Transition-Metal-Oxygen Covalency Controls Ligand-Hole Stability in Li-Rich Cathode

Abstract

Lithium-rich cathodes boast a large specific capacity due to the presence of excess lithium per transition metal. However, making use of that excess Li drives the cathodes to unusually high formal oxidation states, often triggering unwanted side reactions, typically signaled by gas release and commonly associated with oxygen oxidation. By varying the transition metal (TM) chemistry in Li-rich materials, cyclability can vary drastically, with some materials suffering rapid capacity fade while others are able to maintain large and stable voltage windows. Such observations indicate a crucial role for both TM and O activities, but the details remain elusive. Here, we model core-level spectroscopy to understand differences in the charge compensation mechanism between different Li2MO3 (M = Mn, Ru, Ir) cathode materials. We find that ligand holes are important throughout the series, but their stability depends strongly on the transition-metal-oxygen electronic structure. In Li2MnO3, the ground state is already strongly negative-charge-transfer-like, so further delithiation rapidly increases ligand-hole character and promotes instability. Moving from Mn to Ru and Ir increases TM-O hybridization while reducing the tendency toward an extreme negative-charge-transfer state, allowing additional holes to remain delocalized over the covalent TM-O network over a wider range of charge. These results identify TM-O covalency, rather than the absence of oxygen-hole character, as a key factor controlling reversible high-valent redox and suggest a route for stabilizing high-energy-density cathodes without invoking reversible molecular oxygen formation.