An Improved Understanding of Li+ Hopping Pathways and Rates in Li3Fe2(PO4)(3) Using Selective Inversion Li-6 NMR Spectroscopy

Li-6 selective inversion NMR experiments are used to reveal Li ion exchange rates and energy barriers for Li ion hopping in monoclinic Li3Fe2(PO4)(3). The three crystallographically unique Li sites in this material are well resolved by magic-angle spinning, thus allowing for the examination of all three exchange processes. We have revisited this material using selective inversion to probe dynamics, and energy barriers over the temperature range 268-397 K are found to be 0.37 +/- 0.07, 0.53 +/- 0.02, and 0.52 +/- 0.03 eV for the three unique exchange pairs. The results presented here are consistent with the known Li3Fe2(PO4)(3) crystal structure. The selective inversion experiment is more robust than 2D EXSY for the determination of energy barriers by NMR; this can be attributed to the efficiency of the one-dimensional technique, and an exchange model that accounts for multisite exchange and fast spin-lattice relaxation. Moreover, bond valence sum density maps provide a meaningful depiction of lithium ion diffusion pathways in this material that complement the NMR results.