Cationic-anionic redox couple gradient to immunize against irreversible processes of Li-rich layered oxides

The ability to extract/insert more than one Li per formula unit has made Li-rich layered oxides (LLO) one of the most promising cathode materials. However, irreversible transformations triggered by over-delithiation such as phase transitions, oxygen release and Jahn-Teller effects of Mn3+ have limited its practical application. In this work, the irreversible processes during repetitive de/lithiation were found to be diminished by establishing a gradient cationic redox couple of Mn3+/Mn4+ in Li1.2Ni0.2Mn0.6O2. As revealed by STEM, XPS and XAS measurements, the partial substitution of O2- by F- ions promoted nearby Li/transition metal mixing and reduced the valence state of Mn on the surface. Such a configuration shifted the surface redox center towards cationic redox couple (Mn3+/Mn4+), reducing the irreversible oxygen release as well as the ensuing structure and oxidation state changes. As a result of the modification, the product delivered a discharge capacity of 203.4 mA h g(-1) after 80 cycles at 0.2C and achieved capacity retention of 89.6% after 100 cycles at 0.5C. The suppressed irreversible processes during repetitive cycling were investigated through ex situ X-ray absorption energy near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy and DFT calculations, which confirmed the well-preserved oxidation states and atomic configurations in modified Li1.2Ni0.2Mn0.6O2. Overall, this research provided a new avenue to control the irreversible processes in LLO without changing the anion redox behavior of lattice O2- in the bulk area by accommodating the cationic redox couple on the surface.

Automated summary

This study successfully reduced the irreversible processes and improved the cycle stability and electrochemical performance by accommodating the cationic redox couple on the surface of Li-rich layered oxides.