Session: Advances in Materials Synthesis & Characterization of Li-ion, Na-ion & Multi-Valent Batteries:

Li-ion batteries are one of the most promising energy storage sources because of their excellent electrochemical performance and have dominated the field of portable devices for the past 20 years. It has also been widely used in vehicle electrification and grid storage. Thus, the demand for Li-ion batteries with high energy density increased dramatically. The electrochemical behavior of current Li-ion batteries mainly relies on the transition metal oxide cathodes. Mn-based cathode materials have become highly desirable owing to the low cost and high earth abundance. Recently, a nanostructured disordered rocksalt-type Li₄Mn₂O₅was synthesized, which displays the highest reported capacity till date up to 355 mAh/g with relative reversibility as compared to other contemporary Li-Mn-O electrodes. In spite of such intriguing electrochemical performance, the detailed charge compensation mechanism associated with (de)lithiation is still unclear. A previous study predicted that the electrochemistry of Li₄Mn₂O₅ was driven by Oxygen/Mn redox. Further study in Mn via X-ray absorption spectroscopy (XAS) revealed that Mn⁴⁺/Mn⁵⁺ redox couple contributes a small partial capacity in the redox mechanism, indicating the significant involvement of oxygen during cycling. However, the mechanism of oxygen participation in electrochemistry was not studied. In pursuit of understanding the detailed electrochemical mechanism, in this study, X-ray absorption spectroscopic study was performed in both Mn and O K-edge at various states of the charge-discharge cycle. XAS measurements were conducted at different charged and discharged states of Li₄Mn₂O₅ cycled between 4.8V and 1.2V voltage window. The obtained Mn K-edge XAS spectra clearly unravel the change of oxidation state of Mn upon charging and discharging, with a quantitative analysis linking the oxidation state and absorption energy via the integral method. As a complementary probe to Mn K-edge XAS, O K-edge XAS unambiguously demonstrates the involvement of oxygen upon cycling and uncovers the dominant contribution of oxygen to the electrochemical performance at high voltage charging. This finding could shed light on the design of Oxygen-involved Mn-based cathodes with high capacity and reversibility.

Haifeng Li

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