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

Abstract

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 Li_1.2Ni_0.2Mn_0.6O₂. 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 Li_1.2Ni_0.2Mn_0.6O₂. 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.