7Li and 51V MAS NMR study of the electrochemical Behavior of Li1-xV3O8

Li-7 and V-51 MAS NMR spectra acquired during the electrochemical cycling of the layered battery material Li1+xV3O8 have been used to follow the local structural and electronic changes that occur during the different stages of intercalation/deintercalation. Several resonances are observed using V-51 MAS NMR and an attempt to assign them to the different vanadium sites on the basis of the differences in the width of their sidebands manifolds is made. Both the V-51 and Li-7 NMR spectra show multiple vanadium and lithium local environments, and the spectra cannot be explained by using a simple model, based on the number of crystallographically distinct vanadium sites. Deintercalated samples with x close to 0 (prepared at potentials of 4.2 V vs Li) give V-51 spectra with very poor resolution. On lithium intercalation, the V-51 NMR resonances sharpen and shift to higher frequencies; three sharp resonances along with two broader resonances are clearly resolved for the samples prepared at potentials of 3.4 and 3.0 V (x = 0.3 and 0.5, respectively). This behavior is consistent with solid solution behavior in this potential range. Three lithium sites are observed which are assigned to the octahedral site and two tetrahedral sites between the lithium layers. The phase transition that is seen in the electrochemical data, but not with structural probes (such as X-ray diffraction), and that occurs close to 2.8 V (to form Li2V3O8), results in a decrease in the resolution of the V-51 resonances, and the observation of new Li-7 resonances at negative frequencies with much shorter relaxation times. Variable temperature experiments confirm that the shifts and short relaxation times are due to a transferred hyperfine interaction. Based on the Li-7 and V-51 spectra, the phase transition is ascribed to the formation of localized V4+ and V5+ ions. Further intercalation to form Li4V3O8 results in the complete loss of the V-51 resonances and larger Li-7 hyperfine shifts, due to the presence of unpaired (d(1)) electrons.