To improve the energy and power density of Na-ion batteries, an increasing number of researchers have focused their attention on activation of the anionic redox process. Although several materials have been proposed, few studies have focused on the Na-rich materials compared with Li-rich materials. A key aspect is sufficient utilization of anionic species. Herein, a comprehensive study of Mn-based Na1.2 Mn0.4 Ir0.4 O2 (NMI) O3-type Na-rich materials is presented, which involves both cationic and anionic contributions during the redox process. The single-cation redox step relies on the Mn3+ /Mn4+ , whereas Ir atoms build a strong covalent bond with O and effectively suppress the O2 release. In situ Raman, ex situ X-ray photoelectron spectroscopy, and soft-X-ray absorption spectroscopy are employed to unequivocally confirm the reversibility of O2 2- species formation and suggest a high degree of anionic reaction in this NMI Na-rich material. In operando X-ray diffraction study discloses the asymmetric structure evolution between the initial and subsequent cycles, which also explains the effect of the charge compensation mechanism on the electrochemical performance. The research provides a novel insight on Na-rich materials and a new perspective in materials design towards future applications.
Keywords: Ir doping; Mn based Na-rich materials; Na-ion batteries; anionic redox; in situ characterization.
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