The discovery of anionic redox in Li-rich layered transition metal oxides (LLOs) has generated great hopes in the scientific community that this phenomenon could lead to a new generation of high energy density batteries. In such Li2MO3 materials, oxygen ions exhibit an electrochemical activity that cumulates with the classical cationic activity observed in classical LiMO2 materials. In principle, the extra-capacity due to anions could reach 1 electron per oxygen thereby yielding a substantial increase of the material capacity. A literature survey shows that, in rather covalent Li2MO3 electrodes, the anionic activity is associated with the reversible formation of peroxo-like species (with O-O bonds around 2.4Å) while in more ionic materials, an irreversible release of O2 is oberved during the first charge. The peroxo-like bonds formation process is then crucial for LLOs structural stability, yet not fully understood and needs further investigations. Nowadays, most techniques used to study anionic process in LLOs (XAS, RIXS, XPS …) focus on the variation of the metal and oxygen oxidation states upon charge/discharge and do not give an accurate or quantitative analysis of the local bonds. In contrast, the Raman spectroscopy is a powerful and user-friendly technique that allows to follow bonds formation «in situ » during the electrochemical process. The aim of our work is to identify the vibrational signature of the peroxo and peroxo-like species in order to follow the evolution of the M-O and the O-O bonds formation in Li2MO3 cathode materials during the battery operation. Using density functional theory methods we calculate the Raman spectra with the intention to make the connexion between the covalency of the M-O bond and the anionic redox. In order to remove all the ambiguities that arise in LLOs materials and verify whether the covalency is the leading parameter of peroxo bond formation, we study the peroxo bonds (1.5Å) in simple alcali A2O2 series (A = Li, Na, K). To confirm the methodology we benchmarked the influence of the basis sets and of different functionals beforehand. Irrespective of the alcali metal we found that the frequency of a peroxo bond is around 800cm-1 . Thanks to this signature we were able to explore a series of LLOs and to show that true (O2)2- peroxides cannot be stabilized in a transition metal environment, thereby limiting the anionic redox process to 1/3 electrons per oxygen.

Raman spectroscopy for energy materials

2019 Fall National Meeting

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