Synthesis of Li4V(PO4)2F2 and 6,7LiNMR studies of its lithium ion dynamics

Abstract

A single phase, well-crystallized Li₄V(PO₄)₂F₂/carbon nanocomposite has been prepared by an optimized solid-state route via oxidation of Li₅V(PO₄)₂F_2. 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 ų). 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.