounts of Na are inserted and extracted from the host structures. Future directions for realization of commercial Fe-containing layered oxides are discussed.