Oxygen Redox Activity in Cathodes – a Common Phenomenon Calling for Density-Based Descriptors

Lithium-excess transition metal oxide materials are promising cathode candidates for future secondary batteries due to their relatively high energy density, which is commonly related to redox-active oxygen centers. First-principle computations are crucial for the understanding of the underlying redox mechanism in these compounds, with plane-wave density functional theory being the most frequently used setup. An important tool for the assignment of the redox-active species is the projected density of states, although the atomic contributions postulated this way do not strictly correspond to any observable physical quantity. By directly analyzing the computed real-space charge density changes, on the other hand, oxygen redox activity can be found to be substantial in most transition metal oxide compounds, although a projection onto atomic states would suggest otherwise. This can be linked to the shortcomings of the commonly employed spherical approximation for ions in crystalline compounds used to compute the projected density of states, which fails to describe the charge density topology in covalent transition metal oxides and leads to a qualitatively different picture from a charge density-based approach, specifically, the underrepresentation of oxygen contributions and exaggeration of transition metal contributions to the density of states The density based approach, due to the non-spherical nature of Bader domains, is more apt to properly describe oxygen redox contributions. This raises the question how meaningful the descriptors of oxygen redox activity are and how it should be acknowledged for transition metal oxide compounds in general.