Differentiating Lithium Ion Hopping Rates in Vanadium
Phosphate versus Vanadium Fluorophosphate Structures Using 1D <sup>6</sup>Li Selective Inversion NMR
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Abstract
The electrochemical performance of
lithium ion batteries is strongly
correlated with the ion dynamics within the electrode structures.
This study characterizes Li ion hopping rates and energy barriers
in the layered phase, Li<sub>5</sub>V(PO<sub>4</sub>)<sub>2</sub>F<sub>2</sub>, using <sup>6</sup>Li selective inversion (SI) NMR measurements.
Li<sub>5</sub>V(PO<sub>4</sub>)<sub>2</sub>F<sub>2</sub> has six crystallographically
distinct lithium sites giving the possibility of fifteen exchange
partners between nonequivalent lithium environments. Here, <sup>6</sup>Li 1D SI measurements over a variable temperature range were used
to quantify the time scales and energy barriers of ion mobility for
several ion pairs observed to participate in ion hopping. The rates
determined in this material are similar in range to the previously
determined rates found in tavorite Li<sub>2</sub>VPO<sub>4</sub>F
yet considerably slower than results from both α-Li<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> and α-Li<sub>3</sub>Fe<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>. A detailed analysis of
the structural features that enhance or inhibit fast ion mobility
is discussed. This includes a consideration of the bond valence density
maps of the diffusion pathway. Comparison of the ion mobilities in
the phosphates and fluorophosphates shows how the gains in redox potential
come at the expense of fast ion mobility, meaning that any improvements
to the energy output of the lithium ion battery through higher voltage
may be compromised due to slow charge/discharge rates