Structural Characterization of the Li-Ion Battery Cathode Materials LiTi_xMn_2-xO₄ (0.2 ≤ x ≤ 1.5): A Combined Experimental 7Li NMR and First-Principles Study

Titanium doping in lithium manganese oxide spinels was shown to be beneficial for the structural stability of the potential Li-ion battery cathode materials LiTi_xMn_2-xO₄, 0.2 ≤ x ≤ 1.5, yet the distribution of Li/Ti/Mn in the structure and the cation oxidation states, both pivotal for the electrochemical performance of the material, are not fully understood. Our work investigates the changes in the local ordering of the ions throughout this series by using a combination of ⁷Li NMR spectroscopy and ab initio density functional theory calculations. The ⁷Li NMR shifts are first calculated for a variety of Li configurations with different numbers and arrangements of Mn ions in the first metal coordination shell and then decomposed into Li-O-Mn bond pathway contributions to the shift. These Li-O-Mn bond pathways are then used to simulate and assign the experimental NMR spectra of different configurations and stoichiometries beyond those in the initial subset of configurations via a random distribution model and a reverse Monte Carlo approach. This methodology enables a detailed understanding of the experimental ⁷Li NMR spectra, allowing the variations in the local ordering of the ions in the structure to be identified. A random distribution of Ti⁴⁺-Mn^3+/4+ sites is found at low Ti content (x = 0.2); an inhomogeneous lattice of Mn⁴⁺-rich and Ti⁴⁺-rich domains is identified for x = 0.4, and single-phase solid solution is observed for x = 0.6 and 0.8. A mixed Li-Mn²⁺ tetrahedral and Li-Mn^3+/4+-Ti octahedral configuration is determined for the x = 1.0 case. A specific cation ordering in the partially inverse LiTi_1.5Mn_0.5O₄ case is found, which transforms into a two-phase network of disordered Mn³⁺-rich and ordered Mn²⁺-rich domains for x = 1.1-1.4.