O3-type NaNi1/3Fe1/3Mn1/3O2 layered cathode for Na-ion batteries: Structural evolution and redox mechanism upon Na (de) intercalation

Layered sodium-ion battery cathode, O3-type NaNi1/3Fe1/3Mn1/3O2, has been systematically investigated by synchrotron-based analyses to characterize the structural behavior during electrochemical reaction. X-ray absorption spectroscopy shows reversible redox process upon cycling and clearly proves that both Ni and Fe are active in Na1–xNi1/3Fe1/3Mn1/3O2 and that redox couples of Ni2+/Ni4+ via Ni3+ and Fe3+/Fe4+ are responsible for charge compensation. Specifically, the capacity is mainly realized with Ni2+/Ni4+ and slightly from Fe3+/Fe4+ under charging voltage of 4.0 V. At high voltage (>4.0 V), however, Fe redox reaction is dominant and Ni contributes slightly to capacity. In structural evolution, Na1-xNi1/3Fe1/3Mn1/3O2 undergoes phase transformation from O3 to P3 phase below 4.0 V and further reaches OP2 structure above 4.0 V along with a significant contraction of d-spacing. Moreover, quantitative analysis of extended X-ray absorption fine structure suggests that disorder of local structure for Fe is greatly increased in high voltage region. Accordingly, collapse of d-spacing can be considered as being caused by Fe migration in the TM layer into the neighboring Na layer. This study thus provides detailed redox behavior and factor of structural distortions under high voltage region by considering bulk and local structural changes.