Understanding the Structural Evolution and Redox Mechanism of a NaFeO₂-NaCoO₂ Solid Solution for Sodium-Ion Batteries

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• 作者：Kei Kubota ; Takuya Asari ; Hiroaki Yoshida ; Naoaki Yaabuuchi ; Hiromasa Shiiba ; Masanobu Nakayama and Shinichi Komaba
• 刊名：Advanced Functional Materials
• 出版年：2016
• 出版时间：September 6, 2016
• 年：2016
• 卷：26
• 期：33
• 页码：6047-6059
• 全文大小：4520K
• ISSN：1616-3028

文摘

Na-ion batteries have become promising candidates for large-scale energy-storage systems because of the abundant Na resources and they have attracted considerable academic interest because of their unique behavior, such as their electrochemical activity for the Fe³⁺/Fe⁴⁺ redox couple. The high-rate performance derived from the low Lewis-acidity of the Na⁺ ions is another advantage of Na-ion batteries and has been demonstrated in NaFe_1/2Co_1/2O₂ solutions. Here, a solid solution of NaFeO₂-NaCoO₂ is synthesized and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end-members. The combined analysis of operando X-ray diffraction, ex situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations for Na_{1– x} Fe_1/2Co_1/2O₂ reveals that the O3-type phase transforms into a P3-type phase coupled with Na⁺/vacancy ordering, which has not been observed in O3-type NaFeO₂. The substitution of Co for Fe stabilizes the P3-type phase formed by sodium extraction and could suppress the irreversible structural change that is usually observed in O3-type NaFeO₂, resulting in a better cycle retention and higher rate performance. Although no ordering of the transition metal ions is seen in the neutron diffraction experiments, as supported by Monte-Carlo simulations, the formation of a superlattice originating from the Na⁺/vacancy ordering is found by synchrotron X-ray diffraction for Na_0.5Fe_1/2Co_1/2O₂, which may involve a potential step in the charge/discharge profiles.