Operando Mössbauer study of Fe4+/Fe3+ redox couple in NaFeO2

Among the potential positive electrode materials for Na-ion batteries, iron-based compounds have been regarded as promising candidates for the reversible (de)intercalation of Na on the basis of their high abundance in the Earth’s crust and their low environmental impact. The model oxide O3-type NaFeO2, synthesized via the simple reaction of iron oxide and Na2CO3 at 600°C, has been identified as the most interesting one from the viewpoint of both gravimetric and volumetric energy density.[1–3] Na/NaFeO2 cells cycle through a relatively flat potential plateau between 3.3 and 3.4 V vs. Na+/Na, commonly linked with the Fe4+/Fe3+ redox couple. However, if cycling is extended above 3.5 V, other irreversible reaction plateaus appear, leading to the material degradation. 57Fe Mössbauer spectroscopy is thus a method of choice for the study of both (1) the cycling mechanism and (2) the irreversible reactions occurring above 3.5 V.
In this work, operando 57Fe Mössbauer analysis was carried out during the electrochemical cycling of NaFeO2 vs. Na metal using a specifically designed in situ cell,[4] and analysed using an alternative and innovating data analysis approach based on chemometric tools such as Principal Component Analysis (PCA) and multivariate curve resolution (MCR).[5,6] This approach, which allows the unbiased extraction of all possible information from the operando data, enabled the stepwise reconstruction of the “real” spectral components occurring during the cycling of NaFeO2. In this way, a clear description of the electrochemically active iron species could be obtained, allowing a clearer comprehension of the cycling mechanisms of this material vs. sodium.

References
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[2] H. Yoshida, N. Yabuuchi, S. Komaba, in:, ECS Meet. Abstr. MA2012-02, ECS, Honolulu, 2011, p. 1850.
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[4] J.-B. Leriche, S. Hamelet, J. Shu, M. Morcrette, C. Masquelier, G. Ouvrard, M. Zerrouki, P. Soudan, S. Belin, E. Elkaïm, F. Baudelet, J. Electrochem. Soc. 157 (2010) A606.
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