Study of Lithium Dynamics in Monoclinic Li3Fe2(PO4)3 using 6Li VT and 2D Exchange MAS NMR Spectroscopy

Details of Li-mobility in Li3Fe2(PO4)3 are elucidated using solid-state 6Li NMR. Three crystallographically unique Li sites were resolved under magic angle spinning (25 kHz) with paramagnetic shifts arising at 45 ppm, 102 ppm, and 216 ppm. These resonances were assigned to the crystallographic positions based on the degree of the Fermi-contact interaction with the paramagnetic iron center. 6Li 2D exchange NMR experiments were performed under variable temperature conditions in order to determine the activation energies for hopping between lithium sites. Activation energies ranged from 0.59 (± 0.05) eV to 0.81 (± 0.04) eV, where shorter Li internuclear distances and larger Li−O bottlenecks yielded lower activation energies. These results were compared to a previous study on the isostructural Li3V2(PO4)3, which showed similar trends of increased internuclear distance (and constricted bottlenecks) yielding larger energy barriers for Li−Li exchange. Overall, the average activation energy for lithium ion hopping in the iron-based structure is lower than the vanadium analogue, which is attributed to the more open framework of the former.