Structural Evolution and Redox Mechanism of O3-NaNi1/3Fe1/3Mn1/3O2 Layered Cathode for Na Rechargeable Batteries
Authors: Jeong Mihee , Lee Hayeon , Yoon Jaesang , Yoon Won-Sub 
Published: May 2020

Sodium-ion batteries (SIBs) have great potential to alternate Li-ion batteries (LIBs) for large-scale energy storage systems in view of easy accessibility to Na resources and low cost [1–4]. Since Na ions are similar chemical characteristics of Li ions, the knowledge from research in LIBs can be easily applied to Na-based systems. Derived from the equivalent structures of Li analogue, various electrode materials such as oxides, polyanionic compounds, and sulfates, have been researched in SIBs to date [2,4–8]. One of the cathode candidates for SIBs, layered transition metal oxides (Na
x
TMO2, x ≤ 1, TM = transition metals) are of great interest due to their potential of relatively high capacity, simple structure, and easy synthesis [9,10].

In this study, 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–x
Ni1/3Fe1/3Mn1/3O2 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, Feredox reaction is dominant and Ni contributes slightly to capacity. In terms of structural evolution, Na1-x
Ni1/3Fe1/3Mn1/3O2 undergoes phase transformation from O3 to P3 structure 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 will give a better understanding of phase transformation and clear charge compensation of NaNi1/3Fe1/3Mn1/3O2 layered cathode during Na+ deintercalation/intercalation. Furthermore, we propose the factor to bring the structural distortions under high voltage region by examining the local environment changes of each transition metal.

From these experimental results, we will discuss structural evolution behavior and particular redox reaction of layered NaNi1/3Fe1/3Mn1/3O2 cathode material. More detailed results and discussion will be presented in the 237th ECS meeting.

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