Li-6 1D EXSY NMR Spectroscopy: A New Tool for Studying Lithium Dynamics in Paramagnetic Materials Applied to Monoclinic Li2VPO4F

Li-6 selective inversion NMR studies are used to probe details of Li mobility in Li2VPO4F. Two crystallographically unique Li sites were resolved under magic-angle spinning (25-40 kHz) with paramagnetic shifts arising at 46 and -47 ppm (330 K). The rate of exchange between these sites was evaluated using selective inversion (or one-dimensional exchange (1D EXSY)) NMR. This methodology relies on relaxation-based experiments that provide a means for mobility time scales to be determined for materials in which Li+ ions exchange slowly relative to their T-1 spin-lattice relaxation. This situation is particularly relevant to cathode materials for lithium ion batteries, where the unpaired electrons of the transition-metal centers provide a dominant mechanism for rapid relaxation. In Li2VPO4F, Li1-Li2 exchange pair jump rates extend from 24 (+/- 1) to 55 (+/- 4) Hz over a temperature range of 330-350 K The activation energy for this ion exchange process was measured to be 0.44 (+/- 0.06) eV.