Structural Evolution and Redox Mechanism of O3-NaNi_1/3Fe_1/3Mn_1/3O₂ Layered Cathode for Na Rechargeable Batteries

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_xTMO₂, 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 NaNi_1/3Fe_1/3Mn_1/3O₂, 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 Na_1–xNi_1/3Fe_1/3Mn_1/3O₂ 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, Na_1-xNi_1/3Fe_1/3Mn_1/3O₂ 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 NaNi_1/3Fe_1/3Mn_1/3O₂ 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 NaNi_1/3Fe_1/3Mn_1/3O₂ cathode material. More detailed results and discussion will be presented in the 237^th ECS meeting.

References:

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