Direct Visualization of the Reversible O2−/O− Redox Process in Li-Rich Cathode Materials

Abstract

Conventional cathodes of Li-ion batteries mainly operate through an insertion–extraction process involving transition metal redox. These cathodes will not be able to meet the increasing requirements until lithium-rich layered oxides emerge with beyond-capacity performance. Nevertheless, in-depth understanding of the evolution of crystal and excess capacity delivered by Li-rich layered oxides is insufficient. Herein, various in situ technologies such as X-ray diffraction and Raman spectroscopy are employed for a typical material Li_1.2Ni_0.2Mn_0.6O₂, directly visualizing O⁻O⁻ (peroxo oxygen dimers) bonding mostly along the c-axis and demonstrating the reversible O²⁻/O⁻ redox process. Additionally, the formation of the peroxo OO bond is calculated via density functional theory, and the corresponding OO bond length of ≈1.3 Å matches well with the in situ Raman results. These findings enrich the oxygen chemistry in layered oxides and open opportunities to design high-performance positive electrodes for lithium-ion batteries.

A typical Li-rich material Li_1.2Ni_0.2Mn_0.6O₂ is systematically analyzed by in situ X-ray diffraction and Raman spectroscopy. Peroxo OO bonding is directly visualized mostly along the c-axis and a reversible O²⁻/O⁻ redox process is demonstrated. Additionally, the formation of peroxo OO bonds is calculated via density functional theory, and the corresponding OO bond length of ≈1.3 Å matches well with the in situ Raman results.

http://ift.tt/2EAAeQ9