Synthesis of Li 4 V(PO 4 ) 2 F 2 and 6,7Li NMR studies of its lithium ion dynamics

A single phase, well-crystallized Li4V(PO4)2F2/carbon nanocomposite has been prepared by an optimized solid-state route via oxidation of Li5V(PO4)2F2. The Li4V(PO4)2F2 composition exhibits lattice parameters close to those of the oxidized parent (a = 6.898 Å, b = 10.673 Å, c = 9.977 Å; β = 87.84°; V = 734.0 Å3). 1D 6Li 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)2F2. 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)2F2. Importantly, our studies show that upon delithiation, the ion mobility was found to increase significantly vis a vis the parent Li5V(PO4)2F2. We ascribe this to the creation of lithium vacancies within the structure that open up pathways for ion transport.