Identification of Degree of Ordering in Spinel LiNi0.5Mn1.5O4 through NMR and Raman Spectroscopies supported by Theoretical Calculations

The performance of the high voltage spinel LiNi0.5Mn1.5O4 (LNMO) in Li-ion batteries strongly depends on its synthesis conditions, actual Ni/Mn stoichiometry, and degree of ordering of Ni and Mn. Depending on the extent of this ordering, the spinel structure can be described in the conventional space group Fd3¯m as the non-substituted LiMn2O4 or, for the highly ordered phase, in the space group P4332. As previously reported in the literature, using neutron and electron diffraction, a qualitative description of the extent of ordering can be achieved and roughly related to the electrochemical performance of LNMO. To deeper characterize and understand this complex system, in this paper, we will show that Raman spectroscopy, and especially the characteristics of the band located at 160 cm−1 attributed for the first time to a twisting motion of octahedral entities, allow to estimate the degree of ordering in LNMO, whereas NMR spectroscopy allows to give a clear description of the local environments of Li, in relationship with the Ni/Mn stoichiometry and extent of ordering. Theoretical calculations were used to support the analysis and attribution of the Raman and NMR signals/spectra. These spectroscopic characterizations enabled in-depth insights into the complexity of LNMO in stoichiometry, degree of ordering, and purity versus the presence of rock-salt or layered oxides as defects or crystalline domains.