Does Spinel Serve As a Rigid Framework for Oxygen Redox?

Abstract

A major challenge for the development of lithium-ion batteries with high energy densities is to discover a large-capacity cathode material, and an additional oxygen redox capacity has recently been regarded as a new impetus. From the electronic-structure viewpoint, the best-studied oxygen-redox cathode materials, lithium-rich layered oxides possess oxide ions with nonbonding O 2p states along the Li-O-Li coordination axis, which provide the oxygen redox activity. After oxygen oxidation, either a π-type M t 2g-O 2p interaction (M: transition metal) or an O-O bond formation stabilizes the oxidized oxide ions, allowing for reaction reversibility. However, the layered structure is generally unstable after extracting an excess amount of Li ions, leading to the degradation of both capacity and voltage during charge/discharge cycles. It is therefore particularly important to implement oxide ions with nonbonding O 2p states into a stable host framework against excess Li-ion extraction to achieve reversible oxygen-redox capacity. As spinel oxides have a stable three-dimensional framework, it is attractive to explore for spinel oxides capable of the oxygen-redox reactions. Here, we theoretically diagnose the oxygen-redox activity of spinel oxides using density functional theory (DFT) calculations. LiMg0.5Mn1.5O4 is employed as a model material, as its spinel structure is expected to have oxide ions with nonbonding O 2p state owing to the existence of ionic Mg2+ in the framework. The electronic structure change during the oxygen-redox reaction of LiMg0.5Mn1.5O4 is compared with that of a typical Li-rich layered oxide (Li2MnO3), revealing the lability of oxidized oxide ions in a spinel oxide.