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 …