Ex Situ ²³Na Solid-State NMR Reveals the Local Na-Ion Distribution in Carbon-Coated Na₂FePO₄F during Electrochemical Cycling

The potential Na-ion cathode material Na 2 FePO 4 F is investigated here by ex situ 23 Na solid-state nuclear magnetic resonance (ssNMR) in order to characterize the structure and ion mobility as a function of electrochemical cycling. The use of fast magic angle spinning (MAS) speeds of 65 kHz allows for the collection of high-resolution 23 Na NMR spectra that reveal two unique peaks at +450 and −175 ppm, corresponding to the two crystallographically unique Na sites in the material of interest. Two-dimensional NMR exchange spectroscopy results reveal that chemical exchange between the Na ions residing in distinct environments has a maximum hopping rate of ∼200 Hz. The collection of one-dimensional NMR spectra as a function of electrochemical cycling reveals the reproducible formation of a new peak at +320 ppm in the 23 Na NMR spectrum at all intermediate states of charge. The appearance of this resonance at +320 ppm is attributed to the fully oxidized (NaFePO 4 F) phase that is present even upon initial electrochemical oxidation. The simultaneous existence of both the pristine and oxidized phases suggest formation of two distinct phases upon charging, consistent with a two-phase desodiation mechanism. This two-phase arrangement of Na ions persists for multiple charge/discharge cycles and is congruent with high reversibility of Na (de)intercalation in Na 2 FePO 4 F cathodes. These findings imply that the Na 2 FePO 4 F framework is incredibly structurally stable with a robust intercalation process, despite a lack of ideal sodium-ion kinetics.