4d Lithium-Rich Cathode System Reinvestigated with Electron Paramagnetic Resonance: Correlation between Ionicity, Oxygen Dimers, and Molecular O-2

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 (O-2)(n-) (n > 0), molecular O-2, and oxygen p redox. Here, we show using electron paramagnetic resonance (EPR) spectroscopy that in the 4d Li-rich ruthenate compounds, Li2Ru0.75Sn0.25O3 and Li2Ru0.5Sn0.5O3, strong covalency between 4d transition metal and oxygen can inhibit the formation of trapped molecular O-2 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 Li2Ru0.25Sn0.75O3, (O-2)- 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-(O-2)m- (m > 1)-* Ru-(O-2)- or (2) oxygen electron holes-* Sn-(O-2)m- (m > 1)-* Sn-(O-2)--* O-2, and the species to which they will evolve are related to which metal (O-2)- bonds to and whether the ionicity dominates.