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.