Abstract
The anionic redox observed in Li-rich transition metal oxides (Li[LixM1-x]O2) has recently emerged as a lever to increase the energy density of Li-ion batteries. In these electrodes, oxygen activity during electrochemical charging is associated with the O2-/(O2 2-) transformation which, in some cases, can lead to a reaction from oxygen to O2 gas. Despite the enthusiasm about these materials over the last decade, the stability of the peroxide entities in a metallic M(d) environment as well as the reversibility of the O2-/(O2 2-) transformation are still not clearly demonstrated. To characterize the oxidized/reduced species that form during the charge/discharge processes of the battery, Raman spectroscopy can be very powerful. This technique allows to probe locally the structure of the materials and to follow in situ the evolution of the different Mn-O and O-O bonds during the electrochemical process. The aim of this study is to investigate the vibrational properties of these materials, using quantum chemical calculations based on density functional theory (DFT). To identify the spectroscopic response of the O2 n- species (n < 2), the Raman spectra of the whole series of alkali, alkaline earth and post-transitional metal peroxides, M2O2 (M = Li, Na, K, Rb, Cs) and MO2 (M = Mg, Ca, Sr, Ba, Cd, Zn) is discussed in a first part and the structural and electronic factors that govern the stability of O2 2- species in M(s,p) environments are rationalized. In a second part, the vibrational properties of the Li2-yMnO3 phases (y = 0 to 2) are discussed in an attempt to elucidate the most likely electrochemical mechanism for this electrode. The results show that the peroxide entity is not stable in a M(d) environment thus ruling out the electrochemical mechanisms recently proposed in the literature for this electrode.