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 π redox. Here, we show using electron paramagnetic resonance (EPR) spectroscopy that in the 4d Li-rich ruthenate compounds, Li 2 Ru 0.75 Sn 0.25 O 3 and Li 2 Ru 0.5 Sn 0.5 O 3 , 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 Li 2 Ru 0.25 Sn 0.75 O 3 , (O 2 ) - will detach from Sn 4+ , eventually leading to the formation of trapped molecular O 2 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.