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 Na2FePO4F is investigated here by ex situ 23Na 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 23Na 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 23Na NMR spectrum at all intermediate states of charge. The appearance of this resonance at +320 ppm is attributed to the fully oxidized (NaFePO4F) 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 Na2FePO4F cathodes. These findings imply that the Na2FePO4F framework is incredibly structurally stable with a robust intercalation process, despite a lack of ideal sodium-ion kinetics.