Unlike cathodes for lithium-ion batteries, oxygen redox (OR) processes at a high voltage ((Formula presented.) 4.2 V) during the first charge in sodium-ion batteries (SIBs) employ some Li-incorporated Mn oxides that is recovered during subsequent discharge. To determine the intrinsic origin, P2-type Na0.6[Li0.2Mn0.8]O2 exhibiting a reversible OR-induced two-phase reaction was investigated using experiments and first-principle calculations. First, operando X-ray diffraction results in reversible P2-Z phase transformations and thermodynamic analysis show the two-phase reaction features Li migration into the tetrahedral sites from the transition-metal layer in the latter phase. Second, Li-induced decoupling of the oxygen 2p-electron led to selective anion redox activity depending on the oxygen sites that are Li-rich (redox-active) and Mn-rich (redox-inactive) environments. Third, redox-active oxygen coordinated to the Li vacancy predominantly participates in the formation of peroxo-like dimers with distortion of the MnO6 octahedron, as observed in the reversible extended X-ray absorption fine structure spectra during the OR reaction. Considering three physicochemical perspectives, we reveal that Li ions play a role in activating OR reactions and control OR participation in the charge-compensation process. Our findings suggest that the Li/Mn ratio is a critical factor for achieving a reversible OR reaction, and broaden the possibilities of exploiting OR to reach high-energy densities in next-generation SIBs.
Bibliographical noteFunding Information:
D.H.K. acknowledges the supports of a National Research Foundation of Korea (NRF) grant, funded by the Korean Government (MSIT) (No. 2019R1F1 A1052498) and Kyung Hee University in 2020 (KHU-20201108). S.-H.Y. acknowledges the support of the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (NRF-2020R1 C1 C1012308). I.-H.K. acknowledges the support of the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (NRF-2020R1I1 A1 A01072724).
© 2021 Wiley-VCH GmbH
- anion redox
- first-principles calculations
- layered oxides
- oxygen redox
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