Abstract
Titanium doping in lithium manganese oxide spinels was shown to be beneficial for the structural stability of the potential Li-ion battery cathode materials LiTixMn2-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+-Mn3+/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-Mn3+/4+-Ti octahedral configuration is determined for the x = 1.0 case. A specific cation ordering in the partially inverse LiTi1.5Mn0.5O4 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.
| Original language | English |
|---|---|
| Pages (from-to) | 817-829 |
| Number of pages | 13 |
| Journal | Chemistry of Materials |
| Volume | 30 |
| Early online date | 23 Jan 2018 |
| DOIs | |
| Publication status | Published - 13 Feb 2018 |
Bibliographical note
Funding Information:The authors are thankful to Dr. Andrew J. Morris, Dr. Michael Gaultois, and David Halat for useful discussions and to Hajime Shinohara and Dr. Sian Dutton for helping with the SQUID measurements. R.P. acknowledges financial support from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA Grant 317127. C.M. acknowledges the School of the Physical Sciences of the University of Cambridge for funding through an Oppenheimer Research Fellowship. Via our membership of the UK’s HPC Materials Chemistry Consortium, which is funded by EPSRC (Grant EP/L000202), this work made use of the facilities of ARCHER, the UK’s national high-performance computing service. Computational work was also carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. R.P. and A.J.P. acknowledge funding from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. DOE under Contract no. DE-AC02-05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) Program subcontract no. 7057154. Data supporting this work are available from: https://doi.org/10. 17863/CAM.17508.
Publisher Copyright:
© 2017 American Chemical Society.
Data Availability Statement
The Supporting Information is available free of charge on the ACS Publications website at DOI doi/full/10.1021/acs.chemmater.7b04314Additional computational details, synthesis procedure, complete convex hull, additional lattice simulations, and details and results of magnetic SQUID measurements (PDF)