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

6Li 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.