Identifying the Structural Evolution of the Sodium Ion Battery Na₂ FePO₄ F Cathode

Na₂ FePO₄ F is a promising cathode material for Na-ion batteries owing to its relatively high discharge voltage and excellent cycling performance. Now, the long- and short-range structural evolution of Na₂ FePO₄ F during cycling is studied by in situ high-energy X-ray diffraction (XRD), ex situ solid-state nuclear magnetic resonance (NMR), and first-principles DFT calculations. DFT calculations suggest that the intermediate phase, Na_1.5 FePO₄ F, adopts the space group of P2₁ /c, which is a subgroup (P2₁ /b11, No. 14) of Pbcn (No. 60), the space group of the starting phase, Na₂ FePO₄ F, and this space group provides a good fit to the experimental XRD and NMR results. The two crystallographically unique Na sites in the structure of Na₂ FePO₄ F behave differently during cycling, where the Na ions on the Na2 site are electrochemically active while those on the Na1 site are inert. This study determines the structural evolution and the electrochemical reaction mechanisms of Na₂ FePO₄ F in a Na-ion battery.