Redox Potential Paradox in NaxMO2 for Sodium-Ion Battery Cathodes

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• 作者：Yusuke Nanba ; Tatsumi Iwao ; Benoit Mortemard de Boisse ; Wenwen Zhao ; Eiji Hosono ; Daisuke Asakura ; Hideharu Niwa ; Hisao Kiuchi ; Jun Miyawaki ; Yoshihisa Harada ; Masashi Okubo ; Atsuo Yamada
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：February 23, 2016
• 年：2016
• 卷：28
• 期：4
• 页码：1058-1065
• 全文大小：611K
• ISSN：1520-5002

文摘

Raising the operating potential of the cathode materials in sodium-ion batteries is a crucial challenge if they are to outperform state-of-the-art lithium-ion batteries. Although the layered transition metal oxides, NaMO₂ (M: transition metal), are the most promising cathode materials owing to their high theoretical capacity with much more stable nature than Li_1–xMO₂ system, factors influencing the redox potential have not yet been fully understood. Here, we identify redox potential paradox, E(Ni³⁺/Ni²⁺) > E(Ni⁴⁺/Ni³⁺), in an identical structural framework, namely, NaTi⁴⁺_0.5Ni²⁺_0.5O₂ and NaFe³⁺_0.5Ni³⁺_0.5O², which is induced by transition of the oxides from Mott–Hubbard to negative charge-transfer regimes. The origin of the unusually low E(Ni⁴⁺/Ni³⁺) is the surprisingly large contribution (over 80%) of oxygen orbital to the redox reaction, of which the primary effect on the electrochemical property is demonstrated for the first time, providing a firm platform to design better cathodes for advanced sodium-ion batteries.