Abstract
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 T1 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.
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