Metal-Ligand π Interactions in Lithium-Rich Li₂RhO₃ Cathode Material Activate Bimodal Anionic Redox

Li-rich oxide (LRO) cathodes that exhibit anionic redox activity can boost the energy density of Li-ion batteries. Oxygen redox in LROs can originate from the charge compensation of pure O 2p nonbonding (NB) states; however, the high charging voltages cause much safety concerns in practical applications. Exploiting new anionic redox modes that can be used at low voltages is thus imperative. In view of this, a further understanding of the anionic redox behavior with respect to metal-ligand interactions in LROs is highly desired. In this study, by analyzing the orbital combinations of transition metals (TMs) and O in LROs, the prevalence of π-type, σ-type, and NB states is investigated. Highly covalent Li₂RhO₃ with strong π-type interactions is selected as a model material. Owing to the closer energy levels of O and Rh and the orbital vacancy of Rh⁴⁺, oxygen acts as a π-electron donor to central Rh and exhibits high reactivity in the occupied anti-bonding state, showing a novel low-voltage O redox which is distinct from high-voltage NB O redox. This π-type oxygen redox mode expands the fundamental theories of anionic redox and provides a new design route to achieve high-capacity Li-rich cathode materials.