Does Spinel Serve as a Rigid Framework for Oxygen Redox?

The oxygen-redox chemistry of lithium-rich layered transition-metal oxides has recently paved a path for the development of large-capacity positive electrode materials. Although oxygen-redox activity typically arises from the labile 2p states of undercoordinated oxide ions, the instability of layered structures after extracting excess Li ions often leads to transformation to a spinel structure, causing capacity fading and voltage decay after charge/discharge cycles. In this work, we perform density functional theory calculations to determine the oxygen-redox activity of spinel LiMg0.5Mn1.5O4 in an attempt to discover a robust three-dimensional framework for oxygen-redox reactions. The presence of ionic Mg2+ in the framework results in labile O 2p states for oxygen-redox activity. However, in contrast with the dominant Mn–O interactions that could stabilize oxidized oxide ions in a typical lithium-rich layered oxide Li2MnO3, spinel LiMg0.5Mn1.5O4 is dominated by O–O interactions near the Fermi level, which are less able to stabilize holes, leading to decomposition reactions that include release of oxygen gas.