Differentiating Lithium Ion Hopping Rates in Vanadium Phosphate versus Vanadium Fluorophosphate Structures Using 1D 6Li Selective Inversion NMR

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, Li5V(PO4)2F2, using 6Li selective inversion (SI) NMR measurements. Li5V(PO4)2F2 has six crystallographically distinct lithium sites giving the possibility of fifteen exchange partners between nonequivalent lithium environments. Here, 6Li 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 Li2VPO4F yet considerably slower than results from both α-Li3V2(PO4)3 and α-Li3Fe2(PO4)3. 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.