Probing reaction processes and reversibility in Earth-abundant Na3FeF6 for Na-ion batteries. Issue 36 (7th July 2021)

• Record Type: Journal Article
• Authors: Foley, Emily E.; Wong, Anthony; Vincent, Rebecca C.; Manche, Alexis; Zaveri, Aryan; Gonzalez-Correa, Eliovardo; Ménard, Gabriel; Clément, Raphaële J.
• Is Part Of: Physical chemistry chemical physics. Volume 23:Issue 36(2021)
• Page Start: 20052
• Page End: 20064
• Publication Date: 2021-07-07
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/d1cp02763h
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : By correlating the observed capacity with the amount NaF identified through 23 Na NMR, we have unambiguously shown that Na3 FeF6 undergoes conversion to NaF and Fe, despite the metastability of the newly formed Fe nanoparticles. Abstract : Sodium (Na)-ion batteries are the most explored 'beyond-Li' battery systems, yet their energy densities are still largely limited by the positive electrode material. Na3 FeF6 is a promising Earth-abundant containing electrode and operates through a conversion-type charge–discharge reaction associated with a high theoretical capacity (336 mA h g −1 ). In practice, however, only a third of this capacity is achieved during electrochemical cycling. In this study, we demonstrate a new rapid and environmentally-friendly assisted-microwave method for the preparation of Na3 FeF6 . A comprehensive understanding of charge–discharge processes and of the reactivity of the cycled electrode samples is achieved using a combination of electrochemical tests, synchrotron X-ray diffraction, 57 Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, magnetometry, and 23 Na/ 19 F solid-state nuclear magnetic resonance (NMR) complemented with first principles calculations of NMR properties. We find that the primary performance limitation of the Na3 FeF6 electrode is the sluggish kinetics of the conversion reaction, while the methods employed for materials synthesis and electrode preparation do not have a significant impact on the conversion efficiency and reversibility. Our work confirms that Na3 FeF6 undergoes conversion into NaF and Fe(s) nanoparticles. The latter are found to be prone to oxidation prior to ex situ measurements, thus necessitating a robust analysis of the stable phases (here, NaF) formed upon conversion.