The Unique Structural Evolution of the O3‐Phase Na2/3Fe2/3Mn1/3O2 during High Rate Charge/Discharge: A Sodium‐Centred Perspective

The development of new insertion electrodes in sodium‐ion batteries requires an in‐depth understanding of the relationship between electrochemical performance and the structural evolution during cycling. To date in situ synchrotron X‐ray and neutron diffraction methods appear to be the only probes of in situ electrode evolution at high rates, a critical condition for battery development. Here, the structural evolution of the recently synthesized O3‐phase of Na 2/3 Fe 2/3 Mn 1/3 O 2 is reported under relatively high current rates. The evolution of the phases, their lattice parameters, and phase fractions, and the sodium content in the crystal structure as a function of the charge/discharge process are shown. It is found that the O3‐phase persists throughout the charge/discharge cycle but undergoes a series of two‐phase and solid‐solution transitions subtly modifying the sodium content and atomic positions but keeping the overall space‐group symmetry (structural motif). In addition, for the first time, evidence of a structurally characterized region is shown that undergoes two‐phase and solid‐solution phase transitions simultaneously. The Mn/Fe–O bond lengths, c lattice parameter evolution, and the distance between the Mn/FeO 6 layers are shown to concertedly change in a favorable manner for Na + insertion/extraction. The exceptional electrochemical performance of this electrode can be related in part to the electrode maintaining the O3‐phase throughout the charge/discharge process.