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 Mössbauer 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.

In the pursuit of urgently needed, energy dense solid-state batteries for electric vehicle and portable electronics applications, halide solid electrolytes offer a promising path forward with exceptional compatibility against high-voltage oxide electrodes, tunable ionic conductivities, and facile processing. For this family of compounds, synthesis protocols strongly affect cation site disorder and modulate Li⁺ mobility. In this work, we reveal the presence of a high concentration of stacking faults in the superionic conductor Li₃YCl₆ and demonstrate a method of controlling its Li⁺ conductivity by tuning the defect concentration with synthesis and heat treatments at select temperatures. Leveraging complementary insights from variable temperature synchrotron X-ray diffraction, neutron diffraction, cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, density functional theory, and electrochemical impedance spectroscopy, we identify the nature of planar defects and the role of nonstoichiometry in lowering Li⁺ migration barriers and increasing Li site connectivity in mechanochemically synthesized Li₃YCl₆. We harness paramagnetic relaxation enhancement to enable ⁸⁹Y solid-state NMR and directly contrast the Y cation site disorder resulting from different preparation methods, demonstrating a potent tool for other researchers studying Y-containing compositions. With heat treatments at temperatures as low as 333 K (60 °C), we decrease the concentration of planar defects, demonstrating a simple method for tuning the Li⁺ conductivity. Findings from this work are expected to be generalizable to other halide solid electrolyte candidates and provide an improved understanding of defect-enabled Li⁺ conduction in this class of Li-ion conductors.