X-Ray Absorption Spectroscopy Study to Elucidate the Redox Reaction Mechanism of Li4Mn2O5 upon Delithiation

Abstract

Li-ion batteries are one of the most promising energy storage sources because of excellent electrochemical performance and has 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 of Li-ion batteries with high energy density increased dramatically. The electrochemical behavior of current Li-ion batteries mainly relies on the transition metal oxide cathodes1,2,3. Mn-based cathode materials have become highly desirable owing to the low cost and high earth abundance. Recently, a nanostructured disordered rocksalt-type Li4Mn2O5 was 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 electrodes4,5. In spite of such intriguing electrochemical performance, the detailed charge compensation mechanism associated with (de)lithiation is still unclear. 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, 4,5 indicating the significant involvement of oxygen during cycling. However, the mechanism of oxygen participation in the 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 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 unravels the change of oxidation state of Mn upon charging and discharging, with a quantitative analysis linking the oxidation state and absorption energy via 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.