The Unique Structural Evolution of the O3-Phase Na_2/3Fe_2/3Mn_1/3O₂ 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_2/3Fe_2/3Mn_1/3O₂ 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/FeO₆ 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. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.