4d Lithium-Rich Cathode System Reinvestigated with Electron Paramagnetic Resonance: Correlation between Ionicity, Oxygen Dimers, and Molecular O₂

Layered lithium-rich (Li-rich) oxide cathodes with additional capacity contribution via oxygen redox are promising high energy density cathodes for next generation Li-ion batteries. However, the chemical states of the oxidized oxygen in charged materials are under fierce debate, including the O^2- with stable electron holes, O-O dimer (O₂)ⁿ⁻ (n > 0), molecular O₂, and oxygen π redox. Here, we show using electron paramagnetic resonance (EPR) spectroscopy that in the 4d Li-rich ruthenate compounds, Li₂Ru_0.75Sn_0.25O₃ and Li₂Ru_0.5Sn_0.5O₃, strong covalency between 4d transition metal and oxygen can inhibit the formation of trapped molecular O₂ but not suppress the formation of O-O dimer. As the covalent bond of Ru-O weakens and the ionic bond Sn-O becomes dominant in Li₂Ru_0.25Sn_0.75O₃, (O₂)⁻ will detach from Sn⁴⁺, eventually leading to the formation of trapped molecular O₂ during the deep oxygen redox. We propose two possible evolution paths of oxidized oxygen as (1) oxygen electron holes → Ru-(O₂)^m− (m > 1) → Ru-(O₂)⁻ or (2) oxygen electron holes → Sn-(O₂)^m− (m > 1) → Sn-(O₂)⁻ → O₂, and the species to which they will evolve are related to which metal (O₂)⁻ bonds to and whether the ionicity dominates.