4d Lithium-Rich Cathode System Reinvestigated with Electron Paramagnetic Resonance: Correlation between Ionicity, Oxygen Dimers, and Molecular O<sub>2</sub>
Xiang Wu, Hui Liu, Xiaobing Lou, Fushan Geng, Jingxin Li, Chao Li, Bingwen Hu
Abstract
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.