Lithium-ion batteries operate on cationc redox reactions accompanied by insertion-deinsertion of Li+. They are nowadays primary energy source for portable electronics and of great importance for electric transportation. To meet increasing market requirements, improvement of batteries parameters, in particular the energy density, is an ongoing research in the field of electrochemical energy storage. Recently the Li-rich layered transition metal oxides (LLTMOs), of general formulation Li[LixM1-x]O2, used as cathode materials were shown to be promising for increasing energy density of Li and Na ion batteries. In this systems M in the metallic layer is partially substituted by Li, which enables a cumulative cationic (M4+/M5+) and anionic (2O2-/(O2)n- oxo to peroxo transformation) redox activity, thus increasing the capacity. However, the oxidation of oxygen can lead to irreversible structural transformations, voltage hysteresis or even oxygen release, which preclude LLTMO-cathodes from commercialization. Recent theoretical results suggest that the electronic ground state of the lithiated oxide (Mott-Hubbard or charge transfer) is an important parameter that govern the mechanism of the anionic process in charge. Nonetheless, no clear consensus has yet been reached on the reversibility of the anionic process and the associated structural reorganization of the oxygen network. In our work, we want to answer whether the X-ray absorption spectroscopy (XAS) can be a technique of choice to address this question. In the X-ray absorption spectroscopy the core electron of a chosen atom is excited to the available unoccupied states with well defined symmetry and therefore any variations in the electronic and the local structure of LLTMO upon cycling can be seen in the measured spectrum. We used density functional theory (DFT) methods and its DFT+U variant to simulate the XAS spectra for different LLTMO displaying different electronic ground state and follow the XAS evolution at various delithiation stages. We will show that true peroxides (O2)2- species at 1.5 Å cannot be stabilized in these compounds when oxygen is surrounded by a transition metal. This is correlated to the charge transfer gap of oxides with respect to O2-/(O2)2- redox couple. This rationalization allowed us to propose M/M' chemical substitutions that can limit or suppress these irreversibilities. Moreover, our approach allowed us to benchmark various DFT methods in their ability to reproduce XAS spectra.

Ab initio simulations of XAS spectra for Li-rich transition metal oxides

2019 Fall National Meeting

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