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Author(s): Vincent, Rebecca C; Shen, Jimmy-Xuan; Preefer, Molleigh B; Lin, Justin; Seeler, Fabian; Schierle-Arndt, Kerstin; Persson, Kristin A; Seshadri, Ram

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. Na3FeF6 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 Na3FeF6. 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, 57Fe Mossbauer spectroscopy, X-ray photoelectron spectroscopy, magnetometry, and 23Na/19F solid-state nuclear magnetic resonance (NMR) complemented with first principles calculations of NMR properties. We find that the primary performance limitation of the Na3FeF6 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 Na3FeF6 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.

A Li-rich polyanionic compound based on V3+ with a previously unknown structure, Li5VF4(SO4)2, has been developed as a high-voltage cathode material for Li-ion batteries. The solvothermal preparati...

PNb9O25, a Wadsley–Roth compound whose structure is obtained by appropriate crystallographic shear of the ReO3 structure, is a high-power electrode material that can reach 85% of the equilibrium ca...

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. Na

A Li-rich polyanionic compound based on V

The effects of shear planes in perovskite materials have been studied in order to identify their role in the electrochemical behavior of Li+ intercalation hosts. These planes modulate the structural stability and ionic transport pathways and therefore play an intimate role in the characteristics and performance of shear compounds. Herein, two Nb-based compounds, NbO2F and Nb3O7F, were chosen as representative perovskite and shear derivatives respectively to investigate the role of crystallographic shear. A series of operando measurements, including X-ray diffraction and X-ray absorption spectroscopy, in conjunction with structural analysis, Raman spectroscopy, and detailed electrochemical studies identified the effect of shear planes. It was found that shear planes led to increased structural stability during Li+ (de)intercalation with shear layers being maintained, while perovskite layers were seen to degrade rapidly. However, disordering in the shear plane stacking introduced during delithiation ultimately led to poor capacity retention despite structural maintenance as Li+ diffusion channels are disrupted.