Relationship Between the Structural Evolution and Electrochemical Properties of NaxFe0.5Mn0.5O2 (0.67≤x≤0.83) As the Cathode Materials for Na-Ion Batteries

Motivated by the demand for the high energy density battery and low cost, Na-ion batteries have attracted much interest in recent years. Search for the new Na-ion insertion materials are the key elements to achieve the practical Na-ion batteries. Following the concept of Li-ion batteries, a series of layered NaMO₂ (M=Co, Mn, Cr, V and Fe) materials have been proposed to show the good electrochemical performance. [1]

Na_xFe_1/2Mn_1/2O₂ has been considered as one of the most perspective cathode materials for Na-ion batteries.[1] Based on the coordination of Na ions, Na_xFe_1/2Mn_1/2O₂ shows two different types of crystal structures: P2 type for Na_0.67Fe_0.5Mn_0.5O₂ (space group P6₃/mmc) and O3 type for Na_0.83Fe_0.5Mn_0.5O₂ (space group R-3m). Furthermore, the variation of the valence state and the effect of Jahn-Teller distortion for high-spin Fe⁴⁺ (t_2g³e_g¹) and Mn³⁺ (t_2g³e_g¹) ions, induced by different content of Na ions, have a strong impact on the electrochemical performance for Na_xFe_1/2Mn_1/2O₂. Hence, it is very meaningful to study the structural evolution between P2-Na_0.67Fe_1/2Mn_1/2O₂ and O3-Na_0.83Fe_1/2Mn_1/2O₂ and its influence on the electrochemical performance as the cathode materials for Na-ions batteries.

In this study, we presented a comprehensive study on the synthesis, crystal and electronic structure, and electrochemical properties for Na_xFe_1/2Mn_1/2O₂ (0.67≤x≤0.83) by means of Rietveld analysis, Raman spectra, XPS, magnetic susceptibility, discharge/charge and CV measurements. We focus on the structural insights we have obtained for Na_xFe_1/2Mn_1/2O₂ with different Na ions contents using conventional, as shown in Figure 1, and synchrotron X-ray diffraction. We also discuss the impact of structural evolution on the electronic structure and electrochemical properties for Na_xFe_1/2Mn_1/2O₂ (0.67≤x≤0.83). The possible implications of our findings for the practical application of these materials as the cathode materials are also evaluated.

Reference:

[1] Naoaki Yabuuchi, Masataka Kajiyama, Junichi Iwatate, Nature materials, 11, 512, 2012