Ex situ NMR and neutron diffraction study of structure and lithium motion in Li7MnN4

Li7MnN4 was prepared and characterised by both long and short range probes of structure. The results of the Rietveld refinement of its crystal structure from Powder Neutron Diffraction data are in agreement with previous determination from single crystal X-ray diffraction data. The Li-6 MAS-NMR spectrum of this compound contained five main resonances, which could be assigned to the five different crystallographic sites based on the multiplicities of the different sites and an analysis of the mechanisms that contribute to the observed NMR shifts. Variable temperature and two-dimensional NMR spectra showed that the Li ions in three out of the five sites are extremely mobile in this material, and a model to explain this mobility was developed involving jumps via nearby empty octahedral sites. In order to get a deeper insight in the structural changes taking place during the reversible lithium extraction/insertion redox process in this compound, partially oxidised samples were electrochemically prepared and studied both by Li-6 MAS-NMR and powder neutron diffraction. On extracting Li, distinct Li-6 NMR resonances due to the five sites were no longer resolved, exchange occurring rapidly between the different crystallographic positions. Comparison of the experimental neutron diffraction patterns of partially oxidised samples with calculated neutron diffractograms assuming lithium vacancies in the different sites, coupled with an analysis of the Li-Mn distances, indicated that lithium extraction takes place from the Li1 and Li5 sites. (c) 2005 Elsevier B.V. All rights reserved.