Synthesis of Li4V(PO4)(2)F-2 and Li-6,Li-7 NMR studies of its lithium ion dynamics

A single phase, well-crystallized Li4V(PO4)(2)F-2/carbon nanocomposite has been prepared by an optimized solid-state route via oxidation of Li5V(PO4)(2)F-2. The Li4V(PO4)(2)F-2 composition exhibits lattice parameters close to those of the oxidized parent (a = 6.898 angstrom, b = 10.673 angstrom, c = 9.977 angstrom; beta - 87.84 degrees; V - 734.0 angstrom(3)). 1D Li-6 solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) studies identified which of the six lithium ions are removed from the lattice of the parent Li5V(PO4)(2)F-2. The results are in perfect accord with previous NMR-based predictions of which sites would be the most mobile and thereby most easily extracted upon cycling. Variable-temperature NMR studies and 2D exchange spectroscopy (EXSY) are used to probe the Li ion dynamics in Li4V(PO4)(2)F-2. Importantly, our studies show that upon delithiation, the ion mobility was found to increase significantly vis a vis the parent Li5V(PO4)(2)F-2. We ascribe this to the creation of lithium vacancies within the structure that open up pathways for ion transport.