Sodium ion batteries (SIBs) have emerged as suitable alternative energy storage systems to Li ion batteries (LIBs) due to the cost-effective and ample Na source. Sodium layered oxides have considerable attention as cathodes for Na-ion batteries due to easy synthesis and simple structure.

O3-type NaTMO₂ materials, where single transition metal is an oxidizable element such as Cr, Mn, Fe, Co, Ni [1], have the capability to reversible intercalation reaction of Na ions [2,3], which is different from their Li analogues system, where only nickel and cobalt do reversible intercalation of Li ions [4]. Furthermore, various mixed transition metals can be realized in the TM layer to create new oxide compounds [5,6]. Among them, O3-NaNi_1/3Fe_1/3Mn_1/3O₂ shows a capacity around 130 mAh/g with good capacity retention when cycled to 4.0 V [7].

In this study, layered sodium-ion battery cathode material, O3-type NaNi_1/3Fe_1/3Mn_1/3O₂, was systematically investigated by using synchrotron-based x-ray techniques to characterize the detailed redox mechanism during electrochemical process and to understand the role of each transition metal structural behavior in the ternary-element material. In Figure 1, reversible changes in the Ni, Fe, and Mn K-edge X-ray Absorption Near Edge Structure (XANES) spectra show that reversible electronic structural changes in the local level during Na⁺ deintercalation/intercalation in the voltage range of 2.0 – 4.0 V. Moreover, Ni and Fe elements are both active in Na_1–xNi_1/3Fe_1/3Mn_1/3O₂ cathode and redox couples of Ni²⁺/Ni³⁺/Ni⁴⁺ and Fe³⁺/Fe⁴⁺ are responsible for the charge compensation mechanism. High Resolution Powder Diffraction (HRPD) results reveal that O3-type (R-3m) phase transforms into a P3 (R3m) structure coupled with Na⁺/vacancy ordering during charge and further elucidate the final P3-OP2 of phase transformation on over-sodiated state. Furthermore, in the structure with the small quantity of sodium, internal Fe ion migration occurs from octahedral site into tetrahedral site, which is possible due to formed vacant space by highly deintercalation of Na ions and this atomic level of movement causes the layered structure to have structural distortions. An in-depth analysis of the structural behavior and reaction mechanism for NaNi_1/3Fe_1/3Mn_1/3O₂ cathode material when the Na ions are used of the entire composition widens an electrochemical perspective and suggests a direction where we better understand the nature of structure which can be used as the cathode for the advanced rechargeable batteries with high energy density.

From these experimental results, we will discuss structural evolution behavior of layered NaNi_1/3Fe_1/3Mn­_1/3O₂ cathode material. More detailed results and discussion including reaction process of Ni, Fe, and Mn in the material will be presented in the AiMES 2018 meeting.

Figure 1. XANES spectra of (a, b) Ni K-edge, (c,d) Fe K-edge, and (e,f) Mn K-edge during the 1^st cycle of Na⁺ deintercalation/intercalation process.

References:

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[4] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, et al., Energy Environ. Sci. 4 (2011)

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[7] D. Kim, E. Lee, M. Slater, W. Q. Lu, S. Rood, C. S. Johnson, Electrochem. Commun. (2012), 18, 66.